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What You Need to Know About Twin Screw Extruders and Compounding Machines

Twin screw extruders and compounding machines are the center of many industrial processes. These machines use two screws that interlock with each other and are placed within the same cylinder to blend, compound or melt materials. What makes them different from other designs is their ability to control such a large amount of process variables, which makes them suitable for all kinds of material handling. This article will explore their mechanics further along with operation principles as well as applications in different fields.

Understanding Twin Screw Extruders

Understanding Twin Screw Extruders
Understanding Twin Screw Extruders
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Basics of a Twin Screw Extruder

Double screw extruders have their roots in single-screw extruder design. The significant distinction between the two lies in the number of screws used – one vs. two. These screws revolve closely intermeshed and in co-rotating direction, creating an action of conveyance and shearing that mixes, melts as well as pumps feedstock through them effectively. By changing pitches, helical angles or depths of these screws which are unique each other could be designed thus altering the intensity with which they shear thereby affecting mix quality too. Additionally this construction allows for excellent self-cleaning ability resulting into reduced downtimes caused by maintenance activities hence increasing operational efficiency levels.

Process of twin screw extrusion

The following is a step-by-step summary for the process involved during double screw extrusion:

  • Feeding: Input materials such as pellets, granules or powder are fed into an extruder via its hopper.
  • Conveying: Material transport through barrels happens when those intermeshed screws rotate at some speed; screw type and speed determine rate at which material can be acquired.
  • Melting/Plasticizing: As it travels along heated barrels where temperatures are known to be high enough for melting plastics etcetera, shear forces produced by rotating blades assist it become plasticized until turning into highly viscous fluid molten state.
  • Mixing/Compounding: Further compounding takes place while still molten; additives may also be introduced here if need be.
  • Degassing: In this stage any trapped volatiles within product being formed get removed out so that only nonvolatile matter remains behind – sometimes done under vacuum conditions called vacuum degassing or deaeration depending on type being processed and requirements thereof.
  • Shaping: Dies shape streams of softened material coming out from a machine like strands/sheets/tubes/pellets etcetera depending upon desired final form .
  • Cooling & Cutting: At the end, plastics are usually cooled down using water baths, especially if they come out as strands. Then, they are cut into desired lengths by cutters.

All these steps can be controlled individually to produce materials with desired properties and qualities, thus illustrating how twin-screw extrusion is versatile and efficient.

Twin screw extruder configurations

Different processing needs call for various configurations of twin screw extruders which mainly revolve around screw design types as well as orientations:

  • Corotating Twin Screw Extruders – In this case both screws rotate in same direction; it ensures enough conveyance of material through extruder plus high speed & shear rates possible (ideal for mixing, compounding etc.)
  • Counter-rotating Twin Screw Extruders – Screws turn opposite each other; used where there’s need for high pressures e.g., PVC compounding.
  • Intermeshing Twin Screw Extruders– Close interlocking between adjacent flights helps clean them during operation hence widely used in recycling industry alongside their ability to process wide range of materials having different viscosities at low temperatures .
  • Non-Intermeshing Twin Screw Extruders – Here clearance exists between not intermeshing screws giving room or space for high throughput rates predominantly applicable during devolatilization processes.

Every configuration boasts its own unique features making double screw extrusion highly adaptive procedure.

Advantages of a twin screw extruder

For many industrial applications, twin-screw extrusion offers several advantages that reflect its unmatched versatility in such fields. Some include;

  • Effective Mixing: Distributive mixing within a polymer melt is achieved more easily due to the better dispersive nature provided by twinscrew setups, which results in product uniformity.
  • Self-cleaning: The screws have the ability to clean each other, especially in the intermeshing design which is very advantageous when dealing with materials that stick or are compact.
  • Highly versatile: Twin screw extruders can be used in a wide range of applications from compounding to devolatilization as stated earlier due to their various configurations.
  • High throughput: Generally twin screw extruders have higher output capacities than single screw ones thus increasing overall productivity.
  • Robustness: Twin screw extruders are made strong and with tough features therefore can last for many years under normal conditions which makes them cheap in the long run for most industries.

In summary, this unique configuration of twin-screw extruder along with its efficiency, versatility and durability; it becomes very useful during different operations within industries.

Twin screw extruder applications

Twin screw extruder applications

Due to its robust design and flexible functionality, the twin screw extruder is widely used in many industries. Some of the most important applications are:

  • Plastic compounding: In plastic compounding, twin screw extruders are used to melt and mix various materials into a homogeneous composite.
  • Food processing: They cook, shape and improve food products effectively in the food industry. This includes breakfast cereals, pet foods and confectionery among others.
  • Pharmaceuticals: Twin-screw extruders are also essential for some pharmaceutical applications such as hot-melt extrusion which allows for the production of complex drug delivery systems.
  • Polymer devolatilization: Polymer devolatilization refers to removing volatile substances from the polymer matrix using a twin-screw extruder.
  • Biodegradable material production: Twin screw extruders have been adopted for biodegradable materials production in response to increased environmental concerns thereby promoting sustainable manufacturing practices.

