What you have to know | Cause of problem with injection mold

Time:2019-11-22 09:32:49 / Popularity: / Source:

Cause Analysis of cracking of injection molded products

injection mold 
Cracking, including filament cracks on the surface of part, microcracking, whitening, cracking, and damage caused by sticking of part, sticking of flow path. Cracking is divided into split cracking and application cracking according to cracking time.. There are mainly following reasons:

1 processing:

(1) If processing pressure is too large, speed is too fast, more filling, excessive injection and holding time, all will cause excessive internal stress and cracking.
(2) Adjusting mold opening speed and pressure to prevent demoulding cracking caused by rapid drawing of workpiece.
(3) Appropriately increase mold temperature, make parts easy to demould, and appropriately reduce material temperature to prevent decomposition.
(4) Prevention of cracking due to weld line, degradation of plastic causes mechanical strength to become low.
(5) Appropriate use of release agent, attention should be paid to elimination of substances such as aerosols attached to mold surface.
(6) Residual stress of workpiece, internal stress can be eliminated by performing an annealing heat treatment immediately after molding, thereby reducing generation of cracks.

2 mold aspects:

(1) Eject should be balanced, such as number of ejector pins and cross-sectional area are sufficient, draft angle should be sufficient, and cavity surface should be sufficiently smooth to prevent cracking due to external force causing residual stress concentration.
(2) Structure of workpiece should not be too thin, and transition part should adopt arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Minimize use of metal inserts to prevent internal stress from increasing due to difference in shrinkage between insert and part.
(4) Appropriate stripping intake passages shall be provided for deep-bottomed parts to prevent vacuum underpressure.
(5) Main runner is large enough to demould gate material when it is not cured, so that it is easy to demould.
(6) Main runner bushing and nozzle joint should prevent chilling material from being dragged to make workpiece stick to fixed mold.

3 materials:

(1) Content of recycled materials is too high, resulting in too low strength of parts.
(2) Humidity is too large, causing some plastics to react with water vapor, reducing strength and causing cracking.
(3) Material itself is not suitable for environment that is being processed or bad quality. If it is contaminated, it will cause cracking.

4 machine aspects:

Plasticizing capacity of injection molding machine should be appropriate. If plasticization is too small, it will not be completely mixed and become brittle. When it is too large, it will degrade.

Causes analysis of bubbles in injection molding products.

Gas of bubble (vacuum bubble) is very thin and belongs to vacuum bubble. In general, if a bubble is found at the moment of mold opening, it is a gas interference problem. Formation of vacuum bubbles is due to insufficient filling or low pressure. Under rapid cooling of mold, fuel at the corner of cavity is pulled, resulting in a volume loss.


(1) Increase energy of injection: pressure, speed, time, amount of material, and increase back pressure to make filling full.
(2) Increasing material temperature and flow smoothly. Lower material temperature to reduce shrinkage, appropriately increase mold temperature, especially local mold temperature at which vacuum bubble is formed.
(3) Gate is placed in thick part to improve flow of nozzle, runner and gate, reduce, consumption of press.
(4) Improve mold exhaust condition.

Causes analysis of Warpage of Injection Molding Products

Deformation, bending and twisting of injection molded products are mainly caused by fact that shrinkage rate of flow direction during plastic molding is larger than that of vertical direction, so that shrinkage ratio of parts is different and warped. Moreover, warpage is inevitably caused by a large internal stress remaining in the inside of workpiece during injection filling, which is a manifestation of deformation caused by high stress orientation. Therefore, fundamentally speaking, mold design determines warping tendency of part. It is very difficult to suppress tendency by changing molding conditions. Finally, problem must be solved from mold design and improvement. This phenomenon is mainly caused by following aspects:

1 mold aspect:

(1) Thickness and quality of parts should be uniform.
(2) Design of cooling system is to make temperature of each part of mold cavity uniform. Pouring system should make flow symmetry to avoid warpage caused by different flow direction and shrinkage rate, properly thicken split runners and main runners which are difficult to form, and try to eliminate density difference, pressure difference, temperature difference in cavity.
(3) Transition zone and corner of workpiece should be sufficiently rounded and have good demolding properties, such as increasing demoulding margin, improving polishing of die face, ejector system should be balanced.
(4) Exhaust should be good.
(5) Increase wall thickness of part or increase direction of warping resistance, strengthen rib to enhance anti-warping ability of part.
(6) Strength of material used in the mold is insufficient.

