Dents are caused by local internal shrinkage after the gate is sealed or due to insufficient injection.
Depression or micro-depression on the surface of injection molded products is an old problem in the injection molding process.
Dents are generally caused by the increase in the wall thickness of the plastic product, which causes a local increase in the shrinkage rate of the product.
It may appear near external corners or at sudden changes in wall thickness, such as bulges, ribs or behind supports, and sometimes in some uncommon parts.
The root cause of dents is the thermal expansion and contraction of the material, because the thermal expansion coefficient of thermoplastics is quite high.
The degree of expansion and contraction depends on many factors, among which the properties of the plastic, the maximum and minimum temperature ranges and the cavity holding pressure are the most important factors.
The size and shape of the injection molded part, as well as the cooling rate and uniformity, are also influencing factors.
The amount of expansion and contraction of plastic materials during molding is related to the thermal expansion coefficient of the processed plastic. The thermal expansion coefficient of 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 mold cavity. At this time, the cooling efficiency decreases.
After the molded part continues to cool, the molded part continues to shrink. The 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 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 solidifies, as the melt near the center of the part cools, the molded part will continue to shrink.
The plane between the sharp corners can only be cooled on one side, and its strength is not as high as the strength of the material at the sharp corners.
The cooling and 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 greater degree of cooling inward. In this way, a dent is produced on the surface of the injection molded part.
The existence of the dent indicates that the molding shrinkage here is higher than the shrinkage of its surrounding parts. If the shrinkage of a molded part is greater in one place than in another, then the molded part will warp.
Residual stress in the mold will reduce the impact strength and temperature resistance of the molded part. In some cases, adjusting the process conditions can avoid the formation of sink marks.
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 the rest of the part.
When the molded part is still hot and continues to shrink, the small gate has already solidified. After solidification, the pressure holding has no effect on the molded part in the cavity.
Semi-crystalline plastic materials have high shrinkage rates for molded parts, which makes the dent problem more serious; non-crystalline materials have lower molding shrinkage, which will minimize dents;
filled and maintained reinforced materials have lower shrinkage rates and are less likely to produce dents.
Thick injection molded parts have a long cooling time and will shrink more, so the thickness is the root cause of dents. Attention should be paid to this during design. Thick wall parts should be avoided as much as possible.
If thick walls cannot be avoided, they should be designed to be hollow. Thick parts should be smoothly transitioned to the nominal wall thickness. Using large arcs instead of sharp corners can eliminate or minimize dents near sharp corners.
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