Solution to Side Wall Dents in SMC Plastic Parts

　　Dents "are caused by local internal shrinkage or lack of material injection after the gate is sealed. Dents or micro dents on the surface of SMC products are an old problem faced by SMC composite material manufacturers during the injection molding process. Dents are usually caused by an increase in wall thickness of plastic products. It may appear near external sharp corners or sudden changes in wall thickness, such as protrusions, ribs, or supports, and sometimes in some uncommon parts. The fundamental reason is the thermal expansion and contraction of the material, as the coefficient of thermal expansion of SMC composite materials is very high.

　　The expansion and shrinkage during the molding process of manufacturing SMC composite materials are related to the thermal expansion coefficient of the processed plastic. The coefficient of thermal expansion during the molding process is called the "molding shrinkage rate". As the molded parts cool and contract, they lose close contact with the cooling surface of the cavity. At this point, the cooling efficiency decreases.

　　After the SMC composite material continues to cool down the molded parts, the molded parts continue to shrink. The amount of shrinkage depends on the combined effect of various factors. The sharp corners on formed parts cool faster and harden earlier than other parts. The thicker part near the center of the molded part separates the cooling surface of the mold cavity further, becoming the part of the molded part that releases heat at the corners.

　　If the shrinkage rate of the formed component is higher than that of another component, then the formed component is the cause of warping. The residual stress in the mold can reduce the impact strength and heat resistance of the formed part. In some cases, SMC composite material manufacturers can avoid dents by adjusting process conditions. For example, during the process of maintaining pressure on molded parts, additional plastic material is injected into the mold cavity to compensate for mold shrinkage. In most cases, the gate is much thinner than the other parts of the part. When the molded part is still hot and continuously shrinking, the small gate has solidified.