2 plastics:

Crystal type has more chance of warpage deformation than amorphous plastic, and crystal type plastic can be used to correct warpage deformation by using a crystallizing process in which crystallinity decreases as cooling rate increases and shrinkage rate becomes smaller.

3. Processing:

(1) Injection pressure is too high, holding time is too long, melt temperature is too low, and speed is too fast, which causes internal stress to increase and warpage.
(2) Mold temperature is too high, and cooling time is too short, so that parts during demolding are overheated and ejection deformation occurs.
(3) Reducing screw speed and back pressure reduction density while maintaining minimum charge amount to limit generation of internal stress.
(4) If necessary, for parts that are easy to warp and deform, it can be be soft-shaped or demolded and then annealed.

Color analysis of injection molding products

injection molded products 
Occurrence of such defects is mainly caused by problems of plastic parts colored with masterbatch. Although color masterbatch is superior to dry powder coloring and dyeing in terms of color stability, color purity and color migration, distribution property, that is, color particles in diluted plastic is relatively poor in uniformity, and finished product naturally has a regional color difference.

Main solution:

(1) Increasing temperature of feeding section, especially temperature at rear end of feeding section, so that temperature is close to or slightly higher than temperature of melting section, masterbatch is melted as soon as possible when it enters melting section to promote uniform mixing with dilution, and increase chance of liquid mixing.
(2) When screw rotation speed is constant, increasing back pressure causes melt temperature and shearing action in barrel to be improved.
(3) Modify mold, especially pouring system. If gate is too wide, turbulence effect is poor when melt passes, temperature rise is not high, so it is not uniform. Ribbon cavity should be narrowed.

Causes analysis of shrinkage and depression of injection molded products

During injection molding process, shrinkage of product is a common phenomenon. Main reasons are:

1 machine:

(1) Nozzle hole is too large to cause melt to recirculate and shrink. When it is too small, resistance is insufficient to shrink.
(2) If clamping force is insufficient, flash will also shrink. Check whether clamping system has any problems.
(3) If amount of plasticization is insufficient, a machine with a large amount of plasticization should be used. Check whether screw and barrel are worn.

2 mold:

(1) Design of parts should be uniform in wall thickness to ensure consistent shrinkage.
(2) Cooling and heating system of mold should ensure that temperature of each part is consistent.
(3) Pouring system should be ensured to be unobstructed, and resistance should not be too large. Such as, size of main runner, sub-runner, and gate should be appropriate, finish should be sufficient, and transition zone needs to be circularly transitioned.
(4) For thin parts, temperature should be raised to ensure smooth flow, and mold temperature should be reduced for thick-walled parts.
(5) Gate should be opened symmetrically, as far as possible in thick part of part, and volume of cold well should be increased.

3 plastics:

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

4 processing aspects:

(1) Temperature of barrel is too high, and volume changes greatly, especially temperature of front furnace. For plastics with poor fluidity, temperature should be properly increased to ensure smoothness.
(2) Injection pressure, speed, back pressure is too low, injection time is too short, so that amount or density is insufficient, contraction pressure, speed, back pressure is too large, and time is too long, causing flash to shrink.
(3) Feeding amount, that is, injection pressure is consumed when cushion is too large, and feeding amount is insufficient when cushion is too small.
(4) For parts that do not require precision, after injection pressure is maintained, parts whose outer layer is basically condensed and hardened, core part is still soft and can be ejected, mold should be demoulded early. So that it can be slowly cooled in air or hot water. It can make its contraction depression smooth and less conspicuous without affecting use.

