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Dry-air conveying: why it's catching on.

It doesn't pay to dry resin if it regains moisture on the way to the molding machine, Using dried air in a pneumatic conveying system makes more sense to quality-conscious processors nowadays.

There has been a mini boom in dry-air conveying systems during the last two years. More processors are deciding that this way of moving pellets from remote dryers to molding machines is required to keep pace with rising demands for quality parts and the growing need for speed and flexibility in manufacturing.

Conveying with dried air guards pellets against moisture regain, thus ensuring that truly dry resin is delivered to a molding machine's hopper. In essence, it protects the processors, investment in raw materials and in the meticulous drying needed to prepare them for molding.

Dry-air conveying systems work like typical pneumatic systems that deliver resin from a drying hopper via tubes to a machine hopper. But unlike standard pneumatic systems which use ambient plant air to convey material, dehumidified air is the medium that moves pellets along a dry-air conveyor. The system usually is fully contained or "closed-loop," completely shielding the already-dried pellets from the moist environment. Most common usage of these systems is in injection molding.

Dry-air conveying technology is not new--it's been available to processors for more than 20 years. But today there is a heightened awareness by processors of the importance of properly drying resins and how it affects quality and profit margins. However, materials-handling equipment suppliers candidly admit that dry-air conveying systems are not right for every processor. In effect, it's a form of insurance where quality is critical. The added investment may be unnecessary for some operations, depending on the product being molded, the resins being used, the quality expectations of your customers, and the cost/performance risk factors your manufacturing system can tolerate.


Equipment suppliers say processors today are more willing to pay for dry-air conveying as a way to reduce process variation resulting from resin pellets contaminated by moisture regain. Lyn Depew, president of Filterless Conveying Inc., says dry-air conveying helps deliver more uniform material to the manufacturing process, providing a more consistent melt that enables the molder to cut down on the constant adjustment of operating temperatures, pressures, and plasticating parameters.

Dry-air conveying also protects processors, investment in time and energy to properly dry resin by protecting it from picking up moisture from ambient air on the way to the molding machine or during extended residence time in the conveying line. This focus on "front-end" process quality translates into dollars saved from reduced scrap and rejected parts caused by moisture contamination in resin pellets, Depew points out.

Two executives from Novatec Inc. Robert K. Crothers Jr., product manager for dryers, and Richard E. Gilmore, systems sales manager, believe custom molders today are willing to pay for equipment that allows them to "cover all the bases" in controlling their processes. They explain that in this age of world-class quality and just-in-time (JIT) manufacturing, business will be gotten by those who can "completely satisfy customers" by building in additional safeguards into a processing system.

Resin producers for years have complained privately that they receive much undue criticism for poor processing quality and inconsistency of materials when the fault is often improper drying and/or conveying procedures on the part of molders. Equipment suppliers say processors, in their pursuit of better quality. are inclined to pay more attention to investments in molding machinery and computer controls than to resin drying and other materials-handling solutions.

Apart from quality concerns, the most common reason processors choose dry-air conveying systems is to remove large drying hoppers from the tops of injection molding machines, according to Charles Sears, president of Dry-Air Industries. Until recently, mounting dryers right on top of molding machinery was considered the best way to ensure that only dry pellets would enter the machine feed throat. But by relocating drying hoppers, either to the floor beside the machine or to a remote central-drying location, there is now a need to reliably convey dry pellets to the machine feed throat.

Processors today want to get large drying hoppers off of injection presses to improve overall plant safety and maintenance accessibility, Sears says. "Putting all that bulk on top of a machine creates safety and access problems. These are unreasonable heights at which to work and control a material handling system," agrees John H. McLeod, v.p. of Thoreson-McCosh.

One supplier tells of a recent dry-air conveying installation where the molder said he could almost justify the investment for the system through lowered insurance premiums resulting from the removal of tall drying hoppers from above injection presses.

