Problems resulted from a non-uniform wall thickness of a plastic injection molded part are more than imagined. This article tries to analyze typical defects resulting from the part design using non-uniform wall thickness. Hopefully, a common sense of the most important principle of part design is uniform part thickness can be established.

Sink marks or hollow
Sink marks and shrinkage occurs most frequently in bulge area (including ribs, snap, etc.), the wall thickness equivalent in these area is greater than the thickness of the ribs and the panel (see Figure below).

When the injection mold start to cool down, the green area solidify first, the red area continue to cool done, it will suck in the other area and it becomes to be sink mark or sink hollow.

To avoid these failures, the wall thickness of the bulge area design at 50 to 75% of the wall thickness is preferably. By CAE simulation, you can forecasts the shrinkage on bulge area with different wall thickness designed to understand the different effects on sink marks or shrinkage.

Uneven wall thickness cause air trap
If wall thickness is uneven and the difference is large, melt flow will obtain favor to fill the thicker wall zone with smaller plastic flow resistance first. If the thin-wall area is surrounded by thick wall area, the air in thin wall area filled will be surrounded and trapped. the air trapped inside will be Oxidation, and even burnt.

Uneven wall thickness cause weld line
During the injection process, slower and faster cooling create different melt flows, weld line will be formed while the melt flow met, the weld line is not just cause obvious appearance defect but also very fragile.

Uneven wall thickness cause deformation
Picture below show a cell phone case made in PC+ABS, main wall thickness is only 1/3 of the side wall, the difference is huge, the shrinkage on different area is different so the internal stress created cause deformation.