The amount of expansion and contraction of plastic materials in the molding process is related to the thermal expansion coefficient of the processed plastic, which is called "molding shrinkage".
With the cooling shrinkage of the molding part, the mold part loses close contact with the cooling surface of the mold cavity. At this time, the cooling efficiency decreases. After the mold part continues to cool, the mold part shrinks continuously. The amount of shrinkage depends on the comprehensive action of various factors. The sharp corner on the mold part cools fastest, hardens earlier than other parts, and the thick part near the center of the mold part is farthest from the cooling surface of the mold cavity, becoming the mold part In the last part of the heat release, after the material at the corner solidifies, as the melt near the center of the part cools, the molded part will continue to contract. The plane between the sharp corners can only be cooled at one side, and its strength is not as high as that of the material at the sharp corners.
The cooling shrinkage of the plastic material at the center of the work piece pulls the relatively weak surface between the partially cooled and the sharp corner with larger cooling degree inward. This creates a dent in the surface of the injection part. The existence of dents indicates that the shrinkage rate of the molding is higher than that of the surrounding parts. If the shrinkage of the molded part is higher in one place than in another, the cause of warpage of the molded part is. The residual stress in the mold will reduce the impact strength and temperature resistance of the molded parts. In some cases, the formation of dents can be avoided by adjusting the process conditions. For example, during the pressure maintaining process of the molding part, an additional plastic material is injected into the mold cavity to compensate for the molding shrinkage. In most cases, the gate is much thinner than the other parts of the part. When the molded part is still very hot and continues to contract, the small gate has been solidified. After curing, the pressure retaining has no effect on the molded part in the cavity.
Possible causes of problems:
(1) . insufficient plastic in cavity.
(2) . the melting temperature is either too high or too low.
(3) . the runner is unreasonable and the gate section is too small.
(4) . whether the mold temperature is compatible with the plastic characteristics.
(5) . the surface contacting the plastic is overheated during the cooling stage.
(6) . the cooling effect is not good, and the product continues to contract after demoulding.
(7) . the product structure is unreasonable (too high, too thick, obviously different in thickness)
Remedy:
(1) . increase injection volume.
(2) . adjust the temperature of the injection cylinder.
(3) . reduce mold surface temperature.
(4) . try to keep the product cool enough.
(5) . improve the product structure if allowed.
(6) . adjust the screw speed to obtain the correct screw surface speed.
(7) . the mold temperature shall be properly controlled according to the characteristics of the plastic used and the product structure.
(8) . check if the check valve is installed correctly, because abnormal operation will cause pressure loss.
(9) . correct the flow passage to avoid excessive pressure loss; properly expand the section size according to the actual needs.
(10) . ensure to use correct padding; increase screw forward time; increase injection pressure; increase injection speed.
