Tilt mixer rotor evolution.

In the years after Charles Goodyear's successful vulcanization of rubber and the ensuing commercially viable applications of this new wonder substance, Europe and the United States entered into the "Golden Age" of the industrial revolution, and there was considerable interest in developing a machine that could automate the successful compounding of rubber products in a consistent and repeatable manner. However, it was not until the dawn of the 20th century and the assembly line production of the automobile that this interest became a need. More cars meant more tires and more tires meant more tire compounds, and this need for increased productivity is what drove the development of a rubber compounding machine. The clear winner in this contest was the mixer created by Fernley Banbury in 1916, and this became the standard by which all other rubber mixers are measured.

He embraced the existing concept of an enclosed chamber housing a pair of counter-rotating rotary elements, and he added two key features that distinguished this machine from others of its time: (a) double-flighted rotors with the flights placed at angles on the surface (Banbury observed that the "sliding action of the blades tend to impart a smearing action on the rubber"), and (b) a ram or weight to press the rubber into the mixing chamber while the rotors are in operation. The first mixers of this design were named the #3 mixer and the #9 mixer, with the numerical prefix designating the number of 60" mills that each mixer could replace in terms of productivity (lbs./hour). In fact, the mixers became so successful that Farrel introduced larger Banbury mixers, the #11 and #27, to the market to meet the need for increasing productivity.

Subsequent developments in rubber mixers (non-tilt) include:

* 1930s--Francis Shaw Intermix--intermeshing rotors;

* 1950s--Stewart Bolling mixer--interchangeable rotors;

* 1980s--KSBI and Pomini--Farrel licensing ended;

* 1980s--KSBI "swirl-H" rotors;

* 1980s--Farrel "ST" rotors;

* 1990s--Pomini "VIC";

* 1990s--Skinner "Co-Flow" mixer;

* 1990s--KSBI "6-wing" rotor;

* 2002--Farrel "wing function technology."

In part, these advances in rotor/mixer design were made possible by improvements in computerized control systems, modern control devices, better quality raw materials and storage practices and more efficient cooling of equipment.

With all these advancements, why would anyone consider a tilt mixer for compounding? There are numerous applications where the tilt mixer offers a viable alternative to the more conventional drop/slide door mixers for batch mixing.

Tilt mixer applications

The development of the tilt mixer began when a large audio tape manufacturer approached Moriyama in Japan with dispersion problems they were experiencing for their tape compounds. The end result was the creation of the design commonly known throughout the industry as the "tilt mixer," which Moriyama named the "dispersion mixer." In many ways, Moriyama borrowed heavily from Fernley Banbury's work: They took the tape manufacturer's standard compounding machine, a sigma blade mixer (tilts to discharge), and beefed up the chamber design and added the same two key features as Banbury did--double flighted rotors, and a "ram" to keep materials in the chamber. Some of the tilt mixer applications include:

* Pharmaceuticals--full access for complete recovery of batch materials;

* color change outs--full access for complete clean out of mixer;

* small batch production--many models under 100 liter capacity;

* temperature sensitive compounds--superior heat sink;

* high value-added compounds--all the above;

* oil sensitive compounds--self-lubricating dust seals require no oil; and

* transition from mill mixing--easy and economical upgrade.

As noted, the development of the internal mixer was driven by the need for increased productivity (quantity), whereas the tilt mixer development was driven by the need for improved dispersion (quality). Although similar features were added (double flighted rotors and a pressurized ram), the design philosophy of the component differs significantly and reflects the impetus behind the development.

Design differences between tilt mixer and internal mixer

The difference between the two types of mixer are:

* Rotor geometry--the tilt mixer focuses on long and short (depth) blades of similar length (length on shaft) with a relatively large helix angle for the blades. Internal mixers focus on a long blade twice as long as the corresponding short blade.

* Rotor tip-to-bore clearance--the production-sized tilt mixer standard is 3 millimeters (1/8"), and the standard internal mixer is 3/16" on similar sized machines.

* Rotor land width and contour--the land (rotor blade tip) width on a tilt mixer design is about 1" on mid-sized mixers (35, 55, 75 and 110 liters) with sharply defined leading and trailing edges. An internal mixer of corresponding size has a relatively rounded tip that is roughly 1/4" wide.

* Dust seals--tilt mixers employ a self-lubricating, phenolic resin instead of the typical brass seal (with lubrication fittings) used on the more traditional mixers.

* Pressurized ram/lid--the ability to tilt and effectively discharge is aided by a wide chamber throat, which needs a broad ram to enclose and properly pressurize the mixing vessel. Many tilt mixers use a snug fitting ram with flat slat packings around the perimeter of the ram for positioning guides and better seal. The flat bottom promotes exchanges of materials between the two sections of the chamber. The internal mixer ram is very narrow within the chamber throat and allows for considerable lateral movement of the ram to aid the exchange of materials within the chamber.

The internal mixer rotors were designed primarily to meet production demands by the tire industry, and the first tilt mixer rotors (Moriyama's) were designed to meet the demands for increased quality. The first generation (G-I) tilt mixer had certain features, which distinguished it from other mixers on the market, including:

* Tip cooled rotors;

* low temperature mixing;

* hardened wear rings;

* dry dust seals;

* tilt discharge;

* unitized assembly;

* no hydraulics;

* low HP requirements; and

* compact "footprint."

