NOVEMBER/DECEMBER 2006
Small is beautiful to granulators, which turn bigger pieces of scrap into smaller ones. Here’s how to get the most from these versatile processing machines.
BY LYNN R. NOVELLI
A granulator—as its name suggests—essentially processes bigger pieces into smaller pieces, called granulate. In the scrap world, granulators make material small to more easily separate it from contaminants and, ultimately, to have it meet a desired specification.
Granulators first appeared on the scrap recycling scene during World War II. The wire chopping industry implemented the technology to recover rubber from insulated wire and cable for use in the war effort. Though the basic design of granulators has changed little in the ensuing 60 years, these machines do boast some engineering refinements and incorporate the latest wear-resistant steel. And the granulator market has expanded over the years, with processors using them not only for scrap wire and cable but also for scrap tires, electronics, plastics, and more.
Chop, Chop: Granulator Basics
Granulation systems come in a variety of sizes and processing capacities. At one end are single-operator mini-machines for small, low-volume scrap chopping or granulating operations. At the other end are multistage processing systems for large, high-volume facilities. To maximize production, these bigger plants use high-speed granulators, which can operate at speeds of 500 to 1,200 rpm.
In general, granulation systems work best when incoming scrap material is preprocessed. This can mean size reduction—shearing or shredding scrap into relatively short, uniform lengths. Wire choppers, for instance, feed bundles of bulky wire and cable into preshredders that cut the material into lengths suitable for granulation. Or, in tire-processing lines, a preprocessing step removes the bead wire so that the rubber strips or shreds contain only rubber, tread wire, and fabric when they enter the granulator.
Next, the preprocessed material moves by conveyor or grapple into the primary granulator, basically a closed chamber containing one or more rotors with cutting knives attached. As the material enters the chamber, the rotor—powered by an electric motor—turns and cuts the material between a fixed knife and the rotating blades. The high speed of the rotor generates heat during processing, so systems use water or pressurized air to cool the chamber. Some manufacturers claim air systems are best because they also can improve material flow and circulation through the chamber for more uniform cutting.
As material becomes granulated, pieces that are small enough pass through a perforated grate to a hopper or another conveyor. Larger pieces remain in the chamber for further processing until they are small enough to pass through the grate.
Most wire-chopping lines and some tire-recycling systems include a second granulator or series of granulators, depending on the system’s capacity and the size specifications for the finished product. For wire and cable, the secondary granulator is a more precise machine than the primary granulator, calibrated to chop the scrap into small enough pieces to “liberate” the metal from the insulation.
A separator follows the first granulator in tire recycling and the secondary granulators in wire chopping. At this stage the material consists of intermixed fractions—metal and insulation in the case of wire; rubber, metal, and fiber in the case of tires. For wire, separation of the clean metal from the insulation is the final step. In tire recycling, the clean rubber crumb can proceed through additional processing steps. To reduce further in size, rubber crumb runs through a secondary granulator that processes the material to a fine powder. For some applications, the material passes through a cracker mill, which produces an extremely fine, clean powder.
Capacities for granulators in the wire-chopping industry range from 1,000 to 15,000 pounds an hour. The size of the equipment is one determinant of capacity and production, but another is the gauge of the infeed wire: finer-gauge wire yields a lower hourly production in terms of pounds recovered.
For a typical rubber granulator processing ¾-inch pieces of scrap rubber, the capacity is reportedly about 5,000 pounds an hour.
Staying on the Cutting Edge
“Anyone running a wire-chopping or tire-recycling line is looking for three things,” says a spokesman for a granulator manufacturer: “efficient cutting, high production, and easy maintenance.”
For those three requirements, the granulator’s knives make all the difference. Most granulator models have both fixed and rotating knives. Knives engineered for cutting wire have a 90-degree tip and a blunt edge in a multiple-edge blade, which engineers consider the ideal configuration for processing the soft metals—primarily copper and aluminum—in wire. Knives intended for cutting rubber usually have a 50- to 55-degree tip and a sharp edge. Other engineers recommend a slight rake angle, 80 to 85 degrees, with a sharp tip for clean, straight cutting of fine-gauge wire.
The various granulator brands differ in the number, configuration, and design of their knives and rotors. The number of knives ranges from fewer than 10 to more than 140, depending on the size of the equipment and the application. Manufacturers that use fewer knives say their granulators still cover the full cutting width of the rotor and require less time to change knives.
Likewise, each manufacturer has its own design for the cutting chamber and for positioning the knives. The four basic rotor configurations used today are straight, V-shape, helical, and scissor cut. The rotor configuration determines the granulator’s cutting geometry—that is, the shape of the cut the knives make. In the competitive wire chopping industry, processors are always seeking greater efficiency and production, and mechanical engineers have shown that changes in cutting geometry have a significant effect on both. “The newer machines offer many more choices in cutting geometry,” one granulator vendor notes.
