Improved productivity in the rubber industry.

Improved productivity means different things to different people. Here, improved productivity in the rubber industry is defined by:

* Mixing a better quality rubber in the same amount of time or less: and/or

* mixing more of the same quality rubber in less time.

There is a number of ways to improve productivity, and some of the methods to do so are separated into two groups, equipment (mechanical) and mixing parameters. Equipment or mechanical parameters that can increase productivity are the use of internal mixers, the mixer size and rotor design (and any combination thereof). Some mixing parameters that affect productivity are rotor speed and the way the raw materials are added during the mixing process.

Equipment (mechanical) parameters

Mill vs. internal mixer On the equipment or mechanical side of things, mills have been a staple of the mixing industry for years. However, mixing with a mill has a number of drawbacks. A mill is dangerous, has limited productivity, lacks batch to batch consistency and is dirty, especially when mixing highly filled systems. The next step up from a mill is an internal mixer. Internal mixers come in many sizes ranging from chamber volumes of 0.3 liters to 699 liters (batch sizes ranging from 0.5 to 1,150 pounds based upon a specific gravity of 1.0).

Internal mixers generally consist of six major components (figure 1), including: a stationary chamber, which houses the rotors (which do the mixing); the hopper, through which the raw materials are added; the ram cylinder which houses a floating weight (the weight forces the material into and keeps it in the chamber during mixing) and a discharge door, through which the finished mix exits the mixer.

Due to room, size or height restrictions, a typical internal mixer may not fit into the manufacturing space available. In this case, a tilt discharge mixer is the answer. A tilt discharge mixer has all the major components of an internal mixer, but does not need a discharge door, as the entire chamber actually tilts forward to discharge the product. The chamber volume for tilt discharge mixers ranges from 4 to 300 liters and, based upon a specific gravity of 1.0, the batch sizes are between 6 and 500 pounds.

Two examples of productivity improvement by mixing with a tilt mixer (an 80 liter mixer in this case) versus mill mixing are shown in table 1. In the first example, a highly loaded natural rubber compound was mixed on a mill for 25 minutes (1,500 seconds) to make a 160 pound batch. When mixed in the 80 liter tilt discharge mixer (fitted with an exhaust system to capture airborne particles), the cycle time for the same 160 pound batch was 7.5 minutes (450 seconds). With the tilt mixer, the productivity or throughput (pounds/hour) increased from 384 to almost 1,300 pounds per hour (a 233% improvement).

In the second example, 60 pounds of a highly filled synthetic rubber compound were mixed on a mill in 30 minutes (1,800 seconds). When mixed in an 80 liter tilt mixer, a 200 pound batch took 20 minutes (1,200 seconds) to complete. The productivity improved by 480%, from 120 to 600 pounds per hour. Not only are these prime examples of making more of the same quality rubber in less time, but the mixer is safer, cleaner and less manpower intensive than a mill.

Mixer size

Another equipment-based method to increase productivity is to simply install more mixers, either the stationary intensive mixer or the tilt discharge type. However, where space and financing may become an issue, the next step would be to go to a larger sized mixer (if possible) than what is currently used. An example of the difference in batch weights between different sized mixers is displayed in table 2. Using a specific gravity of 1, the batch weights (in pounds) for two common production sized intensive mixers, a BB-80 and a BB-270, are compared. Also taken into account are the different types of tangential rotors that are used for mixing. Each type of rotor has a specific volume and fill factor associated with it that gives the different batch sizes. Regardless of the type of rotor installed in the mixer, the BB-270 yields approximately 3.5 times more product than the smaller sized BB-80 mixer.

