Pellets of plastic resin generally absorb the moisture in the air to some extent.
If the amount of moisture absorbed is high, the plastic resin can undergo hydrolysis in the process of being melted and kneaded inside the cylinder of the injection molding machine (there are also plastics that undergo chemical dissociation with water as the trigger), or silver-colored lines (silver streaks) appear on the surface of the molded item at the time of injection molding, or defects such as air bubbles, insufficient glossiness, transfer defects, etc. can occur.
In view of this, it is necessary that moisture in the pellets of the molding material is removed beforehand by putting them through a drying unit.
If this preliminary drying is not done properly, it can cause variations in the fluidity or reduction in the material properties, and molding defects.

The following are the main types of drying units used for this purpose.

(1) Hot air dryer
The hopper dryer and the box type drying oven are typical of this type dryer. This is a method in which the moisture content is evaporated by blowing hot air on the pellets.
Although this is very common and simple drying method, it is not suitable for thoroughly removing the moisture content.

(2) Dehumidified hot air dryer
This is the method in which the moisture content in the pellets is evaporated by blowing a hot air the moisture content in which has been removed in advance.
Since the hot air used for drying can be used again after dehumidifying it again, heat loss is low, and it is possible to carry out rational drying.
This method is suitable for drying PBT, etc.

(3) Reduced pressure heat transfer type dryer
This is the method of evaporating the moisture content in the pellets by heat transfer in a reduced pressure environment. Drying becomes possible at a low temperature, and it is possible to prevent oxidization of the plastic and to reduce the effect of additives inside the pellet.
In addition, this is also a method in which heat loss is small.
This type of dryer is attracting a lot of attention as the drying unit of the future.

* Reference: "An Encyclopedia of Injection Molding", 'Peripheral and Auxiliary Equipment', p. 214, (Hideki Kubo, Industrial Research Center of Japan, (2002)) (in Japanese)