How to solve the dent of injection molded parts?

Sometimes there is shrinkage and dent in the processed injection molded products, what is it? What are the causes of these situations?

 

1. Machine

The injection nozzle hole is too large to cause the melt material backflow and shrinkage, too small resistance to the large amount of material is not enough to appear shrinkage.

Insufficient clamping force will also cause shrinkage due to flying edge, the clamping system should be checked to see if there is a problem.

Insufficient plasticizing volume should be chosen from a machine with large plasticizing volume, check whether the screw and barrel are worn.

 

2. Mould

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

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

The pouring system should ensure the smooth flow and the resistance should not be too large, such as the main flow channel, diversion channel, gate size should be appropriate, the finish should be enough, and the transition area should be rounded transition.

The temperature should be increased for thin parts to ensure smooth material flow, and the mold temperature should be reduced for thick-walled parts.

The gate should be opened symmetrically, as much as possible in the thick-walled part of the part, and the volume of the cold material well should be increased.

 

3. Plastic

Crystalline plastic shrinks more than non-crystalline plastic, so it is necessary to increase the amount of material or add nucleating agent in the plastic to speed up the crystallization and reduce the shrinkage depression.

 

4. Processing

The barrel temperature is too high and the volume changes a lot, especially the temperature of the front furnace. The temperature should be increased appropriately for the plastics with poor fluidity to ensure smoothness.

Injection pressure, speed, back pressure is too low, injection time is too short, so that the amount of material or density is not enough and shrinkage pressure, speed, back pressure is too large, too long caused by flying edge and shrinkage.

The amount of material added that the cushion is too large when the injection pressure is consumed, too small, the amount of material is not enough.

For the parts that do not require precision, in the injection pressure is completed, the outer layer of the basic condensation hardening and the sandwich part is still soft and can be ejected parts, early out of the mold, let it cool slowly in the air or hot water, can make the shrinkage depression gentle and less conspicuous and does not affect the use.