Process advantages by improved rotor cooling--part 1.

The technology of mixing rubber compounds is more than 100 years old. Since the introduction of internal mixers, ongoing developments have been focused on improving the internal mixer. The heart of the mixer, the rotor, has been the focus of improvement. One important aspect of rotor design is the rotor geometry in terms of mixing efficiency and achievable compound quality. The cooling ability of the rotors is another vital aspect of rotor design.

Today, all inner parts of the internal mixer, which are in direct contact with the compound, are temperature controlled, with the mixing chambers and the door top having drilled passages in close proximity to the inner surface.

Mixer inner parts with surfaces in direct contact with the compound include mixing chambers, rotors, door top, floating weight and rotor end plates.

The rotors, which are of substantial significance in terms of cooling efficiency, should ideally be of a two-piece design. The rotor body should have cooling channels near the surface. A cored type, cooled ram and rotor end plates with drilled passages should be used in a process sensitive machine. It is also desirable to have the thickness of the walls between the inner surface of the mixer and the cooling water passages in the chamber sides as thin as possible, without compromising structural integrity to obtain the best cooling efficiency.

Overall energy balance (simplified)

In examining the energy flow of a 110 L intermeshing production mixer, the total energy input is transferred to about 10% loss into mechanical losses (gearbox, motor, bearings, etc.) and about 38% is used for the enthalpy rise of the compound. Fifty-two percent goes into the mixing chambers, rotors, rotor endplates, floating weight and the door top. About 50% of this energy is removed directly by the cooling water. The remaining 50% is stored in the metal volume (mass) between mixer surface and the cooling channels.

The values are similar for a tangential mixer. However, the mixing application has a big influence on the whole energy balance. Looking at a typical final mix with a mixing time of three minutes, it is even possible to have 50% and more of the energy going into the compound. Looking at a mixing application with a longer mixing time and with a constant batch temperature at the end, almost 0% of the energy will go into the compound during the final stage of the mix.

Specific heat flow

To analyze the heat exchange in an internal mixer, the heat flow will be examined. In general, a convective heat flow is seen between the compound and the mixing chamber/ rotor surface, which can be described as follows:

q" = [[alpha].sub.CCS] x ([T.sub.CS] - [T.sub.C]) q" - [[alpha].sub.CRS] x ([T.sub.RS] - [T.sub.C]) [[alpha].sub.CRS] : Heat flow coefficient between compound and rotor surface

[[alpha].sub.CCS]: Heat flow coefficient between compound and mixing chamber surface

[T.sub.CS]: Mixing chamber surface temperature

[T.sub.RS]: Rotor surface temperature

[T.sub.C]: Compound temperature

Between the rubber and the metal surface, the heat flow coefficient tends to approach a value between 100 and 200 w/[m.sup.2]K.

Convective heat flow is also seen between the mixing chamber/rotor and the cooling channels:

[q.sub.RW]" = [[alpha].sub.RW] x ([T.sub.CCSR] - [T.sub.W]) [q.sub.CW]" - [[alpha].sub.CW] x ([T.sub.CCCS] - [T.sub.W])

[[alpha].sub.RW]: Heat flow coefficient between water and cooling channel surface rotor

[[alpha].sub.CW]: Heat flow coefficient between water and cooling channel surface mixing chamber surface

[T.sub.CCSR]: Rotor cooling channel surface temperature

[T.sub.CCCS]: Mixing chamber cooling channel surface temperature

[T.sub.W]: Cooling water temperature

The heat flow coefficient on the water side could be estimated at about 5,000 to 10,000 w/[m.sup.2]K.

Temperature influence

To understand the mixer temperature influence on the intake behavior of the mixer, the material is forced into the mixer by the floating weight with the pressure [P.sub.st]. To move through the rotors, it has to change from the width [H.sub.3] to [H.sub.2] to [H.sub.1]. Besides the pressure of the floating weight, this is done by the shearing stress [[tau].sub.w] at the rotor surfaces. Rubber has a distinctive temperature related yield point. The coefficient of friction is temperature and lubricating film related. If the rotors are too cold, the coefficient of friction (steel/rubber) will drop and slippage occurs between the rubber and the rotors. If the rotor temperature is too high, a lubricating film will build up and slippage occurs between the rotors and the rubber. Either way, the feeding time will increase, as well as the ram setting time for the mixer. To measure the ram setting time, the mixer was filled with rubber bales and the time was measured to move down the ram into final position.

The ram setting time for both rotor speeds is more than 100 seconds if the rotor temperature is 30[degrees]C/86[degrees]F. The ram setting time decreases to about half if the rotor temperature is 60[degrees]C/140[degrees]F. No significant improvements can be seen for a rotor temperature of 90[degrees]C/194[degrees]F, and it has to be assumed that due to slippage, the ram setting time will increase for even higher rotor temperatures.

