What are the Different Types of Screw Extruders and Their Functions?

What are the Different Types of Screw Extruders and Their Functions?

What is a screw extruder? The extruder is one of the types of plastic machinery. It is the most common machinery in the modified plastics industry and is widely used in the polymer processing industry and other production and processing fields. Extruders can be divided into single-screw extruders and twin-screw extruders, and their processing has advantages and disadvantages.
Published: May 11, 2022
What are the Different Types of Screw Extruders and Their Functions?

What Is a Single Screw Extruder?

As common extruder equipment, a single-screw extruder is used in the plastics processing industry. The screw rotates in a heated barrel. Because of its simple structure, easy manufacture, high processing efficiency, and low price, it is widely used. It is the type of extruder with the most mature technology and the most consumption. The single-screw extruder is generally divided into three sections in terms of effective length. The effective length of the three sections is determined according to the diameter of the screw, the pitch, and the depth of the screw, which are generally divided into one-third each.

The single-screw extruder is mainly composed of three parts: extrusion system, transmission system, and heating and cooling system. The single-screw extruder has developed from the initial basic spiral structure to various types of structures such as damping screw blocks, vented screws, slotted screw barrels, pin barrels, and building block structures. The machine takes up little space, and it has become the main equipment used in the field of composite processing and plastic blown film.

Relevant Processing Characteristics and Principles of Single-Screw Extruder:

  • Plasticizing ability: A single screw is suitable for plasticizing extrusion of polymer, suitable for extrusion processing of pellets; the shear degradation of polymer is small, but the material stays in the extruder for a long time.
  • Material conveying mechanism: The material conveying in the single-screw extruder is a drag-type flow, the solid conveying process is friction drag, and the melt conveying process is a viscous drag. To a certain extent, it determines the size of the conveying capacity of the single-screw extruder.
  • Processing speed field: The velocity distribution in a single-screw extruder is relatively well-defined and easy to describe, the situation in a twin-screw extruder is rather complex.
  • Self-cleaning: Single screw extruders do not have this feature.
  • Applicable fields of Single Screw Extruder: Single screw extruders are mainly used in the fields of pipes, plates, sheets, and profiles. Suitable for PP-R pipe, PE gas pipe, PEX cross-linked pipe, aluminum-plastic composite pipe, ABS pipe, PVC pipe, HDPE silicon core pipe, and various co-extrusion composite pipes; suitable for PVC, PET, PS, PP, PC, etc. Extrusion of profiles and sheets, extrusion of other various plastics such as wire, rod, etc.; adjusting the speed of the extruder, and changing the structure of the extrusion screw can be applied to the production of various plastic profiles such as PVC and polyolefins.
Advantages of Single Screw Extruder:
  • Compared to twin-screw extruders, they are far superior in extruding common materials.
  • Other screw extruders are much cheaper.
  • Compared with twin-screw extruders, the sheer force of single-screw extruders is smaller.
  • Can be used for operations on a variety of different types of materials.
  • Has high throughput.
Disadvantages of Single Screw Extruder:
  • Since the transport of plastic material is by friction, there are limitations in feeding performance.
  • Some materials, such as pastes or powders, are difficult to mix. This makes it unsuitable to use the machine during certain processes.

The Basic Structure and Function of a Single Screw Extruder:

  1. Extrusion System: The function of the extrusion system is to melt and plasticize the polymer material to form a uniform melt and realize the transition from a glass state to a viscous flow state. And in this process, a certain pressure is established, and the screw is continuously extruded and conveyed to the die. Therefore, the extrusion system plays an important role in the molding quality and output of extrusion processing. The extrusion system includes parts such as a feeding device, screw, and barrel, which is the most critical part of the extruder. The material can only move in the barrel through the rotation of the screw, and get pressurized and part of the heat.
  2. Transmission system: The transmission system is usually composed of a motor, a reducer, a bearing, etc., and its function is to drive the screw and supply the torque and torque required by the screw in the extrusion process. In the extrusion process, the screw speed is required to be stable and not change with the change of screw load to ensure uniform product quality. However, on different occasions, the screw is required to be able to achieve variable speed, to achieve the requirement that one equipment can adapt to the extrusion of products of different materials or different shapes. In most extruders, the change of screw speed is achieved by adjusting the motor speed, and the transmission system is also equipped with a good lubrication system and a rapid braking device.
  3. Heating and cooling system: The heating and cooling system of the extruder consists of a heating device and a cooling device, which is a necessary condition for the extrusion process to proceed smoothly. The cooling device is generally arranged in the extruder barrel, screw, and the bottom of the hopper and other parts. Barrel cooling can be water-cooled or air-cooled. Generally, small, and medium-sized extruders mostly use air-cooling; large-scale extruders mostly use water-cooling or a combination of the two. The screw cooling mainly adopts central water cooling, the purpose is to increase the solid conveying rate of the material, stabilize the output, and at the same time improve the product quality. The cooling device at the bottom of the hopper is mainly to strengthen the conveying effect of solid materials, to prevent the material particles from sticking due to temperature rise, blocking the material opening and affecting the feeding. Generally, for extruders and high-speed extruders with a screw diameter of more than 90mm, a cooling device must be installed at the bottom of the hopper.

