Patent Publication Number: US-6990307-B2

Title: Device for transporting particles

Description:
FIELD OF THE INVENTION 
     The invention relates in general to an electrographic, electrostatographic or an electrophotographic printer or copier and specifically a device for the transport of particles, in particular marking particles, in a photographic element in such printer or copier. 
     BACKGROUND OF THE INVENTION 
     Typically, marking particle transport devices are used to transport developers from a storage area to a photographic element. The photographic element is usually an image carrier, which is stretched over one or several rollers, for example an endless band, which already has a latent image thereon. The developer is a mixture, composed of at least magnetic carrier particles and non-magnetic marking particles, in which the marking particles attach to the magnetic carrier particles through triboelectric charging. As soon as the non-magnetic marking particles attach to the latent image on the photographic element, the magnetic carrier particles are removed again. Finally, the image composed on the marking particles is transferred to a paper-like or endless print substrate, and the non-magnetic marking particles are fixed to such substrate in a fixing station through pressure and heat. 
     To transfer such a developer mixture to the substrate, typically a so-called magnetic brush is used, which is known in the state of the art. For example, in U.S. Pat. No. 4,357,103, such a magnetic brush developer device is described. The magnetic brush developer device shown there displays a non-magnetic cartridge, which situated near the photographic element and slowly rotates. The developer is transported to this cartridge. The developer attaches to the cartridge because there is a multi-polar magnetic roller inside the cartridge, which attracts the magnetic carrier particles and presses on the surface of the cartridge. In this specification, a further magnet is located opposite the photographic element, which reduces the magnetic field of the multi-polar magnet roller in the contact area of the developer and the photographic element and thereby eases the transfer of the developer. 
     In the European disclosure specification EP 929 006 A2, a cartridge containing a multi-polar magnet roller is shown, in which the cartridge and the multi-polar magnet roller rotate at the same time. In addition, there has also been added an electromagnet on the opposite side of the photographic element, which is controlled in such a way that the desired magnetic field is formed in the contact area between cartridge and photographic element. A comparable device can also be seen in the Japanese specification JP 57118269 A2. 
     In the Japanese specification JP 57078575 A2, two multi-polar magnet rolls are used, which are situated on both sides of a photographic element, in order to bring the magnetic developer on to the photographic developer. In this case, the multi-polar magnet rolls turn in an opposite direction to each other. 
     In order to gain more influence on the spatial distribution of the magnetic forces in a typical developer station or other device for the controlled transport of magnetic particles or mixtures of magnetic and non-magnetic particles, in order to stem or compress the developer for example or to remove it from a surface, it would be advantageous to have a stationary, i.e., constant in its strength, magnetic field. 
     If a force from the magnet roller is not wanted locally, then an additional magnet must be added. The field is then calculated from the absolute value of the vector totals of the magnetic inductions. This overlapping of a stationary magnetic field with a rotating multi-polar field is alternately constructive and destructive and therefore, the absolute value is not constant. It behaves in a similar way if the second magnet is a roller, which turns in an opposite direction. In this instance, it is not possible to create a stationary magnetic field. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is clear that there is a need for devices for the transport of particles in a developer station of a digital printer or copier. Therefore, the invention provides an improved device for the transport of particles, in particular, for the transportation of magnetic developers in a developer station of an electrographic, electrostatographic or electrophotographic printer or copier. 
     The rotation of the multi-polar magnetic rollers in the same direction has the following physical reasons. The potential energy E of a magnetic dipole, which is already directed toward the external field direction, is equal to the negative product of the absolute value of its dipole momentum m and the absolute value of the magnetic induction B. E=−|m||B|. The translative force F=−degree E=|m| degree |B| is determined with the gradients of the absolute value of the magnetic induction. If the absolute value |B| is constant at any given location (i.e. although the field vectors can still turn, they cannot alter their length), then a temporal constant magnetic field for aligned magnetic dipoles results. Permanent magnetized or magnetizable particles, as are found for example in the 2-component developer or magnetic marking particles such as toner, of an electrographic or electrophotographic copier or printer, are produced by such magnetic dipoles. Below, such powders that contain magnetic dipoles will be termed as developers. In the magnetic field, the magnetic particles are aligned in the direction of the magnetic field and are ordered in chain-like structures along the field lines. 
