Page 189 - Injection Molding Advanced Troubleshooting Guide

19.3 Buildup Troubleshooting 179

19.3.4.3„ Material: Type
There are some materials, especially wide-spec materials, that will tend to have
more issues with buildup. In some cases, these materials may have components
and monomers that are not fully reacted which end up being volatilized during the
melting process. The volatiles will then end up trying to escape the mold through
the vents, where these residual byproducts will then leave buildup.
An example would be caprolactam in nylon. Sometimes this unreacted caprolac-
tam will result in a white buildup on the mold and/or part surface. This buildup in
vents will lead to burning of the part, and the white buildup on the part will be a
visual defect. Due to a wide range of moisture content in nylon and its low viscosity
it can be challenging to provide adequate venting to allow all volatiles to escape
while the mold is filling.
Polycarbonate can be another material that tends to form buildup while running.
Polycarbonate is more forgiving than many materials for vent depth so it is often
easier to get adequate venting with polycarbonate. Evaluate the mold per the rec-
ommended venting guidelines.
Many ABS and PPO materials will also have volatiles that will result in a liquid that
builds up on parting lines.

Adequate venting is critical to successful molding with all materials. If the mate-
rial being molded seems to be causing more issues than normal make sure to pro-
vide optimized venting.

19.3.4.4„ Material: Additives
Plastics are produced with a wide range of additives that can migrate out of the
material during processing. Additives that may contribute to buildup on the mold
include:
ƒ Flame retardants
ƒ Colorants
ƒ Lubricants
ƒ Release agents
ƒ UV stabilizers
There will be cases where the additive package used in a material will lead to
buildup problems. Unfortunately, these additives may be necessary to provide the
plastic with specific required properties. If a particular material is causing an on-
going issue verify that all processing recommendations are being followed and
verify that venting is maximized (see Chapter 7 on venting).
When using any additives as a masterbatch it is important to verify that the re-
quired let down ratio is being used. If an excess amount of an additive is being