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SINK MARK PREVENTION FEATURE FOR INJECTION MOLDED PLASTIC PARTS

IP.com Disclosure Number: IPCOM000008465D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2002-Jun-17
Document File: 2 page(s) / 95K

Publishing Venue

Motorola

Related People

Michael H. Brannan: AUTHOR [+2]

Abstract

Sink marks are depressions on the surface of injection molded plastic parts caused during the plastic cooling process. Thicker sections of plastic will cool at a slower rate than others, and will yield a higher percentage of shrink in that local area. The extra shrinkage in that local area is what causes the depressions.

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MOTOROLA Technical Developments

8

SINK MARK PREVENTION FEATURE FOR INJECTION MOLDED PLASTIC PARTS

by Michael H. Brannan and Jerry R. Nichols

INTRODUCTION

  Sink marks are depressions on the surface of injection molded plastic parts caused during the plastic cooling process. Thicker sections of plastic will cool at a slower rate than others, and will yield a higher percentage of shrink in that local area. The extra shrinkage in that local area is what causes the depressions.

  A molded plastics industry design guideline for maximum rib thickness to eliminate the possibility of a sink mark on an appearance surface has been in place for many years. The guideline suggests that rib thickness should be fifty-percent or less of the nominal part thickness. This article outlines a feature that will reduce, or eliminate sink marks when used at the rib/wall intersection with a rib thickness over 50% of the nominal part thickness.

DESCRIPTION

  The feature is a local reduction of the nominal wall thickness on either side of the rib. Remaining, is a smaller section of plastic that will cool at a faster rate, yield less shrink, and show no sink mark in that local area. This reduced section can now tolerate a rib thickness greater than the fifty percent guideline mentioned for sink mark prevention.

  A graphical method is used to show why sink marks are more likely when the industry guideline for a maximum fifty percent rib is violated. A sketch is generated of a circle in the rib/wall cross section tangent to both rib corner radii and the outer appearance wall. This illustrates the volume of plastic present at the rib/wall intersection, A correlation has been developed to aid in sink mark prediction which relates the diameter of the circle in the rib/wall cross section to the thickness of the part nominal wall.

  Figure 1 shows a rib of fifty percent of the nom- inal wall and a thirty-one percent comer radius. The thirty-one percent radius was used to give margin over the recommended fifty percen...