Page 322 - Injection Molding Advanced Troubleshooting Guide

316 32 Glass Fibers on Surface

be lower due to hydrolysis degradation of the material. This material will give a
much better looking surface than will material at the other end of the spectrum
(0.01% moisture content, for example). Keep this in mind when developing pro-
cesses in the summer. It is often a good idea to allow for some extra drying time to
help drive down the moisture content.
One of the challenges in injection molding is dealing with environmental factors. It
is nice to have a climate-controlled molding operation but many shops do not have
this luxury. Because summer climate versus winter climate can have a large im-
pact on a molding process it puts greater importance on doing an adequate job
during process development to try to accommodate potential seasonal variation.
This accounts for why there is a large range of drying times for nylon: saturated
summer material may require 12 hours to drive the moisture percentage to an
equivalent of what 2 hours does in the dry winter months.
Further information on drying can be found in Chapter 9.

32.3.4.2„ Material: Viscosity Increase
If the incoming material is varying in viscosity from lot to lot the ability of the
material to flow well may be compromised. If running a material that is considered
to be “wide spec” there will be a much greater likelihood that the material will ex-
perience enough of a shift in viscosity that the process will be impacted. “Wide
spec” material by its very nature is going to have greater variation from lot to lot.
If the issue seems to correspond with a lot change in material, it is a good idea to
try an additional lot of material to help determine if there is truly a problem. Occa-
sionally there will be processes that have very good correlation in molding window
with melt flow index (MFI).

32.3.4.3„ Material: Glass Content

If the incoming material is experiencing variation in glass content there may be
variations in the appearance of the part surface. As the percent content of glass
increases the likelihood of seeing glass fibers on the part surface will also increase.
Also if the glass percentage increases so will the viscosity of the material, which
may lead to concerns discussed in Section 32.3.4.2.
There are other factors involved in the use of glass such as the sizing agent that
allows the glass to be wetted out by the polymer. These sizing agents often have a
silane component that helps the plastic to bond with the glass fiber. If the glass
fibers are not being encapsulated or wetted out by the polymer they will tend to be
more visible on the part surface.