     Therefore, in a suitable configuration of the device, the magnet rollers are multi-polar magnet rollers with a sine magnetic profile. A rapidly rotating magnet roller is especially used with the SPD (small particle dry) development process of Eastman Kodak Company. This is a two-dimensional multi-pole including several transversal magnetized magnets, which are attached to the surface of a cylinder in such a way that the radial magnetization alternates harmonically around the circumference of the cylinder. That means that if the roller rotates, a stationary hall probe, which measures the radial or tangential component of the magnetic induction, shows a sine or cosine course. In this case, the absolute value of the magnetic field is independent of the turning angle (because of sin(Φ) 2 +cos(Φ) 2 =1), but decreases sharply with the distance from the axis. The force on an aligned magnetic dipole is always radial in the direction of the axis. 
     In an especially suitable configuration of the device, the rotation of the magnetic rollers is synchronized as the magnetic rollers each rotate around their axes that both have the same pole changing frequency, so that the absolute value of the total magnetic field of the two magnetic rollers is temporally constant. With only one magnetic roller, the force which an aligned magnetic particle experiences is only dependent on one single coordinate, namely on the distance from the axis. This force is always directed toward the axis of the magnetic roller. If the magnetic field of a rotating magnetic roller overlaps the fields of other magnets that are not synchronized, then the absolute value of the total magnetic induction is no longer constant at a given location. This means that the chain-like patterns along which the magnetic particles are arranged are no longer stable. 
     In contrast, by synchronizing the magnetic rollers, there is the possibility of creating a stationary magnetic field by overlapping the magnetic fields of several magnetic rollers. This field can have a considerably more complex spatial distribution than is possible with only one magnetic roller. 
     The dynamics of the developer can be controlled in an advantageous manner with such a stationary magnetic field. The spatial distribution of the field can be influenced by the end arrangement of the magnetic rollers, namely by the number of poles of the magnetic rollers used, the radius of the magnetic rollers, the maximum field strength of each magnetic roller and their relatives distances and turning angles to each other. Thus, even with two magnetic rollers, an attracting and a repelling region in the space between the rollers can be created. The configuration possibilities of the field, which controls the dynamics of the magnetized particles, can be expanded even further by adding further synchronized magnetic rollers. Therefore, in the scope of the invention, it is possible to synchronize a greater number than two magnetic rollers with each other. 
     Magnetic fields that overlap increase vectorially. The magnetic field is strengthened with equally aligned field vectors and weakened—or in extreme cases eradicated—with oppositely aligned field vectors. So that the absolute value of the vector totals is temporally constant, the vectors of the individual magnetic fields must have a fixed angle correlation. This can only be achieved if the magnetic rollers are synchronized, i.e. rotating in the same direction and with the same pole alternation frequency. For example, two magnetic rollers, A and B should be synchronized with each other, where magnetic roller A has 10 poles (alternating between 5 north and 5 south poles) and magnetic pole B has 4 poles. If both magnetic rollers rotate clockwise and magnetic roller A rotates at 1000 rotations per minute, a frequency of 10,000 pole alternations per minute follows for magnetic roller A. This means that magnetic roller B must turn at 2,500 rotations per minute in a clockwise direction. 
     In a further suitable configuration of the device, the second magnetic roller is set up as dual polar. Only configuring one of the magnetic rollers with two poles is done in order to get as near as possible to the developing zone with the limited range. How quickly the magnetic field decreases with the distance depends on the number of poles. The more poles that are arranged on one magnetic roller, the lower the range of the resulting magnetic field. The largest range of the combined magnetic field from the first and second magnetic field can be attained if one of the magnetic rollers is only set up with two poles. 
