Page 303 - Injection Molding Advanced Troubleshooting Guide

296 30 Flash

30.3.4„ Flash Troubleshooting Material Issues

Material factors to consider are:
ƒ Viscosity decrease
ƒ Moisture content
ƒ Regrind

30.3.4.1„ Material: Viscosity Decrease
If the viscosity of a material decreases it will flow easier. The result of this
easier-flowing material is that it will fill through thinner gaps, which may result in
flash.
All materials will have variation over time. The normal measurement that is avail-
able to detect variation in material viscosity is the melt flow index (MFI). Unfor-
tunately, MFI only gives results for very low shear rate (grams/10 minutes), which
may not translate well into the molding environment. Some materials will show
good correlation between MFI and flow in the mold. Large increases in MFI may
indicate a shift in material viscosity that leads to a risk of flash.
The various additives in a material can also impact its viscosity. For example, a
decrease in glass percentage could lead to a material that flows easier and poten-
tially leads to flash.
If the flash started immediately after changing to a new box or lot of material, the
material is the first place to start investigating. Switch to another lot if one is avail-
able to see if the problem persists. If a lot change impacts the issue the root cause
may very well be a material issue.

30.3.4.2„ Material: Moisture Content
Materials that experience hydrolysis will also experience viscosity shifts as the
molecular weight is reduced during hydrolysis. Because hydrolysis is breaking
down the chain length of the molecules of the material the result is a material that
will flow easier.
When processing wet material, the risk for flash is much increased. This easi-
er-flowing material will flash through parting line gaps that the material typically
does not. Always when starting a molding machine and purging out material watch
how runny and thin the material is as it leaves the nozzle. Wet degraded material
will be very watery and will not maintain any shape of the melt stream in the
purge puddle. Also wet material will tend to have a lot of bubbles from the extra
gas content. If the melt does not look normal it is a good idea to check out the mois-
ture content before shooting the material in the mold.
For more information on drying see Chapter 9.