Page 327 - Injection Molding Advanced Troubleshooting Guide

322 33 Gloss Variation

33.3.1.2„ Molding Process: Second-Stage Time
Second-stage time that is not long enough to ensure gate seal can lead to a glossy
area near the gate. If the plastic is not retained in the cavity until gate seal there
will be a depressurization of the plastic near the gate. The release of plastic near
the gate will create a localized low-pressure area that will typically have a different
gloss level than the rest of the part.
Verify that the second-stage time is set correctly per the documented process. A
gate seal study should be conducted during process development to determine the
appropriate second-stage time. If a gate seal study was not conducted take the time
to complete a gate seal study to understand the correct time required for establish-
ing gate seal.

33.3.1.3„ Molding Process: Fill Velocity
Generally, a faster fill velocity will provide less pressure drop and yield better sur-
face replication. Faster fill rates tend to provide a glossier surface whereas surfaces
obtained with slow fill rates tend to be duller. Faster fill velocities also lead to a part
that will pack out more uniformly.
Verify that the fill time is correct according to the documented process. Fill velocity
settings may not match, but fill time should. If fill time and fill only weight repli-
cate the desired process that ensures that the same volumetric flow rate occurred.

33.3.1.4„ Molding Process: Fill Only Weight
If the fill only (95–98%) shot is too light due to an early transfer there may be a
gloss difference that forms in a sharp line where transfer occurred. An early trans-
fer can create a very distinct area at end of fill that looks different.
Check the fill only weight to ensure that the part is filled to 95–98% during first
stage. If the shot is short the transfer position should be adjusted to provide an
adequate fill shot. Be aware that filling too short can lead to a lag in second-stage
pressurization. Figure 33.2 shows process monitoring data on a process with an
early transfer. Note how the screw travel curve moves significantly after transfer.
As the screw continues to move forward the machine is trying to reach the set sec-
ond-stage pressure but it takes several seconds. Early transfer creates many issues
but gloss variation is an often-overlooked problem.