Analysis of the causes of shrinkage and depression in injection molded products

During the injection molding process, shrinkage and indentation of products are common phenomena. The main reasons for this situation are:

1. Regarding the machine:

(1) The nozzle hole is too large, causing the molten material to flow back and shrink. When it is too small, the resistance is high and the material volume is insufficient, resulting in shrinkage.

(2) Insufficient locking force can cause shrinkage of the flash, and the locking system should be checked for any issues.

(3) If the plasticizing amount is insufficient, a machine with a large plasticizing amount should be selected to check whether the screw and the barrel are worn.

2. In terms of molds:

(1) The design of the parts should ensure uniform wall thickness and consistent shrinkage.

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

(3) The pouring system should ensure unobstructed flow, and the resistance should not be too high. For example, the size of the main flow channel, diversion channel, and gate should be appropriate, and the smoothness should be sufficient. The transition zone should have a curved transition.

(4) For thin parts, the temperature should be raised to ensure smooth material flow, while for thick walled parts, the mold temperature should be reduced.

(5) The gate should be symmetrically opened, preferably at the thick wall part of the workpiece, and the volume of the cold material well should be increased

3. Regarding plastics:

Crystalline plastics are more prone to shrinkage than non crystalline plastics. During processing, it is necessary to increase the amount of material appropriately or add a changing agent to the plastic to accelerate crystallization and reduce shrinkage depression.

4. In terms of processing:

(1) If the temperature of the material barrel is too high and the volume changes greatly, especially the temperature of the front furnace, the temperature of plastics with poor fluidity should be appropriately increased to ensure smoothness.

(2) Injection pressure, speed, back pressure too low, injection time too short, resulting in insufficient material quantity or density and shrinkage pressure, speed, back pressure too high, time too long causing flash and shrinkage.

(3) When the buffer pad is too large, the injection pressure is consumed. When it is too small, the material amount is insufficient.

(4) For parts that do not require precision, after the injection molding is completed and the outer layer is basically solidified and hardened, while the sandwich part is still soft and can be pushed out, early release of the mold and slow cooling in air or hot water can make the shrinkage depression smooth and less noticeable without affecting use.