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A Comparison of Underwater Pelletizers with Other Types of Pelletizers

Pelletizers play a crucial role in the plastics industry, enabling the conversion of plastic resins into uniform pellets for further processing. Different types of pelletizers are available, each with its own unique features and advantages. In this article, we will compare underwater pelletizers with other types, such as strand pelletizers and hot face pelletizers, to understand their differences and applications.

Introduction

Pelletizers are specialized machines used in the plastics industry to transform plastic resins into small, uniform pellets. These pellets are easier to handle, transport, and store, making them highly desirable for various plastic processing applications. While different types of pelletizers exist, underwater pelletizers, strand pelletizers, and hot face pelletizers are among the most commonly used.

Pelletizers are machines designed to cut or shape plastic resins into uniform pellets. They consist of cutting mechanisms, cooling systems, and discharge units that work together to produce high-quality pellets. Pelletizers are essential in the plastics industry as they facilitate the downstream processes of plastic manufacturing. By converting bulk resin into small pellets, pelletizers enable efficient transportation, storage, and handling of plastic materials.

Underwater Pelletizers

COWELL Underwater Pelletizers
COWELL  Underwater Pelletizers

Working Principle

Underwater pelletizers operate by extruding molten plastic through a die plate into an underwater chamber. As the plastic enters the chamber, it is cooled and solidifies, forming cylindrical strands. The strands are then cut into uniform pellets using an underwater cutter.

Advantages

Underwater pelletizers offer several advantages. First, they provide excellent pellet quality with minimal fines and dust. The underwater cooling process ensures rapid solidification, resulting in pellets with uniform shape and size. Second, underwater pelletizers have high production efficiency, allowing for continuous operation and increased output. Finally, these pelletizers are suitable for a wide range of polymers and applications, offering versatility to manufacturers.

Limitations

Despite their advantages, underwater pelletizers have some limitations. They require a significant initial investment due to their complex design and underwater cooling system. Additionally, underwater pelletizers may have higher maintenance requirements compared to other types, as the underwater environment poses certain challenges.

Strand Pelletizers

This cutting system divided into water cooling strand, air cooling strands and under water strand pelletizing systems.

Under water strand pelletizing system

COWELL Strand Pelletizers
COWELL Strand Pelletizers

Working Principle

Strand pelletizers involve extruding molten plastic through a die plate to form continuous strands. The strands are then pulled through a set of rotating knives, which cut them into desired pellet lengths. The pellets are subsequently cooled and collected. The strand will go into the pelletizing pushed by the water flow which drive by a water pump. The water flushed into a helical water bath, the strands fall into the water and go to the pelletizer.  After the pelletzier, the granules will go to the centrifuge dryer with water, the centrifuge dryer will separate the water and granules. The whole process is automatic without manpower.

The typical application for this type of pelletizing system is PET recyling, high filler and materials which is not strong enough.

Air cooling strands pelletizing system

Air cooling strands pelletizing system

Air cooling strand pelletizing system is specially designed for some materials which are sensitive to water/moisture. Such as bio-degradable materials(PLA, PBAT, etc)The strands go to the pedrail and cooled by air.  Then go to the pelletizer and cut them into granules. The whole process without water involved. Customer can save the cost of drying system.

Water cooling strand pelletizing system

Water cooling strand pelletizing system

This is the most common pelletizing system for plastic compounding. It is composed of strand die head water bath, air knife, strand pelletizer. The strands come out from the die head, operator needs to take them manually through the water bath & air knife, putting it into the strand pelletizer to start.  After the pelletizer, the compounds will fall into the vibrator directly.

Advantages of Strand Pelletizers

Strand pelletizers offer several advantages in the plastic pelletizing process. Firstly, they are versatile and can handle a wide range of polymers, making them suitable for various applications. Secondly, strand pelletizers provide good pellet quality with consistent shape and size. The cutting process ensures minimal fines and dust, resulting in high-quality pellets. Additionally, strand pelletizers have relatively lower initial investment costs compared to underwater pelletizers, making them more accessible to smaller-scale manufacturers.

