Material Handling, continued

In the previous tip the mixing of materials and or additives, was discussed.  Now let us look at the drying process for materials that may need to be dried.
The following are the most common types of dryers out in the work place.
  1) Hot Air
  2) Refrigeration
  3) Desiccant
  4) Other
The real difference between these systems is the Dew Point that they are capable of achieving, which is the key to drying of materials.
Hot Air:
It heats the air but does not reduce the moisture content of the air that it heats. This type of dryer is typically used to remove surface moisture.

Refrigeration:
This type of dryer can reduce the moisture content of the air. It can lower the dew point to 32° F, which is approximately 6,000-PPM moisture.

Desiccant:
The ideal for hygroscopic materials, these are a closed loop system and can achieve a dew point of -40, which equates to 125-PPM moisture. This type of dryer does demand maintenance attention.

Other:
This would include the molecular sieve type dryers, the new ones introduced at NPE, that use other media to remove the moisture from the air stream. Further, the use of vacuum, and or the venturi effect dryer.

Notes for Drying
Dryers are used to remove moisture from plastics materials prior to processing. These yield a part that is both cosmetic and physically correct since moisture in plastics while processing may alter both. The physical process of drying is achieved by creating a vapor pressure differential between the moisture in the pellet and the atmosphere around it. In short, heating the pellet and lowering the dew point of the air around it, than using the airflow to swept away the moisture.

There are four factors involved in drying materials.
1) Air Flow- CFM (cubic feet per minute) should equal the throughput of the material in pounds per hour processed on the machine.

2) Dew Point- this is the dryness of the air and refers to the temperature at which moisture forms, i.e. the lower the better.

3)  Air Temperature- the temperature of the air as measured at the inlet to the hopper and specified by the manufacture per material.

4)  Residence time- the time the resin must be at temperature to dry properly.

The above give the basic for the dryers, type and how they function.  Please consult the resin suppliers data per the requirements and specifications for the material you are processing.

Thanks again for the time, to be continued.

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Distribution