What is a Twin Screw Extruder?

A twin screw extruder, as its name suggests, is an extruder equipped with two screws. It is primarily used in processes such as mixing, melting, reacting, and forming materials. In particular, co-rotating, fully intermeshing twin screw extruders excel in advanced material processing due to their ability to move materials forward while performing intense mixing through complex screw movements.

Screws in Twin Screw Extruders

The twin screws in a twin screw extruder operate using a modular element system, where various screw elements are combined to achieve material transport and mixing. By adjusting the configuration of screw elements to match the extrusion process, the equipment offers high flexibility in its setup.

the Size of Twin Screw Extruders

The size of a twin screw extruder is generally defined by the diameter of its two screws. As the screw diameter increases, the internal volume of the barrel also increases, leading to a higher production capacity. On the other hand, smaller extruders used for material development prioritize ease of handling. In research and development settings, twin-screw extruders with screw diameters typically ranging from 8 to 20 mm are commonly used

Features of Twin Screw Extruders

High Mixing Capability and Suitability for Multiple Materials

The twin screws interact with one another to efficiently move, mix, disperse, and distribute materials. This ensures superior capability to uniformly blend multiple materials. Even complex materials like composites and nanomaterials can be homogenized, resulting in high-quality mixtures.

High Shear Stress

The design and arrangement of the screws enable the application of strong shear stress to the materials. This allows efficient processing of high-viscosity materials and those that are challenging to melt. This feature is particularly beneficial for reactive extrusion and resin compounding.

Flexible Process Configuration

Twin screw extruders can adapt their barrel and screw configurations to optimize production efficiency for various extrusion processes. For example, vents can be added for effective degassing, or the screws and barrels can be extended to increase reaction time.

Excellent Temperature Control

Twin screw extruders are equipped with precise temperature control throughout the extrusion process. This ensures accurate management of material melting and chemical reactions, resulting in high-quality final products.

Self-Cleaning by Wiping Action

Fully intermeshing twin screw extruders prevent material stagnation or gelation by ensuring the screws wipe each other. This minimizes contamination adherence to the barrel’s inner surface, maintains high thermal conductivity, and ensures smooth material melting. Additionally, material transitions during mixing of different substances are seamless, ensuring consistent quality.

Mid-Process Material Addition (Side Feeding or Liquid Injection)

During the extrusion process, additional solid or liquid materials can be introduced. Using side feeders, materials can be added at different stages for efficient mixing. Liquid additives, lubricants, or reactants can also be introduced at optimal timings, enabling the creation of specific product characteristics.

Adjustable Mixing Performance via Screw Element Selection

By varying the type and arrangement of screw elements, the extruder’s mixing performance can be optimized. Screw elements are available in various shapes for transport, mixing, or dispersion. Customizing these elements allows adjustment of shear and mixing levels to align with process efficiency and desired product properties.

Effective Degassing with Vent Structures

Vent structures are incorporated to efficiently remove gases and volatile components generated during the process. This removes bubbles or volatiles within the material, enhancing product quality and preventing defects caused by air pockets or component volatilization

Basic Structure of a Twin Screw Extruder

A twin screw extruder operates by transferring power from a motor, through a gearbox, to the screws at a controlled rotational speed. The two screws rotate within the barrel, intermeshing to convey, melt, and mix the raw material. Heaters attached to the barrel ensure precise temperature control, maintaining the material in an optimal state for processing. The quality and performance of the final product are heavily influenced by the screw design, rotational speed, and barrel temperature. Below are the key components and auxiliary equipment that constitute a basic twin screw extruder:

①　Hopper

The entry point for raw materials. The process begins as materials are fed into the extruder through the hopper.

②　Barrel

The structure housing the screws and where material is transported. The barrel controls the temperature and pressure, often equipped with heating or cooling jackets. Typically composed of multiple sections, the temperature is adjusted based on the processing stage.

③　Heater

Mounted on the barrel, the heater regulates the temperature of the material, ensuring it is processed at the correct temperature.

④　Screw

The most critical components of a twin screw extruder. Two screws rotate within the barrel, with their design and arrangement significantly affecting the mixing efficiency. By combining various screw elements, materials can be conveyed, dispersed, mixed, and transported to the exit.

⑤　Gearbox

The mechanism that transfers the motor’s rotational force to the screws, controlling their speed and torque.

⑥　Motor

The power source for the extruder. By adjusting the speed and torque, the motor influences extrusion performance.

