Page 206 - Injection Molding Advanced Troubleshooting Guide
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196 21 Cloudiness
Sometimes if hang up areas are the root cause, it can be possible to help break the
residue material free by raising temperatures and using a purging compound be-
fore running the new material through the barrel.
Something as simple as a nozzle tip can create hang up spots especially when us-
ing general-purpose tips, which have a dead spot that can trap material between
color changes and will drag out as the machine runs. This is more critical on nozzle
tips with small orifices because the dead spot is larger.
21.3.4 Cloudiness Troubleshooting Material Issues
Some possible causes of cloudiness in parts can be:
Contamination
Moisture content
Semi-crystalline nucleation
21.3.4.1 Material: Contamination
One of the most common root causes for a cloudy appearance is a contaminated
material. Contamination can come from a number of sources such as:
Not keeping material containers covered, which can lead to various contami-
nants getting into a gaylord or drum. Any storage containers should have a lid to
prevent contamination.
Unclear labeling, which can lead to a person mistakenly adding the wrong mate-
rial to a hopper or container. Labeling must be obvious to avoid people needing to
try to figure out if they have the right material.
Regrind, which can easily be mixed if it is not handled effectively. Immediate re-
use at the point of scrap generation is ideal.
Poor material changeover procedures, which can easily lead to a cross contami-
nation of materials. Whether it is poor purging procedures or lack of thorough-
ness in cleaning hoppers the opportunity is there on every changeover.
Case Study: Material Mixing
In this case study a blend of crystal polystyrene and SBC (styrene -butadiene
®
copolymer such as K resin ) was being used for a clear product. The crystal
polystyrene was used to create a lower cost blend than running straight SBC.
Even though both materials were naturally clear, creating a blend with too
much polystyrene resulted in cloudy parts. The ratio of polystyrene to SBC
had to be controlled to optimize cost, cosmetics, and performance.
