Plastic resin properly processed will be strong and void of physical defects. These desirable characteristic are achieved by adhering to the resin manufacturer's recommendations, which will include a specified level of ‘resin dryness’. When resin is heated past its melting point, moisture in the resin breaks the molecular polymer chains into shorter pieces. This can lower the intrinsic viscosity of the resin and degrade the performance characteristics of the finished piece.
The purpose of a resin dryer is to remove moisture from resin pellets to an acceptable level before processing. There is an attraction between water and polymer chains and it takes effort to separate them from each other. Heat reduces the attraction between water and polymer molecules making it easier to drive them apart during the drying process. Sufficient drying will keep the intrinsic viscosity high by maintaining those long molecular chains. Manufacturing quality issues that commonly arise with moisture are removed when properly dried.
Process air heaters play an important role in resin drying environments. They warm resin pellets, loosen water-polymer bonds, drive up the temperature of low dewpoint air and in some processes they help ‘regenerate’ a special drying chemical called ‘desiccant’.
Tutco-Farnam provides standard and custom engineered process air heaters for resin drying equipment.
Resins are subject to four drying parameters. Temperature, dewpoint, airflow and time are all critical factors that we can control. The resin manufacturer will suggest conditions to be achieved within the drying hopper based on these parameters. For example, a manufacturer of a particular resin may recommend a dewpoint of -40°F (-40°C) at 350°F for 4 hours.
We see plastic process drying temperatures ranging from 180°F to 400°F. A temperature too low and the moisture will stay with the resin. Too high and the pellets may degrade or start to clump. Before dehumidified air enters the hopper it is often heated. This hot and dry air acts like a sponge soaking up moisture from the pellets. High temperature air is also used to help regenerate desiccant by driving off moisture so that the desiccant can be reused.
The moisture content of the air is described by its dewpoint. It is the temperature at which condensation takes place. The value goes down as the air gets dryer. -40°F (-40°C) is considered an ideal level of dryness for many resins. It means that the air is so dry that the temperature would have to be -40°F (-40°C) for condensation to begin. It is important to note that dewpoint is not a description of the moisture content of the resin we are ultimately drying. It is a description of the air used in the drying process.
Air needs to move for dry heat to reach the pellets and to carry moist air away from the system. The rate of airflow needs to be sufficiently high for the warm dehumidified air to effectively work within the hopper but not so high that energy is wasted or material starts melting. The ideal airflow in any given situation is going to be based on many variables that will be unique to each drying event including the throughput of material and the size of the hopper.
Pellets must be thoroughly heated for water molecules at the center of the pellets to free themselves from polymer chains. Water molecules that move around more freely will be able to migrate to the surface of a pellet when introduced to dehumidified air.
Polymer resins are poor conductors and it takes time for heat to reach the center of a pellet. It then takes awhile longer for the warm moist air to diffuse from the pellet into the dry air of the hopper. Under proper conditions most of the moisture will leave readily. Moisture deep within the pellet however will take more time. While the majority of moisture may leave in the first hour, there may still be three hours or more to go before a particular resin is suitably dry.
Types of Resin
Resin pellets are either Hygroscopic or Non-hygroscopic.
Moisture penetrates the surface of hygroscopic pellets and does so until a moisture equilibrium is reached. The moisture inside the pellet is attracted to the molecular polymer chains. Heat can loosen the bonds between the moisture and the polymer chains but heat alone will not be enough to remove the moisture from deep inside the pellets. There needs to be very dry air surrounding the pellets so that the moisture will migrate out of the pellet seeking a new equilibrium.
Moisture does not penetrate the surface of the non-hygroscopic pellet but rather condenses on the surface. Subjecting the pellets to a sufficiently heated airflow is usually enough to dry the pellets. Non-hygroscopic resins include PVC, Polystyrene, Polyethylene and Polypropylene.
There are many different types of plastic process drying apparatus and innovative new approaches are in the works. The most common solutions are Hot Air, Desiccant and Compressed Air Dryers. They all use heaters but in slightly different ways.
Hot Air Dryers
These are relatively inexpensive dryers with simple arrangements used primarily for removing surface moisture from non-hygroscopic resins. The basic components include a heater, blower and controller. They are sometimes used with select hygroscopic resins when the application has less stringent moisture specifications and the factory is located in a dry climate. A hot air dryer configuration is not a closed loop system. They have a more difficult time achieving moisture consistency in hygroscopic resins especially when the local climate has varying amounts of moisture in the air. Heat-loss can be reduced with a heater insulation blanket. Learn more about energy savings with heater insullation blankets in our Engineering Talk article, Insulation Blankets Make Cents by Dexter Diepholz.
Desiccant is a material that attracts and holds moisture on its surface through a process called adsorption. When the material becomes saturated it can no longer remove moisture from the air. The desiccant gets reused after going through a process called ‘regeneration’ where the moisture covered desiccant is heated and the moisture is released and evacuated.
Robust process heaters are used in desiccant dryers for material drying as well as desiccant regeneration. The process heaters used in a desiccant dryer will depend upon several factors including watts, airflow and allowable space. A minimum of 350°F is required to effectively release moisture from the surface of desiccant pellets. A desiccant heater-blower assembly may throw temperatures as high as 600°F. The moving air carries the moisture away leaving the desiccant dry and ready for reuse.
Tower dryers use separate compartments filled with desiccant beads, one for adsorbing moisture and the other for regeneration. Carousel dryers are similar but have a mechanism to make it easier to swap between moist and dry desiccant beds. A third resin dryer innovation uses a porous wheel made of desiccant. This ‘rotor’ design has advantages including better airflow through the desiccant material and greater moisture adsorbing surface area. Learn more about desicannt air heaters here.
Compressed Air Dryers
When air goes from being compressed to a normal atmospheric pressure the dewpoint of that air drops by a significant measure. Heating that same air will make it even more like a sponge. That warm dry air is then passed through the hopper. Good airflow is the key to quality drying results. A variation on the traditional compressed air dryer is a compressed air dryer with a moisture removing membrane. The addition of a membrane may be essential if your factory compressed air is marginal. A compressed air configuration may be more cost effective with short runs, high temperatures and lower throughput.
These units dry hygroscopic resins by creating a vacuum that lowers the boiling point of water and then draws moisture vapor from the pellets. Heaters initially raise the temperature of the pellets so that moisture within will release from their bond with the polymer molecules. Moisture vapor escapes from the pellets under the repeated application of a strong vacuum. Since the boiling point of water will be much lower, drying temperatures can be lower too.
Heating Solutions for Resin Dryers
Tutco-Farnam has decades of experience designing and manufacturing heaters for the plastic process industry. From resin dryers to band heaters we offer durable solutions that reduce cost and improve product performance.
We’ll work with you to configure one of our standard models or design something completely custom. Tell us about your project and see what Tutco-Farnam can do for you.