CHAPTER 28: Properties and Handling of Particulate Solids 959 Mixers for Cohesive Solids Some of the most difficult of all mixing problems involve cohesive solids such as pastes, plastic materials, and rubber. In some ways these substances resemble liquids, but their enormously high viscosity means that the mixing equipment must be different from and much more powerful than the mixers described in Chap 9. With cohesive solids the mixing elements cannot generate flow currents; instead they shear, fold, stretch, and compress the material to be mixed. The mechanical energy is applied by moving parts directly to the mass of material. In the closed type, such as Banbury mixers, the inner wall of the casing acts as part of the mixing means, and all mixing action occurs close to the moving parts. Clearances between mixing arms, rotors, and wall of casing are small. The forces generated in these mixers are large the machinery must be ruggedly built, and the power consumption is high. The heat evolved per unit mass of material may be sufficient to require cooling to prevent the temperature from reaching a level dangerous to the equipment or the material.