What is a Twin Screw Extruder?

A twin screw extruder, as the name suggests, is an extruder equipped with two screws and is mainly used for material mixing, melting, reaction, and forming processes.　In particular, co-rotating fully intermeshing twin screw extruders are able to convey materials forward while performing strong and effective mixing through the complex motion of the screws. For this reason, they show excellent performance in advanced material processing.

In addition, twin screw extruders allow flexible modification of screw configurations, making it possible to set optimal mixing conditions according to the properties of the material. For example, even materials with high viscosity or compounds with a high filler content can be uniformly melted and mixed by controlling the mixing intensity and residence time.

Furthermore, by adjusting extrusion parameters such as screw diameter, L/D ratio, and barrel temperature, twin screw extruders can be used for a wide range of purposes, from small scale material evaluation to process optimization and scale-up to mass production.

In this way, twin screw extruders are not only mixing machines, but also essential equipment for advanced material processing technologies and for the evaluation and development of new materials, and they are widely used in many industrial fields.

Screws in Twin Screw Extruders

A twin screw extruder is a highly flexible machine in which two element type screws can be freely combined according to the application and extrusion process.After the raw materials are stably fed into the feeding section, they are conveyed forward in the material conveying section, where they are heated and compressed. In the mixing section, the materials are fully melted and homogenized by strong shear stress generated by the fully intermeshing twin screws, allowing additives and fillers to be uniformly dispersed. The processed material is then discharged through the outlet section and extruded into a die, from which it is continuously supplied to downstream processes such as pelletizing or film and sheet forming.

In this way, a twin screw extruder can continuously perform conveying, melting, mixing, and extrusion in a single integrated process, making it suitable for a wide range of applications, including the production of compounded materials and polymer modification.

the Size of Twin Screw Extruders

The machine size of a twin screw extruder is generally defined by the diameter of the two screws. As the screw diameter increases, the internal volume of the barrel also increases, allowing more efficient melting and mixing of materials and resulting in higher production capacity. In addition, a larger screw length-to-diameter ratio (L/D ratio) makes it possible to better control residence time and shear stress during the extrusion process, thereby improving material dispersion.

On the other hand, small scale twin screw extruders used for material development and evaluation of new materials are required to have machine sizes that emphasize ease of handling and flexibility in installation. These laboratory scale machines allow evaluation using small sample quantities. In research and development applications, twin screw extruders with screw diameters generally ranging from 6 to 20 mm are commonly used.

For example, a small twin screw compounding extruder with a screw diameter of 15 mm can achieve a throughput of several hundred grams to approximately 5 kg per hour for general purpose materials, making it an ideal size for obtaining samples for post extrusion analysis and evaluation.

Even in small scale machines, handling performance, operability, and customization comparable to those of production machines are required. It is important that the feeding methods, temperature settings, and optimization of mixing conditions can faithfully reproduce the processes used in production scale extruders. In addition, in-house manufacturing capabilities and the ability to design and produce auxiliary equipment contribute significantly to improving the accuracy of material development and prototyping.

As a result, knowledge obtained at the research and development stage can be smoothly transferred to production scale machines, enabling efficient scale up with a clear view toward mass production processes.

Features of Twin Screw Extruders

High Mixing Capability and Suitability for Multiple Materials

A fully intermeshing twin screw extruder applies strong shear to materials as the two screws rotate in the same direction while intermeshing with each other. This structure allows the materials to move forward while being efficiently exchanged between the screws, resulting in highly uniform mixing.

Because stable dispersion and distributive mixing can be achieved even when multiple materials are processed simultaneously, this type of extruder is suitable for difficult to mix systems such as polymer alloys, composite materials, and materials containing nanofillers. Furthermore, as dispersion and distribution are improved, the mechanical properties of the final product are significantly enhanced.

High Shear Stress

By appropriately designing the screw geometry and element arrangement, high shear stress can be applied to the material. This makes it possible to obtain a uniform molten state in a short time, even for high-viscosity resins or materials that are difficult to melt.

In particular, in the field of reactive extrusion, mixing accompanied by chemical reactions is required, and the balance between shear and residence time is critical. Fully intermeshing screws offer high energy transfer efficiency, enabling both promotion of reactions and effective dispersive and distributive mixing while suppressing excessive molecular chain scission.

For this reason, they demonstrate high processing performance in a wide range of processes, including polymer modification, additive dispersion, and blending operations.

Flexible Process Configuration

One of the key features of a twinscrew extruder is the ability to freely reconfigure the screw arrangement and barrel configuration according to the objectives of the process. For example, when reactive extrusion or devolatilization is emphasized, configurations that extend residence time and optimize vent locations can be applied.

In contrast, when rapid mixing or efficient dispersive and distributive mixing is required, efficiency can be increased by designing the process with an expanded mixing zone. In addition, independent temperature control for each barrel section allows fine and precise optimization of the entire process.

Thanks to this high level of flexibility, a single twin screw extruder can accommodate a wide range of processing conditions, from product development to mass production.

Excellent Temperature Control

Each barrel of a twin screw extruder is equipped with an independent temperature control zone, allowing highly precise heating and cooling. This enables strict control of material melting behavior and chemical reaction rates, making it possible to maintain advanced extrusion processes.

