A new concept for a continuous mixer under an internal mixer.

From 1965 to 1979 about 50 of the first generation transfermix equipment (ref. 1) were installed worldwide in the millrooms of tiremakers. Of these, 30 were of the size 460/380 mm (18/15[inches]) under about 350 liter internal mixers (e.g. No. 11 or Intermix K7) and the rest of size 610/530 mm (24/21[inches]) under about 600 1 mixers (No. 27 or Intermix K 10). That is not a big number compared to the many mill-lines and dump-extruders installed, where the latter save manpower, but do not mix.

Most of these transfer-mixers are no longer in use now, because of insufficient mixing capacity for present day compounds and of excessive heat build-up.

Recently a tandem (double) internal mixer (ref. 2), i.e., a masterbatch internal mixer followed by a ramless second internal mixer one size larger than the first one, for cooling and final mixing, has been marketed by Francis Shaw Ltd.

These facts, together with the advances in transfer-mix technology made since the first generation and continuing pressure from users for ways to use installed internal mixers more effectively, have prompted a radical reconsideration of the transfer-mix concept for millrooms.

Multi cut transfer-mix (MCT)

Cold feed extruders

Since about 1980, several hundred of these have been supplied in sizes from 50 to 250 mm, having a much improved plasticizing capacity for difficult compounds.

This requires an improved mixing-capacity cross-sectionally, as defined here with reference to figures 1 and 2, which is caused by a much tighter concentration of helical grooves (refs. 3-6) in the transfer-sections.

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The licensees were AZ Formen- und Maschinenbau, of Munich, mainly for tire retreading, RCM AG of Hori/Zurich, Switzerland of the MCT for a wide range of compounds including the more difficult ones, and Berstorff of Hanover, for the Pin-Convert Extruder (SC) for the compounds which the pin-barrel extruder cannot handle (refs. 7 and 8). All constructions have adjustable throttles which widen the range of compounds.

Mixing capacity of the MCT

In April 1994, a research program of the IKV (ref. 9) (Institute fur Kunststoff Verarbeitung of the State Technical College at Aachen, Germany) into the mixing capacity of the MCT was concluded: This showed that with certain combinations of screw- and barrel-geometries and the use of throttling, the improvement of carbon black dispersion and the final mixing were as good as with a conventional internal mixer.

One feed was masterbatch-pellets of different sizes, in each case preblended in a ribbon blender with vulcanizing chemicals. An alternative feed was two strips of masterbatch, one incorporating sulfur and the other the accelerators.

This represents academic validation of some practices of customers of AZ in the retreading industry.

For a number of years already one firm had combined feed-strips from different suppliers into a truck-tread compound directly built onto the buffed casing for subsequent mold-curing. Additionally, for retreading with precured tread, AZ supplies a cushion-gum mixer-extruder for a sulfur and an accelerator feed-strip, which separately have a good shelf life. After direct extrusion onto the casing surface - at about 85[degrees]C - and tread application very fast cure in the autoclave is obtained.

However, both preblended pellets and two-feed strips secure uniform levels of pre-dispersion in the feed section of the MCT which are not provided in the drop from an internal mixer especially if this is run with a reduced cycle time.

Transfer-mix with longitudinal mixing capacity

This third generation (ref. 10) is shown in figures 3 and 4 in a comparison with earlier generations, shown in figures 1 and 2. These also serve to help visualize the ordered mixing capacity of the system in two ways.

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Figure 2 illustrates the pack of cards model of mixing in a transfer-mix geometry of the first two generations. This starts from four packs at the beginning of four giver-grooves being transferred in ordered sequence to form four mixed packs at the end of the four taker-grooves. With the "cards" being transferred being marked in black, clearly none of the cards are transfer-worked (sheared) twice or are left out.

The cards in the four packs at the beginning of the section are exactly those which occur mixed at the end, which is why this is referred to as cross-sectional mixing.

It will also be realized that the widths of the crossing grooves - determined by the numbers of the grooves in the giver and the taker component around the circumference - determines the area of the cards, marked in black, which is being transferred. Analogously, the developed length of the transfer-zone, divided into the depth of the giver-groove, determines the thickness of the cards or volume-elements being worked.

Figure 2 illustrates what may be considered an overlaying effect: That of the vortices in the helical grooves being transferred and turned inside-out layer by layer, where the layers may be considered sequences of the cards. The inner core of a vortex in the giver-groove, indicated by dotted lines, at the beginning of the transfer-section ends up being the outermost layer of the vortex at the end of the taker-groove.

