Injection molding is an industry with a wide range of knowledge, strong technical and experience requirements. It involves knowledge of plastic properties, injection mold structure, injection molding machine function, injection molding process adjustment, coloring technology, nozzle material recovery/utilization, quality control and production management. In process of injection molding production, some phenomena (such as: nozzle casting, glue leakage, nozzle drawing, mold sticking, plasticizing noise, screw slippage, mold opening difficulties, etc.) and product quality defects (such as: shrinkage, glue shortage, peaking, water ripples, water ripples, air ripples, flow ripples, material flowers, cracking, mold sticking, ejection mark, drag flowers, glue leakage, internal stress, bubbles, color difference, blind holes, broken columns, warping deformation, etc.) often occur. How to quickly and effectively improve these poor injection molding phenomena is not enough based on past experience alone. It is necessary to comprehensively and systematically master professional and technical knowledge of injection molding and accumulate rich practical experience, learn methods and techniques of scientifically analyzing and dealing with problems.

Injection molding products are manufactured in principle according to specifications/standards and customer requirements, but changes in actual injection molding production process are still quite extensive and complex; sometimes when injection molding is going smoothly, shrinkage, deformation, cracks, silver streaks or other defects may suddenly occur. During injection molding, we need to accurately analyze/determine problem points from defects of plastic parts and find effective solutions to problems. This is a kind of professional technology and experience accumulation. Sometimes, problem can be solved by changing injection molding process conditions, making slight adjustments and improvements to mold/machine, and replacing raw materials or color powder used.

A. What kind of defect is produced? When (at beginning of injection molding or during production process) and where? What is degree?
B. How often does defect occur (every time, or occasionally)? How many defects are there?
C. How many mold cavities are there? Do injection molding defects always occur in same mold cavity?
D. Does defect always occur in same position during molding?
E. Has defect been predicted to occur during mold design/production? (Mold flow analysis is generally performed)
F. Has defect occurred obviously at gate? Or is it far away from gate?
G. Will defect still occur when replacing new raw materials or color powder?
H. Try changing an injection molding machine to see if defect only occurs in one injection molding machine or in other injection molding machines

Define→Analysis→Measurement→Improve→Control
Definition: What kind of defect occurs, when and where it occurs, how often, and what is number of defects/defective rate?
Analysis: What are relevant factors that cause defect? What are main factors? What is root cause?
Measurement: MAS (measurement system analysis) analysis, visual inspection of appearance quality, short shot analysis of internal quality, size measurement, visual color or colorimeter
Improvement: Develop an effective plan/plan to improve injection molding defects (what method to use), and organize implementation and follow-up
Control: Consolidate improvement results (record complete injection molding process conditions), summarize/standardize defects caused by this type of structure, and apply this improvement method to other similar products, so as to draw inferences from one example and apply it to other similar products

Factors affecting injection defects in injection molding should be considered from following four aspects

A. Raw materials B. Injection molding machine C. Injection mold D. Molding conditions

 

There are many abnormal problems in injection molding process, mainly in two categories: "injection molding product quality defects" and "abnormal phenomena in injection molding production".
1. Common quality defects of injection molding products include: lack of glue, shrinkage, silver streaks (material flowers), flash (burrs), burning, bubbles (shrinkage holes), water ripples, jetting patterns (snake patterns), flow patterns (flow marks), water inclusion patterns, cracks (turtle patterns), ejection mark, surface matte, warping, black stripes, color mixing, drag patterns, insufficient transparency, size deviation, peeling (peeling), cold material spots, black spots, air patterns, color difference, blind holes, broken columns, etc.
2. Specific abnormal phenomena in injection molding production include: nozzle flow (runny nose) glue leakage, glue parts sticking to mold, nozzle sticking to mold, nozzle drawing, poor inserts, excessive glue, broken needles, blocked nozzles, ejector position perforation, mold imprinting, mold pressing, plasticizing noise, poor material feeding, screw slippage, mold opening difficulty, etc.
Following analyzes main causes of various injection molding defects and abnormal phenomena, and explores effective methods to solve problems.

Insufficient filling is phenomenon that molten plastic does not completely fill every corner of mold molding space. Reasons for insufficient filling are
a. Improper molding conditions
b. Unreasonable mold design and production:
c. Thickness of molded product is too thin, etc.
Countermeasures for molding conditions are:
Increasing melt temperature (melt barrel temperature), increasing mold temperature, increasing injection pressure/injection speed and increasing fluidity of melt; in terms of mold, main channel or branch channel size can be increased or gate position, size, number, etc. can be reviewed to make molten material flow easily to every corner of cavity. In order to allow gas in molding space to be evacuated smoothly, an exhaust groove/exhaust needle can be opened at an appropriate position.

Depressions and internal cavities on the surface of injection molded parts are shrinkage, which not only affects appearance quality of plastic parts, but also reduces strength of product; shrinkage is related to molding process, mold design and type of plastic used.
1. Plastic materials
Different raw materials have different shrinkage rates. Usually, most raw materials that are prone to shrinkage are crystalline plastics (such as nylon, 100% plastic, etc.). During injection molding process, when crystalline plastic is heated and becomes a flowing state, molecules are arranged irregularly; when injected into colder mold cavity, plastic molecules slowly arrange neatly to form crystals, resulting in a large volume shrinkage, and its size is smaller than specified range, which is so-called "shrinkage".
2. Injection molding process technology
In terms of injection molding technology control, shrinkage occurs in following situations: a. Insufficient holding pressure; b. Too slow injection speed; c. Too low mold temperature/material temperature; d. Insufficient holding time, etc. Therefore, when setting injection molding process conditions, it is necessary to review whether molding conditions are correct and whether holding pressure is sufficient to prevent shrinkage problems. Different mold flow processes have different shrinkage rates. Extending holding time can ensure that product has sufficient time to cool and fill glue.
3. Mold and product design. Fundamental reason for shrinkage is uneven wall thickness of product. Surface of column and rib of product is prone to shrinkage. Flow channel design, gate size and cooling effect of mold also have a great impact on finished product. Due to low heat transfer capacity of plastic, the farther away from mold wall or the thicker it is, the slower it solidifies and cools. There should be enough plastic to fill mold cavity. When screw of injection molding machine is injected or held, molten material will not reduce pressure due to backflow; on the other hand, if flow channel is too thin/too long and gate is too small, if condensation is too fast, semi-solid plastic will block flow channel or gate, causing pressure to drop, resulting in shrinkage of finished product.

