During injection molding process, it is a common phenomenon for products to shrink and dent. Main reasons for this situation are:

(1) If nozzle hole is too large, melt will flow back and cause shrinkage; if it is too small, resistance will be large and amount of material will be insufficient, causing shrinkage.
(2) Insufficient clamping force will cause flash to shrink, so check whether there is any problem with clamping system.
(3) If amount of plasticizing is insufficient, a machine with a large amount of plasticizing should be used to check whether screw and barrel are worn.

(1) Design of parts should make wall thickness uniform and ensure consistent shrinkage.
(2) Cooling and heating system of mold should ensure that temperature of each part is consistent.
(3) Pouring system must be smooth and resistance must not be too large. For example, size of main runner, runner, and gate must be appropriate, smoothness must be sufficient, and transition area must be an arc transition.
(4) For thin parts, temperature should be increased to ensure smooth material flow, and for thick-walled parts, mold temperature should be lowered.
(5) Gate should be opened symmetrically, try to open it in thick-walled part of part, and cold slug well volume should be increased.

Crystalline plastics shrink more than amorphous plastics. During processing, amount of material should be appropriately increased, or a replacement agent should be added to plastic to speed up crystallization and reduce shrinkage dents.
Processing
(1) Temperature of barrel is too high and volume changes greatly, especially temperature of front furnace. For plastics with poor fluidity, temperature should be appropriately increased to ensure smoothness.
(2) Injection pressure, speed, and back pressure are too low, and injection time is too short, resulting in insufficient material quantity or density, resulting in shrinkage caused by excessive pressure, speed, and back pressure, or too long time, causing flash and shrinkage.
(3) When amount of material added is too large, injection pressure will be consumed; if it is too small, amount of material will be insufficient.
(4) For parts that do not require precision, after injection and pressure-holding is completed, outer layer is basically condensed and hardened but sandwich part is still soft and can be ejected. Eject mold as soon as possible and let it cool slowly in air or hot water, which can make shrinkage dents gentle and less conspicuous without affecting use.

Rigid plastic parts such as PS form dense ripples centered on gate on their surfaces near gate, sometimes called vibration lines. Reason is that when melt viscosity is too high and mold is filled in a stagnant flow, front material quickly condenses and shrinks as soon as it contacts surface of mold cavity, subsequent melt expands shrunk cold material and continues process. Continuous alternation causes material flow to form surface vibration patterns as it advances.

(1) Increase temperature of barrel, especially nozzle, and mold temperature.
(2) Increase injection pressure and speed to quickly fill mold cavity.
(3) Improve size of flow channel and gate to prevent excessive resistance.
(4) Mold must be well vented and a large enough cold slug well must be installed.
(5) Do not design parts too thin.

Some plastic parts will soon develop swelling or bubbling on the back of metal insert or in particularly thick areas after being molded and demoulded. This is caused by expansion of gas released by plastic that is not completely cooled and hardened under action of internal pressure.

1. Effective cooling. Reduce mold temperature, extend mold opening time, reduce drying and processing temperature of material.
2. Reduce mold filling speed, shorten molding cycle, and reduce flow resistance.
3. Increase holding pressure and time.
4. Improve situation where wall surface of part is too thick or thickness changes greatly.

 

(1) Too much processing pressure, too fast speed, more filling, injection, too long pressure holding time will cause excessive internal stress and cracking.
(2) Adjust mold opening speed and pressure to prevent demolding and cracking caused by rapid forced drawing of parts.
(3) Increase mold temperature appropriately to make parts easy to demould, and lower material temperature appropriately to prevent decomposition.
(4) Prevent cracking due to lower mechanical strength due to welding marks and plastic degradation.
(5) Use release agents appropriately and pay attention to frequently removing aerosol and other substances attached to mold surface.
(6) Residual stress of part can be eliminated by annealing heat treatment immediately after molding to reduce generation of cracks.

