Understand The Difference between Single Screw Extruder And Twin Screw Extruder in One Article

Twin screw extruders are developed on the basis of single screw extruders. Due to their excellent feeding performance, mixing and plasticizing performance, exhaust performance, and extrusion stability, they have been widely used in the molding and processing of extruded products. So, what are the advantages and differences between single screw and twin screw extruders?

01 Introduction and Application of Single Screw Extruder

(1) Introduction to Single Screw Extruder

As the name suggests, a single screw extruder means that there is a screw inside the cylinder. Generally, the effective length is divided into three sections, which are determined based on the diameter, pitch, and depth of the screw. Generally, each section is divided into one third.

The first section: Starting from the last thread of the material outlet, it is called the conveying section. The material here is required not to be plasticized, but to be preheated and compacted under pressure. In the past, the old extrusion theory believed that the material here was a loose body. Later, it was proven that the material here was actually a solid plug, which means that after being squeezed, it was a solid like a plug. Therefore, as long as the transportation task was completed, its function was to complete it.

Section 2: Compression section. The volume of this screw groove gradually decreases from large to small, and the temperature needs to reach the degree of material plasticization. Compression occurs here (from the volume of the conveying section to 3, which can be compressed to volume 1, which is called the compression ratio of the screw -3:1, and some machines also have different values), and the material that completes plasticization enters the third section.

Third paragraph: Measurement section. The material here maintains a plasticizing temperature, just as accurately and quantitatively conveying the melt material as the metering pump to supply the machine head. At this time, the temperature cannot be lower than the plasticizing temperature, usually slightly higher.

(2) Application of single screw extruder

Single screw extruders are mainly used for the extrusion of pipes, sheets, plates, and profiles, as well as for the granulation of partially modified materials.(3) Maintenance of single screw extruder

① Due to the high requirements of the electrical control system for environmental temperature and dust prevention, the electrical system should be separated from the production site and ventilation or ventilation fans should be installed. It is recommended to place the electrical control cabinet in a simple room, maintain indoor cleanliness and ventilation, and ensure that the indoor temperature does not exceed 40 ℃.

② The single screw extruder is not allowed to run empty to prevent the screw and barrel from rolling. When the host starts idling, it is not allowed to exceed 100r/min; When starting the main engine, start at low speed first. After starting the main engine, check for any abnormal sounds, and then slowly increase the main engine speed to within the allowable range of the process (it is best to adjust it to the optimal state). When the new machine is running in, the current load should be between 60% -70%, and it is best not to exceed 90% during normal use. Attention: If abnormal noise occurs during the operation of the extruder, it should be immediately stopped for inspection or repair.

③ Turn on the oil pump first when starting up, and then turn off the oil pump after shutting down; The water pump should remain in working condition throughout the entire production process, and the operation of the water pump should not be stopped to avoid the temperature rise of the machine barrel causing the decomposition and carbonization of materials inside the machine barrel; The asbestos fan cover of the main motor fan needs to be cleaned regularly to avoid excessive dust adhesion and blockage of the fan cover, which may cause insufficient heat dissipation of the motor and cause tripping due to overheating.

④ Timely clean the dust, tools, and debris on the surface of the unit.

⑤ Strictly prevent metal or other debris from falling into the hopper to avoid damaging the screw and barrel. To prevent iron debris from entering the barrel, magnetic components or magnetic racks can be installed at the feeding port of the barrel to prevent debris from falling. The material must be screened beforehand.

⑥ Pay attention to the cleanliness of the production environment, and do not let garbage and impurities mix with materials to block the filter plate, affecting product output and increasing machine head resistance.

⑦ The gearbox should use the lubricating oil specified in the machine manual, and add oil according to the specified oil level height. If the oil level is too low, insufficient lubrication will reduce the service life of the parts; Excessive oil, high heat generation, high energy consumption, and easy deterioration of oil can also lead to lubrication failure and damage to parts. The oil leakage part of the gearbox should be replaced with a sealing gasket (ring) in a timely manner to ensure the amount of lubricating oil.

02 Introduction and Application of Twin Screw Extruder

(1) Introduction to Twin Screw Extruder

There are various types of twin screw extruders, which can be divided into two categories: meshing and non meshing. Meshing twin screw extruders can also be divided into two categories: co rotating and counter rotating.

The twin screw extruder includes the following multiple systems, and the screw system mainly completes the plasticization and conveying process of materials, which has a significant impact on the performance and quality of the finished product.

① Feeding system: including hopper, mixing motor, and feeding motor. It can prevent material accumulation and facilitate its smooth entry into the feeding port.

② External heating system: mainly uses heating rods and cylinders to efficiently heat up materials and promote plasticization.

③ Cooling system: A heat exchange system composed of thermal oil or water is used to reduce the heat of the body, thereby effectively controlling the temperature of the cylinder.

④ Hydraulic screen changing system: Use replaceable filter screens to intercept impurities, improve plasticization, and ensure the uniformity and stability of the quality of the produced materials.

⑤ Vacuum system: Extract moisture and other low molecular volatile substances from the material.

⑥ Electric control system: responsible for monitoring and controlling the relevant equipment of the main and auxiliary material systems.

⑦ Screw system: The most important component of an extruder, which can be divided into a conveying section, a melting section (exhaust port), a plasticizing section (vacuum port), and a discharge section.

