Warpage in an injection molded part is caused by shrinkage variations throughout the part.
Shrinkage of plastic part is driven by the volumetric change of material as it cools down from the melt state to solid. It must be noted that shrinkage itself does not cause warpage. A high uniform shrinkage will give a perfectly shaped part that is simply smaller in size. However, if regions of the part shrink unequally, stresses are created within the part. D epending on the part stiffness, this may cause the part to deform (i.e. warpage) or even crack in the long term.
The fundamental causes of shrinkage variations (and consequently warpage) are:
1) Differential Cooling
Non-uniform cooling causes shrinkage variations through the thickness of a part. Aspects which influence the shrinkage variations due to differential cooling are:
2) Differential shrinkage
This results in shrinkage variation across the part surface (i.e. parallel and perpendicular to the flow). Differential shrinkage can be caused by:
3) Material orientation effects
This results in different amounts of shrinkage parallel and perpendicular to the flow. Shrinkage due to orientation effects can be caused by:
Using Moldflow, we are able to isolate each individual cause of shrinkage variations (see list above) and determine its effect and contribution on part warpage. By identifying the primary cause of warpage, we can suggest specific part- or mold design modifications, or optimize processing parameters (for example modified packing profile).
The part- and mold designer can improve the dimensional stability of a product by considering the overall contributers of part warpage. These contributers are devided into the following (genaral) groups:
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