In plastic products, especially ABS, PP and PC, there are bright stress marks on the surface of the products, such as thimble stress marks, seed stress marks and meat thickness difference stress marks.
These apparent bright and white marks are the result of internal stress, which is no more than orientation stress and cooling shrinkage stress. The results show that the irregular stress mark near the gate is caused by the typical orientation stress, and the meat thickness difference is the common result of the orientation and shrinkage stress, while the stress trace of the thimble and inclined pin is caused by more factors.
In general, with the increase of holding pressure and time, the first one is the stress mark of meat thickness difference, the stress mark of thimble oblique pin, then the stress mark of meat thickness difference, and finally the irregular stress mark near the gate (which will be white and bright in the side view), which also depends on the specific structure of the product and the pouring position.
Therefore, when these stress marks appear, reducing the packing pressure and holding time is the most important. To understand this, we should start from here, and it is more effective to modify the product design and mold design than to adjust the molding
1. Product design:
It is necessary to add glue or steal meat at the end of flow because of too thin meat thickness on the whole or thick meat at the flow end, resulting in excessive filling pressure and holding pressure. That is to say, when the holding pressure is reduced, the shrinkage can be improved by stealing meat. If not, it is necessary to add glue to the large surface;
Try to avoid poor meat thickness, if there is a need for large differentiation;
The rib of male die should not be too large to cause rib stress mark on female die.
2. Mold design:
The gate is too small or too few, uneven distribution;
The mold temperature is higher due to loose mold arrangement of moving parts or unreasonable water channel configuration.
3. Molding conditions:
Reasonable setting of holding pressure and time (decrease);
Increase or decrease of mold temperature (increase can improve filling and reduce packing pressure; decrease is to make biting surface bright and close to stress mark. Generally speaking, reducing mold temperature is the most choice)
