Page 98 - Injection Molding Advanced Troubleshooting Guide

9.2 Keys to Drying 83

9.2.1„ Temperature

Too low a temperature can come from the following problems:
ƒ Drying temperature set too low:
It is vital to follow the material suppliers recommended dryer temperature set-
tings. If the temperature is set too low the moisture will not be released by the
plastic resulting in lack of drying.
ƒ Incorrect location of dryer control thermocouple or RTD:
The temperature should be measured at the hopper inlet. If the temperature is
measured at the dryer outlet there will be a drop in temperature before the air
reaches the material. The temperature set point must account for this tempera-
ture drop if you are controlling based on dryer outlet rather than hopper inlet.
For improved efficiency use insulated hoses between the dryer outlet and the
hopper inlet as this will limit the amount of heat loss.
ƒ A burned-out heater can prevent a dryer from achieving the set process tempera-
ture. If the dryer is alarming for low temperature the heater may need to be
checked and possibly replaced.
Bear in mind that a higher drying temperature is not the route to faster drying! If a
material is dried at too high of a temperature it will become tacky or even melt,
which will result in what is often called a “hard ball”, which is when the material
pellets stick together and will not feed through the drying hopper. A “hard balled”
or “rocked” drying hopper means hours’ worth of difficult work trying to remove
the stuck plastic. This is an experience that creates a wonderful learning opportu-
nity, and the person who set the temperature too high should be the one who has
to remove the melted plastic; chances are they will not repeat this mistake!

9.2.2„ Dry Air

As the moisture is released from the material by heating the moisture must be
carried away. Moving dry air through the drying hopper will allow the moisture to
be carried away from the plastic. Without providing the dry air the water mole-
cules have nowhere to go and as a result the material will stay wet.
The dryness of air is measured by its dew point, the temperature at which moisture
in the air will condense. The dew point of the air should be between −20 and −40 °F
for effective drying. Common reasons for not reaching the required dew point
include:
ƒ Bad desiccant, either due to the age of the desiccant or desiccant that has been
contaminated with plastic fines or byproducts not filtered out of return air. As
desiccant pellets can and will go bad, use a dew point meter to determine if there