Why can't a single-screw extruder replicate the mixing performance of a twin-screw?

People often ask: Why can't the mixing or compounding performance of a counter-rotating parallel twin-screw be replicated on a single-screw extruder?

The core advantages of twin screw:

Precise control of mixing and shearing throughout the entire screw groove

First, twin screws can transfer the entire channel filled with polymer from one screw to the other multiple times, thus achieving full-groove mixing.

While achieving this full groove mixing transfer, the majority of the polymer being transferred can be subjected to very low shear, while only a small portion is subjected to extremely high shear, by simply changing the depth of the opposing grooves or using mixing ridges.

Furthermore, the screw runs idle, allowing ample space for this transfer to occur. Due to the meshing of the screw elements, this transfer occurs with minimal pressure drop and, consequently, minimal loss of production.

Limitations of single screw:

The contradiction between pressure drop and shear distribution

In a single screw, the pressure drop across the high shear zone (which is necessary for intensive mixing) is a limiting factor because it leads to a loss of throughput and an increase in melt temperature . By repeating this process multiple times, twin screws can achieve fairly intensive and thorough mixing without overheating.

Regardless of the extruder type, high shear powerful mixing is required to fully disperse additives and even other polymers (because many materials are only partially compatible with each other or form agglomerates that require high shear to break up).

Mixing in a single screw is primarily limited by the passage of the polymer down the screw. The shear rate and resulting downstream velocity is greatest at the top of the barrel or channel and is minimum or possibly zero at the root of the screw.

Mixing in a single-screw is primarily limited by the flow of the polymer along the screw channel.The shear rate and the resulting downstream velocity are maximum at the top of the barrel wall or groove,and are minimal or potentially zero at the screw root. Because the melted polymer adheres strongly to the barrel and screw surfaces, the shear forces generated by the polymer as the screw rotates within the barrel are the primary driving force. Essentially, in the force balance of polymers, the barrel rotates around the screw, with the moving surface of the barrel in contact with the polymer providing transport. In the attached diagram, these velocities are represented as Vb and V0.

Velocity stratification also results in only partial “flipping”of the polymer in the screw groove. This alone makes it difficult to completely mix the material inside the screw groove. Furthermore, due to the continuous shear thinning of viscosity from the barrel wall to the screw root, the material within a single-screw groove tends to remain at a relatively constant radial position.

Flexible twin screw design:

Balance between multiple high shear and overall shear

To overcome this limitation, various types of mixers and additional threads are used to interrupt and redirect the polymer melt flow. However, these devices create flow resistance, reducing output and increasing melt temperature. All polymers must flow through such devices repeatedly to achieve mixing uniformity.

Twin screws can apply high shear in small increments by varying the depth of the screw groove multiple times and/or using mixing ribs, while limiting the overall shear experienced by the entire melt. This is difficult to achieve on a single screw, and in most cases not even possible, as it requires tight clearances or flow restrictions to produce high shear rates.

A single screw must have a specific groove depth to achieve the desired throughput and melt temperature . This limits the use of multiple high shear zones, which is very useful for twin screws because the material exchange between the screws allows high shear to be applied to only a small portion of the polymer by transferring the polymer to the other groove.

Features of Maddock Mixer

There are many single-screw mixing and barrier flight designs that can add some mixing to a single screw, although most primarily aid melting by preventing the passage of unmelted material. Interestingly, one of the earliest and most popular single-screw mixers was the Maddock-type mixer, which combined two principles of a twin-screw.One is complete "turnover" of the entire melt and the application of short bursts of high shear as the polymer passes through the barrier from the inlet to the outlet groove.

Maddock mixers are designed with the characteristics of twin-screw mixing in mind: full polymer turnover and limited high shear.

Although quite effective in principle, it is primarily limited to “single application”because its inherent pressure drop reduces throughput and significantly increases melt temperature, making it difficult to use multiple mixing sections. Spiral Maddock designs are also available, but they offer minimal improvement in pressure drop. Many other single-screw mixers simply split the melt at low shear, primarily to homogenize the temperature, but lack the high shear required for powerful mixing.