Weld line is caused by cooling of front part of molten resin from different directions and failure to completely fuse at the junction. Fusion occurs at confluence of resin. Fusion occurs when two resin flows meet. At this time, the lower temperature of the two, the more obvious fusion.

 

Since two resin flows at fusion point do not mix with each other (because they are semi-cured while moving forward in jet), if temperature is low, surface will become thicker, texture will be obvious, and strength will also decrease. This is because bonding between two becomes weaker. On the contrary, if temperature of two resin flows is higher, bonding will be enhanced and appearance will become less obvious. At fusion point, two molten resins are squeezed, and bonding condition here depends on pressure applied at this point.
The lower holding pressure, the more obvious fusion and the lower strength. If we consider not only setting of holding pressure, but also condition that actual pressure applied at fusion point will decrease, then above (i) to (iv) are almost equally applicable. This is because as solidification proceeds, pressure transmission becomes more difficult. In addition, if gate size becomes smaller and gate position becomes worse, appearance and strength of fusion will deteriorate. Fusion is confluence point of resin and may also be the end of flow. At this time, if an exhaust port is not set well at this position to exhaust gas, appearance and strength of fusion will deteriorate. In general, it mainly affects appearance, has an impact on painting and electroplating. In severe cases, it affects strength of product (especially in the case of fiber-reinforced resins, it is particularly serious). You can refer to following items for improvement:
l) Adjust molding conditions to improve fluidity. For example, increase resin temperature, increase mold temperature, increase injection pressure and speed, etc.
2) Add venting grooves. Setting ejector rods at weld mark is also conducive to venting. Venting steel is also a good choice, but it is expensive.
3) Set process overflow and use it as weld mark, and then cut it off after molding.
4) If it affects appearance, position of glue injection can be changed to change position of weld mark.

1. Injection rate is too high. When melt is ejected, due to elasticity of melt, when melt flows rapidly from barrel through mold gate to mold cavity, melt recovers too quickly due to elasticity, causing melt to rupture and produce radial patterns.

 

●Solutions: ① Change process conditions. Using high pressure and low speed injection can increase flow time of elastic melt on same flow length, increase degree of elastic failure, and thus reduce appearance of radial patterns.
② Change shape of mold gate. Enlarging gate or changing gate to a fan shape can slightly restore elasticity of melt before it enters mold cavity to avoid melt rupture.
③ Lengthen length of main runner of mold. Before melt enters mold cavity, elasticity of melt is invalidated first, which can also avoid melt fracture.
④ Replace equipment with an extended nozzle. Lengthening flow path of melt before entering mold cavity increases degree of melt elasticity failure and avoids appearance of radial lines due to melt fracture.

1. Apparent grey or black clouding may occur near gate, in the middle of flow channel and in the part far from gate. It can only be seen in transparent parts, and often occurs in products made of PMMA, PC and PS materials.
Physical reason: If metering process starts too early, air wrapped in particles in screw feeding zone does not overflow feeding port, and air will be squeezed into melt. However, pressure in feeding zone is too low to move air to back. Air squeezed into melt in barrel will cause grey or black clouding in product. Just like what happens in a compression ignition diesel engine, coking phenomenon caused by air squeezed into barrel is sometimes called "diesel effect." Coking phenomenon can explain that place where melt and extruded bubbles meet generates high temperature due to compression, and at the same time, oxygen in the air causes melt to break through oxidation. Process debugging should start melting process in the middle of feeding zone, where melt pressure is already high, forcing air between particles to move backward and overflow feed port. Reasons and improvement measures related to processing parameters are shown as below:

Reasons and improvement measures related to design are shown as below:

It is necessary to figure out whether it is a water spot or other pattern. In addition, water spots may not be produced if rubber is dried thoroughly. Other factors will also cause water spots, such as mold freezing water, dew due to temperature difference effect, mold mixed water flow, material mixing, etc.

1. Apparently, there is a discernible ring around sprue - if a central gate is used, it is a central circle, if a side gate is used, it is a concentric circle. This is because ring size is small and looks like a halo. This phenomenon mainly occurs when processing high-viscosity (low-flow) materials, such as PC, PMMA and ABS.
Physical reasons
If injection speed is too high, melt flows too fast and viscosity is high, some of surface materials near sprue are easily dislocated and infiltrated. These dislocations will appear as halos on outer layer. Near sprue, flow speed is particularly high, then gradually decreases. As injection speed becomes constant, front end of flow body expands into a gradually widening circle. At the same time, in order to obtain a low fluid front flow speed near sprue, multi-stage injection must be used, such as: slow-faster-fast. Purpose is to obtain a uniform melt front flow speed throughout filling cycle. It is generally believed that halos are caused by melt dislocation during holding pressure stage. In fact, role of front flow effect is to move melt into product during pressure holding stage. Reasons and improvement measures related to processing parameters are shown in the table below:

Reasons and improvement measures related to design are shown in the table below:

1. Void inside product appears as a round or elongated bubble. Only transparent products can see void inside from outside; opaque products cannot be measured from outside. Voids often occur in products with relatively thick walls and are in the thickest place.
Physical reasons
When bubbles are generated in product, they are often considered to be bubbles. Air in mold is wrapped by molten material flowing into mold cavity. Another explanation is that water vapor and bubbles in barrel will find ways to enter inside of product. Therefore, there are many reasons for formation of such "bubbles". At the beginning, product will form a hard outer skin, and it will develop faster or slower depending on degree of mold cooling. However, in thick-walled area, center part continues to remain sticky for a long time. Outer skin has enough strength to resist any stress shrinkage. As a result, melt inside is stretched outward, and a gap is formed in still plastic center part of product. Reasons and improvement measures related to processing parameters are shown in following table:

Reasons and improvement measures related to design are shown in following table:

1. There are unmelted particles near apparent material head. It is impossible to obtain a smooth surface for thin-walled products.
Physical reasons: Due to short molding cycle of thin-walled products, plasticization must be carried out at a very high screw speed to shorten residual time of melt in screw barrel. When encountering production of thin-walled products, usually including PE, PP, PC, etc., mold workers will try to lower melt temperature to shorten cooling time, and particles that are not completely melted will be injected into mold. Reasons and improvement measures related to processing parameters are shown in the table below:

Reasons and improvement measures related to design are shown in the table below:

Apparent color unevenness is that color of product surface is different, which can appear near and far from material head, and occasionally in sharp edge material flow area.
Physical causes
Uneven color is caused by uneven distribution of pigment, especially when adding color through masterbatch, color powder or liquid color. Masterbatch or color cannot be completely homogenized at temperatures below recommended processing temperature. When molding temperature is too high or barrel is left for too long, it is easy to cause thermal degradation of pigment or plastic, resulting in uneven color. When material is plasticized or homogenized at correct temperature, if it is injected too quickly through cross section of head, friction heat may be generated, causing degradation of pigment and change of color. Usually when using masterbatch, you should ensure compatibility of chemical and physical properties of pigment and its solution with resin to be colored. Reasons and improvement measures related to processing parameters are shown in the table below:

Reasons and improvement measures related to design are shown in the table below:

1. Silver or black streaks appear on the surface of the apparent product, radiating outward from gate or a point nearby. If low viscosity (high fluidity) material and high molding temperature are used, lines are mostly black, and if high viscosity (low fluidity) material is used, lines are mostly silver-white.
Physical reason
This is caused by another type of bubble being squeezed in and compressed. If screw pressure reduction amplitude is too high (screw retraction), pressure reduction speed is too fast, and melt in front of screw head is released too much, negative pressure will be generated in melt. When melt temperature is too high, bubbles are easily formed in melt. These bubbles will be compressed again in subsequent injection stage, resulting in the formation of black lines in product, which eventually becomes "diesel effect". If gate is a central gate, lines will radiate outward from material head. In the case of hot runner injection, lines will only appear after a certain section of runner, because material in hot runner does not contain any bubbles, so material will not produce burnt marks. Only melt at barrel head will produce burnt marks. If it is a low-viscosity melt, lines are grayer and larger than those of high-viscosity materials, because the former is prone to vacuum and gaps during screw pressure reduction process. Reasons and improvement measures related to processing parameters are shown in the table below:

Moisture streaks are long silver threads on the surface of product. Opening direction of moisture streaks is along direction of material flow. In places where product is not fully filled, front end of fluid is very rough.
Physical reasons
Some plastics such as PA, ABS, PMMA, SAN and PBT are easy to absorb water. If plastic storage conditions are not good, moisture will enter particles or attach to surface. When particles melt, moisture will turn into steam to form bubbles. During injection, these bubbles will be exposed on the surface of fluid front, burst and then produce irregular lines. Reasons and improvement measures related to processing parameters are shown in the table below:

Deep grooves can be seen in the entire material flow direction and even at the end of flow channel. This phenomenon occurs when using high-viscosity (poor fluidity) materials and thick-walled products. These grooves look like lines on a record. They are very clear on products made of PC materials, but larger and gray on ABS products.

Physical reasons
If melt contacting mold surface solidifies too quickly during injection process, especially at low injection speeds, and flow resistance is too high, distortion will occur at the front of fluid. Solidified outer layer of material will not completely contact cavity wall and form waves. These wavy materials will freeze, and holding pressure will no longer be able to flatten them. Causes and improvement measures related to processing parameters are shown in the table below:

Causes and improvement measures related to design are shown in the table below:

(1) What is cold slug?

 

1. Cold slug refers to phenomenon that solidified resin at the front end of nozzle is mixed into molded product.
2. Cold slug appears as poor gloss or spray marks on the surface of molded product.
3. Although they look similar, however, due to different countermeasures, it is necessary to pay attention.
(2) Causes of cold material generation
(2-1) Nozzle temperature is lower than front end of nozzle of molded product. In order to prevent drooling (phenomenon of resin flowing out of hole at the front end of nozzle), resin temperature is generally lowered appropriately. Therefore, first part of resin injected into mold will become solidified or semi-solidified resin. This is called cold material. In order to prevent resin from entering mold cavity, a resin reservoir is generally made at the root of main channel and branch channel on one side of mold as a receiving container for cold material. However, if resin temperature is too low, amount of cold material will increase, and it may enter mold cavity. These will appear as spray marks or poor gloss.
(3) Countermeasures for cold material
(3-1) Increase nozzle temperature to prevent cold material. Increasing nozzle temperature is very effective. However, occurrence of drooling must be taken into account. In order to prevent drooling, set temperature of barrel and nozzle should be gradually increased. If nozzle is fixed to mold, it is also effective to increase mold temperature.
(3-2) Enlarge cold material well (resin accumulation area)
Enlarging the cold material well can prevent cold material from entering the molded product. It is generally recommended to use a cold material well with a length of about 1.5 times root diameter of main channel.
(3-3) Retract barrel once per cycle
In the case of molding with nozzle fixed to mold, it is also effective to retract barrel once per cycle and remove nozzle from mold. However, it is also necessary to pay attention to drooling. Since drooling also depends on viscosity and fluidity of resin, nozzle model and aperture, you must repeat actual experiments (including above-mentioned temperature adjustment). Depending on situation, you should also consider in advance methods such as reducing nozzle aperture (i.e. using another nozzle) to suppress drooling and increasing temperature accordingly to suppress cold material.

This refers to a piece of cold material stuck or stuck on the surface near material head. Cold slugs will cause marks on the surface of product, and in severe cases, they will also reduce mechanical properties of product.

Physical reasons
Cold slugs are often generated when melt can cool near machine nozzle or hot runner. Since melt injected first always gathers near gate, defects will occur in this area. Its cause is unreasonable temperature control around machine nozzle or hot runner nozzle. Reasons and improvement measures related to processing parameters are shown in the table below:

4. Reasons and improvement measures related to design are shown in the table below:

Plastic products are usually made of polymers or a mixture of polymers and other components. After being heated, they are molded into a certain shape under certain conditions, then cooled, shaped, and trimmed. This process is molding and processing of plastics. Since plastics and thermosetting plastics behave differently after being heated, their molding methods are also different. There are dozens of plastic molding methods, among which the most important are extrusion, injection, calendering, blow molding and compression molding. Weight of products processed by them accounts for more than 80% of all plastic products.
Extrusion molding - It is the most important molding method for thermoplastics, and about half of plastic products are extrusion molded. Extrusion method can mold almost all thermoplastics. Products mainly include continuous production, pipes of equal cross-section, plates, films, wire and cable coatings, and various special-shaped products. Extrusion molding can also be used for plasticizing, granulating, coloring and blending of thermoplastics. After thermoplastic polymer is evenly mixed with various additives, it is subjected to mechanical shear force, friction heat and external heat in plastic barrel of extruder, so that it is plasticized and melted, then pushed by screw, it passes through filter plate into molding mold and is extruded into products.
Injection molding - This molding method is to heat and melt plastic (generally granular material) in barrel of injection molding machine. When it is in a flowing state, molten plastic is compressed and moved forward under pressure of plunger or screw, then injected into closed mold with a lower temperature at a very fast speed through nozzle at the front end of barrel. After a certain period of cooling and shaping, mold is opened to obtain product. Injection molding is developed based on the principle of metal die-casting. Since injection molding can produce products with complex shapes, precise dimensions, or with metal inserts in one molding, it has been widely used and currently accounts for more than 20% of the total molding processing. Injection molding process usually consists of five stages: plasticization, mold filling (i.e. injection), pressure holding, cooling and demolding. General injection molded products have waste materials such as gates and runners, which need to be trimmed and removed. This not only consumes time, but also wastes raw materials. Gateless injection molding developed in recent years not only overcomes above disadvantages, but also helps to improve production efficiency.

Barrel temperature setting is unreasonable. Partial storage of material in barrel. Resin intrudes into joint between barrel and nozzle (long-term storage). A backflow valve or backflow ring is installed. Hydrolysis caused by insufficient drying. Capacity of injection molding machine is too large.
Treatment method: Reduce barrel temperature. Avoid *angle structure. Try to eliminate gap in joint. Avoid using backflow valves and backflow rings. Pre-dry according to specified conditions. Choose an injection molding machine with appropriate capacity.

Warping and deformation of injection products are very difficult problems. It should be solved mainly from mold design aspect, and adjustment effect of molding conditions is very limited. Causes and solutions of warping and deformation can refer to following items:

 

1. When residual stress causes deformation due to molding conditions, stress can be eliminated by reducing injection pressure, increasing mold and making mold temperature uniform, increasing resin temperature or using annealing methods.

2. When stress deformation is caused by poor demolding, it can be solved by increasing number or area of push rods, setting demolding slopes, etc.

3. When cooling is uneven or cooling time is insufficient due to inappropriate cooling methods, cooling method can be adjusted and cooling time can be extended. For example, a cooling circuit can be set as close to deformation as possible.

