For the big jobs a shredder is better.

If you need to grind up large scrap--whether in-plant scrap or post-consumer waste--size-reduction equipment suppliers say a shredder can make the job easier. Whether it be gaylord-sized purgings, refrigerator liners, 55-gal drums, auto fuel tanks, bumper fascias, or heavy-wall pipe, a shredder grinds them all with less noise, fines, energy consumption, and up-front capital cost than a conventional granulator.

What makes shredders different is that they operate at low speed with high torque. Opposing rows of cutting rings with hook-like protrusions gradually pull parts in between the knives at constant speed, rather than chopping them with high-speed rotating knives.

This difference accounts for lower noise and heat generation, as well as up to 8096 less dust and fines--even with brittle styrenics. And unlike granulators, shredders reportedly are not prone to surging.

The shredder not only handles material too demanding for a granulator, but also eliminates the need to precut large parts with a band saw, thereby avoiding a labor-intensive and hazardous size-reduction step.

Although shredders have been cutting up plastics for over 15 years, they still haven't gained the recognition they deserve, some suppliers maintain. However, the recycling boom of recent years has brought them to more processors' attention. At the same time, the number of firms marketing shredders to plastics processors has doubled since 1988.

"Many processors aren't familiar with shredders because they've done everything with granulators. But shredders are another step in efficiency," says Tim Hanrahan, v.p. of sales and marketing at Erema North America.

"Processors do have a need for a shredder. Recyclers, in particular, can shred scrap that may have otherwise gone to a landfill," says Harold L. Huffman Jr., product manager et Louts P. Batson. He believes processors who have become more conscious of fluctuating resin prices will find the shredder an effective way to broaden the scope of what they can recycle. (Batson sells shredders from Sant' Andrea Novara in Italy. In Canada, they're sold by Hamilton Avtec.)

NOT JUST FOR BIG JOBS

Although large shredders show the biggest cost advantage over similarly sized granulators, a shredder can also handle small jobs like film or sheet edge trim, molding flash, and extruder drool. "A big area for shredders in the last few years is medical waste and plastic containers," says Mitch Katz, size-reduction business manager at Entoleter Inc.

Shredders can handle the toughest engineering plastics, elastic and brittle materials, nonwoven fibers, products that are foam-filled or glass-reinforced, and even reinforced thermoses composites.

Some processors put a wide variety of scrap sizes and geometries into the same shredder. Resource Recycling in Rockingham, N.C., puts 750,000 lb/month of all sorts of LDPE and HDPE post-industrial and post-consumer scrap into its Saturn shredder. "Our incoming scrap can be in bales, chunks, rolls, really any form imaginable. Sizes too, will vary," says plant manager Stan Synkula.

Similarly, WNI Industries in Waynesboro, Va., uses a Saturn machine to shred 7000 lb/hr of nylon scrap ranging from 1-cu-ft chunks to as large as 36 cu ft.

Shredders are best when tailored for a particular use, says Ken Lewis, technical sales specialist at Shred-Tech. Choice of blade style and drive type are the major design variables.

PRECUTTING SAVES $$$

Although a shredder may sometimes be a stand-alone alternative to a granulator, more often the two are used together. Many equipment makers say a shredder fits best on the front end of a two-stage size-reduction process, in which a granulator is the second stage. Approximately 95% of all shredders sold to plastics processors are part of a two-stage system, says Shred-Tech's Lewis. "It's a good method of pre-sizing parts or purgings to fit in granulators. Pre-sizing saves wear and tear on grinders," says Conair Martin product manager Bob Harrison. "A shredder is typically run in conjunction with a granulator because it helps the throughput capacity of a granulator by making uniform pieces of scrap material," says Damon Dedo, sales director at Saturn Shredders.

Users say shredders are faster at reducing large, thick chunks. In a granulator, a large block of plastic will tend to bounce around and be slowly chipped away.

Although not all suppliers agree that a shredder increases throughput, they do say initial size reduction with a shredder should enable a processor to use a smaller granulator. Chilton Metal Plastics in Chilton, Wis., uses an AEC/Nelmor shredder to precut entire portable toilet stalls.

Steadiness of throughput is another virtue: "Shredders deliver consistent metered feed at [predictable] rates to the granulator," Conair's Harrison says. Many shredders have a ram feeder to push material into the shredder.

A two-stage size-reduction system can be more cost-effective than a granulator alone, depending on the equipment size. For example, if you need a machine with a 32 x 50 in. throat opening to ingest large scrap, you could pay as much as $300,000 for a stand-alone granulator, says Harrison. Instead, you could spend $120,000 for a similarly sized shredder plus $80,000 for a smaller granulator, saving $100,000, he says.

Equipment suppliers also note that granulator blade life may be extended three- or four-fold in a two stage system. "If you throw something bad into a granulator you can destroy the blades in seconds," warns Erema's Hanrahan. Although shredder knives may be just as expensive as granulator knives, they can last up to 15,000 hr, whereas granulator knives in a stand-alone system may need replacing after 100 hr.