Rotor design

When in the market for a new or replacement mixer, one will usually look into the newest or most advanced rotor designs available (if they have not already done so). There are over 30 different types of rotors available, from 2-wing to 6-wing tangential rotors and from 3-lobe to 4-lobe intermeshing rotors (IM rotors). One reason there are so many different rotors and rotor designs available is that no one rotor/design is a cure-all for all rubber mixes (or plastic, for that matter). However, the more advanced rotor designs can significantly improve productivity in the majority of rubber mastication. The most recent rotor design offered by Kobelco Stewart Bolling since the late 1990s is the patented 6-wing VCMT tangential rotor (figure 3), where the VCMT stands for various clearance mixing technology (ref. 1).

[FIGURE 3 OMITTED]

Unlike most rotor designs, which feature a constant rotor tip width and rotor tip to chamber wall clearance, each long wing of the 6-wing VCMT rotor is split into three distinctively different rotor tip widths and rotor tip-to-chamber wall clearances (as illustrated in figure 4). One third of the long wing is a small tip width and clearance, another third of the long wing is a medium tip width and clearance and the remaining third is a large tip width and clearance. The tip configuration of the long wing, shown in figure 4, is a small/ large/medium configuration. The second long wing is a medium/small/large and the third long wing is a large/medium/ small configuration of the width and clearance.

[FIGURE 4 OMITTED]

The three short wings of the 6-wing VCMT rotor are different than the long wings; as the tip width and tip-to-chamber wall clearance, along each of their lengths, are constant. However, one short wing is a small tip width and clearance (as seen in figure 4), another is a medium tip width and clearance and the third is a large tip width and clearance.

The advantage of the small tip clearance is that it reduces the dead areas on the mixer sides, leading to improved cooling capacity. The small rotor tip-to-chamber wall clearance gives the high shear mixing, which leads to excellent dispersion and quicker mixing of the material. The large tip-to-chamber wall clearance allows greater movement of material in the mix. This decreases frictional heat and improves the heat homogeneity throughout the batch. The medium tip-to-wall clearance does a little of both. The medium tip gives more shear than the large tip clearance, aiding dispersion, but shears the material less than the small tip clearance, which aids heat homogeneity. The overall advantages of the 6-wing VCMT rotor are:

* Modification of drop temperature;

* fastest cycle times;

* highest throughput (pounds/hour);

* excellent dispersion;

* reduced energy consumption:

* reduction of mixing steps; and

* reduced Mooney viscosity.

The drop temperature can be modified in either direction. Since scorch and curing are time/temperature dependent, the taster the mix gets to the drop temperature, the shorter time it spends at the higher temperatures. This allows product discharging at higher temperatures, if so desired. When compared with other rotor designs, the 6-wing VCMT rotor gives the fastest cycle times, which leads to the highest throughput. The shorter cycle time also leads to less energy consumption per pound of material mixed (kWh/lb.). The small tip clearances create the shearing action within the mix that yields excellent dispersion. The use of the 6-wing rotor often leads to decreasing the number of steps needed during a mixing cycle eliminating exotic addition schemes). An example of this is demonstrated in table 3.

Products mixed with the 6-wing VCMT rotor have shown reduced Mooney viscosity when compared to material mixed with other rotor designs. A comparison between the 6-wing VCMT rotor and other rotor designs is shown in table 4. A natural rubber compound with a high loading of carbon black was mixed in a 16 liter pilot plant mixer with four different types of rotors in automatic control mode (to remove human inconsistencies) using identical mixing conditions. The only difference in the four examples was the type of rotor used for mastication. The 6-wing VCMT gave the fastest cycle time, highest throughput and lowest Mooney viscosity.

A second example comparing the 6-wing VCMT rotor with other rotor designs is exhibited in table 5. An EPDM rubber compound with a high loading of carbon black and oil was mixed with four different types of rotors in a 16 liter pilot plant mixer. Again, the 6-wing VCMT rotor gave the fastest cycle time, the highest throughput and the lowest Mooney viscosity.

A third example, in a compound that combines carbon black and a high loading of silica with natural rubber, is illustrated in table 6. Likewise masticated in a 16 liter pilot plant mixer, the 6-wing rotor reduced the cycle time, which led to higher throughput while giving a good Mooney viscosity.