     In a further suitable configuration of the device, the first magnetic roller is eccentrically arranged inside a transport cylinder made of non-magnetic metal. By doing this, the developer can be run through areas of high and low magnetic force with only one magnetic roller, because the distance from the axis of the magnetic roller decreases the attractive power. 
     In a further suitable configuration of the device, the transport cylinder arranged around the first magnetic roller rotates with lower speed than the first magnetic roller. The higher rotation speed of the magnetic roller essentially contributes toward an improvement in the mixture of the developer, while the rotation speed of the transport cylinder is in contrast essentially responsible for adapting the transport speed of the developer in the direction of the photographic element. The developer has a speed relative to the transport cylinder, whereby the rotating magnetic field vectors force the magnetic particles or groups of magnetic particles to rotate around them. The pole changing frequency is selected so that in time, while the developer moves through the contact area with the photographic element, several pole changes occur. 
     In a further suitable configuration of the device, the temporally constant field of the amount of the whole magnetic induction of the two magnetic rollers shows a region, which acts repellently on the aligned magnetic dipoles, inside the transport cylinder arranged around the first magnetic roller tight under its casing surface. In this case, the developer experiences a force, which lifts it off the transport cylinder surface. This can be used to bring the developer in contact with the photographic element earlier on the one hand and therefore increase the size of the contact surface and on the other hand, to remove the developer from the transport cylinder after passing through the development zone through repulsion without mechanical contact. 
     In a further suitable configuration of the device, the temporally constant field of the amount of the whole magnetic induction of the two magnetic rollers shows a region, which acts repellently on the aligned magnetic dipoles, on the other side of the photographic element. In this case, there is an additional force, which repels the magnetized particles from the photographic element. This reduces an unwanted phenomenon, the so-called developer pick-up (DPU), where some magnetized particles remain stuck to the film. 
     In a further suitable configuration of the device, the temporally constant field of the amount of the whole magnetic induction of the two magnetic rollers shows a region, which acts repellently on the aligned magnetic dipoles, between the transport cylinder and the photographic element. This represents a magnetic obstacle for the developer. A logjam is caused. The size of the contact surface with the film can be increased in this way, which improves the transfer to the photographic element. In addition, the magnetic forces are reduced in the area of the magnetic minimum, so that the developer can mix more easily. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawing, in which: 
       Preferred embodiments of the device will be described in more detail below with reference to the illustration. The FIGURE shows the device in a schematic representation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the FIGURE, a preferable embodiment of the device  100  includes one transport cylinder  10 , which can be rotated and is stored ready to be used in contrast to a support cylinder  20 . Drive and/or operation means which are generally known about and are necessary for the operation of the device  100 , and cam discs and control aids are not shown or only described in a general sense to illustrate the method of action of the device  100 . 
     The transport cylinder  10  and the support cylinder  20  are arranged parallel to each other along the axis and have a narrow transfer gap between them. A flat, endless photographic element  1 , basically an image transfer band as is well known in state of the art digital printers and copiers, moves through this transfer gap in the direction of the arrow marked as  2  in the drawing. 
     Inside the transport cylinder  10 , a first magnetic roller  11  is arranged eccentrically but parallel to the axis, which shows a number of north poles  40  and south poles  50 ; in this example there are five north poles  40  and five south poles  50 . The first magnetic roller  11  shows a sine formed magnetic profile. The first magnetic roller  11  turns at speed in the direction of the arrow marked as  13  in the diagram, whereby the rotation speed is more than the speed of the transport cylinder  10 . In addition, the rotation direction  13 , as shown by the arrow in the FIGURE, of the first magnetic role  11  and the rotation direction  12  of the transport cylinder  10  are opposite to each other. The opposite rotation  13  of the first magnetic roller in terms of the rotation direction  12  of the transport cylinder  10  is not fundamentally necessary; it is enough if both movements result in a relative movement. The rotation direction  12  of the transport cylinder  10  fortunately has the same direction in the transfer zone as the movement direction  2  of the endless photographic element  1 . 
