There are many different plastic pelletizing system designs on the market, but all pelletizing lines fall into two main categories: cold pelletizing systems and die-faced hot pelletizing systems. The main difference between the two is the timing of the pelletizing process. Both types of pelletizing systems have their advantages and disadvantages, so follow wanrooetech to learn more about them!

Cold cutting system

Cold pelletizing system includes the mouth die, cooling zone (air-cooled or water-cooled), drying zone (if water-cooled) and pelletizing chamber. There are two main types of cold cutting pelletizing systems, namely, flake pelletizers and strip pelletizers.

a: Flake pelletizers molten polymer flows from the mixing equipment through a belt die or a roll refiner to be calendered into polymer flakes of a certain thickness. The flakes are solidified and cooled over a distance during transport and then cut into round or square pellets in a chamber with a pelletizing knife.

Flake pelletizing system is a particularly old method of manufacturing pellets for a variety of polymers from nylon to PVC. Advantages: High throughput. This is a cold pelletizing method with a higher noise emission than pelletizing from molten polymers. The cutter life of solidified cutting polymers is short and powder generation is often a problem.

b: Strip pelletizers have been in use for almost as long as flake pelletizers. It includes a mouth die, a cooling section (water bath or blower), a drying section (if water cooling is used) and a pelletizing knife. The molten polymer is passed through a horizontally mounted die with a machine or gear pump to form the bars (modern dies are precision machined and uniformly heated to produce consistent quality bars). Once the strips are discharged from the mouthpiece, they are cooled with a blower or air/vacuum facility, or with a water bath. If water cooling is used, the strips are passed through a drying section with forced ventilation to remove moisture and then the strips are delivered to the pelletizing chamber. The strips are cut to the exact length required by the shearing action of a pair of fixed and rotary knives. The pellets have a diameter of 3.175 mm and a length of 3.175 mm with clear angles.

c: The traditional method of drawing strips is to stretch the strips through a cooling section (commonly a water bath), which sometimes causes the strips to fall or be inconsistent in size. This is common for polymers with poor melt state strength, such as polypropylene, polyester and nylon. When a strip falls, the material is scrapped, so the operator needs to pay close attention. Downstream pellets need to be sieved if the strips are inconsistently drawn.

Other modes of strip formation can be used without close operator monitoring by using an electric motor-driven slotted feed conveyor to support and split the strips from the die to the pelletizer. The strips conveyed by the rotary force are more uniform in size, do not fall and therefore have less scrap. Some of these methods can achieve a production capacity of 6803.89 kg/h, compared to about 1814.37 kg/h for the stretch strip method, since the operator can only supervise a limited number of strips.

The strip line is inexpensive, easy to operate, and easy to clean. This has its advantages for color mixing since the change of two different color batches requires a thorough cleaning of the equipment. However, the disadvantage of the strip method is the space required for the cooling section, the length of which is determined by the temperature requirements of the polymer.

Die-cutting systems

There are three basic types of die-cutting systems, namely spray granulators, water jet (water ring) granulators and underwater granulators. While these systems can vary in design, a typical system includes a mouth die, a cutting chamber, a motorized rotary blade, a cooling medium and a method of drying the pellets (if water cooling is used).

The mouth die is an important component of a die-surface thermal pelletizing system. It is mounted vertically or horizontally and is usually heated with oil, steam or a cartridge or belt-type electric heater. Electric heat is usually used for smaller dies; however, larger dies are usually heated with steam or oil. Die construction materials vary, but regardless of the material or heating medium used, the die orifice diameter needs to be uniform; there needs to be enough heat to maintain the polymer temperature throughout the process, and the die surface against which the pelletizing knife rotates needs to be tough and smooth – all of which are needed to create uniform pellets.

As the molten polymer is mouthed into the die, the pelletizing knife rotates at a very high speed to cut it into pellets. Typically, the pelletizing knife is either in contact with or very close to the die surface. Once the pellets are cut, they are thrown off the knife by centrifugal force and transported to the cooling medium. The size, shape, material and mounting of the knife can vary. Some systems have a spring applied load to automatically adjust the distance between the cutting knife and the mouth die; while some systems have to manually adjust the distance from the cutting knife to the mouth die. Since the life of the cutting knife depends on the knife a die alignment accuracy, the abrasiveness of the polymer and the aggressiveness of the operator, cutting polymer pellets in the molten state is desirable.

a: Spray pelletizer system s are recommended for polymers that are sensitive to heat and long residence times, such as PVC, TPR and cross-linked polyethylene. Pelletizing rates up to 4989.52 kg/h The flow path of polymer from the machine to the pelletizing chamber should be kept as short as possible, and a small amount of heat should be used. As the polymer passes through the die, it is cut into pellets by the rotational force of the die surface. After the pellets are cut, they are thrown off the rotating knife and captured by the forced circulation of air in the specially designed pelletizing chamber. The air flow initially quenches the pellet surface and carries it out of the cutting chamber to the cooling zone.

Fluidized bed dryers are often employed to cool the pellets. The pellets are slid down an adjustable slope while a circulating fan blows air through the pellets. Adjusting the inclination of the slope can lengthen or shorten the residence time of the granules in the dryer. Another common cooling method is to discharge the pellets from the cutting chamber into a water tank and then remove the water with a fluidized bed dryer or centrifugal dryer.

b: Water ring pelletizer system for most polymers except those with low melt viscosity or viscous properties. This type of equipment, also known as water ring cutter, achieves a granulation rate of 13607.77 kg/h.

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

c: The underwater pelletizer system is similar to the spray pelletizer and the water ring pelletizer, except that it has a smooth stream of water flowing over the die surface and is in direct contact with the die surface. The size of the pelletizing chamber is just large enough to allow the pelletizing knife to rotate freely across the die surface without restricting the water flow. The molten polymer is removed from the die, the rotating knife cuts the pellets, and the pellets are carried out of the cutting chamber into the centrifugal dryer by the tempered water. In the dryer, the water is discharged back to the storage tank, cooled and recycled; the pellets pass through the centrifugal dryer to remove the water.

The underwater pelletizer needs to use the mouth die with uniform heat distribution and special insulation facilities. Small pelletizing knives are electrically heated; large pelletizing knives require oil- or steam-heated dies. Process water is routinely heated to higher temperatures, but not hot enough to have a detrimental effect on the free flow of the pellets. Underwater pelletizers are used for most polymers and some models can achieve a pelletizing capacity of 22,679.62 kg/h. The way the water flows over the die face is a major advantage when used for pelletizing low viscosity or adhesive polymers, but for some polymers such as nylon and some brands of polyester this feature can cause die freezing. Other advantages are lower noise emission because the pellets are cut in the molten state and the water acts as a sound barrier, and fewer knife changes compared to cold cutting systems.

Wanrooetech’s products include intelligent equipment with parallel twin screws, strip pelletizing, filling, co-mingling modification The core products include intelligent equipment of co-rotating twin-screw, strip pelletizing, filling, co-blending modification, water-ring hot cutting, air-cooled die surface hot cutting, underwater pelletizing, continuous dense extrusion line, providing customers with intelligent production environment solutions based on rubber and plastic, dedicated to creating energy-saving, stable and intelligent manufacturing application scenarios.