A Slower Approach

By Brian Taylor 

There is more than one way to skin a cat, the old saw informs us, and there is certainly more than one way to break an obsolete item into smaller pieces. 

While hammermills can work quickly to reduce all but the heartiest of materials into transportable pieces, it is not the only configuration on the market for doing so. 

In fact certain recyclables, such as scrap tires and some types of plastic, would render a hammermill ineffective and require a very different shredder configuration. 

SHEARING WHILE SHREDDING

Shear shredders combine the cutting action of shears with the rotating action of shredders. 

Unlike hammermills, shear shredders rely less on brute force and more on a cutting or scissoring action. The machine’s advocates tout their quietness and energy efficiency when compared to their louder counterparts. 

The recycling industry, for the most part, has matched the shear shredder with the processing and retained the hammermill—noise, energy-consumption and all—as the favorite for the processing of other materials. 

Scrap tires, soft plastics (such as plastic film) and nylon carpet are among the materials that are being processed with shear shredders. 

“They are very popular as the pre-shredder in tire recycling,” says Mike Hinsey, director of sales with Granutech Saturn Systems, Grand Prairie, Texas. “They are also widely used in nonferrous metals recovery to process things such as aluminum and copper wire and cable and with dealer grade scrap, where it acts as kind of a pre-breaker in the cleaning process,” adds Hinsey. 

Jeff Taylor, president of Hempstead Industries, Troy, Mich., says his company has had success selling his company’s shredders into the plastics industry, where manufacturers use the machines to process their own prompt scrap. “They’re used to process automotive plastic scrap, consumer goods factory trimmings, purgings, and skeletons from die-cut material, among other things,” says Taylor. “The plastic industry is as diverse as anything on Earth.” 

Because of that diversity, a wide range of shredders have been designed and marketed to handle different types of plastic. “The ideal design is based on the size of the pieces being processed, the type of material and the volume of material,” says Taylor. “Are you running 500 pounds per hour, or 5,000?” 

Perhaps most critical to consider is the texture of the material being fed to the shredder. “A pliable plastic requires a different configuration than, say, a PVC that is hard and that fractures when you break it,” notes Taylor. 

Among the variables that affect which machine suits a given application are the type of drive system, the decision between a keyed shaft or a hex shaft, and the amount of engine power required. 

Sundance Products, Gainesville, Ga., is a custom compounder of recycled polypropylene resin (#5 plastic) that uses a low-speed, high-torque shredder to process the scrap plastic items it takes in as feedstock. The company purchases scrap #5 plastic in all shapes and sizes, processes the material and then extrudes the plastic into pellets for resale to injection molding businesses. Approximately 80% of the scrap it purchases goes through the low-speed shredders the company operates. 

“We drop about 160,000 to 200,000 pounds a day through our shredders,” says Steve Hungerford, vice president of raw materials procurement and sales for Sundance Products. “The shredder allows us to take large plastic sheets in different forms and bring them down to a size that can be handled through the separation processes that follow.” 

OPTIONS AND CONSIDERATIONS

Many shredders offer an option of using either a direct electric drive or a hydraulic drive. Direct electric shredders dominate the market for applications that require 100 horsepower or less, according to Hinsey. The shredding of softer goods less likely to jam the rotor is considered compatible with the electric drive. Hinsey lists textiles, waste paper and secondary tire shredding as ideal applications. 

Hydraulic drive shredders dominate in those markets requiring shredders with more power and the ability to reverse rotor direction quickly due to overloads. Among the recyclables frequently processed in hydraulic-drive shredders are whole tires, nonferrous metals, rigid and film plastics, and polyurethane foam. 

“Some companies focus only on power and the size of the cutter,” notes Hungerford of Sundance Plastics. “With us the cutter configuration is very important. We look at how aggressive the hook is, how much material it can grab, the hook height and width, cutter material and the number of hooks per wheel. We place a lot of importance in the cutter configuration so the shredder doesn’t take more than it can handle.” 

Sufficient power supply does not have to be a problem with hydraulic drive machines, which can be powered by up to 800 horsepower for applications where large or tough material is being processed, or for applications where the blades are reversing frequently. 

Hydraulic systems, notes Hinsey, can be either open-loop or closed-loop. The open-loop systems pumps fluid from a reservoir through a valve package, out to the hydraulic motor and back to the reservoir. A closed-loop system maintains a continuous flow of oil between the hydraulic pump and the hydraulic motor. A small amount of oil is continuously drained from the loop and cooled and is replenished by a charge pump adding oil into the loop. Flow direction is controlled by a swash plate in the hydraulic pump. 