Silver streaks are generally formed because injection starts too quickly, so that air in melt and mold cavity cannot be discharged, and air is mixed in rubber, causing silver streaks on the surface of product. Silver streaks not only affect appearance, but also reduce mechanical strength of plastic parts. To avoid this defect, cause must be found and improved.
1. Plastic itself contains moisture or cavitation agents.
Due to exposure of plastics to air during manufacturing process, water vapor/oil agents are inhaled, or wrong proportion of ingredients are added during mixing, so that these volatile substances are transformed into gas under high temperature during sol.
2. Decomposition of melt due to heat
If temperature, back pressure and melt speed of melt tube are set too high, or molding cycle is too long, heat-sensitive plastics (such as PVC, Saigang and PC, etc.) are easily decomposed by high temperature to produce gas
3. Air
Plastic particles contain air between particles. If temperature of melt tube near hopper is set very high, surface of plastic particles will melt and stick together before compression, and air between plastic particles cannot be completely removed (poor degassing)

Flash (flash) around plastic parts during injection molding process is a common injection molding problem. If injection pressure/holding pressure is too high, injection speed at the end is too fast, clamping force is insufficient, ejector hole or slider (slider) is worn, mold surface is uneven (there is a gap), and viscosity of material is too low (such as nylon material), flash will occur around product and ejector/core puller (slider). In particular, periphery and ejector position of PP and PA plastic parts are most likely to produce flash. Flash is the worst situation in injection molding operations. If flash sticks to mold parting surface and remains and is directly clamped, it will damage mold parting surface. Damaged area will cause new flash, and there is no way to solve it. Therefore, special attention should be paid to occurrence of flash, and injection molding process parameters should be adjusted reasonably, and use/cleaning/lubrication/maintenance of injection mold should be done well during production process.

If mold is poorly vented or injection is too fast during injection molding process, air in mold will not be discharged in time, and temperature will rise sharply to more than 300℃ under instantaneous high pressure compression, causing trapped air, burning, and yellowing at column, corner, or end of molten plastic flow of product.

During injection molding process, due to large change in wall thickness of plastic part, shrinkage (vacuum bubble) is easy to occur in the part where plastic part is too thick. Principle of shrinkage is same as shrinkage. Difference is that shrinkage is concave on the surface of plastic part, while shrinkage is formed inside. Shrinkage usually occurs in thick-walled parts, which is mainly related to cooling speed of mold. Cooling speed of molten material in mold is different, and shrinkage degree is different; if mold temperature is too low, surface of molten material cools sharply, and hotter molten plastic in thick wall part is pulled to surrounding surface, causing shrinkage (vacuum bubble) inside. Shrinkage cavities not only affect strength and mechanical properties of product, but also appearance of transparent products. The key to improving shrinkage cavities is to control mold temperature, and the other measures are same as those for improving shrinkage.

Water ripples are traces of molten material flow, and are shaped like water ripples with gate as center (mostly seen in smooth molds). Water ripples are wrinkles formed when molten material that initially flows into cavity cools too quickly, and hot melt injected later pushes cooling surface to slip. This can be improved by increasing melt temperature/mold temperature, speeding up injection speed, and increasing holding pressure. If cold material remaining at the front end of nozzle directly enters molding space, it will also cause water ripples; therefore, a material retention hole (cold material well) should be opened at the end of main channel to effectively prevent occurrence of water ripples, size of runner or gate can also be increased to improve it.

If form, size, and position of gate are not suitable or injection speed at gate is too fast during injection molding process, a snake-like jet pattern will appear on the surface of plastic part (in front of side gate).

During injection molding process, if material temperature/mold temperature is too low, thickness of product section is too thin, and injection speed of corresponding part is too fast, turbulent flow lines (flow marks) will appear on the surface of plastic part
Turbulent flow lines often appear in front of gate, wall thickness of plastic part changes greatly, or thick wall part

Weld line is a thin line formed at junction of cold material front when molten plastic flows. Causes of weld lines:
1. Flow mode of molten material caused by shape of molded product (mold structure)
2. Poor fluidity of molten material
3. Foreign matter such as air, volatiles or release agents are involved in confluence of molten material
Weld lines are caused by low temperature of front end of flowing molten material, which makes confluence part unable to fully fuse (mostly occurs in the window and hole parts of molded product). They are usually divided into cold weld lines and hot weld lines (air entrapment). If fluidity of molten material is particularly good, weld line can be almost invisible. Increasing temperature of molten material, temperature of mold and injection speed can minimize degree of weld line.
Changing position and number of gates, moving location of weld line to other places, setting exhaust grooves at fusion part, quickly evacuating air and volatiles in this part, or opening a molten overflow hole near fusion part, moving weld line to cold material hole, then cutting it off afterwards are also effective treatment measures.
Weld lines not only hinder appearance of molded product, but also reduce strength of molded product. Strength of weld line of non-reinforced plastics without glass fiber and other fillers is almost same as that of other parts, but strength of glass fiber in glass fiber reinforced plastics at fusion part is much lower.

