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Twin Screw Extruder Working Principle

The structure of a twin-screw extruder is quite similar to that of a single-screw extruder, but their operating principles are significantly different. In a single-screw extruder, material transport relies on the friction and viscous drag of the material, resulting in a wide residence time distribution.

In contrast, material transport in a twin-screw extruder depends on the positive displacement conveying of the screws, leading to a narrow residence time distribution. The raw materials for the twin-screw extruder are fed through the feed port by a metering feeder. Some additives (such as glass fibers) need to be added through the middle feed port of the barrel and are conveyed to the die head by the screws.

In this process, the movement of the material varies depending on the screw engagement method and the direction of rotation. This article will introduce the specific differences in their operating principles from this perspective.

How Does a Twin Screw Extruder Machine Work?

First, we will take the application of twin-screw extruders in the field of plastics as an example to briefly describe the working principle of twin-screw extruders. Then we will specifically introduce their differences from the perspective of “different screw meshing methods and different rotation directions”.

twin screw extruder

Here's how it works:

1. After starting the twin-screw extruder, plastic pellets or powder are first fed into the screw grooves through the feeding system. The feeding system typically consists of a feed port, feeder, hopper, and feed screw. The plastic raw material is evenly delivered to the feed screw by the vibration of the hopper or the rotation of the feeder.

material feeding

2. Once the plastic raw material enters the screw grooves, the twin screws begin to rotate. By controlling the pitch and depth of the screw grooves, the plastic raw material is advanced from the front end to the rear end. During this advancement, the plastic raw material is compressed by the outer part of the barrel and the screw grooves, generating shear force and friction. This results in high-speed friction and heating of the plastic raw material. The heating system provides additional thermal energy, gradually melting the plastic raw material into a thermoplastic molten substance.

screw rotating process

3. Extrusion Process: When the molten plastic reaches a certain level, it enters the extrusion section of the barrel. This section typically consists of widened screw grooves and an extrusion die. In the extrusion section, the pitch of the screw grooves gradually decreases, creating increasingly high pressure, which further enhances the melting of the plastic. The extrusion die, through specific structures and channels, shapes the molten plastic into the desired cross-section and length. The extrusion outlet is usually equipped with a cooling system, which quickly cools and solidifies the molten plastic.

extrusion process

In the operation of a twin-screw extruder, the control system plays a crucial role. Through the control system, parameters such as feeding, screw speed, temperature, and pressure can be monitored and adjusted in real-time to ensure the stability and controllability of the extrusion process. The control system can also adjust the screw speed, temperature, and shape according to product requirements to achieve the desired extrusion results.

The Differences in Working Principles Among Different Twin-Screw Extruders

Intermeshing twin-screw extruders can form closed or semi-closed cavities, enabling positive displacement conveying conditions. The extent of positive displacement conveying is influenced by the degree of closure.

What Is Positive Displacement Conveying Conditions?

“Positive displacement conveying conditions” refer to a mode of material transport where the twin screws of an extruder push or displace the material forward through the barrel. This method ensures that a specific volume of material is moved forward with each rotation of the screws, regardless of the material’s viscosity or other flow characteristics. It contrasts with methods where material movement relies on factors like friction or gravity, as in some single-screw extruders.

Intermeshing Co-Rotating Twin-Screw Extruder

  • The material can flow from the screw groove of one screw through the gap in the intermeshing zone into the screw groove of the other screw, forming a rotating inverted “8” shape moving forward.
  • The screws rotate at opposite speeds at the intermeshing point, creating significant shear on the material and scraping off any material buildup inside the grooves, thus providing good mixing and self-cleaning effects.
  • The shearing action on the material at the screw intermeshing point continuously renews the surface layer of the material, resulting in excellent venting and degassing performance.
  • The two intermeshing screws exert a forced conveying effect on the material, allowing the material inside the grooves to be transported forward without completely filling the grooves. This setup can balance the pressure in the upper and lower intermeshing zones of the screws, ensuring even load distribution on the screw bearings and the barrel, thus enabling operation at high speeds.

By employing various structural mixing and shearing elements, the mixing capability of the screws can be enhanced, making them widely used in material compounding and reactive extrusion processes.

various structural mixing and shearing elements

Intermeshing Counter-Rotating Twin-Screw Extruder

  • The two screws rotate in different directions, and the spiral path of the material in one screw is blocked by the other screw, so an “8”-shaped motion cannot be formed.
  • At the meshing point, the threads of one screw are inserted into the screw grooves of another screw, so that the continuous screw grooves are divided into C-shaped chambers isolated from each other.
  • Every time the screw rotates one circle, the C-shaped chamber moves forward a lead distance, so there is no continuous channel for material from the barrel feeding port to the machine head.

The material entering the meshing gap generates a separation force on the screw while being squeezed, causing the screw to bend and wear the barrel, which is called the “calendering effect“. The calendering effect causes uneven force on the barrel wall and bearings, which can easily cause local pressure wear, so it can only work at a lower speed. The mixing shearing effect is weak, with positive conveying characteristics, and is mainly suitable for profile extrusion.

Non-intermeshing Twin-Screw Extruder

The center distance between the two screws is greater than the sum of their radius. The practical value lies in non-intermeshing counter-rotating twin-screw extruders. These extruders cannot form closed or semi-closed cavities and lack positive displacement conveying conditions. Material transport is similar to that of a single-screw extruder, with the main difference being the exchange of material from one screw to the other.

Forward conveying capability is less than that of a single-screw extruder, while reverse mixing performance is superior to that of a single-screw extruder. It is mainly used for compounding and plasticizing polymers.

Non-intermeshing Twin-Screw Extruder

Intermeshing Conical Counter-Rotating Twin-Screw Extruder

  • The cross-sectional area of the screw at the end of the metering section is reduced, resulting in lower axial pressure at the same die head pressure, thereby reducing the load on the thrust bearings.
  • The axes of the two screws are separated at the rear end, allowing for the installation of larger load-bearing bearings, capable of withstanding higher torque and having a longer lifespan.
  • The screw diameter in the feeding section is large, providing a large heating surface area beneficial for plasticization. In the metering section, the screw diameter is small, resulting in a smaller heating surface area, which is advantageous for achieving low-temperature extrusion.
Intermeshing Conical Counter-Rotating Twin-Screw Extruder
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