Enhanced mixing in the intermeshing batch mixer.

An internal batch mixer has a mixing chamber and a driving mechanism which causes a pair of rotors to rotate in opposite directions to mix materials.

In internal batch mixers, a batch of materials to be mixed into a homogeneous mass is fed into a mixing chamber through a vertical chute and is pushed down into the mixing chamber under pressure by a ram or floating weight located in the chute. This ram may be driven hydraulically or pneumatically. When the ram is moved down to its operating position during the mixing of a batch of ingredients, the lower face of the ram forms an upper closure of the mixing chamber. The mixture of ingredients which is produced can be removed from the mixing chamber by opening a door to a discharge opening at the bottom of the chamber. The door is then closed prior to introducing the next batch of materials into the mixing chamber.

Internal batch mixers mix materials through the use of a pair of rotors which are rotated in opposite directions from one another as a batch of materials is mixed. The rotors are rotated by a drive system which includes a motor, a gear mechanism for connecting the motor to the rotors and controls for stopping, starting, controlling the speed of the motor and other processing parameters. Each of the rotors has a drive-end connected to the drive system and water-end through which coolant such as water is circulated through the rotors. Each of these rotors has lobes, that is, extensions or protrusions of the rotor beyond its smallest or minor diameter, which mix the materials. The lobes may be either wings, that are elongated extensions having generally narrow tip-width surfaces along their lengths, or nogs, which are elongated extensions having generally wide tip-width surfaces.

The processing of polymer in the internal batch mixers has been around for a long time. The earliest mixers were fitted with a pair of non-intermeshing rotors (ref. 1). Further developments brought about the mixers fitted with a pair of intermeshing rotors. Although many mixer improvements are going on, the basic structure of either the non-intermeshing or intermeshing mixer has not changed substantially. On the contrary, the design of a pair of rotors has changed substantially based on customers' demand for quality, productivity and efficiency.

The non-intermeshing mixers (refs. 1-4) have dominated the tire industry due to the following reasons:

* Large available mixing chamber volume;

* high fill factor;

* quick injection and discharging of material; and

* high productivity per energy usage.

On the other hand, the conventional intermeshing mixers (refs. 5-8) are known to perform very well in the technical goods industry, but have failed to penetrate the tire industry due to the following reasons:

* Small available mixing chamber volume;

* low fill factor;

* slow injection and discharging of material; and

* low productivity per energy usage.

The new intermeshing batch mixer has a pair of squeeze-flow rotors. Each rotor moves, squeezes and relieves the material back and forth within individual mixing chamber cavities. The intermeshing squeeze-flow rotors are constructed and installed within the new mixer so that the lobes of one rotor fit into spaces between or within the lobes of the other rotor. The new intermeshing batch mixer is developed to resolve the problems of the conventional intermeshing mixers, and also to improve on the performance of the non-intermeshing mixers.

Batch mixer basic terminologies

Terminologies used by polymer processing equipment manufacturers vary from manufacturer to manufacturer. An attempt is being made to define some of the terminologies and show different types of rotors, mixers and mixing concepts.

Rotor types

Rotors are the rotating members of a mixer. The lobes of the rotors perform the mixing function by shearing the polymer and other ingredients against the stationary walls of the mixing chamber. In some cases, shearing occurs between the two rotating rotors. The lobes transfer material from one chamber cavity to the other chamber cavity. Figure 1 shows two different types of rotor lobes. Most of the mixing occurs on the leading surface and over the tip-width of a pair of rotors. Figure 2 shows different categories of rotors.

[Figures 1-2 ILLUSTRATION OMITTED]

Mixer types

There are two types of internal batch mixers. The type of a mixer is determined by whether the rotational paths of the major diameters of the two rotors intercept or not. For instance, a non-intermeshing mixer has a pair of rotors, where the rotational path of the first rotor's major diameter does not intercept the rotational path of the second rotor's major diameter. In the case of an intermeshing mixer, the rotational paths of both rotor major diameters do intercept. The process of mixing varies from company to company.

Mixing types

There are many types of mixing phenomena, but for this article, the mixing types are broken down into two generalized groups as follows:

* Intensive mixing; and

* extensive mixing.

Intensive mixing (dispersive, shear, etc.) is a function of reduction of the particle size equally in the batch. Two methods that are used include:

* High shear milling which results in high pressure and temperature - shearing by breaking a big particle into small pieces by fracturing; and

* low shear milling which results in low pressure and temperature - shearing by breaking a big particle into small pieces by stretching.

Extensive mixing (distributive, blending, squeezing, etc.) is a function of distribution of the particles uniformly within the batch.

