Page 420 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting 419
42.3.1.4 Molding Process: Back Pressure
Back pressure is an important process setting to generate a high-quality melt. A
lack of back pressure will result in an inhomogeneous melt that may contain air.
Part of the melt compression that takes place during melting forces the air that is
trapped between unmelted pellets to be compressed and forced out of the melt.
Back pressure is typically set from 500 to 3000 psi depending on how much shear
and mixing is desired for a given process. When running below and even some-
times at the low end of this range splay can be the result.
One thing to watch carefully is when people reduce back pressure while purging a
machine. This is done to help the screw recover but it must be changed back to the
set point when the machine is running parts. This can often be overlooked, and an
alternative to lowering the back pressure is to purge and use the manual decom-
pression button to retract the screw to the desired shot size. If you change the back
pressure there will come a time when you forget to reset it to the desired set point.
When back pressure is set too high it is possible to overheat the material, which in
turn degrades the material. As mentioned in Section 42.3.1.2 melt temperature is
important and the shear created from back pressure is a critical component of the
melt process. High back pressure settings are often used as an aid to enhance color
mixing to compensate for an inadequate screw design, and the result of this pro-
cessing around items is a narrowed process window in other areas.
42.3.1.5 Molding Process: Residence Time
Residence time is defined as the time the plastic spends heated in the barrel of the
molding machine and is determined by two parameters:
1. Shot size of mold versus barrel capacity of molding machine
2. Cycle time
As both of the above numbers increase so does the residence time. Many materials
will degrade when held at melt temperatures for too long a time. The hotter the
melt temperature the less tolerance there is for excess residence time.
Ideally the shot size for a given mold should be between 25–75% of the rated barrel
capacity depending on the material being molded. When residence time goes up
the risk of degrading the material rises. The best option when this situation occurs
is to try to keep the melt temperature on the lower end of the recommended range,
and this is done through shear input and barrel temperature set points.
42.3.1.6 Molding Process: Screw Recovery
When the screw is recovering there is a large amount of energy input to the plastic
from the shear between it and the barrel. As the screw recovery rate increases the
amount of shear also increases, which can drive the melt temperature of the mate-