Page 262 - Injection Molding Advanced Troubleshooting Guide

26.3 Dimensions Troubleshooting 253

One of the challenges with plastics is that many factors can influence the shrink
rate of the material, which is why suppliers always give a range of shrink rates.
Some of the factors to consider when discussing shrink include:
ƒ Wall stock
ƒ Flow length
ƒ Distance from gates for distributing pressure
The above factors relate back to part and mold design and may need to be compen-
sated for. Another factor to keep in mind is that plastics will change size based on
the temperature the part is exposed to. Even the difference between measurements
in the hot factory environment as compared to an air-conditioned quality lab can
produce results that are significantly different. If experiencing parts that do not
match from summer to winter investigate the linear thermal expansion character-
istics of the material. Some size limits will be set too tight to accommodate sea-
sonal variation in size. The variation of plastic should be considered up front when
designing parts as well as when dimensional tolerances are set.
Be aware of anisotropic shrink, which is differential shrink that occurs with flow
and across flow. A challenging part of designing appropriate shrink numbers on
plastic parts is knowing that anisotropic shrink may occur leading to cutting a
mold with different shrink factors depending on the properties of the part. This
concern can magnify when molding filled materials. Warp analysis on CAE can
help provide insight into this area.