Drying more with less

Proper drying of resin is something most processors take for granted, until there’s a breakdown. When this happens, the molder can find himself with a major mess on his hands.

“A dryer is much like a chiller or water tower in that you size it for worse case conditions,” says Pete Stoughton, product specialist, dryers, Conair. “However, it is obvious when you have a problem with a chilling system, but sometimes not so obvious with a dryer.”

Stoughton relates a true story in which hundreds of pop bottles loaded inside a tractor trailer travelling across a desert highway began cracking under what is considered normal ambient conditions. The entire shipment was scrapped as a result of residual moisture in the PET during blow molding.

Such a problem can cost you more than money, says Stoughton. It can cost you the trust of your customer, or even result in a lawsuit.

In North America worst case conditions for drying start in June, or earlier depending on the region, when high ambient humidity becomes a factor. A drying system should be specified to meet the materials’ supplier recommended drying specifications for a given hygroscopic material under these conditions. Different hygroscopic materials can have vastly different drying requirements. PET, for example, typically needs to be 40 times dryer than nylon used in a general molding application.

Joe Corturillo, dry product manager at Nucon Wittmann, notes that the most important factor affecting a resin’s moisture content is proper control of dewpoint. The lower the moisture level requirement, the more important it is to accurately maintain a low dewpoint. The company’s Drymax line of compact dual tower dryers comes with a SmartRegen function that minimizes heating time of the regeneration phase and prevents spikes in dewpoint. The system measures the temperature of the regeneration exhaust air and compares it against a known value for a properly dried desiccant bed to ensure the desiccant is completely dry. The heater is then shut off and the bed is put into its cooling cycle.

“SmartRegen saves energy and prevents the type of dewpoint temperature spikes you see with dual tower dryers when the desiccant bed hasn’t been cooled sufficiently,” says Corturillo.

The portable drying and conveying (PDC) option is available on Drymax dryer models C30, C60 and 100. Drymax PDC dryers consist of a standard dryer equipped with a single drying hopper and a three-phase conveying blower. The blower supplies a loader mounted on the drying hopper. Dry air is used to convey dried material to the just-in-time loader mounted on the injection molding machine. Both the loader on the drying hopper and the JIT loader on the molding machine are included in the PDC package.

DRYERS UP TO HIGH HUMIDITY, SHORT RUN DEMANDS

Maguire Products Low Pressure dryers are ideal for high-humidity environments, according to Pat Smith, vice-president of marketing and sales. The LPD dryer uses a vacuum, instead of hot, dry air, to reduce the boiling point of water and quickly turn moisture into water vapor, allowing for fast removal of water from the resin pellet. The LPD dryer carries out heating and vacuum drying simultaneously in three separate indexing stations, making possible small batches. The small batch size and short drying cycle of the LPD dryer make it possible to shorten cold startups from several hours to less than an hour. Additionally, the LPD saves money by eliminating the desiccant, which requires more heat to regenerate, and eventually must be replaced as it degrades over time. The dryer, introduced by Maguire in 2000, requires one-sixth of the time and 20% of the energy compared with desiccant dryers, says Smith.

Dri-Air Industries has continued to add features to its PDII hopper design in order to meet the changing need of molders. The PDII can help eliminate the machine downtime associated with material changes. While running material in one hopper, the second hopper dries the material for the next run. With the PDII, material change-over times become a function of tooling changes, rather than resin drying time. This feature makes the dryer ideal for short-run, multi-material molding operations. For two-shot molding, the PDII is equipped with a dual closed-loop loading system to feed the receivers on each injection unit.

The Speedryer Mini is a new mini hopper dryer designed for molding applications consuming one to 20 lb./hr. of resin. Made by Canam Manufactured Products, Inc., based in Surrey, BC, the dryer is ideal for small extruders or injection molding machines. Utilizing desiccant-free “heatpipe” technology, heat energy is transferred directly to the plastic pellets. Rapid heating increases vapor pressure in the pellet and forces moisture out, where it is stripped away by counter air flow. According to the company, energy consumption is less than half that used by desiccant/hot air dryers. The Speedryer Mini is designed to be mounted directly on the feed throat of the molding machine.

Thoreson McCosh’s three-desiccant bed dryer system can lower the cost per pound of dried material. The desiccant beds are cooled with dry air, which keeps the beds fully charged and ready to perform work. As well, heat is reclaimed and returned to the process air stream, saving energy.

Conair’s redesigned carousel dryer, the Carousel Plus, has been in full-scale production since January. The Carousel Plus is the result of a project to upgrade the technology and design of Conair’s original Carousel dryer. While the old indexing Carousel weighed as much as 320 lb., the Carousel Plus weighs only 40 lb., providing a significant reduction in thermal mass and thus minimizing energy consumption. As well, the number of parts between the old and new dryers has been reduced from about 190 to just 19. The Carousel Plus uses molecular sieve desiccant impregnated into and bonded onto a glass-fibre substrate. This material is then formed into a continuously rotating wheel that constantly brings fresh desiccant online while the rest of the wheel is being regenerated.

Stoughton says he sees an increasing tendency for processors to under-specify equipment for a given application. Sometimes a molder, intent on reducing material change-over times, will opt for a smaller capacity hopper.

“If you’re running polycarboante, you need to dry it at 250F for three to four hours,” says Stoughton. “A 200-lb. hopper will not be sufficient to meet this requirement if you are running 100 lb./hr.”

In other words, better to be safe than sorry.