Page 216 - Injection Molding Advanced Troubleshooting Guide

22.3 Color Swirls Troubleshooting 207

22.3.4.3„ Material: Raw Component Issues
Occasionally there will be cases where a raw component of a material or concen-
trate will yield problems with distribution throughout the material. A potential is-
sue can occur if the raw component does not disperse well with the color con-
centrate. Sometimes, raw pigments can agglomerate and clump together, which in
turn can lead to areas of swirls.
This may especially be a factor when a mold has been running successfully and
starts to suddenly experience color swirl issues when a new lot of color concen-
trate is introduced. If a problem starts to occur immediately after a new lot of mate-
rial or color concentrate has been introduced then the material is a good place to
start the troubleshooting process. There will be times when something in the ma-
terial or the color concentrate is directly leading to the part defect.
One way to check for material issues is to switch to a different lot of material or
even switch to another color material. If the issue is isolated to specific lots then it
is critical to contact the material supplier to help with the investigation. Another
important thing to keep in mind is that if the issue is in the raw material or color
concentrate you will probably experience problems on any job that is running the
material. Keep in mind that there are cases where certain molds will show a defect
much faster and to a greater severity than others, so what may create 10% scrap on
one mold may only cause 1% on another.

Case Study: Raw Component Issues
This example was using a concentrate for a polypropylene automotive com-
ponent. In this case the new automotive interior specification called for a
high loading on antistatic compound in the concentrate. It quickly became
apparent that the 25 : 1 loading that was utilized on multiple parts was pro-
ducing a wide range of final colors, varying in lightness (L value) as well as
shifting red (a value), whereas the blue to yellow (b value) was ok. Visual
confirmation of the L*a*b* color values showed that there was a definite
shift in color on specific parts.
Root cause investigation was undertaken, which led to data that seemed to
indicate the color was shifting based on barrel residence time. Further in-
vestigation with the material supplier determined that to meet the antistatic
requirements they had to load the concentrate heavily with glycerol
monostearate, which degraded with heat and residence time. The molds
that ran with hotter melt temperatures and longer residence time delivered
the worst results.
After many discussions regarding the specification and the concentrate
impact it was determined that the specification was actually not the specific
one that was dictated for this particular application. After the material
supplier made some formulation adjustments the “new” formula was able to
produce consistent and quality parts that met the customer’s need.
