Mixer with variable intermeshing clearance.

The variable intermeshing clearance (VIC) mixer combines all the advantages of both the open roll mill (quality) and the internal mixer (productivity) in a single unit which makes the VIC a flexible machine capable of responding to the most critical requirements of the technologist, who can now adapt the mixing process to suit each compound while at the same time safeguarding the specific characteristics of the compound.

All this has been made possible through development of one operating variable that no other internal mixer has - the ability to vary the clearance between the rotors.

Technical features

We will outline the main technical features which are the characteristics of the VIC. More information can be found in company catalogues.

The rotors are intermeshing and the distance between them can be increased or decreased. The distance between their external surfaces (clearance) is adjusted between a minimum and a maximum level by rotating an eccentric sleeve in which the bearings are housed. A reference scale is on the end frames of the VIC, indicating the various settings for the distance between the rotor axes.

The numbers corresponding to the various settings have a symbolic value only, and bear no relation to the effective distance between the outer surfaces of the rotor (clearance). The increasing/decreasing of the distance between the rotors can also be accomplished during mixing operation.

The dust seals between the rotors and the chamber use the traditional ring system, suitably adapted to the feature of the variable distance between the rotor axes. The rotating part of the seals is made of hard material whereas the fixed part consists of bronze rings easily replaceable.

The cooling zones of the machine have been studied to achieve the maximum thermal performance. For a given capacity, the heat exchange surfaces of the VIC are more than 70% greater than those of a "PN" tangential mixer. The installed power of the VIC is 25% higher than that of the equivalent tangential mixer. The total volume and weight of the VIC are also higher between 20-35%) in respect to a tangential mixer with the same capacity.

Technological aspects

In the VIC, the shear values that ensure processing of the rubber are produced between the rotors, rather than between the chamber and the rotors, as in tangential-type machines.

For a better understanding of what has been stated above, we will examine the behavior of the rubber inside the mixer, paying particular attention to the shear rate of the particles of rubber during the mixing process. When in contact with the rotors, the speed of the rubber is equal to die peripheral speed of the rotors, whereas the speed of the particles on the fixed walls of the chamber is zero. This produces a shear rate y.) which, in the case of tangential-type mixers (shearing takes place between rotors and chamber), is extremely high, whereas in the case of intermeshing-type mixers (shearing takes place between the rotors) the shear rate is much lower, since the difference between the peripherel speed of the tips of one rotor and that of the root of the other rotor is to be considered.

In mixers with intermeshing rotors the average shear rate of the processing compound is lower than the one in tangential-type mixers. In technological terms this represents a limitation for the known intermeshing mixer.

On the other hand, the greater contact surface between the metal and the processing batch already mentioned makes it possible to achieve a greater transfer of the shear stress values to the batch, thus compensating for the disadvantage described, but still with some limits.

In fact we have to consider the relationship between the shear rate [gamma]. and the shear stress [tau] as follows

[tau] = [gamma] * [eta] where [eta] is the viscosity of the compound.

At this point it becomes evident that the possibility of varying the clearance between the rotors in an intermeshing-type mixer gives the opportunity to increase the value of the shear rate in machines with intermeshing rotors. This means that the VIC has this additional operating variable that by reducing the clearance between the rotor surfaces improves the value of the shear rate (see figure 1).

The sketch shown in figure 2 is intended to clarify the way in which the VIC incorporates the basic operating principles of both the open mill and the internal mixer, retaining the positive aspects of both machines and eliminating the negative ones.

Tests carried on the VIC

Since the start-up of the first VIC in February 1988 until today, several tests were carried out with the aim to explore as deeply as possible the performances of this new mixer. Performance studies were done mainly using mechanical goods compounds since the first VIC 115 was installed in a rubber mechanical goods facility. The opportunity to run tests on this industrial size of machine gave us the possibility to thoroughly explore the performance of the VIC with tire compounds. The results were so encouraging that we can confirm that VIC presently meets the interest and mixing requirements of tire companies.

VIC sizes considered for the tests

In order to have a tests apparatus as complete as possible to achieve reliable results, different sizes of machines were used. The sizes used for the tests were the VIC 1.8, VIC 115 and the VIC 265 whose technical characteristics are listed in table 1. The main formulations of die compounds tested on the VIC are given in table 2. The last item on the list is the pure natural rubber used to perform the mastification tests.

