The 3 types of plastic pelletizing in detail

Plastic pelletizing plays a crucial role in the recycling process. In this step, the melt is converted into pellets. There are three different types of pelletizing:

• Hot die face pelletizing (HP)
• Strand pelletizing (SP)
• Underwater pelletizing (UWP)

The choice of pelletizing system depends on

• the material to be processed and
• the logistics conditions in production.

In this blog article we explain the various types of plastic pelletizing and their areas of application.

Hot die face pelletizing

Hot die face pelletizing involves the hot melt being cut by the knife (see also cold pelletizing). There are two types of hot die face pelletizing for processing plastics.

• Water ring hot die face pelletizing
• Air pelletizing

This form of pelletizing produces lenticular pellets.

Water ring hot die face pelletizing

1. Transformation of melt into pellets
   After filtration the plastic melt is fed to the die plate. It is pressed through the holes and converted into pellets by a cutting knife. The pressure that is used to press the melt through the die plate is decisive for a consistent shape and size of the pellet.
   
   For this reason, some manufacturers, such as PURE LOOP, use sensors that measure the pressure behind the die plate. This makes it possible to adjust the blade speed for plastic pelletizing.
   
   
2. Initial cooling by the water ring
   When cutting the pellets, the knives reach up to 3,000 rpm. As a result, the material is ejected into the water ring. This is created by multiple nozzles, which produce a fine film of water around the inner wall of the chamber. In the water ring, the pellets are already undergoing initial cooling. This phase is an important step in plastic pelletizing. As a result of cooling, the material becomes hard on the outside and the heat is retained on the inside.
   
   
3. Separating the pellets from the water in a pellet dewatering screen
   The pellets and water are fed into a dewatering screen. In this basin, a shaker with a screen separates the pellets from the water. The water flows back into the basin through the screen holes. Meanwhile, the pellets are transported to the centrifuge for further processing.
   
   The water in the basin must be replaced regularly and properly disposed of or treated. This maintenance work is important for high-quality plastic pelletizing. This is the only way to keep the system clean and prevent damage.
   
   
4. Further drying with the centrifuge.
   The centrifuge effectively separates the pellets from the remaining water. Depending on the manufacturer, the centrifuge can be aligned either vertically or horizontally:
   • Vertical centrifuges often require a bearing or motor at the bottom. However, this results in higher maintenance costs, as water can penetrate these components. In addition, a fan is required to keep the material moving.
   • Horizontal centrifuges dry the pellets as well as transport them to the final air-cooling line.
   During this step, the pellets are still warm on the inside. This thermal energy helps to remove residual moisture during transport through the air-cooling line.
5. Final cooling and drying in the cyclone
   The pellets are then transported through a transport line into a cyclone (also known as a centrifuge). The final cooling and drying stage takes place inside this cyclone. After that, the pellets are ready for packaging. Some manufacturers also offer individual expansion options for color measurement or a weighing system. For specific adaptations as part of plastic pelletizing, talk to your manufacturer.

Air pelletizing

How does plastic pelletizing with air work?

1. Transformation of melt into pellets
   As with water ring hot die face pelletizing, the plastic melt is fed to the die plate after filtration. It is pressed through the holes, cut off with knives, and converted into pellets.
   
   The pressure when pressing the melt through the die plate is decisive for the consistent shape and size of the pellets. Find out more about pressure during plastic pelletizing here.
   
   
2. Cooling by air
   The pellets are cut directly on the die plate with knives at up to 3,000 rpm. The speed of the blades propels the material into the air chamber. Here, initial cooling takes place through ventilation.
   
   
3. Final cooling and drying in the cyclone
   The fan transports the pellets through a transport line into a cyclone (centrifuge). Inside the cyclone, the material is finally cooled and dried before it is ready for packaging.
   
   This last phase completes the plastic pelletizing process.

 

Strand pelletizing

Strand pelletizing is a form of cold pelletizing. This means that the melt is cooled down before it is cut into pellets (see also hot die face pelletizing). This method covers two types of plastic pelletizing:

• Conventional strand pelletizing
• Automatic strand pelletizing (ASP)

The pellets are cylindrical.

How does conventional strand pelletizing work?

1. Transformation of the melt into strands
   In the first step, the filtered melt is pressed through a melt strand head, which produces long plastic strands. These are also known colloquially as “spaghetti.”
   An operator pulls these strands through a water basin, where they cool down. This direct and manual process highlights the extent of operator intervention required for this method of plastic pelletizing.
   
   
2. Strand pelletizer
   Cooling is followed by removing the water from the surface of the strands by drawing moisture away under suction.
   
   Once the material is separated from the water, it enters the strand pelletizer. Here, two rollers cut the material with knives. During this step, the strands become pellets, which are used for further processing or as an end product.
   
   
3. Drying and transporting the pellets
   The plastic pellets are transported to the packaging area by an air blower or vibrating table. During transport, the material dries. However, with this type of pelletizing, most residual moisture remains in the pellet.

Underwater pelletizing (UWP)

How does UWP work?

1. Starting up the machine and adjusting the melt diverter
   When starting up the system, it is crucial to set the melt diverter correctly. This prevents premature overfilling of the pelletizing system with the material. The melt is fed into a basin below to avoid overloading the system.
   
   At the same time, the cutting chamber is filled with water. As soon as the machine is ready, the melt diverter is switched over and the plastic pelletizing process begins.
   
   
2. Transformation of melt into pellets
   During pelletizing, the melt is pressed through a die plate and cut directly by a knife. The cutting chamber, which is filled with water, ensures immediate and effective cooling of the material.
   
   
3. Continuous cooling along the transport line
   The material is then transported in water along a line to the centrifuge. The cooling process continues while the material is being transported.
   
   Just before the centrifuge, there is a separation chamber. Inside it, the pellets are separated from the water. The excess water flows into a basin underneath.
   
   
4. Final drying in the centrifuge
   After final drying in the centrifuge, the pellets are ready for packaging. The carefully coordinated process used during the UWP method guarantees a high-quality end product from plastic pelletizing.

The various types of plastic pelletizing at a glance.

Plastic pelletizing is not only a crucial step in the recycling process, but also an area of continuous technological progress. By assessing the three main methods in detail

• hot die face pelletizing,
• strand pelletizing, and
• underwater pelletizing

— it becomes clear how diverse the options for processing a wide variety of types of plastic are. Each method offers specific advantages that make it ideal for specific applications and materials.

The choice of pelletizing system depends largely on the material requirements and logistics situation on site. Each process has its place and plays an important role in the overall context of plastic pelletizing.