Shredder Selection Basics

If you are starting to research a shredder for your operation, look out! There are so many types of shredders today, it’s confusing for everyone. Even the names for the different types of technologies are inconsistent; grinders, granulators, mills and shredders can refer to the same type of shredder or to completely different types depending on whom you talk to. The word "shredder" generally refers to a single-, double-, three- or four-shaft system.

On the surface, the perfect solution is readily available as you watch videos of shredders process various infeed materials at record rates.

Unfortunately, reality is a different story. Despite all the equipment available and its increased capabilities, no one machine is perfect for all applications. There isn’t a single shredder that can do it all.

Finding the best shredding equipment solution requires a more thorough approach today. A buyer-beware attitude is appropriate given all of the different types of equipment available and their seemingly unlimited capabilities.

The basic criteria for buying shredding equipment has not changed. The specific application requirements of the customer need to be clearly identified prior to the evaluation of any type of shredder. This includes a thorough analysis of the type of material to be processed, the feed method, the mix or percentage of material processed, the desired particle size, discharge requirements and budget.

Will the material be presented to the shredder in boxes, bales, rolls, slabs or loose? Will the material stream contain contaminants? Is general volume reduction required or is a specific particle size required?

Typically the higher the production rate and the smaller the particle size desired, the more expensive the solution, so be certain about your requirements. For example, if you request the shredder produce a 50-millimeter particle size, be certain that a 50-millimeter strip of material is not adequate. The difference can be significant, as the 50-millimeter-by-50-millimeter size will likely require a system with a screen to recirculate the material. Using a system with a screen has trade-offs.

While screens are useful at producing a uniform particle size, the screen may produce some undesirable side effects. Screens may reduce the throughput rate significantly, and some materials may blind the screen. Also, because the screen restricts material flow, damage to the screen or equipment may occur if non-shreddable material is processed.

Recycling applications can often involve one-, two- or four-shaft shredders.

Low-speed, two -shaft shredders called rotary shear shredders have been used for years to process material across a wide range of applications. The two-shaft shredder is well suited for applications such as composting, municipal solid waste, tires, metals, industrial waste, electronic scrap, plastics and paper.

The advantages of the two-shaft shredder are many; but, perhaps most notable is its ability to process contaminated waste steams that may contain metal or non-shreddable materials.

Because of the large heavy-duty, low-speed cutting disks and the automatic reversing capabilities of the shredder, the chance of damage to the equipment is minimal when the shredder encounters difficult-to-shred materials. These large-diameter cutters also are known for long life, sometimes lasting years in non-abrasive applications.

A two-shaft shredder of the proper size and design can be reliable, provide low operating cost and process a high volume of material under many feed conditions and operating environments.

Companies such as SSI Shredding Systems of Wilsonville, Ore., provide shredders in sizes ranging from 10 horsepower to 800 horsepower to meet the many applications for which a two-shaft shredder is suitable.

For some materials and for applications that require a smaller more uniform particle size, the two-shaft shredder is less suitable. Two-shaft shredders are somewhat limited in the particle size they can produce. Most materials tend to be shredded into strips of various lengths. Strip width depends on the cutter width. Document shredding applications that require a higher level of security typically use a 5/8-inch wide strip. Most other applications use a cutter 1 to 2 inches wide on small and mid-range systems (20 horsepower to 100 horsepower), while larger systems (150 horsepower to 500 horsepower) use 2-inch to 6-inch cutters.

Particle length using a two-shaft shredder varies widely, depending on the material, and can range from a few inches to a few feet. This particle size may not meet the need in some applications to produce a consistent and smaller particle size.

One-shaft (or single-shaft) shredder technology is one option if stringent particle-size control is required. This technology has advantages for certain applications. Single-rotor shredders offer the advantage of better particle-size control through the use of a sizing screen. This technology typically operates at rotor speeds in the 80-to-110-rpm range, with some shredder styles operating at much higher rpms.

The single-shaft shredder is ideal for processing "clean" material streams, such as plastic purgings, foam, fiber, paper, rubber and even some metals like aluminum. Most use a horizontal ram to feed the material to a single drum rotor. Rotor sizes vary and are typically 300 millimeters to 500 millimeters in diameter. The cutting action is achieved as 30 to 90 small (40- to-50 millimeter) square cutters pass a fixed cutting anvil. If a sizing screen is used, oversize material is re-circulated and processed again by the rotor until the desired particle size is achieved. The small cutters often can be rotated two to four times and replaced when worn or damaged. Screen sizing is typically 5/8 inch to 6 inches.

Achieving a uniform particle size is a major advantage of the single-shaft shredder, but this technology has its limitations, too. Because the system is processing at a higher speed with a smaller hardened steel cutter, some damage is likely to occur should the cutter encounter heavy metal or non-shreddable material. The damage can range from minor to extensive. Cutter changing is frequent when processing abrasive materials such as carpet. Some materials can plug or blind the screen, and the higher-rpm rotor can generate heat and fines with some materials.

The single-shaft shredder is a viable solution with certain types of clean material, but it can be high maintenance when processing material that contains contaminates or abrasives.

Four-shaft, low-speed shredders also are available from some suppliers. This technology combines the two-shaft shredders’ reliability and uses a sizing screen for particle size control.

The four-shaft shredder is a top-fed machine, like the two-shaft shredder. The cutting disks shear the material against the opposing cutter. The lower sets of cutters process material bi-directionally while operating in either direction. The upper two sets of cutters clean the primary shafts and also provide some shearing. All the cutters sweep the sizing screen, which produces the desired particle size.

SSI Shredding Systems has recently introduced the Quad 140(63) 500-horsepower system for high-volume applications that require a more uniform particle size than a two-shaft shredder is capable of producing. It is suitable for processing mixed materials streams, including challenging materials such as computer scrap.

No one piece of size-reduction equipment is the all-in-one, do-it-all machine. Each technology has its strengths and weaknesses, and each machine excels in certain applications and is less effective in others.

When selecting equipment, choose to work with suppliers who are experienced with the equipment, support the system with parts and service and provide a high level of application knowledge. A shredder supplier that has proven success with your application is important, and testing your material on the system you are considering also is recommended.