Page 213 - Injection Molding Advanced Troubleshooting Guide
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204 22 Color Swirls
22.3.3.3 Machine: Hang Up in Screw or Barrel
If there is damage to the screw, barrel, end cap, adapter, or nozzle, material can
become trapped and not clean out well during color changes. These hang up areas
can in turn lead to color swirls where the previous color drags out of the hang up
location causing contamination and color swirls.
This is an area that can be checked after other key areas have been evaluated. Hang
ups are more likely a root cause if the defect shows up after a color change and
then gradually improves as the mold runs.
If hang ups are suspected it will be necessary to pull and examine the screw and
barrel components. To help highlight hang up locations conduct a color change
before pulling the screw and look for locations where the previous color is trapped.
22.3.4 Color Swirls Troubleshooting Material Issues
There can be a variety potential material issues that lead to color swirls including:
Color concentrate
Contamination
Raw component issues
22.3.4.1 Material: Color Concentrate
The first potential root cause that can come from color concentrate is an incompat-
ible material to concentrate problem (keep in mind that there are no “universal
carriers”) [1]. This can simply be an issue of using a concentrate that is intended
for another material. If, for example, a styrenic concentrate is added to a PP base
resin there may be color swirls due to poor intermixing between the materials.
Therefore, the appropriate color concentrate should be used with a given base
resin.
When using color concentrate the letdown ratio must match what is required for
optimal coloring, for example 25 : 1 or 50 : 1. If the letdown percentage is low the
color will not be dispersed adequately in the base resin. Also make sure that the
correct size auger and settings are used with volumetric additive feeders. See Fig-
ure 22.2 for an example of a volumetric additive feeder. Different auger sizes will
require adjustments to the feeder set point to achieve the correct letdown ratio.
