Co-Rotating Vs. Counter-Rotating Twin-Screw Extruders: How Much Do Rotation Directions Impact Performance?

In the world of material compounding and extrusion, Twin-Screw Extruders is the heart of production. Its performance directly dictates mixing quality, production efficiency, and the mechanical properties of the final product.

At JWELL, we understand that choosing the right technology-Co-rotating or Counter-rotating-is critical for your specific application. While the difference may seem to be just the direction of rotation, our Computational Fluid Dynamics (CFD) simulations and field tests reveal significant distinctions in processing characteristics, mixing capabilities, and product adaptability.

This article breaks down these technical differences to help you make an informed decision for your production line.

The core distinction lies in how the screws interact with the material:

Co-rotating Twin Screw Extruders:

• Rotation: Both screws turn in the same direction.

• Mechanism: The thrust from the screw flights is superimposed. Material is transported in an "∞" (figure - eight) path, creating an enforced conveying effect.

Co-rotating twin-screw mating mode

Counter-rotating Twin Screw Extruders:

Rotation: Screws turn in opposite directions.

Mechanism: The thrust forces offset each other. Material travels in a "C"-shaped trajectory within closed chambers, undergoing repeated mixing and reaction.

Counter-rotating twin-screw mating mode

Based on the pressure cloud map (Fig. 3) and pressure variation curve (Fig. 4), the following observations were made:

• Co-rotating Twin Screw: The pressure within the flow channel exhibits rhythmic fluctuations. These fluctuations correspond with the position of the screw flights (as seen in Fig. 4). Since the screw flights are raised ridges, the material undergoes intense compression and shear forces as it passes over them, resulting in higher pressure peaks at the flight locations.

• Counter-rotating Twin Screw: The pressure initially rises and then decreases along the extrusion direction, characterized by a significant localized high-pressure phenomenon in the middle section. This high pressure occurs at the intermeshing zone (referencing Fig. 2). The opposite rotation of the screws causes material to flow unevenly or become blocked, leading to material accumulation in the center, which generates this localized high pressure.

Pressure cloud diagram in the runner during the extrusion process 

pressure history

Based on the shear rate contour (Fig. 5) and variation curve (Fig. 6), the observations are as follows:

• General Behavior (Both Types): In both co-rotating and counter-rotating processes, the shear rate is consistently higher at the screw flights and lower in the screw channels. This occurs because the clearance between the screw flight and the barrel wall is minimal, accelerating the material flow and inducing high shear forces. In contrast, the larger gap in the screw channels results in lower shear rates.

• Counter-rotating Twin Screw: This type exhibits localized spikes in shear rate. This phenomenon is specifically caused by leakage flow (typically occurring through the tight clearances in the intermeshing region).

Cloud graph of shear rate during the extrusion process 

Shear rate variation curve

Mixing is divided into two primary categories: Distributive and Dispersive.

Distributive Mixing: The process of material redistribution and reorientation to achieve homogeneity.

Dispersive Mixing: The process of reducing the size of material particles through stretching and shearing forces.

The Tracer Particle Method is used for quantification, analyzing trajectory parameters like Residence Time (RT), Separation Scale, and Maximum Shear Stress to evaluate mixing differences.

Axial Mixing Performance: Residence Time Distribution (RTD)

The Residence Time Distribution (RTD) is a critical metric for axial mixing, describing the statistical spread of time that material spends within the extruder. It is represented by probability and probability density functions.

The cumulative residence time distribution curve (Fig. 7) illustrates the cumulative probability of fluid or material remaining inside the extruder.

In the co-rotating twin-screw system, tracer particles start exiting the channel at 1.00 s and fully exit by 54.82 s, giving a residence time span of 53.82 s.

In the counter-rotating twin-screw system, tracer particles first exit at 1.48 s and completely exit by 59.80 s, resulting in a residence time span of 58.32 s.

The cumulative curve of the co-rotating twin-screw remains above that of the counter-rotating system, indicating a higher proportion of particles exiting the channel at any given time.

Cumulative stay time distribution

The residence-time probability density curve illustrates how likely the material is to remain inside the extruder during different time intervals. A higher density indicates a greater probability of material staying within that specific time window, while a lower density reflects fewer occurrences.

According to the probability density function (Fig. 8):

Most particles in the co-rotating twin-screw extruder fall within 1.00–1.99 s, whereas in the counter-rotating system they concentrate within 1.48–2.97 s. The co-rotating curve shifts further left and shows a higher peak, indicating stronger conveying performance. This is likely due to the forced "∞-shaped" transport path characteristic of co-rotating screws.

In contrast, the counter-rotating extruder drives the material along a "C-shaped" trajectory, where repeated mixing and re-circulation within the C-chamber extend the residence time.

residence time distribution

Distributive Index

The distributive index reflects the rheological behavior and flow characteristics of the material during extrusion. As shown in the distributive index curve (Fig. 9), the counter-rotating twin-screw extruder demonstrates better distributive uniformity compared with the co-rotating system.

Distribution index

Separation Scale

The separation scale characterizes the progress of distributive mixing. As shown in Fig. 10, the initial separation scale is large because the two types of particles enter from opposite sides. As mixing progresses, the separation scale decreases due to the screw-induced dispersive action, indicating deeper surface-level mixing. The fluctuations observed are caused by particle aggregation during flow.

The separation-scale curve of the co-rotating twin-screw extruder remains consistently below that of the counter-rotating system, demonstrating its stronger distributive mixing capability.

In co-rotating twin-screw extrusion, both screws rotate in the same direction and generate strong shear at the intermeshing zone. This promotes frequent material exchange between the screws, delivering more uniform distributive mixing.

In contrast, counter-rotating extrusion retains most material within the C-shaped chamber. Only a small portion enters the gap region where shear and elongation occur. The higher degree of enclosure reduces irregular flow but also lowers overall mixing uniformity.

Dispersive Mixing Performance

Dispersive mixing is defined by the progressive reduction of particle size, driven mainly by shear and elongational stresses. The maximum shear stress experienced by tracer particles reflects the intensity of the dispersive process. A higher proportion of particles subjected to high shear indicates stronger dispersive capability.

As shown in Fig. 11, the counter-rotating twin-screw exhibits a higher probability curve, suggesting it exposes more particles to elevated shear levels.

Probability of maximum shear stress

Fig. 12 shows the probability density of maximum shear stress, where the peak indicates the stress level most frequently experienced by the particles.

The curve characteristics for both systems confirm that the counter-rotating twin-screw provides stronger shear and elongation forces, resulting in superior dispersive mixing compared with the co-rotating design.

Figs. 13 and 14 summarize the tensile and impact test results.

Material extruded by the co-rotating twin-screw shows slightly higher tensile strength and elongation at break.
Conversely, samples from the counter-rotating system exhibit marginally higher impact energy absorption and impact strength.

Tensile test data

Impact test data

Advantages of Co-rotating Twin-Screw Extruders:

More stable flow field, especially in terms of pressure control.

Strong distribution-mixing performance with high material uniformity.

Shorter residence time and higher conveying efficiency, ideal for heat-sensitive formulations and minimizing thermal degradation.

Extrudates exhibit better tensile properties.

Advantages of Counter-rotating Twin-Screw Extruders:

Higher pressure-building capability (with attention to localized pressure peaks).

Stronger shear and stretching effects, delivering superior dispersive mixing.

Longer and broader residence-time distribution, suitable for processes requiring extended reaction or mixing time.

Extrudates show higher impact strength and lower melt viscosity due to more extensive chain scission.