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The reasons of warpage in injection molding

Warpage is usually caused by molded-in stresses. The issue of warpage is related to being a semi crystalline material, HDPE, being semi crystalline polymer, will have differential shrinkage along and across the flow leading to warpage. It is difficult to eliminate this problem just by optimizing processing parameter. If use of multiple gates is possible, that might help multidirectional flow leading to lesser warpage.

The two areas that will have greatest affect are injection mold temperature and gating being both location and type. Not knowing configuration or flow difficult to give specific advice, but think of the part on a molecular scale. Imagine the surface of part being interconnected crystals with cooling starting at coldest surface of injection mold which will be multiple points and spreading out in a circular shape all within fractions of a second. You want the coldest surface possible and repeatable and a flow front that fills mold as evenly as possible. The more even the flow front in relation to part configuration from one side across to other and a plastic injection mold surface temperature that is a uniform temperature will minimize warp. There are some design tricks that would also minimize warpage but you may past that point. As to type of gate, as large as possible so you are not adding heat through shear. Also monitor melt temp at nozzle, looking for lowest temp that will enable complete mold fill.

Hire a thermal imaging camera, as I've done on many occasions, including in the last fortnight. Though the semi-crystallinity will accentuate warpage tendencies compared to amorphous materials, just see how temperatures vary across all surfaces of the part on ejection. Those that are hottest will take longer to cool, shrinking all the while. Simple physics will tell you that variable stresses (a mixture of tensile and compression) are bound to pulls the shape out-of true. Within seconds of seeing the images, you will appreciate why the part shape you actually get once the part has reached ambient temperatures differs from design intent. Flow-induced and variable crystallinity-induced effects will only add to the net effect. I'm dealing with an injection molding in PP with a >15mm wall section and, the forces at play as it cools are huge. Part, tool and process (cooling included) simulation by someone who understands the interrelationships will give you great insight into how to understand reasons for - and to minimize by good part.

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