Page 417 - Injection Molding Advanced Troubleshooting Guide

416 42 Splay

Some additives such as talc can result in non-hygroscopic materials needing to be
dried.
Material suppliers will provide a recommended drying temperature and time for
hygroscopic materials. It is critical to follow these drying specifications to ensure
that the material is dry enough to successfully process.
Successful drying requires the following:
ƒ Temperature
ƒ Dry air
ƒ Air flow
ƒ Time for the material to dry under the drying conditions
To ensure adequate drying it is critical to have all four of these conditions met.
Four hours of drying time is meaningless if the temperature requirement is not
met. A typical desiccant dryer will provide an air dew point of −40 °F. Keep in mind
that the dryer temperature will help release the moisture from the pellets, the low
dew point will allow the air to pick up the moisture, the air flow exposes more of
the pellets to the warm/dry air, and the time is required to give the moisture a
chance to leave the pellets and provide an adequate moisture content. For further
details on drying see Chapter 9.
Material moisture content can be verified prior to molding by utilizing a moisture
analyzer. The common types of moisture analyzers utilize two different techniques:
1. Carl Fischer titration method
This technique 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 technique 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.
It is important to understand that just because a material has been dried to an
adequate level does not mean it will stay there. If you expose dried material to am-
bient air that has humidity in it the material will begin to absorb the moisture like
a sponge. Many materials will be too wet to effectively process if exposed to ambi-
ent humidity for longer than 15 minutes. To avoid this creating splay problems,