Limitations

However, strand pelletizers also have certain limitations to consider. One drawback is the limited production efficiency compared to underwater pelletizers. The batch process of cutting strands into pellets requires intermittent operation, leading to slower production rates. Additionally, strand pelletizers may have higher maintenance needs due to the wear and tear of the cutting knives. Manufacturers need to ensure regular maintenance to prevent any degradation in pellet quality and machine performance.

Hot Face Pelletizers

Hot face pelleting system includes hot face air cooling and hot face water cooling pelletizing system.

Hot face air cooling pelletizing system

Hot face air cooling pelletizing system

Obviously, hot face air cooling pelletizer means that the compounds will be cut and cooled with air.  The cutter is installed on the die head by a shaft. The knives touch the die face. The materials come out and cut by the cutter. The air blower will blow the compounds to the cyclone separators.  All the pipelines and silos will be water jacket design to help to cool the compounds.

Hot face water cooling pelletizing system

The die head of hot face water cooling system is the same as air cooling die head. The cutter will be installed on the die head with knives touching the die face.  After the cutting, the granules will fall into the water  and go to the centrifuge dryer with water, centrifuge dryer will separate the granules and water.

Hot face water cooling pelletizing system

Advantages

Hot face pelletizers offer their own set of advantages in the pelletizing process. They provide precise control over pellet size and shape, ensuring consistency in the final product. The use of a heated die plate allows for efficient and uniform heat transfer, resulting in excellent pellet quality. Hot face pelletizers are also capable of handling a wide range of polymers, making them versatile for different applications.

Limitations

Despite their advantages, hot face pelletizers have some limitations to consider. The heating element requires energy consumption, making them less energy-efficient compared to other types of pelletizers. Additionally, the heating process may cause some thermal degradation of certain polymers, affecting the overall quality of the pellets. Manufacturers should carefully select suitable polymers and optimize operating parameters to mitigate these limitations.

Comparison of Underwater Pelletizers with Other Types

When comparing underwater pelletizers with strand pelletizers and hot face pelletizers, several factors come into play. Let’s explore the key aspects for comparison:

Pellet Quality

In terms of pellet quality, underwater pelletizers have the advantage of producing uniform pellets with minimal fines and dust. The underwater cooling process ensures rapid solidification, resulting in pellets with consistent shape and size. Strand pelletizers also offer good pellet quality, but the cutting process may generate slightly more fines. Hot face pelletizers provide precise control over pellet shape and size, but there is a possibility of thermal degradation, which can affect the quality.

Production Efficiency

Underwater pelletizers excel in production efficiency due to their continuous operation and high output rates. They are designed for continuous pellet production, leading to increased productivity. Strand pelletizers, on the other hand, have a batch process that requires intermittent operation, resulting in slower production rates. Hot face pelletizers fall somewhere in between, offering relatively higher production efficiency than strand pelletizers but not as high as underwater pelletizers.

Maintenance and Operation

Underwater pelletizers may have higher maintenance requirements due to the complexity of their design and the underwater environment. Regular maintenance is essential to ensure optimal performance. Strand pelletizers generally have lower maintenance needs, with the main focus being on the cutting knives. Hot face pelletizers require routine maintenance and monitoring of the heated die plate to maintain optimal temperature control.

Application Flexibility

Underwater pelletizers are known for their versatility and ability to process a wide range of polymers and applications. They can handle different types of resins with varying viscosities, making them suitable for diverse industries such as automotive, packaging, and construction. Strand pelletizers are also versatile and can process various polymers, but they may have limitations with highly viscous or heat-sensitive materials. Hot face pelletizers offer flexibility in terms of pellet size and shape control, making them ideal for applications that require specific pellet characteristics.

Conclusion

In conclusion, underwater pelletizers, strand pelletizers, and hot face pelletizers each have their own unique features and advantages in the plastic pelletizing process. Underwater pelletizers excel in pellet quality and production efficiency, although they require higher maintenance and have a higher initial cost. Strand pelletizers offer good pellet quality, affordability, and versatility, but their production efficiency is relatively lower. Hot face pelletizers provide precise control over pellet size and shape but may have higher energy consumption and potential for thermal degradation.

When choosing the right type of pelletizer, manufacturers should consider factors such as the desired pellet quality, production efficiency requirements, maintenance needs, cost considerations, and application flexibility. Each type of pelletizer has its strengths and limitations, and the decision should be based on specific production requirements and preferences.

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