Abstract:
An image forming apparatus is provided in which a printhead structure is held in contact with the outer surface of a toner carrier, the contact force therebetween being maintained constant and relatively low, notwithstanding mechanical variations of the toner carrier. The contact surface between the toner carrier and the printhead structure is preferably located in the vicinity of the apertures in order to ensure a constant gap distance between the apertures and a toner layer carried on the outer surface of the toner carrier.

Description:
RELATED APPLICATIONS  
       [0001]    The present application is a continuation of International Application No. PCT/SE98/02296, filed on Dec. 11, 1998, which designates the United States Patent and Trademark Office as a designated office and as an elected office, and the present application claims the benefit of priority under 35 U.S.C. §120 of International Application No. PCT/SE98/02296, which is incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention is within the field of electrographical printing devices. More specifically, the invention relates to an image forming apparatus including an apertured printhead structure brought into cooperation with a particle source to modulate a stream of toner particles from the particle source through the apertured printhead structure. The invention further relates to improved means for accurately positioning the printhead structure in relation to the particle source to maintain a constant gap distance therebetween.  
           [0004]    2. Description of the Related Art  
           [0005]    U.S. Pat. No. 5,036,341 granted to Larson discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. The Larson patent discloses a method in which an electrode matrix, arranged between a back electrode and a rotating particle carrier, generates a pattern of electrostatic fields which, due to control in accordance with an image information, modulate a transport of toner particles toward the back electrode. An electrostatic field on the back electrode attracts the toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected individual control electrodes to create electrostatic fields which either permit or restrict the transport of toner particles from the particle carrier through the electrode matrix. In effect, these electrostatic fields “open” or “close” either permit or restrict the transport of toner particles from the particle carrier through the electrode matrix. In effect, these electrostatic fields “open” or “close” selected apertures in the electrode matrix to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged toner particles allowed to pass through the opened apertures impinges upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.  
           [0006]    An electrode matrix for use in direct electrostatic printing devices may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or a screen-shaped, apertured printed circuit. Generally, the matrix is formed of a thin, flexible substrate of electrically insulating material, such as polyimide, provided with a plurality of apertures and overlaid with a printed circuit of control electrodes arranged in connection to the apertures, such that each aperture is surrounded by an individually addressable control electrode.  
           [0007]    An essential requirement of the aforementioned method is to maintain a constant, uniform gap distance between the control electrodes and the toner particle layer on the particle carrier. The gap distance can vary from machine to machine because it is determined by a combination of independent factors such as manufacturing variations in the size and placement of the particle carrier and the electrode matrix, as well as the thickness of the toner layer on the particle carrier. Because the gap distance is only in the order of 10-30 microns, even the slightest mechanical imperfections can result in a drastic degradation of the print quality. For instance, the particle carrier is a rotating cylinder having a rotation axis which is not perfectly centered, or an outer surface which is neither perfectly round nor perfectly smooth, along which various surface imperfections cause variation in the gap distance. Further, the toner particles themselves may vary in their diameter and degree of sphericity, and the toner particle layer may vary in thickness along the surface of the cylinder. Thus, to accommodate all of these independent dimensional variations, there is a need for improved means for positioning a printhead structure in conjunction to a particle source.  
           [0008]    U.S. Pat. No. 5,552,814 discloses an apparatus comprising a biasing device by which corresponding portions of the particle carrier and the printhead structure which are adjacent to the apertures are biased against each other for contact therebetween. In such an apparatus, the printhead structure preferably comprises an elastic substrate made of electrically insulating material so that the control electrodes are formed on one surface of the substrate. The biasing device may include tensioning means for applying a tension to the printhead structure so that the printhead structure is held elastically curved along a part of the outer surface of the toner carrier such that the printhead structure is in pressing contact with the toner carrier. The tensioning means are used to apply a tension on the printhead structure in a direction of feed of the recording medium. A drawback associated with a biasing device such as that disclosed in U.S. Pat. No. 5,552,814 is that variations in the relative position between the toner carrier and the printhead structure considerably influence the contact force therebetween. For example, if the toner carrier is not perfectly centered about its rotation axis, the contact force will vary along the outer surface of the toner carrier, causing a degradation of the print uniformity. Moreover, the surface of the printhead structure facing the toner carrier is worn away or deteriorated by abrasion forces due to frictional contact with toner particles. This may result in higher friction coefficient and higher surface roughness of the printhead structure, which in turn degrades the toner layer supplied to the apertures.  
           [0009]    Therefore, there is still a need to improve means for positioning a printhead structure in conjunction to a toner carrier, so as to ensure a relatively low and constant contact force therebetween, that contact force being independent of mechanical imperfections or manufacturing variations of the toner carrier.  