Use of a low-speed shredder may also save energy. According to Ed Ronca, group v.p. of sales and marketing for AEC/Nelmor, "Energy savings up to $6000 annually can be realized by replacing a large granulator with a shredder." His estimate is based on electricity cost of 9[cts.]/kwh and use of a 75-hp motor with 87% motor efficiency.

SHREDDER ANATOMY

A shredder typically has two hexagonal knife shafts that counterrotate at speeds from 5 to 200 rpm (in contrast to granulators' 600-1300 rpm). The shafts turn at different speeds--usually a differential of 10-20%--so that the knives don't intermesh at the same point on every rotation. One shaft is commonly driven by the motor and the other by a gearing system, but larger machines are also supplied with dual driven shafts.

The shafts are fitted with hook-like knives or cutters on disks. Unlike a granulator, there is usually no screen at the bottom of a shredder. However, Franklin Miller Inc. recently developed a shredder with a screen that automatically recirculates shredded material through the machine until it is small enough to pass through.

Each disk of a shredder can have one to six hooks or lobes that enable the knives to grab the scrap and pull it in between the shafts. Mono-lobed disks are generally used for larger parts, multi-lobes for smaller parts. "More cutter lobes restrict the shredder's ability to grab a big part, causing it to dance atop the blades," notes Ball & Jewell sales manager John Oleksy.

Blade thickness can be varied to change the width of shredded material. Thin blades can shred material down to 0.5-tn.-wide strands, though rigid material could require blades as wide as 4 in., according to Saturn's Dedo. "If you have thinner knives, they are doing more work, so you may need a larger drive," adds Erema's Hanrahan.

The height of the hooks also depends on the product. "If you are shredding film, a less aggressive knife design is used, but a blow molded product may require a more aggressive knife design," notes Lewis of Shred-Tech.

In addition, a shredder can be powered electrically or hydraulically, depending on the horsepower required. "The general rule of thumb is if you need 75 hp or less use an electric motor and if you need more, go hydraulic," says Entoleter's Katz.

A special feature of shredders, as contrasted with granulators, is that they can temporarily reverse the direction of the shafts in case of a jam. A sensor reads the amperage of the turning shafts, automatically reversing rotation when the amperage levels rise beyond a cutoff point--an indication that the shafts are stuck. After clearing the jam, the machine resumes normal operation. Hydraulic motors can handle frequent reversing better than electric drives, suppliers note. But electric-driven machines are roughly 20-30% less expensive than hydraulic shredders, Dedo says.

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There are at least two unusual pieces of size-reduction equipment on the market that differ in design from shredders but reportedly share the benefits of low-speed cutter rotation--reduced noise, fines, and energy consumption, as well as long blade life.

One device is the WLK machine from Weima in Germany, available in a range of sizes from All-Grind Inc. and Size Reduction Specialists Corp. The WLK has a single rotor shaft with a staggered row of diamond-shaped blades. Each blade can be rotated to expose eight cutting surfaces before it is discarded. Special concave cutters have four cutting surfaces. These blades nibble or chip the plastic, rather than shredding it. The rotor turns at 80-90 rpm.

The Weima machine also has a quick-release sizing screen and a hydraulic ram to push material against the rotor. A large amount of material can be loaded into the hopper at one time. The ram will push scrap against the rotor until a max. amperage level is reached and then push no harder. As the scrap is ground down, the ram will advance until it reaches a limit and then it will retract, allowing more scrap to fall into the cutting zone.

Another special machine is the "S" cutter from Nissui of Japan, sold here both by Nissui Corp. and Size Reduction Specialists. It's more similar to a granulator, having a single rotor (turning at 30 rpm) and also bed knives, and is designed specifically for press-side grinding of injection molding sprues and runners. It has two large rotating hooks that chop the scrap into smaller particles. Then multiple rows of small chipper blades cut plastic into cubes of 4, 5, 6, 7, or 9 mm (depending on model) without the use of a sizing screen.

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Durakon Industries Inc. in Lapeer, Mich., thermo-forms truck-bed liners from HDPE sheet, producing along the way some 15 million lb of scrap a year. "We were always behind in our reprocessing," says project engineer David Thomas. The vacuum forming operation ran five days a week, but regrinding was a seven-day operation. "From our process, we can count on roughly 30% of every sheet going back through the reclamation system," he says. "In 1993 we realized a need for a shredding/grinding system. Our previous method of grinding just couldn't keep up."

Durakon's sheet is roughly 100 to 280 mils thick. The firm's granulators could not handle the load. "We had meltdowns on several occasions due to overworking the granulating and evacuation equipment, causing us to pull molten polymer out of the granulator," Thomas recalls. The problems led to granulator blade changes as often as once a week.

Durakon purchased a two-stage shredder/grinder system from AEC/Nelmor designed for 6000-7000 lb/hr, along with a conveyor system to feed the shredder. The shredder is mounted atop the granulator and feeds the granulator at a uniform rate, so output is predictable and has increased from previous levels, Thomas reports. Scrap reprocessing now is accomplished during the regular five-day workweek, and the company changes granulator blades every 30 to 45 days.