These are common problems in plastic parts colored with masterbatch. Although masterbatch coloring is superior to dry powder coloring and dye paste coloring in terms of color stability, color purity and color migration, its distribution, that is, uniformity of mixing color particles in diluted plastic, is relatively poor, and finished product will naturally have color differences.

1. Increase temperature of feeding section, especially temperature at rear end of feeding section, so that its temperature is close to or slightly higher than melting section temperature, so that masterbatch melts as soon as it enters melting section, promotes uniform mixing with dilution, and increases chance of liquid mixing.

2. When screw speed is constant, increase back pressure to increase melt temperature and shearing effect in barrel.

3. Modify mold, especially pouring system. For example, if gate is too wide, turbulence effect is poor when molten material passes through, and temperature rise is not high, so it is uneven, and color band cavity should be modified to have regional color differences.

It refers to a phenomenon in which molded product cannot be removed from mold. Depending on size and temperature of molded product, poor demoulding will occur when demoulding resistance increases. Although it will also be affected by resin properties and molding conditions, main reason is still shape of molded product and structure of mold. Special attention should be paid to situation when it is accompanied by tight clamping of slender reinforcing ribs, hubs and plastic hole plugs. When molding shrinkage around plastic hole plug is too large, clamping force on plastic hole plug will become larger, which is prone to poor demoulding. In terms of molding conditions, low mold temperature and high holding pressure are prone to poor demoulding. When overfilling occurs in vertical molded products such as long reinforcing ribs or hubs, these reinforcing ribs or hubs will become difficult to fall off, resulting in poor demoulding. In terms of molding conditions, low mold temperature and high holding pressure can easily cause poor demolding. This is opposite of plastic hole bolt holding tight. When demolding slope of mold cavity product part is small, poor demolding is likely to occur due to large demolding resistance. Position of ejector pin also has a great influence (it is best to set ejector pin at the place where demolding resistance is large). In addition, finish or damage of mold cavity surface also affects demolding resistance, resulting in poor demolding. During injection molding, finished product will have mucous film. First of all, consider whether injection pressure or holding pressure is too high. Too much injection pressure will cause finished product to be over-saturated, causing plastic to be pressurized into other gaps, causing finished product to be stuck in mold cavity and difficult to demold, and it is easy to have mucous film when taking it out. When temperature of material pipe is too high, two situations usually occur. One is that temperature is too high, causing plastic to decompose and deteriorate, losing its original characteristics; and it will break or tear during demoulding process, causing sticking. The other is that rubber is not easy to cool after filling into mold cavity, and cycle time needs to be extended, which is not economical. Therefore, it is necessary to adjust its operating temperature appropriately according to characteristics of rubber. As for problem of the mold, if feed port is unbalanced, it will cause finished product to stick when demolding. At this time, improvement measures must be taken on mold. Following table shows the possible causes and treatment measures of finished product sticking:
Table 4 Fault Cause Treatment Method

Table 5: Causes of failure and treatment methods

When demolding resistance caused by plastic hole bolt is too large, it is also effective to suppress molding shrinkage by increasing holding pressure and mold temperature. On the contrary, if size is increased and embedded in mold cavity due to overfilling, holding pressure and mold temperature should be reduced to increase molding shrinkage. When demolding resistance caused by plastic hole bolt is too large, it is also effective to suppress molding shrinkage by increasing holding pressure and mold temperature. Increase demolding slope at periphery of mold cavity, wheel hub and reinforcing ribs. Especially when parts are already firmly embedded, ejector pins should be added to parts that need to increase ejection strength.

Air traps refer to air in mold cavity being enclosed before melt is melted. It occurs when melt converges from different directions before melt is melted, or when air cannot escape from vents or gaps of embedded parts. Air traps usually occur in last filled area. If vents in these areas are too small or there are no vents, air traps will occur, causing voids or bubbles inside plastic part, short shots of plastic part, or surface defects. In addition: When thickness of plastic part varies greatly, melt tends to flow to thick area and cause a race-tracking effect, which is also main cause of air traps. To eliminate air traps, injection speed can be reduced to change filling mode; or vent position can be changed or vent size can be increased. Air traps caused by race-tracking effect can be improved by changing thickness of plastic part or changing vent position. Methods to improve air entrapment are as follows:

1. Change design of plastic parts: Reducing wall thickness ratio can reduce competitive flow effect of molten plastic.

2. Mold design should be changed: Venting can be improved by setting vent holes in appropriate position. Vent holes are usually located in last filled area, such as junction between molds, parting surface, between embedded parts and mold wall, position of ejector pin and mold slider. Redesigning gate and melt transfer system can change filling pattern so that last filling area falls at appropriate vent hole position. In addition, it should be ensured that there are vent holes large enough to allow air to escape during filling; but be careful that vent holes are not too large to cause burrs. Recommended vent hole size is 0.025 cm (0.001 inch) for crystalline plastics and 0.038 cm (0.0015 inch) for amorphous plastics.

3. Adjust molding conditions: High injection speed will cause jet flow, resulting in air entrapment. Using a lower injection speed allows sufficient time for air to escape.

On the surface of finished product, with CATE as center, there are many silver-white streaks, which are basically produced along flow direction of raw materials. This phenomenon occurs after many adverse conditions accumulate, and sometimes it is difficult to grasp real cause.

 

If there is water or other volatile components in raw materials and they are not fully dried, many silver stripes will appear on the surface.

When other raw materials are accidentally mixed into raw materials, blisters will also form. Its shape is mica-like or pin-shaped, which is easy to distinguish from blisters caused by other reasons.

This situation is also easy to occur when raw materials or material pipes are not clean.

Injection time is long, and temperature of raw materials injected into mold cavity at the beginning is low. As a result of curing, volatile components will not be eliminated. This situation often occurs for raw materials that are sensitive to temperature.

If mold temperature is low, raw materials will solidify quickly and it is easy to occur (1.4), so that volatile components will not be discharged.

When mold exhaust is poor, gas is not easy to be discharged when raw materials enter, which will cause blisters. In this case, top of finished product will often burn black.

If there is moisture on mold, heat brought by filling material will evaporate it and fuse with molten material to form a boil, which is an egg white mist.

If there is cold material or small in cold material pocket of rubber channel, cooled material will be brought into mold cavity during injection, and part of it will quickly solidify to form a thin layer. Low mold temperature at the beginning of production will also cause boils.

During filling process, raw material forms a thin layer due to rapid cooling of contact part of mold cavity surface, which is melted and decomposed by subsequent raw materials, forming a white or stain-like shape, which is more common in thin shell products.

During filling, raw material forms a turbulent flow, which prolongs flow path of raw material and generates friction due to influence of structure in mold cavity. In addition, filling speed is faster than cooling speed of raw material. GATE position is at ribs or small, which is easy to cause boils. The place where finished product meat thickness increases sharply is also prone to boils.

GATE and flow channel are small or deformed, and filling speed is fast, which instantly generates friction, causing temperature to rise sharply and cause raw material to decompose.

Raw material contains recycled material, which is not fully dried and decomposes during injection, resulting in sores.

Raw material stays in material tube for a long time, causing partial overheating and decomposition.

Insufficient back pressure, air is drawn in (insufficient compression ratio).

Molding machine
Insufficient plasticizing capacity. Resin overheating and decomposition (barrel temperature). Raw materials in barrel stay for a long time, causing partial overheating. Injection pressure is too high. Screw entrains air (insufficient back pressure).
Mold
Poor exhaust in mold. Low mold temperature. Small storage of cold material nest in rubber channel. GATE is too small or deformed. There is moisture on mold surface. Poor shape of mold cavity (cross-section or wall thickness changes more and more rapidly).
Raw materials
Raw materials contain moisture and volatile components. Raw materials are not dried enough. Other raw materials are mixed in.

1. Characteristics of defects in injection molded parts
Changes in weight and size during injection molding process exceed production capacity of mold, injection molding machine, and plastic combination.

2. Possible causes of problems

Plastic input into injection cylinder is uneven.

Temperature of injection cylinder or fluctuation range is too large.

Capacity of injection molding machine is too small.

Injection pressure is unstable.

Screw reset is unstable.

Operation time changes and solution viscosity is inconsistent.

Injection speed (flow control) is unstable.

Using a type of plastic that is not suitable for mold.

Consider impact of mold temperature, injection pressure, speed, time and holding pressure on product.

3. Remedial measures

Check whether there is sufficient cooling water flowing through hopper throat to maintain correct temperature.

Check for inferior or loose thermocouples.

Check whether thermocouple used with temperature controller is of correct type.

Check injection volume and plasticizing capacity of injection molding machine, then compare it with actual injection volume and amount of injection material used per hour.

Check whether there is stable molten hot material in each operation.

Check whether backflow prevention valve is leaking and replace it if necessary.

Check for incorrect feed settings.

Ensure that screw return position is stable in each operation, that is, change is no more than 0.4mm.

Check for inconsistency in operation time.

Use back pressure.

Check whether hydraulic system is operating normally and whether oil temperature is too high or too low (25-60oC).

Choose type of plastic that is suitable for mold (mainly considering shrinkage and mechanical strength).

Re-adjust the entire production process.

Main cause of defect is interference of gases (mainly water vapor, decomposition gas, solvent gas, and air).If screw speed is too fast and back pressure is low, amount of air involved in plasticizing resin will increase. As a result, striped bubbles appear on the surface of molded product and silver streaks are easily formed. Resins are chemical substances, so they will gradually decompose as temperature increases. The higher resin temperature, the more it decomposes, and the more silver streaks will appear. If material is not pre-dried enough, moisture and original gas components in resin will be brought into molded product intact, which will easily form silver streaks. When gas is not completely exhausted, bubbles will remain on the surface of molded product, which will easily cause silver streaks. If components different from original resin are mixed in due to insufficient cleaning, and temperature of resin is low, gas will sometimes be generated and silver streaks will be induced.

1. Machine:

Barrel and screw are worn or there is a flow angle between rubber head and rubber ring, which causes decomposition due to long-term heat.

Heating system is out of control, causing decomposition due to excessive temperature. Check whether there are problems with heating components such as thermocouples and heating coils. Improper screw design causes individual decomposition or easy air intake.

2. Mold:

Poor exhaust.

Friction resistance of runner, gate, and cavity in mold is large, causing local overheating and decomposition.

Unbalanced distribution of gate and cavity and unreasonable cooling system will cause unbalanced heating, local overheating or blockage of air channel.

Water leaks into cooling channel into cavity.

3. Plastic:

Plastic has high humidity, too much recycled material is added, or contains harmful scraps (scraps are very easy to decompose). Plastic should be fully dried and scraps should be eliminated.

It absorbs moisture from atmosphere or from colorant. Colorant should also be dried. It is best to install a dryer on machine.

Amount of lubricant, stabilizer, etc. added to plastic is too much or mixed unevenly, or plastic itself contains volatile solvents. Decomposition will also occur when mixed plastics are heated to a degree that is difficult to balance.

Plastic is contaminated and mixed with other plastics.

4. Processing:

Setting temperature, pressure, speed, back pressure, and melt motor speed too high will cause decomposition, or pressure and speed are too low, injection time and holding pressure are insufficient, and back pressure is too low. Due to failure to obtain high pressure, density is insufficient and gas cannot be melted, resulting in silver streaks. Appropriate temperature, pressure, speed and time should be set, and multi-stage injection speed should be used.

Low back pressure and fast speed can easily allow air to enter barrel and enter mold with molten material. When cycle is too long, molten material is heated for too long in barrel and decomposes.

Insufficient material quantity, too large material buffer, too low material temperature or too low mold temperature will affect flow and molding pressure of material and promote formation of bubbles.

According to causes of bubbles, there are several countermeasures:

When wall thickness of product is large, cooling speed of outer surface is faster than that of the center. Therefore, as cooling progresses, resin in the center shrinks and expands toward surface, causing insufficient filling in the center. This situation is called vacuum bubbles. Main solutions are:

Determine reasonable gate and runner size according to wall thickness. Generally, gate height should be 50% to 60% of product wall thickness.

Leave a certain amount of supplementary injection material until gate is sealed.

Injection time should be slightly longer than gate sealing time.

Reduce injection speed and increase injection pressure.

Use materials with high melt viscosity grade.

Solutions to bubbles caused by generation of volatile gases are:

Fully pre-dry.

Reduce resin temperature to avoid generation of decomposition gas.

Bubbles caused by poor fluidity can be solved by increasing temperature of resin and mold, increasing injection speed.

There are many bubbles on the surface and inside of apparent product - mainly near material head. In the middle of flow channel and away from sprue - not only in thick part of product wall. Bubbles have different sizes and shapes.

Physical reasons
Bubbles mainly occur in heat-sensitive materials that must be processed at high temperatures. If required molding temperature is too high, material will decompose through molecular splitting, melt will be in danger of thermal degradation, and bubbles will be easily generated during molding process. If cycle time is long, it may usually be due to too long residual time and insufficient stroke utilization. It may also be because melt in barrel is overheated. Reasons and improvement measures related to processing parameters are shown in the table below:

Reasons and improvement measures related to design are shown in the table below:

1.Rays

In point gate design perpendicular to direction of part, a radiation system composed of different color depths and glosses centered on gate appears on the surface of part during injection molding, which is called a ray. There are generally three manifestations, namely dark lines on a dark background, dark lines on a dark background, dense and whitish dark lines around gate. This type of defect mostly occurs when injecting a mixture of polystyrene and modified polystyrene, and is related to following factors: two materials have differences in rheology, colorability, etc., flow rate and heating conditions of stratosphere and turbulent layer of pouring system are different; plastic generates burnt wire due to thermal decomposition; and gaseous substances interfere when plastic enters mold.
Solutions:

When using mixed plastics, mix plastics well and make plastic particles of same and uniform size.

Plastic and colorant should be mixed evenly, and if necessary, add appropriate dispersants and mix them mechanically.

Plasticization should be complete and plasticization performance of machine should be good.

Reduce injection pressure and speed, shorten injection and holding time, at the same time increase mold temperature, increase nozzle temperature, and reduce fore furnace temperature.

Prevent degradation of plastics, cause molten material and coking materials with increased viscosity: For example, pay attention to whether the screw and barrel are worn and there is a * angle, or heating system is out of control, and improper processing operation causes plastic to be heated for a long time and decompose. This can be done by polishing inner surface of screw and the front end of barrel.

Improve gate design, such as enlarging gate diameter, changing gate position, changing gate to a rounded transition, trying to locally heat gate, and adding a cold well at runner end.

