Shredders play an integral role over a wide range of applications. It is important to take the time to ensure that you have identified not only the right size of machine, but also the right style of shredder to meet your needs.
By Nathan Wilt
Over a broad range of materials—from general waste like MSW to specialty materials such as manufacturing scrap or insulated copper wire waste streams—shredders are typically used for two reasons. The first is to provide separation of a varied material stream in order to upgrade the material to a more marketable product or recover material to divert it away from disposal, such as at a landfill. A raw stream that is mixed and co-mingled will need to be singulated so it can be sorted, and the desired commodity(s) separated from other material. The second reason to use a shredder is to provide general size/volume reduction. Examples of this might be to reduce transportation expense, product destruction, debulking scrap for easier handling within the plant or even reducing the size of material to optimize foundry operations.
Upgrading material is increasingly becoming the more common reason for shredding and can take many different paths. An important thing to keep in mind is that a shredder is not just equipment to break material down, it is preparing it for downstream equipment, so matching the most appropriate shredder to the downstream equipment is critical. Most sorting equipment requires material to be presented to it in a consistent manner to maximize production. Surges of material and incomplete liberation will hinder sorting equipment from being able to effectively classify the material, reducing overall system throughput or increasing contamination. This same principle applies over a wide range of applications. If you are to the point where you are evaluating the benefits of a large capital project, it is essential to look at equipment like the shredder from a system approach in that it will not only size reduce material at a given rate, but that it will also make the rest of the system perform optimally.
The Right Time for Purchase
Consider investing in a shredder when you have enough volume to where it is not economical to break down and separate/sort your inbound material by hand, if you need to physically liberate encapsulated or laminated material, or you need to provide a consistent particle size of material. It is more than just purchasing the shredder itself—the shredder is usually just a part of a bigger process and you really have to look at your goals and the system as a whole in order to match the shredder to the rest of the overall recycling system.
Understanding your markets is critical for maximizing the benefit of a shredder. Equipment like a shredder may be able to meet all of the technical goals that you have for it, but if you do not have enough supply of material to process, or you cannot realize enough net benefit for processing it, you might not be quite ready for the investment into the equipment.
An example of this might be illustrated with electronic scrap. Early on, an E-Scrap processor may begin small with 10 to 20 people manually pulling computers apart to recover the valuable boards. Once there is enough volume, however, people cannot keep up and it makes more sense to use a shredder to break apart whole components and sort the shredded material with different equipment. The business may continue to expand into other complimentary markets that requires additional equipment to enable that capability. Continuing the E-Scrap example, breaking down desktop computers is a very different beast from breaking down larger electronic devices such as a standalone copier/printer workstation that would be in an office environment. Because of this, an E-Scrap Shredding line may need to add a Pre-Shredder to further automate the process and allow for the original equipment to process this new material stream.
When you are investigating purchasing a shredder, keep in mind material handling requirements and configurations as well. Difficulty in feeding the shredder or downstream equipment can significantly hamper the benefits that the shredder can bring to the table. If the shredder is preparing material for downstream equipment, the shredder is not usually the bottleneck, other equipment (especially sorting equipment) often is. A screen, optical sorting system, or even robotics can only process material so quickly, and how you present the material to those system components can have a significant impact on how close you are to reaching your goals. One of the benefits of a shredder is that it not only sizes material, but also naturally meters it to the equipment downstream. Even so, some downstream equipment will require additional surge bin or metering material handling equipment to provide as consistent a flow as possible.
Differences Between Shredders
Each type of shredder has its own strengths; there are very few applications where a “one size fits all” attitude can be applied to the shredder selection. Sometimes individuals have had very good success with a specific type of shredder in one application, only to find that the same piece of equipment is disappointing in another. Because of this, it is important to not to try to force a fit.
Low speed/high torque shredders include shear shredders such as two, three, and four shaft shredders. This style of shredder typically operates at ~20 RPM and has a very tight clearance (on the order of a few thousandths) between cutters. Along with shear shredders, single rotor shredders, which operate at ~100-200 RPM and primary reducers are available.
Single Shaft Shredders
Single shaft shredders are fantastic for plastics or paper and other dry materials. They use replaceable internal screens and can offer high throughput to a controlled particle size for an economical price point on the applications where they excel. The higher speed that they operate at is a two-edged sword—it increases throughput, but also means that they are less forgiving to things like tamp metal. A consistent, smaller particle size often helps this type of shredder immensely, which is why they are often used as a secondary shredder. Maintenance on a single shaft shredder is more frequent, but consumable parts are less expensive on a per-replacement basis. Maintenance intervals are more frequent, but also shorter in duration typically. Throughput on this type of machine will often vary quite a bit between maintenance intervals. After cutters have been replaced or flipped (to expose fresh cutting surfaces), throughput is very high, but can taper off quickly as the cutters wear. If you were to graph it, the throughput on a single shaft shredder often looks like saw teeth—very high at first, but then tapering off until the cutters are flipped or replaced.
