Parallel Twin Screw Extruder VS. Conical Twin Screw Extruder

In the world of plastic processing and polymer manufacturing, twin screw extruders have emerged as a pivotal technology. They play a crucial role in melting, blending, compounding, and shaping various materials. Among twin screw extruders, two prominent designs take the spotlight: the parallel twin screw extruder and the conical twin screw extruder.

These designs are distinguished by the orientation of their twin screw axes. The one with parallel axes is known as a parallel twin screw extruder, while the one with crossed axes is referred to as a conical twin screw extruder.

Twin-Screw Configuration
Twin-Screw Configuration

In this article, we will conduct a comparative analysis of parallel twin screw extruders and conical twin screw extruders, elucidating their similarities, differences, and the key considerations for choosing between these two extrusion technologies.

The common characteristics shared by parallel twin-screw extruders and conical twin-screw extruders

Parallel twin-screw extruders and conical twin-screw extruders are both types of twin-screw extruders used in various industrial applications for materials processing. They share several common features and functions:

  1. Twin-Screw Configuration: Both extruder types feature a dual-screw setup, with the screws rotating in the same direction inside a barrel. This twin-screw configuration is a fundamental characteristic of these machines.
  2. Material Conveying: Both extruder types feature a conveying mechanism driven by the rotating screws that forces the plastic material forward through the barrel. This mechanism ensures consistent material transport and processing.
  3. Dehydration Capabilities: Both types of extruders have the ability to remove moisture and volatiles from materials. This is particularly important in processes like compounding and extrusion of moisture-sensitive materials.
  4. Versatility: These extruders exhibit adaptability to a wide range of materials and plastic product molding processes. They can process various materials, including thermoplastics, thermosetting polymers, rubber compounds, and more, making them suitable for a diverse set of applications.
Twin-Screw extrusion

Parallel twin screw extruder VS. Conical twin screw extruder

While these shared characteristics are noteworthy, it’s essential to recognize that there are substantial distinctions between the parallel twin-screw extruder and the conical twin-screw extruder. Following are their main differences:

  • Screw geometry

Parallel Twin-Screw Extruders: These extruders have two screws running side by side, maintaining the same diameter along their entire length. This design is highly effective for uniform mixing and compounding of materials. The even shear flow in parallel screws is advantageous for applications where consistent dispersion is crucial, such as in the production of filled or reinforced plastics.

Conical Twin-Screw Extruders: Conical extruders, in contrast, feature screws that taper toward the discharge end, resembling a cone. This unique design results in self-wiping action, preventing material buildup on the screws. The tapered design generates higher pressure and shear forces, making conical extruders suitable for applications requiring intense mixing and dispersion, such as PVC compounding.

screw design
  • Pressure generation and screw service life

Parallel Twin-Screw Extruders: Parallel extruders typically generate lower pressure due to their uniform screw diameter. This makes them less suitable for high-pressure applications where materials need to be forced through restrictive die openings. The pressure generated is lower, and the pressure endured by the screw is lower, which is beneficial to maintaining the service life of the screw.

Conical Twin-Screw Extruders: The tapered design of conical extruders allows for the generation of higher pressure. This is advantageous for tasks like PVC processing, where high pressure is needed to achieve optimal dispersion and extrusion through narrow dies. However, because the head and tail diameters of the conical twin screws are different and the high pressure it generated, under the same use time, the wear of the conical twin-screw is more serious than that of the parallel twin-screw, which enable shorter screw life than parallel twin screw extruder.

  • Thrust bearings

Parallel Twin-Screw Extruders: In parallel twin-screw extruders, the screws are positioned very close to each other. This proximity poses a challenge when it comes to providing adequate thrust bearings. To address this, one of the screw shafts must extend beyond the other to accommodate the necessary thrust bearing. This arrangement can be complicated and requires additional engineering solutions.

Conical Twin-Screw Extruders: Conversely, conical twin-screw extruders offer a solution to the thrust bearing challenge. This is made possible by having wider spacing between the screws, thanks to the increased diameters at the feed end. The wider spacing allows for the use of standard thrust bearings and a simpler, more robust distribution gear system. This design provides greater structural stability and ease of maintenance.

Twin-Screw
  • Energy efficiency

Parallel Twin-Screw Extruders: In cases where the output is equal, parallel extruders are frequently perceived as more energy-efficient, mainly due to their lower pressure generation. This can result in potential energy savings.

Conical Twin-Screw Extruders: Conical extruders may consume more energy due to the higher pressure they generate. However, their efficiency in high-pressure applications can still make them more energy-efficient than other alternatives for specific tasks.

  • Output

Parallel Twin-Screw Extruders: Parallel twin-screw extruders are known for their efficient and consistent output. They excel at mixing and compounding materials, making them suitable for a wide range of applications. However, the output of parallel twin-screw extruders can be limited by their design, and in some cases, they may have lower output capacity compared to conical extruders, particularly in applications that require high-pressure processing. However, due to the longer screw length in the parallel twin-screw extruder compared to the conical twin-screw extruder, the plasticizing effect of the parallel twin-screw extruder is enhanced, resulting in more even kneading of the material.

Conical Twin-Screw Extruders: Conical twin-screw extruders are often capable of higher output rates, especially in applications that require intense mixing and high-pressure dispersion, such as PVC compounding. The tapered design of conical screws can generate greater pressure, allowing for increased throughput in certain scenarios. This can make conical extruders a preferred choice when high output is a key requirement.

Twin-Screw extrusion

The differences between parallel twin-screw and conical twin-screw extruders significantly impact their advantages and suitability for specific applications.

Parallel twin-screw extruders excel at mixing, have a broad material compatibility, and are energy-efficient, making them ideal for general compounding and material processing. Conical twin-screw extruders, with their higher pressure generation and self-wiping action, are well-suited for applications demanding intense mixing, such as PVC compounding, but may consume more energy. 

Factors to consider when choosing between the two screw extruder

  • Material compatibility

Consider the type of materials you will be processing. Parallel twin-screw extruders are known for their versatility and can handle a wide range of materials. In contrast, conical twin-screw extruders are more specialized and may be better suited for certain materials like PVC. Choose the extruder that aligns with the materials you’ll be working with.

Twin-Screw extrusion
  • Space and layout

Assess the available space in your facility. Conical twin-screw extruders tend to have a more compact design, which can be advantageous when space is limited. Parallel extruders, on the other hand, have a more elongated configuration and may require more floor space.

  • Die and product specifications

Examine the die design and product specifications you need. The die design at the extruder’s exit can impact the final product’s shape and dimensions. Ensure that the chosen extruder aligns with your die requirements and product specifications.

  • Budget and cost considerations

Factor in the initial purchase cost, operating costs, and the overall budget. Conical twin-screw extruders may have a higher initial cost due to their specialized design and capabilities. Assess your budget constraints and determine which extruder aligns with your financial considerations.

Both parallel and conical twin screw extruders have their unique advantages, making them indispensable tools in the plastic and polymer processing industry. The decision between the two should be based on the specific requirements of the application, considering factors like material compatibility, throughput, space constraints, and energy efficiency. When chosen wisely, these extruders can enhance the efficiency and quality of polymer processing, contributing to the ever-evolving world of plastics manufacturing.

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