Page 427 - Injection Molding Advanced Troubleshooting Guide

426 42 Splay

ƒ Cracked feed throat
ƒ Damaged screw, barrel, or non-return valve
ƒ Lack of process control

42.3.3.1„ Machine: Temperature Control
Issues with heat control on the molding machine can be a frequent cause of splay
on molded parts. Both too high and too low a material temperature can result in
degradation of the material that may result in splay. When temperatures are too
high thermal degradation of the material will occur, which will generate outgas-
sing that leads to splay. When the temperatures are too low excess shear will occur
in the screw and barrel, which can also cause material degradation. Most newer
machines have heater band burnout detection, which will help with these potential
issues. If you do not have heater band burnout detection on the machine you will
need to check current draw on the heater bands with an “Amp Clamp” type of
meter.

42.3.3.2„ Machine: Cracked Feed Throat
A crack in the feed throat can allow feed throat cooling water to leak and get into
the incoming stream of material. This rarely occurs and is difficult to diagnose be-
cause you will have to clear the feed throat and remove the hopper to see if there is
a crack and water leaking. No matter how well you dry the material if you are add-
ing water back in at the feed throat you will have splay problems.

42.3.3.3„ Machine: Screw Design
Poor screw design can lead to poor melt quality, which in turn can result in splay.
The primary example of this would be running a semi-crystalline material in a
general-purpose (GP) screw with an inadequate compression ratio or L/D ratio.
This combination will lead to poor melt quality where in the worst case the melt
stream has unmelted pellets that can often leave a surface defect with a trail of
splay. At a lesser level the air that is between the unmelted pellets will not be
compressed out of the melt stream, which will result in gas being compressed into
the melt stream.
Keep in mind that a GP screw is at best a compromise trying to be a “one size fits
all” item. Depending on the material being run a specific compression ratio, L/D
ratio, and mixing elements may be required to provide an adequate melt without
gas entrapment. Figure 42.4 shows an example of a standard GP screw.