Our previous blog, The Importance of Drying Plastics: Plastic Part Making, we discussed the necessity of drying resins in the processing phase to benefit the quality of our customers parts. This article explains the different types of resin dryers available in the plastic injection market and which one Midstate Mold currently uses.
Hot Air Dryer
One of the most economical ways to dry plastic resin, these dryers simply use heated ambient air to pull moisture away from the resin. These have been the natural choice for resins that have no affinity to moisture, like non-hygroscopic resins. For these resins, this dryer will remove moisture from the surface of the pellets.
With mildly hygroscopic resins, the repeated flow of hot air through the resin changes the gas pressure between the inside and outside of the resin pellets. While the pellet tries to balance with the pressure, the moisture moves from inside the pellet to the surface, eventually reaching equilibrium with the dry air stream.
Compressed Air Dryer
This dryer is essentially a hot air dryer consisting of mainly the same parts: heater, controller, and hopper. However, a compressed air dryer, uses compressed air from a plant’s processor. Compressed air generally holds less moisture than what is considered “normal” air at atmospheric pressure. This dryer is perfect for smaller molding applications.
This sounds like a simple and energy efficient process, however this dryer still requires the same amount of energy to run as is used with a desiccant system. Molders have found they may need to add a compressor because this dryer consumes so much air.
Essentially the “gold standard” dryer for resins that tend to absorb moisture. Desiccant dryers can quickly achieve the low moisture content dictated by resin manufacturers. Some types of desiccant dryers use desiccant beads that absorb and release moisture (like a sponge) repeatedly. The beads eventually become saturated and need to go through a regeneration process to remove any moisture to start absorption again. The regeneration process passes extremely hot air, 400-500 degrees F, over the beads to release moisture.
We personally own the HPD Series, 4-Bed Portable Desiccant Dryer. This is the most energy efficient dryer in the market that dries materials faster while adapting to ambient and material conditions.
Effective Drying Procedure
Proper drying requires attention to the following areas: air’s volumetric flow rate across pellets, temperature and moisture content of the air, the time the material is in the drying hopper, and temperature of the air coming back from the hopper to the drying unit.
An elevated drying temperature is desirable since it can increase the rate of moisture being removed from the resin. Except, it’s important to note that the drying temperature must remain below the melting or softening point of the resin material.
What Resin Dryer is Best for You?
It’s all about how the dryers are going to be used in an application. This is where a close eye on your operation is important. What is the most important to you: lowest price, low energy use, or high performance? Are you trying to solve a specific problem? Will the dryer be able to work with a variety of resins?
For us, the desiccant dryer has proven to still be the most energy efficient, versatile, and rugged type of dryer that has the flexibility to work in aggressive work environments with numerous types of resins. Each job is unique, and we specialize in helping to design, create, and perfect your next part no matter your requirements. Contact us today!