Page 196 - Injection Molding Advanced Troubleshooting Guide
P. 196
186 20 Burns
When a mold has a hydraulic leak, it is usually possible to detect oil dripping some-
where on the mold surface or from the cylinder itself. When this oil is carried into
the melt stream the contamination can lead to plugged vents. If a cylinder is leak-
ing oil it may need to be rebuilt or replaced. There can also be cases where a hy-
draulic connection is located on the top of the mold, and when the fitting is un-
plugged there can be drips of oil that are left behind and will often drip down into
the mold cavity. This is a good reason to keep hydraulic connections off the top of
the mold whenever possible. Also make sure that standard operating procedure for
technicians and mold setters is to wipe off any leaked oil.
Excessive grease on moving components can end up in the mold cavity and result
in problems including plugged vents. Frequently when a mold comes back from
service it will be “over-greased” as mold shops will have a tendency to go with the
approach of if a little grease is good a lot is better. Work with your tool shops to
establish guidelines for the appropriate amount of grease to be used on a mold.
Also, anyone in your shop who applies grease to a mold should be familiar with
proper application of grease on mold components.
20.3.3 Burns Troubleshooting Machine Issues
Verify machine conditions to ensure that the machine is not running out of control
on temperature, back pressure, or recovery speed. Any overheating of the material
may lead to issues with excess gas that can in turn lead to burning.
The main machine-related causes are:
Buildup on screw, end cap, check valve
Wrong screw design for material
Overriding barrel heaters
Damage to screw, barrel, check valve
20.3.3.1 Machine: Buildup
Buildup is a condition on the machine where material is collecting and degrading
over time. As this material degrades it can release additional outgassing and by-
products that can be difficult to vent in the mold. If these additional gases are not
vented from the mold they could become a gas trap that in turn leads to a burn.
Buildup can often become an issue when a machine is allowed to sit idle with ma-
terial in the barrel, or when transitioning between materials.
Frequently when buildup is a problem there will be other symptoms including
black specks, splay, and brown streaking that appear on the molded parts. Thor-
ough purging at material changes or after extended downtime will help minimize
this issue.
