Causes of shrinkage and depression in the molding process of Plastic Injection Mold
1. In terms of machines
(1) If the nozzle hole is too large, the melt will flow back and shrink, and when the resistance is too small, the amount of material will be insufficient to shrink.
(2) If the clamping force is insufficient, the flash will also shrink. Check whether there is any problem with the clamping system.
(3) If the amount of plasticization is insufficient, a machine with a large amount of plasticization should be used to check whether the screw and barrel are worn.
2. Mould
(1) The design of the part should make the wall thickness uniform and ensure uniform shrinkage.
(2) The cooling and heating system of the mold must ensure that the temperature of each part is consistent.
(3) The gating system must be unobstructed and the resistance must not be too large. For example, the size of the main runner, runner and gate must be appropriate, the smoothness must be sufficient, and the transition zone must be arc-transitioned.
(4) The temperature should be increased for thin parts to ensure smooth material flow, and the mold temperature should be lowered for thick-walled parts.
(5) The gate should be opened symmetrically, and should be opened in the thick wall part of the product as much as possible, and the volume of the cold slug well should be increased.
3. Plastic
Crystalline plastics are more harmful than non-crystalline plastics, so the amount of material should be appropriately increased during processing, or a replacement agent should be added to the plastic to speed up crystallization and reduce shrinkage.
4. Processing
(1) The temperature of the barrel is too high and the volume changes greatly, especially the forehearth temperature. For plastics with poor fluidity, the temperature should be appropriately increased to ensure smoothness.
(2) The injection pressure, speed, back pressure is too low, and the injection time is too short, so that the material volume or density is insufficient and the contraction pressure, the speed, the back pressure is too large, and the time is too long to cause flashing and contraction.
(3) The feeding amount means that the injection pressure is consumed when the cushion is too large, and the amount is insufficient when it is too small.
(4) For parts that do not require precision, after the injection and holding pressure is completed, the outer layer is basically condensed and hardened, and the sandwich part is still soft and can be ejected, and the parts are ejected early and allowed to cool slowly in air or hot water. , Can make the contraction depression gentle and not so conspicuous without affecting the use.
