Twin Screw Comparison: Parallel Vs Conical Performance

When it comes to twin-screw extruders, the two prominent designs are parallel twin screw extruders and conical twin screw extruders. These extruders are essential in the world of plastic processing and polymer manufacturing, playing a pivotal role in melting, blending, compounding, and shaping various materials. The choice between the two designs depends on specific requirements and characteristics of each application. This paragraph will delve into the key differences, advantages, and considerations when selecting between parallel and conical twin-screw extruders.

Parallel Twin Screw vs Conical Twin Screw

Structure: Parallel twin screws have the same diameter, conical twin screws have different diameters at each end

The structure of twin screws is an important consideration when choosing between a parallel twin screw extruder and a conical twin screw extruder. The key difference in structure lies in the diameter of the screws:

Parallel Twin Screws

Parallel twin screws have the same diameter throughout their length. This means that 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 processing different materials.

Conical Twin Screws

Conical twin screws, on the other hand, have different diameters at each end. The screws are arranged in a conical shape, tapering towards the discharge end of the extruder. This design provides a gradual reduction in 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.

Comparison

The difference in screw diameters results in several performance differences between the two types of extruders. Conical twin screws offer better self-cleaning capabilities and are suitable for processing materials with high viscosities and complex formulations. Parallel twin screws provide more uniform mixing and are often used for precise compounding and blending operations. The choice between the two depends on the specific requirements of the application, including material characteristics, processing requirements, cost considerations, and maintenance.

Mixing performance: Conical twin screws offer better self-cleaning, parallel twin screws offer more uniform mixing

When it comes to mixing performance, there are distinct differences between conical twin screws and parallel twin screws. Conical twin screws offer superior self-cleaning capabilities, while parallel twin screws provide more uniform mixing.

The self-cleaning advantage of conical twin screws is due to their tapered design, which prevents material buildup on the screws. This design also results in higher pressure and shear forces, making conical extruders ideal for applications that require intense mixing and dispersion, such as PVC compounding. The conical shape of the screws generates a self-wiping action, ensuring efficient and thorough cleaning.

On the other hand, parallel twin screws excel at uniform mixing and compounding. The parallel arrangement of the screws allows for consistent shear flow, which is crucial in processes where uniform dispersion is essential, such as in the production of filled or reinforced plastics. The even shear flow prevents uneven distribution and ensures a homogeneous mixture.

The choice between conical and parallel twin screws depends on the specific requirements of the application. Conical twin screws are preferred when dealing with materials that have high viscosities and complex formulations. Their ability to handle high-viscosity materials and generate higher pressure makes them suitable for processing heat-sensitive materials, such as PVC, food, and pharmaceutical items.

Parallel twin screws, on the other hand, offer more flexibility in processing different materials. They are well-suited for precise compounding and blending operations, as well as a wide range of applications, thanks to their uniform mixing capabilities. This versatility makes them a popular choice in various industries, including polymer compounding, masterbatch production, and food processing.

Pressure generation: Conical twin screws generate higher pressure, parallel twin screws are better for gentle pressure build-up

When it comes to pressure generation, there are distinct differences between conical and parallel twin screws. Conical twin screws, due to their tapered design, generate higher pressure, which is advantageous for applications like PVC processing that demand intense mixing and extrusion through narrow dies. On the other hand, parallel twin screws are better suited for controlled and gentle pressure build-up, making them a preferred choice when a more gradual increase in pressure is required.

The conical twin screw's tapered structure results in a self-wiping action, preventing material build-up on the screws. This design feature, along with the higher pressure generation, makes conical extruders ideal for tasks requiring vigorous mixing and dispersion. The increased pressure also enables improved throughput in certain scenarios, making conical extruders desirable when high output is a critical factor.

In contrast, parallel twin screws maintain a uniform diameter, which leads to lower pressure generation. While this makes them less suitable for high-pressure applications, it has the benefit of prolonging screw service life. Parallel twin screws are highly effective for uniform mixing and compounding of materials, and their energy efficiency is often perceived as superior when compared to conical screws, especially in cases where output levels are equivalent.

The decision between conical and parallel twin screws ultimately depends on the specific requirements of the application. If higher pressure, intense mixing, and increased output are key considerations, then conical twin screws are the preferred option. However, if gentle pressure build-up, uniform mixing, energy efficiency, and longer screw life are more important, then parallel twin screws are the better choice.

Application: Conical twin screw extruders are used for PVC pipes, parallel twin screw extruders are used for polymer compounding

Conical twin screw extruders are used in the production of PVC pipes and profiles. They are designed with screws in a conical shape, tapering towards the discharge end of the extruder. This design provides a gradual reduction in screw channel volume, resulting in increased pressure and improved compounding. The conical shape of the screws offers better self-cleaning capabilities and suits the processing of materials with high viscosity and complex formulations.

PVC pipes made using conical twin screw extruders include various formulations of PVC environmental protection water supply pipes, UPVC drainage pipes, CPVC hot water pipes, UPVC square rain downpipes, PVC double-wall corrugated pipes, PVC power cable sheathing pipes, and PVC industrial trunkings.

On the other hand, parallel twin screw extruders are commonly used in polymer compounding, masterbatch production, and other applications. In this configuration, the two screws are arranged side by side and rotate in the same direction, offering excellent mixing and compounding capabilities. The material processed is conveyed forward between the screws, undergoing thorough melting, mixing, and homogenization. The parallel arrangement allows for higher throughput and more flexibility in processing different materials.

Parallel twin screw extruders are designed to handle a wide range of materials, including plastics, rubber, and food products. They are particularly advantageous when precise compounding and blending operations are required.

Maintenance: Conical twin screw extruders are easier to maintain, parallel twin screw extruders are more complicated

When it comes to maintenance, conical twin screw extruders have an edge over their parallel counterparts due to their structural design. The conical arrangement of the screws in a conical twin screw extruder provides wider spacing between the screws, thanks to the increased diameters at the feed end. This design feature has several implications for maintenance:

• Thrust Bearing Installation: The wider spacing in conical twin screw extruders allows for the use of standard thrust bearings. In contrast, parallel twin screw extruders have screws positioned very close to each other, which complicates the provision of adequate thrust bearings. To address this issue, one of the screw shafts in a parallel extruder must extend beyond the other to accommodate the necessary thrust bearing, requiring additional engineering solutions.
• Gear System Simplicity: Conical twin screw extruders offer a simpler and more robust distribution gear system due to the increased space between the screws. This structural advantage contributes to easier maintenance and enhanced stability.
• Bearing Capacity: Conical twin screw extruders typically have larger installation spaces for bearings and gear shafts. This feature enables the use of larger-specification radial and thrust bearings, resulting in improved bearing capacity.
• Transmission System Complexity: The transmission system of a parallel twin screw extruder is relatively complex. The gearbox manufacturing cost is higher, and maintenance is more complicated compared to conical twin screw extruders.

In summary, the structural differences between conical and parallel twin screw extruders significantly impact maintenance requirements. The conical arrangement in conical twin screw extruders provides advantages in terms of thrust bearing installation, gear system simplicity, bearing capacity, and reduced complexity of the transmission system. As a result, conical twin screw extruders are generally easier to maintain, while parallel twin screw extruders present more complicated maintenance challenges.

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