Page 468 - Injection Molding Advanced Troubleshooting Guide

468 46 Warp

46.3.3.2„ Machine: Robot Handling
When a robot removes a part from a mold there is a possibility of warping the part.
If the robot pushes the part against the mold with enough force it can cause distor-
tion of the part resulting in a warped part.
Another concern with robot handling is how the robot extracts the part from any
mold details or components such as lifters. If the robot does not clear the mold
component as it retracts, the part can actually catch on the mold component
and become warped as the robot causes the part to deflect away from the mold
component.
If the robot handling of the part is suspected as the root cause of an issue, try run-
ning a cycle in semi-automatic mode and hand removing the part from the mold. If
the part that was removed by hand does not have the defect it may be a good indi-
cator that the robot extraction is the problem.

46.3.4„ Warp Troubleshooting Material Issues

Material factors can have a significant impact on warp including:
ƒ Material type
ƒ Part packaging
ƒ Filler content and type

46.3.4.1„ Material: Material Type
The number one difference for warp impact is between amorphous and semi-crys-
talline materials. The orderly crystalline structure of semi-crystalline materials
will lead to more contraction of the space between the molecules during cooling.
When semi-crystalline materials cool in the mold the resulting shrink will be
higher due to the way the crystallinity builds in the material. Increasing amounts
of crystallinity will lead to increased amounts of shrink and greater likelihood of
differential shrink. In the melted state semi-crystalline materials will actually be
in an amorphous stage, and freezing the material in more of an unordered amor-
phous state will lead to reduced shrinkage. Keep in mind if using a cold mold to
minimize crystallinity the part may experience warp at a later point in time when
it is exposed to temperature higher than the mold temperature.
Semi-crystalline materials will also tend to experience anisotropic shrink, which
means that the shrinkage in the flow direction will be different to the shrink across
the flow direction. This can often lead to differential shrink, which results in warp
that is difficult to predict. Injection molding CAE software can help predict this
anisotropic shrink and allow a molder to cut a mold with differential shrink rates.
When using a semi-crystalline material make sure to account for the increased
shrink and warp during part and mold design.