Hopper dryers, also called hot air dryers, are the ideal choice for removing moisture from the surface of pellets of non-hygroscopic resin prior to melting. Manufacturers who use these products know that this is an ongoing struggle and that failure to properly dry pellets will result in an unacceptable level of moisture and possibly cause flaws in the finished product.
Non-hygroscopic resins are relatively easy to dry because water molecules only collect on the surface rather than penetrating into the pellet. For these substances, sufficient drying can usually be achieved with a hopper dryer in which the pellets are placed in a hopper and dried with warm air.
This is the purpose for which hopper dryers are generally used, but this is not to say that they are completely ineffective for use with hygroscopic materials. If hygroscopic substances are reground so that their surface area is sufficiently increased, a hopper dryer can be used successfully to reduce water content.
Ambient humidity is another factor that influences the efficiency of hot air dryers, so better results will be achieved if they are used in conditions of constant low humidity. In locations where humidity is very low, hot air dryers can be used to dry hygroscopic substances with some success.
A hopper dryer can be used to great advantage in situations where materials have been stored in large containers for long periods of time, allowing the buildup of condensation. Significant condensation can happen due to normal day-to-night temperature fluctuations, especially in a humid climate.
A hopper dryer can also be useful where materials have been stored at a cool temperature and suddenly brought into a warmer area, resulting in condensation. When used for this application, a hot air dryer is sometimes referred to as a preheater.
A hopper dryer will usually be equipped with a loading device which loads the resin pellets into the hopper. While hot air dryers often have the option of manual loading, this is practical only when the amount of pellets to be dried is relatively small. For larger jobs, an autoloader is certainly a worthwhile investment and is sometimes included as a standard feature.