What Is a Twin-Screw Extruder?

Twin-screw extruder is a common machine in the modified plastics industry and is widely used in the polymer processing industry and other production and processing fields. The twin-screw extruder consists of a transmission device, a feeding device, a barrel, and a screw. There are many kinds of twin-screw extruders, which can be divided into meshing types and non-meshing types.

Related Processing Characteristics and Principles of Twin-Screw Extruder:

  • Plasticizing ability: The twin-screw has good mixing and plasticizing ability, and the material stays in the extruder for a short time. It is suitable for powder processing.
  • Material conveying mechanism: The material is conveyed by positive displacement in the twin-screw extruder. With the rotation of the screw, the material is forcibly pushed forward by the intermeshing threads. The proximity of the screw grooves. Maximum positive displacement delivery can be achieved with closely intermeshing counter-rotating twin-screw extruders.
  • Processing speed field: The twin-screw extruder has a meshing zone, and the complex flow in the meshing zone makes the twin-screw extruder have many advantages, such as sufficient mixing, uniform heat transfer, strong melting ability, and good exhaust performance, but it is difficult to accurately analyze the meshing zone. flow state.
  • Self-cleaning: The twin-screw extruder has a relatively high shear speed. Due to the opposite direction of the speed of the screw edges and the screw grooves in the meshing area, it can scrape off any accumulated material adhering to the screw and has a very good self-cleaning. Therefore, the residence time of the material is short, and it is not easy to cause local degradation and deterioration.
  • Applicable fields: Twin-screw extruders are widely used in physical and chemical modification of matrix resins, such as filling, strengthening, toughening, reactive extrusion, etc. It can be used in glass fiber reinforced, fuel resistant granulation (such as PA6, PA66, PET, PBT, PP, PC reinforced flame retardant, etc.). High filler granulation (such as PE, PP filled with 75% CaCO3). Heat-sensitive material granulation (such as PVC, XLPE cable material), concentrated color masterbatch (such as filling 50% toner). Anti-static masterbatch, alloy, coloring, low-filling blending granulation. Cable material granulation (such as sheath material, and insulation material). XLPE pipe material granulation (such as masterbatch for hot water crosslinking). Thermosetting plastic mixing and extrusion (such as phenolic resin, epoxy resin, powder coating). Hot melt adhesive, PU reactive extrusion granulation (such as EVA hot melt adhesive, polyurethane). K resin, SBS devolatilization granulation, etc.

What is the Difference Between the Working Principle of the Co-Rotating Twin-Screw Extruder and the Single-Screw Extruder?

The structure and function of the co-rotating twin-screw extruder are very similar to that of the single-screw extruder, but there are still differences in the working principle.

  • Forced delivery: In meshing co-rotating twin screws, the two screws move in opposite directions at the meshing point, one screw pulls the material into the meshing gap, and the other screw pushes the material out of the gap, resulting in the material from a screw groove. Turning to the other screw groove, the material is forcibly conveyed in the direction of the head along the screw in a ∞ shape.
  • Homogenization and mixing: The co-rotating twin-screw have a small gap at the meshing point, the speed of the screw and the screw groove is opposite, and the relative speed is large, so the meshing area has a high shear speed, a large shear force, and the mixing effect is much better than that of a single screw. Screw extruders and counter-rotating twin-screw extruders.
  • Self-cleaning: The co-rotating twin-screw extruder has a relatively high shear speed due to the opposite direction of the speed of the screw and the screw groove in the meshing area. It has a very high sheer speed, which can scrape off any accumulated material adhering to the screw. The good self-cleaning effect, so that the residence time of the material is short, and it is not easy to produce local degradation and deterioration.
  • Plasticization of materials: The size of the screw gap has a great influence on the plasticization quality of the material. The smaller the gap, the greater the shearing force, but the less material passes through. The larger the gap, the greater the amount of material passing through, but the less shear force.
  • Compression of materials: The co-rotating twin-screw extruder has many more methods for compressing materials, and the comprehensive effect is good.
  • Feeding method: The co-rotating twin-screw extruder requires uniform and quantitative feeding and adopts the method of metered starvation feeding.
  • Exhaust: Since it is starvation feeding, a large-lead screw conveying element can be used so that the screw groove is not full and is in a zero-pressure state so that an exhaust section can be set.

