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Is it better to choose a parallel twin-screw extruder or a conical twin-screw extruder?

Parallel twin-screw extruder

Working Principle

In a parallel twin screw extruder, the two screws are arranged side by side and rotate in the same direction. This configuration offers excellent mixing and compounding capabilities. The material being processed is conveyed forward between the screws, undergoing thorough melting, mixing, and homogenization. The parallel arrangement allows for higher throughput and more flexibility in terms of processing different materials.

Advantages

  • Enhanced mixing capabilities
  • High throughput
  • Suitable for a wide range of materials
  • Flexibility in processing different formulations

Disadvantages

Higher initial cost compared to single-screw extruder. When talking about the initial cost, the parallel twin screw extruder is much higher than that of single screw.

  • With aggressive shearing heat due to the inter-meshed screw structure, two screw will shear the materials, help to melt and homogenize them. During this process, the extra shearing heat may damage the materials which is more heat sensitive.
  • The maintenance of parallel twin screw extruder is a bit more complicated than that of single screw extruder.  Operators should be a big more educated and skilled to do the maintenance.

Conical twin screw Extruder

Working Principle

In a conical twin screw extruder, the two screws are arranged in a conical shape, tapering towards the discharge end of the extruder. This design provides a gradual reduction in the screw channel volume, resulting in increased pressure and improved compounding. The material is transported and mixed as it moves from the larger end to the smaller end of the conical screws.

Advantages

  • Increased pressure and compounding capabilities
  • Improved melting and devolatilization
  • Efficient handling of heat-sensitive materials
  • Compact design, requiring less space

Disadvantages

  • Lower throughput compared to parallel twin screw extruders

    Due to the structure restrictions, conical twin screw extruder gives a lower rotation speed which conveys less materials in a rated time. Thus the throughput is less compare to twin screw extruders.

  • Limited flexibility in processing different materials

   Since the conical screw is a integrated screw, the profile is fixed during the design process, and the structure is designed to some specific applications. Once it is designed and manufactured, customer can not transfer the raw materials. If they wants to produce other materials, then a new designed screw is a necessity.  And whats more, the conical twin screw extruder is limited to be used in some applications, such as PVC compounding,  PVC profile direct extrusion, high filler compounds,etc

  • May require additional equipment for specific applications

Factors to Consider When Choosing

When deciding between a parallel twin screw extruder and a conical twin screw extruder, several factors should be taken into account:

Material Characteristics

Consider the properties of the materials you will be processing. Some materials may require the enhanced mixing capabilities offered by a parallel twin screw extruder, while others may benefit from the increased pressure and compounding abilities of a conical twin screw extruder. Also, the throughput is a key consideration. Parallel twin screw has a dominant advantage on this aspect.  Parallel twin screw extruder, it is configured like a Lego. Customer can adjust the parameters freely to adapt their different applications.

The screw speed could be up to 1200rpm to get a fast rotation speed to limit the duration time in the barrel. The Do/Di could be up to 1.8 in order to have a bigger free volume  to achieve more throughput. L/D could be changed in order to process different materials just by changing the shaft L/D, removing/add extra barrels and screw elements to match the desired screw L/D. A flexible screw profile enables the extruder to be an versatile machine for many different application, thanks to its modular design.

In order to have more components to be mixed in the extruder, efficient side feeder could be added to the extruder for some fillers like CaCo3, Talc, TiO2, SiO2, some low temperature ingredients could be added by side feeder at the end of the barrel in order to get less heating and shearing heat.

Processing Requirements

Evaluate the specific processing requirements of your application. Determine if you need higher throughput, better mixing, improved devolatilization, or other specific functionalities. This will help guide your choice between the two types of extruders.

Normally, customer considers parallel twin screw extruder purchase is based on different materials and formulations achieved on it. They may not use 1 parallel twin screw extruder just for one material or one formulation. That is not a economical way to invest on the parallel twin screw extruder. For example, people wants to produce PE with 80% CaCo3 at 500kg/hr, he can chose both twin screw extruder to produce the filler masterbatch. But as a filler masterbatch producer, he should consider the possibility to produce other kinds of filler masterbatches, lilke ,PVC, PS, PP etc.  Furtherly, he may also consider to transfer his business to other applications like engineering plastics, bio-plastics or others.   So the right decision will be a parallel twin screw extruder for sure if he has the schedule for those kinda applications.

Cost Considerations

Budgetary limits play a significant role in the decision-making process. Parallel twin screw extruders generally have a higher initial cost compared to conical twin screw extruders. Consider the long-term benefits and potential return on investment when evaluating the overall cost-effectiveness.

Maintenance and Service

Extruder maintenance and service are crucial for ensuring optimal performance and longevity. Evaluate the ease of maintenance and availability of spare parts for the chosen extruder type. Consider the support and expertise provided by the manufacturer or supplier.

Applications of twin screw extruders

twin screw extruders find applications in various industries, including:

  • Plastic compounding and extrusion
  • Food processing and extrusion
  • Pharmaceutical manufacturing
  • Rubber and elastomer processing
  • Powder coatings and paints
  • Chemical and petrochemical industries

The choice between a parallel twin screw extruder and a conical twin screw extruder depends on the specific requirements and characteristics of each application.

Comparison and Decision-Making Process

To make an informed decision between a parallel twin screw extruder and a conical twin screw extruder, carefully assess the factors discussed above. Consider the material properties, processing requirements, cost considerations, and maintenance aspects. Additionally, consult with experts or manufacturers to gain further insights and recommendations tailored to your specific application.

The similarity between parallel twin screw extruder and conical twin screw extruder

What parallel and conical twin screw extruders have in common: the conveying mechanism of forced forward push for plastic products, excellent mixing plasticizing ability and dehydration ability, it has basically the same applicability to the forming process of raw materials and plastic products. 

The difference between parallel twin screw extruder and conical twin screw extruder

The following are the differences between parallel and conical twin screw granulators: 

1. Diameter: the diameter of the parallel twin screw is the same, and the diameter of the small end of the conical twin screw is different from that of the big end. 

2. Concentric distance: the centre distance of the flat twin screw is the same, the two axes of the conical twin screw are at an intersection angle, and the size of the centre distance changes along the axis. 

3. The ratio of length to diameter: parallel twin screw refers to the ratio of the reasonable part length of the screw to the outer circle of the screw, and the tapered twin screw refers to the ratio of the reasonable part length of the screw to the average diameter of the big end and the small end.

Conclusion

Choosing the right type of twin screw extruder is essential for achieving optimal processing results. Both parallel twin screw extruders and conical twin screw extruders offer unique advantages and disadvantages. By considering the material characteristics, processing requirements, cost factors, and maintenance considerations, you can make a well-informed decision that aligns with your specific needs and objectives.

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