There is a shrinkage on the surface of the injection molding product. The normal thinking should be to increase the pressure to fill the cavity, but occasionally adjust the shrinkage of some places in the plastic product. Why is the pressure greater and the shrinkage pit larger?
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analysis:
Shrinkage: The reason is also the same as the filling deficiency. In principle, it can be solved by excess filling, but there is a danger of stress. The design should pay attention to the uniform wall thickness, and the ribs and studs should be reduced as much as possible. Wall thickness.
Adjusting plastic products: Why is the pressure greater, and the shrinkage pit is bigger?
The figure above demonstrates an ideal filling model for injection molding. In the model we see that the plastic melt with higher temperature and lower viscosity preferentially passes through the center of the cavity, and the lower temperature plastics on both sides are gradually dispersed around. At the end of the injection phase, the plastic melt just reaches the end of the cavity. At this time, the plastic with lower temperature on both sides is not completely attached to the mold wall. If the pressure is not large enough, the product will cause local shrinkage.
Adjusting plastic products: Why is the pressure greater, and the shrinkage pit is bigger?
But we also need to consider the product structure. There is a local "wall thickness" in some molded parts, which causes the following problems:
The special position causes a large gap in the "wall thickness" of the injection phase;
The raw material of the plastic shrinkage during the pressure-holding phase cannot be filled with the void at the "wall thickness";
Large shrinkage at the "wall thickness" of the cooling stage causes the shrinkage pit to become larger
At this time, if the pressure is increased, not only can the gap be completely shrunk, but the two ends of the "wall thickness" are excessively filled due to the race effect, so that the shrinkage pit becomes larger during the cooling phase.
to sum up:
In this case, the first place to look at the location of the shrink pit, rather than directly modify the process parameters without analysis. Before solving the injection molding defects, it is necessary to correctly understand what defects the product is, why it is produced, and to clarify these two problems, and the injection molding defects will be solved.