The amount of expansion and contraction in the molding process of a plastic material is related to the coefficient of thermal expansion of the plastic being processed. The coefficient of thermal expansion of the molding process is called 'mold shrinkage'.
As the molded part cools and shrinks, the molded part loses close contact with the cooling surface of the mold cavity. In this case, the cooling efficiency decreases. After the molded part continues to cool, the molded part shrinks continuously, and the amount of shrinkage depends on various factors. The sharp corners on the part cool the fastest and harden earlier than other parts. The thick part near the center of the molding separates the cooling surface of the cavity farthest, and becomes the last heat release part of the molded part. The material at the corner After curing, as the melt near the center of the part cools, the molded part will continue to shrink. The plane between the sharp corners can only be cooled on one side, and the strength of the material is not high at the sharp corners.
The cooling contraction of the plastic material at the center of the part pulls the relatively weaker surface between the partially cooled and the sharper corners inwards. This creates a dent on the surface of the injection molded part. The presence of dents is explained here Mold shrinkage is higher than the shrinkage of its surrounding parts.
If the shrinkage of the molded part is higher than that of the other part, the molded part will cause warpage. The residual stress in the mold will reduce the impact strength and temperature resistance of the molded part.
In some cases, process conditions can be adjusted to avoid dents. For example, during the pressure holding process of the molded part, additional plastic material is injected into the cavity to compensate for mold shrinkage. In most cases, the gate area is greater than the part size. The other parts are much thinner. When the molded part is still hot and shrinking continuously, the small gate has been cured. After curing, the holding pressure will not work on the molded part in the cavity.
Possible causes of the problem:
● insufficient plastic in the cavity
● The melting temperature is not too high or too low
● Unreasonable flow path, too small gate cross section
● whether the mold temperature adapts to the plastic characteristics
● The surface that touches the plastic during the cooling phase is overheated
● The cooling effect is not good, the product continues to shrink after demoulding
● The product structure is irrational (enhanced into the ancient over-high, too thick, obviously thick and thin)
Remedy:
● increase the injection volume
● Adjust the temperature of the shot cylinder
● reduce the mold surface temperature
● Try to make the product have enough cooling
● Improve product structure if allowed
●Adjust the screw speed to get the correct screw surface speed
●According to the characteristics of the plastic used and the structure of the product, the mold temperature should be properly controlled
● Check that the stop valve is installed correctly, because abnormal operation will lead to pressure loss
● Correct the flow path to avoid excessive pressure loss; properly expand the section size according to actual needs
● Ensure correct use of dunnage; increase the screw forward time; increase injection pressure; increase injection speed