Page 318 - Injection Molding Advanced Troubleshooting Guide
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312 32 Glass Fibers on Surface
hazards like cooling line ruptures. High-temperature lines are a critical factor
when running with high mold temperatures. Do not try to cut costs on this. Hot oil
is another option for achieving even higher mold temperatures.
Clearly label all of the thermolators in the shop. This way if a mold needs a
high-temperature thermolator it will be clear to everyone and when a replacement
thermolator is swapped for repair the correct replacement will be used. If someone
swaps a standard thermolator for a high-heat unit it will not be possible to achieve
the mold temperature needed to keep the surface looking good.
32.3.2 Glass Fibers on Surface Troubleshooting Mold Issues
A few mold-related issues will potentially impact the amount of glass fibers show-
ing on a part surface such as:
Lack of venting
Hot runner temperatures
Flow length
Gating
32.3.2.1 Mold: Venting
Lack of venting in a mold will cause the flow front velocity to slow as the mold is
filled. If the mold is not adequately vented the gas will be trapped ahead of the melt
flow stream and act as resistance to the plastic filling the mold.
The other key constraint from lack of venting is the possibility of burning occur-
ring at the faster fill speeds that for glass-filled materials provide the best cosmet-
ics. If the mold cannot be filled at an optimized velocity due to lack of venting the
mold must be optimized for the process. Trying to process around a lack of venting
will at best produce a part that is borderline on quality and will be a constant prob-
lem in manufacturing.
Knit lines can be especially challenging with glass-filled materials and the appear-
ance at the end knit line or end of fill areas will tend to look worse than the rest of
the part. To combat this there will be cases where the end of fill knit line location
actually needs an overflow to allow the material to have wide open venting and
create more of a meld line than a straight butt weld line.
Case Study: Glass-Filled Venting
In this particular part molded with 33% glass-filled nylon 6, the process was
optimized and providing good surface appearance everywhere but the end
of fill knit lines (two locations). A sample was run with one of the two gates
