What is a screw extruder? A screw extruder is a type of pump that is used in a plastic injection machine to pump molten plastic. It is one of the most common types of machinery used in the modified plastics and polymer processing industry. Extruders can be divided into single-screw extruders and twin-screw extruders, each with their own processing advantages and disadvantages.
What Is a Single Screw Extruder?
A single-screw extruder is a type of plastic injection pump that uses a screw which rotates inside a heated barrel. It is the most mature form of technology used for plastic injection. Its simple structure is easy to manufacture, and has high processing efficiency and low price. Single-screw extruders are generally comprised of three sections, each performing a separate function; the extrusion system, the transmission system, and the heating and cooling system. Extruders are categorized according to the length, diameter, pitch, and depth of the screw in each of these sections.
Relevant Processing Characteristics and Principles of Single-Screw Extruder:
- Plasticizing ability: A single screw is highly suitable for plasticizing polymer pellets for extrusion because the degradation of polymers during processing is low.
- Material conveying mechanism: The material is conveyed through the single-screw extruder through a process called friction drag. The melting properties and viscous drag determine the conveying capacity of the single-screw extruder.
- Processing speed: The velocity of distribution in single-screw extruders is relatively well-defined but determination of velocity in twin-screw extruders is rather complex.
- Self-cleaning: Single screw extruders do not have this feature.
- Applicable fields of Single Screw Extruders: Single screw extruders are mainly used in the production of pipes, plates, and sheets. They are widely used for the manufacturing of PP-R pipes, PE gas pipes, PEX cross-linked pipes, aluminum-plastic composite pipes, ABS pipes, PVC pipes, HDPE silicon core pipes, and various co-extrusion composite pipes. The speed and structure of the extrusion screw can be adapted for the extrusion of sheets, wires, rods etc.
Advantages of Single Screw Extruders:
- Single screw extruders can be used for operations on a variety of different types of materials.
- They have high throughput.
- They are much cheaper.
Disadvantages of Single Screw Extruders:
- Compared with twin-screw extruders, the sheer force of single-screw extruders is smaller.
- 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 in single screw extruders making them unsuitable for use in certain processes.
The Basic Structure and Function of a Single Screw Extruder:
- Extrusion System: The function of the extrusion system is to melt and plasticize the polymer material to form a uniform melt while the material is transitioning from a glass state to a viscous flow state. And in this process, a stable pressure is maintained, and the screw continuously conveys the material to the die. The extrusion system includes a feeding device and a screw and barrel. The heat, rotation speed of the screw, and pressurization all need to be coordinated according to the material being processed.
- Transmission system: The transmission system is usually composed of a motor, a reducer, and bearings, and supplies the torque required to drive the screw in the extrusion process. To ensure uniform product quality. the screw speed must remain stable during processing, even when there is a change of screw load. In some situations, it may be necessary for the screw to vary its speed during different phases of the injection. In most extruders, the change of screw speed is achieved by adjusting the motor speed. The transmission system requires a good lubrication system and a rapid braking device.
- Heating and cooling system: The heating and cooling system of the extruder consists of a heating device and a cooling device which are applied to maintain consistent temperature of material during the extrusion process. The cooling device is generally in the extruder barrel near the bottom of the hopper. Barrel cooling can be either water-cooled or air-cooled. Small, and medium-sized extruders generally use air-cooling while large-scale extruders generally use water-cooling or a combination of the two. The cooling device strengthens the conveying effect of solid materials, and prevents material particles from sticking to the barrel or screw due to excessive temperature rise.
What Is a Twin-Screw Extruder?
A twin-screw extruder consists of a transmission device that uses two screws. 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 Extruders:
- Plasticizing ability: The twin-screw has good mixing and plasticizing ability. Because the material only 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 is important. Maximum positive displacement delivery can be achieved with closely intermeshing, counter-rotating, twin-screw extruders.
- Processing speed: The twin-screw extruder has a meshing zone, which is designed to assure sufficient mixing, uniform heat transfer, strong melting ability, and good extrusion performance. The twin-screw extruder has many advantages, but it can be difficult to accurately analyze and design the meshing zone.
- Self-cleaning: The twin-screw extruder has a relatively high shear speed. Due to the opposing direction of the screw edges and grooves in the meshing area, the twin-screw extruder can scrape off any accumulated material adhering to the screw allowing for very good self-cleaning. Because the time the material is in the barrel is short, degradation and deterioration of material is reduced.
