shrinkage of injection molded parts

Dents are caused by local internal shrinkage after gate sealing or lack of material injection. The depression or micro depression on the surface of injection molded products is an old problem in the process of injection molding.

The dent is generally caused by the local increase of product shrinkage caused by the increase of plastic product wall thickness. It may appear near the external sharp corner or at the sudden change of wall thickness, such as the back of protrusion, stiffener or support, and sometimes in some uncommon parts. The root cause of dents is the thermal expansion and cold contraction of materials, because the coefficient of thermal expansion of thermoplastics is quite high.

The degree of expansion and contraction depends on many factors, among which the performance of plastic, the maximum and minimum temperature range and the pressure holding pressure in the mold cavity are the most important factors. The size and shape of injection molded parts, as well as 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 shrinks continuously, and the shrinkage amount depends on the comprehensive action of various factors.

The sharp corner on the molded part cools fastest and hardens 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 to release heat on the molded part. After the material at the corner solidifies, the molded part will continue to shrink with the melt cooling near the center of the molded part, and the plane between the sharp corners can only be cooled on one side, Its strength is not as high as that of materials at 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 corners with a large degree of cooling inward. In this way, dents are generated on the surface of the injection molded part. The existence of dents indicates that the molding shrinkage here is higher than that of its surrounding parts. If the shrinkage of the molded part in one place is higher than that in another, the cause of 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, adjusting process conditions can avoid dents. For example, during the pressure holding 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 maintaining has no effect on the molded part in the mold cavity.

The shrinkage of semi crystalline plastic molded parts is high, which makes the dent problem more serious; The molding shrinkage of amorphous materials is low, which will minimize the dent; Filling and maintaining reinforced materials have lower shrinkage and less possibility of dents.

Thick injection molded parts have long cooling time and will produce large shrinkage. Therefore, large thickness is the root cause of dents. Attention should be paid in design to avoid thick wall parts as much as possible. If the thick wall cannot be avoided, it should be designed as hollow. The thick parts will smoothly transition to the nominal wall thickness and use large circular arc instead of sharp corners, Dents near sharp corners can be eliminated or minimized.