The above mentioned advantages of twin screw extrusion – efficient mixing, high throughput, self-cleaning mechanism and durability – are utilized at each stage of these applications to optimize production processes as well as improve final product quality.

Common Problems in Twin Screw Extrusion

Common Problems in Twin Screw Extrusion

Issues in the design of the screw

The screw design is an essential aspect of extruder operation, and any problems with it can have significant consequences. The primary function of the screw in a twin-screw extruder is to transport, mix, melt and pressurize the material as it moves along the barrel. However, poor or inefficient screw design can lead to various issues such as poor mixing, inadequate melting, or uneven pressure distribution along the barrel. These problems may result in product defects like voids or inclusions within the processed material. Additionally, improper screw design can cause excessive shearing forces on sensitive materials leading to degradation and reduced quality of finished products. Therefore, it is crucial that designers consider factors such as material characteristics (e.g., viscosity), desired output rate, and energy requirements, among others, when coming up with suitable designs for screws used in extrusion processes.

Temperature control problems

Temperature control is another critical factor that affects the performance of extruders. Inadequate temperature regulation within different zones along the barrel can cause variations in processing conditions, resulting in poor product quality or even equipment damage due to overheating or under cooling effects, respectively. For instance, if temperatures are too high at certain stages during processing, this may lead to thermal degradation of heat-sensitive materials. In contrast, low temperatures could result in incomplete melting, thereby affecting the physical properties (e.g., strength)of products being produced. Also; inadequate heating may lead to increased viscosity hence affecting flowability which further affects homogeneity achieved through mixing action by screws inside extruders . To solve these challenges, modern control systems should be employed for precise monitoring and adjustment.

There are also problems with screw design that arise during the extrusion process. The screw takes care of stirring the polymer in a molten state and conveying it along the extruder, so any inaccuracies in its configuration can result in processing inefficiencies, inconsistent output, and compromised quality of products. Among other things, these pitch lengths may not be correct, diameters may differ or flight depths might have errors; all affecting times polymers reside here, when they melt and mix within an extruder respectively. Moreover, abrasion caused by materials being processed or high operating loads can wear away at screws thus further complicating matters for extruders. For this reason, therefore, selection must be done cautiously while designing screws such that, among others, one considers the type of polymer used, desired rate of production as well as process temperatures involved. Inspection needs to become a regular activity coupled with maintenance especially if wear resistant materials were employed during screw manufacture

Critical Components of Twin Screw Extruders

One-step Twin-screw extruder casting system
One-step Twin-screw extruder casting system
1. metering feeder 2. twin-screw extruder 3. screen changer 4. melt pump 5. Die 6. Casting roll 7.After cool roll
8. Pull roll station 9. Winding system

Twin screw extruder barrel and screw

The barrel and screw constitute the heart of the twin screw extruder. The barrel houses the screws and is designed to withstand high pressures and temperatures. It is typically lined with a wear-resistant material to minimize damage from the abrasive materials being processed. On the other hand, the screws are the central components that drive the extrusion process. They are carefully engineered with specific profiles and pitches to control the movement and mixing of the polymer. The rotation of the screws in the barrel creates zones of high pressure and temperature, enabling the melting, mixing, and pumping of the plastic. The design of the screws is crucial in determining the efficiency and quality of the extrusion process. Consequently, they need to be selected and designed with consideration to the specific polymer and the desired processing conditions.

Torque generation in twin screw extruders

Torque generation is a fundamental aspect of the operation of twin screw extruders. Torque is essentially the turning force applied by the screws to move the material through the barrel. It’s generated through the interaction of the rotating screws with the polymer, creating shear powers that aid in melting and mixing the material. The amount of torque required varies with the characteristics of the polymer and the specific processing conditions. High-viscosity materials and high processing pressures tend to require more torque, while lower-viscosity materials and lower pressures require less. A motor typically supplies the torque, and its capacity needs to be sufficient to handle the maximum torque demand of the extrusion process to avoid stalling or damage. The accurate control of torque is critical to ensure consistent processing conditions and advanced control systems are often employed for this purpose. Monitoring torque levels can also provide valuable information about the extrusion process and assist in troubleshooting and optimization.

