Page 413 - Injection Molding Advanced Troubleshooting Guide

412 41 Sink

41.3.4.2„ Material: Nucleating Agent
If the material being run is a nucleated semi-crystalline material the level of nucle-
ation will impact the crystallinity of the material, which in turn can impact the
shrink rate of the material. If the shrink rate is increased the result could be sink
on the part surface.
Verify that the correct material is being molded. If using a nucleating agent mas-
terbatch additive, check that the letdown ratio is correct.

41.3.4.3„ Material: Blowing Agent
If a blowing agent is used to help pack out a part it must be added at a consistent
level. If the blowing agent is not added at a consistent percentage the ability of the
material to foam will vary. Verify that the blowing agent is added at the correct
letdown ratio.

Also when using a blowing agent make sure not to pressurize the plastic with sec-
ond-stage pressure. Over-pressurizing the material in the cavity will restrict the
ability of the blowing agent to foam in the material. In some cases people will add
second-stage pressure to the process, which can lead to additional sink because
the function of the blowing agent is restricted.