The Difference Between Co-Rotating Twin-Screw Extruder and Counter-Rotating Twin-Screw Extruder

1. Co-rotating Twin Screw Extruder

In a co-rotating twin screw extruder, two parallel screws rotate synchronously, creating a uniform shear effect at their contact points and between them and the barrel. The intensity of this shearing effect can be controlled by adjusting the screw design and spacing to meet different processing needs. The geometric shape of the screw elements combined with the co-rotating feature gives the screws excellent material distribution and mixing capabilities, making them particularly suitable for compounding operations.

When material enters the barrel and softens, due to the opposite direction of the screws at the meshing points, one screw draws the material into the gap while the other pushes it out, resulting in an "∞" shaped movement path between the two screws. In this process, the material experiences significant changes in relative speed, which is advantageous for its mixing and homogenization. Additionally, the very small gap at the meshing area and the opposite direction of the threads and grooves generate high-intensity shear actions, aiding in the uniform plastification of the material.

The screws and barrel adopt a modular design and can be equipped with various types of thread elements, such as conveying, kneading, shearing, reverse threading, and pressure-increasing threads, each with its specific function. By combining these elements in a manner similar to building blocks and optimizing the design, the extruder can meet the processing needs of different materials and various process formulations flexibly.

Furthermore, the co-rotating twin screw extruder also functions as a dynamic reactor. Once the material melts inside the barrel, a series of chemical reactions can occur, including but not limited to polymerization and grafting. Reactive extrusion processing is not limited to the polymerization of monomers or oligomers (such as free radical polymerization, addition polymerization, condensation polymerization, and copolymerization), but also includes controlled crosslinking and degradation of polyolefins, polymer graft modification, and forced blending modification and physical modification (such as filling, compounding, toughening, and reinforcement). This makes the co-rotating twin screw extruder a versatile processing equipment widely used in the field of polymer materials science.

2. Counter-rotating Twin Screw Extruder

In contrast, the design of the counter-rotating twin screw extruder is relatively simpler. Since the two screws rotate in opposite directions and have fewer contact points between them, the precision requirements for the screws' fit are relatively low. In a counter-rotating twin screw extruder, the material conveyance is similar to the positive displacement method in a gear pump. This type of extruder features lower rotation speeds, reducing heat generation from shear, which decreases the risk of material degradation and makes it suitable for processing heat-sensitive materials (like PVC).

Compared to a co-rotating twin screw extruder, a counter-rotating twin screw extruder has higher conveying efficiency, better venting, and superior melting effects. Although its dispersive mixing performance is slightly inferior, it can better establish a stable head pressure, making it an ideal choice for direct extrusion of products. Additionally, pre-plasticization pelletizing procedures can be omitted, allowing direct processing of PVC powders, thereby reducing product costs.

Traditionally, counter-rotating conical twin screw extruders have been primarily used for the extrusion of polyvinyl chloride (PVC) products. However, recent domestic practices have shown that counter-rotating conical twin screw extruders can also be successfully applied to the extrusion of polyolefin (like polyethylene, PE) pipes. The main motivation for using counter-rotating conical twin screw extruders for polyolefin pipe extrusion is to save energy and reduce costs.

For example, a conical twin screw extruder with a 65mm diameter and a screw spacing of 132μm has a drive motor power of only 37kW, whereas a 75mm or 80mm single screw extruder typically has a drive power of no less than 90kW. This means that for similar or slightly different output levels, the counter-rotating conical twin screw extruder may offer better energy efficiency.

It should be noted that due to the significant differences in flow properties between polyolefin melts and PVC melts, applying a counter-rotating conical twin screw extruder originally designed for PVC to the extrusion of polyolefin pipes requires adjustments to the screw design to ensure effective processing of polyolefin materials. Additionally, modifications to the extrusion die (head) may also be necessary.

Due to the highly customizable nature and high technical content of co-rotating twin screw extruders, they are more precisely and complexly designed. Maintenance also requires more time and effort for meticulous management. Conversely, the simplicity and clarity of the design of counter-rotating twin screw extruders have won favor among some users, especially excelling in applications that seek stability and reliability.

Allen

allen.chen@kymach.com

With over two decades of experience in the foreign trade industry, I possess deep expertise in plastic processing and compounding technologies. Leveraging extensive professional knowledge and market insights, I am dedicated to providing global clients with precise and efficient products and tailored solutions.