Tandem process revolutionising tyre industry
By Prof. Dr. Andreas Limper, Dr. Harald Keuter, HF Mixing Group
The tandem process has already improved the mixing process considerably. In the future, the machines will have to be further optimised with regard to dispersion, distribution, temperature control and process consistency with improved economy
Demanding formulas require highly developed mixing technology. The requirements for tyre mixtures are increasing and from 2020 onwards another 20 per cent drop in fuel consumption should be implemented. This requires complex rubber formulations for tyres in which liquid polymers, new types of filler and other additives are likely to play an important role. The tandem process has already improved the mixing process considerably. In the future, the machines will have to be further optimised with regard to dispersion, distribution, temperature control and process consistency with improved economy.
In the tyre industry, compounds are traditionally produced in several stages. This is due to the relatively high viscosities of the compounds, which must be reduced from one mixing stage to the next, so that further processing can be carried out without problems. The number of mixing stages ranges from 2 to 5, depending on whether you are dealing with motorcar tyres, truck, bus or “off-the-road” tyres (OTR).
Tangential mixers of the sizes 270-litre, 370-litre and 620-litre class are often used in the tyre industry, so-called Banbury machines. The term Banbury has become a synonym for mixers. A great advantage of this type of machine is its good intake behaviour. Comparatively high filling ratios can also be used. These two factors enable short mixing times and thus high throughputs.
The intermeshing mixer has also been in use in the tyre industry for the last 10 to 15 years. A major reason for this is due to the use of silicas in compounds, which are often difficult to disperse. Furthermore, the cooling ability of the mixer is of greater importance due to the chemical reaction taking place between the silica-silane complex – the mixer becomes a reaction vessel.
At this point, the intermeshing mixer has advantages over the tangential mixer due to the additional mixing effect between the rotors and the increased cooling surface area at the same chamber volume (approximately 20 per cent larger). The greater time required for the material intake is less important due to the longer mixing cycles, necessary for silica compounding.
Demand for compunds
The demands on tyre compounds are steadily increasing. The European Union, for example, has introduced the EU tyre label on 01.11.2012, which – similar to the energy efficiency of electrical appliances – has specified the three important tyre properties; grip behaviour in wet conditions, noise emission and fuel consumption in different classes from A to G. The aim is to reduce fuel consumption by 20 per cent by 2020 . The requirements are then to continue to increase these properties significantly from 2020 onwards, which is likely to lead to even more complex formulas and mixing processes. It is likely that liquid polymers, new types of fillers and other additives will play an important role.
At the same time, of course, the price pressure in the production process of the tyre also increases; a process accelerated by continued globalisation. For the mixing process, this means that the demands on the machine with respect to dispersion, distribution, temperature control and process consistency must increase with improved economy.
New rotor geometries, process control systems and a holistic approach for mixing room design and automation solutions provide opportunities.
Idea and implementation of tandem process
Dr. Julius Peter patented the so-called tandem mixing process in 1987. Dr. Peter was the Technical Director of Continental in Hannover at the time. The idea of a tandem line is to combine an upper internal mixer with a lower mixer without a ram, as Image 2 shows. The original approach was to be able to run mixing processes in a single stage. Dispersion was supposed to take place in the upper internal mixer and viscosity reduction plus any necessary cooling steps as well as addition of accelerator systems was to take place in the lower ramless mixer.
It is easy to understand that this raised the hope to achieve significant cost savings, as all intermediate storage steps would be eliminated and the overall mixing time from base to final mixing would be shortened.
To convert the tandem idea into a working mechanical engineering solution, some critical points had to be overcome first. One of the core problems was to hold the compound within the mixing chamber of the lower mixer, where, depending on the rotor system, further processing will be carried out. If the intake behaviour is poor there will be little additional mixing and the possibility of degradation.
One of the core problems was to hold the compound within the mixing chamber of the lower mixer, where, depending on the rotor system, further processing will be carried out. If the intake behaviour is poor there will be little additional mixing and the possibility of degradation
Tangential and intermeshing rotor systems have specific advantages and disadvantages. The intake behaviour of tangential rotors is better at holding the compound in the chamber of the lower mixer. In contrast, intermeshing rotors have a better cooling behaviour and have advantages with respect to the degradation behaviour. The disadvantage is the less favourable intake behaviour.
In contrast to tangential rotors, intermeshing rotors can be run underfilled, as image 3 shows. These have a significantly broader, optimum filling ratio range, resulting in more flexible use. However, if the maximum filling ratio is exceeded, the quality decreases rapidly.
In the case of tangential rotors, this sensitivity to filling ratio is present in both directions – overfilling and underfilling. Although the filling ratio is higher for the tangential mixer the optimum ratio is much tighter.
In this case, it is possible to speak more or less of an optimum operating point, which is a few percentage points higher than in intermeshing rotors, though.
Thus, the solution came to the fore to build a larger lower mixer and to underfill it so that the advantages of better cooling and better degradation behaviour could be utilised.
In addition, the intermeshing rotor geometry had to be optimised with regard to the intake behaviour, which leads to the birth of the “tandem rotor.”
In principle, both tangential and intermeshing rotors can be used in the upper mixer. This should be determined by the application.
(Appeared in February-March 2018 issue of Tyre Asia. Second part of the article will appear in Tyre Asia’s April-May issue)