If you know anything about plastics processing, you know that it begins with resin pellets. There are many different types of plastic resins that offer a wide range of different physical and chemical characteristics. These differences in characteristics make every resin unique. The environment and seasonal changes also play a role making a resin behave differently from location to location, or season to season.
Plastics get wet?
Believe it or not, some plastic resins actually get wet; not just on the outside but deep down at the molecular level. Some plastic resins actually have polarity that attracts water molecules that bond with the polymer on a molecular level. These are known as hygroscopic resins. Plastic resins that don’t attract moisture – or essentially only get wet on the surface – are called non-hygroscopic resins.
Even small amounts of moisture in hygroscopic resins, detectable only with instruments, can lead to quality problems when plastic resins are molded into parts. Problems include:
- Cosmetic/appearance problems: Plastics that should be clear appear cloudy or have opaque streaks.
- Strength or performance problems: One example involves polyethylene terephthalate (PET), a plastic widely used for water bottles. If PET resin is thoroughly dried before it is molded into bottles, the bottles can be very thin-walled, yet very strong and leak-resistant. If not, they’ll break or leak easily.
How plastic resin gets dried
The two most popular types of resin dryers are hot-air dryers and desiccant dryers. Hot-air dryers pull ambient air from the surrounding environment, heat it with a heating element, and then pass it through a hopper filled with plastic resin. The hot air picks up the moisture from the resin pellets and carries the moisture out of the hopper. Hot air dryers are typically used only for non-hygroscopic materials that collect moisture only on the surface… like dew on your windshield or water droplets on a raincoat.
To dry hygroscopic resins that bind water at a molecular level, a hot-air dryer probably won’t be enough. For these materials, a desiccant
dryer is needed. Desiccant dryers are “the gold standard” for preparing materials. They also use hot air like hot-air dryers, but they have another important ingredient: desiccant.
Desiccants are materials that adsorb, or take on, moisture. A good example is a packet placed in shoe boxes or electronic products to collect moisture and prevent damage. We have all seen those “DO NOT EAT” little packets in products we’ve purchased. In desiccant dryers made by Conair, desiccant is formed into the shape of a continuously rotating wheel. A closed loop of air moves from the dryer, through the hopper to collect moisture, then back through the desiccant wheel, which removes the moisture, leaving the air stream dry and ready to recirculate. The wheel rotates constantly so that, while one part of it is removing moisture from process air, another section is being heated (or regenerated) to remove that moisture from the desiccant. Thus, the desiccant wheel acts like a kind of sponge, constantly taking on and then getting rid of water through regeneration. A closed-cycle desiccant dryer provides an airflow that has a very low moisture content so it provides far better, more consistent drying performance for hard-to dry hygroscopic resins – regardless of location or season.
To the right is a schematic of the drying process. Dry air, marked in red (1), leaves the desiccant wheel and is heated before being passed into the base of the resin hopper. As it flows upward through the hopper, the air absorbs water before circulating out of the hopper into the return-air circuit, where it is cooled and passed through the desiccant wheel, where it gives up its moisture. A separate circuit (2) heats ambient air and passes it through the wheel to regenerate the desiccant. Finally (3), the regenerated desiccant must be cooled (so it can absorb moisture again). This is accomplished using cooled air returning from the drying hopper.
Once the plastic resin is properly dried, it can be molded or extruded into high-quality parts.
Hopefully this basic explanation was an education that helps you better understand the importance of properly drying resin prior to processing, OR was a refresher on why your process does or does not require desiccant drying.
Every application is unique – a resin that never needs dried in Arizona may need dried all year or part of the year if used in Michigan. If you’re not sure if your resin needs to be dried, refer to the information from the resin supplier (typically printed on the label of the resin container). As always, feel free to contact the experts at Conair with all your questions on application/processing requirements to ensure you have the right equipment to keep you making quality parts.