Each granulator manufacturer has developed a proprietary blade design to maximize cutting efficiency and reduce downtime for blade sharpening and changes. “This is an incredibly rough-use machine, and everyone tries to come up with a knife design that combines the best cutting characteristics with the longest service life,” says a granulator sales representative.
Blade-Runner Realities
Granulators must withstand the abuse of processing tons of wire or rubber for one or more shifts every day. The knives are the essential component of granulators, and recyclers who want to maximize their investment in the equipment must treat their blades with care. Their profitability, in fact, can depend on striking a balance between processing speed and knife wear. Processing faster increases output, but only up to a point because it also dulls the blades more quickly, which then slows processing.
“Ultimately, the goal is to maximize output at the highest speed possible without causing excessive wear and tear on the blades,” says another industry expert. “At $2,000 to $3,000 for a set of blades, processors need to use their blades wisely.”
Knife placement in the rotor and the angle of the blade’s cutting edge can make a difference in the blade’s chip and nick resistance and its ability to retain a sharp edge. Granulator knives can require sharpening at vastly different intervals, depending on the density and cleanliness of the material they are processing. According to wire chopping experts, decent-quality knives can process 100,000 to 500,000 pounds of clean copper and aluminum wire between sharpenings. An easy way to check blade sharpness, they say, is to inspect the final granulate, looking for clean cuts without excessive bends or hooks. This becomes more important as wire diameter decreases.
Sharpening or replacing knives means production downtime, so granulator manufacturers have devised all kinds of strategies for minimizing sharpening requirements. One vendor has designed its granulators with fewer knives, which translates to less time required for sharpening. Another offers a four-sided blade, basically a rectangular block with four sharp edges. When one edge dulls, the processor rotates the blade to the next edge, which extends the time between sharpenings. Alternatively, some processors simply hold a set of new, sharp knives in inventory so they can switch them out with minimal loss of production time.
A set of high-quality granulator blades should be able to take half a dozen sharpenings before they must be replaced, manufacturers say. That translates into buying about two sets a year for most applications, one product manager says. He estimates that his company’s blades will last an average of six months, two shifts a day, when they’re processing clean, nonabrasive material.
Another vendor suggests owning multiple sets of blades with varying hours and degrees of wear. “This allows the processor to change the blades based on the material being chopped and still get maximum production,” he explains. “The other way to do it is to change the infeed material as the knives get duller.”
Screening Out Problems
The knives might be the most important part of a granulator, but other wear parts need regular attention to keep the equipment in top running condition.
After the knives, the cutting-chamber screen—the grate through which processed material passes—is the part that most frequently wears out. Screens are less costly to replace than knives, but processors still must stop production to replace them. For the longest service life, granulator experts advise buying a cutting-chamber screen made of high-grade, heat-treated steel. Precision drilling of each hole provides consistency in product size and adds to separation efficiency—and, ultimately, material recovery.
To keep a granulator running at its best, manufacturers also recommend frequently inspecting and adjusting the gap between the rotor blades and fixed-bed blades to ensure clean, consistent cuts and prolong blade life. And operators should check the drive-belt tension regularly to ensure it is at the manufacturer-recommended level.
Though monitoring electrical systems and noise levels does not fall under maintenance, processors need to perform regular monitoring and reporting to satisfy OSHA regulations.
Minding Maintenance and Efficiency
Employee training in granulator operation and maintenance is one of the keys to a long, productive life for this hard-working equipment.
Maintenance should include inspecting equipment for signs of wear as well as cleaning and making adjustments daily. “Processors who maintain their equipment and replace or build up wear parts or areas as needed will be rewarded with extended service life,” a granulator vendor notes. This is critical, he adds, because “once a granulator’s maintenance is neglected to a certain point, it can go beyond reparability—and that gets expensive.”
Controlling the infeed makes all the difference in maintenance, output quality, and productivity. The infeed should always be as clean as possible. Excessive dirt or contaminants can reduce blade life and put extra wear on the shaft bearings, which causes vibration in the rotor and reduces cutting efficiency.
Prechopping or shredding the infeed can help maximize productivity and reduce maintenance. “When everything is prechopped, there’s nothing to cause the granulator’s rpm to increase and overload,” one vendor notes. “Granulators operate best when they have a nice, steady load and avoid surge feeding.”
For further control, granulators can be equipped with a built-in load-sensing monitor that automatically manages the infeed rate to maintain maximum productivity without stressing the machine. Load-sensing controls on the infeed system signal whether to increase or decrease the feed rate.
With proper care and maintenance, granulators can reportedly last 15 to 20 years, even under the hardest operating conditions. In fact, one industry insider says, “the old machines are one of our biggest competitors.” With new machines carrying hefty price tags, keeping an older machine in top operating condition makes good financial sense.
Lynn R. Novelli is a writer in Russell, Ohio.