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202 22 Color Swirls
material and should be documented for future process verification and troubleshoot-
ing. Always keep in mind that melt temperature is a result of many interacting
factors including barrel temperature, screw design, back pressure, screw recovery
rate, residence time, etc. Do not rely on barrel temperature settings as a predictor
of melt temperature.
22.3.2 Color Swirls Troubleshooting Mold Issues
The mold is not typically the root cause of color swirls. The most likely tooling-
related color swirl problem will be from color contamination from a hot runner
system. This really is more of a contamination issue but highlights the importance
of proper color changeover procedures. A hot runner system must be treated as
part of the melt delivery system, which means that it must be cleaned out during
color changes. A helpful hint when conducting a color change on a hot runner sys-
tem is to raise the hot runner temperature settings by 30–50 °F to expedite the
color change through the hot runner manifold and drops.
22.3.3 Color Swirls Troubleshooting Machine Issues
The molding machine itself can be the root cause of problems with color swirls.
Some of the main concerns related to the molding machine include:
Screw design
Inadequate back pressure
Hang up in screw or barrel
22.3.3.1 Machine: Screw Design
A general-purpose screw design is not an ideal screw for mixing of material and
color concentrate. There are a wide variety of mixing style screws that will provide
a more thorough mix as the machine melts and conveys the plastic. These screw
designs have a variety of mixing elements incorporated to help create a more uni-
form melt. These “melt quality” screws will provide additional melt turbulence
which helps distribute the color concentrate within the base resin.
Processors will often try to apply a “Band-Aid” to the problem of poor melt unifor-
mity. These “Band-Aid” approaches include using mixing nozzles and mixing tips.
These nozzles and tips have mixing elements that will provide additional shear
and mixing as the material travels through these locations. These items can help
with color concentrate mixing but will not be as effective as having a dedicated
mixing screw. When using mixing nozzles and mixing tips the machine will expe-
rience additional pressure drop through the nozzle due to the restriction to flow.