Dry-air conveying also supports manufacturing operations practicing JIT and related quick-mold/quick-materials changeovers. A machine-mounted drying hopper takes too long to empty out, clean, refill with a different resin and then dry that resin for several hours before it can be molded. With either central drying or mobile beside-the-press drying systems, one batch of material can be dried while another can be on its way to the molding machine--without picking up ambient moisture, provided that dry-air conveying is used. This flexibility, say the Novatec executives, facilitates shorter job runs and eliminates the machine downtime that would occur while waiting for the next resin batch to dry--which would negate the "quick" aspect of quick mold changing.


Molders considering dry-air conveying as a tool to improve their process usually are supplying parts to customers imposing the most rigid world-class quality standards--like those in automotive, medical, and electrical/electronics. It's also likely to benefit those who mold PET and polycarbonate, usually identified as the two most difficult resins to dry and keep dry, followed by polyetherimide (GE's Ultem), nylons, liquid-crystal polymers, and other TP polyesters. Not all hygroscopic resins are as susceptible to rapid moisture regain as the foregoing examples.

But despite the many reasons to consider dry-air conveying, equipment suppliers say they often question--and sometimes even discourage--customers interested in this technology. Some suppliers acknowledge that processors often can achieve desired results by conveying with ambient air as long as they closely monitor processing parameters.

Joe Fitzgerald, manager of product and system development for Whitlock/AEC, cautions that even though more processors today are "looking to play it safe," dry-air conveying is sometimes just overkill. Fitzgerald, echoing the sentiments of most equipment suppliers, says faithfully controlling drying times and temperatures and residence times in hoppers and lines, along with frequent purging of lines, can allow processors to remain close to the -40 F dewpoint recommended for most resins and avoid the pitfalls of moisture regain. Greg C. Lewis, assistant sales manager for Matsui America, says it's even possible to convey most of today's engineering resins with ambient-air systems as long as a process is closely monitored.


Dry-air conveying typically is described in terms of cellular/integral systems, central systems, or a stand-alone dryer for a single injection press.

Cellular/integral systems commonly serve up to eight machines in a "cell," with the drying system located close by, resulting in relatively short piping lengths for dry-air lines. Sometimes the dryer can be a portable type, serving two injection presses. One example of this is the new MDC Mobile Dryer/Conveyor from Conair Franklin, which includes a dehumidifying dryer, insulated drying hopper, and compact hopper loader mounted on a wheeled platform. It supplies dry air for both drying and conveying and recycles the conveying air back to the drying hopper in a closed loop. (It comes in three sizes, from 30 to 90 lb/hr, with microprocessor control. For more drying and conveying equipment news, see New Products section.)

A central system involves a larger number of machines fed by a central bank of dryers usually located in a separate room some distance from the molding area. Central systems often imply a longer, more extensive arrangement of resin feed lines, with more elaborate air-drying equipment and controls.

In general, dry-air conveying involves a desiccant dryer, drying hopper, purge valves, direct-feed/"sight-tube" loader on the throat of the molding-machine barrel, vacuum pump, air cooler, dust collector, and computer controls. Many equipment suppliers, like Matsui America and Dry-Air Industries, emphasize "dual" closed loops: separate dry-air sources for conveying and resin drying. Some suppliers recommend drying and conveying resin with a single dry-air source. In most dry-air conveying systems, maintaining at least a -40 F dewpoint is considered the standard (although PET usually requires a -50 F dewpoint).

Conair product sales manager Pete Stoughton says most dry-air conveying systems operate in a "dilute" phase, meaning there is a high volume and velocity of air carrying the resin. Maximum velocity in a dilute dry-air conveying system should not exceed 5500-6000 ft/min in order to avoid pellets plating out in the high-friction bends of the conveying lines, which creates "snake skins" and "angel hairs."

