Why is there a stress mark on injection molded parts?


When molding an Injection Mold, there is a problem that often plagues the producer, that is, the injection molded part will have the ejector stress mark.

What is the reason for the ejector stress mark of the injection molded parts, how to eliminate it?
The main performance of the ejector stress mark plastic parts is that there is a relatively obvious whitening phenomenon on the surface of the plastic parts, that is, frosty micro cracks appear.

Causes of defects and elimination
The ejector stress mark , also called the top white. The reason for the appearance of whitening is mainly due to the excessive force applied at the top white area. The chemical resistance and mechanical properties of the plastic parts appear to be greatly weakened.
The specific analysis is as follows.
1)The plastic injection mold
①If the surface of the cavity is rough, the mold release resistance is increased, resulting in whitening. For this, the inner surface roughness of the cavity should be reduced.
②If the top of the ejector pin is in the weak part of the plastic part, the whitening phenomenon will be aggravated. For this, the ejector device should be placed at the wall thickness.
③If the draft angle of the mold is too small, the mold release resistance of the plastic part is increased, and the force of the plastic part is increased at the time of ejection, and the whitening phenomenon is likely to occur. Therefore, the draft angle should be appropriately increased.
④In order to increase the strength of the ejector portion, local strengthening can be performed here, and reinforcement can be performed from the structure and the wall thickness.
⑤If the contact surface of the ejector pin and the plastic part is too small, the local stress of the plastic part is large, so that the white color is generated. For this, the cross-sectional area of the end of the push rod should be appropriately increased.
⑥For plastic parts with core, demoulding is difficult, and the ejector stress mark is easy to occur. In this case, it is a good method to release the vacuum between the core and the plastic part, such as installing a valve in the core to release the vacuum.

2)Injection molding process
①If the temperature difference between the melt and the mold is too large, the residual stress in the plastic part is increased, and whitening is likely to occur when subjected to local force. For this, the melt temperature should be lowered and the temperature difference between the melt and the mold should be reduced.
②If the cooling time is too short, the stress in the plastic part is difficult to be fully balanced. When the local force is too large, the ejector stress mark phenomenon occurs, and the cooling time should be appropriately extended.
③If the injection pressure is too large, the residual stress in the plastic part is also large and causes the ejector stress mark. For this, the injection pressure should be appropriately lowered.
④If the holding pressure is too high, the stress in the plastic part is large, and when it is subjected to a large ejector force, the ejector stress mark phenomenon is likely to occur, and the holding pressure should be appropriately reduced.
⑤If the holding time is too long, residual stress is also likely to occur, and the ejector press mark occurs. For this, the holding time should be shortened.