Optimization strategies for the mixing room.

Rubber manufacturers offer their customers tailor made solutions. This flexibility is achieved by the manufacturer providing not only product and processing know-how, but also material development (e.g. compounding). However, this advantage of the rubber manufacturing industry implies also the necessity of mixing batch by batch, probably in very small lot sizes.

In practice, deviations between single batches (manufacturing characteristics, composition, etc.) cause fluctuations in quality of the final product and refuse. Many of those problems emanate from the mixing room. In general the occuring problems, like fluctuations in viscosity, dispersion, etc., cannot be compensated in subsequent processes.

Therefore, the mixing room in rubber industry is at present more in the focus than ever before. On the one hand the assessment of mixing quality and its consistency being substantial preconditions for high-quality rubber products has come out on top. On the other hand especially thereby the complexity of mixing rooms has been increased steadily in recent years (ref 1).

System correlations in the rubber mixing room

The investor of a new mixing room has to be aware that the main costs and also the utility of the plant decisively depend on the entire concept. If the mixing room is regarded as a black-box, the different sizes such as raw material, energy, etc., enter the plant and leave it as a final rubber compound. Among others, the following works have to be done: storing; drying; precutting; dosing; weighing; dispersing; distributive mixing; forming and cooling.

The concept of many users today consists of transferring as many tasks as possible into a mixer. However, there is no doubt that the mixer is not an optimal precutting unit for rubber or that one and the same mixer does not represent an optimal unit for masterbatch and final mixing.

First of all, regarding the mixing plant itself, it has to be distinguished between tangential and intermeshing internal mixers. The size of the machine is mainly influenced by the number of shifts planned and the splitting-up of the compounding on a phased basis. The respective temperature not only for the internal mixer but also for the subsequent units has to be adjusted to the machines themselves as well as the materials which are to be mixed. The material handling system is essentially determined by the analysis of the material requirement. Depending on the material, different versions of delivery and storage are possible.

The conveying and weighing units are influenced by the material. The automatization system has to guarantee an effective balancing of functions between programmable computer control, process control and weighing computers. Moreover, there must be the possibility of integrating a communication to higher-ranking computer systems, e.g. to PPS-systems. Besides the temporal and space arrangements, the planning of the erection has to consider that in addition to the machine parts themselves also pneumatic and hydraulic piping as well as electric wiring have to be installed. Only by the integration of the whole work necessary for erection can a trouble-free process of work be brought about. For the commissioning of the mixing plant not only the serviceability of the single units themselves is of importance but also the interaction of the single components. The service after commissioning certainly is as much important as the items mentioned above. The guarantee of fast service is becoming more and more important especially with regard to complex mixing rooms. During production the material testing is an important part of quality control. For the testing is generally done off-line, batch fluctuations are recognized with a time delay of several minutes or hours.

Parameters for on-line control of internal mixer

Many parameters are influencing the mixing result of the process in the mixing room. To control the internal mixer, only a few parameters are available. Common parameters detected at the mixer are mass temperature, power, rotor speed, torque and ram pressure. Since the mixing process is not continuous, the boundary conditions are very important too. The wall temperature gives information on the thermal situation. Another important parameter is the ram movement. Uncontrolled fluctuations in the ram movement lead to fluctuations in the power curve. The ram movement is generally neglected as a mixing parameter.

Process technology

As already mentioned at the beginning, a very important influence on quality is based on the process technological concept of the entire plant. Figure 1 shows the different steps and their relative advantages to optimize the mixing process. A great reserve can also be seen in the surrounding of the mixer. A cost against profit analysis has to be made to prove the efficiency of each step. In the following a few single steps will be discussed.