Table 1 - mixer technical characteristics
VIC 1.8
Geometric volume                                 1.8 litres
Net volume (*)                                  1.13 litres
Fill factor (net vol./geom. vol.)               0.63 litres
D.C. motor power                                  22 kw
Rotor speed (max.)                               230 rpm
  * Constant torque 0-1 00 rpm
  * Constant power 100-230 rpm
Specific pressure on compound (5 bar)           5.08 bar
Temperature control: 3 independent circuits:
  * rotors
  * sides
  * door top
with a water tempering system permitting a temperature
range of 20 [degrees]/120 [degrees] C.
Thermocouple fixed in the end plate
Microprocessor for automatic control of the mixing cycle
The machine is complete with a 150 x 300mm roll-mill.
(*) the net or dynamic volume was determined by the introduction
of a batch consisting exclusively of natural rubber of the
type SMR 5L (ML 1'+ 4' = 70) with a rotor speed of 35 rpm.
VIC 115
Geometric volume                                115 litre
Net volume (*)                                   72 litre
Fill factor (net vol/geom. vol.)               0.63 litre
D.C. motor power                                580 kw
Rotor speed (max.)                               60 rpm
Specific pressure on compound (7 bar)          4.08 bar
Temperature control : 3 independent circuits:
  * rotors
  * sides, floating weight and end plates
  * door top
with a water tempering system permitting a temperature
range of 18 [degrees]/80 [degrees] C.
Two thermocouples mixed in the door top and in the end
plate.
Microprocessor for automatic control of the mixing cycle.
Downstream equipment:
  * conveyor belt
  * 560 x 1500 mm roll-mill
  * batch-off for 400 mm wide strip
(*) the net or dynamic volume was determined by the introduction
of a batch consisting exclusively of natural rubber of the
type SMR 5L (ML 1'+ 4' = 70) with a rotor speed of 35 rpm.
VIC 265
Geometric volume                                275 litre
Net volume (*)                                  173 litre
Fill factor (net vol/geom. vol.)               0.63 litre
D.C. motor power                                970 kw
Rotor speed (max.)                               40 rpm
Specific press, on compound (7 bar)            4.08 bar
Temperature control: 3 independent circuits:
  * rotors
  * sides, floating weight and end plates
  * door top
with a water tempering system permitting a temperature
range of 18 [degrees]/80 [degrees] C
Two thermocouples fixed in the door top and in the end
plate.
Microprocessor for automatic control of the mixing cycle.
Downstream equipment:
  * 30 x 100" variable speed two roll mill
  * Batch off
(*) the net or dynamic volume was determined by the introduction
of a batch consisting exclusively of natural rubber of the
type SMR 5L (ML 1'+ 4' = 70) with a rotor speed of 35 rpm.

[TABULAR DATA 2 OMITTED]

Results

The following results refer to tests run on industrial sized machines (over 150 liters of net capacity) directly at a customer's shop.

In regard to the trials carried out on the VIC 1.8 laboratory mixe (ref. 6), this shows an interesting study about the influence of the variation of different operating parameters during a mixing cycle carried out both on the tangential type mixer and on our VIC 1.8 with intermeshing rotors and variable clearance.

We want to highlight the results relating to the use of the clearance variation parameters, since this is the aspect which constitutes the principal advantage of the VIC over the known intermeshing internal mixers on the market.

The variable clearance between rotors offers advantages of two kinds - technical advantages and technological advantages.

For a preliminary description of such advantages see reference 5, while the following discussion relates to the latest results on mechanical goods compounds processed on a VIC 265 by our customer.

The production data and numerous tests made on our VICs of laboratory and industrial size (VIC 265) have enabled us to reach die following conclusions regarding the effect of the clearance on the mixer operating parameters and the final characteristics of the product.

Widened clearance

In this situation there is more space between the rotors, with the following consequences:

* faster machine loading;

* faster raw material incorporation and consequent drop of the floating weight to lower position;

* higher average power absorption, which at the end of the cycle means an increase in the specific energy of the compound;

* higher instantaneous power peak;

* higher opportunities that mixing effect start at the beginning of the cycle intermeshing a greater part of the batch into the chamber and between the rotors.

Narrow clearance

In this situation the phenomena described above have exactly opposite effects.

An advantage using the narrow clearance in the faster cooling time of the batch due to the fact that there is more room for the compound between the rotors and the sides, that are the zones with the most effective overall coefficient of heat exchange.

