Causes of Warpage

Warpage is due to non-uniform shrinkage which has many causes. Moldflow isolates 3 causes of warpage: area shrinkage, orientation effects and cooling effects. 3D Shapes can help you identify a 4th intrinsic and probably most common form: geometry induced warpage.

Our experience evaluating 100's of parts, is that one of the most common and easily corrected causes is area shrinkage. The volumetric shrinkage result is a excellent indicator for area shrinkage problems which are related to improper packing. Often, just changing the packing profile, gate size or gate location(s) can greatly improve packing and warpage.

Although orientation effects can occur with unfilled materials, the most significant orientation effects often happen with fiber filled materials. This is because shrinkage varies greatly in the direction of fiber orientation versus across the direction of orientation - often by a factor of at least 2-to-1.

With most well designed tools non-uniform cooling is a small contributor to warpage although the cooling layout may have a big effect on cycle time and local hot spots.

Many times the most overhwelming effect is geometry induced warpage or, "the maddening effects of form, fit and function". Most plastic parts violate design rules, the most common being uniform wall thickness. This is due to design requirements and is often difficult to change. This may cause high residual (molded-in) stress or thick areas with high shrinkage.

The rigidity of any part is based on the part geometry and the material. Many times, no matter what design changes are made, the best result from analysis is to minimize rather than eliminate warpage. This is also very useful information. First of all, unecessary tooling changes can be avoided if you know they will not yield further improvements. Secondly, warpage can be anticipated and dealt with in other ways.