Here's a fascinating post on the IDSA Materials and Processes blog about the things you can learn about injection moulding from studying the "ejection marks" on the surface of plastic objects:
So I noticed the marks on the lid of my mother-in-law's trash can and thought about what that says about how this part was made and how this might be something an industrial designer would need to understand. What I was looking at was the ejection mark placed on an angled surface. Because this large HDPE (high-density polyethylene) part will be somewhat soft when it is ejected or pushed out of the mold, the molder needed to be able to bear on several points close to the perimeter of the part (because just pushing on the middle would probably permanently distort the warm part).
Further inspection of this part also showed that the gates (injection points for the part) was on the underside, or opposite side of the part, which told me that the part probably rode back with the moving half of the mold and then was ejected after the side action (see the lip used to lift the lid?) retracted.
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