Resin-bonded friction lining compounds are normally made from a mixture of different fibers together with organic and inorganic solids. To some extent, the density of these materials varies over a very wide range. Precisely defined fiber opening must take place during the mixing process. In addition, all of the components (often more than 40) must be uniformly embedded into an agglomerate to prevent de-mixing during downstream processing. Long mixing times are normally needed on conventional mixers to get the best results. With EIRICH mixing technology, processing times are reduced by a substantial margin.
Friction linings, often used as clutch and brake linings, nowadays must meet very demanding mechanical and tribological requirements. As a result, the first production stage, which is the mixing, decisively determines the quality of the friction lining and thus plays a vital role. Resins used as binders, for example high-viscosity phenol resin mixtures (also known as liquid resins), form the matrix in a multi-component system into which fibers and fillers are embedded. Fiber opening must take place in a defined manner. The binder, which has a higher viscosity than honey, must be sufficiently well distributed. This presents a difficult challenge for conventional mixers, and mixing times are long.
The mixing time on a double-shaft mixer was 50 minutes at a friction lining manufacturer, so the company approached EIRICH. It was possible to reduce the mixing time to only five minutes. The inherent system characteristics are the reason for the difference. EIRICH mixers have a rotating pan which transports the mix constituents to the mixing tool. In combination with the stationary material deflector at the pan walls, an intensive three-dimensional flow is generated in the mixing chamber. The unique feature of the EIRICH mixer is the variable speed of the mixing tool (rotor) which can run at up to 30 m/s in the same direction as the mixing pan or in the opposite direction. Mixing power input can be adapted to the specific process and requirements.
Depending on the rotor tool geometry and speed, very high specific mixing energies (> 10 kW/100 kg) can be achieved. Because the mixing pan and rotor tool normally move in opposite directions, the speed difference is significant, creating high shear forces. In the case at hand, only five minutes are needed to produce a defined opening of fibers, add the dry constituents and add the liquid resin instead of the previous 50 minutes.
Despite the short mixing time, the mixing quality on EIRICH equipment is generally higher than on other mixing systems. The operating principle based on a rotating mixing pan and material deflector reduces the amount of dead space in the mixer where mixing results are less than optimal, because each partial volume of the mix is transported to the rotor tool. Also, there is certainly no possibility of de-mixing, which is occurring with other mixing systems.
In many cases, there are also other advantages. If heat treatment follows the mixing process, this can take place right in the mixer using inductive heating (with explosion protection if needed). This eliminates the need for material transfer and possibly other work steps, simplifying the entire process. Total processing time is reduced even further.
Mixers with inductive heating in the mixing pan have been used for several years in other industries. Heat is generated directly in the pan walls. This eliminates heat loss at the transition between the heating medium and the pan walls. Inductive heating generally has good dynamic regulation characteristics, ensuring optimal temperature control and reproducibility. Due to the rapid heat transfer between the intensive-flow mix and the rotating pan walls, the mix temperature can be regulated right down to the exact degree if necessary. Mixers with inductive heating are available in sizes ranging from 75 liters up to 400 liters effective capacity. The temperature of the mix can be as high as 250 °C. As a result, mixing with molten resin instead of resin solution is possible in the refractory, carbon and graphite industries. This in turn reduces porosity, which is an essential quality criteria in these industries. Depending on the requirements, explosion protection versions of the mixers can be supplied (Zone 21 or 20 internal, external Zone 22) in accordance with the European ATEX regulations.
The mixers can also be cooled, and various options are available. Dry ice pellets, carbonic acid snow and liquid nitrogen have proven to be effective as cooling agents. The solid or gaseous cooling media are fed into the area around the rotor tool where they are immediately mixed in. This eliminates the risk of local over-cooling in the mix.
Some well-known friction coating manufacturers have already conducted successful pilot trials to test these highly useful heating and cooling options.
Oliver Zeitner, e-mail: firstname.lastname@example.org