How Does a Twin Screw Extruder Work?

The twin-screw extruder is developed on the basis of the single-screw extruder. When single-screw extruders are used to process blends (fiber and resin) and filling reinforcement materials (red mud, slag, etc.), a series of problems such as difficulty in discharging materials, difficulty in melting, and poor plasticization have arisen. In order to improve these problems, twin-screw extruders were produced. This article will mainly introduce the working principle of a twin-screw extruder to help you understand its advantages compared to a single-screw extruder.

How Does a Twin Screw Extruder Work?

Twin-screw extruder

The twin-screw extruder includes an extrusion system, a transmission system, and a heating and cooling system. Its basic functions are the same as those of a single-screw extruder, but its working principles are very different. It can continuously output solids and melts more stably, with a higher delivery rate. The material is melted and plasticized more completely and uniformly. The main difference from a single-screw extruder is that it has two twin-screws and a double-hole barrel with an exhaust system. 

There are different types of twin-screw extruders based on screw structure, meshing mode, and rotation direction. This article will also explain some of the differences in the working principles of different twin-screw extruders.

How does a twin-screw extruder work?

Step1: Material Intake

The twin-screw extruder adopts forced feeding method. For example, as soon as PVC material enters the extruder, it is subjected to strong shearing, stirring and calendering when passing through the radial gap between the two screws.

Step2: Material Conveyance

Non-intermeshing twin-screw extruders cannot form a closed or semi-closed cavity and have no positive displacement transportation conditions. The material conveyance mechanism is similar to that of a single-screw extruder.

The intermeshing twin-screw extruder mainly transports materials through forced conveying (positive displacement conveying). Different types of twin-screw extruders have different positive displacement conveying capabilities, and there will be slight differences in conveying mechanisms. The degree of positive displacement depends on the proximity of the flight of one screw to the opposing flight of the other screw. The closely interlocked screw configuration of the counter-rotating extruder produces a significant level of positive displacement conveying characteristics.

What is positive displacement conveying?

In positive displacement conveying, the twin screws of the extruder positively displace or move material along the barrel by trapping it within the intermeshing screw channels. This mechanism ensures a consistent and controlled flow of material through the extruder, preventing backflow or leakage. It is a fundamental principle that contributes to the reliability and precision of material processing in the extruder.

Since the co-rotating twin screws have opposite speeds at the meshing position(the meshing area typically refers to the region where the intermeshing screws actively engage with each other to convey, mix, and process the material), one screw will pull the material into the meshing gap, while the other screw will push the material out of the gap. As a result, the material will be transferred from one screw to the other. , advancing in an “∞” shape. This change in speed and the larger relative speed in the meshing area are very conducive to the mixing and homogenization of materials.

Due to the minimal gap in the meshing area, the threads and grooves at the point of engagement exhibit opposing speeds, resulting in a high shearing speed. This configuration imparts an effective self-cleaning capability, allowing it to scrape away any material accumulation adhered to the screw. Consequently, the residence time of the material is notably brief. As a result of these characteristics, the intermeshing co-rotating twin-screw extruder finds its primary application in mixing and granulation processes.

Step3: Melting and Mixing

As the material progresses through the barrel, it undergoes a remarkable transformation due to the increasing temperature and pressure. The dynamic movement of the rotating screws, combined with the heating elements integrated into the barrel, gradually elevates the material’s temperature, leading to its melting. This controlled melting process plays a pivotal role in achieving the desired molecular structure and guaranteeing the utmost quality of the final product.

In addition, the twin-screw extruder excels at blending components through the interlocking action of the screws, creating turbulence and shear forces. This results in a highly efficient mixing process as the material undergoes collisions and compressions. Whether it involves harmonizing polymers, dispersing additives, or integrating fillers, the parallel co-rotating twin-screw extruder consistently achieves precise and consistent results.

Step4: Venting and Degassing(Optional)

Some advanced twin-screw extruders have a venting section to eliminate volatile components and gases released during the melting and mixing process. This ensures a purer and higher-quality final product.

In certain plastic modification applications, a degassing section can be included in the extruder to remove any residual gases or volatile substances from the molten material. This ensures superior product stability and reduces the risk of unwanted reactions or degradation.

Step5: Die and Cooling

The material passes through a meticulously crafted die after leaving the extruder barrel. This die molds the material into the desired shape, whether it be continuous strands for granulation or a specific profile for modifying the plastic. The choice of die configuration is crucial as it determines the outcome of the extrusion process. After emerging from the die, using air or water rapidly solidifies the plastic. This cooling step preserves the shape and prevents distortion.

Step6: Cutting and Collection

Following the cooling phase, the solidified plastic may undergo further processing, particularly in plastic modification applications. At this stage, precision becomes paramount. The cured plastic undergoes a cutting process, usually performed by specialized equipment such as a granulator, to ensure uniformity in the size of the resulting chips or granules. The choice between cutting into smaller pieces or pelletizing depends on the specific requirements of the application. These processed materials are then collected for further use in downstream manufacturing processes.

By adhering to this working principle, the twin-screw extruder excels in plastic modification and granulation, offering versatility in the preparation of plastic material for diverse industrial applications.

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