Between the two extreme working conditions any other one can be selected.

All these gap variations can be established and preselected and automatically adjusted and controlled during the cycle in direct relationship with the process.

In this connection the experience so far obtained allows us to state that in general it is best to start the cycle with the widened clearance so as to decrease the times of incorporation of the ingredients and to size it during the final stage when the temperature of the compound must be kept under control and its dispersion further improved.

They show the temperature and power charts recorded during manufacture of an NBR based compound (n. 13) used as a substrate for high pressure steel reinforced oil hydraulic hoses. Density = 1.3 kg/l 98 Mooney 83 Shore A

In view of the absolute criticality of this compound, which must guarantee perfect adhesion with the metal, mixing is carried out in two passes in order to obtain optimal dispersion of some adhesive critical components.

The abscissae show time which progresses from right to left (1 cm = 30 secs), while the ordinates indicate both temperature (1 cm = 25 [degrees] C) and power (1 cm = 200 kW). The diagram refers to the first stage of processing.

The cycle was carried out by time with discharge at temperature according to the procedures outlined in table 3.

Table 3 - cycle procedure
Time             T            Operation
(secs)      ([degrees] C)
0                             Polymers
60                            Fillers (white, black, oils)
160                           1st cleaning
210                           2nd cleaning
145          Discharge
RPM = 25; Pressure = 5 bars; W.T.S. = 21 [degrees] C;
F.F. = 0.59

In both cases shown in the two diagrams the clearance was varied during mixing.

In figure 3 the process was started with wide clearance during the polymer's mastication stage, and then narrowed when the fillers were incorporated.

Figure 4 shows the opposite situation with a start with narrow clearance, and incorporation of the charges with wider clearance.

From an examination of the charts it can be clearly seen that:

* the effect of varying die clearance is evident when the machine is full, whereas it is insignificant when it is partly loaded (polymer mastication stage);

* the power peak with enlarged clearance is 33% higher (800 kW against 600 kW);

* the temperature curve slope during the final stage of the cycle after the second rise of the cleaning piston, is higher with large clearance. The speed of rise is 0.83 [degrees] C/sec. against 0.68 [degrees] C/sec., with a 22% increase;

* the cycle time with large clearance is 9% less (284 secs. against 312 secs.) By observing the trend of the power curves in the two different situations, it can be noted that with the narrow clearance the compound was discharged immediately after the last rise in the power values whereas with the wide clearance the discharge (always carried out at temperature) occurred 25 seconds after such rise. This gives reason to believe that the compound was already mixed even before the discharge and it would thus be possible to gain a total of 28 + 25 = 53 seconds (-17%) as compared with the situation with narrow clearance. In this case the sufficiently high viscosity of the compound (98 Mooney) enabled good results to be obtained in terms of dispersion of the ingredients even with the widened clearance, in spite of the lower times. With compounds of lower viscosity (98 Mooney) experience has on the other hand shown better dispersion necessarily only with the narrow clearance;

* the specific energy given to the compound is about the same in the two cases (0.120 kWh/kg with narrow clearance, 0.125 kWh/kg with wide clearance), but in view of the shorter cycle time with the widened clearance it can be stated that in the latter case a greater value of average power - about + 14% - is transmitted to the compound.

Tests with optimized clearance

The example reported above shows that the possibility of carrying the clearance between the rotors by adjusting it to the characteristics of each individual compound constitutes a significant variable in a mixing cycle. It is a supplementary parameter available to the technologists' continuing search for a higher quality standard combined with uniformity.

We studied optimization of the clearance through innumerable tests and these enabled us to reach the conclusion that the VIC series of mixers constitutes a new generation of machines with high flexibility and performance.

This is supported by the following results achieved with an SBR/CR/EPDM based compound processed on the VIC 265 (n. 16) compared with the same compound processed in a tangential mixer having the same effective capacity.

Density = 1.48 kg/l 110 Mooney 78 Shore A

The compounds were manufactured in a single stage, with incorporation of the accelerants directly into the mixer during the final stage of the cycle.

The comparison was made according to two different procedures in order to better demonstrate the different features of the machines.

In the first series of tests an examination was made of cycles conducted in the same way on both machines.

In the second set of tests variants were introduced into the cycle on each type of machine, and these took into account the different characteristics of the two units but with the scope of obtaining equal quality characteristics from the compound on discharge.

