The most common mixers in rubber manufacturing are the Banbury and the Kneader, both internal mixers, and the rolling mill. The construction of these two internal mixers is similar in regard to their mechanical characteristics, where- as each machine has its preferences for different proce- dures, and they can be used complementarily for optimized time and cost efficiency. Both mixers basically consist of counter-rotating pairs of rotors, a mixing chamber, a float- ing weight (ram), and a feed hopper. The two mixers have different rubber output mechanisms: the rubber exits the Kneader directly from the mixing chamber, which is tilted backwards in order to discharge the material. The Banbury has a drop door, through which the rubber is dropped onto the rolling mill.Both machines are available in different chamber sizes.
Contrary to popular industry belief, a large mixer is not directly linked to a more efficient process or a high quality of the final outcome: with thermally sensitive polymers such as NR and SBR, a certain temperature should not be exceeded during the mixing process. The key advantage of smaller mixers is a more favorable chamber volume to cooling surface ratio. Smaller mixing systems have a superior surface-cooling ratio compared to larger systems: The volume increases in the cube, whereas the cooling surface increases in the square. Consequently, the mixing temperature will always be considerably higher in a larger mixer. Furthermore, in larger mixers it takes longer to mix the large batches, which can damage the rubber’s molecular structure and can harm the quality of the final compound. The quality decreases with extended mixing time and finally negatively affects the physical properties of the final product. As a matter of fact, a larger mixer is economi- cally beneficial due to its higher output rates, but following the principle of quality as a priority, a middle-sized mixer should be the first choice to put quality over quantity.
Despite the similarities of both internal mixers, each ma- chine provides certain benefits that make it more suit- able for one production step than another (for a detailed overview see table 1). The Banbury is the superior mixer for masterbatch production with its high-performance engine, adjustable rotor speed, and automated weigh- ing system. High degrees of dispersion and mixing high viscosity rubber can be achieved within a short time frame. Since the Banbury operates at higher temperatures than the Kneader, it is not the best solution for mastication and finalization, although it is possible. The higher temperature of the Banbury is compensated by a shorter mixing time. However, there are certain advantages to favor the Kneader over the Banbury.
Selecting the right mixer for the respective production step and its correct operation requires a lot of experience; using both mixers can be a huge advantage.
The rolling mill consists of two parallel, counter rotating rolls with a gap in between that can be adjusted. The roll- ing mill can be used for all steps of the mixing process. For the masterbatch production other compounding ingredi- ents are added into the rubber. The extremely high shearing forces of the rolling mill at a low temperature lead to su- perior dispersibility of the ingredients, but unlike the inter- nal mixers with their sealed chambers, the rolling mill is an open mechanic system and the raw materials would scatter.
This would create a dusty and unsafe production environ- ment, which is a strong argument for the use of internal mix- ers. Additionally, a longer mixing time reduces the efficiency of the rolling mill and it is therefore most commonly used for sheeting, which will be described in the following section.
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