Uniform Wall Thickness for Good Plastic Injection Molded Parts

By Protomold

Good injection molded part design relies on consistent wall thickness to minimize the potential for warped or distorted parts.

Avoiding Warp | Avoiding Sink | Rib-to-Wall Thickness Ratios

Avoiding Warp

Sink Marks
As the plastic solidifies in the mold it freezes from the outside (near the mold surface) toward the inside. In thick sections this results in inward pulling stresses (due to contraction) that can cause sink marks in the outer surfaces of the part.

Part Warpage
In addition, because thinner sections will freeze faster than thicker sections there is also the possibility of stresses building up between thick & thin sections, resulting in part warpage. 

Avoiding warp by minimizing, the use of ramps and gussets. 
Warpage due to stresses in step transitions between wall thicknesses can be improved through the use of a ramp. 

The use of gussets can be helpful to provide support in corners to avoid warping.

A. High stress concentrations
B. Reduced stress concentrations
C. Thinner walls result in shrinkage during cooling
D. Gussets provide additional support to reduce warpage

Avoiding Sink

Thicker and non-uniform wall thicknesses can often result in sinks in the material due to the same solidification physics described above. 

The use of thinner, uniform wall thicknesses helps to avoid sink. 

A. Boss in corner causes sink
B. Thinner walls on boss eliminates sink
C. Thick walls cause sink, warp & excess shrink
D. Thinner walls give accurate part

Rib-To-Wall Thickness Ratios

Thin ribs on thicker walls may provide stiffness but also can result in sinking on the outside of the wall. This rule-of-thumb guideline should help keep this from happening.

Watch rib-to-wall thickness ratios—To prevent sink, the thickness of the rib should be about half of the thickness of the wall.