MCTD extruders: From theory to practice.

Screw conveying systems, or screw extruders, have been used by the rubber industry as a means of mixing, homogenizing, plasticizing and shaping a great variety of elastomeric compounds since the early 1900s. In the past, much of the screw and barrel development had been based largely on intuition and experience. In order to achieve greater performance, designers had often adopted ideas from elsewhere, with early cues sometimes coming from such diverse industries as meat processing.

Early operational designs were generally hot-feed machines, where the rubber compound was first preheated by mill mastication and thereafter fed into the extruder. Development of cold-feed extruders began in the 1930s, although notable success was only recorded after WWII. The desired mixing, homogenizing and plasticizing of the elastomer was to be achieved in a variety of purpose designed and sequentially spaced sections, cues for the design of which were now often being inherited from the plastics industry. Since then, the designs have become much more specialized and purpose developed. However, many rubber extrusion systems still rely on various sequentially spaced sections to achieve adequate performance.

MCTD principles of operation

One of the mixing, homogenizing and plasticizing systems able to perform by itself and not in conjunction with a variety of different other sections is the MCT or Multicut Transfermix. The MCT extruder relies on a highly interactive screw and barrel design, or transfer zone, to mix and blend the rubber compound intensively. Figure 1 shows a schematic representation of the interacting transfer threads of an MCT extruder. The receding flights of the screw intersect with the increasing channels of the barrel, so that the extrudate is partly forced out of the screw into the barrel and hence sheared normal to the flow already transferred into the barrel. The mixing action of the MCT is thus achieved by dividing (shearing), differentially transporting and re-uniting the compound flow in the transfer zone. In so doing, the compound is mixed both radially, as well as axially.

[Figure 1 ILLUSTRATION OMITTED]

Figure 2 shows a CAD-generated three-dimensional model of a Multicut Transfermix generation D (MCTD) screw and barrel. The changing geometry of the screw interacts with the development of the opposite-handed thread of the barrel (only feed and Transfermix section shown) and consequently transfers and shears the extrudate from screw to barrel, promoting the radial and axial mixing.

[Figure 2 ILLUSTRATION OMITTED]

Figure 3 demonstrates the compound within the extruder, with figure 3b highlighting the opposite-handed flow development within the transfer zone.

[Figure 3 ILLUSTRATION OMITTED]

MCTD performance

The development of new tire tread compounds is leading towards CB/Si[O.sub.2] modified elastomers with increasing levels of fillers. High concentrations of reinforcing fillers (typically carbon black or silica) result in higher compound viscosity (refs. 1 and 2). Such highly filled elastomers thus increasingly strain or exceed the capabilities of more conventional extruders. The demands of obtaining homogeneity in plasticization, while keeping within the closely defined temperature range of the compound extrudate, often do not permit sufficient output from these cold-feed extruders, so that oversized and hence economically disadvantageous extruders are frequently used.

The MCT, and particularly the latest generation MCTD range of extruders, has however proven to be successful at mixing a variety of these modified elastomers, both in practice and scientific tests. At the Insitut fur Kunststoffverarbeitung (Institute for Polymer Processing), IKV, at the University of Aachen, Germany, up to 10% by weight of a process oil test liquid was continuously dispersed in the matrix of an elastomer, with adequate homogeneity, in the Transfermix section of an MCTD extruder (ref. 3). Earlier work at the IKV showed the final mixing quality to be essentially the same as that obtained in an internal mixer (ref. 4).

Throttling

Despite the superior results achieved compared to more conventional extruders when processing most compounds, the extrusion of others, typically nervy natural rubber, has posed more of a challenge. This may be ascribed to the shear-work required for plasticization of these different compounds being vastly greater. For a single machine to be able to process such varying compounds, it is thus necessary to change the amount of shear working performed on the extrudate. This may be obtained by adjusting the pressure and consequently the pressure dependent shear work during transfer. This pressure adjustment is controlled by a recently developed throttle system. Notably, the throttle is variable from 0 to 100% (practically 100% closure). Furthermore, no open position dead spots are caused by this new system. The throttle consists of an adjustable conical ring, which may be axially positioned so as to allow its front edge to interact with the zero-depth ring of the rotor-screw. It thus becomes possible to vary the flow cross-section from zero throttling to close to 100% closure. This variable throttling also occurs at the most effective position, immediately following the front and most active shear zone of the Transfermix.