Mixing parameters

Many parameters are used in the design of a pair of rotors and a mixer. The focus is on design parameters which directly affect mixing. Table 1 shows some of the few critical design parameters and their effects.

Table 1 - mixing parameters

Design element       Effect

Tip-width            Dispersion, shear, temperature
Tip clearance        Dispersion, shear, temperature
Leading angle        Size of rolling bank, entry effects over tip
Trailing angle       Relief after tip, direction for distributive
                       mixing
Minor diameter       Strength, fill factor, flow
Choking              Block flow of rolling bank
Squeezing            Blending or distributive mixing
Wing length          Shear, temperature
Helix angle          Shear, temperature
Rotor internal       Cooling, heating, strength, deflection
L/D                  Strength, deflection, residence time
 (rotor length-to-major
 diameter
 ratio)

Mixing result

The following three measurements of the processed material were used extensively to evaluate the mixing capabilities of the new mixer:

* Quality: is a function of the processed material's tensile strength, Mooney viscosity reduction, homogeneity, temperature uniformity, etc.;

* Productivity: is a function of the mixer's available mixing chamber volume, fill factor, time for loading, injection of raw materials, mixing, discharging, etc.; and

Efficiency: is a function of productivity per energy usage.

New mixer

The new mixer is developed by using the good features of non-intermeshing and intermeshing mixers. The new mixer is called CoFlow, which is derived from the counterclockwise and clockwise flow of its rotors. The mixer has a large mixing chamber based on its long chamber length. The ratio of the mixer's chamber length-to-chamber internal diameter is larger than the conventional intermeshing mixers. This large ratio allows the new mixer to have a large productivity capability. The same large size ram and door openings are used for quick feeding of raw materials and discharging of processed material. This mixer has a pair of new rotors called squeeze-flow. Figure 3 shows a partial cross-section of schematic view and horizontal section of the new mixer.

[Figure 3 ILLUSTRATION OMITTED]

New rotors

Each of the new rotors has four lobes. The circumference of each rotor is divided into first, second, third and fourth sequential 90 [degrees] quadrants, which are spaced in a direction opposite to the intended direction of rotation of that rotor. Each of the rotors has a pair of lobes located in every other quadrant of the rotor. In order to intermesh the two rotors, the right rotor has a pair of lobes in each of its first and third quadrants, while the left rotor has a pair of lobes in each of its second and fourth quadrants. One of the lobes of each pair of lobes is longer than the other. The first pair of lobes of each rotor is diagonally symmetric to the second pair of lobes of each rotor. Figure 4 shows the unwrapped circumferential envelope of the new rotor lobe tips. For uniform shear rate over the long lobe tip as well as the short lobe tip, the same cohelix angle, which is the complimentary angle of the helix angle, is used for both long and short lobes. The use of equal cohelix angle allows the squeeze-flow effect within each chamber to be minimized and the relief effect to be maximized. This configuration is recommended for tough materials.

[Figure 4 ILLUSTRATION OMITTED]

Material flow of the rotors is unique. The long and short lobes push material axially towards each other. The material is then squeezed and flows out of the space between the ends of both long and short lobes. The rotor's name - squeeze-flow - is derived from the squeeze and flow out between the space of the long and short lobes. The axial push of the long lobe dominates the axial push of the short lobe in the same quadrant. Material flow in the rolling bank on the leading surfaces of the long lobes of each rotor, as observed from the water-end of the mixer, is in the form of N-shape. Therefore, the dominant material flow of both right and left rotors is in the form of N-shape as shown in figure 5. At the center region of the mixer, the interaction of right and left rotor lobes allows for automatic open and closed spaces; as shown in figure 5c. During mixing, the automatic open and closed spaces enhance the injection of raw materials and discharging of processed material. The feature of open and closed space between rotors has already been determined to be effective by manual means (ref. 8).

[Figure 5 ILLUSTRATION OMITTED]

The transfer of material from the right chamber cavity to the left chamber cavity is enhanced by two types of circulations (counterclockwise and clockwise flow).

Mixer comparison

Analysis of processing material behavior in mixers is used to evaluate the mixing capabilities of the new mixer against the conventional intermeshing and non-intermeshing mixers. Mixing types in the following regions of the mixers are evaluated in detail:

* Between the rotors and chamber walls;

* between the two rotors; and

* rolling bank.

