Page 469 - Injection Molding Advanced Troubleshooting Guide

46.3 Warp Troubleshooting 469

Materials with fillers will shrink less. The difference between a filled polypropyl-
ene and an unfilled could be 0.015 in/in shrink or more. The higher the shrink
potential of the material the more likely it will be that there are regions of differen-
tial shrink resulting in warp. Using a filled material where appropriate can help to
reduce shrinkage and as a result improve part warp (see Section 46.3.4.3).
When running a fiber-filled material, orientation will potentially lead to anisotro-
pic shrink. Fibers will establish an orientation relative to flow direction with the
fibers lining up in the direction of flow. This orientation can result in a restriction
of shrink in the direction parallel to flow whereas the shrink perpendicular to flow
occurs at more normal rates.

46.3.4.2„ Material: Part Packaging
This is really a handling issue but the packaging that is used for a part may con-
tribute to warp of a part. If parts are packed in a manner that applies force to the
warm molded part the result can be part warp. This may result from packaging
that does not fit the part well, or bulk packing parts that are too warm; also layer
packing can put weight on lower layers resulting in warp.
Selecting the appropriate pack is as important to providing a quality part as any
other part of the process. Pack selection during process development must be eval-
uated to keep a part protected through storage and delivery to the customer. Make
sure that the part fits the packaging without being flexed or bent. Evaluate if bulk
packaging is appropriate for a given part because many times the part must be
better protected than what bulk packaging provides.

46.3.4.3„ Material: Filler Content and Type
Fillers will reduce the amount of shrink that a part experiences. If a material has a
large amount of filler variation or does not have the correct loading of filler the
resulting increase in shrink could lead to warp.
Verify that the correct material is being used along with the correct filler content.
Many times the material supplier will provide filler content on the certificate of
analysis for the filled material.
If using an unfilled material review if a filled material would be acceptable and
evaluate the impact on warp. It is a simple experiment to run some filled material
in a mold and compare the results to the unfilled material. If the filled material is
better investigate the cost, appearance, and impacts on dimensional and physical
properties. After verifying all important factors work with the customer to switch
materials to the filled version.
It is also important to note that fillers will act as nucleating agents and affect the
way semi-crystalline materials crystallize. This effect can be positive or negative
but must be factored in when troubleshooting warp. Nucleated materials typically
will not experience as much shrink as would a non-nucleated material.