Page 291 - Injection Molding Advanced Troubleshooting Guide

284 30 Flash

Verify that the set pressure matches the established process. Also verify that the
actual pressure exerted by the machine is equal to what is specified in the docu-
mented process.

30.3.1.2„ Molding Process: Heavy Fill Only Weight
®
Using RJG Decoupled Molding techniques requires transferring from first-stage
fill to second-stage pressure at a 95–98% full cavity. This is referred to as a fill only
shot. Fill only weight should be documented and matched every time the mold is
run. If the fill only weight is heavier than normal, i. e. filling further than 98%,
there will be a good likelihood that the mold will flash. Filling a mold to 100% will
cause a very rapid rise in cavity pressure that can overcome the clamp force result-
ing in flash. shows a simple plaque that was flashed from transferring too late.

Figure 30.3 Part flashed from transferring too late

To check the fill only shot, set second-stage pressure to zero or minimum allowed
on the machine controller. Some controllers will also need to have second-stage
volume, speed, or velocity turned off to completely stop any impact from second-stage
pressurization. The resulting short shot should be weighed and compared to the
documented fill only weight. If the part is too heavy the transfer position should be
increased to put less plastic into the cavity during first-stage fill. Also examine the
part at this point for flash because a fill only shot with flash is a good indicator of a
tooling issue (see Section 30.3.2).