(2) Segmentation of twin screw extruder

The screw system of a parallel twin screw extruder is generally divided into four sections: conveying section, melting section (exhaust port), plasticizing section (vacuum port), and discharge section.

① Conveying section: used to transport materials and prevent backflow.

② Melting section: This section allows the material to fully melt and mix through heat conduction and frictional shear.

③ Plasticization section: further melt and mix the various components of the material, with the function of distribution and dispersion mixing.

④ Discharging section: Conveying and pressurizing, forming a certain pressure to make the material tighter, and further mixing to achieve the purpose of extrusion granulation.

(3) Conveying element

① The types are divided into: large lead and small lead.

② Effect of lead usage: The more lead, the higher the screw extrusion amount, the shorter the material residence time, and the weaker the material mixing ability.

③ The usual method of using small lead is to gradually reduce it in combination, used in the conveying section, melting section, and plasticizing section to increase pressure and improve melting; Improve the homogenization degree of the mixture and the stability of the conveying capacity.

(4) Shear element

① Direction classification: There are forward and reverse directions. Positive, promoting material flow and achieving its functions; Reverse rotation, also known as reverse rotation, has a reflux effect on the transportation of materials, prolongs the residence time of materials, enhances plasticizing ability, and improves mixing effect.

② Angle classification: generally divided into 30 °, 45 °, 60 °, and 90 °.

③ Function and effect: When in the positive direction, increase the staggered angle, reduce the conveying capacity, extend the residence time, and improve the degree of plasticization. For dispersion, the larger the angle, the more obvious the effect; Dispersion mixing is generally best at an angle of 45 °, followed by 30 °, and worst at 60 °.

④ The effect of head count

In the forward direction, the fewer the heads, the greater the extrusion and conveying capacity, the greater the torque, and the better the mixing effect, but the less the shear effect; When in reverse, the fewer heads, the smaller the extrusion conveying capacity, and the better the mixing effect.

03 Differences in Single and Twin Screw Extruders

Next, let's have a detailed understanding of the differences between these two extruders in terms of price, operability, screws, and other aspects.

(1) Single screw extruder with lower price

The single screw extruder has a simple structure and low price.

The twin screw structure is complex and expensive. Twin screw extruders are about twice as expensive as single screw extruders. Some may be higher, depending on the manufacturer and model.

(2) Single screw formula is simple

For operations, the difference between the two is not very significant. However, the process and formula of a single screw extruder are relatively simple, while the process and formula of a twin screw extruder are more complex.

(3) Twin screws can be freely combined with screws

Twin screw threads can be combined freely according to the material situation and the operator. Single screw is relatively rigid and cannot be combined with screws.

(4) Twin screw has higher efficiency

Twin screw extruder has a large output, fast extrusion speed, and low energy consumption per unit output, while single screw extruder is inferior. The efficiency of twin screw extruders is mostly about twice that of single screw extruders. It depends on the manufacturer and model.

(5) Twin screw mixing with strong plasticizing ability

Single screw is suitable for plastic extrusion of polymers and extrusion processing of granular materials, such as molding, film blowing, injection molding, etc., with a wide range of materials used.

The twin screw extruder has excellent mixing and plasticizing ability, making it more suitable for plastic modification. And the twin screw extruder can produce dual colors.

(6) Single screw for easy maintenance

In terms of maintenance, a single screw is simpler than a double screw because its structure is relatively simple and easy to repair.

(7) Transport mechanism

The transportation of a single screw extruder mainly relies on the friction force generated between the material and the barrel. The counter rotating twin screw extruder is a forward conveying machine, which has the effect of forcing the material forward. Additionally, the counter rotating twin screw extruder also has a shearing effect on the material at the intersection of the two screws. (For counter rotating twin screw extruders)

(8) Velocity field

The velocity distribution in a single screw extruder is relatively clear and easy to describe, while the situation in a counter rotating twin screw extruder is quite complex and difficult to describe. This is mainly because the screw has a meshing zone, and the complex flow that occurs in the meshing zone makes it have many advantages such as sufficient mixing, uniform heat transfer, strong melting ability, and good exhaust performance. However, it is difficult to accurately analyze the flow state in the meshing zone. (For counter rotating twin screw extruders)

04 Application Examples

(1) Application examples of twin screw extruders

Glass fiber reinforced, fuel retardant granulation (such as PA6, PA66, PET, PBT, PP, PC reinforced flame retardant, etc.), high filling material granulation (such as PE, PP filled with 75% CaCO3), thermosensitive material granulation (such as PVC, XLPE cable material), thick color masterbatch (such as filled with 50% color powder), anti-static masterbatch, alloy, coloring, low filling blend granulation, cable material granulation (such as sheath material, insulation material), XLPE pipe material granulation (such as masterbatch used for hot water crosslinking), Thermosetting plastic mixing and extrusion (such as phenolic resin, epoxy resin, powder coating), hot melt adhesive, reactive extrusion granulation (such as EVA hot melt adhesive, polyurethane), K resin, SBS devolatilization granulation, etc.

(2) Application Example of Single Screw Extruder

Suitable for 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 extruded composite pipes; Suitable for extrusion of PVC, PET, PS, PP, PC and other profiles and plates, as well as extrusion of various other plastics such as wires and rods; Adjusting the extruder speed and changing the structure of the extrusion screw can be applied to the production of various plastic profiles such as PVC and polyolefins.

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