Figure 6a shows the mixing types of the new mixer, while figure 6b shows the mixing types of the new mixer without milling at the center of the mixer. The lack of milling at the center of the new mixer is due to a decrease in the rotor minor diameter. Figure 7 shows that high intensive mixing occurs between rotors and low intensive mixing between rotors and chamber walls of the conventional intermeshing mixers. For the conventional non-intermeshing mixers, the following attempts have been tried to eliminate the material stagnation at the center of the mixers, but to no avail:

[Figures 6-7 ILLUSTRATION OMITTED]

* Friction ratio configuration to synchronized configuration (ref. 2);

* long wings were made longer and short wings were made shorter (ref. 3); and

* different designs for the top of the drop door and bottom of the ram.

Figure 8 shows the stagnation of the conventional nonintermeshing mixers. For comparison purposes, the mixing types of all the mixers are tabulated in table 2.

[Figure 8 ILLUSTRATION OMITTED]

Table 2 - mixing type comparison

Mixer type        Fig.              Between
                            chamber walls and rotors
                          Rotor tip         Rolling bank

CoFlow mixer        6a    High intensive    High extensive
 (new)                     mixing            mixing
Conventional        7     Low intensive     Low extensive
 intermeshing              mixing            mixing
 mixer
CoFlow mixer        6b    High intensive    High extensive
 without milling           mixing            mixing
 at center
Conventional        8     High intensive    Extensive
 non-intermesh-            mixing            mixing
 ing (tangential)
 mixer

                            Mixing type
Mixer type                 Between rotors

                     Rotor tip          Rolling bank

CoFlow mixer         High intensive    High extensive
 (new)                mixing            mixing
Conventional         High intensive    High intensive
 intermeshing         mixing            mixing
 mixer
CoFlow mixer         None              High extensive
 without milling                        mixing
 at center
Conventional         None              Extensive
 non-intermesh-                         mixing
 ing (tangential)
 mixer

                        Transfer from
Mixer type            chamber cavity to
                       chamber cavity

CoFlow mixer
 (new)                    Completely
Conventional              Completely
 intermeshing
 mixer
CoFlow mixer              Completely
 without milling
 at center
Conventional              Partially
 non-intermesh-
 ing (tangential)
 mixer

Discussion

The analysis has shown that the new mixer has very efficient mixing types. The material stagnation observed at the center of the conventional non-intermeshing mixers has been eliminated completely by the new mixer. The new mixer has a larger mixing chamber length-to-diameter ratio than the conventional intermeshing mixer. The new mixer has the option to eliminate milling of material at the mixer center by decreasing the rotor minor diameter. The decrease of the minor diameter yields very large available mixing chamber volume. The same large size ram and door openings of the new mixer allow for quick loading of raw materials and discharging of processed material.

A pair of the new rotors has an additional capability of squeezing and relieving of the processing material. The new rotors circulate material in the mixing chamber both counterclockwise and clockwise while squeezing and relieving of the material. The injection of raw materials and discharging of processed material are enhanced by the automatic open and closed spaces between a pair of the new rotors. Both the drop door top and the ram bottom are self cleaned by the new rotor tips. The cooling and heating of material have been improved due to a better tip cooled design.

Conclusion

A new breakthrough concept has been used successfully to develop the new enhanced intermeshing mixer called CoFlow. The good features of non-intermeshing and intermeshing mixers were used in the design of the new mixer. The new mixer has a larger available mixing chamber volume than all the conventional non-intermeshing and intermeshing mixers. A pair of the new rotors called squeeze-flow has a higher fill factor and productivity than all the conventional intermeshing rotors being used in the polymer processing industry today. A pair of the new rotors has higher quality, higher productivity, better strength and better efficiency than all the conventional non-intermeshing rotors. The new rotors have the same fill factor as the conventional non-inter-meshing rotors.

References

(1.) Banbury, F.H., "Machines for treating rubber and other heavy plastic material," U.S. Patent 1,200,070 (1916).

(2.) Nortey, N.O., "Optimized four-wing non-intermeshing rotors for synchronous drive at optimum phase relation in internal batch mixing machines," U.S. Patent 4,834,543 (1989).

(3.) Sato, N., "Mixing and kneading machine," U.S. Patent 4,284,358 (1981).

(4.) Wiedmann W. et al, "Mixing apparatus for kneading of plastic substances," U.S. Patent 4,234,259 (1980).

(5.) Cooke, R.T., "Improvements in rubber mixing or preparing machine," British Patent 431,012 (1935).

(6.) Lasch A. et al, "Modeling clay machine for raw rubber or other similar malleable material," German Patent 641,685 (1937).

(7.) Johnson, F. et al, "Mixer with streamlined nogs and cut back long nogs," European Patent 170,397 (1990).

(8.) Passoni, G.C., "Closed Parallel rotor mixer with adjustable interaxial separation," U.S. Patent 4, 775,240 (1988).