This post is the 11th article in our series about common issues that you can encounter during your plastic production. In this installment, we're talking about shrinkage marks. If you want to start from the beginning, you can check out our article on short shots.
What is the shrinkage mark?
At the boundary where the wall thickness of the plastic part varies greatly, there are obvious marks due to the uneven thickness contraction in the two places. Since it due to uneven contraction, it is called a shrinkage mark. Some pictures follow below:
Reasons and solutions
1. Improper control of molding process.
Under this condition, the following should occur :
The injection pressure and speed should be appropriately increased.
The compression density of the melt should be increased.
The time for maintaining the pressure of the injection should be extended.
The contraction of the plastic melt should be compensated for.
The injection buffer should be increased.
The pressure, however, should not be too high, otherwise, it will cause a bump. If the sunken and sink mark occurs near the gate, it can be solved by extending the holding time. Additionally, the cooling time of the plastic parts in the mold should be extended when the plastic parts are recessed at the thickness of the wall. If there are sunken and sink mark around the insert due to partial contraction of melt, which is mainly caused by the low temperature of the insert, we should try to increase the temperature of the insert. If the surface sunken is caused by insufficient feed, the amount of feed should be increased. In addition, the molds must be sufficiently cooled.
2. Mold defects
When there are issues with the mold, the gate and runner section should be appropriately expanded according to the specific situation. The gate location should be set as far as possible in a symmetrical place while the feed inlet should be set at the back wall of the plastic part. If the sunken and sink mark occur far from the gate, it is usually due to poor melt flow at one part of the mold structure that impedes the pressure transfer. In this regard, the structural dimensions of the mold casting system should be appropriately expanded, and it is best to extend the flow channel to the area where the sunken marks occur. For the plastic parts where the walls are thick, a wing gate is preferred.
3. Raw materials do not conform to the molding requirements
For plastic parts with higher surface requirements, plastics with low shrinkage characteristics should be used as much as possible. A suitable lubricant can be added in the raw materials to assist with these issues.
4. The shape structure design of plastic parts is not well designed
When designing the shape structure of plastic parts, the wall thickness should be consistent as much as possible. If the wall thickness of the plastic part varies greatly, it can be solved by adjusting the structural parameters of the casting system or changing the wall thickness distribution. The following picture shows and example: