Comparison of static and dynamic mixing systems

For the mixing of 2K materials, static mixers are usually the preferred choice due to their simple design, ease of use and relatively low cost. In certain cases, however, static mixers may not provide the required mixing performance. Dynamic mixers provide an alternative solution, particularly for applications where a high mixing quality is required or where rheologically demanding materials need to be mixed.

For example, mixing and metering with static mixers can be challenging when the components have significantly different viscosities or high mixing ratios. In such cases, a large number of mixing elements may be required to achieve sufficient mixing quality, resulting in long mixers, high discharge forces, and limited handling. In addition, these sometimes very long mixers have to be disposed of after use, which increases both the amount of plastic waste and the waste volume of the material remaining in the mixer. Moreover, due to the high pressure drop in such long static mixers, very high-viscose materials often cannot be discharged at the required flow rate because the discharge devices (such as dispensing guns or dosing machines) do not provide the sufficient discharge forces.

1.1.Advantages of dynamic mixers

Dynamic mixers overcome these problems. Unlike static mixers, which have an aligned layered mixing pattern, the rotating blades of dynamic mixers create a chaotic, multi-directional pattern that enables fast dispersion and mixing (see Figure 1). 

Figure 1 - Typical mixing pattern in static (left) vs dynamic (right). Dynamic mixing allows multi-directional mixing and dispersion

For materials with a high mixing ratio or with significant viscosity differences, this prevents the so-called channeling effect. Channeling describes the compression of the lower-viscosity component by the higher-viscosity component and the formation of narrow channels through which the thinner component is forced through the mixer at high speed and almost unmixed.

Furthermore, the fast rotating blades of dynamic mixers generate high shear rates in the mixer. On one hand, the higher shear energy accelerates mixing, while on the other hand it significantly decreases the viscosity of shear-thinning materials by orders of magnitude, resulting in much lower metering forces. Figure 2 illustrates this behavior of dynamic mixers impressively using the viscosity curve of a typical adhesive.

Figure 2 - Rheological behavior of a typical shear-thinning material. Light blue and grey area indicate typical shear rates of static and dynamic mixers

As a result, dynamic mixing increases mixing efficiency and reduces pressure drop. This allows comparable dynamic mixers to be designed more compact, which in turn leads to less plastic waste, less waste volume and better handling (shorter mixer).  

The lower pressure drop in the axial direction also allows very high viscosity materials to be metered.

1.2.Applications of dynamic mixing systems at medmix

Dynamic mixing systems are widely used in metering machines to mix high-viscosity two-component adhesives or silicones at high flow rates. Such applications typically use metal mixers that are cleaned or flushed after each use. To avoid flushing with expensive material and time-consuming cleaning, sometimes also disposable dynamic mixers are used, here. 

However, there are also cartridge-based applications where dynamic mixing systems are used. medmix offers systems for both the dental and industrial segments. 

Figure 3 - Examples of dynamic mixers in the medmix portfolio

In the dental segment medmix offers a dynamic mixing system for the application of high-viscosity silicone materials in trays to assist dentists in the production of highly accurate dental impressions. These materials are often offered at a mixing ratio of 5 to 1.

1.3.Why are dynamic mixers less common than static ones?

Despite the undeniable advantages, there are only a few cartridge-based applications where dynamic mixing systems are used. The reason is the more complicated design of the dispensing system required. Since dynamic mixers require an additional drive to rotate the mixer blades, this must also be housed in a discharge unit, making them heavier, bulkier, and more expensive. In addition, finding the optimum combination of flow rate and rotational speed can be challenging and needs additional training of the operator. And for industrial applications in particular, ease of operation and cost competitiveness are often key criteria.  

In short, dynamic mixing increases mixing quality, reduces waste, and improves application accuracy due to the reduced overall length of the mixer, but at the expense of additional complexity and cost. Therefore, dynamic mixing systems are used in applications where no other solution is possible to accomplish the mixing task or where the benefits outweigh the additional cost and effort.

About the author:

"Joachim Schöck has been working as a senior technology expert at Sulzer Mixpac and medmix Switzerland AG for 12 years. His main activity is the optimization and further development of high-precision application and mixing systems. This is done to a large extent using modern simulation tools such as CFD and FEM. Another focus is on the further development of test methods for predicting the mixing quality of 2K adhesives and sealants."