The Reasons for Uneven Shrinkage of Injection Molded Parts
Jun 22, 2020

There are many common quality defects when it comes to injection molding. Such as flow lines, warpage, air trapped and sink marks, etc. Among them, uneven shrinkage is one of the most common of defects in injection molding, let’s talk about what causes uneven shrinkage of injection molded parts in this article.

There are some reasons for uneven shrinkage as below:

1. Melting Temperature

The different melting temperature will lead to different shrinkage rate. The higher melting temperature, the larger shrinkage.

2. Cooling Rate

As the molecules with slow cooling rate have more time to arrange, the shrinkage rate of its is greater than that of the quick cooling speed.

3. Melting Direction (amorphous material)

Generally, in-flow shrinkage ( internal stress is large ) is greater than the cross-flow shrinkage, as the plastic polymer chain in the flow direction is stretched seriously, and the trend of restoring the unstretched state is greater.

4. Product Size & mold design

More wall thickness, more shrinkage. It has higher requirements on cooling time and holding pressure in the thick areas. There will be local high temperature and continuous cooling after demolding, so, it’ll easily cause sink marks in the thick areas ( like ribs ). Increasing the holding pressure and prolonging the holding time appropriately can improve the marks.

For plastic parts, the position and size of gates will also affect the sink marks, they can be arranged in the thick areas and design with large enough size, that will be benefit for holding pressure. Also, it also can reduce the thickness by core out, which is also helpful for improving marks.

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