In practice, some compounds can be plasticized with the throttle open, while compounds requiring greater shear work may need a closure of over 90% of the area of flow. The viscosity of these high throttling compounds is, however, reduced to such an extent that even with the high but short restrictions of the flow cross-section, adequate throughput and processing become possible.

Industry tested

Hundreds of Transfermix-type extruders are currently operational worldwide. Transfermix-type extruders for both the new tire, as well as the retread industries, are available, from laboratory to 10 inch size. Apart from being able to extrude a diversity of compounds at higher outputs and better quality, the Transfermix extruders are also extremely compact in size. Not only is the mixing section much shorter than that of a conventional extruder, but the gearbox and motor are integrally designed so as to optimize space requirements.

The advantage of reduced size is most relevant when operating with multiple extruder systems, such as duplex, trip- or quadruplex systems. Figure 4 is a side view drawing of the extremely compact arrangement of three MCTD extruders around a triplex extrusion head.

[Figure 4 ILLUSTRATION OMITTED]

Another advantage of A-Z extruders is their relatively low power consumption. This is especially true when extruding CB/Si[O.sub.2] modified elastomers, where average power savings of around 20% have been reported.

Transfermix type extruders also form the heart of a range of specialized hot and cold retreading machinery, well over 100 units of which are already operational in North America alone. In the case of the CTC-SB I and CTC-SB II extruder/builders, the Transfermix extrudes cushion gum directly onto tire casings, resulting in significant labor and materials savings, as well as quality increases.

Development potential

As has been described above, the latest generation of MCTD extruders has demonstrated significant output, energy and size advantages when extruding a variety of rubber compounds. The introduction of a new throttle system has enabled the Transfermix extruder to process even nervy natural rubber adequately, so that the current range of MCTD extruders may be regarded as broad-range, able to process a large variety of compounds. However, the challenge is to design an extruder that is capable of processing all of the usual industrially used compounds at similarly superior output, quality and energy savings. Researchers are currently actively addressing this issue, with a particular revolutionary solution proposal deserving special mention: Recent experiments performed at the independent Deutsches Institut fur Kautschuktechnologie E.V. (German Institute of Rubber Technology), DIK, have indicated that subjecting the extrudate to high frequency ultrasonic stimulation may improve the flow behavior of elastomers. In particular, tests showed the experimental elastomer viscosity decreasing when being subjected to the (as yet underpowered and far from optimized) ultrasound stimulation. It may thus be assumed that the compound will consequently become much easier to extrude.

Conclusion

Screw extrusion development has steadily progressed to the point that many compounds which have previously been considered the domain of hot-feed extruders or not extrudable at all, are now being processed with cold-feed extruders. However, increasingly hard-to-process compounds are being formulated by the tire developers, so that the designer of the related machinery must constantly find innovative new ways of enabling the extruders to keep up with this pace of development. The Transfermix extruders may be considered as an example of such innovation, and continuing development and optimization of this type should result in its superior performance across the full spectrum of future rubber compounds.

References

(1.) Fultz, William C. and Evans, Larry R., J. M. Huber Corp., "Tire tread compounds with silica/CB blends" Rubber World, April 1998.

(2.) Meyer, Paul, Frenkel C-D Central Co. Ltd., U.S. Patent No. 5,421,650, Mixing Machinery of the Transfermix Type, June 6, 1995.

(3.) Haberstroh E., I.K.V. University of Aachen, Das IKV und die Kautschuktechnologie, 19. IKV Kolloquium 11th to 13th of March 1998.

(4.) Meiertoberens U. and Michaeli W., I.K.V. University of Aachen, Untersuchung eines Konzepts zum Fertigmischen von Kautschukmischungen auf Basis des neuen Transfermix, April 1994.