Understanding the properties of a resin and the impact moisture can have on the finished part are important. Drying resin is key to the injection molding process to improve part quality, and there are several types of dryers that are prevalent on shop floors today.
Older dryers are much less efficient than more recent models, or today’s ultra-efficient models. For example, an older dryer may use 100 to 150 watts of electricity for every pound of material dried. More recent models are using about 60 watts per pound. Today’s ultra-efficient models are using about 20 watts per pound of material dried.
For most shops, hot air dryers prove the most cost-effective way to dry resin. They heat ambient air to draw moisture from the resin. As a result, they are great options for non-hygroscopic resins. They also can be effective with slightly hydroscopic resins, repeating the flow of hot air through the resin and changing the gas pressure between the inside and the outside of the pellets, allowing them to be dried.
A compressed air dryer is very similar, with the same basic parts – a heater, controller and hopper. These dryers uses compressed air, which generally holds less moisture than air at normal pressures. It is a good option for smaller molding applications.
Compressed air dryers use a rapid pressure drop to quickly remove any humidity contained in the drying air stream. These dryers are designed for production systems with very low resin throughput because their ability to remove large amounts of moisture is very limited.
Desiccant dryers are considered the premier dryer for resins that absorb moisture. These systems draw air from the drying hopper, chill it to room temperature, and then dry the air stream using a molecular sieve or cartridge. They can quickly achieve the low moisture point set by the resin manufacturer.
Many of these dryers use desiccant beads that absorb and release moisture. As they become saturated, they go through a process to remove the moisture and begin absorption. Extremely hot air is passed over the beads for this step.
For operators, correct drying requires attention to the air’s volumetric flow rate over pellets, temperature and moisture content of the air, the time the resin is in the drying hopper and the temperature of the air returning from the hopper to the dryer.
An elevated drying temperature is desirable since it can increase the rate of moisture being removed from the resin.
Oven drying is among the oldest and simplest methods for drying hygroscopic resins. These dryers expose material to a predetermined temperature in shallow trays stored horizontally in a rack. Often, no pre-treatment or pre-drying of air exists with these types of systems. The principle behind oven tray drying is that at elevated temperatures, moisture will be liberated from the granulate.
Vacuum ovens can provide improved drying performance, and are equipped with negative pressure evacuation that removes moisture from the pellet as it is heated in the oven chamber. One key to the success of this method is the equipment’s ability to pass enough air or other drying media through the material.
It is important to employ an abundance of caution when using vacuum dryers. They are not practical for deep oven storage trays because of their limited ability to remove humidity completely. In questionable situations, it is advisable to test the efficiency of the vacuum dryer for resins with varying hygroscopic characteristics.