Various ways of pelletizing production line and their advantages and disadvantages

There are many different pelletizing line designs on the market, but all pelletizing lines fall into two main categories: cold pelletizing systems and die face hot pelletizing systems. The main difference between the two is the timing of the pelletizing process.

Cold pelletizing system

The use of bar granulator has a long history. Includes die, cooling section (water bath or blower), drying section (if water cooled) and pelletizing knife. The polymer melted by the machine or gear pump passes through a horizontally installed die to form a strip (the modern die is precision machined and uniformly heated to produce a strip of stable quality). After the strip is discharged from the mouth, it is cooled by a blower or air/vacuum facility, or cooled by a water bath. If water cooling is used, the strip passes through a drying section, where the water is removed by forced ventilation, and then the strip is sent to the pelletizing chamber. Using the shearing action of a pair of fixed knives and rotating knives, the strip is accurately cut to the required length. The pellets had a diameter of 3.175mm and a length of 3.175mm with sharp edges.

The cost of the strip material production line is not high, the operation is simple, and the cleaning is convenient. This has advantages for color compounding because the replacement of two different batches of color requires thorough cleaning of the equipment. However, the strip-making method has the disadvantage that the cooling section takes up space, the length of which is determined by the temperature requirements of the polymer.

die surface heat-cutting system

There are three basic types of die face heat-cutting systems, namely spray granulator, water jet (water ring) granulator and underwater granulator. Although this type of system can have different designs, a typical system includes a die, a cutting chamber, an electric rotating blade, a cooling medium, and a method of drying the pellets (if water cooling is used).

Spray granulator is recommend used for polymers that are sensitive to heat and long residence times, such as polyvinyl chloride, TPR and cross-linked polyethylene. Cutting rates up to 4989.52 kg/h The flow path of the polymer from the machine to the cutting chamber is kept as short as possible and a small amount of heat is used. As the polymer passes through the die, the force of rotation of the die surface cuts it into pellets. After being cut, the pellets are immediately thrown off the rotating knife and captured by the forced circulation of air in a specially designed pelletizing chamber. The air stream initially quenches the pellet surface and carries it out of the pelletizing chamber to the cooling zone.

Fluidized bed dryers are often employed to cool the pellets. The pellets slide down an adjustable ramp, and a circulation fan blows air through the pellets. Adjusting the slope angle can lengthen or shorten the residence time of the pellets in the dryer. Another common cooling method is to discharge the pellets from the pelletizing chamber into a water bath and then remove the water with a fluid bed dryer or centrifugal dryer.

Water jet granulator, in addition to low melt viscosity or viscous polymer, suitable for most polymers. This type of equipment is also called water ring pelletizer, and the pelletizing rate reaches 13607.77 kg/h.

The molten polymer is cut into pellets from the hot die by a rotating knife rotating against the die surface. The special feature of this granulation system is its specially designed water jet pelletizing chamber. The water flows in a spiral until it flows out of the grain chamber. After the pellets are cut, they are thrown into the water flow for preliminary quenching. The pellet slurry is discharged into a pellet slurry tank to be further cooled, and then sent to a centrifugal dryer to remove moisture.

The underwater granulator is similar to the spray granulator and the water jet granulator, except that it has a smooth water flow through the mold surface and is in direct contact with the mold surface. The size of the pelletizing chamber is just enough to allow the pelletizing knife to rotate freely across the die surface without restricting the flow of water. The molten polymer is cut from a die, a rotary knife, and the pellets are carried out of the pelletizing chamber by tempered water into a centrifugal dryer. In the dryer, the water is drained back to the storage tank, cooled and recycled; the pellets are removed of water by a centrifugal dryer.

Underwater pelletizers need to use a die with uniform heat distribution and special insulation facilities. Small pelletizing knives are electrically heated; large pelletizing knives require an oil-heated or steam-heated die. The process water is regular heated to a higher temperature, but not hot enough to adversely affect the free flow of pellets. Underwater pelletizers are used for most polymers, and some models can reach a pelletizing capacity of 22679.62kglh. The manner in which water flows through the die face is a major advantage when used for pelletizing low viscosity or adherent polymers, but this feature may cause the die to freeze for some polymers such as nylon and certain brands of polyester. Other advantages are: because in the molten state of cutting pellets, and water plays a sound barrier role, noise emission is low; Compared with the cold cutting system, the number of times to replace the cutting knife is less.