The temperature of the mixer wall has to actually be the same at the beginning of each mixing cycle (and not the temperature of the tempering water) in order to minimize the influence. Otherwise, the first batch effect cannot be avoided, which also occurs after long interruption times. Attention has to be paid to the fact that mixers of production size can change their wall temperature by 20 [degrees] C during an interruption time of five minutes. The wall temperature needs about 4-5 cycles to reach the equilibrium stage. In addition to that it is evident that the water temperature has a major influence on the equilibrium stage of wall temperature and thus the mixing behavior. The influence of water temperature to the BIT (black incorporation time) is demonstrated in figure 2.

Reproducible ram pressure is a demand, based on the correlation between dispersion, mass temperature and ram movement (ref. 2). Using a pneumatic ram, pressure fluctuations of the supply lead to variations in the mixture as well. A hydraulically driven ram solves this problem and also has the advantage of less energy consumption.

Apart from these influences on quality by peripheral units, decisive importance is attached to the mixing sequence. The plasticizer injection is shown in figure 3 as an example. The power diagrams of a production mixer are specified which had to incorporate large quantities of plasticizer into the compound. It is a common practice that the plasticizer injection is switched on by the operator after having achieved a certain torque, as well as switched off at a certain minimal power consumption. If the point of time for switching off is missed, wall sliding effects and extremely long mixing times will result. The important progress did not only consist in a cut of the average mixing time from 15 to 8.5 minutes, but also in the abolition of the peak value with an extremely long mixing time, i.e., in a regularity of the mixing quality.

In the year 1900, Werner & Pfleiderer delivered a kneader with a variable rotor clearance. Though the mixer has not been used in the rubber industry, the advantages of this system could easily be seen:

* The processing window can be extended in a wide range because of the additional parameter rotor clearance;

* Improvement of the feeding behavior;

* Change of this parameter shifts the mixing process from the chamber wall to the intermeshing region.

Because the internal mixer has many other parameters, which are often not used (figure 1), this concept has not been carried through. Another reason is that the concept is generally considered mechanically susceptible. In 1989 a new approach was presented with the VIC system.

In the last few years a new concept was made for tangential kneaders to run the rotors at the same speed (even speed). Figure 4 shows different rotor positions of a tangential system. Position 1 shows a maximum volume between the rotors. The kneader has a good feeding behavior. Position 2 shows a minimum volume between the rotors and high relative velocities are generated. The kneader has good dispersion and less feeding behavior. By this concept the universal internal mixer becomes a more specialized machine for individual mixing processes.

Mixing plant

The aforementioned examples prove the importance of a general concept. They also show that only the installation of a modem mixer cannot comprehensively solve all problems. The right mixer is, nevertheless, a necessary precondition for an optimal system.

The tangential mixer system offers its advantages mainly in the feeding and discharging area as well as in the machine's degree of efficiency. The intermeshing system shows advantages with regard to the specific energy supply at optimized thermal behavior, the mixing effect in view of dispersion and distribution, blending one rubber with another or with thermoplastics, as well as the absorption of plasticizer.

For both systems different rotor geometries have been built in the past. For the intermeshing type today there are only a few designs in the market. One major difference between those is the design of the cooling passages. Here the cooling system with a forced flow through an almost constant cross-section, like that provided with WP's patented PES-3 geometry (ref. 3), gives a very good cooling performance and a homogenous temperature distribution on the rotor surface. This is very important, as hot spots on the rotor surface significantly affect the mixing process.

Different approaches have been recognized in the last years for the tangential mixer. The ZZ-2 rotor design, which was patented in the 80s (ref. 3), has been successfully introduced and is now working in both the tire and the technical rubber goods areas worldwide. The ZZ-2 rotors balance the forces on the rotors and establish a constant average shear rate. In addition to that there is also the patented flow path between the long wings and the rotor end plates that allows optimized mixing behavior. This is especially true for cooled rotor end plates.

Finally, the decision for one of the systems can only be made related to the formula. Once the concept of the internal mixer is chosen, the next step is to determine the size of the machine. With an increasing size of machine the number of charges generally decreases and, thus, administration and personnel expenses as well as inspection costs. On the other hand, the extreme case, a one-charge order can be unsatisfactory regarding process technology and energy.