In the course of the transfer, fresh layers of the material are continually being led along the walls of both grooves, which for rubber as a very bad heat-conductor is a precondition for real heat-transfer with the metal surfaces.

This will clarify how, with a transfer-mixer geometry, the size of the elements and the intensity of their being worked is in the hands of the designer.

Therefore such a geometry enables the quality of mixing to be numerically expressed with precision, without any probability-calculations. Also the energy-inputs will be minimized compared to other devices in which volume-elements which are not exactly defined geometrically are subject to being worked under Gaussian probability distributions - some too little, some too much - so that for all of them to be worked at least once, the majority will have to be worked over a varying number of times.

Figures 3 and 4 illustrate the corresponding effects for the third generation transfer-mix geometry, at the same time clarifying its special features: In the particular design shown, only every second crossing-area of the opposite handed grooves is a transfer-area (marked black), as distinct from the above figures where every crossing-point has this function. This accounts for the 'longitudinal mixing,' which is also capable of numerical expression through the geometry applied.

Figure 9 shows an example where every fourth crossing area will be a transfer- or working area, indicating much more longitudinal mixing relative to the cross-sectional mixing.

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First practical application of Transfermix III

The placement of the plasticizing action of a cold feed extruder into a single transfer zone according to this new concept instead of into two MCT-zones has resulted in notably improved throughput and a flattening of the extrusion-temperature curve with increasing screw-speed.

This has been applied by RCM in their new MCTD-extruder, figure 5 showing outputs and temperature-curves. The outputs of such different compounds as NR-based truck tread and sidewall have approached one another and they differ more by their extrusion temperatures, although the range of these has also been narrowed.

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The self-cleaning feature of the MCT has also been maintained.

Approaches to the major problems of millroom use

Stock temperature

Reduction of temperature together with work-input for mixing has so far mainly been effected by mills, though not as a simultaneous action.

Table 1 shows, as a rough guide to the range of diameters needed, a comparison between the cooling surfaces of milltrains and of transfer-mix. For the mills, the cooling surface has been taken as that of one roll and for the transfer-mix as that defined by the radii of the smoothed surfaces on the internal and the external component, without taking into account the additional surface formed by grooving.

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The practical realization in the transfer-mix requires a departure from the approximately proportional scale-up, certainly in relation to groove-depths, that has been basic to dump extruder design.

Based on the relative circumferential speeds which produced acceptable results on smaller transfer-mixers, one can calculate much smaller groove-depths and ranges of rotational speeds based on the periodic drops from the internal mixer which define a range of throughputs.

Distribution around a large circumference

The adjustable throttle, which is being used to good effect on the different types of transfer-mix, can be used even more freely when adequate cooling can take place simultaneously with mechanical work-input for mixing. Thereby the working sections of the transfer-mix can be kept running full independently of the throughput and within acceptable ranges of rotational speed. The feature of self-cleaning can be maintained as established for the MCT.

Predispersion from an inhomogeneous drop

This is where longitudinal mixing comes in. The longitudinal displacements of the actual transfers, of which only two are shown in figures 3 and 4, can be multiplied as required for any number of adjacent helical grooves. In the extreme case, there could be as many as there are grooves.

In this way, an inhomogeneous lump dropped from the internal mixer will be transfer-mixed as predetermined slices into the continuously increasing mass within the 'taker' grooves, whereby dispersion is effected. This procedure can be repeated in subsequent transfer-zones.

Proposed new concept

Mechanical features

Figures 6-9 schematically show the proposed dump-mixer. With its effective mean diameters considerably larger than that of horizontal dump-extruders, and its grooves much more numerous and shallower, it can effect a multiple of the cooling-effect previously brought about by mill-trains.

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Figure 8 particularly indicates a way in which this cooling surface-area can be increased many times over and above the effect of the large mean diameter with the accommodation of many transfer-zones in a squat design.

The stationary internal column with its enlarged top surface together with the outer casing rotated in the manner of a rotary kiln, makes possible, as indicated in figure 7, a method of feeding from the drop-shaft of the internal mixer without the use of ram-feeders, a costly and maintenance intensive feature. Likewise, feed-channels for powders enable lower transfer-mix-zones to be weigh-fed with vulcanizing chemicals. Also, pipes can perform the same service for metering in under pressure of liquid vulcanizing chemicals, which may be a saving on the materials side.