Rotor design, monobloc (single casting) versus two-piece design

Today, there are several hundred tangential 270s in the tire industry. The main reasons to operate tangential mixers in the tire industry are the fast feeding rates, the discharge characteristics and the standardization over years of 270 mixers in the industry in general. As mixer design evolved to the current common 270-liter size, the rotors were lengthened by nearly 10% (from 32" to 35"), thereby becoming slimmer. The rotors are manufactured as castings in the monobloc style. The rotors consequently have relatively thick walls due to the casting process itself, and also to resist the heavy forces during the mixing process. This construction with relatively thick walls and a spray cooling system inside has limited cooling efficiency.

Today, there are several hundred tangential 270s in the tire industry. Today's restrictions of the monobloc construction have been overcome by a completely new manufacturing technique. These manufacturing techniques have been used successfully for more than a decade for intermeshing rotors. One of the results of these new manufacturing techniques is the HESC rotor (high efficiency super cooling). The HESC rotor, with a wing ratio of 3:1, was specially designed for a four-wing rotor, and the shorter flights are on the opposite side. A major advantage of the HESC rotor is its conformity with standard full-four-wing rotors in terms of rotor geometry. The increased cooling capacity provides an additional parameter to improve mixing efficiency by improving mixing procedures. In general, this design can also be adapted to other rotor geometry and machine sizes. The new HD-SC two-piece tangential rotor is another example. The HD-SC is a high shear, four-wing rotor similar to other standard rotors in the industry with a 4:1 wing ratio, but with the advanced cooling due to the two-piece design.

Investigation of HESC rotor cooling capacity

In a previous study (ref. 3), the cooling behavior of the HESC rotor was investigated. The rotors were heated with hot water, and after a waiting period of about one hour, they were cooled rapidly with cold water. For this procedure, two temperature control units (TCUs) were connected to the rotors, one with a temperature of 90[degrees]C/194[degrees]F and a second with an adjusted temperature of 18[degrees]C/64.4[degrees]F.

The distribution of the surface temperature was recorded for each type of rotor during the cooling phase utilizing infrared thermography. After a cooling time of five minutes, the HESC rotor had cooled down uniformly to a temperature at the surface of about 20-30[degrees]C/68-86[degrees]F, while the conventionally configured rotor cooled down unequally, with the heat retention greatest at places inside the wings, where the wall is very thick.

To investigate the cooling effect of the rotors during production, a measurement of the cooling capacity was done. The cooling water flow through the rotors and the inlet and outlet temperatures were measured. Using these values, the cooling capacity was calculated as follows:

Q = V x [rho] x cp x ([T.sub.Outlet] - [T.sub.Inlet]) In this equation, Q is the heat flux out of the two rotors, V is the volume flow of the coolant through the rotors and the two physical parameters [rho] and cp are the density and specific heat of the coolant. The temperature difference between outlet and inlet is characterized by the term ([T.sub.Outlet] - [T.sub.Inlet]).

A comparison of the cooling efficiency in the production shows that the HESC rotor has more than double the cooling effect as the conventional rotor design (ref. 3). Due to the improved cooling rate and the more uniform surface temperature, HESC rotors are therefore not only better suited for the production of silica compounds, but also for carbon black formulations in general.

Experimental

Recipe

For the test series of this investigation, three different tough final batch compounds with Mooney values ML (1+4), 100[degrees]C/212[degrees]F of approximately MU ~ 70 (Recipe C) were used. These compounds represent a wide range of different black compounds in the tire industry. In order to get a constant process, ten batches from each compound were produced. Furthermore, a silica compound (compound D) was investigated.

Mixing equipment and mixing procedure

The experiments were carried out on a typical 270-liter tangential mixer, often used in the tire industry, having a net chamber volume of 257 liters (ref. 1). This machine was equipped with standard full-four-wing rotors. A comparison between the old rotor design and the new HESC rotors was made on an internal mixer which was equipped with the old rotor system (standard full-four-wing), and later on one rebuilt with HESC rotors. The adjustments of the machine, for example the cooling temperature, the ram pressure and the mixing process, were kept constant in both experimental series. Both sets of rotors were operated with even speed.

A data acquisition system was installed at the internal mixer. The most important mixing parameters of rotor speed, power consumption, ram position and batch temperature were recorded during the trials.

For temperature control of the compound during the mixing process, the machine was equipped with three different temperature control systems. During the experiments, a coolant temperature of 30[degrees]C/86[degrees]F was set for all circuits of the machine.

With a hydraulic hopper system, constant ram pressure was realized during the mixing process. Four hydraulic cylinders were responsible for the movement of the ram in order to achieve the maximum ramp pressure of 5.1 bar/74 psi on the batch. The mixing procedure of the black compounds A, B and C was almost the same in each case, and can be described as follows: a) load MB + cure system + accelerators; b) mixing for x s; c) ram lift; d) mixing for y s; e) ram lift; f) mixing up to z [degrees]C; and g) dumping. Note that x, y and z are variable parameters for each compound.

A conventional mixing procedure was used for the silica compound D. After a mastication phase, where the polymer was fed into the mixer together with small chemicals, the silica, carbon black and zinc oxide were added. The fillers were mixed for 30 seconds.

This article will be continued in next month's Rubber World (October 2005), where results and conclusions of this experimental will be examined.

Fabian A. Schmahl, Andreas Limper and Harald Keuter, HF Rubber Machinery