By means of variable speed drives and temperature control units the number of mixing steps can often be reduced by controlling the increase in temperature of the compound by suitable rotor speeds, as well as the temperature of the mixing chamber. However, the number of mixing steps is, of course, an initial condition to determine the size of the machine.

Material handling system

The demands made to such a system are substantially determined by the raw materials to be processed and the respective quantities of raw material. The basis of the design has to be the analysis of the actual or planned demand of material, certainly the type of storing depends on it. There are many arguments like reduction of repacking, reduction in the number of raw material inspections, the use of a central ventilation system and the reduction of the emissions that plead for a purchase of big packing like silo, container or big sack.

Apart from these general considerations, it has to be examined for each kind of raw material whether the above general approach is not limited by material specific characteristics. This is to be shown in the following by some practical examples.

*.Bulk material of a high specific weight, e.g. oxide of zinc, tend to agglomerate in bins which are too big. In such cases, the bulk material has to be loosened up in the bin before processing;

* Pelletized carbon black can better segregate in big silos than in bags. That means high concentrations of a particular size of grain can locally occur in silos;

* Carbon blacks delivered in silo wagons may have a temperature of far more than 60'C on delivery, if the supplier has just taken them from production. This means that carbon blacks of different temperatures can possibly be fed into the mixer;

* Chemicals such as antioxidant agents or accelerators can be superannuated. The danger of superannuation certainly increases by choosing a storing bin that is too large.

Automation

The automation of a mixing room comprises the fields described in detail in table 1.

    Table 1 - task division of a manufacturing
      process control for a mixing room
Production control              Mixer control
Facility lay-out                Mixer sequencing control
Recipe preparation              Temperature control unit
Material inventory control        operation
Production scheduling           Material requirement
Data handling                     control
Host communication
Weighing control                Additional information
Silo control                    Rolling mill sequencing
Automatic weighing and            control
  metering                      Information/data
Computer aided and                monitoring
  hand weighing                 Set point for operation of
Material preparation              downstream equipment

Particularly in the field of weighing control the interlocking with the previously discussed material handling plant is evident. The complexity of this sub-section proved in practice that only the design of local intelligent devices is flexible and efficient enough to meet the requirements of the weighing control. Referring to this sub-section it means that the functions have to be distributed to:

* The computer system for the conversion of formula data to scale orders;

* The PLC (programmable logical control) for driving the conveying and dosing units;

* The weighing computers for the self-supporting handling of the weighing orders.

The handling of all these functions in a central computer system involves the danger of being unable to run the plant in case of a malfunction of this computer system. As far as this concept of local devices is concerned, the manual operation by means of the PLC and the weighing computer is still possible in case of a malfunction of the computer system.

Erection

Not only the erection of a completely new mixing room, but also the integration of retrofits into a mixing room is a prerequisite to the very detailed planning of the works to be done. In the first step, either the conditions of space are to be determined or preliminary plans for the building are to be prepared. However, this first step already implies that the planners exactly know, even at this point of time, how such an erection has to proceed in each step. A variety of machines or machine parts, hydraulic and pneumatic pipes and flexible tubes, cables for electric and electronic units, as well as units for the material handling have to be installed on site. A coordination of individual erection plans by a general supplier offers the possibility of optimization. The erection course can now be planned work-orientated and no longer supplier-related.

Training

Training of the operating personnel has to be provided before commissioning plants. According to experience, a two-step concept showed the best success. In the first step, the handling of the computer system is tested at a simulating control system. As additional units do not follow, the approach to the new system is distinctly easier. In the second step, an automized mixer line is actually run under technology center-conditions. During these training steps all functions of the automation system can be tested. The production start is therefore considerably simplified.

