Page 105 - Injection Molding Advanced Troubleshooting Guide
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90 9 Drying
9.5 Material Concerns
As mentioned in Section 9.1 the type of material has a dramatic impact on the dry-
ing process. Every material has a moisture absorption rate, a maximum moisture
content, and a specific required temperature to achieve optimum drying.
It is important to understand that the additives in a non-hygroscopic material can
absorb moisture and in turn require drying prior to molding. Many mineral-filled
polypropylenes require 2 hours of drying to remove the surface moisture from the
filler content.
For materials that are extremely moisture sensitive the time that the material can
be exposed to humid air can be very short. TPU is one of the worst, and studies
have shown that within 5 minutes of leaving a drying hopper TPU can actually
absorb enough moisture to make it too wet to mold successfully. Take care to mini-
mize the time materials spend outside the drying hopper.
Do not forget that splay is not the only result of molding wet material. Some mate-
rials will experience hydrolysis when molded with excess moisture content, which
will lead to reduction of physical properties that may cause part failure. Also
understand that some materials such as TPU or polyesters will become sticky and
weakened when molded wet. When molding wet TPU or polyesters the first sign of
problems may be a part stuck to the mold, or cracking parts during ejection from
the mold.
It is also vital to understand that when a plastic suffers from hydrolysis the molec-
ular weight of the material will be reduced. A reduction in molecular weight will
lead to reduced physical properties that may compromise the performance of the
molded part. Beware that hydrolysis is not a reversible process: re-drying a mate-
rial that has been processed wet will not put the molecular chains back together.
Once a material has undergone hydrolysis from being processed wet it should be
scrapped or otherwise it can cause processing and performance problems.
References
[1] Bozzelli J., “Bozzelli’s Guide To Specifying A Dryer”, Plastics Technology, April 2014.
