Page 168 - Injection Molding Advanced Troubleshooting Guide
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17.3 Brown Streak Troubleshooting 157
17.3.2.2 Mold: Manifold Leaks
If a hot runner manifold or drop is leaking material there will be a trap area for
material to collect in and degrade. These leaks can allow trapped degraded mate-
rial to contaminate the melt stream and create brown streaks. Unfortunately, the
mold must be pulled to examine the hot runner manifold if there are suspected
leaks. One sign of a hot runner leak can be when the process is losing cushion; if
the process is losing cushion the plastic is going somewhere, either leaking at the
nozzle, leaking past the non-return valve, or leaking in the hot runner manifold.
Running a mold with a hot runner leak will eventually lead to failure of heater
zones and require a great deal of work to repair.
17.3.2.3 Mold: Lubricant Related Issues
Sometimes brown streaks on molded parts can come from grease contamination
on the mold surface. The grease that is used to lubricate the mold components can
show up on the surface of the parts and appear as a brown streak. Sometimes
when this is the cause you can follow the streak back to a location on the mold that
has a greased surface such as an ejector pin, lifter, or a slide. Over-greasing a mold
can create a great deal of scrap, so care should always be taken to ensure that
components are greased but not over-greased. Be especially careful to never spray
the ejector half of the mold with mold cleaner because this will thin the grease and
cause it to bleed into the cavity for an extended time period. Even mold savers with
high solvent levels may cause greases to degrade and become more prone to bleed-
ing. Always keep a clean mold!
Another factor in the mold that can lead to brown streaks on the molded parts is
bleeding caused by mold saver getting trapped in between mold inserts. Certain
mold savers can bleed from these insert lines and carry dirt and rust into the mold
cavity. If this is the cause of the brown streaking the streaks will likely lead back
to the witness lines from where the insert is located. Inspection of the mold may
well show dark lines along these insert lines. This is another case where spraying
with mold cleaner may cause additional contamination to bleed from the insert.
Case Study: Bleeding at Insert
In this case brown streaks were appearing on the part near an insert seam
line. The initial thought on this issue was that there was a water leak, but
examination of the mold showed that there was none. During the tooling
examination an oily substance was detected on the mold surface. It was
determined that the “wet” mold saver used was seeping into the insert
seam line and then bleeding into the part. Switching to a dry mold saver
solved this issue.
