Polypropylene is very susceptible to pin push (ejector marks) where other materials are not. This requires more focus on ejection with pins and lifters. Changing from a 0.250-in.-diam. ejector pin to a 0.375-in.-diam. pin more than the doubles the ejection surface area. But even more important is to focus on pushing on the details of the part versus nominal wall stock.
If you put an ejector pin next to a rib detail, you will be stressing the part while trying to pull the plastic out of a rib or detail. But if you put the ejector pin or blade on the bottom of the detail, you will have much lesser tendency toward pin push or stress marks. This will also allow venting in areas where gas traps can contribute to splay and erosion. In most cases, PP is not considered a material that will erode steel. However, I have been involved with some significant erosion on tools running PP from severe gas traps.
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