Central or beside-the-press dryers: Which is right for you?
Damp resins are about as useful as an ashtray on a motorcycle, making it hard to overstate the importance of a good resin drying system to the success of your plastics processing. Typically made up of the dryer itself and the drying hopper,...
Damp resins are about as useful as an ashtray on a motorcycle, making it hard to overstate the importance of a good resin drying system to the success of your plastics processing. Typically made up of the dryer itself and the drying hopper, dryers are simple enough in theory, but as a practical application there’s an “X factor” involved that every processor has to solve: where to put them? Machine-mounted or beside-the-press, or off-machine in a central location?
Turns out there’s no one correct answer. Both configurations have their strengths and, depending on your processing needs, either — or a combination of both — could be the right fit for you. To help take the guesswork out of deciding, we’ve asked some of the experts to lay out the case for each.
“Central drying systems have been around for a long time, but beside-the-press units have been around even longer,” said Mark Haynie, drying product manager with Novatec Inc. It’s not complicated: The units either sit beside or are mounted on a mezzanine directly above the process machines they supply. Benefits include little opportunity for material loss, and for contamination or exposure to ambient air that might cause moisture pickup. Odds are, it’s the configuration many facilities — especially the older shops — were built on. “The majority of processors probably started with a beside-the-press system, buying dryers for each machine,” Haynie said.
But if, in this brave new millennium, beside-the-press strikes you as your grandfather’s technology, you might be surprised to learn that equipment suppliers still report strong sales. “Beside-the-press dryers remain a good way for small processors to establish themselves and grow their plants, because they offer the flexibility to take on a variety of jobs from a variety of customers,” Haynie said.
And even once the training wheels are off, beside-the-press dryers can still make good sense. First of all, small-budget operations with an existing setup of several individual dryers may not want to switch to a central system for simple economic reasons.
Second, beside-the-press remains an excellent approach for handling short processing runs. “Compared to central systems, which don’t like to move small amounts of material on a frequent basis, it can be hard to beat the flexibility that a beside-the-press dryer offers to custom molders doing either short runs or part prototyping,” said Charles Sears, president of Dir-Air Industries Inc. “It’s a good approach for molders who treat their presses as separate, individual work cells.”
And in the increasingly important area of handling regrind, beside-the-press dryers might just have the edge over a central system. “While both systems can handle regrind, the regrind — which has a different shape and bulk density than a pellet — can separate in a central system because of the longer vacuum conveying distances, cancelling out the time, money, and effort you put into creating a homogenous blend,” said Rob Miller, president of Wittmann Canada Inc. “Our solution is to put a metering valve on the bottom of the dryer hopper, and then close that valve at the end of the conveying cycle; if there’s regrind left behind in the elbows, it ensures that the whole mix gets to the machine. It works, but it also represents an additional investment.”
Finally, personal or organizational experiences can factor into the choice. “Central drying is often perceived as a slightly higher risk approach because, generally, the processor is putting all of his eggs in one basket, and can lose production on all the processing machines fed by a dryer if it shuts down,” said John Fleischer, vice president of sales and marketing with Universal Dynamics Inc. (Is it a fair cop? Probably not. “Most central systems now have more than one dryer, so you can limp by with the second unit if the main dryer goes down,” said Charles Sears.)
Having given their props to beside-the-press units, many drying system suppliers still say that the central system is the better option for most processors in most circumstances. Some are pretty clear about it. “If I owned a molding facility, I would select a central system,” said Scott Harris, vice president of sales at Motan Inc. “I think it’s the most efficient way to dry material, even for short runs — I think today’s central systems are efficient enough to handle these applications.”
An obvious benefit of a central drying system is that it can be designed to provide the same drying capabilities as several individual drying systems but requires less equipment. That’s a bonus, given the downsizing trend throughout industry, which results in many companies having fewer personnel to maintain dryers and other equipment. “If you have a crowded plant, a central system allows you to open up space to add machines and become more profitable, without increasing the plant size,” said Jerry Muntz, vice president at Thoreson McCosh. “You can also minimize the fork truck traffic and really clean up your plant floor; that might not sound important, but a shop that’s neat, clean, and uncluttered makes a positive impression on prospective customers.”
Speaking of uncluttered, a central system is a good way to minimize resin spillage. “Resin is the biggest cost to all processors, and material handling in a central system leads to considerably less spillage and loss from emptying and changing Gaylords, and also from mislabeling,” said Brian Davis, general manager of Maguire Canada.
But fears about central drying systems still persist. “Cross-contamination of materials, or sending the wrong material to the wrong machine, is a common concern voiced about the multiple hopper central drying arrangement,” said Rob Miller. Purging each line after each conveying cycle, he continued, eliminates the potential for cross-contamination. “And a good control system for a central dryer is crucial; it removes the human error of sending the wrong material to the wrong press or not letting it dry properly,” he added.
A related concern centres around central systems and the potential for moisture regain during conveying from the dryer to the machine feed throat. “Processors have legitimate concerns about hydroscopic materials picking up moisture when they’re conveyed from a central drying system, especially in damp conditions,” said Jamie Jamison, dryer product manager at The Conair Group. “For roughly 80 per cent of the resins, purging from the conveying lines with ambient air is sufficient, since resin doesn’t remain in the lines for very long.” But for the most moisture-sensitive materials, which can’t be exposed to ambient at all, Jamison continued, a solution is to move the dry resin from the central drying system using a dry air source instead of ambient air. “In this case, a just-in-time hopper on the processing machine is a good idea, since it allows you to minimize the amount of material that’s someplace other than in the dryer or the machine feed throat,” he said. “If you must use a large hopper on the press, you can always use a small machine-side dryer to give the material a final blast of dry air before it enters the machine.” Alternately, according to Mark Haynie, a closed loop convey can provide the same advantages as dry air and causes less inconsistencies in the dryer performance while still eliminating moisture regain.
THE MIDDLE GROUND
Some processors who wanted the benefits of a central drying system without giving up their beside-the-press dryers have tried combining the two. Converting to a central drying system by usin
g a beside-the-press dryer to feed banks of multiple hoppers can cost up to 25 per cent less than purchasing an individual dryer for each material hopper, vendors say, but it remains a quasi-central drying system at best. “We’ve relocated beside-the-press units into central systems on many occasions,” said Scott Harris. “It opens up new floor space, but it doesn’t provide the energy savings of a central system, and doesn’t allow the processor to balance incoming and outgoing air flows with the same precision.”
Another option? “A mobile drying and conveying system gives you the opportunity to apply drying capacity where and when you need it,” said Jamie Jamison. “If you have a machine that runs hydroscopic material only part of the time, you can use the mobile system when needed, and otherwise store it off the production floor. You also have the benefit of being able to pre-dry material away from the press so that it’s ready to use when a mold or material change is completed.”
And there’s a final possibility available, if you’re lucky enough. “Depending on their finances and how their processing machines are configured, some facilities can configure a central drying system in one location that supports either the entire plant or a specific machine cell or cells, and still maintain a cluster of beside-the-press units for trials and quick changeovers,” said John Fleischer. “It might be the best scenario of all.”