A further essential simplification of the commissioning can be achieved by handling or modifying the formulas even in the forefield. For this reason, the GK 45 E installed in the technology center in Freudenberg is especially interesting for producers without any experience regarding intermeshing mixers.

On-line material testing

One of the greatest problems of rubber processing is the delay between rubber production and rubber testing. For industrial rubber mixing processes, it is desirable to evaluate on-line the distribution of different ingredients in the polymer matrix (i.e. homogeneity). Together with two partners (Krupp Forschungsinstitut and Pirelli) WP developed a laboratory system capable of in-process monitoring the homogeneity of rubber slabs on the roll mill or the batch off. The method is based on remote laser-microanalysis (RELMA) (refs. 4-6). The light of a pulsed UV laser is focused onto the rubber surface producing a hot and bright plasma. The optical plasma emission is detected spectroscopically with high time resolution. At WP's research center in Germany a RELMA system has been attached to a laboratory two-roll mill. Here the focused laser beam is guided with the aid of a linear scanner onto the moving slab, giving a raster-type arrangement of the measuring points. Thus, for example, rubber homogeneity can be characterized by the distribution of individual chemical elements. Another year is planned for testing including practice.

The compound's element concentrations are measured in the fraction of a second. At the focal spot the laser beam only ablates a tiny amount of material (about 5 mg) without any vulcanization. The detectable elements and/or related oxides (present status) are: S, C, P, Zn, Co, Al, Mg, Si, Ca, Fe, Ti, K, Na. The intensity of the emission can be correlated to the element content. Taking the example of ZnO, figure 5 shows the result of measurements taken on a well mixed and a poorly mixed rubber plate.

The second system, named Rheomill, is a sensor system which enables it to stipulate the flowing behavior of rubber mixtures when they are still worked on the roll mill (ref. 7). During the rolling process, a wedge is switched on the rubber slab. The wedge gap accommodates through sensor units the power/pressure ratio of the wedge gap flow, as well as the temperature and the swelling behavior of the mixture. The temperature is measured with an infrared sensor and the thickness of the rubber sheet by a laser. For the tests represented in figure 6 the measured values of the tangential force correlate very well with the Mooney viscosity. Here different amounts of a soft EPDM-compound were blended with a hard compound. This system will be tested in production in 1993.

References

[1.] Limper, A., Hader, W., "The mixing room - system aspects," meeting of the Rubber Division, American Chemical Society, Detroit, MI, October 8-11, 1991. [2.] Wiedmann, W.M., Schmid, H.M., "Optimierung tangierender und ineinandergreifender rotorgeometrien von gummiknetern," Kautschik + Gummi, Kunststoffe 34 (1981) 6. [3.] Wiedmann, W.M., Schmid, H.M., German patent DE 28 36 940 C2: Innenmischer zum Kneten von plastischen Massen, insbesondere von Gummi. [4.] Leis, F., Sdorra, W., Ko, J.B., Niemax, K., "Basic investigations for laser microanalysis," Mikrochim. Acta (1989) No. 2, p. 185. [5.] Lorenzen, C.-J., Carlhoff, C., Hahn, U., Jogwich, M., "Applications of laser-induced emission spectral analysis for industrial process and quality control," J. of Analyt. Atomic Spectroscopy (1992), pp. 1029-1035. [6.] Hader, W., Lorenzen, C-J., "RELMA: On-line quality control in the rubber mixing room using laser microanalysis," Technische Mitteilungen Krupp 2/1992, pp. 97-102. [7.] Limper, A., Hader, W., Verfahren und Vorrichtung zum Messen der viskoelastischen Eigenschaften von polymerem Material DE 4032375.

Acknowledgements

"Compounding studies using a new DOX," is based on a paper given at the October, 1992 meeting of the ACS Rubber Division.

"Tandem mixing." is based on a paper given at the October, 1991 meeting of the ACS Rubber Division.

"Optimization strategies for the mixing room," is based on a paper given at the October, 1991 meeting of the ACS Rubber Division.