The difference between a single-screw extruder and a twin-screw extruder

The twin-screw extruder is developed from the single-screw extruder, and the two are both related and different. This article will talk about the difference between single-screw and twin-screw extruders in terms of their transfer methods and flow rates.

Here is the list of contents

• Material transfer method

• Flow velocity field of the material

• Differences in production process control

• The difference in recipe requirements

Material transfer method

In single screw extruders, the material transfer is dragging. Frictional drag in the solid conveying section and viscous drag in the melt conveying section. The frictional properties of the solid material and the viscosity of the molten material determine the conveying behavior. If some materials have poor frictional properties, it is more difficult to feed the materials into the single screw extruder if the feeding problem is not solved. So granular raw materials are suitable for single screw extruder feeding.

In twin-screw extruders, especially meshing twin-screw extruders, the material transfer is to some extent a positive displacement transfer, and the degree of positive displacement depends on the proximity of the screw prongs of one screw to the relative screw grooves of the other screw. The screw geometry of a closely meshed anisotropic rotary extruder yields a high degree of positive displacement conveying characteristics. A forced feed is formed and the powdered material facilitates the extrusion feed.

Flow velocity field of the material

The flow velocity distribution of the material in a single-screw extruder has been described fairly well by researchers, whereas the flow velocity distribution of the material in a twin-screw extruder is quite complex and difficult to describe. Many researchers have analyzed the material flow velocity field without considering the material flow in the engagement zone, but the results of these analyses are very different from reality because the mixing characteristics and overall behavior of the twin-screw extruder depend mainly on the leakage flow occurring in the engagement zone, which is quite complex. The complex flow spectrum of the material in the twin-screw extruder shows macroscopic advantages that are not comparable to those of the single-screw extruder, such as adequate mixing, good heat transfer, high melting capacity, high venting capacity, and good control of the material temperature.

Differences in production process control

Due to the differences in structure and working principle between the single-screw extruders and twin-screw extruders, there are also great differences in the production process control of PVC profiles, specifically in temperature control, screw speed control, and two aspects. Due to the limitation of space, this article will not describe it in detail.

The difference in recipe requirements

Since the material flow state is different between single-screw and twin-screw extruders, the material is subjected to different shear forces in the screw, and the plasticization time and history are different, so the formulation requirements for the PVC system are also different. The sheer force of the material in the twin-screw extruder is much greater than the shear force in the single-screw extruder, which requires higher internal lubrication of the PVC system. However, the plasticization time of the material in the twin-screw extruder is short, the plasticization history is short, and the thermal stability of the PVC system does not require as long as the extrusion plasticization time of the single-screw extruder, which requires a long stabilization time for the heat stabilizer. In addition, the twin-screw extruder plasticizing capacity, material plasticization uniformity are far greater than the single-screw extruder, twin-screw extruder extrusion products are also of higher quality than the single-screw extruder, mainly in the material tensile strength, impact strength and weld angle strength. Specific performance in the single-screw extruder extrusion PVC profile formulations used in the heat stabilizer, processing aids, modifiers is more than those used in the twin-screw extruder PVC profile formulations.

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