There are two groups of polymers: hygroscopic and non-hygroscopic. Each has with a set of processing characteristics and an attraction to moisture.
Both types of materials have the propensity to collect moisture. For any shop, it is important to have a general understanding of the water content to ensure the successful production of parts while minimizing any variation in the process.
Hygroscopic polymers include nylon, ABS, acrylic, PET, PBT, polyurethane, polycarbonate, and others. Each of these materials absorb moisture internally and release moisture through the air. Resins moved from storage to the molding machine often must be dried because of these properties. They must be processed quickly after drying.
Nylon 6 is one of the more hygroscopic resins and is capable of containing as much as 9% of its weight in water.
Non-hygroscopic polymers include PVC, polypropylene, polystyrene, polyethylene and others. They do not absorb moisture internally into the pellet. Dampness can be collected on the surface of pellets, and applying heat is an important part of removing that moisture.
Resin manufacturers in many instances dry the resin through the production process and then package the material in sealed plastic bags. Some suppliers package the material in polyethylene bags or in vacuum-sealed bags that can act as moisture barriers. However, the PE bags are porous and will allow some amount of water into the bag.
It is important to take the right steps on the shop floor to control moisture. When this isn’t done correctly, moisture is a key hurdle to achieving part quality.