Mixing parameters

An area often overlooked when trying to improve productivity is the mixing sequence itself. A number of different parameters make up the mixing procedure. Material addition, rotor speed, drive speed (even or friction) and inlet water temperature (temperature of the water circulated through the mixer's sides, discharge door and rotors) are but a few. Some mixing parameters that can improve productivity, whether alone or in combination with others, are the speed at which the rotors mix and the method of material addition during the mixing process.

Rotor speed

Increased rotor speed is definitely not an option if the rotors mix at a single fixed speed (will mix at one constant rpm setting) and only a slightly better option if the mixer is dual speed (will mix at one of two fixed rpm settings). However, variable speed mixers (allowing mixing over a range of rotor speed settings) give the most flexibility when experimenting with increasing the speed of the rotors during mixing.

Many equate faster rotor (mixing) speed to poorer product properties and performance, but this is not the case for all rubber mixes. Some mixes respond better to faster mixing speeds than others. A number of examples is shown in table 7.

Five different rubber compounds (base rubbers unknown) were mixed in one pass in a BB-270 mixer at rotor speeds ranging between 18-22 rpm (compound 5 was initially mixed at 20 rpm with the speed decreasing to 18 rpm during the mix cycle). The same five compounds were then mixed at speeds 5-10 rpm faster than the original rotor speeds. With the faster rotor speeds, not only are the cycle times improved by anywhere from 32-53%, but the product made was well within specifications and practically indiscernible from what was made at the slower rotor speeds. Obviously, these rubber compounds were amenable to being mixed at faster speeds.

Another example of improved productivity via increasing the rotor speed is shown in table 8 for a one pass EPDM rubber compound with a high loading of carbon black and oil. From the data in table 8, it is apparent that this EPDM compound (in this case masticated in a 16 liter pilot plant mixer with 6-wing VCMT rotors) responded well to a faster mixing speed. Each time the speed was increased, up to 40 rpm, not only did the cycle time decrease and the throughput increase, but the viscosity improved.

When mixed at a constant speed above 40 rpm, the cycle time was too short and the material yielded poorer properties. However, by initially masticating the rubber at 60 rpm for a period of time and then lowering the rotor speed to 40 rpm for the remainder of the mix, the mixing cycle proved sufficient. This set of conditions gave the best combination of minimum viscosity (66) and productivity (1,287 pounds/hour, an 80% improvement).

One is not going to know how a rubber mix will respond to faster rotor speeds, especially when using the more advanced rotor designs, unless one is willing to experiment and test the rubber mix at different rotor speeds.

Material addition

The same can be said for using a different material addition procedure, with or without the use of advanced rotor designs. The basic addition procedures inferred here are the right side up (RSUP, rubber in first, then additives) and the upside down (UPSD, add most or all additives with the rubber going in last).

Examples of the effects of employing these different material additions are evident in table 9. Three different nitrile rubber mixtures with a high loading of carbon black are normally mixed with 2-wing rotors and a right side tip addition. Upon trying these nitrile recipes in a 16 liter pilot plant mixer with the 6-wing VCMT and with intermeshing rotors, the testing included an upside down material addition. In all three examples, an improvement in the cycle times and throughput was demonstrated.

However, the most noticeable improvement was shown by the 6-wing VCMT rotor using the upside down addition. When mixed with the upside down addition, the carbon black dispersion was significantly better (81% vs. 59%) in a shorter cycle time than that obtained when the rubber was masticated with a right side up scheme. This is a prime example of mixing a better quality rubber compound in the same amount of time or less.