         SUMMARY OF THE INVENTION  
         [0010]    It is therefore an object of the present invention to provide an image forming apparatus in which a printhead structure is held in contact with the outer surface of a toner carrier, the contact force therebetween being maintained constant and relatively low, notwithstanding mechanical variations of the toner carrier. The contact surface between the toner carrier and the printhead structure is preferably located in the vicinity of the apertures in order to ensure a constant gap distance between the apertures and a toner layer carried on the outer surface of the toner carrier.  
           [0011]    According to the present invention, the printhead structure comprises a sheet-like substrate of flexible material, such as polyimide or the like, having a substantially rectangular shape, a first surface facing the toner carrier, a second surface facing the recording medium, a longitudinal axis in the feed direction of the recording medium, a transversal axis extending perpendicular to the feed direction of the recording medium, a front part located downstream of the transversal axis with respect to a feed direction of the recording medium, a rear part located upstream of the transversal axis with respect to a feed direction of the recording medium, and a plurality of apertures aligned in at least one row extending preferably parallel with the transversal axis.  
           [0012]    Portions of the front and rear parts of the substrate are coated with a film of rigid material, so that the substrate has a variable flexibility along its longitudinal axis. Accordingly, the substrate has a first rigid portion on its front part, a second rigid portion on its rear part, and a bendable portion therebetween, in the following referred as free portion. One of said front and rear parts of the substrate is transversally fastened along a fastening line by means of a securing device, and the opposite part is supported at a supporting point by means of a supporting device. The fastening line and the supporting point are positioned on each side of the toner carrier in such an arrangement that the substrate is held arcuate along its longitudinal axis so that the free portion of the substrate which contacts the outer surface of the toner carrier has a curvature radius substantially equal to the radius of the toner carrier. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic view of an image forming apparatus according to the present invention,  
         [0014]    [0014]FIG. 2 a  is a schematic section view across a print station of the image forming apparatus of FIG. 1,  
         [0015]    [0015]FIG. 2 b  is an enlargement of a part of FIG. 2 a,  showing a print zone in a print station,  
         [0016]    [0016]FIG. 3 a  is a schematic plane view of a printhead structure in an image forming apparatus as that shown in FIG. 1,  
         [0017]    [0017]FIG. 3 b  is an enlargement of a part of FIG. 3 a,    
         [0018]    [0018]FIG. 4 is a schematic perspective view of a printhead structure and its position in a print station,  
         [0019]    [0019]FIG. 5 is a schematic section view of the printhead structure of FIG. 4, across the longitudinal axis thereof,  
         [0020]    [0020]FIG. 6 is a schematic view of conventional positioning of a printhead structure in an image forming device according to prior art,  
         [0021]    [0021]FIG. 7 is a schematic view of positioning of a printhead structure in an image forming device according to the present invention,  
         [0022]    [0022]FIG. 8 is an embodiment of the positioning of a printhead structure in conjunction to a toner carrier,  
         [0023]    [0023]FIG. 9 is another embodiment of the positioning of a printhead structure in conjunction to a toner carrier,  
         [0024]    [0024]FIG. 10 is another embodiment of the positioning of a printhead structure in conjunction to a toner carrier, in which spacer means are provided to maintain a constant gap distance therebetween,  
         [0025]    [0025]FIG. 11 is an embodiment of the present invention in which the printhead structure is arranged on a mounting frame. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    1. General Description of the Apparatus Referring to FIG. 1  
         [0027]    The present invention relates to an image recording apparatus such as that schematically illustrated in FIG. 1, in which an image receiving medium, such as an intermediate transfer belt  1 , is conveyed in a longitudinal direction (arrow D) successively past four print stations (Y, C, M, K), each corresponding to a specific toner color (generally yellow, cyan, magenta and black), to intercept a modulated stream of toner particles from each print station whereby the so obtained four image configurations are directly superposed onto the transfer belt  1 , forming a visible four color toner image. That toner image is subsequently brought into contact with an information carrier (P) in a transfer unit (TU), whereas the toner image is transferred to the information carrier and thereafter made permanent in a fusing unit (not shown).  