⑦　Die Head

The section where the extruded material is shaped. As the material passes through the die, it is formed into the desired shape. Dies can be replaced or customized according to the product specifications.

⑧　Ancillary Facility

Auxiliary equipment is essential for twin screw extruders, with each component offering unique functions. Feeders ensure a stable supply of raw materials, screen changers remove foreign matter, and gear pumps stabilize pressure and flow. Cooling devices cool the material after extrusion, while pelletizers cut the material into pellets.

Operating Principle and Process of Twin screw Extruders

The material is fed through a hopper and conveyed forward by the rotation of the screws. Because the screws are intermeshing, the material is compressed and mixed as it moves, resulting in a uniform state. Precise control of pressure and temperature is essential, and heat is applied as needed to facilitate melting or promote chemical reactions.

Feeding

Raw materials are supplied from the hopper to the screws using a feeder. The screws’ rotation moves the materials forward.

Heating and Melting

Raw materials are heated in the barrel by the heaters, transitioning from a solid to a molten state. Temperature management is crucial and requires precise control of the melting temperature.

Dispersive and Distributive Mixing

As materials are conveyed, they are simultaneously mixed by the screws. Screw configurations enhance dispersion and homogeneity, optimizing material properties.

Degassing

During processing, gases or moisture in the resin materials are released. A degassing zone expels unwanted gases and moisture through vents, improving product quality.

Extrusion

Finally, the molten material is extruded through the die, taking on its final shape. Cooling systems are used to solidify the material, resulting in the finished product.

Applications of Twin Screw Extruders

Twin screw extruders are used in various fields, including pelletizing, recycling, filler compounding, polymer alloy and blend production, degassing, and the manufacturing of biodegradable resins. Applications also extend to sheet and film formation, chemical reactions, drying, and dehydration.

Kneading and Pelletizing

This is one of the fundamental uses of twin screw extruders. Polymers and additives are kneaded into a uniform mixture and shaped into pellets for further processing like injection or blow molding.

Recycling

Twin screw extruders are utilized for recycling plastics and composites into new materials. Features like impurity removal and degassing efficiently handle gases and impurities, enabling the production of high-quality recycled materials. Advanced chemical recycling, such as depolymerizing PMMA, also uses twin screw extruders.

Filler Compounding

Twin screw extruders mix fillers like glass fiber, carbon fiber, or calcium carbonate with polymers to enhance material properties. Efficient mixing ensures uniform filler dispersion, directly influencing material performance.

Polymer Alloys and Blends

In this application, different polymers are mixed to create materials with novel properties. Twin screw extruders facilitate efficient blending and promote molecular-level interactions.

Natural Resource Compounds

With rising environmental awareness, compounds combining polymers and natural resources (e.g., cellulose) are gaining attention. Twin screw extruders efficiently mix these materials to produce composites like cellulose nanofiber-reinforced plastics.

Sheet and Film Formation

High-precision mixing enables twin screw extruders to produce sheets and films with consistent material properties, offering precise control over thickness and strength.

Chemical Reactions, Drying, and Solvent Removal

Twin screw extruders are also used in polymer reactions, dehydration, and solvent removal processes. Their degassing function efficiently removes byproducts or unwanted substances, improving material quality and reaction efficiency.。

Alternative Proteins (Soy-Based Meat)

Twin screw extruders are used to produce high-moisture soy meat (HMMA) and textured vegetable protein (TVP). By adjusting water content and die design, manufacturers create products with distinct textures and moisture levels.

　

Types of Twin screw Extruders

Briefly introduced the features of the co-rotating fully intermeshing twin-screw extruder. As shown in the diagram above, twin screw extruders can be classified into various types. The main classification depends on two factors: whether the two screws are intermeshing or not, and whether the screws rotate in the same direction (co-rotating) or in opposite directions (counter-rotating). These two points broadly define the types of twin-screw extruders.

Based on Rotation Direction

Twin screw extruders can be classified into co-rotating and counter-rotating types. Due to the difference in screw rotation direction and screw design, the characteristics of these two types differ significantly. In co-rotating twin-screw extruders, high shear stress is generated between the screws, enabling strong and efficient mixing. On the other hand, counter-rotating twin screw extruders produce lower shear stress compared to co-rotating types, making them suitable for processing materials where heat generation from shear needs to be minimized.