The second example, this time with intermeshing rotors, showed the same quality of rubber being made with the upside down scheme, but with a much shorter cycle time. The third compound is an example that not all rubber mixes respond the same. Even though the cycle time was considerably shorter, the product was much poorer in quality (a 72% carbon black dispersion) with the upside down addition, when compared to the right side up mix (94% dispersion). The third example could still show improved productivity if it was mixed at a slower speed (faster rotor speed gave an even poorer carbon black dispersion in the upside down mode for this recipe).

The use of a different method of addition is especially interesting when attempting to make a multiple pass system successfully mix in one pass. Different nitrile rubbers with high loadings of carbon black were initially mixed in a 16 liter pilot plant mixer with a two pass, fight side up material addition. The same recipes were then mixed in a single pass with both a fight side up and upside down addition.

For the first example in table 10, the material was mixed using 2-wing rotors at 30 rpm in a two pass system. The material was dropped at 235[degrees]F for the first pass and at 200[degrees]F for the second pass. The same recipe was then mixed in one pass with 2-wing rotors, using both a right side up and an upside down addition at 50 rpm rotor speed and a 260[degrees]F drop temperature. The fight side up one pass mix gave improved productivity, but a poorer product (39% carbon black dispersion). The upside down one pass mix gave the same quality rubber as made with the two pass mix, but in substantially less time (over 5.5 minutes faster) with a substantial improvement in productivity (256%).

The second example in table 10 exhibits the same trend. The nitrile/carbon black recipe was mixed using 6-wing VCMT rotors at 30 rpm in two passes with a right side up addition. The material was dropped at 235[degrees]F on the first pass and at 200[degrees]F for the second pass. The same recipe was mixed in one pass with the 6-wing VCMT rotors using both a right side up and an upside down addition at a 60 rpm rotor speed and 240[degrees]F drop temperature. The second set of results in table 10 shows that the one pass fight side up mix again gave improved cycle time/throughput, but a poorer product (62% carbon black dispersion). The one pass upside down mix gave the same quality rubber as made with the two pass mix, but in substantially less time (over three minutes faster) and with improved throughput (247%).

Improved productivity via the combination of various parameters

What are the possibilities when a number of methods to improve productivity is combined? The data in table 11 show how the productivity and properties in a mix react to combining some of the equipment and mixing parameters mentioned earlier. An EPDM rubber compound with high loadings of carbon black and oil was originally masticated in a 16 liter pilot plant mixer with 2-wing rotors as a two pass mix with a fight side up addition. The rotor speed was 40 rpm for the first pass and 20 rpm for the second pass. Keeping the same mixing procedure, simply changing from 2-wing rotors to the 6-wing VCMT rotors (example 2), yielded a reduction of 156 seconds (over 2.5 minutes) in cycle time, a 39% increase in throughput and a lower minimum viscosity (64 MU). This exhibits the improvement gained by switching to an advanced rotor design (equipment).

The third example of table 11 shows how the EPDM recipe responded to the 6-wing VCMT rotors as a single pass, right side up addition at 40 rpm rotor speed. In this case, there was a continued improvement in cycle time and throughput, but a drop-off in the product's properties with a significant jump in the minimum viscosity (to 76 MU).

At this point, a material addition change was introduced, going from a right side up mix to an upside down mix. Keeping the one pass mix scheme, the rotor speed at 40 rpm with 6-wing VCMT rotors, the change in the material addition culminated in still further improvements. The cycle time was reduced to only 90 seconds and the throughput was improved over 300% when compared to the original 2-wing data. The minimum viscosity was improved, but it was still higher than that obtained with the 2-wing rotors. This fourth example shows what a change in rotor type, coupled with a different material addition, can accomplish.

The EPDM mix was then masticated at different rotor speeds to see what effect the speed had on the process. It was found that by initially mixing the rubber at 60 rpm, then reducing the rotor speed to 40 rpm for the remainder of the mix, the target viscosity was obtained (66 MU), along with an even faster cycle time (85 seconds) and greater throughput (1,287 pounds/hour).

