Abstract:
A roll mill for comminuting and homogenizing viscous masses, in particular for dispersing and uniformly distributing solid particles suspended in a binding agent. The roll mill has at least two rolls pivoted around their longitudinal axes, wherein the rotational axis of the first roll is fixed in place, and the rotational axis of a second roll is movably mounted, as well as at least one pressing device for pressing at least one roll against the other roll. Roll pressing takes place by way of a mechanical-pneumatic pressing device, which has a force transducer and a pneumatic drive. The roll surfaces or process surfaces are made out of a metal-free ceramic material.

Description:
[0001]     This application is a continuation of International Application No. PCT/CH2005/000539, filed Sep. 12, 2005, which claims priority from German application 10 2004 052 084.4 filed Oct. 26, 2004, the entire disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a roll mill, in particular to a three-roll mill, for comminuting and homogenizing viscous masses, in particular for dispersing and uniformly distributing solid particles suspended in a binding agent. Such a roll mill has at least two rolls pivoted around their longitudinal axes, wherein the rotational axis of the first roll is fixed in place, and the rotational axis of a second roll is movably mounted. At least one roll is pressed against the other by means of at least one pressing device.  
         [0003]     Known devices for roll pressing include spindle drives or hydraulic pressing devices. A roll mill with a hydraulic roll pressing device is known from EP 0 151 997 B1.  
         [0004]     Spindle drives represent a costly and voluminous type of roll pressing device, in particular those intended to be automatic, and not manual.  
         [0005]     While hydraulic drives to constitute a relatively compact type of roll pressing device, operation is repeatedly accompanied by hygiene problems caused by exiting hydraulic fluid. This poses a problem in particular during the processing of pastes for cosmetic, pharmaceutical or nutraceutical applications, or the processing of pasty materials under clean room conditions.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of the invention is to provide a roll mill of the basic design mentioned at the outset that enables improved product quality, while averting the disadvantages of roll pressing via spindle drives or hydraulic drives on the one hand, and avoiding any metallic contamination of the product during its comminution on the other.  
         [0007]     This object is achieved in the roll mill of the present invention by using a mechanical-pneumatic pressing device as the pressing device, which represents a cost-effective solution that satisfies hygiene requirements, and by forming the roll surfaces or processing surfaces out of a metal-free ceramic material.  
         [0008]     According to the invention, the roll surfaces or processing surfaces are made out of a metal-free ceramic material, wherein the rolls preferably have a ceramic cylinder fit onto a hollow metal cylinder. This prevents the product from becoming metallically contaminated by roll abrasion in the comminuting process. This is particularly important while processing pastes for applications in electronics, and for the manufacture of insulating bodies based on fine ceramics.  
         [0009]     The mechanical-pneumatic pressing device preferably has a force transducer that acts on a moving journal bearing of the movable roll(s), and a pneumatic drive that acts on the force transducer. This combination of pneumatic drive and force transducer is particularly cost-effective. The force transducer intensifies the actuating power generated by the pneumatic drive. In this way, the high pressures necessary for roll pressing can be applied by the pneumatic drive without any problem. At the same time, the force transducer reduces the path traversed by the pneumatic drive, so that a much small distance is covered during roll pressing than for the pneumatic drive. This makes it possible to achieve a level of accuracy during nip adjustment that is far greater than a level of accuracy prescribed for the pneumatic drive. Obtained as a result is a high positioning accuracy of the movable roll(s) using relatively cost-effective means.  
         [0010]     The pressing device acting on the movable roll best has a first force transducer that acts on a first movable journal bearing of the moving roll, and a first pneumatic drive that acts on the first force transducer, as well as a second force transducer that acts on a second movable journal bearing of the moving roll, and a second pneumatic drive that acts on the second force transducer. As a result, the at least one moving roll can be pressed at both of its end points. This makes it easier to correct roll pressing given an unsymmetrical roll wear.  
