Principle of Waste Plastic Recycling Extruder Granulator


Principle of Waste Plastic Recycling Extruder Granulator:
Most polymers must be blended and then granulated to become marketable materials before they can be made into a final product. There are much different designs of waste recycled plastic extruder granulators, but all plastic extruder granulators can be divided into two categories: cold granulation system and die surface hot granulation system. The main difference between the two is the arrangement of granulation time. Cold granulation system, at the end of the processing process from the solidified polymer granulation; In the die surface pelletizing system, the molten polymer is pelletized as soon as it emerges from the die surface, and the pellet is cooled downstream. The two granulation systems have their own advantages and disadvantages.

Waste Plastic Recycling Extruder Granulator Video:

Cold granulation system
The cold plastic extruder granulator consists of a die, a cooling zone (air-cooled or water-cooled), a drying zone (if water-cooled) and a pelletizing chamber. There are two kinds of cold granulation systems, that is, sheet granulator and strip granulator.
Sheet granulator The molten polymer is calendered into sheet polymer of a certain thickness from a mixing facility through a belt die or roller mill. The pellets are solidified and cooled over a distance during transportation, and then cut into round or square pellets with a pelletizing knife in a chamber.
Sheet granulation is the oldest method of making pellets and can be used with a variety of different polymers from nylon to PVC.
Big weight. The accuracy is reported to be quite good, with a pelletizing capacity up to 1843.69kg/h. This is a cold pelletizing method with a higher noise emission than the pelletizing method from molten polymer. The solidified polymer cutter has a short life and the power generation is often a problem. Some “grain chain” phenomena can be seen in some polymers.
The use history of strip granulator is almost as long as that of sheet granulator. Includes mouth die, cooling section (water bath or blower), drying section (if water cooling is used), and granulating knife. The molten polymer is extruded by an extruder or a gear pump through a horizontal mounting die to form a bar (modern die is precision machined and uniformly heated to produce a stable quality bar). After the strip is discharged from the mouth, it is cooled by a blower or an air/vacuum device, or by a water bath. If water cooling is used, the strips shall pass through a dry section, remove moisture with forced ventilation, and then be sent to the pelletizing chamber. Using the shear action of a pair of fixed knives and a rotating knife, the bar is cut to the desired length precisely.
The diameter of the granule is 3.175mm, the length is 3.175mm, and the edges and corners are clear.
The traditional method of drawing the bar is to stretch the bar through the cooling section (most commonly a water bath), sometimes causing the bar to fall or size inconsistencies. This is most common in polymers with weak melting strength, such as polypropylene, polyester and nylon. When the strip is dropped, the material is scrapped, so the operator needs to pay close attention. If the strip drawing is not consistent, the downstream granules need to be screened.
Other modes of strip forming can be done without the operator’s close supervision by using a motion-driven slotted feed conveyor to support and split the bars from the die to the granulator. The size of the material transported by the rotary force is more uniform, and it will not fall, thus less scrap. Some of these methods can yield up to 6803.89 kg/h, compared with about 1814.37 kg/h for the stretcher method, because the operator can only look after a limited number of strips.
The strip production line has the advantages of low cost, simple operation and convenient cleaning. This has advantages for color mixing, as two batches of different colors must be replaced by a thorough cleaning of the equipment. However, the drawback of the strip making method is that the cooling section takes up space, the length of which is determined by the temperature requirements of the polymer.

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Die surface cutting system
There are three basic types of die heating system, that is, airflow granulator, water-jet granulator and underwater granulator. Although different designs may be available for such systems, typical systems include mouth die, cutting chamber, electric rotary blade, method of cooling medium and drying pellets if water cooling is used.
The die is an important part of the die surface thermal granulation system. It is installed vertically or horizontally and is usually heated by oil, steam or by a cylinder or belt heater. Electric heating is usually used for smaller die; Larger dies, however, are usually heated by steam or oil. Mold structure materials have different materials, but no matter what kind of material or what kind of heating medium, die orifice diameter must be uniform; There must be enough heat to maintain the temperature of the polymer throughout the extrusion process; The knife must be strong and smooth against the rotating die surface – these are necessary to produce a uniform pellet.

