What are the differences and characteristics between single and twin screw extruders?

The extrusion machines in the domestic plastic industry are roughly divided into two categories: single screw extruder and twin screw extruder. Because these two types of equipment have their own characteristics, they are widely used in the plastic industry.

So what are the advantages of these two extruders and what are the differences between them? How to select screw parameters based on different raw materials?

1.Introduction and Application of Single Screw Extruder

(1) Introduction to Single Screw Extruder

As the name suggests, a single screw extruder has a screw inside its barrel. Generally, the effective length is divided into three sections, determined by the diameter, pitch, and depth of the screw, each accounting for one-third of the total effective length.

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.

The second section: called the 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 is generated here (from the conveying section three, which can be compressed to one, which is called the compression ratio of the screw -3:1, and some machines also have different values), and the material that completes the plasticization enters the third section.

The third section: Measurement section. The material here maintains the plasticizing temperature, only accurately and quantitatively conveying the melt material like a 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.

 

2.Introduction and Application of Twin Screw Extruder

(1) Introduction to Twin Screw Extruder

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.

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

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

③Cooling 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.

④Hydraulic 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.

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

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

⑦Screw 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 the number of heads: In the forward direction, the less the number of 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.

3.The difference between single and double screw extruders

Let's have a specific understanding of the differences between these two types of extruders in terms of price, operability, screws, and other aspects.

(1) Single screw extruders are less expensive

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 freely combined according to the material situation and the operator's mood. 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

Simply put, a 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) conveying 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)

(9) Application examples

Examples of applications of twin screw extruders include fiberglass reinforcement, fuel blocking 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), Granulation of XLPE pipe materials (such as masterbatch used for hot water crosslinking), mixing and extrusion of thermosetting plastics (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.

Examples of single screw extruder applications: 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.

Nanjing Haisi is a professional manufacturer of granulating extruders for coloring/dyeing, filling masterbatch production, as well as plastic modification and compounding , plastic recycling. Innovative machines are built as per the concept of durability, high efficiency and economical power consumption.