         [0011]     As an alternative, the pressing device can have a force transducer and a pneumatic drive that acts on the transducer, wherein the force transducer acts on a forked device with two forked ends, wherein the first forked end acts on a first movable journal bearing of the moving roll, and the second forked end acts no a second movable journal bearing of the moving roll.  
         [0012]     This makes it possible to press the at least one moving roll using only a single pressing device for this roll.  
         [0013]     Possible force transducers include a lever arrangement, in particular a toggle mechanism or cam plate, as well as combinations of levers and cam plates. As an alternative, use can also be made of purely pneumatic force transducers, e.g., having a large piston and a small piston, which interact, or a gearbox can be used as the force transducer.  
         [0014]     In a particularly advantageous embodiment of this invention, the roll mill according to the invention is a so-called “three-roll mill” with three parallel rolls. The rotational axis of the middle roll is here fixed in place, while the rotational axis of the front roll or feeder roll and the rotational axis of the back roll or transfer roll are movable. To this end, it has a front mechanical-pneumatic pressing device for pressing the front roll against the middle roll, as well as a rear mechanical-pneumatic pressing device for pressing the back roll against the middle roll. This provides for two roll nips. In this way, the operating conditions for both roll nips can be independently adjusted by setting the nip distance, the differential velocity and the pressure in the respective nip.  
         [0015]     The rolls are best cooled from the inside. For example, this is important while processing organic pigments, in particular with respect to certain yellow pigments.  
         [0016]     Both the front roll and the back roll are pressed against the middle roll by means of a mechanical-pneumatic pressing device. This makes it possible to adjust the front and back roll nip. The mechanical-pneumatic pressing device preferably has a control means for setting the nip. Since the force transducer, as explained above, enables a “force transmission” and “path reduction”, the relatively weak force of a pneumatic device can be multiplied for purposes of roll pressing, while at the same time greatly increasing the accuracy of nip adjustment prescribed by the pneumatic device.  
         [0017]     The transfer roll is best abutted by a stripper that strips away the comminuted, homogenized mass, wherein the stripper also preferably consists of a metal-free material, in particular of a ceramic material or polymer material. This also prevents the product from becoming metallically contaminated in any way as the result of stripper abrasion while being stripped from the transfer roll.  
         [0018]     A tarpaulin preferably covers at least the feed area of the roll mill. This prevents undesired contaminants from the factory building from getting into the product and vice versa, i.e., undesired volatile product constituents form getting into the air of the factory building. This improves “product hygiene” on the one hand, and “workplace hygiene” on the other.  
         [0019]     The space under the tarpaulin is preferably connected with a gas vent. This makes it possible to keep volatile substances contained in the product solvent from getting into the air of a factory building.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Additional advantages, features and possible applications of the invention may be gleaned from the following description of exemplary embodiments of the invention, which are not to be regarded as limiting in any way, wherein:  
         [0021]      FIG. 1  shows a diagrammatic side view of a first exemplary embodiment of the roll mill according to the invention;  
         [0022]      FIG. 2  shows a top view of the rolls in the first exemplary embodiment on  FIG. 1 ;  
         [0023]      FIG. 3  shows a diagrammatic side view of a second exemplary embodiment corresponding to  FIG. 1 , and  
         [0024]      FIG. 4  shows a top view of the rolls of the second exemplary embodiment on  FIG. 3 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     In both the first and second exemplary embodiment, the roll surfaces or roll processing surfaces S 1 , S 2 , S 3  or S 1 ′, S 2 ′, S 3 ′ are made out of ceramic material. The stripper  9  shown on  FIG. 3  can also consist of ceramic material or polymer material. These or other metal-free materials for the roll processing surfaces and the stripper knife are of particular interest for processing pastes in the electronics industry.  
         [0026]     The ceramic rolls are rounded at the end of the roll processing length.  
