Page 147 - Injection Molding Advanced Troubleshooting Guide

134 15 Black or Brown Specks

15.3.2.1„ Mold: Debris from Mold Wear or Shutoff Surfaces
Sometimes black specks do not arise from material degradation. An often-over-
looked root cause of black specks on plastic parts is dust from the mold wear sur-
faces. Many times, the miter locks, wear plates, or shutoff surfaces on molds will
create a dust of metal that can make its way into the mold cavity where it will be
molded into the part. This metal dust will appear as black specks if it is molded
into the surface of a light-colored part. Note that when this occurs the location of
the problem area is normally above the cavity, which allows the dust to drop into
the cavity.
Mold cleanliness is critical to avoid this potential root cause. Wear surfaces should
be kept clean and examined for metal “flaking” because this could indicate a larger
problem with the mold.
Caution should be exercised whenever offset mold temperatures are used. If the
mold halves are being run at differing temperatures thermal expansion will lead to
differential expansion that may contribute to excessive wear on shutoffs or lock
angles. Especially with larger molds this differential expansion may lead to metal
debris from wear dropping into the cavity leading to black specks.
If larger flakes of metal are detected a closer inspection of the mold is warranted.
These metal flakes often indicate that something is damaged or wearing at an un-
usual rate. Detection of abnormal items like metal flaking can help eliminate
expensive mold damage. Always look for areas on the mold where galling may be
happening. Wear plates made from bronze may be required on certain wear loca-
tions on a mold.

15.3.2.2„ Mold: Hot Runner Temperatures
Similar to high melt temperature this will usually not be the root cause of the black
specks but can be a contributing factor. If the hot runner temperatures are set high
or running hot for any reason the material will be pushed closer to the limit where
it will degrade. It is important to keep in mind that the time spent in the hot runner
manifold is normally much shorter than the time spent in the barrel, but depend-
ing on the manifold design the residence time in the manifold can be several shots.
Verify that the mold’s hot runner temperatures are set correctly and running to set
point. Also check the amp (current) draw on the hot runner controller to determine
that the zone is under control. If the current to a given zone is not cycling it may be
an indication that the zone is not wired correctly. If a zone is drawing current con-
tinuously it may be because the thermocouples for two zones are swapped, result-
ing in the controller providing constant power to a zone without seeing the tem-
perature rise.