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How Does a Twin Screw Extruder Work?

The twin screw extruder has traditionally been used to produce pellets so that the material can be continuously fed into secondary processing equipment, such as injection molding machines or single-screw extruders. In addition, twin-screw extruders can also produce films, fibers, sheets or profiles, which is called direct extrusion. The two forms of products are obtained through basically the same production steps. Based on this, this article mainly introduces how twin-screw extruders work so that you can understand the production of products.

Demand for Plastic Compounding and Pelletizing:

Twin screw extruders are mainly used for compounding and pelletizing of some plastic products:

  • Flame-retardant plastics: PA6, PA66, PET, PBT, PP, PC and other materials are improved in flame retardancy by adding flame retardants
  • Highly filled plastics: PE and PP are filled with 75% CaCO3 to enhance rigidity and stability
  • Insulating plastics: Different additives (such as flame retardants, antioxidants, etc.) are evenly dispersed in the matrix resin to ensure the uniformity and stability of insulating plastics
  • Thermoplastic plastics: BPAT materials are compounded with materials such as PLA and EPS to have good toughness, ductility and high temperature resistance.
  • Thermosensitive materials: C-PVC products are added with heat stabilizers and lubricants to facilitate subsequent extrusion molding. The functionality of the material is ensured through precise temperature control
  • Degradable materials: PLA and PBAT, ESR and other degradable materials are modified to improve their elongation and other problems

How Does a Twin Screw Extruder Work?

1. Material Intake

Working components: feed hopper, feed throat

In a twin-screw extruder, the hopper plays a vital role in conveying solid materials to the feed zone of the extruder. Specifically, the hopper pushes the solid material into the extruder through the feeding hole at the bottom of the hopper where it connects to the barrel. This process is usually accompanied by a cooling jacket to ensure the stability of the material flow and prevent overheating.

In the feed zone, the solid material is pushed forward by the rotation of the screw and gradually melted due to the screw gap and the friction between the screw and the barrel. This melting mechanism is different from the conductive melting and dispersion melting in the single-screw extruder, and the melting mechanism of the twin-screw extruder is completely different.

In addition, the side feeder can also be used to push the solid material into the molten polymer in the extruder. The side feeder consists of a hopper and a set of conveying screws, which are designed to push the solid material into the feed zone of the extruder and connect the side feeder to the vacuum port of the air trapped in the solid through a combined barrel, thereby reducing the entrained air.

Twin screw extruder hopper1
Twin screw extruder hopper2
Twin screw extruder hopper3
Twin screw extruder hopper4

2. Material Conveyance

Working components: screw, barrel

After the material enters the extruder barrel, it is conveyed along the barrel axial direction by the rotating screw. The conveying stage is mainly to convey the material from the feed port to the compression zone and start heating.

Non-meshing twin-screw extruders cannot form a closed or semi-closed cavity, and there is no positive displacement conveying condition. The material conveying mechanism is similar to that of a single-screw extruder.

Meshing twin-screw extruders mainly convey materials by forced conveying (positive displacement conveying). Different models of twin-screw extruders have different positive displacement conveying capabilities and slight differences in conveying mechanisms. The degree of positive displacement depends on the proximity of the flight of one screw to the relative flight of the other screw. Counter-rotating extruders with tightly interlocked screw configurations produce a significant level of positive displacement conveying characteristics.

screw

3. Compression Stage

Plastic gradually starts to melt from a solid state in the compression section while being compressed. In this section, the plastic is subjected to the effects of compression ratio and barrel heating + friction heat, the material temperature rises quickly, the friction effect inside the material is large, and the compression effect is large.

4. Melting Stage

When the material enters the melting section, it gradually heats up and begins to melt under the action of the barrel heater. At this stage, the screw continues to rotate, further compressing and advancing the molten material. The rotation of the screw not only provides propulsion, but also makes the material evenly distributed in the screw groove through the design of the thread, thereby ensuring the quality and stability of the molten material.

5. Mixing Stage

Working components: twin screws, mixing elements

The mixing stage includes distributive mixing and dispersive mixing, which ensures that the components are evenly distributed and aggregates are eliminated through the shearing action of the screw.

In addition, twin screw extruders excel at mixing components through the interlocking action of the screws, generating turbulence and shear forces. This results in an efficient mixing process as the material undergoes collision and compression. Whether it involves coordinating polymers, dispersing additives, or integrating fillers, parallel co-rotating twin screw extruders always achieve precise and consistent results.

twin screws1
twin screws2

6. Exhaust Stage

Due to the hungry feeding, a large lead screw conveying element can be used to make the screw groove in an unfilled state and in a zero pressure state, so that the exhaust section can be set. In order to ensure product quality, the gas and volatiles in the melt need to be discharged at this stage. The exhaust zone is usually equipped with a vacuum port to effectively remove the gas and volatiles in the melt.

7. Homogenization and Controlled Molding Stage

Working component: mold

The homogenized melt is pushed to the mold by the extruder and molded into the desired shape, such as sheet, tube or film through the mold. The mold design depends on the shape and specifications of the final product.

8. Cooling and Solidification Stage

Working component: water bath, cooling fan

The extruded product is quickly cooled and solidified by the cooling system to maintain its shape and dimensional stability. Cooling methods include water bath cooling, air cooling, etc.

cooling system

9. Cutting and Post-processing Stage

After the cooling stage, the solidified plastic can be further processed, especially in plastic modification applications. At this stage, precision becomes critical. The solidified plastic goes through a cutting process, usually done by specialized equipment such as a granulator, to ensure that the resulting pieces or pellets are uniform in size. The choice of cutting into small pieces or pellets depends on the specific requirements of the application. The processed material is then collected and used in downstream manufacturing processes.

Working process of twin screw extruder

Conclusion

Twin-screw extruders are widely used in plastic processing and modification. Their excellent mixing, melting and reaction capabilities enable them to process various types of plastics and give them new properties, and improve the comprehensive performance of materials. Plastic pellets made by twin-screw extruders can meet the needs of different application fields.

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