When the molten polymer is extruded out of the die, it is cut into pellets by a pelletizing knife rotating at a high speed. Typically, the granulator is either in contact with or very close to the die surface. After the granule is cut off, it is thrown away from the knife by the centrifugal force and conveyed to the cooling medium. The size, shape, material and installation of granulators can vary. In some systems, the granulator is loaded by a spring to automatically adjust the spacing between the granulator and the die. In some systems, the spacing between the granulator and the die must be manually adjusted. Because the life of the cutter depends on the precision of the cutter mold alignment, the abrasive properties of the polymer and the aggressiveness of the operator, it is desirable to cut the polymer pellets in the molten state.
Airflow granulators are recommended for use with heat – and long-residence time sensitive polymers such as PVC, TPR and crosslinked polyethylene. The flow path from the extruder to the granulation chamber is kept as short as possible with the least amount of heat. As the polymer is extruded through the die, the rotational force of the die surface rotation cuts it into pellets. After the pellets are cut off, they are then thrown away from the rotary knife and captured by forced circulation of air in a specially designed pelletizing chamber. The airflow quenched the surface of the pellet and carried it out of the granulation chamber to the cooling zone.

Fluidized bed driers are often used to cool granules. The pellets are passed down an adjustable bevel and the circulating fan is used to blast air through the pellets. The residence time of granule in drier can be prolonged or shortened by adjusting the incline Angle. Another common cooling method is to remove the pellets from the pelletizing chamber and feed them into a water tank, where the water is then removed using a fluidized bed dryer or a centrifugal dryer.
Water jet granulator is suitable for most polymers except those with low melt viscosity or viscosity. This kind of equipment is also called water ring granulator, granulating rate reaches 13607.77 kg/h.
The molten polymer is extruded from the hot die and cut into pellets by a rotary knife that rotates on the surface of the die. The glue-granulating system features a specially designed water-jet granulating chamber. Water flows in a spiral until it flows out of the granule chamber. After the pellets are cut, they are thrown into the current for preliminary quenching. The granular water slurry is discharged into the granular slurry tank where it is further cooled and then fed into a centrifugal dryer to remove moisture.
The underwater plastic granulator is similar to the airflow granulator and the water jet granulator, except that it has a smooth flow of water flowing through the surface of the mold, and direct contact with the surface of the mold. The granulation chamber is of just enough size to allow the granulation knife to rotate freely across the die surface without restricting the flow of water. The molten polymer is extruded from the die, and the pellets are cut with a rotary knife. The pellets are taken out of the granulation chamber by the temperature water and enter the centrifugal dryer. In the dryer, water is drained back into the storage tank, cooled and recycled for reuse. The pellets are dehydrated through a centrifugal dryer.
Underwater plastic granulation extruders should use mouth dies with uniform heat distribution and special insulation facilities. Small granulation knife using electric heating; Large granulation knives need to be heated by oil or steam die. Process water is heated to the highest temperature under conventional conditions, but the heat should not be enough to cause harmful effects on the free flow of granules. Underwater granulators are used for most polymers, with some models capable of granulating up to 22679.62 KGLH. The way water flows across the surface of the die when used for granulation of low viscosity or adhesive polymers is a major advantage, but for some polymers such as nylon and some brands of polyesters, this feature may cause the die to freeze. Other advantages are: because in the molten state of granulation, and water and play a sound barrier effect, noise emission is low; Fewer times of change of grain cutter compared to cold cutting system.