These first tilt mixers found a home in many of the small general rubber goods operations in Asia. Their size, flexibility, focus on quality, compactness, low power requirements and low cost (to buy, install and operate) made this machine ideal to replace two-roll mills and the reliance on the skills of a mill operator.

Tougher, drier compounds became increasingly common, as did some exotics (silicones, fluoroelastomers, new fillers, etc.), and rotors evolved to reflect the changing market demand. The G-II mixer, introduced in the mid-1980s, features a fatter, more squatty rotor providing greater torque capacity, an improved dust seal design, a narrower throat and improved cooling structures in the rotors and chamber.

At this same time, tilt mixers became better known and more widely accepted as the advantages of this design were recognized. With the recognition came the competition, and Moriyama's virtual monopoly on the market was over. Although some initial clones were substandard, legitimate contenders did come forth.

Tilt mixers were now being looked at for both specialty operations and for small production operations. As the emphasis on productivity from these machines grew, the mixer manufacturers responded and the rotors evolved in predictably divergent paths. Many embraced the conventional non-tilt mixer approach towards productivity by employing rotors with more aggressive designs, including larger blade ratios and narrower blade tips. A few, Moriyama among them, adhered to the original quality-oriented approach. The turn of the new millennium finds rubber goods manufacturers, like so many other businesses, in search of greater efficiencies, especially ergonomic efficiencies.

The latest and most advanced tilt mixer rotor design is G-III, technology introduced by Moriyama. An engineer collating data from compounding trials on different mixer models noticed that one model showed consistently better quality results and the findings were confirmed by a repeat customer that had a number of different model mixers in operation. The primary reason was determined to be that this specific model had an optimal distance between the rotor minor diameter and mixing chamber bore in that there was optimal surface area to volume ratio for the rubber to cool. Rubber does not easily give up its heat history, and the more surface area that can be exposed on a given mass of rubber to a cooled surface, the better that core heat can be dissipated. This gave rise to an R&D effort focusing on high quality mixing, while maximizing the heat sink, thus establishing ideal conditions under which to single-pass mix (figure 1).

[FIGURE 1 OMITTED]

The G-III mixer successfully meets these criteria. The improved cooling jacket passages in the chamber and more elaborate cooling structure in the rotors, combined with a much larger surface area-to-volume ratio inside the chamber, yield a notably improved heat sink. The rotors have a bigger minor OD (rotor body diameter) and stubbier rotor blades of shorter axial length with a larger helix angle to enhance material flow patterns corresponding to the change in relative rotor diameters (major to minor diameters).

Tests conducted on a variety of compounds show conclusively that the G-III is a superior vehicle in which to achieve single pass mixing (table 2; figures 2-4). The process lab at Midlands Millroom Supply has a seven-liter mixer with G-III technology for trials to be conducted to evaluate performance on a given compound.

[FIGURES 2-4 OMITTED]

ACM mixing

In the comparison with the same mixing time, the additional mixing with an open mill was not required with the GIII technology mixer, and equal dispersion was achieved. The dump temperature was approximately 30% lower than with the conventional type dispersion mixer.

CR mixing

In the same process procedure, the mixing time with GIII technology was approximately 20% shorter than with the conventional type dispersion mixer, and the dump temperature was 27-36[degrees]F lower.

NBR mixing

In the same process procedure, the mixing time with GIII technology was approximately 20% shorter than with the conventional type dispersion mixer, and the dump temperature was 58-61[degrees]F lower.

In all of the compounds, the states of dispersion were almost the same. It was proven that the mixing temperature was lower with GIII technology, even if the mixer is larger. This confirmed that the GIII technology enables one pass mixing.

Table 1--tilt mixer rotor evolution

Rotor features          G-I         G-II        G-II

Rotor L/D              Large      Smaller       Same
Rotor land width       Large       Larger       Same
Tip-to-bore            Small        Same        Same
Wing ratio             55/45        Same        Same
Rotor profile          Skinny      Squat       Bulky
Wing helix angle       Large       Larger       Same
Surface area/vol.       Good       Better       Best

Table 2--recipes used for the test

Recipe            ACM base      CR base     NBR base

Polymer              100          100          100
Carbon black         45           50           70
Plasticizer           0           28           30
Miscellaneous        15           20           25
Total (phr)          160          198          225

(For mixes in the conventional dispersion mixer, the accelerator
was added on an open mill for the ACM and
NBR recipes. All others were single stage mixers.)

Figure 2--ACM physical data

                            Conven-         GIII
                             tional      technology

Mooney viscosity               55            53
Scorch time (min.)             16            16
Tensile strength (Mpa)        10.1          10.8
Elongation (%)                630           630

Figure 3--CR physical data

                             Conven-       GIII
                             tional     technology

Mooney viscosity               35           34
Scorch time (min.)              9            9
Tensile strength (Mpa)        15.4         15.8
Elongation (%)                400          400

Figure 4--NBR physical data

                             Conven-       GIII
                             tional     technology

Mooney viscosity               55           63
Scorch time (min.)              7            7
Tensile strength (Mpa)        16.5         17.0
Elongation (%)                360          330