         [0028]    2. General Description of a Print Station Referring to FIGS. 2 a  and  2   b    
         [0029]    A print station includes:  
         [0030]    (a) a particle delivery unit  2  for conveying charged pigment particles to a position adjacent to the image recording medium  1 , said particle delivery unit  2  including a particle carrier  21 ;  
         [0031]    (b) a background electrode  3  positioned in conjunction with the particle carrier  21 ;  
         [0032]    (c) a background voltage source VBE for producing a background electric field between said background electrode  3  and said particle carrier  21 , which background electric field enables a transport of charged pigment particles from the particle carrier  21  toward the background electrode  3 ;  
         [0033]    (d) a printhead structure  4  positioned in said background electric field between the toner carrier  21  and the background electrode  3 , said printhead structure  4  including: a substrate  41  of electrically insulating material; a plurality of apertures  42  arranged through the substrate  41 ; and control electrodes  43  arranged in conjunction with the apertures  42 ;  
         [0034]    (e) variable voltage sources  5  connected to said control electrodes  43  to produce a pattern of electrostatic control fields influencing said background electric field in accordance with an image information for selectively permitting or restricting said transport of charged pigment particles through the apertures  42 ; and  
         [0035]    (f) a positioning device for positioning the printhead structure  4  in relation to the particle carrier  21 , including a fastening element  61  and a supporting element  62 .  
         [0036]    3. General Description of the Different Parts of the Print Station  
         [0037]    (a) The particle delivery unit  2  (as shown in FIG. 2 a ) includes a casing  20 , a partition  22  which separates the casing  20  into a toner container  23  for storing toner particles T, and a buffer chamber  24  for regulation of the amount of toner to be supplied. Provided in the toner container  23  is a rotating stirring member  231  for supplying toner from the toner container  23  to the buffer chamber  24 . The buffer chamber  24  includes a rotating, substantially cylindrical toner carrier  21  having a transversal rotation axis, a predetermined radius R and a peripheral outer surface on which a toner layer is formed by means of a supply element  211  for delivering toner to the toner carrier, and a regulation element  212  which ensures a uniform toner layer thickness.  
         [0038]    (b) The background electrode  3  (as shown in FIG. 2 a ) is a rotating, substantially cylindrical roller supporting the image transfer belt  1 . The background electrode roller  3  has a transversal rotation axis.  
         [0039]    (c) The background voltage source V BE  (as shown in FIG. 2 a ) produces an electric potential difference between the particle carrier  21  and the back electrode roller  3  for generating a uniform background electric field therebetween, which exposes the toner layer for an attraction force toward the background electrode roller  3  thereby enabling a toner transport through the selected apertures  42 . The electric potential difference can be in the order of +1500 V.  
         [0040]    (d) The printhead structure  4  (as shown in FIGS. 3 a  and  3   b ) includes a sheet-like substrate  41  of electrically insulating, flexible polymer material, such as polyimide or the like, having a substantially rectangular shape with a transversal axis  410  along the Y-axis and a longitudinal axis  411  parallel to the X-axis. The substrate  41  has a predetermined thickness, a first surface facing the particle carrier  21 , a second surface facing the background electrode roller  3 , a front part  413  located upstream of the transversal axis  410  with respect to the rotation of the particle carrier  21 , and a rear part  414  located downstream of the transversal axis  410  with respect to the rotation of the particle carrier  21 . The printhead structure  4  has a plurality of apertures  42  arranged through the substrate  41  and aligned in at least one transversal row, for example extending along the transversal axis  410 . The first surface of the substrate is overlaid with a first printed circuit including a plurality of control electrodes  43  each of which surrounds a corresponding aperture  42  in the substrate. The first printhead circuit is coated with a first cover layer (not shown) of electrically insulating material.  
         [0041]    According to a preferred embodiment of the present invention, the front and rear parts  413 ,  414  of the substrate  41  are at least partially laminated with a thin sheet of rigid material. A central portion  415  of the substrate  41 , located between the front part  413  and the rear part  414  remains unlaminated and thus flexible. The apertures  43  are arranged on the flexible central portion  415  of the substrate  41 . Accordingly, the substrate  41  has a variable flexibility along its longitudinal axis  411 , such that the central portion  415  can be given an arcuate shape following the curvature of the particle carrier  21 .  
         [0042]    (e) The variable voltage sources (not shown) are generally conventional IC-drivers supplying a stream of control voltage pulses, defining an image information, to the control electrodes for electrostatically opening or closing corresponding apertures in the substrate, so as to permit or restrict the influence of the background electric field on the toner layer through the opened apertures, and thus modulate a selective toner transport through the printhead structure toward the background electrode. The control voltage pulses have a magnitude and a pulse-width dimensioned to control the amount of toner particles allowed to pass through the corresponding aperture during a print sequence.  
         [0043]    (f) The positioning device (as shown in FIG. 2 a ) includes a fastening element  61  for securing the front part of the substrate to the particle delivery unit along a transversal fastening line, and a supporting element  62  for supporting the rear part of the substrate at a predetermined supporting point  620  on the longitudinal axis of the substrate.  
