Page 419 - Injection Molding Advanced Troubleshooting Guide

418 42 Splay

Figure 42.3 Microscopic picture of slug of degraded material that created a splay streak
behind it

When troubleshooting a process for splay it is critical to verify that the plastic melt
temperature is within the recommended specification. Keep in mind that both too
high a temperature as well as too low can lead to splay issues although too hot is
the most typical culprit. Always keep in mind that temperature settings on the
machine may not reflect the actual melt temperature of the material. There are
cases where the melt temperature can be 50 °F or more over the barrel set points
all due to shear heating from the screw.
There will be times when a heater band is burned out. You can detect these burned
out heaters by checking the current draw using a clamp-style ammeter. If a heater
band is burned out the barrel will have a cool spot and will not be uniformly heated.
Uniform heating with consistency from run to run is critical in maintaining a
repeatable, scrap-free process.

42.3.1.3„ Molding Process: Decompression
Sometimes running with too much decompression (suck back) on the screw can
cause ambient air to be drawn into the nozzle orifice of the machine. This ambient
air will then often be deposited on the surface of the molded part as splay. If a large
decompression is required to avoid drool from the nozzle tip it may be necessary to
work through why so much decompression is required, and often times the cause
is either too much heat or degraded material.