Especially for reactive materials, even small temperature variations can affect the final properties, so this high-precision temperature control is a key factor in ensuring stable product quality. In addition, by optimizing heat transfer within the barrel, unnecessary energy loss can be reduced, achieving both process stability during long-term operation and high production efficiency.

Self-Cleaning by Wiping Action

Fully intermeshing screws mesh with each other like gears, creating a “wiping effect” that automatically removes material adhering to the screw surfaces and the inner wall of the barrel. This effect suppresses material stagnation and helps prevent gel formation and thermal degradation.

In addition, contamination (residual material) is less likely to adhere to the inner surface of the barrel, allowing stable and efficient heat transfer from the heaters. As a result, melt temperature uniformity and energy efficiency are improved, and the influence of residual material during material changeover is minimized, ensuring stable product quality.

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

Twin screw extruders are designed to allow the addition of solid materials and liquids during the extrusion process. By using side feeders, materials can be introduced at different stages, enabling stepwise reactions and compounding.

For example, the main raw material can be mixed in the initial stage, and fillers, stabilizers, plasticizers, or other additives can be added in downstream sections, making reaction control easier. In addition, when combined with liquid feeding systems, reactants, lubricants, and other liquids can be precisely injected, allowing material properties to be optimized according to specific requirements.

These controlled feeding methods enhance process design flexibility and reproducibility, making it possible to handle complex material systems.

Adjustable Mixing Performance via Screw Element Selection

Screw elements are available in various shapes, such as conveying elements and mixing elements, and process performance can be freely adjusted by combining these elements according to their respective functions. In addition, element selection based on experimental data and simulation results enables process design tailored to material properties and target products.

As a result, residence time and pressure distribution can be precisely controlled, achieving both high product quality and production efficiency.

Effective Degassing with Vent Structures

To efficiently remove gases and volatile components generated during the extrusion process, twin-screw extruders are equipped with venting structures. Through devolatilization under vacuum or atmospheric conditions, air bubbles, residual monomers, moisture, and other volatile substances in the raw materials can be effectively discharged.

As a result, the transparency, density, and mechanical properties of the final product are improved, and defects such as poor molding and gas-related imperfections can be prevented. In particular, for processes such as polymer recycling and reactive extrusion, where volatile by-products are likely to be generated, this venting structure is an essential factor for stable operation.

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.

Compounding and Pelletizing (Pellet Production)

Recycling

Filler Compounding

Polymer Alloys and Polymer Blends

Compounding with Natural Resources

Direct Sheet and Film Extrusion

Reaction, Drying and Dehydration, De-monomerization, and De-solventization

Alternative Meat (Soy-Based Meat)

　

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 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.

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.

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.

Basic Experiments Using the ULTnano15

In this experiment, a small twin screw compounding extruder, ULTnano15, was used to visually observe how changes in the Q/N index (the ratio of throughput Q to screw rotation speed N) affect the filling ratio in the plasticizing and melting section. By varying the throughput and screw speed to adjust the Q/N value, differences in the filling behavior were compared.

These findings indicate that the balance between feed rate and screw rotation speed has a sensitive influence on resin residence behavior and filling characteristics. This supports the conclusion that the Q/N index is an effective parameter for understanding and controlling the extrusion process.
Further details on the experimental setup and results can be found on the linked page explaining “filling ratio and residence time in twin-screw extruders.”

Practical Overview of Operating Conditions in Twin Screw Extruders

The process behavior of a twin screw extruder arises from the energy input state formed by the combination of operating variables, which can be set and adjusted during operation (soft conditions), and structural variables, which are predetermined as part of the equipment specifications (hard conditions). As a result of these settings, various process indicators—such as melt temperature, melt pressure, specific energy, and residence time—are generated, enabling quantitative evaluation of material melting and mixing states.

Therefore, in optimizing a twin screw extrusion process, it is crucial not to consider individual operating conditions or machine specifications in isolation. Instead, one should systematically organize the relationship between operating and structural variables from the perspective of energy input, and design process conditions based on a clear understanding of their causal relationships with each process indicator.

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.

In the plasticizing and melting section of the ULTnano15 small　scale twin-screw extruder with a special vertically split barrel manufactured by Technovel, the plastic material can be observed being mixed by the screws and gradually melting. In a twin-screw extruder, the plasticization and melting of the resin are considered to progress as various heat generation mechanisms caused by the high shear stress produced by the two screws act on the plastic material in a complex manner.

　

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.

Scale Up of Twin Screw Extruders

Scale up of twin screw extruders cannot be achieved through simple geometric enlargement or the direct application of scaling equations. As the machine size increases, throughput, shear history, residence time behavior, and heat transfer conditions change simultaneously, making it theoretically and practically impossible to preserve all performance indicators at the same time.

The cubic law represents a three-dimensional, throughput-oriented scaling concept, whereas the square law reflects a two-dimensional, energy-based approach. Actual twin screw compounding processes do not strictly conform to either law, but instead operate within the continuum between these two limiting models.

When productivity is prioritized, operation tends to approach the cubic law. Conversely, when reproducibility of material history is emphasized, operation shifts toward the square law. This trade-off is unavoidable and lies at the core of the complexity and intellectual challenge of scale up.

Ultimately, scale up of twin screw extruders is a design process in which the engineer selects the most appropriate operating point from countless possibilities between the square and cubic laws. It is in this decision-making process that the true expertise of the extrusion engineer is revealed.

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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