Application of Twin-Screw Extruder:

The twin-screw extruder can be divided into counter-rotating twin-screw and co-rotating twin-screw according to the different rotation directions of the two screws.

Intermeshing co-rotating twin-screw extruders are widely used in physical and chemical modification of matrix resins, such as filling, strengthening, toughening, reactive extrusion, etc. The co-rotating twin-screw extruder should be used for the blending mainly based on the dispersion and mixing of various raw materials. The speed vectors of the co-rotating twin-screw are opposite to each other. Its shearing effect is much larger, and it is not suitable for mixing and molding PVC.

Commonly used in blending and filling plastics are powdered calcium carbonate, talc, and titanium dioxide. The general filling amount is 30% to 60%, and the filling masterbatch is up to 80%. Since the powdery filler contains a lot of air, the air will be separated during mixing in the kneading zone of the screw, which will cause the material to flow into the screw groove in the reverse direction, affecting the forward conveying of the powder, and finally reducing the extrusion volume.

Processing Advantages of Twin-Screw Extruder:

The combined design of the co-rotating twin-screw extruder can arrange and combine screw elements and barrel elements of various geometric shapes. According to different mixing requirements, the design can be optimized to meet the needs of different process formulations, so that the co-rotating twin-screw extruder has strong adaptability.

  • Hybrid performance: Since the two screws have meshed with each other, various thread elements can be added as required, such as reverse thread elements, meshing disc elements, toothed disc elements, etc., to fine-tune the shear/mixing, so that the mixing strength and mixing quality are improved. can be effectively controlled.
  • Processing flexibility: The co-rotating twin-screw extruder usually adopts metered starvation feeding, and the output depends on the feeding amount, which is independent of the screw speed and is an independent controllable variable, so it can flexibly handle multiple processing functions on one machine, such as melting, mixing, exhaust, reaction, etc.
  • Controllability of process parameters: The co-rotating twin-screw extruder has narrow residence time distribution, better convective heat transfer, and a precisely controlled temperature curve, and can obtain better shear-time-temperature history during extrusion, and input machinery in a wider range. Therefore, better product quality stability can be obtained.

What Is a Counter-Rotating Twin-Screw Extruder?

The input principle of the counter-rotating twin-screw extruder:

In a counter-rotating twin-screw extruder, the material is conveyed by positive displacement like that in a gear pump. The counter-rotating twin-screw extruder has low rotation speed, and small shearing heat, and the material is not easy to decompose, so it does not require high thermal stability of the material and is especially suitable for the molding of heat-sensitive materials (such as PVC).

Compared with the co-rotating twin-screw extruder, the counter-rotating twin-screw extruder has higher conveying efficiency, better exhaust effect, and melting effect, although the dispersion and mixing effect of materials is worse than that of the co-rotating twin-screw extruder. However, the ability to establish a stable head pressure is strong, so it is more suitable for the direct extrusion of products. In addition, the use of a counter-rotating twin-screw extruder instead of a single-screw extruder can save the pre-plasticizing granulation process and directly extrude and process the PVC powder, which reduces the cost of the product. Counter-rotating twin-screw extruders are mainly used to extrude PVC pipes. Profiles, sheets, and pellets. Parallel counter-rotating twin-screw extruders have also been used in large-scale HDHDPE pipe production lines.

Counter-conical twin-screw extruders are traditionally used for the extrusion of polyvinyl chloride products, and the purpose of this practice is to save energy. From the perspective of energy saving, there is no big difference between co-rotating twin-screw and counter-rotating twin-screw, and it is more difficult for co-rotating twin-screw extruder to establish a stable head pressure. It is not as simple and easy as directly using a counter-rotating twin screw.

Published: May 11, 2022 Source :kknews

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