- Applicable fields: Twin-screw extruders are widely used where physical and chemical modification of matrix resins are required, such as with filling, strengthening, toughening, and reactive extrusions. It can be used for glass fiber reinforced materials, fuel resistant materials (such as PA6, PA66, PET, PBT, PP, PC) flame retardant materials, high filler materials (such as PE, PP filled with 75% CaCO3) heat-sensitive materials (such as PVC, XLPE cable material), concentrated color additives (such as filling 50% toner) anti-static, cable, and pipe material, hot melt adhesive, 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 extruders are very similar to those of single-screw extruders, but there are some differences in the working principle.
- Forced delivery: In 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. The material is forcibly conveyed in the direction of the head along the screw.
- Homogenization and mixing: The co-rotating twin-screws have a small gap at the meshing point, and because the speed of the screw rotation is relatively fast, the opposing meshing area has a high mixing effect; much better than that of single screw extruders and counter-rotating twin-screw extruders.
- Self-cleaning: Due to the relatively high shear speed of the co-rotating twin-screws in the meshing area, they can scrape off any accumulated material adhering to the screw. This self-cleaning effect assures that the residence time of the material in the barrel is short, reducing material degradation and deterioration.
- Plasticization of materials: The size of the screw gap has a great influence on the plasticization quality of the material. With a smaller the gap, there will be a greater shearing force, but less material will be able to pass through. With a larger gap, a greater the amount of material will pass through, but the shear force will be less.
- Compression of materials: The co-rotating twin-screw extruder has many options for compressing and mixing materials, so the overall results are good.
- Feeding method: The co-rotating twin-screw extruder requires uniform and quantitative feeding and assumes metered starvation feeding; feeding at a controlled rate below full capacity.
- Exhaust: By using starvation feeding, a large-lead screw is used at the leading end of the conveying element so that the screw groove is not full and is in a zero-pressure state. This allows for the extrusion section of the screw to be set for consistent pressure.
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 for the physical and chemical modification of matrix resins. The co-rotating twin-screw extruder can be used for the blending, dispersion, and mixing of various raw materials. The co-rotating twin-screws oppose each other at their shearing point so the mixing effect is much larger.
Powdered calcium carbonate, talc, and titanium dioxide are commonly used in the blending and filling of plastics. The filler may generally account for 30% to 60% of the batch. The powdery filler contains a lot of air, but the air will be dispersed during mixing in the kneading zone of the screw. As the material flows into the screw groove, the air will flow in the reverse direction, affecting the forward conveying of the powder, and finally reducing the extrusion volume.
Processing Advantages of Twin-Screw Extruders:
The design of the co-rotating twin-screw extruder allows for it to implement and combine screw elements and barrel elements of various geometric shapes according to different mixing requirements, making them very adaptable.
- Hybrid performance: Since the two screws mesh with each other, various thread features can be added as required, such as reverse threads, meshing discs, toothed discs, etc., to fine-tune the mixing.
- Processing flexibility: The co-rotating twin-screw extruder usually adopts metered starvation feeding, where the output depends on the feeding amount. Because the output is independent of the screw speed, the control variables can more flexibly handle multiple processing functions on one machine, such as melting, mixing, exhaust, reaction, etc.
- Controllability of process parameters: Because the co-rotating twin-screw extruder has a narrower residence time distribution, better heat transfer, a more precisely controlled temperature curve, and a wider range of machinery options, it can offer better product quality stability.
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 uses a slow rotation speed, and can operate at lower heat levels, reducing material decomposition. It does not require high thermal stability of the material so 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, and better melting and exhaust performance, but the dispersion and mixing effect of materials is worse. The counter-rotating twin-screw extruder has the ability to establish a strong and stable head pressure, so it is more suitable for the direct extrusion of products. By using a counter-rotating twin-screw extruder instead of a single-screw extruder, you can eliminate the pre-plasticizing granulation process and directly extrude and process materials like PVC powder, thus reducing the cost of the product. Counter-rotating twin-screw extruders are mainly used to extrude PVC pipes, sheets, and pellets. Parallel, counter-rotating, twin-screw extruders have also been used in large-scale HDPE pipe production lines.
Counter-conical twin-screw extruders are traditionally used for the extrusion of polyvinyl chloride products, as these extruders are more energy efficient. There is little difference between co-rotating twin-screws and counter-rotating twin-screws as far as energy consumption, but it is more difficult for co-rotating twin-screw extruders to establish a stable head pressure.