         [0027]      FIG. 1  and  FIG. 2  show a first exemplary embodiment of the roll mill according to the invention. The three-roll mill shown here contains three rolls  1 ,  2 ,  3 , which are aligned parallel to each other, and all arranged in a single plane E. In other words, the rotational axis A 1  of the front roll  1 , the rotational axis A 2  of the middle roll  2 , and the rotational axis A 3  of the back roll  3  are parallel to each other (see  FIG. 2 ), and all lie in one and the same plane E. In the operating mode, the front roll  1  and the back roll  3  are each pressed by a front pressing device  4  or a back pressing device  5  against the middle roll  2 , the rotational axis A 2  of which is fixed in place. The front roll  1  and back roll  3  are movable, i.e., their rotational axes A 1  and A 3  can be pivoted around a swiveling axis D 3 . The jacket surfaces of rolls  1 ,  2 ,  3  each comprise the roll processing surface S 1 , S 2 , S 3 , with which the product to be processed comes into contact. During operation, the product passing between the rolls  1  and  2  pressed against each other creates a roll nip between the processing surface S 1  of the front roll  1  and the processing surface S 2  of the middle roll  2 . In like manner, the product passing between the rollers  2  and  3  pressed against each other creates a roll nip during operation between the processing surface S 3  of the back roll  3  and the processing surface S 2  of the middle roll.  
         [0028]     The front pressing device  4  and back pressing device  5  each have a force transducer  6  and a pneumatic drive  7 . In the first exemplary embodiment shown on  FIG. 1 , the force transducer is a toggle mechanism  6 , which consists of a first lever  6 A and a second lever  6 B, while the pneumatic drive  7  consists of a pneumatic cylinder  7 A and a pneumatic piston. The force exerted by the pressing devices  4  and  5  flows from the pneumatic piston  7 B, which is accommodated in the pneumatic cylinder  7 A, and linked with the first lever  6 A on an articulated axis D 1  by means of a piston rod  7 B. The first lever  6 A is hinged to a second articulated axis D 2  on the second lever  6 B, which in turn is hinged to a pivoting axis D 3 , and forms a respective suspension and mounting arrangement for the front roll  1  and back roll  3 .  
         [0029]     Depending on how the levers  6 A and  6 B are dimensioned and oriented, the toggle mechanism  6 A,  6 B used as the force transducer  6  and roll suspension unit increase the pneumatic force of the pneumatic drive  7  by a factor of about 20 to 50, wherein this increased force is used for purposes of roll pressing. This enables a sufficiently strong roll pressing, even with a pneumatic drive  7 . On the other hand, this force transducer  6  decreases the stroke traversed by the pneumatic drive  7  by a factor of about 1/50 to 1/20, wherein this reduced stroke is used to set the nip.  
         [0030]     Rolls  1 ,  2  and  3  are driven by overdrives or gearboxes by engine M. The roll block  1 ,  2 ,  3  and engine block M are enveloped by a casing G.  
         [0031]      FIG. 2  shows a top view of the rolls  1 ,  2 ,  3  of the first exemplary embodiment of the roll mill according to the invention shown on  FIG. 1 . As evident, the front roll or feeder roll  1  and the middle roll  2  both have the same processing length L 1 =L 2 , while the back roll or transfer roll  3  has a distinctly shorter processing length L 3 &lt;L 2  to avoid undefined edge effects. The back roll  3  is axially arranged relative to the middle roll  2  in such a way that the ends of the processing length L 2  of the middle roll  2  extend axially over the ends of the process length L 3  of the back roll  3  on both sides. This ensures that unabraded or only inadequately abraded product does not pass from the middle roll  2  to the back roll  3  during roll mill operation, making it possible to achieve a distinctly improved product quality.  
         [0032]      FIG. 3  and  FIG. 4  show a second exemplary embodiment of the roll mill according to the invention.  
         [0033]     All elements of the second exemplary embodiment shown on  FIG. 3  and  FIG. 4  that are identical to the elements of the first exemplary embodiment shown on  FIG. 1  and  FIG. 2  or correspond thereto bear the reference numbers of the corresponding element from  FIG. 1  or  FIG. 2  with a quote mark added. How these elements of the second exemplary embodiment work will not be explained again here. In addition the front pressing device  4 ′ and the back pressing device  5 ′ with their respective force transducer  6 ′ and pneumatic drive  7 ′ are shown only diagrammatically.  
