Page 450 - Injection Molding Advanced Troubleshooting Guide
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450 44 Stringers
44.3.4 Stringers Troubleshooting Material Issues
The main causes of material-related stringing are:
Moisture content
Volatiles
44.3.4.1 Material: Moisture Content
If a material is molded with excess moisture the viscosity of the material will drop
and the gas pressure from the volatiles will tend to push the material from the
nozzle or tip. Gas generation in the material can lead to high residual pressures on
the shot, which will tend to cause the material to drool or string.
Check the moisture content on the material. If the material is not adequately dried
the process will need to be shut down to allow for proper drying.
To see more specifics on drying refer to Chapter 9.
44.3.4.2 Material: Volatiles
Many things in the material can result in volatiles in the melt stream such as:
Low molecular weight components
Lubricants
Degraded regrind
Any of the above items might cause gas to evolve from the material. These volatiles
are gaseous in the melt stream and just like with moisture can create a residual
pressure on the shot in the barrel. The gas pressure can force the material to drool
or string.
If the defects are isolated to a specific lot of material, engage the material supplier
to determine possible solutions. If running regrind try running with 100% virgin
material to see if the problem is improved. If the issue is linked to regrind check for
fines in the regrind that may be melting quickly and degrading.
