After the mold is set, some processing of the purchased materials is required before injection molding, mainly drying and dehumidifying (water absorption).

The key process in injection molding is to dry the plastic before injection.

The reason plastics have to be dry is that they contain water, which, if not dried, can affect the quality of the finished product.

Polymers such as polycarbonate, PC, PBT, PET, ABS, acrylic, polyurethane must be thoroughly dried. They are collectively called hygroscopic resins – hygroscopic substances are substances that attract water easily from their surroundings by absorption or adsorption.

If these materials are used and the water is not removed prior to injection, the polymer will reduce the strength of the product produced during the injection. So before we use these materials, we dehumidify them to make sure that they do not contain any moisture when they are used for injection.

Non-hygroscopic resins (such as polyolefin, polystyrene, and PVC) that do not absorb water can simply be removed by a heated ambient airflow, whereas hygroscopic plastics require the use of a desiccant dryer, which uses compressed air and a desiccant material that draws water from the resin.

Some commonly used materials and their characteristics are listed below: