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

2008/8/9 10:29:00 41757

The "dent" is caused by local shrinkage of the gate after sealing or injection.

Sunken or trapping on the surface of injection molded products is an old problem in the process of injection molding.

The dent is usually caused by the increase of shrinkage rate of plastic products caused by the increase of wall thickness. It may occur near the sharp corners or wall thickness changes, such as bulges, stiffeners or supports behind, sometimes also appear in some unusual parts.

The fundamental reason for dents is the thermal expansion and shrinkage of materials, because the thermal expansion coefficient of thermoplastics is quite high.

The degree of expansion and shrinkage depends on many factors, including the performance of plastics, the maximum and minimum temperature range, and the mold cavity holding pressure is the most important factor.

The size and shape of injection molded parts, as well as the speed and uniformity of cooling are also the influencing factors.

The expansion and shrinkage of plastic materials are related to the thermal expansion coefficient of the plastic. The thermal expansion coefficient of the molding process is called "molding shrinkage".

With the cooling shrinkage of moulded parts, the moulded parts lose contact with the cooling surface of the mold cavity. At this point, the cooling efficiency decreases, and the moulded parts continue to cool, and the molding parts shrink continuously. The shrinkage depends on the combined action of various factors.

The sharp corners of molded parts are the fastest cooling, hardening earlier than other components, approaching the farthest part of the center part of the mold, the farthest part of the cooling surface of the mold cavity, and the final heat release part of the mold part. When the material at the corner is solidified, the mold will continue to shrink as the melt near the center of the workpiece is cooled, and the plane between the sharp corners can only be cooled unilaterally, and the strength of the material is not high without sharp corners.

The cooling and shrinkage of plastic materials at the center of the workpiece will pull the part of the cooling surface relatively weakly with the cooling angle.

Thus, dents are formed on the surface of the injection molded parts.

The existence of dents indicates that the shrinkage rate is higher than the shrinkage at its periphery.

If the molding part shrinks at one place higher than the other, then the mold will cause warpage.

Residual stress in mold decreases the impact strength and temperature resistance of molded parts.

In some cases, adjusting the technological conditions can avoid the occurrence of dents.

For example, in molding process, plastic material is injected into mold cavity to compensate molding shrinkage.

In most cases, the gate is much thinner than other parts of the workpiece. When the molding piece is still hot and continues to shrink, the small gate has been solidified. After curing, the pressure retaining will not work on the mold cavity.

The shrinkage of moulded parts of semi crystalline plastic materials is high, which makes the dent problem more serious. The molding shrinkage of noncrystalline materials is lower, which will reduce the dent to the greatest extent. Filling and maintaining the reinforced materials have lower shrinkage rate and less possibility of dents.

The injection molding parts with thick wall thickness have a longer cooling time, which will cause larger shrinkage. Therefore, thickness is the fundamental reason for the occurrence of dents. When designing, attention should be paid to avoiding thick wall components. If we can not avoid thick walls, we should design hollow, thick parts to smooth over the wall thickness, and replace the sharp corners with large circular arcs, which can eliminate or minimize the dents produced near the sharp corners.