Deformation, bending and twisting in plastic injection occur mainly because that the shrinkage rate in flow direction is larger than that in vertical direction during plastic forming. And warpage is also inevitably caused by large internal stress remaining in the parts during plastic injection. These are all deformations caused by high stress. Fundamentally, the die design determines whether the product will warp or not. It is quite difficult to prevent warpage by changing the forming conditions. The final solution to the problem must start with the mold design and improvement. The warpage is mainly caused by the following reasons:
1. Mold
(1) The thickness and quality of the parts should be uniform.
(2) The well-designed cooling system shall uniform the temperature of all parts of the mold cavity, and the pouring system shall make the material flow symmetrical so as to avoid warping caused by different flow direction and unequal shrinkage. The main passages and branches which are difficult to form shall be appropriately thickened. The density difference, pressure difference and temperature difference in the mold cavity need to be eliminated as far as possible.
(3) The transition zone and corners of the parts should be smooth enough and have good demould properties, such as higher demould redundancy, improved die surface polishing and balanced ejection system.
(4) The exhaust system should be agreeable.
(5) The wall thickness or warpage resistance directions of the part should be increased, and the warpage resistance of the part can enhanced by stiffener.
(6) The strength of material used for the die is not enough.
2. Plastic
Crystalline plastics are more likely to warp than amorphous plastics. And crystalline plastics can correct warpage deformation by utilizing a crystallization process in which crystallinity and shrinkage decreases with increasing cooling rate.
3. Processing
(1) If the plastic injection pressure is too high, the pressure is maintained for too long, or the temperature is too low or too fast, it will lead to the increase of inner stress and warpage will occur.
(2) Mold temperature is too high and cooling time is too short, which makes the parts overheated during demoulding, causes ejection deformation.
(3) Lowering screw speed and reducing density through controlling back pressure to decrease internal stress while maintaining minimum charge.
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