Comparison of Extrusion Performance Between Co-rotating and Counter-rotating Twin-Screw Extruders

Based on the direction of screw rotation, twin-screw extruders can be classified into co-rotating and counter-rotating types. The primary distinction between the two lies in the rotational direction of the screws. This clear difference in rotation direction results in significant variations in processing characteristics, mixing capabilities, and product suitability.

The following analysis focuses on the differences between co-rotating and counter-rotating twin-screw extruders during the extrusion process, exploring the material flow characteristics and their impact on final product quality.

1. The main difference between co-rotating and counter-rotating twin screws lies in the interaction of the screws：

In co-rotating twin screws, the pushing forces of the two screw flights are superimposed when conveying material；

In counter-rotating twin screws, the pushing forces of the two screw flights counteract each other.

This determines the basic working principles and performance characteristics of co-rotating and counter-rotating twin screws.

The configuration of co-rotating twin screws is shown in Figure 1. Both screws rotate in the same direction, and the screw threads are both right-handed. The material conveying direction is determined in the same way as for a single-screw extruder.

The configuration of counter-rotating twin screws is shown in Figure 2. The two screws rotate in opposite directions, and the thread directions of the two screws are opposite. Both screws rotate outwards. That is, in the extrusion direction, the screw on the right is left-handed and rotates clockwise, while the screw on the left is right-handed and rotates counter-clockwise.

2. The internal pressure field in the flow channel differs between co-rotating and counter-rotating twin screws：

During co-rotating twin-screw extrusion, pressure fluctuations in the flow channel rise and fall with the position of the screw flights. This is because the screw flight is the raised part of the screw surface. When the extruded material passes the screw flight, it experiences strong compression and shear forces, resulting in relatively higher pressure at the flight position.

During counter-rotating twin-screw extrusion, the pressure within the flow channel initially increases and then decreases along the extrusion direction, with a localized high-pressure phenomenon occurring in the middle region. This localized high pressure appears at the intermediate position along the extrusion direction in the intermeshing zone. It is caused by the opposite rotation directions of the two screws, leading to non-uniform material flow or blockage during extrusion. Material accumulates in the middle region, thereby creating a localized high-pressure phenomenon within the flow channel.

3. The internal shear rate field in the flow channel is the same for co-rotating and counter-rotating twin screws：

In both co-rotating and counter-rotating twin-screw extrusion processes, the shear rate is higher at the screw flights and lower in the screw channels. The reason for this phenomenon is that material flow velocity increases in narrow gaps. The clearance between the screw flight and the barrel inner wall is small, resulting in a higher shear rate, whereas the clearance between the screw channel and the barrel inner wall is larger, resulting in a lower shear rate.

4. The material residence time in the flow channel differs between co-rotating and counter-rotating twin screws：

Residence time is an important indicator for evaluating the axial mixing performance of screw elements, describing the distribution of time material spends in the twin-screw extruder.

During co-rotating twin-screw extrusion, material is forcibly conveyed by the screws along an “∞”-shaped path. The conveying capacity is strong, resulting in a shorter material residence time.

During counter-rotating twin-screw extrusion, material moves in a “C”-shaped trajectory, undergoing repeated mixing and reaction within the C-shaped chambers, leading to an extended residence time.

5. Distributive mixing and dispersive mixing in the flow channel differ between co-rotating and counter-rotating twin screws：

Distributive mixing describes the process of material distribution, during which the material undergoes continuous shearing, rearrangement, and directional movement under the action of the screws.

Dispersive mixing describes the process of changing material particle size, during which material particles are continuously stretched.

During co-rotating twin-screw extrusion, the two screws rotate in the same direction, generating intense shear action in the intermeshing zone. Material exchanges multiple times between the screws, which facilitates achieving uniform distributive mixing. In contrast, during counter-rotating twin-screw extrusion, most of the material is confined within the C-shaped chambers. Only a small amount of material flows into the gaps and is subjected to shear. The enclosed nature reduces irregular material flow within the channel, consequently decreasing mixing uniformity.

6. Summary

Advantages of co-rotating twin-screw extruders：

Relatively stable pressure field.

Short material residence time, high conveying efficiency, suitable for processing heat-sensitive materials, reducing the risk of thermal degradation.

Excellent distributive mixing capability, high material distribution uniformity.

Advantages of counter-rotating twin-screw extruders：

Stronger pressure-building capacity (but attention must be paid to possible localized high pressure).

Longer and broader material residence time distribution, suitable for processes requiring sufficient reaction or mixing time.

Superior dispersive mixing capability, providing stronger stretching action.

As a technology-driven enterprise specializing in the R&D and manufacturing of co-rotating twin-screw extruders, Nanjing Granuwel Machinery Equipment Co., Ltd. is always committed to transforming our core advantages—such as efficient mixing, precise pressure control, and low-heat-loss conveying—into customer productivity. Our equipment features mature modular design and process customization capabilities in the field of co-rotating twin screws, widely used in high-demand applications such as engineering plastics, heat-sensitive materials, and blending modification.

If you are looking for a solution that combines stable extrusion efficiency with excellent distributive mixing capabilities, please visit our official website at https://www.granuwelextruder.com/ for more detailed product information and technical consultation. Granuwel looks forward to creating value with you, making every extrusion process precise and controllable.