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212 23 Contamination
Often times molding shops will provide very little training for material handlers. If
a material handler does not understand the importance of thorough cleaning
during material changes they may cut corners which will lead to contamination.
Provide a formal training for everyone in the shop that is responsible for loading
materials to ensure that changeovers are executed correctly.
23.3.1.2 Molding Process: High Melt Temperature
When plastics are overheated they can degrade and contaminate the material with
black specks or streaks. See Chapter 15 on black specks for more information
about this problem.
23.3.2 Contamination Troubleshooting Mold Issues
Mold-related concerns that can cause contamination include:
Hot runner hang up
High hot runner temperatures
Wear surfaces
Lubricants
Cleaning
23.3.2.1 Mold: Hot Runner Hang Up
Any areas in a hot runner system that can trap material can lead to contamination.
Material trapped in hang up areas can continue to bleed into the melt stream long
after the material change has been completed. When dealing with materials that
are more temperature sensitive the trapped material may degrade and contami-
nate the parts with charred black specks. Figure 23.3 shows contamination from a
hot drop tip.
Hot runner assemblies should be built in a way that at running temperatures there
will be neither ledges nor mismatch locations that would hang up material. Drops
must be securely assembled to the manifold so as to not leave any gaps between
the drop and the manifold body.
Corners in hot runner manifolds can easily lead to dead spots where material
hangs up. Depending on the hot runner design some manifolds will have corners
that are gun drilled at 90° intersections. With a 90° intersection the outside of the
corner will end up as a dead spot and material will be trapped in this location. Bet-
ter designs have machined corner plugs with a curved corner, no dead spots.
