Most plastic resins, such as PA (Nylon), PC and PET, are hygroscopic materials. They adsorb moisture from humid ambient air and give moisture back to dry air. Every type of resin can hold a specific amount of moisture between its molecular chains. Additional amounts of moisture can be condensed on the surface of the pellets (surface moisture). Non-hygroscopic resins, such as PE, PP and PVC, do not adsorb any moisture, but they still can have surface moisture. adsorbed moisture in hygroscopic resins and surface moisture in non-hygroscopic resins are known to cause defects in molded plastics and they might lead to a complete production stop.

Heating the resins, all the way through to the center of the pellets, to the highest possible temperature is very important to achieve good drying results.

The dew point of the air is the temperature at which the air would be saturated with moisture. Lowering the dew point of the air under -20 °C [-4 °F] in resin drying process is not necessary.

The standard compressed air quality has a dew point of 5 °C [41 °F] at a pressure of 7 bar [100 psi] which equals atmospheric air with a dew point of -20 °C [-4 °F] at sea level.

There are two ways to remove moisture from or reduce the amount of moisture in plastic resins:

• Increasing the pressure of the moisture inside the capillaries to drive the moisture out and then carry the moisture away by air (air drying) or
• Dragging the moisture out of the capillaries by setting the resin under low pressure (vacuum drying).

Hot air dryers are very simple devices. The resin is loaded into a drying hopper where heated air is blown at the bottom of the hopper. The hot air rises between the resins heating the pellets and carrying the released moisture before it leaves the hopper at the top. The humidity of the ambient air used to dry the resin plays a big role in the drying results. The dryer performs better when the ambient air is dry but the ambient air humidity is never constant and very seldom low enough. The hot air dryer is not suitable for drying hygroscopic resins.

The desiccant dryer is more complicated. The air is circulated between the drying hopper and a dryer in a closed loop to insure low air humidity at all times. The desiccant is regenerated periodically by hot air and the moisture is released from the dryer to ambient air. The moisture is removed from a hygroscopic material (the resin) and then deposited in a hygroscopic material (the desiccant) before it is released to the ambient air. This leads to many moving parts, high energy consumption, high maintenance and more floor space requirements.

The compressed air dryer is the ideal continuous resin dryer. It combines the advantages of the hot air dryer and the desiccant dryer and avoids all the disadvantages of air dryers. The Compressed Air Resin Dryer (CARD) preheats the material with hot air and then uses a small amount of compressed air to remove the moisture from the resin. The compressed air is decompressed before it is heated and released at the bottom of the drying hopper. Standard quality of compressed air with a dew point of 5 °C [41 °F] or lower at 7 bar [100 psi] or higher is very sufficient for the job. The central air compressor in plastics processing facilities supplies air to multiple dryers saving expensive floor space in the manufacturing area and reducing the maintenance requirements. The energy consumption of the CARD is much lower than that of a desiccant dyer. In some applications, as in blow molding, the energy consumption can be reduced to nearly ZERO. The exhaust air from the process is used for drying and the heat can be recovered from the air compressor.

Vacuum drying is the best way to dry resins in a batch system but continuous vacuum dryers with multiple vacuum chambers in a carousel configuration were proven to be the wrong way to go. Too many moving parts with very high maintenance requirements are necessary to make the system operates in continuous mode. The chambers are difficult to clean when changing to a different type of resin or different color.