Most shear shredders have a twin-rotor configuration, with each rotor containing affixed blades that act as scissors as the two rotors move in opposite directions to pull in and cut material that is fed in. 

At least one manufacturer is now offering a single-rotor shredder that still relies on shearing as a key part of the processing function. “It’s a relatively slow-speed shredder at 80 to 120 rpm,” says Granutech Saturn’s Hinsey of the company’s Grizzly shredder. “It consists of a rotor that contains more than 200 30mm square knives as well as a stationary row of bed knives. As the shaft turns, it is cutting the material against those bed knives as well as with the rotor knives. A screened discharge allows you to control the product that comes out, with screens available down to one-inch in size.” 

Hinsey notes that the Evergreen Nylon Recycling Inc. facility established by AlliedSignal Inc. in Augusta, Ga., is using three Grizzly units to process a stream of nylon-bearing materials ranging from carpet to harder nylon scrap objects. “The horizontal configuration of the machine allows them to load whole bales and gradually feed the product into the loader. They can go from bales to a consistently sized discharge product,” says Hinsey. 

ADDITIONAL APPLICATIONS

Makers of low-speed shredders have continued to seek out new uses for their machines, with some of those in the recycling industry and others in solid waste. 

Portions of the growing stream of obsolete electronic scrap is finding its way into low-speed shredders, according to SSI Shredding Systems Inc., Wilsonville, Ore. “Shredders are used as part of a multi-step recycling process to recapture the precious metals commonly embedded in electronic scrap from computers, circuit boards, motors and switching drives,” says Chris Nichols, SSI senior engineer. 

Butler-MacDonald, Indianapolis, is an electronics recycling firm that uses a SSI low-speed high torque shredder in its operations. The company first shreds or pulverizes their stream of scrap electronic and telecommunications equipment to segregate the steel, aluminum, plastics and nonferrous copper-bearing materials. Later, they use pulverizers and granulators for further size reduction, prior to final material separation. 

“Taking a product and trying to reduce it to half-inch or smaller pieces can’t be done in one bite,” says Mike Thierault, production manager at Butler-MacDonald. He credits the company’s low-speed shredder as being “very effective at taking that first bite and giving us a sized product that the downstream machines can digest.” 

Solid waste generators and handlers have also turned to slow-speed shredders to process bulky items so they will take up less cubic feet in the landfill. 

Art Van, a furniture sales company based in Detroit, uses low-speed shredders to help economically dispose of old box springs, mattresses, couches and debris generated at its 26 stores. As a customer service, the company takes back mattresses from customers to ensure that they don’t end up in the secondary “gray” market. 

“We were using open-top containers from disposal companies that cost about $400 each, and we were filling about 15 of them per day,” says Art Van maintenance supervisor Ed Michael. “By shredding the mattresses, we can get away with three to four per day. The shredder helps us provide a service to our customers without overloading the landfills.” 

TURN IT UP TO MEDIUM

Medium-speed shredders or grinders—with speeds from 100 rpm to 400 rpm—also use knives or cutters to process material. 

The machines, as mentioned by Granutech Saturn’s Hinsey earlier, feature just one rotor and usually have small cutter tips on the rotor. Plastics recyclers and recyclers of electronic scrap, wood waste and light nonferrous metals are among the target markets for this type of machinery. 

According to Hinsey, plastic mold shops will use a medium-speed shear shredder to process batch-fed rejected molded plastic parts, which are screened and fed back into the extruder for repelletizing. 

Generators of prompt scrap wood, such as furniture makers or pallet manufacturers, will also grind scrap through these machines to make a fuel-quality product. 

Larger medium-speed machines, which can use up to 400 horsepower drives and rotors weighing as much as 10 tons, are typically vertically fed and rely as much on the rotor force as on the cutting edges to perform the work. “The mass of the rotor is very much part of the reduction process, unlike lower-speed machines which are more dependent on the shear cutting action,” says Hinsey. 

They are often used as a secondary process. For scrap tires, they can reduce rough-shredded tires to nearly steel-free chips less than ¾-inch in size. “We’re seeing that there is a larger demand for a low-steel content tire-derived fuel (TDF),” says Hinsey, noting that many TDF users are demanding less than 3% steel content. 

“We’re seeing more and more processors whose buyers aren’t satisfied with nominal fuel chips with too much steel,” says Hinsey. 

Other emerging markets for the machine include processors of asphalt shingles, nylon and carpet recycling, and as a secondary step for wire and cable processors. RT