Page 250 - Injection Molding Advanced Troubleshooting Guide

26.3 Dimensions Troubleshooting 241

It is key to understand that the actual pressure applied in the cavity is what drives
dimensions, not the set point on the molding machine. In many cases the best
available data a processor has to work with is the set pressure of the machine, but
an improved tool that gives a much better understanding of what is occurring
inside the cavity is a cavity pressure transducer (see Figure 26.1).

Figure 26.1 RJG cavity pressure transducer

26.3.1.2„ Molding Process: Second-Stage Time
It is critical to determine if a mold is going to run with or without gate seal. If gate
seal is required it is critical to have enough time on second stage to ensure that
gate seal will be consistent. If a mold is running on the edge of gate seal, variation
will be noted in the quality of the parts especially dimensionally.
Always conduct a gate seal study during process development and establish
whether the mold will run with or without gate seal. There are different ways to
establish what second-stage time should be set at: some people will add 1 second to
the gate seal time or add 10% to the gate seal time. Determine a method and make
sure that it is followed. If the gate seals at 8 seconds, setting second-stage time at 8
may lead to times when the gate is sealed or unsealed; therefore, setting sec-
ond-stage time at 9 seconds will make sure that the gate is sealed. See Figure 26.2
for an example of a gate seal study form; note in this example that the gate seals at