Cold material spots mainly refer to foggy or bright-colored spots near gate of workpiece or curved scars starting from gate that look like earthworms sticking on it. They are caused by plastic front entering cavity or cold material that is squeezed into cavity later due to excessive pressure holding. Front material transfers heat due to cooling effect of nozzle or runner, is partially cooled and solidified before entering cavity. When it expands and is injected into cavity through narrow gate, melt breaks, then it is pushed by hot melt later, thus forming cold material spots. Solutions are as follows:

Cold well should be opened well. Form, size and position of gate should also be considered to prevent cooling speed of material from being too different.

Center of nozzle should be adjusted well, matching dimensions of nozzle and mold feed should be designed well to prevent leakage or cold material from being brought into cavity.

Mold has good exhaust. Interference of gas will cause turbid spots on gate.

Increase mold temperature. Slow down injection speed, increase injection pressure, reduce holding pressure and injection time, and reduce holding pressure.

Dry plastic well. Use less lubricant to prevent powder from being contaminated.

1. Machine:
(1) If nozzle hole is too large, melt will flow back and shrinkage will occur. If it is too small, resistance will be large and material volume will be insufficient, resulting in shrinkage.
(2) Insufficient clamping force will cause flash and shrinkage. Check whether there is a problem with clamping system.
(3) Insufficient plasticizing volume. A machine with a large plasticizing volume should be selected. Check whether screw and barrel are worn.
2. Mold aspect:
(1) Wall thickness of part should be uniform to ensure uniform shrinkage.
(2) Cooling and heating system of mold should ensure temperature of each part is consistent.
(3) Pouring system should ensure smoothness and not too much resistance. For example, size of main channel, branch channel and gate should be appropriate, finish should be sufficient, and transition zone should be arc-shaped.
(4) Temperature should be increased for thin parts to ensure smooth material flow. Mold temperature should be lowered for thick-walled parts.
(5) Gate should be opened symmetrically and opened as far as possible in thick-walled part of part. Volume of cold well should be increased.
3. Plastic aspect:
Crystalline plastic shrinks more than non-crystalline plastic. During processing, amount of material should be appropriately increased, or a change agent should be added to plastic to accelerate crystallization and reduce shrinkage depression.

4. Processing aspect:

Barrel temperature is too high and volume changes greatly, especially temperature of fore furnace. For plastics with poor fluidity, temperature should be appropriately increased to ensure smooth flow.

Injection pressure, speed, back pressure are too low, injection time is too short, resulting in insufficient material quantity or density and shrinkage pressure, speed, back pressure, and time are too long, causing flash and shrinkage.

When amount of material added, that is, cushion, is too large, injection pressure is consumed; when it is too small, amount of material is insufficient.

For parts that do not require precision, after injection pressure is completed, outer layer is basically condensed and hardened, sandwich part is still soft and can be ejected, parts should be removed from mold as soon as possible and allowed to cool slowly in air or hot water. This can make shrinkage depression smooth and not so conspicuous, will not affect the use.

"Dents" are caused by local internal shrinkage after gate is sealed or due to insufficient material injection. Depression or micro-depression on the surface of injection molded products is an old problem in injection molding process.

Dents are generally caused by increase in wall thickness of plastic product, which causes a local increase in shrinkage rate of product. It may appear near external sharp corners or where wall thickness suddenly changes, such as protrusions, ribs, or back of support, and sometimes appear in some uncommon parts. Fundamental reason for formation of dents is thermal expansion and contraction of material, because thermal expansion coefficient of thermoplastics is quite high. Degree of expansion and contraction depends on many factors, among which properties of plastic, maximum and minimum temperature ranges, and cavity holding pressure are the most important factors. Size and shape of injection molded part, as well as cooling speed and uniformity, are also influencing factors. Amount of expansion and contraction of plastic material during molding process is related to thermal expansion coefficient of processed plastic. Thermal expansion coefficient of molding process is called "molding shrinkage". As molded part cools and shrinks, molded part loses close contact with cooling surface of cavity. At this time, cooling efficiency decreases. After molded part continues to cool, molded part continues to shrink. Amount of shrinkage depends on combined effect of various factors. Sharp corners on molded part cool the fastest and harden earlier than other parts. Thick part close to the center of molded part is farthest from cooling surface of cavity and becomes last part of molded part to release heat. After material at the corners solidifies, as melt close to the center of part cools, molded part will continue to shrink. Plane between sharp corners can only be cooled on one side, and its strength is not as high as that of material at sharp corners. Cooling shrinkage of plastic material in the center of part pulls relatively weak surface between partially cooled and sharp corners with greater cooling inward. In this way, a dent is formed on the surface of injection molded part. Existence of a dent indicates that molding shrinkage here is higher than shrinkage of surrounding parts. If shrinkage of molded part is higher in one place than in another, then molded part will warp. Residual stress in mold will reduce impact strength and temperature resistance of molded part. In some cases, adjusting process conditions can avoid formation of dents. For example, during holding pressure of molded part, additional plastic material is injected into mold cavity to compensate for molding shrinkage. In most cases, gate is much thinner than the rest of part. When molded part is still hot and continues to shrink, small gate has already solidified. After solidification, holding pressure has no effect on molded part in cavity. Semi-crystalline plastic materials have a high shrinkage rate for molded parts, which makes dent problem more serious; amorphous materials have a lower molding shrinkage, which will minimize dent; filling and maintenance reinforcement materials have a lower shrinkage rate and are less likely to produce dents. Thick injection molded parts take a long time to cool, which will cause greater shrinkage. Therefore, large thickness is the root cause of dents. Attention should be paid to this during design. Thick-walled parts should be avoided as much as possible. If thick walls cannot be avoided, they should be designed to be hollow. Thick parts should be smoothly transitioned to nominal wall thickness. Using large arcs instead of sharp corners can eliminate or minimize dents near sharp corners.

Cracking includes surface filiform cracks, microcracks, top whitening, cracking, and trauma caused by mold sticking or runner sticking. According to cracking time, it is divided into demolding cracking and application cracking. Cracks refer to a phenomenon in which molded products break during mold opening or ejection. Cracks sometimes occur when molded products are brittle or demolding is poor. Basic reasons are same as those for poor demolding, such as products sticking to mold core, reinforcing ribs, and excessive filling of bosses. In addition, speed of ejector pin will also affect generation of cracks. The higher speed, the easier it is to break. There are many reasons, but the first one is resin aging. Particular attention should be paid to hydrolysis of PBT resin in barrel. The second is insufficient crystallization. Special attention must be paid when mold temperature is low. Analysis is as follows:

 

1. Processing:

Excessive processing pressure, too fast speed, too much filling, too long injection and holding time will cause excessive internal stress and cracking.

Adjust mold opening speed and pressure to prevent rapid strong drawing of part from causing demolding cracking.

Properly increase mold temperature to make part easy to demold, and appropriately lower the material temperature to prevent decomposition.

Prevent cracking due to weld marks and plastic degradation causing mechanical strength to decrease.

Appropriately use mold release agents pay attention to frequently eliminating aerosols and other substances attached to mold surface.

Residual stress of part can be eliminated by annealing heat treatment immediately after molding to reduce internal stress and reduce cracking. Comply with recommended molding temperature of each material, shorten residence time to reduce decomposition and aging of resin. If it is a polyester resin such as PBT, strengthening drying conditions is also an effective way to inhibit hydrolysis. Degree of crystallization of product can also be increased by increasing mold temperature and extending cooling time. In addition, following methods are also effective:

• Slow down mold opening speed and ejection speed to reduce load on molded product and reduce cracks.
• Add R (radius) to corners of molded product to prevent cracking.

2. Mold:

Ejection should be balanced, such as number of ejector pins and cross-sectional area should be sufficient, demolding angle should be sufficient, and cavity surface should be smooth enough to prevent cracking caused by residual stress concentration due to external force.

Structure of part should not be too thin, transition part should use arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.

Use as few metal inserts as possible to prevent internal stress from increasing due to different shrinkage rates between inserts and parts.

For deep-bottomed parts, appropriate demoulding air inlet holes should be set to prevent formation of vacuum negative pressure.

Main runner should be large enough to allow gate material to be demoulded before it solidifies, so that it is easy to demould.

Main runner bushing and nozzle should be connected to prevent cold hard material from dragging and causing parts to stick to fixed mold.

3. Material:

Recycled material content is too high, resulting in low strength of parts.

Excessive humidity causes some plastics to react chemically with water vapor, reducing strength and causing ejection cracking.

Material itself is not suitable for processing environment or the best quality, and contamination will cause cracking.

4. Machine:

Plasticizing capacity of injection molding machine should be appropriate. If it is too small, plasticizing is not sufficient and cannot be completely mixed, making it brittle. If it is too large, it will degrade.

Size changes of parts are essentially caused by different shrinkage degrees of plastics. Any operation that causes changes in material temperature, mold, pressure, and production cycle will lead to changes in size of product, especially for PP, PE, nylon, etc. with high crystallinity. Analysis is as follows:

1. Machine:

If plasticizing capacity is insufficient, a machine with a large plasticizing capacity should be selected.

If material supply is unstable, check whether voltage of machine fluctuates, whether components of injection system are worn, or whether there are problems with hydraulic valve.

If screw speed is unstable, check whether motor is faulty, whether screw and barrel are worn, whether hydraulic valve is stuck, and whether voltage is stable.

If temperature is out of control, proportional valve and total pressure valve are not working properly, and back pressure is unstable.

2. Mold:

Mold must have sufficient strength and rigidity, and cavity material must be wear-resistant.

When dimensional accuracy requirements are very high, try not to use a mold with multiple cavities.

Ejection system, pouring system, and cooling system should be set reasonably to ensure stable production conditions.

3. Plastic:

Mixture of new and recycled materials should be consistent.

Drying conditions should be consistent and particles should be uniform.

When selecting materials, effect of shrinkage on dimensional accuracy should be fully considered.

4. Processing:

If plastic processing temperature is too low, temperature should be increased because the higher temperature, the smaller dimensional shrinkage.

For crystalline plastics, mold temperature should be lower.

Molding cycle should be kept stable and should not fluctuate too much.

Amount of material added, i.e. amount of injection, should be stable.

Some plastic parts will soon swell or blister on the back of metal insert or in particularly thick parts after molding and demolding. This is because plastic that has not been completely cooled and hardened releases gas, expands under action of internal pressure.
Solutions:

Effective cooling. Reduce mold temperature, extend mold opening time, reduce material drying and processing temperature.

Reduce mold filling speed, shorten molding cycle, and reduce flow resistance.

Increase holding pressure and time.

Improve condition where wall of part is too thick or thickness varies greatly.

Bubbles are very thin and belong to vacuum bubbles. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. Formation of vacuum bubbles is due to insufficient plastic injection or low pressure. Under rapid cooling of mold, fuel at the corner of cavity is pulled, resulting in volume loss.

 

Bubbles refer to a phenomenon in which surface of molded product bulges. Bubbles are prone to occur in following two situations, namely, when surface of product begins to bulge gradually when it is taken out of mold after injection molding and when surface of molded product bulges due to thermal expansion. In either case, when surface of molded product softens due to high temperature, internal gas will push up surface of molded product due to thermal expansion, thus forming bubbles. If a large amount of air is involved during metering, bubbles are likely to occur. Specifically, bubbles are likely to occur when screw speed is fast, back pressure is low, and amount of plastic extraction is large. In addition, during cavity filling process, some flow patterns sometimes involve air, thus generating bubbles. If bond between surface layer and core layer is weak, or there are tiny voids or cracks, bubbles are likely to form from these holes. Specifically, in molding of thin-walled products, strains are left in products due to forced filling, or cold materials or spray marks are mixed in. In particular, in liquid crystal polymers, bubbles are likely to form because interlayer strength is not high (this is inherent property of resin). From perspective of molding conditions, bubbles tend to worsen when injection speed is fast. In addition, when gate is small, spray marks will be generated, and large shear forces will cause residual strains, so bubbles will also tend to worsen. A large amount of gas generated in resin is also likely to generate bubbles. When barrel temperature is too high and the residence time is too long, amount of gas generated will increase, which is also likely to generate bubbles. In addition, when drying is insufficient and material contains too much water, bubbles will also be generated.
Solutions:

Increase injection energy: pressure, speed, time and material amount, and increase back pressure to make mold full.

Increase material temperature to make flow smooth. Lower material temperature to reduce shrinkage, and appropriately increase mold temperature, especially local mold temperature where vacuum bubbles are formed.

Set gate at thick part of product, improve flow conditions of nozzle, runner and gate, and reduce consumption of pressure.

Improve exhaust condition of mold. To reduce air entrapment during metering, following conditions should be changed: • Reduce screw speed • Increase back pressure • Do not set too much plastic suction

If air entrapment occurs during cavity filling, it is necessary to adjust shape, gate position and injection speed. This should be dealt with specifically according to situation of molded product. Through short shot, grasp flow pattern, then establish corresponding countermeasures based on this. Changing holding pressure, etc. has little effect on improving bubbles. It is more effective to reduce shear force during filling so that material can smoothly fill cavity to eliminate bubbles. Specifically, following molding conditions can be changed: • Increase mold temperature • Slow down injection speed • Increase gate • Increase thickness (only for too thin parts) • Avoid jetting

Appearance in filling method, weld seam refers to a line where front ends of each fluid meet. Especially where mold has a highly polished surface, weld seam on product looks like a scratch or a groove, especially on dark or transparent products. Position of weld seam is always in direction of material flow.

 

Physical reasons
Weld seam is formed where thin streams of molten material fork and connect together again. The most typical is molten flow around core or product with multiple gates. Where thin streams meet again, weld seams and material flow lines will form on the surface. The larger core around molten material or the longer flow channel between gates, the more obvious weld seam formed. Small weld seams will not affect strength of product. However, where process is very long or temperature and pressure are insufficient, incomplete filling of mold will cause obvious grooves. Main reason is that front ends of fluids are not evenly fused to produce weak light spots. Spots may appear where pigment is added to polymer because of obvious difference in orientation. Number and location of gates determine number and location of weld lines. The smaller angle at which fluid fronts meet, the more obvious weld line. In most cases, it is impossible to completely avoid weld lines or flow lines by process debugging. What can be done is to reduce their brightness or move them to an inconspicuous or completely invisible place. Causes and improvement measures related to processing parameters are shown in the table below:

Brittleness, a large part of brittleness of products is caused by internal stress. There are many reasons for brittleness of products, mainly:

1. Equipment

There are corners or obstacles in barrel, which are easy to promote melt degradation.

Plasticizing capacity of machine is too small, and plastic is not fully plasticized in barrel; plasticizing capacity of machine is too large, plastic is heated and sheared for too long in barrel, which makes plastic easy to age and make product brittle.

Ejector device is tilted or unbalanced, and ejector cross-sectional area is small or improperly distributed.

