Granulation, grinding, and shredding are all common terms used to describe the by-product crumbs of plastics processing. Every plastic processing equipment will produce waste and a certain degree of substandard products, and these substandard products will be discarded as waste products.
For many processors, the successful recycling of their waste can make a difference between gains and losses. However, in order to retain its value (whether it is recycled or reused or sold to others for recycling), it is often necessary to reduce the waste to a manageable and uniform size. That’s where the pelletizing and shredding equipment is located.
Granulators are very common in plastic processing plants and can generally be divided into two categories. The model next to the machine is usually used to grind relatively small amounts of film, runners, unqualified parts, and trim the edges of the film production line for immediate recycling and reuse in the process. As the name suggests, the central granulator is usually located in a room separate from the production workshop. They are usually larger and more powerful, and are usually used to shred large amounts of waste, usually from multiple production lines or forming units.
They can be equipped with special feed hoppers to hold long parts (such as pipe or profile extrusions) or wide materials (such as plates), or to de-granulate unqualified film rolls and unrolled film rolls.
The heavy-duty granulator is used to process large, heavy-duty parts and purge. Generally, the pelletizer runs at a high speed with a relatively low torque. Even the so-called “low-speed” granulator has a rotor that can rotate at a speed of 190 rpm, while the operating speed of a standard-speed granulator is 400 to 500 rpm or higher.
On the other hand, shredders tend to run at lower speeds (100 to 130 rpm) and high torque, making them chew almost anything. This explains why paper shredders are so common in recycling applications involving wood, metal and paper, and why they are becoming more and more popular in plastic recycling. These machines can be provided in a single-axis design, which can reduce the cutting of one or more fixed bed knives, or a dual-axis model, which uses two counter-rotating shafts that cut each other to shred waste.
It is believed that the biaxial model is more efficient in shredding large pieces of waste, but is more complex and more prone to damage to the shaft. In addition, the amount of tool maintenance for the dual-axis model has doubled. Single-shaft models generally provide larger, stronger rotors and use fixed bed knives, which simplifies maintenance and provides heavier workloads. Single-shaft shredders are generally considered to be more efficient in production of rigid plastics and film and fiber materials, which are products that are rapidly spreading in the plastics industry.
If you list all the different types of waste that plastic processing plants may produce, such as bottles, runners, runners, large and small moldings, films, sheets, pipes, fibers, purge materials, profiles, etc., then any one All kinds of waste materials can be treated with granulator or shredder as well. So, how to determine the best of the two size reduction techniques?
The decision may have four main factors:
•The amount of waste;
• Feeding method;
• The required final conditions.
How will the waste be handled?
The granulator does not have the lowest throughput rate. In addition to the energy required to start and run the rotor, an appropriately sized pelletizer can shred thousands of pounds of crumbs into several pounds of crumbs. The small granulator next to the machine is specifically designed to process a steady stream of waste materials at speeds up to approximately 1000 lbs/hr (454 kg/hr). Under metered feeding or batch feeding conditions, a larger central device can be installed to handle loads above 9000 lb/hr (4100 kg/hr).
The only real limitation is the size and configuration of the feed port and cutting chamber, as well as the need to avoid overfeeding and jamming of the rotor. To prevent paper jams, pellet mill manufacturers usually configure their machines to absorb smaller debris and use heavy-duty flywheels and high-power drive motors to drive thick-walled parts, but rotor damage and paper jams are always potential problems.
On the other hand, paper shredders often cannot work efficiently at very low throughput (sometimes they may not work at all). This is especially true for single-shaft crushers, which use horizontal hydraulic plungers to drive waste to the cutting area where the rotor and the fixed knife intersect. The more waste in the feed bin and the heavier it is, the easier it is for the plunger to push it into the rotor.
The shredder rotor is usually a sturdy serrated knife, a sturdy steel (or heavy welded part), which meshes with a fixed knife to chop up the debris. Continuously monitor and control the forward pressure of the feed gate and the rotor drive to optimize the feed rate of the gate for active crushing without causing overload. Most models include a control that can quickly reverse the rotor to clear paper jams caused by overfeeding, especially when thick chips or foreign objects (such as metal debris) are present.
Therefore, although the pelletizer can handle a large amount of waste, it is worth considering whether the shredder is equally effective.
