Mold Concerns

Plastics shrink as they cool and solidify during molding. Variations in shrinkage across the part can lead to warpage and distortion. Factors contributing to warpage include mold cooling, orientation effects, packing and thickness variations. Uneven mold cooling can cause areas of the part to shrink at different rates and by differing amounts. Fiber filled materials tend to exhibit much higher shrinkage in the cross flow direction than in the flow direction. Uneven fiber orientation can lead to variations in shrinkage throughout the part. Thick sections tend to shrink more than thin sections. In addition, it is more difficult to pack out thick sections. Insufficient packing in thick sections can further increase shrinkage. To reduce warpage:

Design parts with uniform wall thickness.
Design mold cooling to cool the part uniformly.
Pay special attention to cooling in inside corners.
In fiber-filled materials, place gates to align fibers uniformly along part length.
Make gates thick enough to facilitate proper packing. (see gate recommendations)
When parts must have variations in wall thickness, gate into thicker wall sections.

Refer to our Mold Design Guide for more information.

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