     Inside the support cylinder  20 , a second magnetic roller  22  is arranged eccentrically but parallel to the axis, which shows several magnetic north poles and south poles, but in this example is only arranged dual polar, i.e., exactly one north pole and one south pole. The second magnetic roller  22  turns at speed in the direction of the arrow marked as  23  in the diagram. The rotation speed of the second magnetic roller  22  is synchronized with the rotation speed of the first magnetic roller  11  and set in such a way that the first and second magnetic rollers  11 ,  22  show the same pole alternating frequency. In the example shown, this means that the second magnetic roller  22  turns at five times the speed of the rotation movement of the first magnetic roller. In addition, the rotation direction  23  of the second magnetic roller  22 , as shown by the arrow in the FIGURE, and the rotation direction  13  of the first magnetic roller  11  are the same. 
     In the gap between the transport cylinder  10  and the support cylinder  20 , the developer particles  30  come into contact with the photographic element  10 ; therefore this is termed in this field as the development zone. Here, the magnetic fields of the two magnetic rollers  11 ,  22  overlap. The second magnetic roller  22  is therefore situated on the reverse side of the photographic element, in order to be as close as possible to the development zone with the limited range of the magnetic fields. How quickly the strength of the magnetic field decreases with the distance depends on the number of the poles  40 ,  50  of the magnetic rollers in question  11 ,  22 : the more poles  40 ,  50  there are, the lower the range. Therefore the second magnetic roller  22  is only arranged with two poles. 
     Developer particles, having at least magnetic carrier particles and non-magnetic marking particles, move on the transport cylinder  10  from a storage area, which is not shown on the diagram but is well known to state of the art specialists, in the direction of the arrow marked with 31 toward the photographic element. 
     Chains, which are turned by the rotating magnetic field vectors, are caused by a concentrated amount of developer particles  30  that form with each pole alteration because of the overlapping magnetic fields of the first and second magnetic rollers  11 ,  22 . This leads to the developer particles  30  being mixed up. The developer particles  30 , which are situated on the surface of the transport cylinder slowly rotating in the direction of the arrow  12 , experience an additional relative movement opposite to the rotation direction of the first magnetic roller  11  owing to the rotating magnetic fields. 
     The marking particles, which were previously attached to the carrier particles triboelectrically, are electrostatically attracted by the photographic element  1  and transported out of the development zone by this through its forward motion. The magnetic carrier particles are, in contrast, transported back to the storage area on the surface of the transport cylinder. 
     The construction parameters of the individual magnetic rollers  11 ,  22  (number of poles, radius, maximum field strength) and the distance and turning angle relative to each other, as well as the location relative to the development area are adapted in such a way that a favorable magnetic field results for the developer transport. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
           
               
             
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 1 
                 Photographic element 
               
               
                 2 
                 Direction of motion of the photographic element 
               
               
                 10 
                 Transport cylinder 
               
               
                 11 
                 First magnetic roller 
               
               
                 12 
                 Rotation direction of the transport cylinder 
               
               
                 13 
                 Rotation direction of the first magnetic roller 
               
               
                 20 
                 Support cylinder 
               
               
                 21 
                 Rotation direction of the support cylinder 
               
               
                 22 
                 Second magnetic roller 
               
               
                 23 
                 Rotation direction of the second magnetic roller 
               
               
                 30 
                 Developer particles 
               
               
                 31 
                 Direction of motion of the developer particles 
               
               
                 40 
                 Magnetic north pole 
               
               
                 50 
                 Magnetic south pole 
               
               
                 100 
                 Device for the invention