Page 101 - Injection Molding Advanced Troubleshooting Guide
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86 9 Drying
that impacts material residence time in the hopper. This study can be replicated
by filling a hopper and adding a layer of another color of pellets to the top of the
dryer as tracer pellets. Start loading the material from the hopper and note how
long it takes to see the alternate colored pellets.
Bear in mind that the material supplier recommended drying times may not
account for material that has become very wet due to sitting in open containers
in a high-humidity environment. Additional time may be required to ensure that
the material is dried to an adequate level.
Note that too much time spent drying can have a negative impact on some materi-
als. What is normally considered an over-drying problem is a lack of moisture
content in a material like nylon that in turn leads to a higher material viscosity
(the water was acting as a plasticizer). When nylon is dried to very low levels of
moisture content the viscosity change may actually lead to a pressure-limited con-
dition that impacts the ability to fill the mold. There are cases when materials are
dried much longer than the recommended drying time so that oxidation of the
material can occur, which can lead to a breakdown of the physical properties of the
material. Always try to avoid leaving material at drying temperatures for extended
periods of time; dryer set points can be reduced to maintain a material at a dried
state without risking over-drying problems.
9.3 Moisture Analysis
Material moisture content can be verified prior to molding by utilizing a moisture
analyzer. There are two common types of moisture analyzers which have very dif-
ferent techniques:
1. Carl Fischer titration method:
This method relies on precise analytical equipment and requires chemicals to
run. This test will provide a true moisture reading from a plastic.
2. Loss on weight method:
This method uses a very precise scale to weigh the material at the start of the
test. The material will be heated forcing it to give up moisture and the moisture
analyzer will calculate moisture percentage based on this weight loss. Note that
this method is more convenient but is less accurate because it will also measure
other things that leave the material including residual monomers or low molec-
ular weight additives.
Every plant must evaluate their choice of moisture analyzer. Carl Fischer titration
will provide more accurate results but the cost of that is measured by the fact
that it is a lab-style piece of equipment that takes more skill to run and maintain.
