Rubber internal batch mixers

Internal batch mixers are widely used in the rubber industry. They are also used for processing plastics such as vinyl, polyolefins, ABS, and polystyrene, along with thermosets such melamines and ureas because they can hold materials at a constant temperature. 

The principle of internal batch mixing was first introduced in 1916 with the development of the Banbury mixer (Figure 2.74a). A Banbury-type internal mixer essentially consists of a cylindrical chamber or shall within which materials to be mixed are deformed by rotating blades or rotors with protrusions. The mixer is provided with a feed door and hopper at the top and a discharge door at the bottom. As the rubber or mix is worked and sheared between the two rotors and between each rotor and the body of the casing, mastication takes place over the wide area, unlike in a open mill where it is restricted only in the area of the nip between the two rolls. 

The paddle ridge width of a batch mixer was varied between 0.2-20 mm and the effects on the rubber mixing process were studied photographically. The distribution of carbon black in the mix was uniform with a width below 11 mm, while a width below 15 mm was required for efficient dispersion. 9 refs. Artieles from this journal can be requested for translation by subseribers to the Rapra produced International Polymer Science and Technology. 

International Polymer Science and Technology 29, No.6, 2002, p.T/43-6 INFLUENCE OF THE PADDLE RIDGE WIDTH OF BATCH MIXER ROTORS ON THE EFFICIENCY AND QUALITY OF THE PRODUCTION OF RUBBER MIXES Shirikhev N I Rasskazov A N Trofimov A P Skok VI NUShP... 

International Polymer Science and Technology 27, No.8,2000, p.T/22-6 INFLUENCE OF THE MAIN DESIGN CHARACTERISTICS OF A BATCH MIXER ON THE EFFECTIVENESS OF PREPARATION OF RUBBER MIXES... 

Large amounts of energy are consumed during the mixing process and this gives rise to large temperature rises in the rubber batch. Internal mixers therefore have cooling channels in the chamber walls, rotors, and sometimes the ram head, to dissipate this heat. The inlet water temperature is controlled in most modem machines, but in many companies this is still not the case. 

The final quality and reproducibility of rubber compounds mixed from scratch on a two-roll mill, or finalisation of compound batches mixed in an internal mixer, depend to a large degree on the skill of the mill operative. In order to overcome to some extent the reliance on operative skill and to ensure reproducible mix quality many mills are now fitted with stock blender units. These consist of a driven twin roller system, of similar overall length to the mill rolls, fitted above the mill frame. 

Since internal mixers can be opened for material feed and compound removal, these are generally simpler to clean when compared to screw-type mixers. It is a common practice to run natural rubber or special high-tack, cohesive compounds through a mixer as a final stage of cleaning. Further, the first batch of compound is typically sacrificed or saved for experimental work following clean-out. 

Campanelli et al (2004) discuss a model of rubber mixing in an internal mixer based on kinetic, thermodynamic and rheological equations that is used to determine the extent of dispersion, batch temperature and relative batch viscosity over time. The chemoviscosity model is complex in that a model must be developed for the compaction zone, the incorporation zone and the size-reduction zone, as defined by the mixing-time zones in Figure 6.21. 

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