As the above sequence of test mixes demonstrates, by employing the combination of a change in the type of rotor (2-wing to 6-wing VCMT), material addition (two pass right side up to one pass upside down) and increasing rotor speed (initially running the batch at 60 rpm), significant improvements were made. The cycle time was reduced dramatically (from 415 seconds to 85 seconds) for a time savings of 330 seconds (over five minutes per batch). The productivity or throughput increased by 332% to 1,287 pounds per hour in the 16 liter mixer, an increase of almost 1,000 pounds per hour. The target viscosity was met (66 MU as compared to 67 MU), and the material was only handled once instead of twice.

Summary

There are a number of ways to improve productivity in rubber mixing, whether increasing the quality of the rubber or the hourly throughput. An internal mixer, either a stationary or tilt mixer, is cleaner, safer and less manpower intensive, with increased productivity, compared to a mill. Larger sized mixers, masticating larger batch sizes, significantly increase productivity when replacing smaller units. The incorporation of a more advanced rotor design, such as the 6-wing VCMT rotor, gives substantial improvements in throughput (faster cycle times), a better quality product (reduced Mooney viscosity and/or better carbon black dispersion) and can allow reducing the number of addition steps during the mixing process.

On the other hand, changing the mixing parameters (such as faster rotor speed and adding the raw materials differently) can also substantially improve productivity in many rubber mixtures. Mix cycle time improvements, with product property retention, were seen when increasing the rotor speed just 5-10 rpm on a production-sized mixer. Reductions in cycle time and Mooney viscosity, and an increase in throughput, were seen when an EPDM mixture was masticated at a faster rotor speed. Altering the addition of the raw materials in a nitrile rubber mix from a right side up two pass mix to an upside down one pass mix gave throughput increases of more than 240%. Incorporating a more advanced rotor design with faster rotor speed and a different mode of material addition for an EPDM mix led to a 332% improvement in productivity.

However the goal of improved productivity is reached is not important. What really matters is the willingness to try new things and, most importantly, spend time at the mixer to test the different parameters available to achieve this goal.

Table 1--comparisons of mill and mixer
production

Mixer        Cycle    Batch    Throughput    % productivity
type         (sec.)   (tbs.)   (lbs./hour)    improvement

Mill         1,500     160          384            --
Tilt mixer     450     160        1,280           233

Mill         1,800      60          120            --
Tilt mixer   1,200     200          600           480

Table 2--comparison of batch weights

             Batch weight      Batch weight
Rotor        BB-80 (pounds) *  BB-270 (pounds) *

2 W                120               425
4 WH               111               415
6 WVCMT            103               388

* Based on a specific gravity of 1

Table 3--reduction in mixing steps with the
6-wing VCMT rotors

Mixing
step     2-wing                6-wing VCMT

1        Add rubber            Add everything but oil
2        Add 1/3 black         Add oil
3        Add 1/3 black         Sweep
4        Add remaining black   Discharge
5        Add oil
6        Sweep
7        Add cure
8        Discharge

Table 4--comparison of different rotors with a
natural/carbon black mix

                                     16 liter mixer
               Cycle time   ML 1+4    throughput *
Rotor type     (seconds)    Mooney     (lbs./hr.)

2-wing            128         67          747
4-wing            119         67          772
6-wing VCMT        88         64          992
Intermeshinq      182         69          401

* Calculated for a 16 liter mixer. Multiply by 16.3 for
BB-270 and 25.3 for BB-370 hourly throughput.

Table 5--comparison of different rotors with a
EPDM/carbon black/oil mix

                                     16 liter mixer
               Cycle time   ML 1+4    throughput *
Rotor type     (seconds)    Mooney     (lbs./hr.)

2-wing            187         75          724
4-wing            139         74          839
6-wing VCMT       108         70          993
Intermeshinq      172         77          546

* Calculated for a 16 liter mixer. Multiply by 16.3 for
BB-270 and 25.3 for BB-370 hourly throughput.

