The natural attraction between hygroscopic resins and water molecules causes them to take up and retain water as soon as they are exposed to humid ambient air. Mild to moderately hygroscopic materials include ABS, acetal, acrylic, polycarbonate, PBT, LCP, and some TPOs, TPEs, and TPUs. The most strongly hygroscopic and difficult-to-dry materials are bottle-grade PET and nylon. ETP requires to be in a dry state before usage, but being sensitive to moisture, it picks up moisture easily. ETP is required to be in bone dry state, failing which surface blemishes appear and mechanical properties get adversely affected. It should not have more than few PPMS of moisture at the time of processing, making a dryer essential while processing ETP. Commodity polymers are not sensitive to moisture and do not usually call for drying.

There are essentially five different types of dryers, few of which have sub types:
Hot-air dryers have long been the natural choice for resins that have no affinity to moisture, essentially consisting of a blower, a heater and a temperature control. They are the oldest, simplest and least costly dryers and use heated ambient air to pull moisture away from the resin. Non-hygroscopic resins such as polyolefins, polystyrene and PVC pick up only surface moisture from condensation when exposed to high humidity and changing temperatures, which is easily removed by a stream of heated ambient air. They remove surface moisture from non-hygroscopic resins and preheat resin prior to moulding and may also be suitable for drying some mildly hygroscopic resins. They can be used for several months of the year for some mildly hygroscopic materials such as certain TPEs, polyolefins whose additives don't retain moisture and even ABS for non-critical applications.

Desiccant dryers account for at least 80% of those in use and are available in three basic styles: twin-tower and carousel designs with two or more desiccant beds, or models with rotating "honeycomb" wheel desiccants. Desiccant dryers can achieve -40 degree F dewpoint, considered adequate for drying any hygroscopic resin. Drying residence time is typically 1 to 4 hr, depending on the type of resin.

The newer dryer choices that dry hygroscopic resins include straight compressed air dryers, compressed air units with a moisture removal membrane, and vacuum dryers. A variant of desiccant technology-the continuous desiccant rotor or "honeycomb" design and newer dryers using radiant heat are claimed to have greater drying capacity than standard hot-air units. Each is creating its own place in the market, and may not largely displace desiccant drying any time soon.

In recent years, desiccant dryers have made significant advances in compactness, drying controls and energy savings. State-of-the art features include closed-loop cooling after regeneration, which keeps atmospheric moisture away from the freshly regenerated desiccant bed, avoiding dewpoint spikes during bed changeover. Dewpoint monitoring with regeneration on demand reportedly cuts energy use by up to 25-30%.
Another way recent models save energy and prevent overdrying is by adjusting airflow and heating automatically based on sensing the temperature differential between incoming and outgoing air. Plant compressed air is introduced at the bottom of the dryer and expanded to atmospheric pressure, which immediately drops its dewpoint by 40° to 50° F. The air is then heated to raise its drying capacity further before it passes up through the resin hopper. This type of dryer has no desiccant, no moving parts, no regeneration heater, no valves, and no cooling coils. It requires minimal maintenance of changing air filters every six months. Dewpoint of the drying air depends on the temperature and humidity of the air coming into the plant air compressor. Many of these dryers can achieve a minimum of about +5 F dewpoint and are recommended for drying certain hygroscopic resins such as ABS, acrylic, acetal, and glass-filled nylon. But for drying materials that require a lower dewpoint-such as PET, PBT, or PC an add-on desiccant-bed kit or a moisture-removal membrane is available.
These units are suited to niche markets such as medical/pharmaceutical components and CD/DVD production. In addition to providing low dewpoint, the membrane can help compensate for limited compressor capacity or poor compressor performance that might otherwise require buying a desiccant dryer or upgrading the compressor.

Instead of blowing hot, dry air over the pellets to draw out the moisture, the newest type of dryer on the market uses vacuum to reduce the boiling point of water, pulling the water vapor from the resin pellets, using only 20% as much energy and 17% as much drying time as other dryers. They also permit much lower drying temperatures-e.g., 300 F for PET vs. the usual 350 F. Vacuum dryers can dry most hygroscopic resins.

Maguire offers semi-continuous units that carry out heating and vacuum drying in separate stations simultaneously. Matsui USA offers batch units that combine heating and vacuum drying in one station and are designed for micro-moulding. Because of the lower temperatures used, Maguire recommends vacuum dryers for heat-sensitive wood-filled plastics. Until recently, vacuum dryers were limited to a maximum of 50 -100 kg/hour. But with the recent introduction of Maguire's 500-kg/hour model, an LPD can be used as a central dryer for several processing machines, as is the case with other dryers.

A list of suppliers for various types of dryers is given below.

(Reference: Plastics Technology)