Why sometimes the processed mold will shrink and sag

Update:11-05-2019
Summary:

Some injection mold workers who have been engaged in ma […]

Some injection mold workers who have been engaged in many years will find that sometimes the processed mold will shrink and sag. What is the matter? What are the causes of these situations?

1. Machine side:

1. The nozzle hole is too large to cause the melt to recirculate and shrink. When the pressure is too small, the resistance is insufficient and the shrinkage occurs.

2. If the clamping force is insufficient, the flash will also shrink. Check the clamping system for any problems.

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

2. Mold aspect:

1. The design of the part should make the wall thickness uniform and ensure the uniform 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 be smooth and the resistance should not be too large. For example, the size of the main channel, the runner and the gate should be appropriate, the finish should be sufficient, and the transition zone should have a circular transition.

4. For thin parts, the temperature should be raised to ensure smooth flow, and the mold temperature should be reduced for thick-walled parts.

5. The gate should be opened symmetrically, as far as possible in the thick part of the part, and the volume of the cold well should be increased.

3. Plastics:

Crystalline plastics shrink than conventional non-crystalline plastics. When processing, the amount of material should be increased appropriately, or a replacement agent should be added to the plastic to accelerate crystallization and reduce shrinkage and depression.

4. Processing:

1. 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 properly increased to ensure smoothness.

2. The injection pressure, speed, back pressure is too low, and the injection time is too short, so that the amount or density is insufficient, and the contraction pressure, speed, back pressure is too large, and the time is too long, resulting in shrinkage of the flash.

3. The feeding amount means that the injection pressure is consumed when the cushion is too large, and when the amount is too small, the amount of the material is insufficient.

4. For parts that do not require precision, after the injection pressure is maintained, the outer layer is basically condensed and hardened, and the part with the sandwich part is soft and can be ejected, and the mold is released early, so that it is slowly cooled in air or hot water. The shrinkage depression can be made gentle and less conspicuous without affecting use.

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