2. Mold

Gate is too small, and you should consider adjusting gate size or adding an auxiliary gate.

Runner is too small or improperly configured, it should be arranged as balanced and reasonable as possible or runner size should be increased.

Poor mold structure causes abnormal injection cycle.

3. Process

Barrel and nozzle temperature is too low, increase it. If material is easy to degrade, barrel and nozzle temperature should be increased.

Reduce screw pre-plastic back pressure and speed to make material slightly loose and reduce degradation of plastic caused by shear overheating.

If mold temperature is too high, demoulding is difficult; if mold temperature is too low, plastic will cool prematurely, weld seam will not be well fused, and it will be easy to crack, especially for high melting point plastics such as polycarbonate.

Cavity and core should have an appropriate demoulding slope. When core is difficult to demould, cavity temperature should be increased and cooling time should be shortened; when cavity is difficult to demould, cavity temperature should be lowered and cooling time should be extended.

Use metal inserts as little as possible. Plastics with large cold and hot specific volumes such as polystyrene, which are brittle, should not be added with inserts for injection molding.

4. Raw materials
(1) When raw materials are mixed with other impurities or doped with inappropriate or excessive solvents or other additives.
(2) Some plastics such as ABS will undergo catalytic cracking reaction with water vapor when heated under humid conditions, causing large strain on the parts.
(3) Too many times of plastic recycling or too high recycled material content, or too long heating time in barrel, will cause parts to crack.
(4) Quality of plastic itself is poor, such as a large molecular weight distribution, a large proportion of components with non-uniform structures such as rigid molecular chains, or contamination by other plastics, bad additives, dust and impurities, etc., which are also reasons for brittleness.
5. Product design
(1) Product has sharp corners, notches or parts with large thickness differences that are prone to stress cracking.
(2) Product design is too thin or has too many hollows.
(30) White edge
White edge is an injection defect unique to modified polyethylene and plexiglass, and mostly appears on the edge of part near parting surface. White edge is a collection of countless stretched oriented molecules perpendicular to material flow direction and tiny distances between them. There is still a polymer connection phase in direction of white edge, so white edge is not a crack. Under proper heating, it is possible to restore stretched oriented molecules to a natural curling state and make white edge disappear. Solution:

Pay attention to keeping mold parting surface of template in close fit during production process, especially area around cavity, which must be under a truly sufficient clamping force to avoid longitudinal and lateral expansion.

Reduce injection pressure, time and material volume to reduce orientation of molecules.

Apply oily mold release agent to white edge of mold surface. On the one hand, it makes it difficult for this position to transfer heat and maintain high temperature for a longer time. On the other hand, it can suppress occurrence of white edges.

Improve mold design. For example, use materials with small elastic deformation to make molds, strengthen mechanical bearing capacity of cavity side wall and bottom plate, so that they can withstand high pressure impact during injection and rapid increase in temperature during working process, provide higher temperature compensation for white edge prone area, change direction of material flow, and make flow distribution in cavity reasonable.

Consider changing materials. Frost When demolding some polystyrene parts, a thin layer of white frost-like substances will be found attached to local surface near parting surface. Most of them can be removed after polishing. These white frost-like substances will also adhere to surface of cavity. This is because volatile substances or soluble low molecular weight additives in plastic raw materials become gaseous after heating, are released from plastic melt. After entering cavity, they are squeezed to vicinity of parting surface with exhaust function, and precipitate or crystallize out. These white frost-like powders and grains adhere to mold surface, which will not only scratch next demolding part, but also affect smoothness of mold surface if it happens too many times. Insoluble fillers and colorants are mostly irrelevant to appearance of white frost. Solutions to white frost: strengthen drying of raw materials, reduce molding temperature, strengthen mold exhaust, reduce proportion of recycled materials, etc. When white frost appears, pay special attention to cleaning mold surface frequently.

Main reasons are lack of material and improper injection pressure and speed (including excessive pressure loss caused by resistance). Underfilling refers to inability to completely fill molded product.

 

Normal products will produce products consistent with mold, while underfilled products cannot obtain desired shape. Underfilling occurs when resin fluidity is insufficient or resin metering value is too low.

1. Machine:

Plasticizing amount or heating rate of machine is uncertain. A machine with large plasticizing amount and heating power should be selected. Wear of screw, barrel or glue head causes material return and actual filling amount is not in correct range. Failure of heating system such as thermocouples or heating coils causes actual temperature of barrel to be too low. Wear of sealing components of injection cylinder causes oil leakage or backflow, and required injection pressure cannot be achieved. Inner hole of nozzle is too small or center of nozzle is improperly adjusted, resulting in excessive resistance and pressure consumption.

2. Mold

Temperature of mold is too low locally or as a whole, causing difficulty in feeding. Mold should be appropriately increased.

Distribution of mold cavity is unbalanced. Wall thickness of part is too thin, causing excessive pressure consumption and poor filling. Wall thickness of the entire part or part should be increased, or auxiliary flow or gates can be set near insufficient filling point to solve problem.

Flow channel of mold is too small, causing pressure loss; when it is too large, injection will be weak; when it is too rough, it will cause dissatisfaction of part. Size of runner should be set appropriately, appropriate arc transitions should be applied at transition between main runner and branch runner, gate, or turning point itself.

Poor mold venting. Material entering cavity is blocked by gas pressure that cannot be discharged in time, resulting in insufficient filling. You can make full use of gap of screw to vent or reduce clamping force to vent using parting surface. If necessary, open a venting channel or vent. Product shape and mold structure are also causes of insufficient filling.

When gate size is too small and branch runner is too thin, fluidity will be reduced, which can easily lead to insufficient filling. In addition, if wall thickness of product is too thin, of course, insufficient filling is also likely to occur. Processing and adjustment: injection pressure is too small, speed is too slow, time is too short, temperature is too low, and melt position is too small. If insufficient filling is caused only by insufficient fluidity, it is estimated that there are several conditions:

Resin temperature is too low

Mold temperature is too low

Insufficient material fluidity

Low injection speed

Low injection pressure

If metering value is less than amount required by product, it will inevitably lead to insufficient filling. Sometimes, insufficient filling may occur not only due to insufficient metering values, but also due to errors caused by poor metering. Check whether pre-drying has been performed sufficiently. Recommended drying conditions for each resin are marked on product bag. Please compare actual results with this value. Also check whether vent at the end of flow is intact. If there are signs of burning, it is likely that venting is poor. Try to expand or add vents. If a large amount of gas is generated due to same reasons as above, insufficient filling may sometimes occur at the end of flow. Appropriate drying must be performed in advance to remove moisture, etc. It refers to switching position from injection to holding pressure. If this switching position is too early (that is, it is transferred to holding pressure stage too early), the overall fluidity will decrease, resulting in insufficient filling. Check whether pre-drying has been performed sufficiently. Recommended drying conditions for each resin are marked on product bag. Please compare actual results with this value. Also check whether vent at the end of flow is intact. If there are signs of burning, it is likely that venting is poor. Try to expand or add vents.

Airtrap is also called flash, overflow, flash, etc. Most of them occur at parting and joining positions of mold, such as parting and joining surface of mold, sliding part of slider, gap of insert, hole of ejector pin, etc. If flash is not solved in time, it will be further expanded, so that stamping mold will form a local collapse, causing permanent damage.

 

Flash in the gap of insert and hole of ejector pin will also cause product to be stuck on mold, affecting demolding. Although mold is made with high precision (μm level) and high-pressure mold closing is used during molding, filling pressure of resin is also very high, so there is actually a very small gap. Flash is formed by resin entering this gap. Flash will appear on PL surface, sleeve, slide core interface and exhaust port. Flash is a phenomenon in which resin squeezes into PL surface of mold (parting surface of mold) and causes product to carry excess film. This situation occurs when PL surface cannot withstand pressure of resin and separates, or there is a gap on PL surface.

1. Mechanical equipment:

Actual clamping force of machine is insufficient. When selecting an injection molding machine, rated clamping force of machine must be higher than tension formed by longitudinal projection area of injection molded product during injection, otherwise it will cause mold expansion and flash.

Clamping device is not adjusted well, elbow mechanism is not straightened, resulting in uneven clamping left and right or up and down, and mold parallelism cannot be achieved, resulting in one side of mold being tightly closed while the other side is not tightly attached, which will cause flash during injection.

Mold itself has poor parallelism, or is not installed in parallel, or mold plate is not parallel, or force distribution and deformation of tie rod are uneven, all of which will cause mold to be loose and produce flash.

Check ring is severely worn; spring of spring nozzle fails; barrel or screw is worn too much; inlet cooling system fails to cause "bridging" phenomenon; injection amount set by barrel is insufficient, buffer pad is too small, etc., which may cause flash to appear repeatedly, and accessories must be repaired or replaced in time.

Second, mold aspects

Mold parting surface has poor accuracy. Movable mold plate (such as middle plate) is deformed and warped; there are foreign objects on parting surface or there are protruding skid marks and burrs around mold frame; old mold has fatigue collapse around cavity due to extrusion of the earlier flash.

Mold design is unreasonable. Opening position of mold cavity is too biased, which will cause tension on one side of mold during injection, causing flash; plastic fluidity is too good, such as polyethylene, polypropylene, nylon, etc., which have very low viscosity in molten state and are easy to enter movable or fixed gaps, requiring higher mold manufacturing accuracy; under premise of not affecting integrity of product, it should be placed at the center of mass symmetry as much as possible, material should be fed into thick part of product to prevent situation where there is a shortage of material on one side and flash on other side; when there is a molding hole in the center of product or near it, it is customary to open a side gate on hole. Under a larger injection pressure, if mold is closed Insufficient force. When supporting force of this part of mold is insufficient and slight warping occurs, flash is caused. For example, when there is a movable component on the side of mold, projected area of side is also affected by molding pressure. If supporting force is insufficient, flash will also be caused; sliding core has poor matching accuracy or fixed core and cavity installation position are offset, resulting in flash; cavity venting is poor, there is no venting groove on parting surface of mold, or venting groove is too shallow or too deep or too large, or blocked by foreign matter, which will cause flash; for multi-cavity molds, attention should be paid to reasonable design of each branch runner and gate, otherwise it will cause uneven filling force and flash. Even in a simple two-piece mold, mold is sometimes damaged due to improper ejection of molded product, and flash will appear at damaged location. When using a sliding core, special attention must be paid to fit and gap of sliding surface. In addition, mold is made of steel, mold closing pressure is high pressure, and resin pressure is also high pressure, so in almost all injection molding, mold will generally deform. This is especially significant in the case of large molded products. At this time, presence or absence of support also affects flash (if there is no support, deformation → gap will increase, and flash will also increase).

2. Process

Injection pressure is too high or injection speed is too fast. Due to high pressure and high speed, opening force on mold increases, resulting in overflow. Injection speed and injection time should be adjusted according to thickness of product. Thin products should be filled with high speed quickly, and no more injection should be added after filling; thick products should be filled with low speed, and surface should be roughly fixed before reaching final pressure.

Excessive feeding causes flash. It is worth noting that too much molten material should not be injected to prevent dents. In this way, dents may not be "filled", but flash will appear. This situation should be solved by extending injection time or holding time.

Barrel, nozzle temperature is too high or mold temperature is too high, which will reduce viscosity of plastic and increase fluidity, causing flash in the case of smooth mold entry.

3. Raw materials

Plastic viscosity is too high or too low, which may cause flash. For plastics with low viscosity, such as nylon, polyethylene, and polypropylene, clamping force should be increased; plastics with strong water absorption or water sensitivity will greatly reduce flow viscosity at high temperatures, increasing possibility of flashing, and these plastics must be thoroughly dried; plastics that are mixed with too much recycled material will also have a lower viscosity, and if necessary, retained components should be supplemented. If plastic viscosity is too high, flow resistance will increase, and a large back pressure will be generated, which will increase cavity pressure, resulting in insufficient clamping force and flashing. When resin pressure is too high, mold will separate and flash will be generated. On the contrary, when mold pressure is low, flashing is also likely to occur. Main reasons for increase in resin pressure are as follows: PSS resin has strong fluidity in low shear zone, so resin has disadvantage of easily generating flashing in terms of its own properties. Therefore, compared with other materials, more attention must be paid to preventing flashing when using PPS resin. At this time, requirements for mold precision level are also more stringent than when using other materials.

Injection speed is too fast

Injection pressure is too high

Holding pressure is too high

V-P switching is too slow

Generally speaking, when you want to get a good appearance, sometimes you will set holding pressure too high, especially to prevent dents. This will sometimes cause flash. The better fluidity, the easier it is for resin to enter gap, so the larger flash. Generally speaking, the higher resin temperature and mold temperature, the larger flash; conversely, the lower temperature, the smaller flash.

When particle size of plastic raw material is uneven, feed amount will vary, and product may not be full or have flash.

1 If slider and positioning block are worn, burrs are likely to appear.
2 Burrs will also appear when foreign matter is attached to mold surface.
3 Insufficient clamping force, mold is opened during injection, and burrs appear.
4 If raw material temperature and mold temperature are too high, viscosity will decrease, so burrs are likely to appear on gap between molds.
5 Excessive material supply and excess raw material injection will cause burrs.

Injection molding - Injection molding is also called injection molding. This molding method is to heat and melt plastic (generally granular material) in barrel of injection molding machine. When it is in a flowing state, molten plastic is compressed and moved forward under pressure of plunger or screw, then injected into closed mold with a lower temperature at a very fast speed through nozzle at the front end of barrel. After a certain period of cooling and shaping, mold is opened to obtain product. Injection molding is developed based on principle of metal die-casting. Since injection molding can produce products with complex shapes, precise dimensions, or with metal inserts in one molding, it has been widely used and currently accounts for more than 20% of the total molding processing. Injection molding process usually consists of five stages: plasticization, mold filling (i.e. injection), pressure holding, cooling and demolding. General injection molded products have waste materials such as gates and runners, which need to be trimmed and removed. This not only consumes time, but also wastes raw materials. Gateless injection molding developed in recent years not only overcomes above disadvantages, but also helps to improve production efficiency.

What is poor gloss? Poor gloss means that molded product does not adhere to mold, that is, poor replication.
Causes of poor gloss
(2-1) Caused by jet lines or cold material
When jet lines or cold material appear on the surface of molded product, it looks blurry like fog because it is different from surrounding area. It often appears near gate and in uneven places. (2-2) Caused by gas
This is because gas is blocked between mold and resin and hinders adhesion between the two, resulting in poor gloss. It is more likely to occur when holding pressure is low or exhaust is not smooth.
(2-3) Due to insufficient pressure
Due to low pressure, extrusion force on the mold surface of resin is weak, causing poor gloss. In materials containing fillers such as glass fiber, poor gloss becomes more obvious because filler is easy to float to the surface (resin is easy to sink to inside). For example, when injection speed is low, pressure cannot be transmitted to the end due to solidification of resin, which easily causes poor gloss at the end of molded product. At the same time, when mold temperature is low and the overall solidification is fast, the entire molded product is also prone to poor gloss.