For the very low range of rotational speeds, a prohibitively costly reduction gear can be replaced by a required number of standard geared motors acting on the large diameter gear-wheel, as shown in figure 6.

Figure 9 indicates the application of the feature of longitudinal mixing for making up what will be lacking in predispersion in the drop from an internal mixer operated at reduced cycle-time.

A throttle-device, not shown, will be utilized for adjusting the throughput to that of the internal mixer, independently of the rotational speed, in order that the mixer may run full.

The throttle has a very direct effect, also on qualitative factors of mixing, and is a very quick acting parameter for electronic process control. In fact, several could be used following the several transfer-mix-zones, being sequentially adjusted either by the process-controller or automatically by, for example, pressure-sensors. This measure makes it unnecessary to consider having different transfer-mix geometries for different requirements.

The output will conveniently be formed as pellets from the lower rim of the device, formed as a pelletizer with circumferential knives acting against a relatively rotating component with many grooves distributed around the circumference. These fall onto a rotating table and can be diverted onto a transport-band for direct cooling or for sheeting on an automatically operating mill, followed by conventional cooling, as indicated on figure 6.

The overall squate design may make it possible to replace mill-trains under existing mixers.

Functional features

The mixer could be designed with the following sections from the top down:

* A conically narrowing section, as traced in figure 7, for feeding, distribution and some cooling.

* One or more transfer-mix sections, for longitudinal mixing and for improvement of carbon-black dispersion by cross-sectional mixing at potentially a considerably lower temperature than in the internal mixer.

* A section for mixing extenders or additional chemicals.

* A final mixing section, with vulcanizing chemicals metered in at suitable positions round the circumference, also possibly as suspensions or liquid chemicals.

Mechanical simplifications

For the low rotational speeds, large-diameter rugged bearings such as available for building cranes could be used and these would be mounted so as to be in no danger of contact with the rubber. Seals against the rubber leaking are not needed. Tolerances need not be as fine as for horizontal extruders, as the transfer-mix works on a wider range of clearances.

The vertical disposition will ensure that there will never be any metallic contact, at the beginning or the end of operation, which gives rise to rapid wear in horizontal extruders of larger diameters. This could reduce the need for costly hard-surfacing and generally produce less wear.

The continuous operation eliminates the power-peaks associated with the operation of internal mixers and generally, mixing in transfer-mix devices is more economic of power than in other mixers.

Conclusion

The proposed scheme would appear to meet a genuine need with a continuously operating, self-cleaning device, which can be fully process-controlled and which would overcome existing limitations on mixing quality, temperature control and mechanical simplicity, thereby promising savings on initial and operating expense.

References

(1.)C.M. Parshall and A.J. Saulino, "A practical approach to extrusion, mixing and warming," Rubber World, 5 (1967).

(2.)C. Brown "Tandem mixing for tire compounding," Proc. Int. Tire Conf., Akron, 9 (1994).

(3.)P. Meyer, "The short transfer-mix," presented at the 110th meeting of the Rubber Division ACS, San Francisco 10 (1976).

(4.)P. Meyer, "The multi-cut transfer-mix," Proc. International Rubber Conference, Kiev, USSR, 1978.

(5.)N. Tokita, A.W. Thornton "Analysis of curatives blending and carbon black dispersion of transfer-mix," Proc. International Rubber Conference, Kiev, USSR, 1978.

(6.)P. Meyer, "The development potential of different concepts of rubber extruder," presented at a meeting of the Rubber Division ACS, Houston, 1983; reprinted in Rubber World, 7 (1984).

(7.)P. Meyer, "Practical applications of the short, adjustable MCT cold-feed mixer-extruder" proc. of RAPRA Conf. "Recent developments in rubber extrusion," U.K. Dec. 7. (1989); reprinted Rubber World, 7 (1990).

(8.)G. Capelle, "Der stift convert-extruder im versuch und in der praxis," proc. Asso. Gomma, Milano, Italy, May 26 1994.

(9.)Abschlussbericht Forschungsvorhaben Nr. 8877, "Untersuchung eines konzepts zum fertigmischen auf der basis des neuen transfer-mix," Bearbeiter: U. Meiertoberens; Institut fur Kunststoffverarbeitung (IKV), der Rhein.-Westf. Technischen Hochschule. Aachen, April 11, 1994.

(10.)P. Meyer, "Mixing machinery of the transfer-mix type," European patent application # 0574 172 A1, publ. Dec. 15 1993.