During injection molding process, if gate shape/position is improper, injection pressure/holding pressure is too high and holding time is too long, product demolding is not smooth (forced ejection), finished product internal stress is too large or molecular orientation stress is too large, surface of plastic part will crack (fine cracks), and in severe cases, it will crack.
PS and PC products are prone to cracking. Cracks caused by excessive internal stress can be eliminated by "annealing" treatment.

During injection molding process, if injection pressure/holding pressure is too high, demolding slope of bone position/column position is not enough, demolding is difficult, and core point is not smooth (there is an undercut), surface (ejector position) of plastic part will be ejection mark when it is ejected and demolded, top convex/top explosion will occur in severe cases. Surface of ABS and HIPS products is most prone to ejection mark. Slight top self-protrusion on the surface of plastic part can be improved or eliminated by blowing hot air with a blower.

Surface of molded product loses original gloss of material, forming a milky white film, or is in a fuzzy state (matte color), etc., which are all called dull surface.
Poor surface gloss of molded products is mostly caused by poor surface condition of mold. When mold surface is poorly polished or there is mold scale, surface of molded product will certainly not have a good gloss; using too much release agent or oily release agent is also a cause of poor surface gloss. Moisture absorption of materials or mixing (pollution) of volatiles and foreign substances is also one of reasons for poor surface gloss of product.

There are many reasons for warping of injection molded products, such as demolding too quickly, mold temperature too high, uneven mold temperature and asymmetric melt flow. Two biggest possibilities are:
A. Thickness of plastic part is uneven or corners are not smooth enough, so it cannot cool and shrink evenly, resulting in warping and deformation.
B. For some flat plastic parts, gate position must be set at the corner of product for purpose of beautiful surface. During injection molding, molten plastic can only be injected into mold cavity at high speed from one end, so plastic molecules solidified in mold cavity are straightened and arranged in same direction (called orientation). At this time, orientation stress in plastic part is very large, and these molecules are pulled back to their original state during demolding, resulting in deformation.
In order to allow molten plastic to fill mold cavity smoothly, its design should try to avoid following points:
1. Thickness correlation in same plastic part is too large
2. There are excessively sharp angles
3. Buffer zone is too short, resulting in a large difference in thickness transition
From analysis of gate, design of mold should ensure that plastic can enter mold cavity smoothly, so runner should avoid use of right-angle turns. Transition point is more suitable for use of an isolated transition zone. Therefore, a short and thick runner is the most ideal, which helps to reduce fluid resistance and fluid orientation. However, problem to be considered is that an overly large gate will increase runner waste and also affect appearance of plastic part.
In addition, in order to avoid deformation caused by different tightness during plastic filling, which leads to difficulty in demolding, cross-sectional shape and size of runner should be changed according to injection volume and product shape. After runner of part that is more difficult to form is thickened, main runner should also be relatively enlarged so that cross-sectional area of main runner is equal to sum of cross-sectional areas of runner.
There are two other issues worth noting. One is form of ejector device of plastic part. If ejector pins are too few, it is easy to cause ejection deformation and warping; but if number of ejectors is too large, some of finished products will not be beautiful enough. At this time, push-pull method should be considered. Second is design of cooling water channel of mold cavity, which should allow the overall plastic part to cool and shrink evenly to improve product quality.

Black streaks are phenomenon that molded products have black streaks. Main reason for its occurrence is thermal decomposition of molding material, which is common in plastics with poor thermal stability (such as PVC and POM, etc.).
An effective countermeasure to prevent occurrence of black streaks is to prevent temperature of melt in heating cylinder from being too high and slow down injection speed. If there are scars or gaps on inner wall of heating cylinder or screw, material attached to this part will overheat and cause thermal decomposition. Cracking of check ring (rubber ring) will also cause thermal decomposition due to retention of melt, so plastics with high viscosity or easily decomposed plastics should pay special attention to prevent occurrence of black streaks.
There are two other issues worth noting. One is form of plastic part ejector device. If ejector pins are too few, it is easy to cause ejection deformation and warping; but if number of ejector pins is too large, some finished products will not be beautiful enough. At this time, push-pull method should be considered. Second is design of mold cavity cooling water channel, which should allow the entire plastic part to cool and shrink evenly to improve product quality.

During injection molding process, if color powder is unevenly diffused (poor compatibility), barrel is not cleaned, raw material is mixed with other colors of nozzle materials, return material ratio is unstable, melt is poorly plasticized, etc., surface of plastic part or part where flow direction changes will produce local color deviation (color mixing).

If etched pattern on the side of mold cavity is too coarse, demolding slope is not enough, injection pressure/holding pressure is too high, and there are undercuts (burrs) on the inside of cavity, resulting in grooves of different thicknesses/depths around outer periphery of plastic part during injection molding process, it is called scrapes (strains)

During injection molding process of transparent products, if material temperature is too low, raw material is not dried well, melt decomposes, mold temperature is uneven, or mold surface finish is not good, transparency will be insufficient, affecting its transparency

During injection molding process, if injection molding process is unstable, crystallinity of melt changes, plastic part cools and shrinks unevenly, material is easy to absorb moisture, plastic part or mold is deformed, size of plastic part will deviate (instability).

If melt is poor, there are rubber scraps in mold, and there is cold material drooling in runner during injection molding process, cold material spots will appear inside and on the surface of plastic part.

Transparent plastic parts, white products and light-colored products often have black spot problems during injection molding. Surface of plastic parts will affect appearance quality of products, resulting in high scrap rate, large waste and high cost during production process. Black spot problem is a headache in injection molding. It needs to be controlled from gate material, scraps, ingredients, feeding, environment, shutdown and various links in production process to reduce black spots. Appearance of black spots on the surface of plastic parts is a management problem, which can also be improved by taking effective management measures.

