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23.3 Contamination Troubleshooting 215
When wiping a mold surface, use clean lint-free wipes to minimize the opportunity
for contaminating the mold surface. A dirty wipe not only leaves behind contami-
nants but may also scratch the mold surface.
Be aware that there are cases where the grease in the mold breaks down due to
chemical contact. This can be magnified if someone sprays mold cleaner directly
on the ejector half of the mold. The cleaner solvents can impact the viscosity of the
grease leading to contamination of the molded part from the broken-down grease.
23.3.3 Contamination Troubleshooting Machine Issues
Some of the machine factors that can lead to contamination include:
Hang up areas
Anti-seize
Robot contamination
23.3.3.1 Machine: Hang Up Areas
Any areas in the melt delivery system that have a mismatch can cause material to
hang up. Some of the key areas that should be evaluated for hang ups include:
Hopper to feed throat
End cap to barrel
Nozzle adapter to end cap
Nozzle to nozzle adapter
Nozzle tip to nozzle
Nozzle tip to sprue bushing
Damaged spots on screw, barrel, non-return valve, and end cap
It is a time-consuming process to disassemble these components and inspect for
hang up areas but sometimes it is necessary to get to the root cause. Prior to shut-
ting down for disassembly run another color of material through the barrel. This
alternate color can help highlight spots where the original color is being trapped.
Also look for areas of charred material when conducting this inspection. Again,
this is a time-consuming process so make sure other potential causes are investi-
gated first.
Examination of nozzle tips of different style will show locations where material can
become trapped. See Figure 23.5 for cross sections of common nozzle tips: notice
the potential dead spot at the ball end of the general-purpose style tip. General-pur-
pose tips can cause contamination and streaking issues due to this dead spot.
