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Continuous mixing holds out the promise of efficient and consistent rubber processing This has been recognised for many years, with the last major development activity occurring in the 1970's [1]. Then, the lack of a reliable supply of technologically and economically viable particulate rubber, coupled with a relatively undeveloped mixing technology, caused the movement to founder, despite the best efforts of enthusiasts. Now, continuous mixing is back on the agenda, with major materials suppliers offering a range of elastomers in particulate form. There is also a much better understanding of mixing to draw upon for process design [2,3,4].

In this paper a new continuous mixer is described, followed by presentation of results from a prototype system.

Design of the Single Rotor Continuous Mixer (SRM)

From analysis of existing continuous mixers [3,4] it is clear that some separation of incorporation, distribution and filler dispersion functions is desirable for an efficient design. Similarly, separation of conveying and mixing is necessary for versatility, to bring residence or mixing time under operational control. A layout of the SRM system, equipped with a roller die output device, is shown in Fig 1. Alternatively, a screw extruder can be substituted for the roller die. A schematic cross-section of the prototype, which has a simple fixed die, is shown in Fig 2.

A powder mixer delivers a particulate blend of the rubber compound ingredients to the feed unit. This device, which is essentially a screw extruder, compresses the particulate blend and causes the rubber to flow, effectively encapsulating and incorporating the filler. It also meters the rubber compound to the mixing unit, enabling residence time in the mixing unit to be controlled by adjustment of the feed unit screw speed.

The mixing unit has two zones, distributive and dispersive. The distributive zone is designed to remove residual variations of feedstock composition due either to the powder blending operation or to segregation in transit from the powder blender to the feed unit. Distributive mixing does not require high stresses or energy levels. Progressive subdivision and recombination of flows at modest strain rates is used to restrict the heat generation and rubber compound temperature rise in this zone. Following the distributive zone, the dispersive zone only has to accomplish the micro-scale re-distribution of fractured filler agglomerates. It is designed to subject the rubber compound to a series of high stress events, with a minimum of "wasted" flow between them. A multi-blade rotor is used for this purpose, to accomplish filler dispersion in a very compact unit.

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