Countermeasures for poor gloss

First try to increase holding pressure setting value and holding pressure time setting value. At this time, you should see that situation is improving. If not, it can be considered that actual pressure rise of this part is insufficient due to other reasons. At this time, following countermeasures should be used simultaneously: Increase mold temperature, increase barrel temperature (especially nozzle), increase gate size, increase injection speed, use a material grade with good fluidity

Injection mold

If mold cavity is poorly processed, such as scars, micropores, wear, roughness, etc., it will inevitably reflect on plastic part, making plastic part glossy. For this, mold should be carefully processed to make cavity surface have a smaller roughness, and polishing and chrome plating can be performed if necessary.

If there are oil stains or water stains on cavity surface, or too much release agent is used, surface of plastic part will become dark and glossy. For this, oil stains and water stains should be removed in time, and release agent should be used in limited quantities.

If demolding slope of plastic part is too small, demolding is difficult, or force is too large during demolding, surface gloss of plastic part is the best. For this, demolding slope should be increased.

If mold exhaust is poor, too much gas will stay in mold, which will also lead to poor gloss. In this regard, mold exhaust system should be checked and corrected.

If gate or runner cross-sectional area is too small or changes suddenly, melt will be subject to too much shear force when flowing in it, and it will flow in a turbulent dynamic flow, resulting in poor gloss. In this regard, gate and runner cross-sectional area should be appropriately increased.

It is necessary to check whether gas has been completely exhausted. Check whether thickness and size of exhaust port required for effective gas exhaust are ensured, whether it is contaminated by mold scale, etc. It is also important whether it is configured in appropriate position. If possible, it is recommended to use short shot method to understand actual resin flow condition.

Injection molding process

If injection speed is too low, surface of plastic part is not dense and gloss is poor. In this regard, injection speed can be appropriately increased.

For thick-walled plastic parts, if cooling is insufficient, surface will be hairy and gloss will be dark. In this regard, cooling system should be improved.

If holding pressure is insufficient and holding time is too short, density of plastic part will be insufficient and gloss will be poor. In this case, holding pressure and holding time should be increased.

If melt temperature is too low, fluidity is poor, which can easily lead to poor gloss. In this case, melt temperature should be appropriately increased.

For plastic parts made of crystalline resins such as PE, PP, POM, etc., uneven cooling will lead to poor gloss. In this case, cooling system should be improved to make them cool evenly.

If injection speed is too high and gate cross-sectional area is too small, area near casting will be dark and gloss will be poor. In this case, injection speed can be appropriately reduced and gate cross-sectional area can be increased.

Raw materials

Particle size of raw materials varies greatly, making it difficult to plasticize evenly and gloss is poor. In this case, raw materials should be screened.

Too much recycled material or nozzle material is added to raw materials, which affects uniform plasticization of melt and gloss is poor. In this case, amount of recycled material or nozzle material added should be reduced.

Some raw materials will decompose and discolor during temperature adjustment, resulting in poor gloss. In this case, raw materials with better temperature resistance should be selected.

Moisture or volatile matter content in raw materials is too high. When heated, it volatilizes into gas and condenses in cavity and melt, resulting in poor gloss of plastic parts. In this regard, raw materials should be pre-dried.

Dispersibility of some additives is too poor, resulting in poor gloss of plastic parts. In this regard, additives with better flow properties should be used instead.

Foreign matter, miscellaneous materials or incompatible materials are mixed in raw materials. They cannot be evenly mixed with raw materials and result in poor gloss. In this regard, these miscellaneous materials should be strictly excluded in advance.

If amount of lubricant is too little, fluidity of melt is poor, surface of plastic parts is not dense, and gloss is poor. In this regard, amount of lubricant should be appropriately increased.

1. Appearance

Although mold has a uniform surface material, surface of product still appears gray and uneven gloss.
Physical reasons
Surface of product produced by injection molding is more or less a replica of mold surface. Surface roughness depends on thermoplastic material itself, its viscosity, speed setting, and molding parameters such as injection speed, holding pressure and mold temperature. Therefore, due to roughness of imitation surface, surface of product will appear gray, dark or smooth. Theoretically, when pitted or eroded mold surface has been accurately imitated, light projected onto surface of product will be diffusely reflected. Therefore, dark areas will appear on the surface. For surfaces with less accurate imitation, diffuse reflection phenomenon will be controlled and surface of product will have a good gloss effect. Reasons and improvement measures related to processing parameters are shown in following table:

Reasons and improvement measures related to design are shown in following table:

1. What is discoloration (appearance)?

Discoloration means that color of molded product becomes different from normal color. Since plastic is a chemical substance, when it is heated above its melting point, it will gradually decompose and deteriorate. Discoloration is caused by this process. Main reason is that plastic or added ultraviolet absorber, antistatic agent, etc. overheats and decomposes in barrel, or stays in barrel for too long and decomposes and carbonizes, then is injected into cavity along with molten material.

2. Causes of discoloration

Thermal discoloration inside barrel
Melted resin inside barrel is often in a high temperature state. The higher set temperature and the longer residence time, the more severe discoloration. In addition, shear force applied during metering is also one of causes of discoloration. The higher screw speed, the greater shear force, and the easier it is to discolor. To suppress discoloration of resin in barrel, resin temperature (including nozzle) should be lowered and residence time should be shortened.

1. Machine:

Due to loss of control of heating control system, barrel overheats and causes decomposition and blackening.

Due to defects in screw or barrel, molten material is stuck and accumulated, and decomposes after being subjected to long-term fixed heating. Check whether rubber head kit is worn or whether there are metal foreign objects inside.

Some plastics such as ABS are cross-linked and coked when exposed to high heat in barrel. It is difficult to melt while almost maintaining original particle shape. They are crushed by screw and carried into product.

2. Mold:

When injection speed is too fast, shear force inside mold will also increase. When gate or nozzle is too small, shear force will sometimes increase and cause discoloration. If nozzle, gate and other specific parts of mold change color, injection speed should be reduced. It is recommended to use multi-stage injection at this time.

Mold is not vented and is easy to burn, or size of pouring system is too small, and shear is too strong to cause coking.

There are inappropriate oil lubricants and mold release agents in mold.

Plastic: There are too many volatiles in plastic, too much humidity, too many impurities, too much recycled material, and contamination.

Processing:

Excessive pressure, too high speed, too much back pressure, if discoloration occurs during metering, too fast speed will cause material temperature decomposition.

1. Advantages of hot runner molds

Hot runner molds are widely used in all industrially developed countries and regions in the world today. This is mainly because hot runner molds have following significant features:

Shorten molding cycle of parts. Because there is no cooling time limit for runner system, the parts can be ejected in time after molding and curing. Molding cycle of many thin-walled parts produced by hot runner molds can be less than 5 seconds.

Save plastic raw materials. In pure hot runner molds, there is no production cost because there is no cold runner. This is particularly important for application projects with expensive plastic prices. In fact, major international hot runner manufacturers have developed rapidly in the era when oil and plastic raw materials are expensive in the world. Because hot runner technology is an effective way to reduce material costs and reduce material costs.

Reduce waste and improve product quality. During hot runner mold molding process, plastic melt temperature is accurately controlled in runner system. Plastic can flow into each mold cavity in a more uniform state, resulting in parts of consistent quality. Gate quality of hot runner molded parts is good, residual stress is low after demolding, and part deformation is small. Therefore, many high-quality products on the market are produced by hot runner molds. For example, many plastic parts in familiar MOTOROLA mobile phones, HP printers, and DELL laptops are made of hot runner molds.

Eliminating subsequent processes is conducive to production automation. Parts are finished after being molded by hot runner molds, there is no need to trim gates and recycle cold runners. It is conducive to production automation. Many foreign product manufacturers combine hot runners with automation to greatly improve production efficiency.

Expand application scope of injection molding technology. Many advanced plastic molding processes are developed on the basis of hot runner technology. Such as PET preforming, multi-color co-injection in mold, multi-material co-injection process, STACK MOLD, etc.

2. Disadvantages of hot runner molds

Although hot runner molds have many significant advantages compared with cold runner molds, mold users also need to understand disadvantages of hot runner molds. In summary, there are following points.

Rising mold costs.

Price of hot runner components is relatively expensive, and cost of hot runner molds may increase significantly. If part output is small and mold tool cost ratio is high, it is not economically worthwhile. For many mold users in developing countries, high price of hot runner systems is one of main problems that affect widespread use of hot runner molds.

2. High requirements for hot runner mold manufacturing process equipment

Hot runner molds require precision processing machinery to ensure. Integration and matching requirements of hot runner system and mold are extremely strict, otherwise many serious problems will occur in mold during production process. For example, poor plastic sealing leads to plastic overflow and damage to hot runner components, interrupting production, poor relative position between nozzle insert and gate leads to a serious decline in product quality.

3. Complex operation and maintenance

Compared with cold runner molds, hot runner molds are complex to operate and maintain. If operation is improper, it is very easy to damage hot runner parts, making production impossible and causing huge economic losses. For new users of hot runner molds, it takes a long time to accumulate experience.

4. Composition of hot runner system

Although there are many hot runner manufacturers and a variety of hot runner product series in the world, a typical hot runner system consists of following parts: 1. Hot runner plate (MANIFOLD) 2. Nozzle (NOZZLE) 3. Temperature controller 4. Auxiliary parts
Four. hot runner application main technical key
A successful hot runner mold application project needs to be guaranteed by multiple links. Among them, there are two most important technical factors. One is control of plastic temperature, and the other is control of plastic flow.

Control of plastic temperature

In application of hot runner molds, control of plastic temperature is extremely important. Many processing and product quality problems in production process are directly caused by poor temperature control of hot runner system. For example, problem of poor gate quality of products when using hot needle gate method for injection molding, problem of difficulty in closing valve needle when using valve gate method for molding, problem of inconsistent filling time and quality of parts in multi-cavity molds. If possible, you should try to choose a hot runner system with multi-zone temperature control to increase flexibility and adaptability of use.

Control of plastic flow

Plastics must flow balanced in hot runner system. Gates must be opened at the same time to allow plastic to fill each cavity synchronously. For FAMILY MOLDs with a large difference in part weight, runner size design must be balanced. Otherwise, some parts will not be filled with enough mold pressure, while some parts will be filled with excessive mold pressure, flash will be too large and quality will be poor. Design of hot runner runner size should be reasonable. If size is too small, filling pressure loss will be too large. If size is too large, hot runner volume will be too large, and plastic will stay in hot runner system for too long, which will damage material properties and cause parts to fail to meet use requirements after molding. There are CAE software such as MOLDCAE that specifically helps users to design the best runner in the world.

Application scope of hot runner mold

Types of plastic materials

Hot runner molds have been successfully used to process various plastic materials. Such as PP, PE, PS, ABS, PBT, PA, PSU, PC, POM, LCP, PVC, PET, PMMA, PEI, ABS/PC, etc. Any plastic material that can be processed with cold runner molds can be processed with hot runner molds.

Part size and weight

The smallest part made with hot runner molds is less than 0.1 grams. The largest is more than 30 kilograms. Application is extremely wide and flexible.

Industrial field

Hot runner molds are widely used in various industrial sectors such as electronics, automobiles, medical care, daily necessities, toys, packaging, construction, office equipment, etc.

International hot runner mold production overview

In the world's more developed industrial countries and regions, hot runner mold production is extremely active. Proportion of hot runner molds is increasing. Many small mold factories with less than 10 people are engaged in production of hot runner molds. Generally speaking, North America and Europe have used hot runner technology for a long time, with more experience and higher level. In Asia, Singapore, South Korea, Taiwan, and Hong Kong are in a leading position except Japan. Although North America and Europe have a high level of mold manufacturing, price is high and delivery time is long. In contrast, Asian hot runner mold manufacturers are more competitive in terms of price and delivery time. China's hot runner molds are still in their infancy, but are growing rapidly and proportion is increasing.

Surface of plastic molded products with glass fiber added has various defects: gray, rough, some metal bright spots and other obvious features, especially in convex part of material flow area, near joint line where fluid reunites.
Physical reasons
If injection temperature is too low and mold temperature is too low, material containing glass fiber tends to condense too quickly on mold surface, and glass fiber will never be embedded in melt again. When fronts of two streams meet, orientation of glass fibers is in direction of each thin stream, which will cause irregular surface material at intersection, resulting in formation of joints or flow lines. These phenomena are more obvious when melt in barrel is not completely mixed. For example, screw stroke is too long, resulting in injection of unevenly mixed melt. Reasons and improvement measures related to processing parameters are shown in table below:

Stress whitening and stress increase are found on the side of product facing nozzle, that is, where ejector rod is located on ejection side of mold.

 

Physical reasons:
If required demolding force is too high or surface of ejector rod is relatively small, surface pressure here will be very high, deformation will occur, and eventually ejection part will be whitened. Reasons and improvement measures related to processing parameters are shown in table below:

Reasons and improvement measures related to design are shown in table below:

Surface of product shows very dark stripes of silver and light brown. Gas charring refers to charring phenomenon on the surface of molded product during molding. When resin fills mold cavity, original air will be discharged from exhaust port. At this time, gas generated by resin will also be discharged from exhaust port. However, if exhaust port of mold is blocked, or there is no exhaust port at the end of flow, gas cannot be discharged, and it will form a high temperature after being compressed, causing the resin to be charred. As the resin is filled, the air in the mold cavity and the gas contained in resin will be discharged from exhaust port. However, if injection speed is too fast, gas will not have time to be discharged from exhaust port, and will form a high temperature due to insulation compression, which will cause gas burning. When VP switching is slow, gas burning is also easy to occur due to same reason. Similar to above situation, gas burning will also occur when exhaust capacity of exhaust port is low. There are two reasons: one is that thickness of exhaust port is originally thin, and the other is that exhaust port is blocked by mold scale during use. For same reason, when amount of gas contained in resin is too much, gas burning will also be aggravated. When mold temperature or resin temperature is high (although it is a secondary reason), gas burning also tends to be aggravated.

