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Grain Cutting Method Of Granulating Line

Zhangjiagang Sansu Machine Co.,Ltd | Updated: Jan 25, 2018

Although there are many different pelletizer designs, all pelletizers fall into two broad categories: cold pelletizing systems and die-face thermal pelleting systems. The main difference between the two is the timing of the pelletizing process. Cold pelletizing systems that pelletize the solidified polymer at the end of the process, such as the pelletizing of a water draw process, and in the die-face thermal pelletizing system, the molten polymer is sliced as it emerges from the die Pellets, while the pellets are cooled downstream.

Two kinds of pelletizing systems have their own advantages and disadvantages, the following will be a brief introduction:

Cold pelletizing system

Cold pelletizing systems include dies, cooling zones (air or water cooled), drying zones (if water cooled), and pelletizing chambers. There are two main types of cold pelletizing systems, namely pellet pelletizing and strip pelletizing.

a: Pelletizer The molten polymer is extruded from a mixing apparatus through a belt die or roll mill to a thickness of polymer sheet. Sheets are solidified and cooled by a distance during transport, then cut into round or square pellets using a pelletizer in a chamber.

Sheets Pelletizing is the oldest method of making pellets and can be used for a wide range of polymers from nylon to polyvinyl chloride. Advantages: large output. It has been reported that the accuracy is quite good with a granulation capacity of up to 1843.69 kg / h. This is a cold pelletizing method where the noise emission is higher than the method of pelletizing from a molten polymer. Solidified state cutting polymer knife life is short, generate powder often become a problem. Some polymers can see some "grain chain" phenomenon.

b: The history of pelletizing pelletizers is almost as long as pelletizing pelletizers. Including die, cooling section (water bath or blower), drying section (if water-cooled) and pelletizing knife. The polymer melted with a machine or gear pump is formed into a strip through a horizontally mounted die (the modern die is precision machined and heated evenly to produce a stable strip). After the strip is drained from the die, it is cooled with a blower or air / vacuum facility or cooled in a water bath. If water-cooled, the strip passes through a drying section, blows moisture with forced ventilation, and feeds the strip to the dicing chamber. Using a pair of fixed knife and rotary knife shearing action, the strip cut to the exact length required. The diameter of the pellets is 3.175 mm, 3.175 mm long, with sharp edges and corners.

c: The traditional method of drawing the strip is to stretch the strip through the cooling section (most commonly a water bath), sometimes causing the strip to fall or have inconsistent sizes. This is most commonly found in polymers with poor melt strength, such as polypropylene, polyester and nylon. When the material falls, the material is scrapped, so operators need to pay close attention. If the strip is drawn inconsistently, downstream pellets need to be sifted.

Other modes of striping can be closely monitored without the need for an operator by using a motorized slotted feeder conveyor that supports and divides the strip from die to pelletizer. This is the size of the screw conveying more uniform, will not drop and thus scrap less. Some of these methods give them a throughput of 6803.89 kg / h, compared to about 1814.37 kg / h for drawing into strips because operators can only manage a limited number of strips.

Strip production line cost is not high, easy to operate, and easy to clean. This has advantages for colorant blending because the two batches of different colorant changes must be thoroughly cleaned. However, the disadvantage of the bar-making method is that the cooling section takes up space and its length is determined by the temperature of the polymer.

Die-cutting system

Die-cutting system has three basic types, namely spray granulator, water (water ring) granulator and underwater granulator. Although such systems may have different designs, typical systems include die, cutting chamber, electric rotary blade, cooling media, and dry granulation if water cooling is used.

The die is an important part of the die-face thermal pelletizing system. It is mounted vertically or horizontally and is usually heated with oil, steam or cartridge or band heaters. Electric heat is typically used for smaller dies; however, larger dies are usually heated with steam or oil. Die structure materials have different materials, but no matter what kind of material or heating medium, the die orifice diameter must be uniform; have sufficient heat to maintain the temperature of the polymer throughout the process; The die face must be tough and smooth - these are necessary to make a uniform pellet.

When the molten polymer is die-diced, it is cut into pellets using a pelletizing knife rotating at a high speed. The typical case is pelletizing or touching or very close to the die face. After the pellets have been cut, they are thrown off the knife by the action of centrifugal force and delivered to the cooling medium. Pelletizing knife size, shape, material and installation can be different. In some systems, the pelletizing knife has a spring applied load to automatically adjust the spacing between pelletizing knives and dies; in some systems, the spacing between the pelletizing knife and the die must be adjusted manually. Because knife life depends on knife-to-roll accuracy, abrasiveness of the polymer and operator's aggressiveness, it is advisable to cut the polymer pellets in a molten state.

a: Spray granulators are recommended for polymers that are sensitive to heat and long residence times, such as PVC, TPR and XLPE. The pelletizing rate is up to 4989.52 kg / h The flow path of the polymer slab to the pelletizing chamber should be kept as short as possible with the least amount of heat. As the polymer passes through the die, the rotation of the die-face rotates it into pellets. After the pellets were cut, they were thrown away from the rotary knife and captured for forced circulation of air in a specially designed pelletizing chamber. The air stream preliminarily quenches the surface of the pellet and takes it out of the pelletizing chamber to the cooling zone.

Fluid bed dryers are often used to cool pellets. The pellets slip down an adjustable bevel while the circulating fans blow air through the pellets. Adjusting the bevel angle lengthens or shortens the pellet dwell time in the dryer. Another common cooling method is to discharge the pellets from the pelletizing chamber into a tank, and then use a fluid bed dryer or centrifugal dryer to remove moisture.

b: Water jet granulators, except for polymers with low melt viscosity or viscosity, suitable for most polymers. Such equipment, also known as water ring granulator, granulation rate reached 13607.77 kg / h.

The molten polymer was cut from the hot die into pellets by a rotating knife that rotates in the mold. This sweet granule system features a specially designed water jet pelletizing chamber. The water spirals around due to flow until it flows out of the granule chamber. After the pellets were cut, they were thrown into a stream of water for initial quenching. Pellet water slurry into the slurry tank was further cooled, and then sent to the centrifugal dryer to remove moisture.

c: The underwater pelletizer is similar to the spray pelletizer and water jet pelletizer, except that it has a smooth flow of water through the die face, but in direct contact with the die face. The size of the pelleting chamber is just enough to allow the pelletizing knife to freely rotate across the die without limiting the flow of water. Melt polymer from the die, rotary knife cut pellets, the pellets were tempered water out of the pellet chamber into the centrifugal dryer. In the dryer, the water is discharged back to the tank, cooled and recycled; the pellets are removed by centrifugal desiccator.

Underwater pelletizers require the use of dies with a uniform heat distribution and special insulation. Small pelletizing knife with electric heating; large pelletizing need oil or steam heated die. Conventional process water is heated to maximum temperature under normal conditions but not enough heat to adversely affect the free flow of pellets. Underwater pelletizers are used for most polymers, with some models achieving a pelleting capacity of 22679.62kglh. The advantages of water flowing over the die face when pelleting a low viscosity or adherent polymer are a major advantage, but for some polymers such as nylon and some brands of polyester this may cause the die to freeze . Other advantages include low noise emission due to pelletizing in a molten state while water plays a noise barrier; fewer pelletizing replacements compared to cold-cut systems.