Why are there shrinkage marks on the surface of products

During the production of injection molded products, some products will have uneven surface shrinkage due to uneven wall thickness, resulting in shrinkage marks. So what is a shrink mark? Shrinkage mark refers to the phenomenon of pits, pits or shrinkage marks on the surface of injection molded products. The depth of shrinkage mark is generally small.

The following is an analysis of the reasons of this fault and the troubleshooting methods.

Fault analysis and troubleshooting

1. Improper control of forming conditions.

Properly increase the injection pressure and speed, increase the compression density of the solution, extend the injection and pressure holding time, compensate the shrinkage of the melt, and increase the injection buffer. However, the pressure holding shall not be too high, otherwise it will cause convex marks.

If the depression and shrinkage mark occur near the gate, it can be solved by prolonging the holding time; When the plastic part has a depression at the wall thickness, the cooling time of the plastic part in the mold shall be appropriately prolonged; If the local shrinkage of the melt around the insert causes dents and shrinkage marks, this is mainly caused by the low temperature of the insert. Try to increase the temperature of the insert; If the surface of plastic parts is sunken due to insufficient feeding, the feeding amount shall be increased. In addition, the cooling of plastic parts in the mold must be sufficient.

2. Mold defects.

According to the specific conditions, the gate and runner section shall be appropriately expanded. The gate position shall be set at the symmetrical position as far as possible, and the feed port shall be set at the thick wall of the plastic part. If the depression and shrinkage mark occur far away from the gate, it is generally due to the poor flow of molten material at a certain part of the mold structure, which hinders the pressure transmission.

In this regard, the structural size of the mold gating system should be appropriately expanded, and it is best to extend the runner to the part where the depression occurs. For plastic parts with wall thickness, wing gate shall be preferred.

3. Raw materials do not meet the molding requirements.

For plastic parts with high surface requirements, resin with low shrinkage shall be used as far as possible, and an appropriate amount of lubricant can also be added to the raw materials.

4. The structural design of plastic parts is unreasonable.

When designing the structure of plastic parts, the wall thickness should be consistent as much as possible. If the wall thickness of plastic parts varies greatly, it can be solved by adjusting the structural parameters of the gating system or changing the wall thickness distribution.