The differences between single screw and twin screw extruders go beyond the number of screws. They include variations in material transport mechanisms, self-cleaning capabilities, extrusion stability, and feeding methods.

In both single screw and co-rotating twin screw extruders, material transport relies on the principle of a right-hand screw and nut, where friction pushes materials forward. Co-rotating twin screw extruders feature intermeshing screws with a wiping effect for self-cleaning, enabling efficient material transport. Counter-rotating twin screw extruders use the calender effect, forcibly transporting materials forward like a calender roll or gear pump. Different feeding systems are employed in these extruders to ensure stable material supply and transport.

Material Transport and Propulsion Principles

Right-Hand Screw and Nut Principle

In single screw and co-rotating twin screw extruders, solid material is transported forward similarly to the movement of a nut along a right-hand screw. When a nut is held stationary while the screw rotates, the nut moves forward along the screw. Here, the extruder screw represents the bolt, and the plastic material is analogous to the nut. The material is pressed against the barrel’s inner wall by friction, allowing it to be conveyed forward by the rotating screw to subsequent processing stages.

Wiping-Based Forward Transport

In co-rotating twin screw extruders, self-cleaning through wiping ensures materials are scraped off at intermeshing sections of the screws. This mechanism minimizes material stagnation and facilitates efficient transport.

Calender Effect

In counter-rotating twin screw extruders, the screws intermesh tightly. This design forces materials forward using a principle similar to calender rolls or gear pumps. The mechanical structure ensures powerful and consistent forward transport.

Self-Cleaning Effect

Self-Cleaning in Co-Rotating Twin screw Extruders

The self-cleaning effect is a key advantage of twin screw extruders, reducing material stagnation and ensuring efficient material transport and kneading. This improves product quality stability while minimizing residue and contamination within the extruder.

In co-rotating twin screw extruders, the intermeshing screw design prevents material from accumulating in the screw flights. As the screws rotate, material is continuously transferred between them, enabling self-cleaning. This constant movement prevents stagnation and supports uniform material processing.

Extrusion Stability

Stability in Single screw Extruders

Single screw extruders generally maintain stable pressure and output flow, depending on material flow properties and screw design. However, pressure fluctuations under certain conditions can compromise stability.

Stability in Twin screw Extruders

Twin screw extruders may require gear pumps to achieve highly stable and precise output, especially for applications demanding accurate volumetric extrusion. Gear pumps enable controlled pressure and flow rates for consistent operation.

　

Devolatilization Performance

　

Twin screw extruders excel in devolatilization compared to single screw extruders due to their enhanced kneading effects, which uniformly heat materials and expose large surface areas for efficient evaporation of volatiles. Flexible screw designs also allow for optimal devolatilization zones, ensuring effective removal of volatile components.。

Material Feeding

　

Single screw Extruders: Solid material completely fills the feed zone, often allowing direct feeding through a hopper without the need for a feeder.

Twin screw Extruders: Feeding requires the use of a feeder to supply materials consistently.

Additional Features of Twin screw Extruders

Superior Mixing Capability

Twin screw extruders effectively knead, disperse, and distribute materials due to the interactive movement of the two screws. This makes them highly suitable for mixing multiple materials, including composites and nanomaterials, ensuring uniform and high-quality mixtures.

High Shear Stress

The screw design and configuration can generate strong shear forces, enabling efficient processing of high-viscosity or difficult-to-melt materials. This feature is particularly valuable in applications like reactive extrusion and resin kneading.

Mid-Process Material Addition

Twin screw extruders allow for mid-process addition of solids or liquids. For instance, side feeders can introduce materials at different stages, while liquid additives such as lubricants or reactants can be added at optimal points, enabling the production of materials with specific properties.

Differences Based on Screw Rotation

Co-Rotating Twin screw Extruders

These generate high shear forces between the screws, making them ideal for strong mixing and uniform blending of complex materials. They are particularly effective for applications requiring high-performance kneading.

Counter-Rotating Twin screw Extruders

These exert lower shear forces, reducing stress on materials, making them suitable for applications where shear-induced heat generation must be minimized. They are ideal for processing materials like PVC and are often used for low-speed, stable extrusion.

Twin Screw Extruders by Technovel
– A Dedicated Japanese Extruder Manufacturer

　

As a world-first innovation, we have developed a new type of horizontal multi screw extruder. Our Quad screw and Octa screw extruders are already being utilized across various industries. Even in the field of conventional twin screw extruders, we offer a wide lineup ranging from ultra-compact models—such as the world’s smallest 6 mm diameter screw and our best-selling 15 mm model—to large-scale production machines.

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