Page 252 - Injection Molding Advanced Troubleshooting Guide

26.3 Dimensions Troubleshooting 243

26.3.1.4„ Molding Process: Fill Only Condition
When running with a Decoupled™ process the amount of plastic injected during
first stage will impact how the part is ultimately packed out. Second-stage pressure
should be removed from the process to verify that the fill only weight is accurate to
the required 95–98% specified on the setup sheet. Part of STOP troubleshooting is
to Systematically work through a problem and running a fill only shot is a good
place to start.
If the fill only shot is light and a lack of plastic was injected during first stage, the
resulting cavity pressure will tend to be lower, which will in turn lead to a smaller
part. If the fill only shot is light then the transfer position should be adjusted to
provide a fill only shot that matches the specified weight.
If the fill only shot is heavy there is excessive plastic being injected during first
stage. If the shot is transferred beyond 99% fill only weight then the part is actually
seeing pack during fill, which can lead to increased part size. Again verify that the
fill only shot matches the specified value and is approximately 95–98% full.

26.3.1.5„ Molding Process: Melt Temperature
Melt temperature is a factor that can impact shrink in both directions.

A higher melt temperature will decrease the pressure differential from the gate
across the cavity thereby increasing the ability of the part to be packed out. On the
other hand, a hotter melt will lead to higher shrinkage due to cooling, especially by
post-mold shrinkage.

On a process that has been running well it is important to verify that the melt
temperature matches the documented process. If the baseline melt temperature is
unknown or the process has never produced dimensionally acceptable parts, then
it may be necessary to adjust the melt temperature to determine the impact on the
dimensions. Remember that melt temperature changes can make the dimensions
go either direction.
Also increased melt temperature may be significant enough to impact gate seal
time. Verify that the gate seal time is adequate when changing melt temperature,
fill speed, mold temperatures, or second-stage pressure.
Figure 26.3 shows data from an RJG eDART process monitoring system that shows
one cycle that had an extended rotate time due to a material feed issue. The follow-
ing shot had an increase in cavity pressure that led to a dimensionally inaccurate
part.