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15.3 Black Specks Troubleshooting 135
15.3.2.3 Mold: Hot Runner Hang Up Areas
Anywhere in the melt stream where plastic can be trapped can lead to degradation
of the material. Any spots in the hot runner manifold that have a mismatch in
the steel will potentially lead to material hang ups. Common concerns are corners,
especially if they are 90°, mismatch between nozzles and the manifold, and using
an undersized nozzle tip orifice relative to the sprue opening.
Many of these hot runner issues should be dealt with upfront during the selection
and design of the hot runner. If there are issues with stagnant corners after the
manifold has been built, it will be a major hassle to correct.
The correct sized nozzle tip should always be used with a hot runner manifold. If
the tip orifice is undersized to the sprue opening there will be a shelf and dead
spot at the interface that will trap material potentially leading to black specks. The
process needs to be developed with the correct size tip and this size must always
be used for subsequent molding runs.
15.3.3 Black Specks Troubleshooting Machine Issues
Potential machine-related causes include:
Hang up areas on screw, barrel, or non-return valve
Barrel heater control
Machine performance
15.3.3.1 Machine: Hang Up Areas
A likely cause of black specks is a spot in the barrel, screw, end cap, or nozzle that
is allowing material to hang up and sit for extended periods of time. The two most
common causes of hang up areas are damage and mismatch.
A common area for hang ups to occur is when there is a mismatch between the
barrel, end cap, nozzle adapter, or nozzle (see Figure 15.2). It is critical to ensure
that all of the end components of the machine have uniformly matched surfaces. If
there are any shelves where the components are not matched material will hang
up and degrade. Material that is trapped in these areas will degrade over time and
likely reach a point where there is a heavy buildup of carbon, and this carbon
buildup will tend to break loose over time and be carried along by the melt stream
producing parts with black specks (see Figure 15.3) for an example of mild buildup
on a screw tip). Always utilize components that have the same internal diameters
to eliminate the potential for hang up areas.