Line purging is an option that may help processors involved in JIT manufacturing. A purge system requires more complex valves that will add to hardware and software costs, according to Gilmore of Novatec. Heat boosters at various locations in the system may be required to maintain resin temperatures in long conveying lines, he adds.


While equipment manufacturers offer their own standard lines of dryers, hoppers, valves, controls and blowers, there is no "standard" dry-air conveying package. Virtually every system is custom engineered and integrated to meet a specific requirement. The role of system integrator, delivering a turnkey installation, is the most crucial aspect to consider when choosing a dry-air conveying system.

Here's the advice from suppliers: Go slow, start small, and look for modular equipment that offers the flexibility to upgrade or expand as dictated by future needs. Another word of advice is look for a system integrator willing to take a "strategic partnership" approach to the business relationship. For processors, this means sharing intimate details of the manufacturing operations.

"To properly qualify a dry-air material-handling system, the equipment supplier must get close to the customer and his goals " observes David Cosner, director of sales and marketing for Universal Dynamics Corp. "This requires the customer to provide confidential information to the system integrator. Normally, this is something molders don't like to do." Cosner says the list of proprietary details that enable system integrators to do the best job includes predicted percentage of regrind, and reject rates, cycle times, shot size, operating temperatures and pressures, and some idea of a future growth strategy. Amy Reissener, manager of marketing and communications for Motan Inc., says processors must consider energy savings flexibility for frequent material changes, and maximizing uptime when choosing a dry-air system. If a plant's injection machines average more than one materials change per week and its materials require a typical 3-hr drying time, she would recommend one cellular dry-air system feeding four presses rather than a single portable dryer for each machine.

Pricing estimates are difficult, especially for large-scale central dry-air conveyors, given their custom nature. Some equipment suppliers speak of recently completed dry-air conveying systems ranging in price from $200,000 to over $1 million. A general price estimate for a cellular system serving four to eight injection presses is in a range of $6000 to $20,000.


Bry-Air Systems, Div. of Bry-Air Inc., Sunbury, Ohio (CIRCLE 49) Chicago Conveyor Corp., Addison, Ill. (CIRCLE 50) Colortronic, Div. of K-Tron North America, Pitman, N.J. (CIRCLE 51) Comet Automation Systems Inc., Dayton, Ohio (CIRCLE 52) Conair Franklin, Franklin, Pa. (CIRCLE 53) Dry-Air Industries Inc., Vernon, Conn. (CIRCLE 541 Filterless Conveying Systems Inc., Brantford, Ont. (CIRCLE 55) Foremost Machine Builders Inc., Fairfield, N.J. (CIRCLE 56) Hamilton Avtec Inc., Mississauga, Ont. (CIRCLE 57) HydReclaim Corp., Fenton, Mich. (CIRCLE 58) Labotek Inc., Rochester, N.Y. (CIRCLE 59) MAC Equipment, Inc., Kansas City, Mo. (CIRCLE 60) Matsui America Inc., Elk Grove Village, Ill. (CIRCLE 61) Motan Inc., Plainwell, Mich. (CIRCLE 62) Mould-Tek Industries, Inc., Scarborough, Ont. (CIRCLE 63 Novatec Inc., Baltimore (CIRCLE 64) Pacific Pneumatics, Inc., Ontario, Calif. (CIRCLE 65) Premier Pneumatics, Inc. Salina, Kans. (CIRCLE 66) Process Control Corp., Atlanta (CIRCLE 67) Thoreson-McCosh Inc., Troy, Mich. (CIRCLE 68) Universal Dynamics Inc., Woodbridge, Va. (CIRCLE 69) Walton/Stout Inc., Lithonia, Ga. (CIRCLE 70) Whitlock/AEC, Inc., Wood Dale, Ill. (CIRCLE 71) Young Industries, Inc., Muncy, Pa. (CIRCLE 72)
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Author:Gabriele, Michael C.
Publication:Plastics Technology
Date:Oct 1, 1993
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