During injection molding process, if gate is too small, injection speed is too fast, melt flow changes dramatically, and there is air in melt, air marks (shadows) are likely to occur at gate, corners and steps of plastic parts, especially at gate of ABS, PC, and PPO products.

During injection molding process, if raw material powder changes, amount of water gate material reuse is not controlled, injection molding process (material temperature, back pressure, residual amount, injection speed and screw speed, etc.) changes, machine changes, mixing time is different, raw material drying time is too long, products with matching colors are molded separately (multiple sets of molds), sample changes color and inventory product color is different, it will cause color difference in colors of matching products. Color difference of plastic parts is a common problem in injection molding, and it is also one of the most difficult problems to control, which is a headache for injection molding technology and management personnel. Solving color difference phenomenon is a systematic project, which needs to be controlled from each process (each link) of injection molding production process to be effectively improved.

During injection molding production, if molten material is overheated and decomposed, molten material is poorly plasticized, proportion of sprue material recycled is too large, there is material mixed in sprue material (plastic is contaminated), plastic part is too thin, internal stress is too large, etc., column position/bone position of injection molded part will be insufficient (reduced) in strength and cause sticking/cracking when plastic part is in mold. When strength of plastic part is insufficient, it will crack (break) when it is stressed or used, affecting function, service life and appearance of product.

Bubbles are different from shrinkage holes (vacuum bubbles). They refer to small bubbles that occur in the entire molded product. During injection molding process, if material is not fully dried, injection speed is too fast, there is air in molten material, mold exhaust is poor, and thermal stability of plastic is poor, small bubbles will appear inside plastic part (transparent plastic parts can see it). When there are small bubbles inside plastic part, surface of plastic part is often accompanied by silver streaks (material flowers). Bubbles in transparent parts will affect appearance quality. At the same time, it is also a poor material of plastic part, which will reduce strength of product.

In injection molding production, for some plastic parts with high matching strength requirements, metal inserts (such as screws, nuts, shafts, etc.) are often placed in injection molded parts to make plastic parts or accessories with metal inserts; when injecting metal inserts, problems such as poor positioning of metal inserts, cracking of surrounding glue of metal inserts, flashing of surrounding metal inserts and damage to metal inserts often occur (usually using a vertical injection molding machine)

During injection molding process, molding hole needle of column hole position of plastic part breaks or falls off, and phenomenon of no hole in column position is called a blind hole. Plastic parts with blind holes are defective products and need to be demolded and replaced with molding hole needles.

When plastic melt enters rapidly cooled mold cavity, cooling rate of product surface is much faster than that of inner layer. Surface layer cools rapidly and solidifies. Due to poor thermal conductivity of solidified plastic, solidification inside product is very slow. When gate is closed, central cooling shrinkage cannot be supplemented. Inner layer will be in a tensile state due to shrinkage, while surface layer will be in an opposite state of compressive stress. This stress cannot be eliminated in time after mold is opened and remains in product. We call it residual stress or internal stress. After internal stress of injection molded part is generated, it can be reduced or eliminated by "annealing". Use carbon tetrachloride (CC1) melt or glacial acetic acid solution to detect whether it has internal stress.

When injection molding glass fiber reinforced plastics, floating fibers are likely to appear on the surface of plastic parts, which will deteriorate glossiness of surface of plastic parts and affect appearance quality of injection molded parts.

When injection molding PS, PMMA, and PC materials, due to relatively small compression of injection molding machine screw and poor plasticization of molten material, there are unmelted powders in raw materials, which will cause white spots in transparent plastic parts and affect appearance quality of products.

During injection molding process, phenomenon of molten material flowing out of nozzle when molten material is plasticized is called nozzle drooling (running). In contact injection molding operations, if nozzle drools, molten material flows into main channel, and condensed rubber material will affect smooth progress of injection molding (blocking gate/runner) or cause appearance defects on the surface of rubber part (such as: cold spots, shrinkage, lack of glue, etc.), especially PA materials are most likely to drool.

During injection molding process, phenomenon of hot molten glue flowing out from nozzle head or edge of nozzle thread is called leakage. Nozzle leakage will affect normal progress of injection molding production. In mild cases, it will cause weight or quality of product to be unstable. In severe cases, it will cause shrinkage/lack of glue or burn heating ring of plastic part, affecting appearance quality of product, and number of defective products will increase, wasting raw materials.

During injection molding process, plastic parts often stick to front mold cavity when mold is opened. This phenomenon is called plastic parts sticking to front mold; if plastic parts stick to front mold, it will cause production obstacles or defective products. Cause of its occurrence must be analyzed and improved in time to ensure normal production.

During injection molding process, gate (runner) sticks to mold runner after mold is opened and cannot be separated. This is called gate (runner) sticking to mold. If gate sticking to mold occurs, it will cause production obstacles:
Gate (runner) sticking to mold is mainly due to mismatch between nozzle head and main gate bushing R/aperture, resulting in burrs (undercuts) that cannot be pulled out of gate.

During injection molding process, phenomenon that screw cannot plasticize melt and only produces idle movement is called screw slipping. When screw slips, screw only rotates and does not retreat.

During injection molding process, if plastic parts/sprue materials are not completely taken out or operator fails to find them in time when they stick to mold, plastic parts/sprue materials left in mold after mold is locked will cause mold to be crushed (or molding needle is crushed). Compression molding is a very serious problem that will cause production to stop. Damaged molds such as molding needles, cores or thin structures need to be repaired. Some cores with high dimensional accuracy requirements cannot be repaired and need to be replaced, causing great losses and even affecting delivery time of order. Therefore, in order to prevent compression molding incidents, it is necessary to reasonably set low-pressure protection parameters of mold and install a mold monitoring device to prevent serious compression molding incidents.

