Injection Molding
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Warpage and Deformation of PE Injection Molded Products
Problem Phenomenon
HDPE/LDPE/LLDPE injection molded parts warp and twist after demolding, especially obvious on large flat or frame parts such as bucket lids, turnover boxes and pallets, making assembly and stacking difficult.
Cause Analysis
PE mold shrinkage is as high as 1.5%~4.0%, and HDPE shrinks more due to its high crystallinity; when the melt transforms from an amorphous melt to a highly ordered crystalline solid, the volume shrinks significantly; thick-wall areas cool slowly and thin-wall areas cool quickly, and the differential shrinkage forms residual stress inside, which is released after demolding causing warpage.
Solution
①Separate temperature control circuits for the core and cavity, trying to cool all parts of the product uniformly; ②appropriately increase the mold temperature to slow the crystallization rate and stabilize the final tolerances; ③maximize the holding pressure and extend the holding feeding on the premise of no flash; ④extend the cooling time and constrain with shaping fixtures after demolding; ⑤optimize the wall thickness uniformity (within ±10%).
Key Takeaway
PE warpage = high crystallization shrinkage meeting uneven cooling; the main line is 'uniform temperature control + sufficient holding pressure + slow cooling and shaping'; the more highly crystalline the HDPE, the more time must be spent on cooling and holding; rapid cooling and quick demolding will inevitably warp.
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