As the molding is cooled and contracted, the molding is in close contact with the cooling surface of the mold cavity, and the cooling efficiency is lowered. After the molding is continued, the molding is continuously contracted and the amount of shrinkage depends on the combined effect of various factors The sharp corners of the pieces are cooled faster and harder than the other parts, and the thick portion near the center of the molding is farthest from the cooling surface of the cavity and becomes the portion of the final release of heat on the molding, After curing, with the melt near the center of the melt, the molding will continue to shrink, the corners between the corners can only be unilateral cooling, the strength of the material at high corners without high strength.
The shrinkage of the plastic material at the center of the part, the partial cooling of the surface with a relatively large degree of cooling between the relatively weak surface inward pull. This creates a dent on the surface of the injection molded part. The presence of the indentation of the plastic injection product indicates that the molding shrinkage is higher than the contraction of its peripheral parts.
If the shrinkage of the molded article in one place is higher than the other, the molding member causes warpage. Plastic injection molding products within the mold residual stress will reduce the impact of molded parts and temperature performance.
In some cases, adjusting the process conditions can prevent the generation of dents. For example, during the packing of the molding, an additional plastic material is injected into the mold cavity to compensate for molding shrinkage. In most cases, the gate is much thinner than the rest of the part, and when the molding is still hot and continues to shrink, the small gate is solidified and cured, kick in.


Possible causes of the problem:
(1). Mold cavity plastic shortage.
(2). The melting temperature is not too high or too low.
(3) The surface of the plastic that touches the plastic during the cooling phase is overheated.
(4) Whether the mold temperature is compatible with the plastic properties.
(5). The runner is unreasonable and the gate section is too small.
(6). Cooling effect is not good, the product continues to shrink after stripping.
(7). Product structure is unreasonable (to strengthen into the ancient too high, too thick, obviously different thickness)


Remedy:
(1) Increase the amount of injection.
(2) Adjust the temperature of the feed cylinder.
(3) Adjust the screw speed to obtain the correct screw surface speed.
(4) Try to get the product cool enough.
(5). Under the circumstances permit to improve the product structure.
(6) Reduce the mold surface temperature.
(7). According to the characteristics of plastic used and product structure appropriate control mold temperature.
(8). Correct the flow channel to avoid excessive pressure loss; according to actual needs, the appropriate expansion of cross-sectional size.
(9). Check that the throttle valve is installed correctly because abnormal operation can cause pressure loss.
(10). Make sure to use the correct litter; increase the screw forward time; increase the injection pressure; increase the injection speed.