During injection molding process, when screw rotates to plasticize resin, friction sound of "chi chi" is called plasticizing noise (especially when plasticizing PMMA and PC materials with high viscosity, plasticizing noise is most likely to be generated). Plasticizing noise is mainly caused by large resistance of screw rotation (friction between resins), strong friction sounds in compression section and feeding section.

During the injection molding process, raw materials in drying barrel (hopper) sometimes fail to discharge, causing production obstacles, unstable plasticizing amount, affecting product quality, and in severe cases, glue parts (sticky mold) will occur, resulting in mold compression.

During injection molding process, wire drawing will occur when sprue/main channel head is demolded, causing production obstacles. If wire drawing remains on mold surface, it will damage mold. If it remains in mold, it will affect appearance of product. Sprue is most likely to produce wire drawing when PP and PA materials are injected.

During injection molding production process, sometimes due to excessive clamping force, core misalignment, guide pin wear (excessive friction), and mold is in a high-pressure clamping state for a long time, mold is deformed and produces "bite force", which will cause mold to be unable to open. For larger products, deep cavity molds and elbow clamping, this problem is more likely to occur.

(1) What is unstable plasticization
It refers to a phenomenon in which resin cannot be supplied to barrel or supply amount is unstable.
There are following modes:
(1) No pre-plasticization at all
(2) Unstable pre-plasticization time
(3) Sometimes insufficient filling occurs
Amount of resin supplied to barrel during plasticization is unstable.
(2) Principle of plasticization

 

The key to plasticization:
It is difficult for barrel surface and pellets to slide, while screw surface and pellets can slide easily.
(2) Causes of unstable plasticization
(2-1) 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 also not be able to move forward.
(2-2) Too high or too low back pressure
Back pressure has effect of suppressing gas from being entrained in resin 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.
(2-3) Improper barrel setting temperature
Barrel setting temperature affects pellet temperature in barrel. In other words, since surface state and rigidity of pellets change, it 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 below hopper is low, metering will remain stable. This is because when temperature rises, surface of pellets will melt, and friction between pellets will increase, causing them to intertwine or stick to screw or barrel. Before material becomes viscous, the higher temperature, the greater friction between plastic and metal.
(2-4) Use of recycled materials
Recycled materials are usually irregular in shape, so compared with ordinary pellets, distance between pellets is easy to increase, which can easily cause poor metering. In addition, after powder is mixed in, it will stick to screw, which will weaken conveying force.
(2-5) Inherent problems of grades
In sliding grades, screw rotation force cannot be well converted into forward conveying force because sliding between metal is too good, which can easily cause poor metering. In addition, in impact-resistant grades (as well as PA, LCP, etc.), friction between pellets is likely to increase, which can also easily cause poor metering.
(2-6) Check valve wear or screw wear
Causes melt backflow, making plasticization unstable
(2-7) Material is not dried
(3) Countermeasures for unstable plasticization
(3-1) Adjust screw speed
First, adjust screw speed
If you want to regularly observe whether there is any poor metering phenomenon, measure metering time. Perform 50 to 100 consecutive moldings, change speed in several stages, and make a judgment based on whether metering time suddenly becomes longer.
If adjusting screw speed alone cannot solve problem, you can change back pressure or barrel temperature at the same time.
(3-2) Reduce back pressure
The lower back pressure, the stronger conveying force of pellets and the more stable metering. However, too low a back pressure will increase entrainment of gas and cause unstable resin amount
(3-3) Reduce temperature below barrel hopper
Specifically, temperature below hopper should be gradually reduced little by little. Excessive reduction 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) Recycled materials
Try to make recycled pellets and initial pellets same size. At the same time, remove as much powder as possible.
(3-5) Problems inherent in grades
Sliding grades have property of being easy to slide due to addition of oil or lubricant. If problem cannot be solved by adjusting screw speed, back pressure and barrel temperature at the same time, it is necessary to consider changing grade or screw design.
In impact-resistant grade, adhesion between pellets is the biggest obstacle to metering. In this case, it is particularly necessary to reduce cylinder temperature below hopper or add an anti-slip agent.

(1) What is gate marking (appearance)
Gate marking refers to small flow marks near gate, as shown in figure.

 

(2) What is cause of gate pattern formation:

 

(3) Why does it cause unstable flow pattern:
(1) In terms of process: a. Low mold temperature
b. Fast injection speed
(2) In terms of mold: a. Small gate size
b. Impact gate burnt spots
(3) In terms of material: Low material fluidity
Countermeasures for gate markings
(1) Increase mold temperature
(2) Reduce injection speed (when passing through gate)
(3) Enlarge gate or change the position
(4) Use a grade with better fluidity or add lubricating additives
To reduce speed when passing through gate, it is best to use multi-stage injection

 

(1) What are bubbles (appearance)
Bubbles refer to a phenomenon in which surface of a molded product bulges or gas is trapped inside

 

2) Causes of bubble generation
(2-1) Air inclusion
1. Air inclusion in barrel:
*Screw speed is too fast
*Back pressure is too low
*Resucking volume is too large
2. Air inclusion in mold cavity:
*Injection speed is too fast
*Gate is too small
*Competition flow phenomenon
*Sprue slope is too large

 

(2-2) Resin degradation
1. Barrel temperature is too high
2. Barrel residence time is too long
3. Screw diameter is too large
4. Other easily decomposable materials are mixed
(2-3) Material is not dried
(3) Countermeasures for bubbles
(3-1) Reduce air entrapment
1. Reduce air entrapment in barrel: - Reduce screw speed
- Increase back pressure
- Do not set too much backsucking volume
2. Reduce air entrapment in cavity during filling: - Reduce injection speed
- Adjust gate position, size and shape.
- Adjust main channel demolding slope
The key is to grasp flow pattern through filling test, then establish corresponding countermeasures on this basis.
(3-2) Inhibit material degradation
Reduce barrel temperature (within recommended operating temperature range. Do not reduce it too much)
Choose a reasonable screw diameter
Thoroughly clean barrel and various auxiliary machines to prevent mixing
(3-3) Improve exhaust conditions
(3-4) Use high-viscosity materials or add additives
High-viscosity materials are generally not prone to voids, so trying to use this material is also a method.
Additives such as glass beads can also be added.

