The Method To Solve The Dent On The Side Wall Of Injection Molded Parts

The dent is generally caused by the local increase in the shrinkage rate of the plastic product caused by the increase in the wall thickness of the plastic product. It may appear near the outer sharp corners or at the sudden change of wall thickness, such as protrusions, ribs or behind the support, and sometimes also appears. in some unusual locations. The root cause of dents is the thermal expansion and contraction of the material, since thermoplastics have a fairly high coefficient of thermal expansion.

The degree of expansion and contraction depends on many factors, of which the properties of the plastic, the maximum and minimum temperature ranges, and the cavity holding pressure are the most important factors. There are also the size and shape of the injection molded parts, as well as the cooling rate and uniformity.

The amount of expansion and contraction during the molding process of plastic materials is related to the thermal expansion coefficient of the plastic being processed, and the thermal expansion coefficient during the molding process is called "molding shrinkage". As the molded part cools and shrinks, the molded part loses close contact with the cooling surface of the 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.

The 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 the farthest from the cooling surface of the cavity and becomes the last part of the molded part to release heat. After the solidified material is solidified, as the melt near the center of the part cools, the molded part will continue to shrink, and the plane between the sharp corners can only be cooled on one side, and its strength is not as high as that of the material at the sharp corners.

Cooling shrinkage of the plastic material at the center of the part pulls inward the relatively weak surface between the partially cooled and the more cooled sharp corners. In this way, dents are created on the surface of the injection molded part.

The presence of dents indicates that the molding shrinkage here is higher than the shrinkage of the surrounding areas. If the molded part shrinks more in one place than the other, that is the cause of the molded part warping. In-mold residual stress reduces the impact strength and temperature resistance of the molded part.

In some cases, adjusting process conditions can avoid sink marks. For example, during the holding process of the molded part, the mold cavity is additionally injected with plastic material to compensate for molding shrinkage. In most cases, the gate is much thinner than the rest of the part. While the molded part is still hot and continues to shrink, the small gate has cured. After curing, the holding pressure has no effect on the molded part in the cavity. kick in.

Semi-crystalline plastic materials have high molding shrinkage, which exacerbates the dent problem; non-crystalline materials have lower molding shrinkage, which minimizes dents; fill and maintain reinforcement materials, which have shrinkage rates lower and less likely to dent.