“Dent” is caused by local internal shrinkage after gate sealing or lack of material injection. The depression or micro depression on the surface of injection molded parts is an old problem in the injection molding process.
Dents are generally caused by the local increase of shrinkage rate of plastic products due to the increase of wall thickness of plastic products. They may appear near external sharp corners or at sudden changes of wall thickness, such as the back of bulges, stiffeners or bearings, and sometimes at some uncommon parts. The root cause of dents is the thermal expansion and cold contraction of materials, because the thermal expansion coefficient of thermoplastics is quite high.
The extent of expansion and contraction depends on many factors, among which the performance of plastics, the maximum and minimum temperature ranges and the pressure maintaining pressure of the mold cavity are the most important factors. The size and shape of plastic parts, as well as the cooling speed and uniformity are also influencing factors.
The amount of expansion and contraction of plastic materials in the molding process is related to the thermal expansion coefficient of the processed plastic. The thermal expansion coefficient in the molding process is called “molding shrinkage”. With the cooling shrinkage of the molded part, the molded part loses close contact with the cooling surface of the mold cavity. At this time, the cooling efficiency decreases. After the molded part continues to cool, the molded part continues to shrink. The amount of shrinkage depends on the combined effect of various factors.
Sharp corners on the molded part cool the fastest and harden earlier than other parts. The thick part near the center of the molded part is farthest from the cooling surface of the cavity and becomes the last part of the molded part to release heat. After the material at the corners is cured, the molded part will continue to shrink as the melt near the center of the part cools. The plane between the sharp corners can only be cooled unilaterally, 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 part pulls the relatively weak surface between the partially cooled and the sharp corner with greater cooling degree inward. In this way, a dent is generated on the surface of the injection molded part.
The existence of dents indicates that the molding shrinkage here is higher than the shrinkage of its surrounding parts. If the shrinkage of the molded part at one place is higher than that at another place, then the reason for the warpage of the molded part. The residual stress in the mold will reduce the impact strength and temperature resistance of the molded parts.
In some cases, the dent can be avoided by adjusting the process conditions. For example, during the pressure maintaining process of the molded part, additional plastic material is injected into the mold cavity to compensate for the molding shrinkage. In most cases, the gate is much thinner than other parts of the part. When the molded part is still very hot and continues to shrink, the small gate has been cured. After curing, the pressure retaining has no effect on the molded part in the cavity.
Post time: Nov-15-2022