(1) Ejection must be balanced. For example, number of ejector pins and cross-sectional area must be sufficient, draft slope must be sufficient, and cavity surface must be smooth enough to prevent cracking due to concentration of residual stress in ejection caused by external forces.
(2) Structure of parts should not be too thin, arc transitions should be used as much as possible in transition parts to avoid stress concentration caused by sharp corners and chamfers.
(3) Use as few metal inserts as possible to prevent internal stress from increasing due to different shrinkage rates between insert and finished product.
(4) Appropriate demoulding air inlets should be provided for deep-bottomed parts to prevent formation of vacuum negative pressure.
(5) Sprue is large enough to allow gate material to be demoulded before it has time to solidify, making it easy to demould.
(6) Connection between main flow bushing and nozzle should prevent cold hard material from being dragged and causing part to stick to fixed mold.

(1) Content of recycled materials is too high, resulting in low strength of parts.
(2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing their strength and causing ejection and cracking.
(3) Material itself is not suitable for environment in which it is being processed or is of poor quality. Contamination will cause cracking.
Machine aspect
Plasticizing capacity of injection molding machine must be appropriate. If it is too small, plasticization will be insufficient and it will not be fully mixed and it will become brittle. If it is too large, it will degrade.

Gas in bubble (vacuum bubble) is very thin and belongs to vacuum bubble. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. Vacuum bubbles are formed due to insufficient filling of plastic or low pressure. Under rapid cooling of mold, fuel in contact with cavity is pulled, resulting in volume loss.

(1) Increase injection energy: pressure, speed, time and material volume, and increase back pressure to make mold full.
(2) Increase material temperature and smooth flow. Lower material temperature to reduce shrinkage, and increase mold temperature appropriately, especially local mold temperature where vacuum bubble is formed.
(3) Set gate in thick part of part to improve flow conditions of nozzle, runner and gate, and reduce consumption of pressure.
(4) Improve mold exhaust condition.

Occurrence of deformation, bending, and twisting of injection molded products is mainly due to fact that shrinkage rate in flow direction is greater than that in vertical direction during plastic molding, causing parts to warp due to different shrinkage rates in each direction. Also because there is inevitably a large internal stress remaining inside part during injection molding, causing warping. These are all manifestations of deformation caused by high stress orientation. So fundamentally speaking, mold design determines warping tendency of parts. It is very difficult to suppress this tendency by changing molding conditions. Final solution to problem must start with mold design and improvement. This phenomenon is mainly caused by following aspects:

(1) Thickness and quality of parts should be uniform.
(2) Cooling system should be designed to make temperature of each part of mold cavity uniform. Pouring system should make material flow symmetrical to avoid warpage due to different flow directions and shrinkage rates. Appropriately thicken branch channels and main channels in difficult-to-form parts, and try to eliminate density differences, pressure differences, and temperature differences in mold cavity.
(3) Transition area and corners of thickness of part must be smooth enough and have good demoulding properties, such as increasing demoulding margin, improving polishing of mold surface, and ejection system must be balanced.
(4) Exhaust should be good.
(5) Increase wall thickness of part or increase anti-warping direction, and use reinforcing ribs to enhance anti-warping ability of part.
(6) Material used in mold has insufficient strength.

Crystalline plastics have more chances of warping than amorphous plastics. In addition, crystalline plastics can correct warping deformation by using crystallization process, which decreases as cooling rate increases and shrinkage rate decreases.

(1) Injection pressure is too high, holding time is too long, melt temperature is too low and speed is too fast, which will cause internal stress to increase and warpage deformation will occur.
(2) Mold temperature is too high and cooling time is too short, causing parts to be overheated during demolding and cause ejection deformation.
(3) Reduce screw speed and back pressure to reduce density while maintaining minimum charging amount to limit generation of internal stress.
(4) If necessary, parts that are prone to warping and deformation can be soft-shaped or de-moulded and then de-metered.

Occurrence of this defect is mainly a common problem in plastic parts colored with masterbatch. Although masterbatch coloring is superior to dry powder coloring and dye paste in terms of color stability, color purity, and color migration, distribution properties, that is, uniformity of mixing of color particles in diluted plastics, is relatively poor, and finished products naturally have regional color differences.

 

(1) Increase 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 color masterbatch can melt as soon as possible when entering melting section, promote uniform mixing with dilution, and increase opportunity for liquid mixing.
(2) When screw speed is constant, increasing back pressure increases melt temperature and shearing effect in barrel.
(3) Modify mold, especially pouring system. If gate is too wide, turbulence effect is poor when melt passes through, and temperature rise is not high, so it is uneven and ribbon mold cavity should be narrowed.

