When molding or extruding polymer resins, proper drying of those materials is essential to avoid errors in processing and defects in parts.
Four key concepts are essential for proper resin drying: air flow, temperature, dew point and drying time.
Airflow
Air transfers heat and removes moisture from the resin. Air volume must be sufficient to maintain even distribution through the cross-sectional area of the drying hopper.
With hygroscopic (water-absorbing) resins, low dew point heated air is directed through the material to disengage moisture molecules from the polymer chains. When that moisture moves to the surface, proper airflow carries it away.
Excess air flow is likely to occur if the dryer is oversized for the process and is connected to a smaller drying hopper. In some cases, material will not properly flow into the process machine.
When cool resin enters the drying hopper, that material acts as a heat sink that cools the air returning to the dryer. Cool return air maintains optimal operating temperature of the desiccant. In some cases, if the return air temperature is too high air aftercoolers are used to lower the temperature of the return air.
Recommended airflow can differ depending on the manufacturer of your dryer.
Problems that can arise from excess air flow include:
- High return air temperature
- Reduced desiccant capacity
- Higher dew point
- Blower damage
- Material flow issues
Drying temperature
The drying temperature is a very important consideration in all drying situations.
Heat is manifested in the movement of molecules. As polymer temperature increases, the molecules move more vigorously.
Heating the resin reduces the forces that bind water molecules to polymer chains. Above a certain temperature, water molecules become free to move about.
Temperature controls the rate of drying. For instance, ABS with about 0.35% moisture in an environment with a dew point of 0º F dries to about 0.1% moisture in:
- About 45 minutes at 208º F
- Two hours at 180º F
- Five hours at 160º F
It is always good practice to refer to the material supplier’s technical data for specific information related to your process and your material properties.