Table 6--comparison of different rotors with a
natural/carbon black/silica mix

                                     16 liter mixer
               Cycle time   ML 1+4    throughput *
Rotor type     (seconds)    Mooney     (lbs./hr.)

2-wing            141         80          676
4-wing            115         82          801
6-wing VCMT        95         81          928
Intermeshinq      151         82          474

* Calculated for a 16 liter mixer. Multiply by 16.3 for
BB-270 and 25.3 for BB-370 hourly throughput.

Table 7--increased productivity in a BB-270 mixer
using faster rotor speed

               Rotor speed    Cycle time     % cycle time
Compound          (rpm)       (seconds)      improvement

1                  20            164              --
1                  30             84              49

2                  20            210              --
2                  30             99              53

3                  20            142              --
3                  30             90              37

4                  22            117              --
4                  30             79              32

5                 20-18          131              --
5                  25             89              32

Table 8--comparison of mixing with increased
rotor speed

Rotor     Cycle      Minimum    16 liter mixer        %
speed     time      viscosity    throughput *    productivity
(rpm)   (seconds)       MU        (lbs./hr.)     improvement

25                      76            715             --
30         153          71            943             32
40         116          69          1,216             70
60/40       90          66          1,287             80

* Throughput is calculated for a 16 liter mixer. Multiply by
16.3 for BB-270 and 25.3 for BB-370 hourly throughput.

Table 9--comparison of mixes using different
material addition

                               16 liter
                                mixer             %
               Cycle time    throughput *    productivity
Rotor type      (seconds)     (lbs/hr.)      improvement

6-wing VCMT        127            883             --
6-wing VCMT        105          1,068             17

Intermeshing       254
Intermeshing       160            370             --
Intermeshing       277            588             59
Intermeshing       174            310             --

                  % CB         Material
Rotor type     dispersion      addition

6-wing VCMT        59            RSUP
6-wing VCMT        81            UPSD

Intermeshing
Intermeshing       89            RSUP
Intermeshing       90            UPSD
Intermeshing       94            RSUP

* Calculated for a 16 liter mixer. Multiply by 16.3
for BB-270 and 25.3 for BB-370 hourly throughput.

Table 10--comparison of one and two pass systems
with different material addition

                          16 liter
            Cycle          mixer            %
No. of       time       throughput *   productivity
passes    (seconds)      (lbs./hr.)    Improvement

2            463            250             --
1            184            629            152
1            130            890            256

2
1            284            380             --
1            122            919            142

No. of       % CB         Materia
passes    dispersion      addition

2             51            RSUP
1             39            RSUP
1             54            UPSD

2
1             85            RSUP
1             62            RSUP

* Calculated for a 16 liter mixer. Multiply by 16.3 for
BB-270 and 25.3 for BB-370 hourly throughput.

Table 11--improved productivity by combining changes in
equipment and mixing parameters

            Rotor                         Cycle
            speed           mix            time
Rotor       (rpm)          scheme       (seconds)

2-wing      40/20       2 pass/RSUP        415
6-VCMT      40/20       2 pass/RSUP        259
6-VCMT        40        1 pass/RSUP        123
6-VCMT        40        1 pass/UPSD         90
6-VCMT      60/40       1 pass/UPSD         85

           16 liter
            mixer            %           Minimum
         throughput *   productivity    viscosity
Rotor     (lbs./hr.)    improvement         MU

2-wing                                      67
6-VCMT        298            --             64
6-VCMT        414            39             76
6-VCMT        890           199             69
6-VCMT      1,216           308             66

* Calculated for a 16 liter mixer. Multiply by 16.3
for BB-270 and 25.3 for BB-370 hourly throughput.

References

(1.) K. Inoue, K. Ureshino, N. Yamada, K. Takakura and Y. Kurokawa (to Kobe Steel  Company, Japan) U.S. Patent 5,984,516, Nov. 16, 1999.