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Torsten’s Training Tips: Understanding Shrinkage and Warpage Behaviors

by Torsten Kruse

Minimizing volumetric shrinkages and reducing part warpage are important considerations when designing injection molded parts. By understanding the interaction of part and mold designs, polymer material, and the molding process, preventative measures can be designed into the molding system to minimize shrinkage and warpage.

Polymer materials naturally expand when they are heated and shrink as they cool. If polymer shrinkages are even in all directions, there is no resulting part warpage. Warpage only occurs when there are uneven shrinkages. There are various factors that could create uneven shrinkages in all stages of injection molding, from part design and mold design to polymer material to processing.

Optimizing part design and polymer materials are the crucial first step in minimizing shrinkage and warpage. If the part design is not optimized for the material, the mold design cannot be optimized. And then, once the part reaches the processing phase, a process engineer has very little opportunity to optimize the molded part.

These four images show a rectangular part with various side-wall design features and the resulting warpage for each. In the first design the sides walls are sold without any cutouts, the second part has cut-outs that reach ¾ of the way down to the base plate, the third has cut-outs that reach all the way to the base plate, and the fourth is an I-beam shape.

Warpage Examples

Clearly the warpage behavior changes significantly based on the design. These designs have different volumetric shrinkage range differences in the base and upright walls so the warpages are different. The design components must be allowed to shrink freely so creating cut-outs that extend to the base or changing the upright wall design the warpage was minimized. This proves that “design matters”.

By learning the fundamentals of how changes to simple geometries can impact shrinkage and warpage behaviors, you can expand that understanding to more complex part designs. Designing and building robust molded parts is dependent on understanding the interactions between part and mold designs, polymer material, and the molding process and their impact on shrinkage and warpage behavior.

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