Injection molding plastic parts design tips: How to eliminate sink marks

Designers are always vexed with unsightly sink marks or differential surface gloss on injection molded plastic parts. The following tips, along with good plastic parts’ design, will guarantee elimination of sink marks by design, rather than burden the molder with process control issues of packing or extended in-mold hold cycles.

1. A 1/2 material thickness recessed spherical ring around the base of the boss.

2. Boss core pins should have a spherical end that extends to approximately 1/2 material thickness. It is such a simple tooling feature that it will almost eliminate sink marks on visual surfaces opposite internal bosses.

3. If you are concerned about reducing the boss strength by reducing material around its root, of course ribs can be added.

Put together, the combination of the boss core pin spherical end and the recessed spherical ring, both extending to approximately 1/2 material thickness, will guarantee elimination of sink marks.

By now you will have realized that we are decreasing material buildup, the boss’ root is a triangle’s c² hypotenuse of Pythagorean geometry a²+b²=c², and explains why the plastic melt cools slower causing visual surface sink or gloss differential.

Pier Design has had molded numerous parts including both of these details, all parts have performed successfully and do not have any sink marks or material differential gloss on the visual surface. In fact, we include these details on all injection molded parts’ bosses.

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