Sometimes the processed injection molded products will shrink and dent. What is going on? What are the causes of these situations?
1. Machine Aspects
- If the nozzle hole is too large, the melt will flow back and cause shrinkage; if it is too small, the resistance will be large and the amount of material will be insufficient, causing shrinkage.
- Insufficient clamping force will cause the flash to shrink. You should check whether there is any problem with the clamping system.
- If the plasticizing amount is insufficient, you should choose a machine with a large plasticizing amount and check whether the screw and barrel are worn.
2. Mould Aspect
- The design of the parts should make the wall thickness uniform and ensure consistent shrinkage.
- The cooling and heating system of the mould must ensure that the temperature of each part is consistent.
- The pouring system must be smooth and the resistance must not be too large. For example, the size of the main channel, runner, and gate must be appropriate, the smoothness must be sufficient, and the transition area must be an arc transition.
- For thin parts, the temperature should be increased to ensure smooth material flow, and for thick-walled parts, the mould temperature should be lowered.
- The gate should be opened symmetrically, try to open it in the thick-walled part of the part, and the cold slug well volume should be increased.
Crystalline plastics shrink more than amorphous plastics.
During processing, the amount of material should be appropriately increased, or a nucleating agent should be added to the plastic to speed up crystallization and reduce shrinkage dents.
- The temperature of the barrel is too high and the volume changes greatly, especially the temperature of the front furnace. For plastics with poor fluidity, the temperature should be increased appropriately to ensure smoothness.
- The injection pressure, speed, and back pressure are too low, and the injection time is too short, resulting in insufficient material volume or density, resulting in shrinkage pressure, speed, and back pressure that are too high, and time that is too long, causing flash and shrinkage.
- If the amount of material added is too large, the injection pressure will be consumed. If the amount of material added is too small, the amount of material will be insufficient.
- For parts that do not require precision, after the injection and pressure-holding is completed, the outer layer is basically condensed and hardened but the sandwich part is still soft and can be ejected.
Remove the mold as soon as possible and let it cool slowly in air or hot water. It can make shrinkage dents gentle and less conspicuous without affecting use.
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