         [0034]     The other reference numbers on  FIG. 3  and  FIG. 4  that show elements of the second exemplary embodiment that deviate from the first exemplary embodiment do not bear the quote mark. Their function and importance will be explained below.  
         [0035]     The essential difference between the first exemplary embodiment ( FIG. 1  and  FIG. 2 ) and the second exemplary embodiment ( FIG. 3  and  FIG. 4 ) is that the three-roll mill depicted here has three rolls  1 ′,  2 ′,  3 ′ which, while aligned parallel to each other, are not all arranged in the same plane. Rather, the rotational axis A 1 ′ of the front roll  1 ′ and the rotational axis A 2 ′ of the middle roll  2 ′ are arranged in a first plane E 1 , while the rotational axis A 3 ′ of the back roll  3 ′ and the rotational axis A 2 ′ of the middle roll  2 ′ are arranged in a second plane E 2  that forms an angle γ of about 45° relative to the first plane E 1 . As a result of arranging the three rolls  1 ′,  2 ′,  3 ′ in this way, the product present as a viscous mass with the solid particles (e.g., pigments) distributed therein can be cooled for a longer period of time, and hence more intensively, than in an arrangement in which the rotational axes of the front, middle and back roll lie in a single plane.  
         [0036]      FIG. 4  is a top view of the rolls  1 ′,  2 ′,  3 ′ of the second embodiment of the roll mill according to the invention shown on  FIG. 3 . Here as well, the front roll or feeder roll  1 ′ and middle roll  2 ′ both have the same processing length L 1 ′=L 2 ′, while the back roll or transfer roll  3 ′ has a distinctly shorter processing length L 3 ′&lt;L 2 ′. The back roll  3 ′ is also axially arranged relative to the middle roll  2 ′ in such a way that the ends of the processing length L 2 ′ of the middle roll  2 ′ extend axially over the ends of the processing length L 3 ′ of the back wall  3 ′ on both sides. As already explained, this ensures that no unabraded or only inadequately abraded product gets from the middle roll  2 ′ to the back roll  3 ′ during operation of the three-roll mill, thereby improving product quality.  
         [0037]     The path traversed by the product as it passes through the roll mill according to the second exemplary embodiment is increased by the two additional circular arc lengths at the surfaces S 2 ′ and S 3 ′ of the roll  2 ′ and  3 ′ with radius R that arise between plane E 1  and plane E 2  as the result of angle γ, i.e., an additional path relative to the first exemplary embodiment ( FIG. 1 ) by 2×γ×R.  
         [0038]     A transfer funnel or product trough  8  with stacking wedges extending from the introduction region on either side is arranged over the area of the introduction nip between the front roll  1 ′ and the middle roll  2 ′. As the result of the stacking wedges provided in addition to the conventional wedge gaskets, this product trough increases tightness, thereby ensuring a lower lateral product loss.  
         [0039]     A stripper  9  with a stripping knife is used for removing the product from the back roll  3 ′. The stripper  9  is equipped with an automatic knife adjustment, which is actuated from an SPS controller.  
         [0040]     The pneumatic drive  7  operates at pressures of up to  4  bar, for example, which are brought to bear via the force transducers  6  on the required line pressures in the roll nips. The force transducer  6  make sit possible to increase the pressing force exerted on the rolls by the roll pressing devices  4 ,  5  by a factor of about 10 to about 80. Accordingly, the reduction in the stroke prescribed by the pneumatic drive  7  via the force transducer increase the nip setting accuracy by the same factor.  
         [0041]     The rolls have a diameter of 300 mm, and the back roll  3 ,  3 ′ is about 4 mm to 5 mm shorter than the middle roll  2 ,  2 ′. As a result, the stripper  9  only strips abraded product from the back roll  3 ′.  
         [0042]     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited but by the specific disclosure herein, but only by the appended claims.