What is the density of the waste?
Some waste materials can put a lot of pressure on downsizing equipment. A purge that can be a few inches thick and weighs 30 or 40 pounds is a good example. Putting a central pelletizer with a pig rotor and sufficient horsepower into the pelletizer will also generate a lot of noise, cause power peaks, and may damage the pelletizer. To avoid these problems, companies that often need to clean will first use a band saw or similar tool to cut them into small pieces. On the other hand, for the shredder, there is no problem with purging. In fact, the entire box filled with them can be poured into an open hopper, and the machine will swallow them very efficiently.
The same is true for compact shredded laundry detergent bottles. However, in the uncompressed form, loose bottles will rebound in the shredder, reducing cutting efficiency. However, if the lightweight bottles are fed into the tangential cutting chamber of the granulator with a conveyor, it will not cause any problems.
Substandard plastic film and fiber rolls (each weighing hundreds of pounds) are another example of ideal shredded waste. The entire roll can be put into the shredder, and to process the same waste through the pelletizer, the roll needs to be cut into plates or unwound before it can be pelletized. Special cutters can be provided for these shredder applications to ensure that long strips of film and fiber bundles are not entangled on the rotor.
If you need to dispose of a large amount of heavy and dense waste (whether it is purge parts or thick-walled pipes or thin plates) and want to avoid labor-intensive preparations before pelleting, you can consider using a shredder.
Which feeding method is needed?
As the previous discussion about waste volume and density implies, the shredder requires only the simplest feeding system. In fact, the shredder works best when the heavy, dense waste is simply poured into the hopper. The hydraulic plunger moves laterally in the channel at the bottom of the hopper and pushes the material into the rotor. When the ram retracts, the debris on the top of the hopper will tend to press it down into the feed channel, where the ram can push it forward again. This makes the paper shredder a “disposable” machine.
On the other hand, most granulators are the opposite. As mentioned earlier, filling the cutting cavity with waste can cause power peaks and even block the rotor. Most granulators must be metered and fed by hand, manipulator, conveyor or some other special feeding mechanism. Therefore, compared with a shredder, the amount of labor to dispose of waste materials through a shredder may be less.
What is the final material condition?
The biggest difference between the pelletizer and the shredder may be the reduced size of the waste form. The shredder is a single-stage device that cuts waste into smaller pieces that are easier to handle. The size of these fragments depends in part on the size of the holes in the grading screen, which can range from 2 inches (50.8 mm) in diameter. Less than half the size. This means that shredded products may be much larger than raw plastic particles, and the particle size and shape, dust and fines may also vary greatly.
If the material will be shipped to a recycler for resale and/or reprocessing, this may not be a problem. However, if the material is sent back to the molding machine or extruder with the mixture of raw particles, the shredded waste may need to undergo a secondary pelletizing process to obtain the best size and uniformity. Debris similar to the original material flows more easily and mixes better with the original resin and additives in the processor.
A well-designed granulator, with a sharp blade to maintain a good mechanical condition, is the best tool for producing consistent and uniform granules. In operation, the rotor knife first cuts it into small pieces, and then continues to rotate to cut along the inner surface of the sieving screen, thereby further reducing the size of the small pieces until it passes through the sieve holes and leaves the cutting cavity of the granulator . The diameter of the hole is usually 0.25 to 0.375 inches (6.35 to 9.5 mm), so the particles passing through are often granular, close to the original particle size.
The operation mode of the granulator and the shredder is different, and each has different advantages, which does not mean that the processor must choose one of them. In fact, shredders usually perform rough size reduction tasks, and the chips are then fed into the pelletizer for final sizing to produce a consistent, uniform feed. The biggest advantage of the shredder is that it effectively cuts a large amount of heavy plastic waste into smaller, more manageable pieces with a minimum of operators. Then, the granulator can complete its own work, deal with less demanding grinding tasks, and if necessary, convert the shredded waste into valuable regrind that can be used in the process.
If you carefully observe the types of waste generated in the process and consider how to dispose of the waste after reducing the size, then whether you need the power of the shredder or the fineness of the granulator should quickly become clear. Suppliers who can provide you with these two options will be able to help you analyze the situation and make the best decision based on your needs.
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Link to this article：How to determine the size reduction problem facing the processor?
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