Based on Intermeshing

In fully intermeshing types, the screws rotate while meshing with each other, enabling powerful mixing of materials. This structure allows for uniform blending and high-precision processing, making it suitable for resin compounding and reactive extrusion. It provides high shear and precise mixing. In contrast, non-intermeshing types feature screws that rotate independently without meshing, resulting in lower shear and gentler mixing—ideal for materials that require less aggressive processing.

Screw Configuration in Twin Screw Extruders

The screw design of a twin screw extruder must be carefully optimized based on the properties of the material and specific process requirements. Key parameters such as the L/D ratio (length-to-diameter) and D/d ratio (outer-to-core diameter) must be appropriately set to maximize mixing and reaction efficiency while balancing energy consumption and processing time.

The selection of screw elements also has a significant impact on product quality and production efficiency. Specifically, the screw type, pitch, and groove depth influence material flow, shear force, and residence time—factors that are critical to achieving optimal mixing performance.

By adjusting the combination of screw elements, the extruder can be fine-tuned to deliver the desired performance for a given application, leading to improved product quality and enhanced productivity. Therefore, screw design in twin screw extrusion requires careful and precise customization, and selecting the most suitable configuration for the process is of utmost importance.

Dispersive and Distributive Mixing in Twin screw Extruders

The mixing process in extruders involves plastifying and melting polymers while incorporating different polymers, fillers, and additives to produce a uniform compound. This process can be categorized into two types of mixing mechanisms: dispersive mixing and distributive mixing. Understanding the mechanisms of mixing in twin screw extruders is critical for optimizing the processing method. A balanced and effective combination of these two types of mixing ensures the production of homogeneous compounds with added fillers or polymers.

Material flow in the extruder is further divided into shear flow and extensional flow. While shear flow is generally considered the dominant factor in twin screw extruders, extensional flow also plays a crucial role in dispersion mixing. Therefore, both shear and extensional flow must be taken into account for effective mixing.

Differences Between Single Screw and Twin Screw Extruders

The distinction between single-screw and twin screw extruders lies not only in the number of screws but also in their material transport mechanisms, self-cleaning properties, extrusion stability, and feeding characteristics.

In single-screw extruders and co-rotating twin screw extruders, material transport is based on the principle of a screw and nut, where frictional force moves the material forward. In co-rotating twin screw extruders, the intermeshing screws create a wiping effect, enabling self-cleaning and efficient material transport. In counter-rotating twin screw extruders, material is forced forward through a calendering effect. Each of these extruders employs different feeding systems to ensure stable material supply and transport tailored to the specific extrusion process.

Filling Ratio and Residence Time in Twin screw Extruders

The relationship between residence time and filling ratio significantly impacts aspects such as material flow, mixing, temperature distribution, and reaction efficiency within the extruder. Proper management of these parameters is essential for improving extrusion efficiency and product quality.

Higher filling ratios result in longer residence times, which can enhance mixing and temperature uniformity but may also increase the risk of material degradation. Conversely, lower filling ratios reduce residence time, potentially leading to insufficient mixing. Therefore, selecting an appropriate filling ratio based on the extrusion process objectives is critical. By optimizing the relationship between residence time and filling ratio, stable and efficient extrusion processes can be achieved while minimizing product quality variations.

Plastification and Melting Behavior in Twin Screw Extruders

In twin screw extruders, plastification and melting occur through the complex interaction of various heat generation mechanisms caused by the high shear stress generated by the screws. This process plays a crucial role in determining the dispersion state of different types of plastics and nanoparticles and accounts for most of the energy consumption within the extruder.

The plastification and melting process can be classified into four primary heat generation mechanisms:

1. Heat generation through plastic deformation
2. Viscous dissipation
3. Frictional heating
4. Heat transfer from external heaters

In the solid transport and melting zones of twin screw extruders, these mechanisms influence the melting process and material processability. The interplay of these factors determines process efficiency and the quality of the final product.

　

Tip Clearance of Twin Screw Extruders

　

The design of optimal tip clearance significantly affects dispersive mixing performance, thermal history, and self-cleaning capabilities during extrusion.

Smaller tip clearances generate higher shear stress, improving dispersion but also increasing the risks of thermal degradation and localized overheating due to plastic deformation, viscous dissipation, and frictional heating. However, self-cleaning efficiency improves, minimizing material buildup on the barrel and screws, which helps suppress thermal degradation during extended operations.

In contrast, larger tip clearances allow for greater material throughput but reduce shear stress, potentially compromising dispersion efficiency. Therefore, designing tip clearance based on material characteristics and mixing requirements is key to achieving an efficient and stable extrusion process.

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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