Page 237 - Injection Molding Advanced Troubleshooting Guide

228 24 Cracking

24.3.2„ Cracking Troubleshooting Mold Issues

When evaluating the mold look for sharp spots that can create stress concentra-
tions. Note that part designs should always accommodate and specify radius at
corners and intersections. Typical mold concerns are:
ƒ Undercuts
ƒ Sharp corners
ƒ Lack of draft
ƒ Lifter or slide concerns

24.3.2.1„ Mold: Undercuts
Undercuts in the mold steel will create localized spots where the plastic is “die
locked” behind the steel. These undercuts will trap the part making it more diffi-
cult to remove from the mold (either cover or ejector side). The added sticking of
the part may cause higher stress on the part during mold opening or ejection. Also
undercuts may cause stress risers in the molded part.
Undercuts can be caused by several tooling-related issues including EDM or ma-
chining marks, burrs on the parting line, scratches, and damaged or broken steel.
Close inspection of the mold steel for damage and the part for drags or damage
may indicate areas of undercut plastic. These undercuts must be polished out of
the mold.

24.3.2.2„ Mold: Sharp Corners
Plastics do not like sharp corners, which concentrate stress where cracks will tend
to emanate from. Any corners or intersections on a mold should have a radius
rather than be produced in a “dead sharp” condition.
Small radii are hard to detect on mold parts. One quick and easy check is to take a
ball point pen and draw along the radius of the molded part. If the part has no
radius the ball point of the pen will bridge across the 90° and leave behind a dou-
ble line, but if a radius is present the ball point will leave one line as it marks along
the radius.

24.3.2.3„ Mold: Lack of Draft
In cases where the draft angle is inadequate for ejection of the part the result is a
tendency for the part to stick in the mold. If a part is sticking in the mold the
proper draft angles and surface must be selected based on the specific material
being molded.
Before a mold is built a thorough draft analysis must be conducted to ensure that
the part will be moldable. To overcome a lack of draft it may be necessary to add a
secondary coating to the mold surface to prevent the part from sticking.