Plastics Engineering Magazine11/10/2015
Plastic Shredding/Granulation Options for In-House Reclaim
Dana Darley, National Sales Manager, Vecoplan LLC, Archdale, NC USAProducers of extruded products have three main options when dealing with their own internal scrap…re-use, resale, or material disposal. For any of these three, some type of size reduction of the scrap is either required or of some benefit. We will begin with an overview of the most popular shredding technology typically used for extruded product. Single shaft/rotor shredding systems will be reviewed in both vertical and horizontal configurations. Features and benefits of each technology will be outlined, as we look at a variety of extruded products, such as film, fiber, sheet, non-woven, profile and pipe, as well as blow molded containers and extruder purgings. We will look at the options for size reduction primarily for reprocessing, including the integration of conveyors, metal detection/separation and granulators. While focusing on the shredding step, we will discuss the best technology for each product and discuss basic configurations for typical size reductions systems. We will also review the very latest in shredding technology, which features systems designed to size reduce extruded product to the optimum size for reprocessing; all in one step and while maintaining product quality and high production rates. The vertically fed, single shaft shredder is the workhorse of the plastics recycling industry (see figure1, Shredder, with HiTorc magnetic drive). Material is fed either by hand, fork truck, inclined conveyor or a container lift and dump system into the top of the shredder feed hopper. Belt conveyors are often the method of choice, as they allow material to be staged and give the opportunity for metal detection/separation, before entering the system. In most cases, the feed hopper can be filled, without concerns about over-feeding the processing area of the shredder, and a photo eye can be installed to control the infeed conveyor/dumper. Once the shredder is filled with material, the horizontal hydraulic ram takes over the process. Based on the load/torque of the rotor drive, the PLC control system will use the ram to push material into the cutting rotor of the shredder to maintain the highest efficiency during the size reduction process. Material will drop through the sizing screen once the desired particle size is reached, and is then either mechanically or pneumatically transferred from the shredder to further downstream processing equipment. Shredders and granulators are basically opposites of each other in the size reduction world, for this reason make for great partnerships. Shredders work best when completely filled with heavy, bulky materials, and can effectively reduce this material to particle sizes of 1” or smaller. Most granulators need to be continuously fed material, with care taken not to overfeed, which could cause a blockage or stall of the rotor drive. Large, bulky parts could just bounce around and never efficiently feed into the high speed cutting rotor. However, when properly fed, granulators are very effective at size reducing a constant infeed of material to a small, uniform particle, down to ¼” or smaller. The marriage of the two systems often comes when large, bulky or heavy products need to be size reduced to the smallest particles possible, and with a minimum of operator attention. Shredders can be bulk or batch loaded, with any size or amount of material that will physically fit into the feed hopper of the system. The operator can walk away as the shredder continuously processes the material to a comfortable particle size for the granulator, which will further size reduce to the desired level. This is a process often referred to as “Dump and Run.” Film, fiber and non-woven fabrics can be some of the more difficult products to process. Whether introduced to the system in a baled, rolled or loose form, the tenacity of these products and the tendency for long pieces to self-feed into the shredder rotor, special consideration is required for the design of the machine. To handle self-feeding products and the tendency to wrap, a second set of nip cutters are used to keep material from winding around the rotor (see figure 2, FF rotor). A further measure may be required to cover the top of the rotor, to limit material feed and put the hydraulic ram in full control of the process. These materials can also generate heat during the size reduction process, so cored rotors can be applied that allow for cooling water to flow down the entire length of the rotor. High power drives can also be used to better process these demanding material, such as a magnetic drive (see figure 1, Shredder, with HiTorc). For re-pelletizing and densification processes, a 2” – 3” inch particle size is often required, which can be achieved by the shredder alone. If a smaller particle size is required, a combination shredder/granulator system can be supplied.
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