What are voids (appearance)
It refers to a phenomenon in which voids are generated inside a molded product.
Voids generally occur at the thickest part of product wall!
Causes of voids can be roughly divided into two types: one is mixing of a large amount of gas, and the other is shrinkage of resin at thick wall. The former is called "gas voids" and the latter is called "vacuum voids" to distinguish them.

 

Causes of vacuum voids (insufficient holding pressure)
Main reasons for insufficient effective holding pressure are as follows:
Process: (1) Too early pressure change
(2) Low holding pressure setting value
(3) Short holding time
Mold:
(1) Small gate size
(2) Thin runner
Equipment: Wear of screw check valve set
Material: Too much shrinkage of material
When mold temperature is too high, it is easy to form dents, and when mold temperature is too low, it is easy to form voids

 

(3) Countermeasures for vacuum voids
(3-1) Add resin
Technology: confirm turning point, increase holding pressure; extend holding time
Mold: increase gate size, increase diameter of main channel and branch channel, and gate should be located as close to void-generating part (thicker part) as possible
Equipment: check whether screw and three-piece set of check valve are worn
(3-2) Slow down surface curing
If it is a vacuum void, increasing mold temperature can reduce void. But be aware that this method may induce dents
(3-3) Change material grade or add additives

(1) What are dents (appearance)
“Dents” refer to a phenomenon in which a pit is formed due to shrinkage of resin.
When a crystalline resin cools and solidifies, its volume decreases significantly. This is why dents are formed. Since the shrinkage ratio (shrinkage rate) is roughly fixed and the greater thickness, the greater shrinkage, dents are generally formed in thicker parts of molded product.

 

(2) Causes of sink marks
(2-1) Low effective holding pressure: insufficient resin filling
During holding process, amount of resin that shrinks due to cooling and solidification is filled by holding pressure. If for some reason effective holding pressure is low = resin is difficult to fill, sink marks are likely to form when mold temperature is high, and voids are likely to form when mold temperature is low.

 

Main reasons for decrease in effective holding pressure are as follows:
(1) Holding pressure setting value is too low (or gate is unbalanced) (2) Holding time is too short (3) Gate size is too small (4) Runner is too thin (5) Check valve fails
Given importance of gate position, it must be set as close to thick wall as possible.
(2-2) Slow cooling: excessive shrinkage
The greater thickness, the greater shrinkage during cooling; at the same time, the slower cooling, the greater shrinkage. Therefore, the higher mold temperature, the larger dent.

 

(2-3) Insufficient cooling time
Insufficient cooling time causes insufficient rigidity of solidified layer, resulting in dents.
(3) Countermeasures for dents
(3-1) Try to increase holding pressure
It can be considered that applying holding pressure increases amount of resin filling. In order to make it easier to apply holding pressure, an effective treatment method can also be adopted, that is, to expand main channel, branch channel and gate size, and move gate near dent.
(3-2) Reduce mold temperature
If mold temperature value is very high, try to gradually reduce it. Dent can be reduced by reducing amount of shrinkage
(3-3) Increase cooling time
(3-4) Check condition of check valve
a. Is it worn? b. Are there any impurities that prevent check valve from closing?
Move check valve from front end of screw and check contact surfaces. If there is burnt melt, use a copper brush to remove it; do not use a blowtorch to burn it, because this will soften valve metal and accelerate wear.
(3-5) Reduce thickness
If possible, it is recommended to reduce thickness as much as possible. If it is a reinforcing rib, it should reach about 1/3 of base thickness
(3-6) Change material or add additives
High-viscosity materials are generally not prone to voids, so trying to use this material is also a good method. You can also add additives such as glass beads.

(1) What are "black spots" and gray-black streaks
Black spots refer to phenomenon that black spots or streaks appear in molded product.
For some reason, its composition is estimated to be charred resin or carbonized resin, or dirt.
(2) Causes of black spots and gray-black streaks
(2-1) Resin decomposition
Material retention
Retention time is too long (barrel selection is too large)
Temperature is too high

 

In addition, pay attention to matching surfaces between flange and barrel, between nozzle and flange, whether there are steps, unevenness, or damage that may make resin retention possible.
(2-2) Insufficient cleaning
Situation where previously used resin remains in molding machine due to insufficient cleaning is also a cause of black spots. As mentioned in previous item, there are parts such as check rings and thread grooves that are prone to resin retention or parts of screw that are worn.
(2-3) Foreign matter mixing (contamination)
Other easily decomposable materials may be mixed in.
Check material conveying process and condition of recycled material

 

(2-4) Excessive gas
If metering process starts too early, air wrapped in particles in screw feeding area does not overflow feeding port, and air will be squeezed into melt. However, pressure in feeding area is too low to move air to the back. Air squeezed into melt in barrel will cause gray and black spots in product.
(3) Countermeasures for black spots and gray-black streaks
(3-1) First, please clean thoroughly until black spots no longer appear
Common cleaning methods:
(1) High viscosity PE, PP
(2) Chemical cleaning agent:
(3) Remove screw and brush with a copper brush
For optical products or those with higher requirements, method of one material per barrel can be adopted.
(3-2) Try to lower resin temperature
It is recommended to measure actual temperature with a resin thermometer. In particular, parts that are prone to stagnation, such as check rings, are most likely to cause black spots, so pay special attention to temperature near them.
(3-3) Shorten retention time
Use a molding machine that is suitable for mold size.
(3-4) Whether there is contamination
Occasional mixing of other resins or metals may also cause black spots.
(1) Clean thoroughly again
(2) Check whether return material is clean
(3) Check whether auxiliary machine and barrel (material conveying process) are clean
(3-5) Expel excess gas

(1) What are slip marks (appearance)
“Slip marks” refer to a phenomenon in which a once-cured surface layer succumbs to subsequent pressure and moves.
Once-cured surface slides laterally under action of holding pressure or injection pressure and is squeezed against mold again, so that pattern appears on the surface of molded product.

