Page 99 - Injection Molding Advanced Troubleshooting Guide

84 9 Drying

are issues achieving a low enough dew point. Some dryers have dew point meters
built into them or portable units can be utilized to monitor a dryer.
ƒ On dryers with multiple desiccant beds you must verify the dew point from each
of the beds, and you may find one bad and one good desiccant bed. This means
that you will need to verify the dew point over a time period. One way to verify
the dew point over time is to purchase a chart recorder to connect to the dew
point meter. Another method is to connect the dew point meter to a data monitor-
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ing system such as RJG eDART .
ƒ Return air too hot:
For optimal performance of a desiccant the return air should be between 120 and
150 °F. If drying set points are above 180 °F the return air will probably be too
high for optimal performance. When drying at temperatures above 180 °F an
after cooler should be used to cool the return air to below 150 °F. Also keep in
mind that return air hoses should not be insulated as this will allow the return
air to cool as it travels back to the dryer.
ƒ Burned out regeneration heaters will not provide enough heat to remove the ab-
sorbed water from the desiccant. If the regeneration heaters are not working you
will see a high dew point.
ƒ Make sure that there are no leaking seals or holes in the drying hoses that would
allow moist ambient air to be introduced into the drying hopper.

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How to Use an RJG eDART as a Dryer Monitor
Connect the output from the dew point meter to a 0–10 V analog input mod-
ule. You can also use a dew point meter from RJG that will connect directly to
the eDART without the analog input module.
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Anywhere that you wish to collect temperature data from can have a thermo-
couple installed, and the thermocouples can then be connected to an RJG
Quad Temp Module. You could measure hopper inlet temperature, dryer out-
let temperature, and maybe even regeneration temperature.
With the above information you can establish full time monitoring with a per-
manent eDART or setup a portable dryer qualification methodology where
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you monitor the dryer for 24 hours to determine how well it is working.
Checking Desiccant
To verify if a desiccant is working conduct the following experiment: Dry
desiccant can be taken from a dryer desiccant canister or it can be dried in
an oven for 2 hours at 400 °F (placed in an appropriate container). Allow the
desiccant to cool to room temperature and then pour some water into the
container with the desiccant. If the desiccant is active there will be a violent
exothermic reaction (use caution!) as the desiccant absorbs the water, steam
will be observed, and a significant temperature rise will be detected (> 20 °F).
If the desiccant is not active there will be no reaction or temperature rise.