Centrifugal granulator
Centrifugal granulator because the mouth die only use or only need the lowest degree of heat, rather than hot die surface granulator, and because it is on the molten polymer granulation, is not cold cutting system, but its own kind. This kind of granulator is characterized by the use of cylindrical die, extrusion holes along the circumference of the distribution. The molten polymer is fed at atmospheric pressure. The extrusion pressure is formed by the depth of the melt on the rotor, rotor speed and the relative density of the polymer. As the cylindrical die rotates with its mandibular axis at high speed, the centrifugal force causes the molten polymer to flow evenly to the orifice on the die. When the polymer flows out of the extrusion hole, the rotating die directs the outflow of the strip material to the granulator. The pelleting knife may be of a fixed type, often in the form of a “band saw” that turns very slowly on two turntables. This slow rotation helps to keep the grain cutter worn evenly and to keep the temperature low. After the pellets are cut, the pellets are thrown along a straight path into a chamber for cooling by a jet of water due to their momentum, and then dried by a method similar to that used in other hot die pelletizers.

Plastic extruder granulator
The crushing plastic extruder pelletizing machine can reprocess the thermoplastic scraps and pelletizing the material to the appropriate size. Many manufacturers of thermosetting plastics and synthetic materials also use plastic granulation extruders for scrap processing and reprocessing.
Recycling is so important today that environmental advocates want plastic manufacturers to reuse everything they make. As a result, manufacturers have no choice but to reprocess discarded parts and scraps through granulators. This process can reduce production costs and improve resin utilization.
Many municipalities also use granulators and shredders to sell plastic scraps to plastic product manufacturers to cover the cost of landfill disposal.

According to the size, shape, quantity and type of plastic scraps of special requirements, can design and manufacture different granulators. The number and design of pelletizing cutters as well as the speed, rotor type and strainer size of the pelletizing machine are crucial to obtaining high-quality recycled scrap products. The granulator should be placed next to or under the plastic manufacturing machine, or in the center part, in order to deal with the scraps of several plastic manufacturing machines. Therefore, it is best to choose the granulator reasonably according to the type and position of the leftover material to be processed. Depending on the application, such as film, profile, tube, sheet, etc., the extrusion processing machine can be used side or center type granulator.
For automatic injection molding and series hot molding, the use of plastic pelletizing extruder under the press is more common. During the injection molding process, the injector debris falls into the granulator, and the product is collected and sent on a conveyor belt to the central area for packaging. The hot forming granulator is located below the blanking machine to remove the remaining frame material after rushing out of the parts. The frame material goes into the granulator and the parts are sent to the packing station.
The central granulator generally has a larger volume, and the motor power is usually 55KW. It is suitable for pipes, films, plates and large components.
Different from the milled scraps, the scraps used for circulating granulation must meet more stringent size requirements. If the size of the leftover material is too large, it will cause the throat of the extruder to be blocked, or the melting is too slow to reduce the extrusion volume. However, if the size of the leftover material is too small, it will melt too fast, leading to thermal degradation. The size of the recycled material must be uniform to ensure the constant melting speed in the extruder. Not all thermoplastic scraps can be recycled into pellets. Some lighter materials, such as hot-formed sheet roll scraps, must be compacted by off-line regranulation before they can be mixed with other pure resins.
The power range of plastic extruder granulator is between 0.37 – 735.5 KW, but their working principle is very similar. The cutter mounted on the rotor (rotating speed is about 400~700 r/min) moves relative to the fixed cutter mounted on the cutting outside circumference. Granulator generally has 1 to 6 fixed cutters, depending on the rotor, the number of rotating cutters can be up to 32. The plastic extruder granulator is equipped with a filter net under the fixed cutter. The strainer passes the required size of the return and leaves the larger size of the return until they are cut to a sufficiently small size in the cutting chamber.
The auxiliaries of the plastic pellet extruder include a muffler (to reduce noise to below OSHA standards), a hopper to prevent the return of leftover material, and a vacuum cleaner. Although back fly (back fly of leftover material from the hopper) cannot be completely prevented, the extent of back fly can be minimized with specially designed hoppers.

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