 

Physical reasons
Burnt dark lines are caused by excessive thermal degradation of melt. Light brown dark lines are caused by oxidation or decomposition of melt. Silver lines are generally caused by friction between the screw, check ring, nozzle, material head, narrow cross-section or sharp edge area in product. Generally speaking, plastic will undergo severe degradation or decomposition during time when machine stops and barrel continues to heat. If streaks are only found near sprue, reason is not only that hot runner temperature control is not optimized enough, but also related to nozzle of machine. Even if temperature of melt is slightly high, melt remains in barrel for a relatively long time, which will also lead to a decrease in mechanical properties of product. Under action of degradation chain reaction caused by molecular thermal motion, fluidity of melt will increase, so that mold will inevitably overflow. Be especially careful with complex molds. Reasons and improvement measures related to processing parameters are shown in the table below:

Gas burning is caused by rapid compression of gas and formation of high temperature. Therefore, slowing down injection speed can reduce gas burning. When shape of molded product is restricted, please adjust VP switching position or use multi-stage injection. If problem cannot be solved anyway, exhaust condition should be improved by strengthening exhaust port and other measures.

When raw material flows in mold cavity, fine wrinkles in shape of annual rings centered on gate appear on the surface of finished product.
1 Increase temperature of raw material and mold to make raw material flow easily.
2 If filling speed is slow, temperature will drop during filling process, causing this phenomenon.
3 If nozzle is too long, temperature will drop at nozzle, so cooled raw material will be ejected first, resulting in a pressure drop and causing flow marks.
4 Cooling nest is small. At the beginning of injection, raw material with low temperature is filled first, causing flow marks.

Because shrinkage rate in flow direction during plastic molding is greater than that in vertical direction, shrinkage rate of part in each direction is different, resulting in warping. In addition, due to large internal stress that inevitably remains inside part during injection molding, warping is caused. These are all manifestations of deformation caused by high stress orientation. Therefore, fundamentally speaking, mold design determines warping tendency of part. It is very difficult to suppress this tendency by changing molding conditions. Ultimate solution to problem must start with mold design and improvement. Analysis is as follows:

1. Mold:

Thickness and quality of part should be uniform.

Design of cooling system should make temperature of each part of mold cavity uniform. Pouring system should make material flow symmetrical to avoid warping due to different flow directions and shrinkage rates. Runners and main channels of parts that are difficult to mold should be appropriately thickened to try to eliminate density difference, pressure difference, and temperature difference in cavity.

Transition zone and corners between thickness of part should be smooth enough and have good demolding properties, such as increasing demolding margin, improving polishing of mold surface, and keeping ejection system balanced.

Exhaust should be good.

Increase wall thickness of part or increase anti-warping direction, and use reinforcing ribs to enhance anti-warping ability of part.

Material used for the mold is not strong enough.

2. Plastics:

Crystalline plastics are more likely to warp than amorphous plastics. In addition, crystalline plastics can use crystallization process where the degree of crystallinity decreases with increase of cooling speed and shrinkage rate decreases to correct warping.

3. Processing:

If injection pressure is too high, holding time is too long, melt temperature is too low and speed is too fast, internal stress will increase and warping will occur.

If mold temperature is too high and cooling time is too short, part will be overheated during demolding and ejection deformation will occur.

While maintaining minimum filling amount, reduce screw speed and back pressure to reduce density to limit generation of internal stress.

If necessary, mold can be soft-shaped for parts that are prone to warping or de-rice treatment after demolding.

Injection and holding time: In principle, it is set to gate closing time. If injection and holding time is shorter than gate closing time, holding process for fully transmitting and maintaining pressure in mold cavity will be insufficient, and sometimes deformation will occur.

Cooling time: Because shape of molded product is maintained in mold cavity for a longer time, increasing cooling time will reduce deformation in many cases. However, for some shapes, on the contrary, due to tightness of mold (core), sometimes increasing cooling time will cause poor demolding and deformation, so it cannot be generalized. Therefore, when setting cooling time, it is necessary to pay attention to ejection balance, thickness and mold temperature of molded product.
Mold temperature: As with cooling time, in terms of shape retention effect of molded product, lowering mold temperature will reduce deformation in many cases. However, it is not enough to just lower temperature. For some shapes, low temperature will increase temperature difference between mold cavity and core, which is easy to cause deformation. In addition, when mold temperature is lower than ambient temperature of molded product, deformation or dimensional changes will occur due to post-shrinkage. Therefore, it can be said that in terms of mold temperature, it is not high or low temperature that is important, but uniformity (balance) of temperature including the core cooling, so as to achieve uniform molding shrinkage.

Finished product is not completely filled, and there is a part missing. Following are reasons:
1 Finished product area is large, injection capacity and plasticization capacity of machine are insufficient. At this time, a machine with large capacity should be selected.
2 Mold exhaust effect is not good. If air in mold cavity is not exhausted during injection, filling will be incomplete due to residual air, and sometimes burning will occur.
3 In mold cavity, raw material flows for a long distance, or there are thin-walled parts, which will cool and solidify before raw material filling is completed.
4 Low mold temperature is also easy to cause lack of glue, but increasing mold temperature will extend cooling time, resulting in a longer molding cycle time. Therefore, it is necessary to consider appropriate mold temperature from perspective of production efficiency.
5 Temperature of molten raw material is low or injection speed is slow. Raw material solidifies before filling mold cavity, resulting in short shots.

6. Nozzle aperture is small or nozzle is long. Nozzle temperature should be increased to reduce flow resistance. Nozzle should be as short as possible. If nozzle aperture is small or nozzle is long, not only friction resistance of flow will be increased, but also speed will be slowed down due to resistance, resulting in premature solidification of raw materials.

7. Number of finished mold cavities is large and flow is unbalanced. Size of gate should be set to control it. Resistance of small gate cavity is large and often mold cavity is lack of glue. If there is a hot glue channel system, temperature of a cavity can be adjusted separately to control it.

8. Low injection pressure causes insufficient filling.

Melting spots, silver streaks, cracks: Transparent parts of polystyrene and plexiglass sometimes have some glittering, filamentous silver streaks that can be seen through light. These silver streaks are also called sparkle spots or cracks. This is because stress is generated in direction perpendicular to tensile stress, polymer molecules undergo heavy flow orientation and are manifested as a difference in refraction rate with unoriented part.
Solution: (1) Eliminate interference of gas and other impurities, and fully dry plastic.

(2) Reduce material temperature, adjust barrel temperature in stages, and appropriately increase mold temperature.

(3) Increase injection pressure and reduce injection speed.

(4) Increase or reduce pre-molding back pressure and reduce screw speed.

(5) Improve exhaust condition of flow channel and cavity.

(6) Clean possible blockage of nozzle, flow channel and gate.

(7) Shorten molding cycle. After demolding, silver streaks can be eliminated by annealing: keep polystyrene at 78℃ for 15 minutes, or keep it at 50℃ for 1 hour. For polycarbonate, heat it to above 160℃ and keep it for several minutes.

Rigid plastic parts such as PS form dense ripples on the surface near their gates, centered on gate. When it is called shock marks. Reason is that viscosity of melt is too high and when mold is filled in a stagnant flow, front material quickly condenses and shrinks when it touches surface of cavity, subsequent melt expands and shrunken cold material continues to move forward. Continuous alternation of process causes material flow to form surface vibration marks during forward movement.
Solution: (1) Increase barrel temperature, especially nozzle temperature, and also increase mold temperature.

(2) Increase injection pressure and speed to quickly fill mold cavity.

(3) Improve size of runner and gate to prevent excessive resistance.

(4) Mold should have good venting and a sufficiently large cold well should be set.

Characteristics of defect of injection molded part can be easily seen in "air trap" of transparent injection molded part, but it can also appear in opaque plastics. This is related to thickness and is often caused by plastic shrinking away from center of injection molded part.

1. Possible causes of problem

(1 Mold is not fully filled.
(2 Stop valve is not operating normally.
(3 Plastic is not completely dry.
(4 Pre-molding or injection speed is too fast.
(5 Some special materials should be produced with special equipment.

2. Remedies

(1 Increase shot size.
(2 Increase injection pressure.
(3 Increase screw forward time.
(4 Reduce melt temperature.

Shrinkage holes are depressions on molded surface caused by shrinkage. Main reasons are:

Insufficient compression: When injection pressure is low, material cannot be compressed to an appropriate density, gas and air cannot be fully expelled from material, forming shrinkage holes. At this time, injection pressure should be increased, but size and diameter of main and branch runners and gates are too small, and factors that pressure cannot be transmitted to melt should also be considered.

Temperature of the resin is too high: At high temperatures, molecular distance is larger, space occupied is larger, and greater shrinkage occurs during solidification.

Improper adjustment of injection volume: When injection is completed, an appropriate amount of molten plastic must be left between screw head and nozzle to use it as a buffer. Generally, screw can still move forward by several millimeters to more than ten millimeters at the end of injection, and there is still molten material replenished during pressure maintenance.

Uneven mold cooling

1. What is "pockmarks" (appearance)?

"Pockmarks" mainly refer to "pits" that appear on thick walls. "Pockmarks" are caused by resin not adhering to cavity surface. There are many reasons for occurrence of pits, but main reason is insufficient holding pressure.

2. Causes of "pitting"

(2-1) Insufficient holding pressure
When resin is just injected into mold, its surface has solidified, but middle part has not fully solidified. If holding pressure at this stage is insufficient, semi-solidified surface will not be tightly attached to cavity, which is likely to cause pitting and other poor appearance.
(2-2) Resin temperature and mold temperature
When resin temperature or mold temperature is low, solidification of molded product surface will accelerate, and sometimes it is impossible to apply holding pressure. As a result, poor appearance such as pitting is likely to occur.
(2-3) Flowability and injection speed
When material has poor flowability, filling time will be extended, and surface layer of molded product will solidify before holding pressure process, so sometimes it is impossible to apply holding pressure to resin. As a result, poor appearance such as pitting is likely to occur. When injection speed is slow, pitting is also likely to occur for same reason.

3. Countermeasures for "pitting"

(3-1) Try to increase the holding pressure
First, try to gradually increase holding pressure setting value. This will basically solve problem. You can refer to material molding conditions recommended on the Internet. If other faults (such as flash, etc.) occur after increasing holding pressure, you need to set next item "Conditions that are easy to apply holding pressure"
(3-2) Conditions that make it easy to apply holding pressure
Conditions that make it easy to apply holding pressure are as follows:

Increase resin temperature

Increase mold temperature

Increase injection speed

Expand gate

Adjust thickness (1t~2t is sufficient)

Use a material with good fluidity (*b)

After changing these conditions, actual holding pressure applied to mold cavity will increase, making it less likely to have pitting.

What is wiredrawing (appearance)

Wiredrawing refers to a phenomenon in which front end of main channel is not disconnected and stretches into a filament. The biggest problem is that sometimes molded product cannot be removed due to wiredrawing, and is clamped by mold, resulting in inability to form continuously. The better spinnability of resin (property of being able to stretch into a filament when stretched in a molten state), the more severe wiredrawing. Therefore, compared with modified materials, non-reinforced materials are more likely to cause wiredrawing.

Causes of wiredrawing

(2-1) Front end of main runner has not yet solidified.
After injection, main runner will solidify together with product after cooling process. However, since its front end is connected to barrel nozzle and maintains a certain temperature, core may not be completely solidified. If mold is opened at this time, materials with good spinnability (stretching into filaments and not breaking when stretched) are prone to wire drawing at the front end of main runner. In terms of molding conditions, if resin temperature (especially nozzle temperature) or mold temperature is high, it is easy to cause wire drawing.
(2-2) Problems inherent in materials and grades
As we all know, liquid crystal polymers and impact-resistant grades that use certain elastomers are more likely to cause wire drawing than other materials. This is because these materials have good spinnability. In addition, for same material, the lower viscosity, the more likely it is to cause wire drawing.

1. What are whisker marks (appearance)

Whisker marks refer to small flow marks near gate that are common in Duracon. Marks themselves are very thin and difficult to photograph, as shown in figure on the right.

2. Causes of whisker marks

(2-1) Changes in flow pattern
Resin coming out of gate is usually filled in pattern on the left, but when it flows in pattern on the right, "whisker-like patterns" will be produced. In terms of molding conditions, whisker-like patterns are more likely to occur in following situations:

Mold temperature is low

Injection speed is fast

Gate size is small

Material fluidity is low

3. Countermeasures for whisker-like patterns

(3-1) To improve flow conditions at gate, specifically, following countermeasures can be taken:
(1) Increase mold temperature
(2) Reduce injection speed (when passing through gate)
(3) Enlarge gate
(4) Use a grade with good fluidity
To reduce the speed when passing through gate, it is best to use multi-stage injection.

1. What is uneven embossing? Uneven embossing refers to failure of embossed pattern on mold to be neatly replicated on molded product.

2. Causes of uneven embossing

(2-1) Insufficient holding pressure
Uneven embossing is caused by insufficient holding pressure required to adhere resin to mold cavity. Conditions for reduced holding pressure are as follows:

Low barrel temperature (including nozzle)

Low mold temperature

Small gate

Thin runner

Low holding pressure setting value

Low injection speed

Low material fluidity

(2-2) Uneven embossing caused by other molding defects such as spray marks, poor gloss, and dents
Spray marks and other molding defects can also cause uneven embossing.

3. Countermeasures for uneven embossing

(3-1) Increase holding pressure
First try to increase holding pressure. If this does not solve problem, it can be considered that holding pressure is not effectively transmitted to mold cavity even if molding machine output increases. At this time, following adjustments should be made:
・Increase barrel temperature (including nozzle)
・Increase mold temperature
・Increase injection speed
If this still does not work or effect is poor, it is necessary to change mold or material

Once mold deposit (MD) is formed, it is difficult to remove. Therefore, maintenance must be performed before it firmly adheres. Following are some countermeasures to extend maintenance cycle.

1. Strengthen pre-drying.

It is best to achieve basic standard of 100-120℃ and 3 hours. However, if drying temperature is too high, Duracon DS-01M, EB-7 and EB-10 will deteriorate. (It is best to achieve condition of 60℃×18 hours.) Mechanism is not clear at present. It should be paid attention to.

2. Lower barrel temperature.

It should be lowered as much as possible, and the lower limit is usually 180℃.

3. Reduce residence time of resin.

In order to prevent thermal decomposition of material as much as possible, it is recommended to use a smaller molding machine and make barrel temperature gradient.

4. Increase mold temperature.

When resin is injected, gaseous MD main component is also injected into mold at the same time. After this component contacts mold and cools rapidly, it solidifies and adheres to mold, forming MD. It can be seen that increasing mold temperature will help prevent rapid cooling and increase difficulty of adhesion.

5. Reduce injection speed as much as possible.

In order to prevent resin from heating up due to shearing at narrow flow channel such as gate, and also to make exhaust smooth, it is better to slow down injection speed. This measure will bring unexpected results.