Multi-Shaft Shear Shredders
Multi-shaft shear shredders have a wider range of material that they can process in general. The lower speed allows them to be more resistant to contaminants like tamp metal, and the higher torque allows them to process heavier duty material than a single shaft. Sometimes, applications lend themselves more to a multi-shaft shredder due to the material characteristics. An example of this would be if the material is wet and/or sticky waste (such as MSW) A three or four-shaft shredder, because of the way that it is set up with their screen element, is self-cleaning. If you need to shred food waste or MSW that has a high moisture content, and you want a particular size particle, we would generally recommend a three or a four-shaft over a single rotor. When you need to handle materials that are compressible or thin and stringy (like film plastic), multi-shaft shredders tend to work better. Along with that, the physical size of the material going into a shredder has a big impact on the correct unit. Single shaft shredders have to get the material into the side of the cutting chamber, and often use a hydraulic pusher to force it to engage on the cutters. Shear shredders can do the same, but the larger distances between the two shafts create a ‘valley’ in between the cutters that allows for it to grab physically larger material. This effect is increased in three and four shaft shredders due to the wider cutting chambers.
Variable Speed Drives
Newer technologies also can increase the capabilities of different shredders. Using Variable Speed Drives, a two-shaft shredder is able to do things like breaking down baled radiators, where it would jam up a single shaft machine or even a two-shaft machine that was not set up to process them. Features like SmartFeed for two and four shaft shredders, use Variable Speed Drives to slow down one shaft based on how hard the other is working. This prevents the machine from taking ‘too big of a bite’ of the material, metering is so that it doesn’t overload the shredder. As a result, it can process bigger, nastier material in a smaller machine than was previously possible. Shear shredders are a bit like a swiss army knife and can handle a wide variety of materials. This. again, has different strengths than a single rotor. Maintenance intervals are much further apart, but the individual maintenance durations are longer (perhaps a day or two compared to a couple of hours with a single shaft). Throughput on a shear shredder is not as steep a curve as a single shaft. It is more of a plateau that continues until there is significant wear on the cutters at which point it begins to drop off significantly.
Primary reducers are used when you have an extremely mixed materials stream and you anticipate that there will be unshreddables among the waste. C&D is a classic example of where a primary reducer shines because you can throw stuff into it like steel plates and railroad ties. Its design has a big open cutting chamber like a stationary grid and the cutters swing through it, so it is more of a pierce and tear cutting action, rather than a shear cutting action. Once the material is busted up, it can fall through the grid. This allows for a varied material mix that can be very large dimensionally because of the large sweep of the cutters. Since it is not actually cutting the material, maintenance is significantly reduced as well.
There are very few one-size-fits-all machines. Everybody wants that one machine that will do it all, however, there are so many applications in waste that are so specific, you would have a hard time forcing one type of shredder to be able to handle it all, especially if you are trying to get a controlled particle size. It is essential to select the right kind of shredder for the specific materials that you are processing and the goals you have.
Keeping Your Systems (and Employees) in Top Shape
To get the most out of your shredder for a longer period of time, make sure you are performing maintenance within the intervals that are recommended by the manufacturer. In addition, general cleanliness does make a big difference along with periodic adjustments. Many times, people look at a shredder like it is just is a big machine that breaks stuff down. However, it can make your operation much more efficient if you take the time to make the periodic adjustments as required. This can vary quite a bit from machine to machine or shredder style to shredder style.
Because a shredder is a big machine with a lot of torque, you do not want operators close to it. Make sure that you have procedures in place to remove operators from the area as much as possible. Depending on the material, guarding like a hood on the hopper may be required to prevent material from being ejected from the shredder. Ensure that there are safety interlocks that are in place and operational on all access points. Make sure that your maintenance procedures are safe and that maintenance personnel and operators have been trained on how to control any hazardous energy with the unit. Look for a shredder that is easy to work on in a safe way. When you are looking into a purchase, think about not only how frequently an employee will be in it, but also how easy it will be to work on.
An Integral Role
Shredders are an incredibly useful tool in the waste industry. As we move to diverting more and more of a complex material stream away from disposal like landfills, we will continue to grow the need for the equipment to not only sort, but also prepare the material. Shredders play an integral role in this over a wide range of applications, but it is important to take the time to ensure that you have identified not only the right size of machine, but also the right style of shredder to meet your needs.
Nathan Wilt is in technical sales for SSI Shredding (Wilsonville, OR). Nathan has a passion for equipment and system integration from a background as an engineer and project manager in the semiconductor field for more than 16 years. Nathan has been working at SSI for more than four years, focusing on integrating shredders into larger systems such as Material Recovery Facilities and non-ferrous shredding and sort lines. He can be reached at email@example.com.