Resin dryers.

Above all, a dryer must be reliable, maintainable and accurate in temperature control. Some control sophistication is useful--but not too much.

The days of catalogue shopping are over. That's the conclusion of processors and suppliers that we asked recently for advice on how to buy drying equipment. "Close communications with the buyer is the key today," says one dryer manufacturer. "You have to listen to molders about exactly what they require. In the last three years we've custom-built more systems than ever before."

In the not-so-distant past, if a molder needed a new dryer (or for that matter, just about any other piece of auxiliary equipment), the molder would just open some brochures from favorite suppliers, pick out a dryer and hopper that looked good, and order the equipment. But not any more.

IN-HOUSE TESTING

"We did a lot of testing and research before we made our choice," says Doug Thorpe, molding manager for Nypro Inc., Clinton, Mass. "We started by reviewing literature from about a dozen suppliers, narrowed that down to four or five, and then tested all of them here in the plant."

Ultimately, Nypro chose the systems that performed best during those in-house trials--35 portable dryers with twin desiccant beds and automated regeneration. "Those features were essential," Thorpe notes. "A single-bed dryer stops the process. We prefer a continuous process that constantly regenerates the air stream through the material."

Thorpe's sentiments were echoed by almost every processor interviewed for this story. Shutting down the dryer is inefficient and contradictory to their goal of maximum uptime.

Nypro molds parts for critical medical and electronic applications. Like many custom molding houses, Nypro processes a lot of engineering materials, placing a further requirement on its dryers. In order to properly process high-performance resins, they must be dried to a dewpoint below -20 F. To ensure this critical dewpoint is reached, Thorpe and others recommend purchasing dryers that have both dewpoint monitors and sensors to measure the temperature of the material in the hopper.

"It's the only way you know in advance if something is not working perfectly," he says. "Without monitors and sensors you don't know anything is wrong until you've made the product, and then it's usually too late."

Other molders recommend going one step further--using insulated hoppers and installing insulated lines to ensure their resin stays as dry as possible all the way to the machine throat. For every foot of uninsulated hose, they say, the temperature of the resin drops by one or two degrees Fahrenheit.

While dewpoint meters seem to be essential for anyone processing engineering thermoplastics, just what type of dewpoint meter to use is open to debate. Some molders maintain that, beyond a certain point, precise dewpoint accuracy is not vital. If the spec is -20 F, for example, a few degrees below that won't matter. These processors say a simple analog meter with a dial and indicator needle is sufficient. A digital readout tied into a microprocessor control, though, offers processors the option of computer-generated records, and it can interface with other pieces of equipment--important factors as more and more customers require reams of SPC data.

BEWARE OF BELLS & WHISTLES

As microprocessor controls become increasingly common, some users interviewed consider one option superfluous. "Self-diagnostics are probably not necessary on a dryer," says one processor. "Dryers are a type of equipment basic enough that with a voltage meter and a wiring schematic you can usually figure out what's wrong."

Also, this same processor says, if a controller is too sophisticated, it can frighten operators. "What you're dealing with on dryers is usually an entry-level floor person," he says. "I don't want this person to be fearful of a very sophisticated microprocessor with all kinds of bells and whistles."

Some dryer makers agree. "The more buttons you give a guy to push, the more buttons he's going to push and the more that he'll make go wrong," says a Midwest supplier.

While the debate goes both ways on the level of sophistication desirable in controls, few argue that one of the most important features is a dryer's ability to reach and maintain a constant drying temperature.

"You set it for 200 F and if it drifts up to 250 F, the resin will burn," says Tom Opielowski, director of manufacturing for custom molder Precise Plastic Products Inc. in East McKeesport, Pa. "If it goes down to 150 F, the material will not dry properly."

With today's emphasis on JIT manufacturing, processors nowadays have to think hard about a dryer's capacity. As molders find they are making more frequent changes among growing numbers of materials and colors, some thought has to be given to smaller portable units that can roll from press to press with ready-to-go dried material. An extreme of this approach is to buy very small dryers with hoppers that hold no more than 5 or 6 lb and thus can dry just about any material in 30-45 min. Obviously, these units are limited to small-shot applications. Most of these dryers have no moving parts like fans or blowers, and their manufacturers claim they can reduce power consumption by as much as 80% over conventional desiccant units.

The opposite approach is taken by some processors who deliberately buy dryers with larger hoppers than they need and partially fill them with just enough material for a specific press run. They purchase the oversized dryers to be prepared for expanded production runs that may come in the future. Fill switches and level sensors attached to the drying hoppers ensure the correct amount of material at all times.

But regardless of the size of the dryer hoppers, filling them with only the resin you're going to use that day is essential. "I've seen dryers left running on at operating temperatures. If the plastic sits there for even a day, there's going to be some degradation of polymer properties," Opielowski says.

RELIABILITY AND MAINTAINABILITY

The selection factors most often mentioned by buyers of dryers and other auxiliaries is reliability and maintainability. A good dryer, they say, must require minimal repairs and allow for easy access to the beds and other parts that need periodic cleaning.

"Downtime is money," says Bernie Golub, plant manager for Pro Corp. in Florence, Mass. "One of the scarcest commodities in the plastics business is a good maintenance guy--someone who can fix anything. You don't want that person wasting his time fixing a granulator or dryer when he could be fixing a broken molding machine."

Golub adds, "When buying auxiliary equipment, I weigh the initial purchase cost against the maintenance. If one piece of equipment is going to cost $100 more than another but will take more 'watchdogging,' I'll take the more expensive one every time because it will pay for itself in the long run."