Figures 5 and 6 refer to the first series of tests (cycle of equal mixing time) and they reproduce the temperature and power charts with the usual reference scales: on the abscissae (1 cm = 30 secs) and on the ordinates (1 cm = 25 [degrees] C and 200 kW).

The cycle was conducted on a time basis with discharge at temperature according to what is shown in table 4.

Table 4 - cycle procedures
Time        T         Operation
[secs]  [degrees] C
0                     Polymers RPM = 30
                               Clearance = avg. (VIC)
60                    Powders + oils
                               Clearance = max.  VIC)
150                   1st Cleaning
           70         Accelerants RPM = 20
           82         2nd Cleaning
          110         Discharge
W.T.S. = 21 [degrees] C        Pressure = 5 bars
F.F. (tang.) = 0.67            F.F. (VIC) = 0.58

From an examination of the diagrams the following can be noticed:

* The power absorption in the mastication phase of the polymers only is higher with the VIC. This fact demonstrates that the intermeshing rotors work better even with low filling factors. Our experience allows us to state that the foregoing is valid in all cases of conventional crossslinkable elastomers in the physical state of the bale or scraps.

* The reduction of RPM from 30 to 20 led to a significant reduction in the value of the power curve in the case of the tangential mixer, but this was not influent for the VIC.

* The same phenomenon appears in the second cleaning, in which the rise in weight with VIC generates a remarkable lower drop in the power value. The intermeshing rotors in the final batch stage have greater grip into the compound.

* The instantaneous power peaks are higher in the case of the tangential mixer.

* The VIC has given the compound a 16% higher final specific energy (0,81 kWh/kg against 0.70 kWh/kg).

This last aspect has proved important for purposes of quality of the processed product, which showed better behavior in uniformity of the rheometric curves in the factory controls in the case of VIC combined with better dispersion of the carbon black and a better uniformity of the Mooney viscosity.

The scope of this work was also done to optimize the performance of a tangential mixer installed in the same mixing room as the VIC 265.

In order to obviate the drawbacks mentioned on the tangential mixer we had to increase cycle time and to separate incorporation of the fillers into two successive phases: first the black fillers and then, after 40 seconds of processing, the rest.

Operating this way, the problem of power peaks was eliminated; the specific energy into the compound was the same in the two cases, and the quality was in line with that obtained with the VIC.

All this nevertheless involved with the tangential mixer an increase in cycle time of 29% (350 seconds against 270 seconds).

Tire stock

To complete this article hereby are presented the results obtained with a truck tread compound (n. 1) processed both in a tangential mixer and in our VIC.

In the tangential mixer the mixing took place in three stages:

* master batch;

* remilling;

* final batch (productive stock). With the VIC mixer, due to the intensive mixing action, it was possible to load all the carbon black in the first stage and to complete the stock in the second stage. The total cycle time was reduced by 30%.

The viscosity of the compound has been kept the same adding few parts of peptizant agents in the NR without decreasing the tensile strength of the product.

The carbon black dispersion was even better as you can see in figure 7 where two sliced samples are presented with transmitted light.

Conclusions

In this article we have discussed the technical and technological aspects of mixers with intermeshing rotors, highlighting the advantages offered by these mixers with respect to tangential-type mixers as a result of the higher surface/volume ratio in the mixing zone.

We have also mentioned that, while the basic technology used in the VIC is the same as that of the known intermeshing-type mixers, the fact that it has an additional operating parameter (the variable clearance) gives it greater processing flexibility over the others.

This factor opens up new mixing possibilities since the machine can be easily set to meet the specific mixing requirements of any given compound offering greater possibilities for optimizing recipes to the technologists.

We have also presented the results of the tests performed on three different sizes of mixers (VIC 1.8, VIC 115 and VIC 265) which give us every reason to believe that this machine represents a new generation of mixers.

References

[1.] F. Shaw, "Choosing a mixer, " August 1979 [2.] F. Shaw "Mixing polymers" by H.D. Giffin (European Rubber Journal, May 1984) [3.] W&P "Quality and productivity improvements using intermeshing rotor system for internal mixers, " by H.M. Schmid (Rubber World, February 1984) [4.] W&P "Optimization of rubber mixing in internal mixers, 1985. [5.] Pomini , "VIC generation, " by L. Pomini (internal paper November 1988) [6.] Pomini , "Dual mixer: Efficacious tool to control and to optimize the quality of a compound during the mixing process, " by L. Pomini (Elastica no. 26, April 1991)