Increase gate diameter as much as possible. This is especially effective for point gates and tunnel gates, and effect will be more significant when used together with above ⑤.

Clean exhaust port regularly with a mold cleaning agent. As mentioned above, it will be difficult to clean when MD accumulates very thick, but it is easier to clean exhaust port. The first step of MD adhesion starts with blockage of exhaust port. It is recommended to clean it before work in the morning and after lunch.

1. What is gate residue (appearance)?

It refers to a phenomenon in which gate remains on the surface of molded product. Point gate or tunnel gate will automatically disconnect when mold is opened, but if shape and size of gate are not appropriate, it cannot be completely disconnected.

2. Causes of gate residue

(2-1) Insufficient gate solidification
If gate solidifies insufficiently, parts other than part that should be cut off when mold is opened will also become brittle, so gate will also be cut there, resulting in the front end of gate remaining on product side.
(2-2) Gate shape
In the case of a point gate, if taper angle of front end of gate is too gentle, it may not be possible to completely cut off front end. In addition, diameter of front end of gate will also have an impact: generally speaking, the larger diameter, the more likely it is to produce gate residue. Same is true for tunnel gates. In the case of tunnel gates, even entry angle will have an impact. If angle is too small, gate residue is likely to occur; conversely, if it is too large, poor gate cutting will occur. This is because in tunnel gates, size of gate front end hole changes as its angle changes (basically an ellipse).
(2-3) Problems inherent to grade
Impact-resistant grades or alloy materials are more likely to produce gate residues than standard grades. Reasons usually include

Different resins added to these materials solidify slowly;

Due to large shear force near gate, different resins added are stretched into layers.

3. Countermeasures for gate residue

(3-1) Promote gate solidification: Make gate fully solidify to reduce gate residue. Specific methods are as follows:
• Lower mold temperature
• Allow sufficient cooling time

1. What is poor gate cutting?

It refers to a phenomenon that when using point gates and other molding methods, gate connecting molded product, main channel, and branch channel is not easy to be cut off. If a point gate or a latent gate is used, gate connecting product, main channel, and branch channel will automatically break when mold is opened. However, if shape or size of gate is not appropriate, poor gate cutting will occur and remain inside mold.

2. Causes of poor gate cutting

(2-1) Poor balance of forces
In order to ensure that product part and runner part are cut off at gate, it is very important to maintain a balance of three forces: "gate strength", "runner holding force" and "product holding force". When mold is opened, if runner part remains on fixed side and product part remains on movable side, both will be cut off at gate. If gate strength is too large, or holding force between product part and runner is weak, poor gate cutting will occur. Generally, runner is held by a locking pin; its holding force depends on shape and size of locking pin and temperature of runner part when mold is opened. If size or slope of locking pin is insufficient, gate will fall off before it is cut off, so it is better to increase holding force of runner rather than increase gate strength. On the contrary, if holding force of runner is too large, runner will not be able to detach from mold. In addition, strength and rigidity of resin will also change with changes in temperature, so adjustments must be made based on this point. Product part is maintained by friction of side slope or sliding core. When relying on friction of slope to maintain it, it must still reach a strength higher than gate strength. At this time, it will also be affected by temperature. In addition, gate strength will of course be affected by gate design. If gate size is too large, strength will increase, making gate less likely to be cut off. If there are two templates and a tunnel gate is used, it will also be affected by gate angle and setting position. If there are three mold plates and a point gate is used, it will also be affected by slope and grinding of secondary main channel.

Mold temperature is affected by temperature of resin after cooling. If resin temperature changes, strength and rigidity will also change.

Holding pressure and holding time are affected by resin filling amount, product, main channel and branch channel size. Its size has a great influence on side slope friction maintenance. If size is too large, it may even be impossible to demold.

Injection speed is affected by resin filling amount, product, main channel and branch channel size.

(2-2) Problems inherent in grades
In terms of impact resistance grades or alloy materials, resins with added elastomers have a slower curing speed and a lower elastic modulus, so they are more likely to have poor gate cutting than other materials. Therefore, it is necessary to fully study relevant countermeasures at mold design stage.

3. Countermeasures for poor gate cutting

(3-1) Changing strength balance
Adjust molding conditions as follows according to condition of poor gate cutting.
★ When runner remains on movable side
It can be considered that runner locking pin on fixed side is weak, or strength of gate part is too large. Therefore, measures can be taken to increase holding strength of runner locking pin or weaken gate strength. Correcting mold to change size of the two is also a method. If you want to change molding conditions, lowering mold temperature to promote solidification and increase strength around runner locking pin may also be effective. If it is a tunnel gate, you can also consider correcting gate part.
★ When runner remains on fixed side
It can be considered that side slope of product part is weak, or strength of gate part is too large. One of countermeasures is to modify mold to strengthen slope or reduce gate. Another method is to increase holding pressure and increase size of runner to increase holding force.
★ In a three-piece mold, when product and runner remain on the middle plate, there is a high possibility that gate is too strong, so gate size should be slightly reduced, or runner locking pin should be strengthened. In terms of molding conditions, reducing holding pressure may also be effective.

1. What is main runner sticking (appearance)

Main runner retention refers to a phenomenon in which main runner of molded product fails to separate from mold. If main runner size increases due to long-term holding pressure, or main runner part of mold is damaged, resistance will be generated, so that main runner cannot fall off when mold is opened.

2. Causes of main runner sticking

(2-1) Insufficient curing of main runner
When main runner has not been completely cured, shrinkage at this moment is very small, main runner is close to mold, and strength is also low. Therefore, if main runner is to be withdrawn at this time, it will be very easy to break. In this way, main runner remains. Resin temperature, mold temperature and cycle (cooling time) all have an impact on curing of main runner.
(2-2) Excessive application of holding pressure
Runner part is closest to barrel nozzle, so it is easy to apply holding pressure. Therefore, after applying high holding pressure, size of main runner will increase, and resistance during demolding will also increase accordingly, making it easy for main runner to stick to mold.
(2-3) Problems in mold structure
If slope of main runner part is small, resistance to demolding on fixed side will increase. Although a main runner lock is set to pull out main runner from fixed side, if it is fragile, lock will be damaged, causing main runner to fall off from moving side. In addition, if a branch runner lock is used, it is too strong, main runner and branch runner may sometimes stick to fixed side.
(2-4) Problems inherent in grade
Compared with standard grade, impact-resistant grade or alloy material shrinks less, is more likely to stick to mold, and has lower strength. As a result, main runner sticking is more likely to occur.

3. Countermeasures for main runner sticking

(3-1) Allow main runner to fully cool and solidify before opening mold.
Lower mold temperature and extend cooling time. In particular, lowering mold temperature is an effective method for materials with low strength and slow solidification.
(3-2) Reduce holding pressure
Lowering holding pressure is also effective. Holding pressure applied to product cavity becomes 0 after gate is closed. Therefore, if holding pressure is not applied thereafter, main runner will not be subjected to useless holding pressure. However, excessive reduction is likely to cause unstable injection volume, so care should be taken.
(3-3) There is no specific regulation on angle required to improve slope of main runner of mold. However, if there is a possibility that main runner will stick to mold, increasing angle is also effective. Strengthening main runner lock (increasing size and increasing slope) is also effective. On the contrary, since it may become difficult to fall off due to ejection, it is necessary to adjust it during actual molding. If runner lock is too strong, please slightly reduce size of part.

1. What is surface peeling (appearance)?

As name suggests, surface peeling refers to a phenomenon in which surface layer of molded product peels off. Structure of injection molded products is generally divided into a surface layer (called the skin layer) and an inner layer (called the core layer). This is because molten resin enters mold cavity through jet flow, and while surface layer solidifies, inner layer is still flowing. Phenomenon of interface between these two layers peeling off for some reason is called interface peeling.

2. Causes of surface peeling

(2-1) Excessive shear force
Peeling is caused by excessive shear force when resin flows. Conditions under which shear force increases are as follows. This type of molding defect is particularly likely to occur when thickness is small and pressure is high.

Barrel temperature is too low (including nozzle)

Mold temperature is too low

Gate is too small (shear force increases when passing through gate)

Product thickness is too thin

Holding pressure is too high

Injection speed is too fast.

In addition, attention should be paid to whether flow distance and filling time are too long. If filling time is long, temperature difference between solidified layer and flowing layer near gate will increase, which is likely to cause peeling.
(2-2) Mixing Peeling can also occur when different materials and different types of resins are mixed. There are very few combinations of plastics that are compatible (completely mixed), and there are almost no examples of different resins being compatible. During molding process, these resins are stretched and thinned, are layered and dispersed inside molded product, making it easy for surface to peel off. Compared with general grades, sliding grades and alloy materials containing oil are more likely to peel off surface.
(2-3) Peeling can also occur when a large amount of gas is mixed into surface layer. This is because gas trapped below surface layer gathers into a very thin gas layer. Conditions that easily generate gas are as follows:
(1) Barrel temperature is too high (resin has been decomposed)
(2) Insufficient drying (contains a lot of water)
(3) Screw speed is too fast (air is entrained)
(4) Back pressure is too low (air is entrained)
(5) Holding pressure is too high
(6) Injection speed is too fast
(7) Recycled materials are used
(3) Countermeasures for surface peeling
(3-1) There are various ways to reduce shear force, but first start with conditions that are easy to change:
• Increase barrel temperature (including nozzle)
• Increase mold temperature
• Slow down injection speed
• Reduce holding pressure
At this time, if cause is gas, raising barrel temperature may make situation worse. In terms of barrel temperature, recommended operating temperature of corresponding resin should generally be followed. Next, check gate and product thickness. If peeling occurs near gate, cause may be that gate is too small. If product thickness is too thin and shear force is high, consider using a grade with good fluidity. In addition, in terms of gates, side gates are more desirable than point gates or tunnel gates. If possible, changing gate design is also a method. In addition, using multiple point gates is also effective.
(3-2) Suppressing Gases
To prevent molded product from containing unnecessary gases, following points should be checked or corresponding countermeasures should be implemented:
• Check if barrel temperature is within recommended temperature range
• Increase drying temperature
• Reduce excessively high screw speed
• Apply sufficient back pressure
• Shorten molding cycle
• Reduce use ratio of recycled materials

1. What is a jet mark? (Appearance)

Normally, molten resin flows in the form of a jet. However, when flowing from a narrow area to a wide area, if flow rate is too fast, it sometimes flies out in a ribbon shape and flows without touching mold. This is called a jet mark. Depending on how jet mark appears on the surface of finished product, some are in the form of a ribbon, while others are in the form of a mist, but cause is same.

2. Causes of Jet Marks

(2-1) Small Gate Size
The biggest cause of jet marks is gate size. If you imagine a water gun, it is not difficult to understand phenomenon of jet marks. The smaller hole (gate), the more powerful jet is, and the more severe jet mark will become. Reason why hole is small is that it means that pressure at that place is increased and speed is increased.
(2-2) Injection speed is too fast.
When gate diameter is the same, the faster injection speed, the more serious spray mark.
(2-3) Viscosity is too high/fluidity is too low.
When gate diameter and injection speed are same, the higher viscosity of resin (the lower fluidity), the more serious spray mark. Conditions that affect increase in viscosity are as follows:

Low resin temperature

Low mold temperature

Material viscosity

(2-4) Low holding pressure.
Holding pressure will make spray mark less obvious to a certain extent. On the contrary, if holding pressure is not sufficiently applied, spray mark will be very obvious.

3. Countermeasures for spray marks

(3-1) Try to increase gate size.
First, check whether gate size can be changed. Although it depends on shape and size of product, if there is room, spray mark can be eliminated by changing gate size. It is best to use a short and wide gate runner (gateland); a fan-shaped and angled design style is also effective.
(3-2) Try to change gate position.
Next, check whether gate position can be changed. Spray marks are basically caused by strong force of resin flying out. And the wider target space for flying out, the more serious it is. However, if resin flying out of gate hits wall quickly, jetting pattern can be eliminated. Even if gate position cannot be changed, if a needle or wall can be set on the front of gate in product cavity, same effect can be expected.
(3-3) Try to lower injection temperature
Try to lower injection speed setting. Countermeasure is to use multi-stage injection and only slow down speed when passing through gate (not the overall speed).
(3-4) Reduce resin viscosity
Methods for reducing resin viscosity:

Increase resin temperature

Increase mold temperature

Change grade to high flow type

(3-5) Check holding pressure

1. What is drooling (appearance)

It refers to phenomenon that resin leaks from nozzle of molding machine. In general, resin at the front end of nozzle of injection molding machine is not completely solidified. When pressure in barrel is too high or resin viscosity is too low, molten resin will leak out. Dripping occurs when resin viscosity is low or internal pressure of molding machine barrel is high.

2. Causes of Dripping

(2-1) Low Resin Viscosity
Most injection molding machines use open nozzles and adjust conditions to prevent outflow. However, if molecular weight decreases due to decomposition or barrel temperature is set too high, resin will flow out due to decrease in viscosity.
(2-2) High Barrel Internal Pressure
There are two reasons why a certain pressure is applied to resin in barrel: one is expansion of gas, and the other is back pressure during metering. Cause of gas expansion is decomposition gas of resin and moisture in pellets. After they vaporize and expand, pressure with nowhere to go flows to front end of nozzle, forming drooling. On the other hand, as for back pressure, it is necessary to use it to prevent air from being drawn in during metering, so a certain degree of back pressure must be applied. However, if it is applied too much, resin will be compressed into a tight state, resulting in increased internal pressure and drooling. In addition, air will expand due to heating, so if a large amount of air is drawn in during metering, drooling will become more serious.

3. Countermeasures for drooling

(3-1) Increase resin viscosity
Set barrel temperature, especially nozzle temperature, slightly lower. Setting it too low will affect fluidity and produce cold material (mixed with insufficiently melted resin), which will cause poor appearance, so it is recommended to use the lowest limit within recommended temperature range. In addition, changing material grade to a material with higher viscosity is also an effective method.
(3-2) Reduce internal pressure
First, reduce screw speed and back pressure. However, if back pressure is 0, it is easy to entrain gas and may cause other molding defects, so a minimum back pressure of about 0.2Mpa should be applied. Screw speed should be set at around 100rpm. In terms of preventing gas from mixing, following effective measures can be taken: (1) Strengthen drying, (2) Do not raise barrel temperature too high. In addition, increasing amount of plastic extraction is also effective. Plastic extraction can cause screw to retreat, thereby reducing resin overall. This will form a gap at the front, thereby gaining time before drooling occurs.
(3-3) Changing nozzle shape
For the sake of simplicity and cost, general molding machines use open nozzles. For materials that are really prone to drooling, using closed nozzles is also a solution. In addition, the smaller nozzle aperture, the more difficult it is to form drooling. Many molding machine manufacturers have nozzles of different apertures and shapes. Most of them are priced below 100,000 yen, which is worth buying. However, due to low fluidity of material, premise is that there must be a certain degree of leeway in molding conditions.

