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Basic Components of a Twin Screw Extruder Machine

Twin screw extruders are highly versatile machines used in various industries for processing materials such as plastics, food, pharmaceuticals, and chemicals. These twin screw extruder machines are renowned for their ability to handle complex compounding, mixing, and extrusion tasks with precision and efficiency.

This article provides an overview of the key components of a twin screw extruder machine and their functions, helping users better optimizing its performance and ensuring high-quality output.

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Basic Components of a Twin Screw Extruder Machine

The structure of the twin screw extruder is mostly the same as the single screw extruder. The difference lies in the twin screw extruder having two screws within the barrel, and the feeding section involves a screw-driven forced feed into the barrel. Additionally, the specifications and arrangement of the twin screw extruder bearings are more complex.

Following is the component photo of twin screw extruder machine, we will explain the function of each component separately.

twin screw extruder component

1-Connector

Connectors in a twin screw extruder are used to link various parts of the machine, ensuring seamless communication and operation between different sections. They are essential for assembling the extruder and may facilitate the transfer of materials, electrical signals, or mechanical forces.

2-Filter

The filter, typically located near the die, is used to remove any impurities or contaminants from the molten material before it exits the extruder. This ensures the final product is of high quality and free from defects. Filters can also protect the die and downstream equipment from damage caused by foreign particles.

3-Barrel

The barrel is the cylindrical housing that contains the screws. It supports the screws and provides a controlled environment for material processing. The barrel is typically heated to melt the material and may have cooling sections to control the temperature during extrusion.

The barrel structure of the twin screw extruder is similar to that of the single screw extruder and can be divided into integral barrels and segmented combination barrels.

In twin screw extruders, intermeshing counter-rotating twin screw extruders and conical twin screw extruders generally use integral barrels. Only a few large extruders use segmented combination barrels, aiming to facilitate processing and save on expensive alloy steel materials.

Co-rotating twin screw extruders mostly use segmented barrels. These segmented barrels are divided into equal-length sections, with some sections having a feed port, some having a vent port, and some having both a feed port and a vent port. These segments are connected with screws to form a twin screw combination barrel.

4-Screws

There are various types of screw structures in twin screw extruders, classified based on different screw configurations and whether the screws intermesh. According to the composition of the screw’s threaded sections and the shape of the screws, there are several types as well.

Twin screw extruders can be classified into co-rotating and counter-rotating extruders based on the direction of screw rotation. In co-rotating extruders, the two screws rotate in the same direction during operation, while in counter-rotating extruders, the screws rotate in opposite directions.

Based on whether the screw axes are parallel or not, twin screw extruders can be divided into those with parallel axes and those with intersecting axes.

twin screw

Threaded Section Composition

Screws can be divided into integral threads and combination threads based on their threaded section composition.

Integral screws can be further divided into cylindrical screws, where the diameter of the threaded outer circle is completely uniform, and conical screws, where the diameter of the outer circle gradually decreases. Cylindrical screws are of two kinds: one where the pitch gradually decreases from the feeding section to the homogenizing section while the thread pitch remains constant, and another where the thread ridge width gradually increases from the feeding section to the homogenizing section.

Combination screws refer to screws whose threaded sections are composed of several different thread units assembled together. These units are installed on a shaft with a long connecting key, or on a hexagonal core shaft, acting as a special screw for extruding certain plastics. Co-rotating intermeshing twin screw structures often use combination screws.

twin screw

5-Heater

Heaters are installed along the barrel to provide the necessary thermal energy to melt the material being processed. Precise temperature control is crucial for ensuring proper melting, mixing, and flow of the material. Heaters can be electric, oil, or steam-based.

6-Feeder

The feeder, also known as the feed hopper, is where raw materials are introduced into the extruder. It ensures a consistent and controlled supply of material to the screws.The feeder hopper and feeding mechanisms make up the feeding system. The hopper holds the material, while the feeding mechanisms ensure a consistent and controlled supply of material to the screws.

Feeders can be gravimetric (weight-based) or volumetric (volume-based) depending on the material’s properties and processing requirements. A well-designed feed system is essential for maintaining steady extrusion conditions and preventing material surges or shortages.

7-Support

Supports are structural components that provide stability and alignment to the extruder. They ensure that the machine operates smoothly and that all components are held in their correct positions, minimizing vibrations and mechanical stresses.

8-Thrust Bearing

Thrust bearings are used to support axial loads exerted by the screws. They absorb the axial force generated during the extrusion process, preventing damage to the screws and other mechanical components. This extends the life of the machine and maintains efficient operation.

9-Gearbox

The gearbox, or reducer, reduces the speed of the motor to the optimal level required by the screws. It also increases the torque, enabling the screws to handle the viscous material effectively. Gearboxes ensure smooth and synchronized rotation of the screws.

10-Motor

The motor provides the mechanical power necessary to drive the screws. It converts electrical energy into rotational motion. The motor’s speed and torque can be adjusted to match the requirements of different materials and processing conditions.

twin screw extruder details

Auxiliary Equipment

Die and Cutter Assembly

The die shapes the molten material as it exits the extruder, determining the final product’s form. Dies can be designed for various applications, such as producing films, sheets, pipes, or profiles. The cutter assembly, often positioned after the die, cuts the extruded material into the desired lengths or shapes. Cutters can be rotary, strand, or guillotine types, depending on the product specifications and production requirements.

Control System

The control system oversees the operation of the twin screw extruder, ensuring all components work in harmony. Modern extruders use advanced computer-based control systems to monitor and adjust parameters such as screw speed, temperature, pressure, and feed rate. This precise control enhances process stability, product quality, and operational efficiency.

Straightening Device

One of the most common defects in plastic extrusion is eccentricity, often caused by the bending of the core wire. In sheath extrusion, surface scratches are usually due to bent cable cores. Therefore, straightening devices are essential in extrusion lines. The main types include roller (horizontal and vertical), pulley (single and multiple pulleys), winch (also providing traction, straightening, and tension stabilization), and pressure wheel (horizontal and vertical).

Preheating Device

Preheating the cable core is necessary for both insulation and sheath extrusion. For insulation, especially thin layers, preheating at high temperatures removes surface moisture and oil, preventing air pockets. For sheath extrusion, it dries the cable core, preventing air pockets caused by moisture. Preheating also prevents residual internal pressure in the plastic due to rapid cooling and stabilizes extrusion pressure by eliminating the temperature difference between the cold wire and the hot extruder head. Electric heating preheaters are used, ensuring sufficient capacity and quick temperature rise to efficiently preheat the wire and dry the cable core. The preheat temperature should match the extrusion speed and is generally similar to the die head temperature.

Cooling Device

The molded plastic layer must be cooled immediately after leaving the die head to prevent deformation. Water cooling is commonly used, classified into rapid cooling and gradual cooling based on water temperature. Rapid cooling, using cold water, is beneficial for shaping but can leave internal stress in crystalline polymers, causing cracks. PVC layers typically use rapid cooling. Gradual cooling, involving stages of hot, warm, and cold water, reduces internal stress and is used for PE and PP extrusion to ensure gradual temperature decrease and stabilization.

Conclusion

Understanding the basic components of a twin screw extruder machine is fundamental for optimizing its performance and ensuring high-quality output. Each component plays a crucial role in the overall functionality of the machine, from the barrel and screws to the drive system, feed system, heating and cooling systems, die and cutter assembly, thrust bearings, and control system. By comprehensively understanding these components and their functions, operators can effectively manage the extrusion process, leading to improved product quality and production efficiency.

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