Cracking, including filament cracks on the surface of part, microcracking, whitening, cracking, and traumatic damage caused by sticking of part, sticking of runner. It is divided into demoulding cracking and applying cracking according to cracking time. There are mainly following reasons:

1) If processing pressure is too large, speed is too fast, filling is too much, injection and holding time are too long, internal stress will be too large and crack.
2) Adjust mold opening speed and pressure to prevent demoulding cracking caused by rapid and strong drawing.
3) Appropriately increase mold temperature, make parts easy to demould, properly reduce material temperature to prevent decomposition.
4) Prevention of cracking caused by weakening of mechanical strength due to weld marks and plastic degradation.
5) Appropriate use of release agent, pay attention to elimination of substances such as aerosols attached to mold surface.
6) Residual stress of part, it can be reduced by eliminating internal stress through annealing heat treatment immediately after molding to reduce generation of cracks.

1) Ejector should be balanced. For example, number of ejector pins and cross-sectional area are sufficient, draft angle should be sufficient, 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, 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 inlet channels should be provided for deep-bottomed parts to prevent vacuum underpressure.
5) Main runner is large enough to demould gate material before curing, so that it is easy to demould.
6) Main runner bushing and nozzle joint should prevent chilling material from being pulled and workpiece sticking to fixed mold.

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 or quality of processing being processed, pollution will cause cracking.

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

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

 

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, and also warp is caused by intrinsic residual internal stress in 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) 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 avoid warpage caused by different flow direction and shrinkage rate, properly thicken split runners and main runners which are difficult to form, 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, and ejector system should be balanced.
4) Exhaust should be good.
5) Increase wall thickness of part or increase anti-warpage direction, strengthen rib to enhance anti-warping ability of part.
6) Material used in the mold is insufficient in strength.

Crystal form 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.

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) Reduce screw speed and back pressure to reduce density while maintaining minimum injection amount to limit generation of internal stress.
4) If necessary, mold which is easy to warp and deform may be soft-shaped or demolded and then annealed.

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 in the degree of uniformity is relatively poor, finished product naturally has a regional color difference.

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, so that masterbatch is melted as soon as it enters melting section, promoting uniform mixing with dilution, and increasing chance of liquid mixing.
2) When screw speed is constant, back pressure is increased to increase temperature and shearing action of melt in the barrel.
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, and ribbon cavity should be narrowed.

 

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

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

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

Crystalline plastics shrink than conventional non-crystalline 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.

1) Temperature of barrel is too high, 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 and contraction pressure, speed, back pressure is too large, and time is too long, causing flash to shrink.
3) Feeding amount, that is, when cushion is too large, injection pressure is consumed. But when amount is too small, feeding amount is insufficient.
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 should be taken out early to make it slow cooled in air or hot water. Cooling can make shrinkage depression smooth and less conspicuous without affecting use.

Melting, silver, cracked polystyrene, transparent parts of plexiglass, sometimes through light can see some sparkling filament-like silver. These silver streaks are also known as spots or cracks.

1) Eliminate interference from gases and other impurities, dry plastic thoroughly.
2) Reduce material temperature, adjust barrel temperature in stages, and increase mold temperature appropriately.
3) Increase injection pressure and reduce injection speed.
4) Increase or decrease pre-plastic back pressure, 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 pattern can be removed by annealing: polystyrene is kept at 78℃ for 15 minutes, or at 50℃ for 1 hour; polycarbonate is heated to above 160℃ for several minutes.

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 make cooling speed of each part consistent. For parts with large difference in wall thickness, colorants can be used to mask chromatic aberration. For parts with uniform wall thickness, material temperature and mold temperature should be fixed.
4) Shape, gate form of part and position has an influence on filling condition of plastic, so that some parts of will have chromatic aberration and should be modified if necessary.

Under normal circumstances, surface gloss of an injection molded part is mainly determined by type of plastic, colorant and finish of mold surface. However, it is also often caused by defects such as surface color and gloss, and dark surface of product for some other reasons. Reasons for this and solution are as follows:
1) Mold finish is poor, surface of cavity is rusted, and mold is poorly exhausted.
2) Casting system of mold is defective. Cold material 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, and cooling should be sufficient, especially thick-walled.
6) Prevent cold material from entering part, if necessary, use a self-locking spring or lower nozzle temperature.
7) Too much recycled material is 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.

 

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) Barrel, screw wears, material head and apron have a dead angle of material, which is decomposed by heat for a long time.
2) 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.

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

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 from colorant, and coloring agent should also be dried. It is preferable to install a dryer on the machine.
3) Amount of 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.

1) Set temperature, pressure, speed, back pressure, melt motor speed is too high to cause decomposition. or when pressure and speed are too low, insufficient injection time and pressure, low back pressure, due to failure to obtain high pressure, density is insufficient to melt gas and silver streaks appear. Appropriate temperature, pressure, speed, time should be set and multi-stage injection speed should be used.
2) Back pressure is low and rotation speed is fast, so that air enters barrel, and melt enters mold. When period is too long, melt is decomposed in barrel due to excessive heat.
3) Insufficient material, feeding cushion is too large, material temperature or mold temperature is too low, which affects flow of material and molding pressure, promotes formation of bubbles.

 

Molten plastic in cavity meets in the form of multiple strands due to intrusion of insert holes, inconsistent flow rate, region where filling flow is interrupted, a linear weld seam is produced because it cannot be completely fused. In addition, in the case of gate injection molding, a welded joint is formed, strength at welded joint is poor. Main reasons are as follows:

1) Injection pressure and speed are too low, barrel temperature and mold temperature are too low, causing melt entering mold to cool prematurely and weld seam appears.
2) When injection pressure and speed are too high, there will be a spray and weld seam will appear.
3) Rotation speed should be increased, back pressure should be increased to decrease viscosity of plastic and increase density.
4) Plastic should be dried well, 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.

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 at weld seam is poor, and an exhaust system should be provided.
3) If sprue is too large and pouring system is not properly sized, gate should be opened to avoid flow of melt around hole of insert, or 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 should be opened at weld seam to disengage weld joint from part.

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

Rigid plastic parts such as PS form a dense corrugation on the surface near gate, centered on 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, subsequent melt expands contracted cold material to continue the process. Constant alternating causes the stream to form a surface grain in the forward direction.

1) Increase barrel temperature, especially nozzle temperature, and increase mold temperature.
2) Increase injection pressure and speed to 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.

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

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