Melting spots, silver streaks, cracked polystyrene, transparent parts of plexiglass, sometimes through light you can see some sparkling filament-like silver streaks. These silver streaks are also called flash spots or cracks. This is due to stress generated in vertical direction of tensile stress. Polymer molecules undergo heavy flow orientation and folding rate difference between non-oriented parts is reflected.

(1) Eliminate interference of gas and other impurities and fully dry plastic.
(2) Reduce material temperature, adjust barrel temperature step by step, and increase mold temperature appropriately.
(3) Increase injection pressure and reduce injection speed.
(4) Increase or decrease pre-molding back pressure and reduce screw speed.
(5) Improve exhaust conditions of runner and cavity.
(6) Clean possible blockages in nozzle, runner and gate.
(7) Shorten molding cycle. After demoulding, annealing can be used to eliminate silver streaks: for polystyrene, hold at 78℃ for 15 minutes, or at 50℃ for 1 hour. For polycarbonate, heat to above 160℃ for several minutes. .

Main reasons and solutions for uneven color of injection molded products are as follows:
(1) Poor colorant diffusion, which often causes patterns to appear near gate.
(2) Plastics or colorants have poor thermal stability. To stabilize color tone of parts, production conditions must be strictly fixed, especially material temperature, material quantity and production cycle.
(3) For crystalline plastics, try to keep cooling rate of all parts of part consistent. For parts with large wall thickness differences, colorants can be used to mask color difference. For parts with more uniform wall thicknesses, material temperature and mold temperature must be fixed. .
(4) Shape and gate form and position of part have an impact on plastic filling situation, causing color differences in some parts of part, which must be modified if necessary.

Under normal circumstances, gloss of surface of injection molded parts is mainly determined by type of plastic, colorant and smoothness of mold surface. However, defects such as surface color and gloss defects, surface darkness, etc. are often caused by some other reasons. Reasons and solutions for this are analyzed as follows:
(1) Mold finish is poor, there are rust stains on the surface of cavity, etc., and mold exhaust is poor.
(2) Pouring system of mold is defective. Cold slug well should be enlarged, runner should be enlarged, main channel should be polished, runner and gate should be polished.
(3) Material temperature and mold temperature are low. If necessary, local heating of gate can be used.
(4) Processing pressure is too low, speed is too slow, injection time is insufficient, and back pressure is insufficient, resulting in poor compactness and a dark surface.
(5) Plastic must be fully plasticized, but degradation of material must be prevented, heating must be stable, and cooling must be sufficient, especially for thick walls.
(6) Prevent cold material from entering part. If necessary, use a self-locking spring or reduce nozzle temperature.
(7) Too much recycled material is used, plastic or colorant is of poor quality, mixed with water vapor or other impurities, and lubricant used is of poor quality.
(8) Clamping force must be sufficient.

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

 

(1) Barrel and screw are worn or there is a dead corner in material flow in rubber head and rubber ring, which will decompose due to long-term heating.
(2) Heating system is out of control, causing temperature to be too high and decomposing. Check whether there are any problems with heating elements such as thermocouples and heating coils. Improper screw design may cause loosening or easily introduce air.

(1) Poor exhaust.
(2) Friction resistance of runners, gates, and cavities in mold is large, causing local overheating and decomposition.
(3) Unbalanced distribution of gates and mold cavities and unreasonable cooling systems will cause unbalanced heating, resulting in local overheating or blocking of air channels.
(4) Water leaks from cooling passage and enters mold cavity.

(1) Plastic has high humidity, too much recycled material is added, or it contains harmful debris (debris is easily decomposed). Plastic should be fully dried and debris should be eliminated.
(2) To absorb moisture from atmosphere or from colorant, colorant should also be dried. It is best to install a dryer on machine.
(3) Amount of lubricants, stabilizers, etc. added to plastic is excessive or unevenly mixed, or plastic itself contains volatile solvents. Decomposition can also occur when mixed plastics are heated to a difficult balance.
(4) Plastic is contaminated and mixed with other plastics.