Feeder systems for twin screw extruders

Feeder systems play a crucial role in the twin screw extrusion process, responsible for providing a steady and uniform supply of raw material to the extruder. There are several types of feeders, each designed to handle specific kinds of materials. Gravimetric feeders, for example, measure the weight of the material to control the feed rate, ensuring precise control over the amount of material entering the extruder. Volumetric feeders, on the other hand, regulate the feed rate based on the volume of the material. These feeders are typically used when the bulk density of the material is consistent. The choice of feeder system depends on various factors, including the nature of the raw material, the required feed rate, and the specific requirements of the extrusion process. Understanding the characteristics of the material and the demands of the process is essential in selecting the most suitable feeder system.

Screw design in twin screw extrusion

The design of the screws is a critical factor in twin screw extrusion, having a significant impact on the efficiency and performance of the process. The screws are typically designed with a specific pitch, diameter, and length-to-diameter ratio, and these parameters are selected based on the nature of the material being processed and the specific requirements of the process. For example, longer screws with a high length-to-diameter ratio are often used in processes that require intensive mixing or high levels of devolatilization. The shape and arrangement of the screw elements also play a crucial role in determining the degree of mixing and the conveying characteristics of the extruder. In general, corotating, self-wiping twin screw extruders are preferred for compounding applications due to their superior mixing capabilities and ability to handle a wide range of materials. These designs are more complex and require careful selection and arrangement of the screw elements to achieve the desired processing conditions and product characteristics. The screw design is often optimized through a combination of calculations and empirical testing, with the goal of achieving the best balance of throughput, product quality, and energy efficiency.

Plastic processing in twin screw extruders

Twin screw extruders are extensively employed in the processing of plastic materials due to their versatility, robustness, and high operational efficiency. The process typically begins with the feeding of raw plastic material into the extruder, often in the form of pellets or granules. The material then undergoes the crucial function of melting, facilitated by the controlled heating and shearing actions of the rotating screws. The molten plastic, once thoroughly heated and mixed, is then forced through a shaping die, thereby establishing the final form of the product. The process’s efficiency can be notably enhanced by employing screws with an optimized design, enabling the effective handling and processing of various plastic materials, including thermoplastic polymers, thermosets, and elastomers. The ability of twin screw extruders to provide intensive mixing and devolatilization, coupled with their capacity to handle high-viscosity materials, renders them an integral part of the plastics processing industry.

Types of Twin Screw Extruders

Types of Twin Screw Extruders
Types of Twin Screw Extruders
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Corotating screw twin screw extruders

Corotating twin screw extruders are characterized by the synchronized rotation of two parallel screws in the same direction. This configuration promotes high levels of intermesh, which facilitates intensive mixing and high shear rates, making corotating twin screw extruders particularly effective for compounding, reactive extrusion, and devolatilization processes. The design enables efficient self-wiping action, keeping the screw flights clean and enhancing overall operational efficiency. Moreover, with their capacity to handle high fill levels and process a wide array of plastic materials, corotating twin screw extruders have become a cornerstone in many plastic processing applications.

Counter-rotating screw twin screw extruders

Counter-rotating twin screw extruders feature two parallel screws that rotate in opposite directions. This setup results in a more constrained melt flow path, making them particularly suitable for pipe, profile extrusion, and sheeting applications. The counter-rotating design typically experiences a lower intermeshing action compared to corotating configurations, leading to lower shear and mixing capabilities. Counter-rotating extruders often exhibit higher pressure build-up, which can be leveraged for specialized applications that require high-pressure processes. Their design characteristics also facilitate longer residence times, which is beneficial in applications where extended dwelling of materials in the extruder is advantageous. Just like their corotating counterparts, counter-rotating twin screw extruders are a pivotal part of the plastics processing industry.

Modular Twin Screw Extruder Designs

Modular twin screw extruder designs offer unprecedented flexibility and adaptability in plastic processing. The fundamental aspect of these designs resides in the interchangeable screw and barrel sections. These sections, or ‘modules,’ can be rearranged, replaced, or added to meet specific processing requirements. This modular approach allows for a highly customizable configuration, accommodating a variety of processing tasks ranging from compounding to devolatilization. The modular design also makes maintenance and parts replacement significantly more feasible, reducing downtime and enhancing overall productivity. Given these advantages, modular twin-screw extruder designs have gained considerable traction in the plastics processing industry, further underscoring the importance of twin-screw extrusion technology.

Lab-scale twin screw extruders

Lab-scale twin screw extruders serve a crucial role in the research and development (R&D) phase of plastic processing. These scaled-down versions of industrial extruders allow researchers to test new formulations and processes on a smaller, more manageable scale before transitioning to full-scale production. Lab-scale extruders offer cost efficiencies by reducing the amount of material required for experimental runs. They are also designed to mimic the shear, pressure, and temperature conditions of industrial-scale machines, providing accurate and reliable data that can be used to predict full-scale production performance. Lab-scale twin screw extruders, therefore, are instrumental in facilitating innovation, enhancing process understanding, and accelerating the development of new plastic products and applications.