 

(2) Causes of slip marks
(2-1) Mold design
Slip marks are basically caused by improper product shape. Although there are also reasons for molding conditions, impact is not significant. There are three shapes that are prone to slip marks:
(1) Corners without rounded transitions
(2) Slightly raised ejector pins.
(3) Sharp edges
When resin solidified layer in these parts slides, traces are very conspicuous, so slip marks are likely to occur.

 

In addition, after changing gate position and number of points, resin flow direction and resin pressure will change accordingly, so occurrence of slip marks will also change.
(2-2) Contains lubricant
At a certain sliding level, a large amount of oil is contained to ensure sliding properties, but adhesion between melt layers is often insufficient, which makes it easy to produce sliding marks.
(2-3) Influence of injection speed
When injection speed is low, solidification will be accelerated, so pressure will increase accordingly, and sometimes sliding marks will appear under action of this force. On the contrary, even if injection speed is too fast, solidified layer will become easier to move under action of this force.
(2-4) Influence of mold temperature
When mold temperature is low, resin pressure in mold cavity will increase, and sometimes sliding marks will appear under action of this force. On the contrary, when mold temperature is too high, solidified layer will become soft and sometimes easier to move.
& Pay attention to whether gate is balanced
(3) Countermeasures for slip marks
(3-1) Adjust injection speed
Adjust injection speed up or down on existing basis. It is recommended to use multi-stage injection to eliminate slip marks. If this can solve problem, then fine. If not, adjust mold temperature.
(3-2) Adjust mold temperature
Adjust mold temperature up or down on existing basis to eliminate slip marks. If this does not solve problem, then adjust mold shape.
(3-3) Change mold shape
Change shape of part with problem.
Rounded corner transition
Correctly adjust height of ejector pin, etc.
Reduce sharp edges
Also, changing gate position is effective.
(3-4) Change material grade
When problem cannot be solved and mold shape cannot be changed, using a different grade of material is also a countermeasure.
Make corner have R

 

(1) What are silver streaks (appearance)
Silver streaks refer to phenomenon that white spots or silver streaks appear on the surface of a molded product.

 

(2) Causes of silver streaks
(2-1) Gas generated by resin decomposition
Resins are chemical substances and therefore gradually decompose as temperature increases. The higher resin temperature or the longer it stays, the more it decomposes and the more likely silver streaks will appear.
(2-2) Air inclusion
(1) If screw speed is too fast and back pressure is too low, amount of air involved in plasticizing resin will increase. As a result, streaky bubbles appear on the surface of molded product and silver streaks are easily formed.
(2) Runner silver streaks: Taper of main runner is too large, exceeding 10 degrees. Generally, it is 4-6 degrees.
(2-3) Moisture (opening direction of water streak is along material flow direction. In places where product is not completely filled, front end of fluid is very rough)
(1) Insufficient drying of material
(2) Water leakage in mold cavity
(2-4) Small exhaust port
If gas is not completely exhausted, bubbles will remain on the surface of molded product, making silver streaks more likely to appear.
(2-5) Mixing of different materials
If material produced last time or material used to clean barrel is mixed in due to insufficient cleaning, and temperature of resin is low, gas may be generated and silver streaks may be induced.
In addition, materials that are added with dyeing agent white oil, lubricant silicone oil, plasticizer dibutyl ester, stabilizer, antistatic agent are prone to surface peeling and silver streaks.

 

(3) Countermeasures for silver streaks
(3-1) Check metering status
First, check metering status. If screw speed is too fast or back pressure is insufficient, silver streaks are likely to appear. Please adjust gradually and observe whether there are any changes.
If there are, please adjust to optimal value.
(3-2) Check resin temperature: Resin decomposition
Each resin has its recommended operating temperature (indicated on product bag and product catalog). Check whether actual temperature is within range. If it is out of range, it must be adjusted to within range. At the same time, check whether screw size is reasonable
(3-3) Strengthen drying; at the same time, check whether mold is leaking
Confirm drying temperature and drying time
(3-4) Whether there are contaminants
(1) Re-clean. Because retained resin is constantly decomposing.
(2) Check whether return material is clean
(3) Check whether auxiliary machine and material pipe are clean
(3-5) Check exhaust port
Occasionally, silver streaks will occur due to poor exhaust, so check whether exhaust port is large enough,
(3-6) Check taper of main channel

(1) What is color difference (appearance)
Color unevenness means that color of product surface is different. It can appear near and far from material head, and occasionally in material flow area with sharp edges.
(2) Causes
Uneven pigment mixing
Material degradation
Causes and improvement measures related to process parameters:
1. Material is not evenly mixed. Reduce screw speed; increase barrel temperature and increase screw back pressure
2. Melt temperature is too low. Increase barrel temperature and increase screw back pressure
3. Screw back pressure is too low. Increase screw back pressure
4. Screw speed is too high. Reduce screw speed
Causes and improvement measures related to design are shown in table below:
1. Screw stroke is too long. Use a barrel with a larger diameter or a larger aspect ratio
2. Melt stays in barrel for a short time. Use a barrel with a larger diameter or a larger aspect ratio
3. Screw L:D is too low. Use a barrel with a larger aspect ratio
4. Screw compression ratio is low. Use a high compression ratio screw
5. There is no shearing section and mixing section. Provide a shearing section and (or) mixing section

 

(1) What is a welding line (appearance)
Fusing occurs where resins merge. As shown in figure below, there will be welding in part with openings.

