Plastic injection molding is a common phenomenon in the process of injection molding. Shrinkage and depression are also encountered during the production of injection molding. So what causes shrinkage and depression?
In fact, the teacher's more than ten years of injection molding experience has its own solution when facing this problem. We also analyzed the factors that caused this situation.
During the injection molding process, the products shrink and sag. The main reasons are as follows:
1. Machine aspect
(1) If the nozzle hole is too large, the molten material will flow back and shrink, and if it is too small, the resistance will be insufficient and the material will not shrink.
(2) The flashing will also shrink due to insufficient clamping force. Check the clamping system for any problems.
(3) If the amount of plasticization is insufficient, a machine with a large amount of plasticization should be selected to check whether the screw and barrel are worn.
There is also no sloppyness in the configuration and maintenance inspection of machine equipment, CNC machining centers, Jingdiao, Sodick spark machines and other precision mold production equipment, as well as various types of injection molding machines, from design mold opening to injection molding, Every link is strictly controlled, the purpose is to provide a more professional and better service.
2. Mold
(1) The design of the parts should make the wall thickness uniform to ensure uniform shrinkage.
(2) The cooling and heating system of the mold must ensure that the temperature of each part is the same.
(3) The pouring system must be unobstructed, and the resistance should not be too large, such as the size of the main channel, the diverter channel, and the gate should be appropriate, the smoothness should be sufficient, and the transition area should be rounded.
(4) The temperature should be increased for thin parts to ensure smooth material flow, and the mold temperature should be reduced for thick-walled parts.
(5) The gate should be opened symmetrically, try to open it in the thick wall part of the part, and the volume of the cold material well should be increased.
3. Plastics
Crystalline plastics suffer from shrinkage compared to non-crystalline plastics. During 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 depressions.
4. Processing
(1) The temperature of the barrel is too high, and the volume change is large, especially the temperature of the front furnace. The temperature of the plastic with poor fluidity 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 amount of material or density is insufficient and the contraction pressure, speed, back pressure is too large, and the time is too long, which causes the flash to shrink.
(3) The feeding volume is the injection pressure when the cushion is too large. If the cushion volume is too small, the feeding volume 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 sandwich is still soft and can be ejected. Exit the mold early and let it cool slowly in air or hot water. , Can make the shrinkage depression smooth and not so conspicuous without affecting the use.
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