Various clearance mixer technology.

Like Kobelco Stewart Bolling's proven 4-Wing H Swirl tangential rotor, the new 6WI patented rotor is the result of an ongoing rotor design program involving a series of experimental and theoretical studies. The experimental work consisted of three phases. Phase 1 was a two-dimensional sectioal profile study. Phase 2 involved a blade disposition study utilizing a unique three-dimensional experimental mixer. Phase 3 verified Phase 1 and Phase 2 results through actual mix trials using a BB16 liter mixer. The goals of our ongoing rotor development work is to improve the efficiency of the internal batch mixer with increased output, improved dispersion and homogeneity, reduction of Mooney viscosity and efficient cooling from the rotor. These goals have been met with the introduction of the 6WI VCMT patented rotor.

Experimental

Phase 1: Rotor sectional profile (figures 1 and 2)

Laboratory tests were conducted to determine how the sectional profile of a rotor (angle, tip width, tip clearance and number of wings) affects the mastication of rubber (Mooney reduction) and the mixing of a master carbon batch (micro dispersion) of carbon black and silica. The results were used to establish the optimum sectional profile of the rotor.

[Figures 1-2 ILLUSTRATION OMITTED]

Phase 2: Three-dimensional study (figures 3 and 4)

A special test mixer with a transparent Plexiglas chamber was used to simulate the flow and distributive mixing with an internal batch mixer. The rotors of this mixer are fabricated from thin oval section plates. The arrangement of these plates is easily changed to arrive at the optimal configuration of the rotor.

[Figures 3-4 ILLUSTRATION OMITTED]

Phase 3: Laboratory and small production mixer test

The information obtained from the two and three dimensional model tests on dispersive and distributive mixing was applied in the design of the new 6WI rotor with patented VCMT. Trial rotors were manufactured and fitted in a 3.4-liter laboratory mixer and a BB16 liter demonstration mixer. From the ensuing trials, Kobelco was able to study production parameters such as Mooney reduction, carbon black, silica and sulfur dispersion, energy consumption, fill factor and temperature rise. It was also possible to make a comparative study of the productivity of the different types of rotors (example: 6WI vs. Kobelco's 4-wing "H Swirl" rotor).

Advanced mixer technology

The new 6WI rotor is a six wing tangential rotor utilizing Various Clearance Mixer Technology. VCMT is a new design concept which features varying tip to chamber side clearances, and rotor tip to tip clearances. Each of the long rotor wings is designed with three distinct tip clearances along the circumferential and/or longitudinal direction (figure 5). The rotor tip clearances are designated small, medium or large, each a different measured distance between the rotor tip and chamber bore. Each of the short wings features a single tip to side clearance, short wing no. 1 with a small clearance, short wing no. 2 with a medium clearance and short wing no. 3 with a large clearance. The cooling capacity, cross sectional profile, blade arrangement, blade length, blade twist, tip width and tip clearances, are all configured for optimum performance. The most effective tip clearance depends on the viscosity of the mix. A small or tight tip clearance is best for a low viscosity matrix, while a larger tip clearance is most effective for a high viscosity matrix. In the case of the 6WI rotor, optimum clearances are selected for general applications (ref. 1). Figure 6 depicts the mixing mechanism of the patented 6WI VCMT.

[Figures 5-6 ILLUSTRATION OMITTED]

6WI VCMT patented rotor - small tip clearance

The small or tightest tip clearance is the high shear-mixing zone. This zone provides excellent dispersive mixing. This tip clearance area also provides improved cooling capacity by scraping the chamber wall, and reduces material degradation by preventing stagnation on the chamber wall. The small tip clearance contributes to a higher specific energy consumption (ref. 2).

6WI VCMT patented rotor - medium tip clearance

The medium tip clearance is similar to a typical tangential rotor tip to chamber side clearance.

6WI VCMT patented rotor - large tip clearance

The large tip clearance provides several benefits. First, this zone allows for greater movement of material between rotor to rotor and tip to side. The increased material movement improves homogeneity.

Secondly, the large clearance acts to decrease frictional heat buildup (ref. 1).

Cooling capacity

The 6WI rotor is a strong one-piece cast steel rotor with a single large cored cavity for efficient cooling effect (figure 7). The cored cavity follows the contour of the rotor body and wings, with a uniform wall thickness throughout. Unlike a typical tip-drilled design, the 6WI rotor provides an excellent cooling effect, because of the larger cooling coverage area and uniform wall thickness.

[Figure 7 ILLUSTRATION OMITTED]

Also, unlike the typical tip-drilled design with small diameter cooling passages, the large cored cavity is not prone to deposit buildup and the resulting loss in cooling efficiency.

Rotor wing length and orientation

The optimized blade length and blade configuration of the 6WI rotor provides high distributive mixing. The effective distributive mixing is attributed to the excellent inter-rotor material flow and complicated intra-rotor flow (figure 8, ref. 2).

[Figure 8 ILLUSTRATION OMITTED]

Results

Mixing trials

Successful 6WI VCMT rotor mixing trials have been conducted with a variety of compounds. During the initial development of the new technology, a series of tests compared the 6WI rotor to the 4-wing "H Swirl" rotor and the 6WX rotor (a 6-wing tangential rotor with a constant tip clearance) using a typical stock with carbon black.

Figure 9 shows the results of carbon black dispersion (higher carbon dispersion values reflect increased carbon black dispersion) and mixing time(s) (ref. 2). The graph clearly shows improved dispersion in less mixing time compared to the 4-Wing H Swirl rotor.

[Figure 9 ILLUSTRATION OMITTED]

Figure 9 also demonstrates that the new 6WI VCMT rotor provides a higher range of applicable fill factors and reduces ram seat time (ref. 2). The higher range of fill factors is due to an increase in the size of the high compression areas and corresponding reduction in the size of dead space within the chamber. The improvement in ram seat time is attributed to the ability of the 6-wing rotor to pull material into the chamber quickly.

Figure 9 shows a reduction in Mooney viscosity with the 6WI VCMT rotor given equal mixing time(s) (ref. 2).

Summary

The patented 6WI VCMT rotor may be used for masterbatch or final mixing. It is a versatile rotor, effective with tire stocks, technical rubber goods, resin suppliers, or concentrate and dispersion compounders.

The new rotor has demonstrated average increases in output of up to 20%, while providing excellent dispersive and distributive mixing. In some cases, batch cycle times when tested on a BB16 liter mixer in the Kobelco demonstration laboratory have been reduced by 50% with the 6WI rotor vs. traditional 2-wing or the 4-wing "H Swirl" rotor. The 6WI rotor is completely interchangeable with existing Kobelco 2 and 4 wing rotors. Like Kobelco's 4-wing H Swirl rotor, the newly patented 6WI VCMT rotor requires approximately 25% more hp than a traditional 2-wing tangential rotor.

Acknowledgements

1. K. Inoue, Y. Kuroda, Y. Yamane, H. Funahashi and K. Miyake, "Recent developments in polymer mixing and extrusion technology," Kobe Steel , Ltd., 1998.

2. Y. Hamada, "Introduction of new 6W rotor 6WI, "2, 1998.