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32.3 Glass Fibers on Surface Troubleshooting 315
Case Study: Machine Capability Causing Glassy Surface
In this case the process for a 33% glass-filled part was developed with a
different machine than its home production machine. Both machines had a
controller that had a velocity set point of 6 in/sec. The original machine was
able to achieve the 6 in/sec but the home machine could only achieve just
over 5 in/sec. At the 6 in/sec fill velocity the parts looked cosmetically
good but at 5 in/sec the parts had a glassy surface. Velocity investigations
showed that the home machine just flattened out above 5 in/sec; no matter
what number was entered the machine did not get there.
This speaks to processing to the lowest process capability of any machine
that the mold will run in. If the process is developed in a machine that is
more capable the process cannot be repeated in the less capable machine.
This is another good reason to test molding machines (see Chapter 8 on
machine performance).
32.3.3.2 Machine: Machine Performance
See Chapter 8 on machine performance.
32.3.4 Glass Fibers on Surface Troubleshooting Material Issues
Material concerns may include:
Moisture content
Viscosity increase
Glass content
32.3.4.1 Material: Moisture Content
There will be cases where the moisture content of a material has a large impact on
the surface quality of the molded part relative to glass fibers. This is especially a
problem when dealing with nylons or polyesters because both of these materials
experience high levels of hydrolysis when processed with moisture present. If a
material is run on the higher side of moisture content the viscosity of the material
will be reduced, which translates to an easier-flowing material that will help bury
the glass fibers better.
An area to be especially aware of is when processes are developed in summer
weather conditions versus winter conditions. During the summer high humidity
will tend to cause moisture content to run higher than it will in the winter. If a ny-
lon process is developed during the summer months with moisture on the high
end of the specification (0.2%, for example) the viscosity of the material is going to