Causes Analysis of transparent defects in injection molded products

Melting, silver, cracks. Transparent parts of polystyrene and plexiglass, sometimes through light, can see some sparkling filament-like silver lines, also known as spots or cracks. This is because stress in vertical direction of tensile stress is generated, use of polymer molecules in heavy flow orientation is different from unoriented portion.


(1) Eliminate interference from gases and other impurities, and dry plastic sufficiently.
(2) Reduce temperature of material, adjust temperature of barrel in stages, and appropriately increase mold temperature.
(3) Increase injection pressure and reduce injection speed.
(4) Increase or decrease pre-plastic back pressure and reduce screw speed.
(5) Improve runner and cavity exhaust conditions.
(6) Clean up possible blockages in nozzles, runners, and gates.
(7) Shortening molding cycle. After demolding, silver grain can be removed by annealing: hold polystyrene at 78℃ for 15 minutes, or 50℃ for 1 hour, and heat polycarbonate to 160℃ for several minutes.

Causes analysis of uneven color of injection molded products

Main reasons and solutions for uneven color of injection molded products are as follows:
(1) Colorant is poorly diffused, which tends to cause a pattern near gate.
(2) Plastic or colorant has poor thermal stability. To stabilize color tone of part, it is necessary to strictly fix production conditions, especially material temperature, amount of material and production cycle.
(3) For crystalline plastics, try to keep cooling speed of each part consistent. For parts with large difference in wall thickness, coloring agent can be used to mask chromatic aberration. For parts with uniform wall thickness, material temperature and mold temperature should be fixed. .
(4) Shape of part and form of gate. Position has an influence on filling condition of plastic, so that some parts are subject to chromatic aberration and should be modified if necessary.

Causes analysis of color and gloss defects of injection molded products

injection molded products
Under normal circumstances, gloss of surface of an injection molded part is mainly determined by type, colorant of plastic and finish of mold surface. However, due to some other reasons, it causes defects such as surface color and gloss defects, dark surface.

Reasons and solution:

(1) Mold finish is poor, surface of cavity is rusted, and mold is poorly exhausted.
(2) Casting system of mold is defective. Cold well should be enlarged, runner should be enlarged, main runner, sub-runner and gate should be polished.
(3) Material temperature and mold temperature are low, local heating method of gate can be used if necessary.
(4) Processing pressure is too low, speed is too slow, injection time is insufficient, and back pressure is insufficient, resulting in poor compactness and darkening of surface.
(5) Plastic should be fully plasticized, but degradation of material should be prevented, heat should be stable, cooling should be sufficient, especially for thick-walled parts.
(6) Prevent cold material from entering part, if necessary, use a self-locking spring or reduce nozzle temperature.
(7) Excessive recycled materials are used, plastic or colorant is of poor quality, mixed with water vapor or other impurities, and quality of lubricant used is poor.
(8) Clamping force should be sufficient.

Causes Analysis of Silver Plating of Injection Molding Products

Silver pattern of injection molded products, including surface bubbles and internal pores. Main cause of defects is interference of gases (mainly water vapor, decomposition gas, solvent gas, air). Specific reasons are as follows:

1 machine:

(1) Barrel, screw wears, rubber head and apron have a dead angle of material, which is decomposed by heat for a long time.
(2) If heating system is out of control, causing temperature to be too high and decomposed, check whether there are any problems with heating elements such as thermocouples and heating coils. Improper screw design, resulting in decomposition or easy to bring in air.

2 mold:

(1) Poor exhaust.
(2) Frictional resistance of runners, gates and cavities in the mold is large, causing local overheating and decomposition.
(3) Unbalanced distribution of gates and cavities, unreasonable cooling system will cause heat imbalance and local overheating or obstruction of air passage.
(4) Cooling passage leaks into cavity.

