The Analysis of Causes for Dents on Plastic Injection Molding Products

With shrinkage of the plastic injection molding part, the plastic injection molding part are not in close contact with the cooling surface of the mould cavity, resulting in poor cooling efficiency; the plastic injection molding part continues to cool, and the plastic injection molding part shrink gradually. The shrinkage depends on the combined action of various factors. The sharp corners of the plastic injection molding part have the fastest cooling, which harden earlier than the other parts. The thick part that is close to the center of the plastic injection molding part is the farthest part from the cooling surface of the cavity, which becomes the last part of the plastic injection molding part to release heat. With the cooling of the molten plastic near to the center of the plastic injection molding part, the plastic injection molding part will continue to shrink after the material in the corner solidifies. The plane between the sharp corners can only be cooled on one side. Therefore, strength of the material in the corner is lower than that of in the sharp corner.

The relatively weak surfaces of the sharp corners which are partially or fully cooled will be pulled due to cooling and shrinkage of plastic materials in the center of the plastic injection molding part. That's why dents on surfaces of plastic injection molding parts come into being. The existence of the dent on the plastic injection molding product illustrates that the shrinkage of the sharp corner is higher than that of the other parts.

If the shrinkage in one side of the plastic injection molding part is higher than that of the other side, the plastic injection molding part will warp. The residual stress inside the plastic injection molds will make impact strength and heat resistance of the plastic injection molding part poor.

The Possible Causes of These Problems
(1). The plastics in the cavity are insufficient.
(2). The melting temperature is too high or too low.
(3). The surfaces that are in contact with plastics are overheated during the cooling period.
(4). The mold temperature can't adapt to the characters of plastics. 
(5). The runners are unreasonable and the sections of the pouring gates are too small.
(6). The cooling effect is poor and the plastic injection molding products will continue to shrink after demolding.
(7). The plastic injection molding product's structure is unreasonable.

Measures
(1). Increase injection molding quantity.
(2). Adjust the temperature of injection cylinders.
(3). Adjust the screw speed to obtain the right surface speed of screw.
(4). Manage to have products cooled sufficiently.
(5). Improve structures of plastic injection molding products if permitted.
(6). Lower the temperature of the mold's surface.
(7). Appropriately control the mold temperature according to the characters of plastics as well as the plastic injection molding product's structure.
(8). Adjust the runners to avoid excessive pressure loss and properly enlarge the dimension of the section according to practical requirements.
(9). Check whether the stop valves are correctly installed or not, because the abnormal operation will lead to pressure loss.
(10). Increase the operating time of screws and increase the injection molding pressure and speed.