 

(2) Causes of weld marks
(2-1) Low resin temperature
When two resin flows meet, they fuse. 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-solidified while moving forward in jet), if temperature is low, surface layer will become thicker, texture will be obvious, and strength will also decrease. This is because adhesion between two flows is weakened. On the contrary, if temperature of two resin flows is high, adhesion will be strengthened and appearance will become less obvious. Conditions for lowering resin temperature are:
Low mold temperature
Low setting temperature of barrel (especially nozzle)
Slow injection speed
Low fluidity of material
(2-2) Low pressure
At fusion point, two molten resins are squeezed. Adhesion here depends on pressure applied there. The lower holding pressure, the more obvious fusion and the lower strength. As solidification proceeds, pressure transmission becomes more difficult. In addition, if gate runner size becomes smaller and gate position becomes worse, appearance and strength of fusion will deteriorate.

 

(2-3) Weak exhaust at exhaust port
Fusion is confluence point of resin and may also be the end of flow. At this time, if a vent is not set well at this position to exhaust gas, appearance and strength of fusion will deteriorate.

 

Figure 5. Exhaust of fusion part should be sufficient
Note:
(1) Quality of fusion line of plastics without reinforcement is significantly higher than that of plastics with reinforcement.
(2) Quality of fusion line area is closely related to type and content of fillers and reinforcements. Additives such as processing aids and flame retardants have an adverse effect on quality of fusion line.
(3) Containing fiber reinforcement, arrangement direction of fibers in fusion line area is perpendicular to flow direction. This will significantly reduce mechanical properties of part at this point.

 

(3) Countermeasures for bad weld lines
(3-1) Increase resin temperature of welded part
Try to gradually increase mold temperature and barrel temperature. This is expected to achieve maximum effect. Holding pressure effect will also be improved at the same time. In sense that welded part is formed before resin temperature drops, it is also effective to increase injection speed and form welded part quickly.
(3-2) Try to increase effective holding pressure
Although it is possible to simply increase holding pressure setting, it is recommended to use following conditions that make it easier to apply holding pressure
1. Increase resin temperature 2. Increase mold temperature 3. Increase injection speed or multi-stage injection 4. Expand gate 5. Adjust thickness 6. Use a material with good fluidity 7. Check wear of check valve and screw
(3-3) Check exhaust port
Even if weld is at the end of flow, it is necessary to check exhaust port. At the same time, reduce clamping force to minimum value.
Check whether thickness and size of exhaust port ensure smooth exhaust and whether it is contaminated by mold scale. If exhaust is not sufficient, it will cause gas burning and cause other failures.
(3-4) Check gate and runner
If gate and runner are not big enough, even if temperature and pressure are increased, it may not be effective. In this case, if gate and runner are increased in size, resin will flow more smoothly, and it will be easier to apply pressure, which will improve fusion.
In addition, changing gate position and increasing number of points is also an effective method. By changing gate position, position of fusion line can be controlled.
(3-5) Add cold well

 

Note: Due to influence of mold structure, it is impossible to completely eliminate weld mark, so when debugging, do not limit yourself to removing weld mark, but control adverse phenomena caused by weld mark to minimum, which is more important.

(1) Causes of deformation:
a. Uneven temperature
b. Uneven pressure
c. Molecular orientation
(2) Countermeasures for deformation
a. Gate design
(1) Unify flow direction of resin
Figure 4-1-1 Gate design for a slender flat plate

 

(2) Increase gate size (to reduce pressure loss and molecular orientation)
If there is no problem with quality, gate size is sometimes set smaller due to factors such as shortening molding cycle and trimming gate. However, too small a gate size is not good for deformation.
If number or position of gate is inappropriate, flow length will be too long, flow resistance will be too large, and corresponding injection pressure must also be increased. Plastic molecules are stretched and squeezed, and mechanical stress is forced into them. Residual stress is large and it is easy to warp.
Pressure near gate is high, and melt volume shrinkage is small; pressure at last filling is low, and volume shrinkage is large; when flow length is too long, upstream and downstream melt volume shrinkage differences are large, residual stress is large, and it is easy to warp.
Decision of gate position should follow principle of balanced filling, that is, time for each melt wave front to reach the end of cavity and form a weld mark is basically same, filling is thick first and then thin, flat first and then curved.
b. Mold temperature adjustment

 

2. Strengthen cooling of punch and wall thickness

 

3. Mold material
For places where it is difficult to set cooling holes or where temperature may rise, use materials with good thermal conductivity in Table 4-2-2 to achieve good cooling effect. However, due to factors such as cost or strength, careful selection is required.
Table 4-2-2 Thermal conductivity of mold materials (unit: kcal/m.h.℃)

C. Molding condition factors
Injection and holding time: in principle, it is set to gate closing time.
(1) If injection and holding time are shorter than gate closing time, holding process of fully transmitting and maintaining pressure in mold cavity will be insufficient, and sometimes deformation will occur.
(2) If holding pressure is too high, residual shear stress and compressive stress of melt entering holding pressure will be large, which is easy to cause warping. If holding pressure is too low, melt will flow back near gate, which will cause residual shear stress. Since volume shrinkage of product is large [due to low pressure] and shrinkage of surrounding areas is small, difference in volume shrinkage inside and outside will produce residual tensile and compressive stress, which is easy to cause warping.
So: General idea is high mold temperature, low injection speed, low pressure,