Page 293 - Injection Molding Advanced Troubleshooting Guide
P. 293
286 30 Flash
30.3.1.4 Molding Process: Melt Temperature
If the melt temperature of the plastic is running high the reduced viscosity of the
plastic may allow the material to flash easier. Low-viscosity material will flow
through thin cross sections easier even when the thin cross section is a gap in the
parting line.
Low melt temperature will require higher pressures to fill the mold, possibly gen-
erating enough cavity pressure to force the clamp open.
Check melt temperature for comparison to the documented setup. If the melt tem-
perature does not match the documented process then the following settings
should be verified:
Barrel temperature set points
Back pressure
Screw recovery speed
30.3.1.5 Molding Process: Velocity to Pressure Transfer
The way the machine transfers from first-stage velocity control to second-stage
pressure control is an often overlooked cause of flash. If the machine spikes over
the set pressure during transfer a resulting spike in cavity pressure will occur.
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This spike in pressure can lead to flash. depicts an RJG eDART graph showing a
poor pressure response. The sudden spike in pressure can easily flash a mold.
Case Study: Velocity to Pressure Flash
In this case study the part was being molded out of a mineral-filled TPO. The
velocity to pressure transfer was causing a spike at the start of second-
stage pressure, which over-pressurized the cavity resulting in parting line
flash. The hold volume percent was adjusted and the spike at the velocity to
pressure transfer point was almost completely eliminated. The resultant
parts had no flash.
