The Characteristics of Plastics Granulators and Plastics Shredders

Size Reduction

The Characteristics of Plastics Granulators and Plastics Shredders

Nearly all of the different kinds of scrap generated in a plastics plant – bottles, sprues, runners, small and large molded parts, film, sheet, pipe, profiles, fibers, purgings – can be processed using either a plastics granulator or a plastics shredder. So, how in a particular application, can you determine which of these size-reduction technologies is best?

Four key considerations – Volume/throughput, Density, Feeding method, and Material Size/Condition requirements – will usually provide the answer:

Volume/Throughput. Granulators don’t have a minimum throughput limit. When properly sized, they can handle hundreds or even thousands of pounds of scrap as easily as a few pounds. The range of granulators available is extremely broad:

  • Small beside-the-press granulators, like Conair’s Viper 12 Series, 8 Series, 6 Series, and S Series, are designed to handle steady streams of scrap at rates 900 lb/hr (410 kg/hr) down to as little as 9 lb/hr (4.1 kg/hr).
  • Larger, central units, ranging upward from Conair’s Viper 17 Series, through its 23, 32 and 35 Series, can handle from 1350 lb/hr (612 kg/hr) to as much as 13,200 lb/hr (600 kg/hr) using either metered feeds or batch feeds.

So too is the scrap that they will handle. The principal limitation on granulator capacity is the size and shape of the feed opening. If the scrap fits in and doesn’t jam the granulator, then it can probably be size-reduced. However, for frequent high-volume scrap, a granulator may not be the most efficient way to do the job.

Shredders are optimized to handle larger volumes and heavier parts. Most plastics shredders aren’t efficient for handling very low throughput rates. This is especially true of single-shaft shredders, which use a horizontal hydraulic ram to drive scrap material into the cutting area at the intersection of the rotor and the stationary knives. The more scrap there is in the feed bin and the heavier it is, the easier it is for the ram to push it forward into the rotor. Rotors are usually solid steel (or heavy-duty weldments) with rugged saw-tooth cutters that intermesh with the stationary knives to shred the scrap.

Unlike granulators, whose rotors are fed scrap by gravity, shredders monitor and control both ram speed and rotor speed to assure that scrap feed rates are optimized for aggressive shredding without overloading. Most models include controls that can quickly reverse the rotor to clear jams that can occur from overly aggressive feeding, particularly thick scrap, or foreign matter like tramp metal.

Density. Some scrap can put a considerable strain on size-reduction equipment. Purgings, for example, can be several inches thick and weigh thirty or forty pounds. Putting one in a granulator, even a central granulator with a hog rotor and plenty of horsepower, can be noisy, can cause power spikes and can potentially damage the granulator. To avoid these problems, companies who recycle purgings with granulators often cut them into smaller pieces using a band saw or a similar tool.

Big scrap like purgings are no problem for shredders. In fact, a bin full of purgings can be dumped into a shredder hopper and the machine will devour them quite efficiently. The same holds true for a bale of crushed laundry detergent bottles—a shredder would love them. However, in undensified form, a shredder wouldn’t handle them very efficiently; the loose bottles would bounce around too much.  But loose, lightweight scrap would pose no problem if it was manually or conveyor-fed to the cutting chamber of a granulator.

Off-spec rolls of plastic film and fiber, some weighing hundreds of pounds each, are another example of scrap that is ideal for shredding—the whole roll can simply be dropped in. To process the same scrap in a granulator, the roll stock would have to be cut into slabs or unrolled. Special cutters are available for these shredder applications to ensure that long strips of film and fiber strands do not wrap themselves around the rotor.

If you need to process high-volumes of heavy, dense scrap (whether it is purgings or thick-wall pipe or sheet), and you want to avoid the labor-intensive prep work needed to use a granulator, a shredder is likely your best choice.

Feeding method. As you’ve probably noted already, shredders don’t require careful feeding. They work just fine when heavy, dense scrap is simply dumped into the feed hopper, where a hydraulic ram built into the bottom of the hopper pushes the scrap into the rotor. When the ram retracts, the weight of the remaining scrap above tends to force it downward, so the next ram cycle can move it forward.  Thus, shredders are “dump-and-forget” when it comes to feeding.

Most granulators are just the opposite. To avoid jamming, they need to be fed smaller amounts of scrap continuously, either by hand or by an automated feeding system—a robot or conveyor.

Material Size/Condition requirements. Probably the single biggest difference between a granulator and a shredder is seen in the appearance of the scrap after size reduction.

Shredders are single-stage size-reduction machines.  They chop up scrap into smaller, more manageable pieces, with the sizing determined in part by the size of the holes in the shredder classifying screen, which can be as large as 2 inches (50.8 mm) in diameter to less than half that size. Shredded material thus tends to vary in size, contain a lot of dust and fines, and be considerably larger in size than granulated material.

This may not be a problem if the material is going to be shipped off to a recycler. However, if the intent is to blend the regrind with virgin pellets and send it back to a molding machine or extruder, shredding alone is insufficient. Shredded material usually needs a secondary granulation process to have the optimal size and uniformity needed to flow and blend with virgin material and other additives in the processing machine.

A well-designed granulator, kept in good mechanical condition with sharp blades, is the best tool available for producing uniform, process-ready granulate from scrap. In operation, the granulator’s rotating knives cut the scrap repeatedly, first on the edges of the cutting chamber, and then along the inner surface of the sizing screen that lines the inside of the chamber. The material continues to be cut until it passes through the holes in the sizing screen. Holes are usually ¼ to 3/8 inch (6.35 to 9.5 mm) in diameter, so the particles passing through tend to be granular, close to the size of virgin pellets.

When examining the characteristics of each type of granulator and shredder, it is also important to assess the advantages of each one before choosing a piece of equipment for your factory.