1. What is unstable injection volume? It means that there is a deviation between molded products obtained from each mold. Instability of molded product in terms of size, appearance, weight, etc. is caused by differences in molding conditions.

Injection pressure

Mold temperature

Metering

Exhaust

Instability of above molding conditions is main cause.

2. Causes of unstable injection volume

(2-1) Insufficient pressure
General injection molding process is injection → holding pressure → cooling (metering). Injection ~ holding pressure stage should be process of pressing molten resin in by applying pressure. When pressure is too low, amount of resin injected tends to become unstable. There are many reasons for this lack of pressure, including following:
• Low resin temperature
• Low mold temperature
• Slow injection speed
• Low holding pressure
• Short holding time
• VP switching position too early
• Thin channel parts such as main channel, branch channel, gate, etc., resulting in poor pressure transmission
• Poor resin fluidity, resulting in large pressure loss.
• Partially thick parts in thickness.
(2-2) Unstable mold temperature control
Unstable mold temperature control is particularly prone to dimensional eccentricity or deviation. Depending on specific conditions of mold, it is sometimes difficult to adjust temperature of plastic hole plug locally, which aggravates deviation.
(2-3) Unstable metering
If metering is unstable, amount of resin injected is also unstable. This increases possibility of deviation between each injection. For details, please refer to poor metering section.
(2-4) Poor exhaust
When exhaust port is weak and exhaust is not smooth, filling amount may become unstable.

3. Countermeasures for unstable injection volume

3-1) Sufficient holding pressure
Due to some (even local) reasons, actual holding pressure may be unstable. Therefore, following countermeasures should be taken. Since average size will be larger as a result, it is necessary to set different standards for process management.

(3-2) When checking water temperature control of mold temperature controller, please check whether temperature control channel near problematic part is unobstructed. In particular, temperature of plastic hole plug and other places is easy to rise, so temperature should be controlled as much as possible. If it is an electric heater, please check position of heater.
(3-3) Keep metering stable. Please refer to countermeasures for poor metering here and implement them.
(3-4) Improve exhaust condition Occasionally, exhaust port is not vented smoothly and size is unstable. In this case, it is necessary to reduce injection speed or strengthen exhaust port to make exhaust smooth.

1. What is corrugation (appearance)?

Corrugation refers to wrinkles on the surface of molded product. Usually occurs when injection speed is slow and surface solidifies faster than resin filling.

2. Causes of ripples

(2-1) Injection speed is too slow
When injection speed is too slow, jet cannot be formed, and surface is uneven, resulting in "ripples". There is also a secondary reason: if product is thick and gate is small, actual filling speed will be slow, which is easy to form ripples.
(2-2) Low mold temperature
When mold temperature is low, surface solidification will be accelerated, and jetting difficulty will increase, which is easy to produce ripples.

3. Countermeasures for ripples

(3-1) Speed up injection speed
This is the most effective method. It can basically eliminate ripples by increasing injection speed. However, if molding gate is thick and gate is small, actual filling speed will be slow, so this should also be paid attention to.
(3-2) Slow down surface solidification
Specifically, it is necessary to increase mold temperature. In this way, jet will become easier to a certain extent, and it is also less likely to produce ripples.

1. Poor metering: refers to a phenomenon in which resin cannot be supplied to barrel or supply amount is unstable. This phenomenon is generally referred to as poor metering, but there are actually several modes: no metering at all, sometimes prolonged metering time, sometimes insufficient filling. These can all cause poor metering, that is, resin supplied to barrel is unstable during metering.

2. Causes of poor metering

Improper screw speed

Generally, the higher screw speed, the stronger conveying force of pellets. Therefore, if screw speed is too slow, conveying force of pellets will be weakened, resulting in unstable pellet supply and poor metering. On the contrary, if speed is too fast, pellets will move with screw and will not be able to move forward.

High back pressure

Back pressure has effect of suppressing intrusion of its body into book and stabilizing amount of injected resin, but it also has effect of weakening conveying force. Therefore, if back pressure is too high, metering will become unstable.

Improper barrel setting temperature

Barrel setting temperature will affect pellet temperature in barrel. In other words, surface state and rigidity of pellets change, which also affects metering. In particular, setting temperature below hopper and its adjacent areas will have a great impact on metering. Generally speaking, if temperature setting from nozzle to the bottom of hopper is from high to low, and setting temperature of bottom of hopper is low, metering will remain stable. This is because when temperature rises, surface of pellets will melt, and friction between pellets will increase, resulting in intertwining or sticking to screw or barrel.

Inherent problems of grade

In sliding grade, because sliding with metal parts is too good, screw rotation force cannot be well converted into forward conveying force, which can easily cause poor metering. If you want to use a screw to convey particles such as pellets, pellets should be difficult to slide on outer barrels and easy to slide on inner screw surface. It is because of this difference in friction that rotational force becomes force that conveys pellets forward.

Recycled materials are usually used.

Recycled materials are usually irregular in shape, so friction between pellets is likely to increase compared to ordinary pellets, which can easily cause poor metering.

3. Countermeasures for poor metering

(3-1) Adjust screw speed
First, adjust screw speed.
If you want to regularly observe whether there is poor metering, measure metering time. Through 50-100 consecutive moldings, change speed in several stages, and make a judgment based on whether metering event suddenly becomes longer. Screw speed is generally around 80-120RPM. Please select the best range according to specific situation.
(3-2) Reduce back pressure
The lower back pressure, the stronger physical conveying force and the more stable metering. However, if it is reduced too low, gas will be entrained and resin amount will be unstable, so it is not advisable to set it to 0.
(3-3) Barrel temperature
Specifically, temperature below hopper should be gradually lowered. Excessive lowering will make it difficult for pellets to melt and may even clog barrel, so it should be adjusted gradually. (About 10℃ each time)
(3-4) Inherent problems with grade
Since oil or lubricant is added, Huadong grade originally has property of easy sliding. If problem cannot be solved by adjusting screw speed, back pressure and barrel temperature at the same time, grade or screw design should be changed.
(3-5) Recycled materials
Try to make recycled pellets and initial pellets same size. At the same time, remove powder as much as possible.

During injection molding process, shrinkage and dents of product are a common phenomenon. Main reasons for this are:

1. Machine:

If nozzle hole is too large, melt will flow back and shrink. If it is too small, resistance is large and amount of material is insufficient, resulting in shrinkage.

Insufficient clamping force will cause flash to shrink. Check whether there is a problem with clamping system.

If plasticizing amount is insufficient, a machine with a large plasticizing amount should be selected, screw and barrel should be checked for wear.

2. Mold:

Design of part should make wall thickness uniform to ensure consistent shrinkage.

Cooling and heating system of mold should ensure that temperature of each part is consistent.

Pouring system should ensure smoothness and resistance should not be too large. For example, size of main channel, branch channel, and gate should be appropriate, finish should be sufficient, and transition zone should be arc-shaped.

For thin parts, temperature should be increased to ensure smooth material flow, and mold temperature should be lowered for thick-walled parts.

Gate should be opened symmetrically and opened as far as possible in thick-walled part of part. Volume of cold material well should be increased.

3. Plastic:

Crystalline plastics shrink more than non-crystalline plastics. During processing, amount of material should be appropriately increased, or a replacement agent should be added to plastic to accelerate crystallization and reduce shrinkage depression.

4. Processing:

Barrel temperature is too high, and volume changes greatly, especially temperature of front furnace. For plastics with poor fluidity, temperature should be appropriately increased to ensure smoothness.

Injection pressure, speed, back pressure are too low, and injection time is too short, resulting in insufficient material quantity or density, and shrinkage pressure, speed, back pressure are too large, and time is too long, causing flash and shrinkage.

When amount of material added is too large, injection pressure is consumed. When it is too small, amount of material is insufficient.

For parts that do not require precision, after injection pressure is completed, outer layer is basically condensed and hardened, sandwich part is still soft and can be ejected, parts should be removed from mold as soon as possible and allowed to cool slowly in air or hot water. This can make shrinkage depression smooth and not so conspicuous, will not affect use.

It referred to as sink mark, is a common defect in molding process of injection molding products. It is usually a local shrinkage phenomenon caused by sealing of gate or injection of insufficient material. Sink marks may appear near external corners of product, at the place where wall thickness changes suddenly, such as behind protrusions, ribs or supports, or even in some places that are not often noticed. Root cause of this defect lies in thermal expansion and contraction characteristics of material. Since thermal expansion coefficient of thermoplastics is relatively high, shrinkage and expansion of plastics become an inevitable problem during injection molding process. Degree of shrinkage and expansion is affected by many factors, including properties of plastic, operating temperature range, cavity holding pressure, and uniformity of size, shape and cooling rate of injection molded part. Combined effect of these factors ultimately leads to generation of sinks and sink marks on plastic products.
Sink and sink mark defect analysis
In production process of injection molded products, sink and sink marks (Sink Mark) are a common quality defect. This defect usually manifests as a local inward shrinkage phenomenon on the surface of product, which may appear near external corners of product, at the place where wall thickness changes suddenly, or in some places that are not often noticed. Root cause lies in thermal expansion and contraction characteristics of material, as well as influence of various factors during injection molding process, such as performance of plastic, operating temperature range, cavity holding pressure, and uniformity of size, shape and cooling rate of injection molded parts. In order to effectively eliminate this defect, we need to deeply analyze causes of its occurrence and take corresponding measures. This includes optimizing selection of plastics, controlling operating temperature range, adjusting cavity holding pressure, and ensuring uniformity of size, shape and cooling rate of injection molded parts. Through implementation of these measures, we can significantly reduce occurrence of sink and sink mark (Sink Mark) defects and improve quality of injection molded products.
Improper control of molding conditions
Too low injection pressure, too short injection and holding time, too slow injection rate, too high material temperature and mold temperature may cause depressions or orange peel-like fine unevenness on the surface of plastic part. In order to solve this problem, we can appropriately increase injection pressure and speed, increase compression density of melt, extend injection and holding time to compensate for melt shrinkage. At the same time, increasing injection recoil is also an effective method, but it should be noted that holding pressure should not be too high to avoid generation of convex marks. When depressions and shrinkage marks appear near gate, situation can be improved by extending holding time. If plastic part is depressed at wall thickness, cooling time in mold should be appropriately extended. In addition, depressions and shrinkage marks around insert are often caused by insert temperature being too low, so insert temperature needs to be increased. Too small a nozzle hole or partial blockage of nozzle of injection molding machine can also cause depressions and shrinkage marks, which is usually caused by excessive local loss of injection pressure. At this time, nozzle should be replaced or cleaned. Insufficient feed can also cause depressions on the surface of plastic part, so feed amount needs to be increased. In terms of in-mold cooling, sufficient cooling must be ensured. This can be achieved by adjusting barrel temperature and lowering melt temperature. At the same time, improving setting of mold cooling system, lowering cooling water temperature, and appropriately strengthening cooling of depressed parts are also effective measures. If plastic part is demolded under insufficient cooling, it is not only easy to produce shrinkage depressions, but also may cause local depressions on ejector during hard demolding.
Mold problem
Runner and gate cross-section of mold are too small, which will increase filling resistance and affect surface quality of plastic part. Asymmetric gate settings or improper feed port location can also cause uneven filling speed, which can cause dents and shrinkage marks. In addition, poor mold venting can interfere with the feeding, shrinkage compensation and cooling processes, while mold wear may cause pressure relief. These issues require our close attention and corresponding measures. Specifically, we need to appropriately expand gate and runner sections according to actual conditions, ensure that gate position is symmetrical, and set feed port at thick wall of plastic part. If dents and shrinkage marks are far away from gate, it may be that melt flow is blocked somewhere in mold structure. At this time, size of mold gating system should be increased, especially sprue section at "bottleneck" should be widened, and even sprue should be extended to dented part. For thick-walled plastic parts, wing gates are recommended. This method is suitable for plastic parts that are not suitable for setting gates directly on plastic part or are easily deformed at gate after molding. By adding a wing-shaped body to plastic part and setting gate on winglet, we can transfer dent defects of plastic part to winglet, so that winglet can be easily removed after molding. At the same time, it is also necessary to regularly check wear, pressure relief and exhaust of mold. Once a problem is found, wearing parts should be replaced in time or exhaust conditions should be improved to ensure stability of quality of plastic parts.
Raw material problems
When shrinkage rate of raw material is too large, the flow performance is poor, or internal lubricant of raw material is insufficient, or raw material is wet, it may cause depressions and shrinkage marks on the surface of plastic part. In order to produce plastic parts with high surface quality requirements, we should give priority to resin grades with low shrinkage rates. If under-injection depression is caused by poor melt flow, fluidity can be improved by adding an appropriate amount of lubricant to raw material, or structural size of gating system can be increased. If depression problem is caused by moisture of raw material, raw material needs to be pre-dried.
Plastic part shape structure design problems
When wall thickness of plastic part varies significantly, thick wall area may have depressions and shrinkage marks during molding due to insufficient pressure. In order to avoid such problems, designer should strive to have uniform wall thickness when conceiving shape structure of plastic part. If there is a special case with a large difference in wall thickness, it can be dealt with by adjusting structural parameters of gating system.

1. Insufficient injection pressure

Insufficient injection pressure will cause plastic to not completely fill mold and cause injection to be insufficient. Problem can be solved by adjusting injection pressure.

2. Injection speed is too slow.

Injection speed is too slow to cause plastic to be squeezed and deformed, causing speed of plastic entering mold cavity to slow down, resulting in insufficient injection. Problem can be solved by adjusting injection speed.

3. Improper temperature

Plastic needs to be injected into mold at a certain temperature. If temperature is too low, plastic will be rigid and difficult to fill mold. If temperature is too high, plastic will become liquid and fluidity will become very poor. Problem can be solved by adjusting heating temperature.

4. Mold problem

Defects in mold, such as mold cavity blockage or uneven mold surface, are also one of reasons for insufficient injection of injection molding machine products. Mold needs to be inspected and maintained.

5. Raw material problem

If viscosity of the raw material is not high enough, it will affect its fluidity and cause problem of insufficient injection. Problem can be solved by replacing raw materials with better quality.

Too short an injection cycle may also lead to material shortage. Due to short cycle time, material temperature often cannot keep up, especially when voltage fluctuates greatly. Therefore, injection cycle needs to be adjusted accordingly according to power supply voltage, mainly considering adjusting time period from completion of pressure holding to retraction of screw to ensure that filling and molding conditions are not affected, while extending or shortening preheating time of pellets in barrel.