How to fix dents in injection molded parts

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, so you should check whether there is any problem with the clamping system.

If the amount of plasticizing is insufficient, a machine with a large amount of plasticizing should be used to check whether the screw and barrel are worn.

2. Mold aspect

The design of the parts should make the wall thickness uniform and ensure consistent shrinkage.

The cooling and heating system of the mold must ensure that the temperature of each part is consistent.

The gating 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 mold 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.

3. Plastics

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 accelerate crystallization and reduce shrinkage dents.

4. Processing

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 and shrinkage. The pressure, speed, and back pressure are too high, and the time is too long, causing flash and shrinkage. If the amount of material added is too large, the injection pressure will be consumed; if it 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. It can be removed from the mold as soon as possible and allowed to cool slowly in air or hot water. The shrinkage dent is gentle and not so conspicuous and does not affect use.