3 plastics:

(1) Plastic has a large humidity, proportion of added recycled materials is too large or contains harmful scraps (scraps are easily decomposed), plastic should be sufficiently dried and scraps should be eliminated.
(2) Moisture absorption from atmosphere or from colorant, and coloring agent should also be dried. It is preferable to install a dryer on the machine.
(3) Lubricant, stabilizer added to plastic is excessive or unevenly mixed, or plastic itself has a volatile solvent. Decomposition occurs when mixed plastic is difficult to cope with heat.
(4) Plastic is contaminated and mixed with other plastics.

4 processing aspects:

(1) Set too high temperature, pressure, speed, back pressure, melt motor speed to cause decomposition. Or low pressure, speed, injection time, insufficient pressure, low back pressure, due to failure to obtain high pressure and insufficient density, it can not melt gas and appear silver. Appropriate temperature, pressure, speed and time should be set and multi-stage injection speed should be used.
(2) Low back pressure and fast rotation speed make air enter barrel. As melt enters mold, melt is decomposed in the barrel due to excessive heat when cycle is too long.
(3) Insufficient material, feeding cushion is too large, material temperature or mold temperature is too low, which affects flow of material and molding pressure, and promotes formation of bubbles.

Causes analysis of welded joints in injection molded products

Molten plastic in the cavity meets in the form of multiple strands due to intrusion of insert holes, inconsistent flow rate, and region where filling flow is interrupted, resulting in a linear weld seam due to incomplete fusion. In addition, in the case of gate injection molding, a welded joint is formed, and strength at welded joint is poor. Main reasons are as follows:

1 processing:

(1) Injection pressure and speed are too low, temperature of barrel and mold are too low, causing melt entering mold to cool prematurely and a weld seam occurs.
(2) When injection pressure and speed are too high, a spray will occur and a weld seam will appear.
(3) Rotation speed should be increased, and back pressure should be increased to decrease viscosity of plastic and increase density.
(4) Plastic should be dried well, and recycled material should be used less. If amount of release agent is too much or quality is not good, welded joint will appear.
(5) Reduce clamping force and facilitate exhaust.

2 mold aspects:

(1) If there are too many gates in same cavity, gate or symmetrical setting should be reduced, or as close as possible to fusion joint.
(2) Exhaust system should be set if exhaust system at welded joint is poor.
(3) If sprue is too large and pouring system is not properly sized, gate should be set to avoid flow of melt around hole of insert or to minimize use of inserts.
(4) If wall thickness changes too much, or wall thickness is too thin, wall thickness of part should be uniform.
(5) If necessary, a fusion well shall be set at weld seam to disengage weld joint from workpiece.

3 plastics:

(1) Lubricants and stabilizers should be added to plastics with poor fluidity or heat sensitivity.
(2) Plastics contain a lot of impurities, and if necessary, change plastic with good quality.

Causes Analysis of Shock Pattern of Injection Molding Products

injection mold
A dense corrugation formed by a rigid plastic part such as PS on the surface centered near gate, sometimes called a shock pattern. Reason is that when melt viscosity is too large and mold is filled in a stagnant form, front end material quickly condenses and shrinks as soon as it contacts surface of cavity, and subsequent melt expands contracted cold material to continue advancement. Constant alternating of process causes stream to form a surface striate as it advances.


(1) Increase barrel temperature, especially nozzle temperature, and increase mold temperature.
(2) Increase injection pressure and speed to make it quickly fill mold cavity.
(3) Improve size of runners and gates to prevent excessive resistance.
(4) Mold exhaust should be good, and a large enough cold well should be set.
(5) Do not design parts too thin.

Causes analysis of swelling and bubbling of injection molded products

Some plastic parts quickly swell on the back of metal insert or in a particularly thick part after mold is demolded. This is caused by expansion of gas released by internal pressure of plastic which is not completely cooled and hardened.


1. Effective cooling. Reduce mold temperature, extend mold opening time, reduce drying and processing temperature of material.
2. Reduce filling speed, forming cycle and flow resistance.
3. Increase holding pressure and time.
4. Improve condition that wall of workpiece is too thick or thickness changes greatly.

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