Warpage analysis predicts the deformation of an injection molded part caused by non-uniform shrinkage. Part design, mold design and process conditions all contribute to non-uniform shrinkage. If part shrinkage were completely uniform the part would only get smaller and not warp. Warpage is resisted by geometric features of the part and the material properties which affect rigidity.
How Does Moldflow Predict Warpage?
Moldflow isolates warpage based on 3 causes: area shrinkage, orientation effects and cooling effects. This is resisted by the rigidity of the part and material. Therefore, the primary factor on many parts is the part geometry – the ability to resist warpage.
Based on the 3 isolated contributors, variations in area shrinkage is often the most significant factor. The volumetric shrinkage result is an excellent indicator for area shrinkage problems which are usually related to packing. Often, changing the packing profile, gate size or gate location(s) greatly improves packing and warpage.
Although orientation effects can occur with unfilled materials, they are most significant with fiber filled materials. This is because shrinkage varies by 2x-10x in the direction of fiber orientation versus across the direction of orientation.
With well-designed molds non-uniform cooling is usually a small contributor to warpage although the cooling layout may have a big effect on cycle time and local hot spots.
Due to required form, fit and function, most plastic parts violate design rules; the most common being uniform wall thickness. These are due to design requirements and are often difficult to change. This may cause high residual (molded-in) stress or thick areas with high shrinkage. Often, making subtle geometry changes can have a significant impact. These changes can be hard to understand without analysis.
Sometimes, no matter what design changes are made, analysis helps minimize rather than eliminate warpage. Moldflow analysis will prevent unnecessary tooling changes based on best estimates of cause. Whatever warpage that does exist can be anticipated and dealt with in other ways.