Page 210 - Injection Molding Advanced Troubleshooting Guide

22.3 Color Swirls Troubleshooting 201

22.3.1.1„ Molding Process: Back Pressure
Back pressure is a critical setting for applying shear to the plastic during screw
recovery. The higher the back pressure is set the more mechanical work the screw
will impart into the plastic during rotation. If back pressure is too low the melting
material and color concentrate will not achieve adequate mixing, which will in
turn lead to color swirls.
A common mistake relating to back pressure can arise during purging. Because
the screw will often not recover with full back pressure process technicians will
sometimes reduce or completely turn off back pressure when purging a machine,
but if the back pressure is not returned to the set point the machine will often pro-
duce a number of defects, especially color swirls. Rather than dropping back pres-
sure to purge, a way to combat this issue is to use a combination of screw rotate
and decompress to build the shot to the required shot size. If the technicians con-
sistently avoid dropping back pressure this failure mode will be eliminated.
When developing a process, it is critical to remember that higher back pressures
will impart more mechanical energy and lead to a better mixing of material. If color
mixing problems are being encountered higher back pressure will normally lead to
significant improvements. Please remember though that back pressure is not a
setting that should be maxed out for every process, as higher back pressure will
potentially lead to overheated and degraded material as well as increased screw
recovery time, which can cause cycle time increases.

22.3.1.2„ Molding Process: Screw Recovery Speed

The longer the screw is allowed to rotate and mix material the better the color mix
will be. If screw recovery is excessively fast, slowing down the recovery speed will
typically provide a better mix. It is important to not slow down the screw speed to
a point where the cycle time becomes delayed; the normal recommendation is that
the screw should recover 2–3 seconds prior to mold open.
If a long cooling time is required for a part, screw rotate delay may be required to
ensure that reasonable screw recovery settings can be used. If using a very slow
recovery rate the material will not receive adequate shear energy to provide a uni-
form melt. For back pressure, like most settings on a molding machine, there is no
generic set point that will work for every mold and process, so the proper set point
must be established and maintained.

22.3.1.3„ Molding Process: Melt Temperature
If material is not run at the proper melt temperature one potential outcome is poor
mixing of color concentrate and nonuniform melt. Whether using a melt probe or
an IR temperature gun it is critical to know the melt temperature that a process
is providing. Melt temperature should be within the recommended range for the