High torque twin screw extruders

High torque twin screw extruders are characterized by their ability to generate high levels of rotational force (torque) during operation. This feature allows them to process materials with high viscosity or process materials at high throughput rates while maintaining a consistent level of quality. High torque extruders are particularly advantageous in applications where high levels of filler or reinforced materials are used, as they can maintain a stable and uniform processing condition despite the increased resistance caused by these materials. Furthermore, the high torque design enhances the efficiency of the motor, reducing energy consumption and potentially lowering overall operational costs. Consequently, high-torque twin screw extruders are considered a valuable asset in the sphere of plastic processing, especially for operations requiring rigorous material processing conditions.

Applications of Twin Screw Extruders in Compounding

Applications of Twin Screw Extruders in Compounding

Polymer compounding with twin screw extruders

Polymer compounding is a process that uses twin screw extruders to mix molten polymers with other materials to improve or modify their properties. Twin screws provide strong mixing and dispersing actions by which fillers, pigments, or flame retardants are evenly distributed throughout the polymeric matrix, hence making compounds have uniform, predictable characteristics. In addition, this equipment can achieve high temperatures during processing so that reactive or heat-sensitive additives may be incorporated, thereby giving rise to unique polymer systems through compounding reactions of different materials within them at such elevated energy levels.

Twin-Screw Extrusion for Reaction Processing

Reactive processes take place within the barrel of an extruder where there is sufficient heat and mechanical agitation i.e., due to twin screws’ high temperatures during operation coupled with intense mixing ability. These machines allow for such reactions mainly because they have two intermeshed rotating shafts, each fitted with kneading elements along its length, thus promoting molecular rearrangement, leading either to improved or new properties in polymers.

Incorporating Additives During Compounding Manufacture

During the manufacturing of compounds, plasticizers, stabilizers, pigments, etcetera must be added uniformly into them; therefore, it becomes necessary to utilize twin screw extruders which are capable of providing much-needed shear forces required for achieving better dispersion among different components being mixed together. This happens when one substance is uniformly mixed with another so that every part contains some amount of both substances without any region having excess relative to others.

Devolatilisation Of Powders Using Twin Screw Extruders

Powdered materials often need drying before further processing steps can be undertaken. Thus, powder devolatilization is frequently carried out under vacuum conditions to eliminate volatile matter trapped inside particulate solids. Twinscrews afford a large surface area-to-volume ratio, which aids effective devolatilisation and ultimately leads to high-quality finished goods.

Twin Screw Compounding with High Shear Mixing

Twin screw extruders also find wide application in areas requiring intense levels of shear such as high-shear mixing. The rotational energy imparted by the intermeshing screws is converted into thermal energy resulting in significant shear rates being achieved within the system. This enables efficient dispersion of materials, producing uniform compounds with superior physical properties throughout.


Sure, here are ten reliable sources that provide valuable information about Twin Screw Extruders and Compounding Machines:

  1. In the Mix: Continuous Compounding Using Twin-Screw Extruders: This article from MDDI Online discusses how twin-screw extruders are used in plastics processing machinery to mix fillers and additives with the polymer continuously.
  2. Biopolymer Compounding on a Twin-Screw Extruder: Plastics Technology Online provides insights into the best practices for compounding biopolymers using long extruders, such as a 73-mm twin.
  3. Words of Wisdom: Understanding Twin-Screw Extruders: An article on Plastics Today that explains the role of extruder screw rpm and feed rate in optimizing compounding efficiencies.
  4. Twin-Screw Extruder Manufacturer – Compounding Machines: B&P Littleford’s website offers a look at their twin-screw extruders, which represent state-of-the-art compounding.
  5. Single Screw Extruder vs Twin Screw Extruder: Useon provides an informative comparison between single screw and twin screw extruders, explaining their characteristics and applications.
  6. What are the advantages of a twin screw extruder machine?: This Quora thread offers various insights into the benefits of twin screw extruders, such as their high torque, high speed, and lower consumption.
  7. Comprehensive Compounding and Extrusion Solutions for…: Thermo Fisher Scientific’s brochure presents a wide range of customizable options for twin-screw extruders designed to fit demanding and ever-changing environments.
  8. Compounding and Extrusion | Extruders: Another page from Thermo Fisher Scientific offers an overview of how twin-screw extruders are used in the mixing, compounding, and processing of materials.
  9. Twin Screw Extruders: This chapter from an academic book available on Wiley Online Library provides a detailed technical explanation of twin screw extruders.
  10. Twin Screw Extrusion for the Production of Controlled-Release Fertilizers: A research paper on ResearchGate that discusses how twin screw extrusion is used in the production of controlled-release fertilizers.

Each of these sources provides valuable information about Twin Screw Extruders and Compounding Machines, and they have been evaluated for accuracy, credibility, and relevance.

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