         [0044]    4. General Description of the Invention  
         [0045]    The position of the printhead structure  4  relative to the particle carrier according to a preferred embodiment of the present invention is schematically illustrated in FIG. 4 and FIG. 5.  
         [0046]    According to most prior art (FIG. 6), the printhead structure is conventionally positioned on a X-Y plane and arranged between two transversally extending fastening elements which maintains the substrate in a stretched state at a predetermined gap distance from the peripheral outer surface of the particle carrier. According to the present invention, a fastening element  61  extends transversally across the width of the substrate  41  at a predetermined distance Zf from the X-Y plane and at a predetermined distance Xf from the Y-Z plane. The securing element  62  is preferably disposed to support the substrate  41  at a predetermined position on its longitudinal axis  411 . The supporting element  62  is arranged at a predetermined distance Zs from the X-Y plane and at a predetermined distance Xs from the Y-Z plane. Front and rear parts  413 ,  414  of the substrate  41  are. made rigid, whereby only the central part  415  of the substrate  41  is allowed to be bent. Accordingly, the rigid parts of the substrate  41  will extend at angles a and b from the X-Y plane, which in turn results in a curvature of the central flexible part  415  of the substrate  41 . That curvature is determined by the distances Xf, Zf, Xs, Zs which are adapted to the radius R of the particle carrier  21 . Since the substrate  41  is supported at a single point, the whole printhead structure is allowed to pivot in both longitudinal and transversal direction in order to accommodate the mechanical variations of the particle carrier.  
         [0047]    5. Embodiments of the Invention  
         [0048]    Shown in FIGS. 7 and 8 is an embodiment of the invention in which the fastening and supporting elements  61 ,  62  are disposed so as to provide a contact area  63  between the peripheral outer surface of the particle carrier  21  and the first surface of the substrate  41 . The contact area  63  is located between the apertures  42  and the supporting element  62 , downstream of the apertures  42  with respect to the rotation of the particle carrier  21 . The curvature radius of the substrate  41  is made slightly larger than the radius R of the particle carrier  21 , which results in a uniform gap distance Lk between the apertures  42  and the peripheral outer surface of the particle carrier  21 . Since the substrate  41  is held in contact with the particle carrier  21 , the substrate  41  follows the shape of the particle carrier  21 , resulting in that the gap distance Lk remains constant and independent of undesired mechanical variations. The supporting element  62  maintain the substrate  41  in contact with the particle carrier  21  along a transversal contact line onto the peripheral outer surface thereof, and allows the substrate  41  to pivot in order to compensate both radial and transversal variations of the shape of the particle carrier  21 . Accordingly, the pivotability of the substrate  41  ensures a uniform gap distance Lk even when the particle carrier  211  is not perfectly centered, nor perfectly parallel to the X-Y plane.  
         [0049]    Shown in FIG. 9 is an embodiment of the invention, in which the curvature of the substrate  41  is dimensioned to provide a contact area  63  on both sides of the apertures  42 . The curvature radius of the substrate between the contact areas is slightly smaller than the radius of the particle carrier  21 , resulting in that a uniform gap distance Lk is formed between the peripheral outer surface of the particle carrier and the apertures.  
         [0050]    Shown in FIG. 10 is an alternate embodiment of the invention including spacer layers  71 ,  72  arranged between the substrate  41  and the particle carrier  21 . In such an embodiment the gap distance Lk is mainly determined by a predetermined thickness of a spacer layer  71  arranged upstream of the apertures  42  with respect to the rotation of the particle carrier. Preferably, the positioning device includes an upstream spacer layer  71  interposed between the front part  413  of the substrate  41  and the peripheral outer surface of the particle carrier  21 . Spacer layers  71 ,  72  may also be arranged on both upstream and downstream positions as exemplified in FIG. 10. The spacer layer  71  is preferably a removable sheet of flexible material, such as polyimide, having a predetermined thickness corresponding to an appropriate value of the gap distance Lk.  
         [0051]    A positioning device in accordance to the present invention may be constructed as shown in FIG. 11. A mounting frame  80  is arranged in a position parallel to the X-Y plane of FIGS. 4, 5. The mounting frame  80  has two longitudinal portions  81 ,  82  having elements  811 ,  812  for supporting the particle carrier  21 , a first transversal portion  83  on which the substrate  41  is fastened by the fastening element  61 , and a second transversal portion  84  in which a supporting element  62  is arranged. As shown in FIG. 11, the supporting element  62  is an adjustable pivot having a first end brought in contact with the substrate. The pivot can be adjusted, for example moved in a X-direction, in order to accurately optimize the contact point  620  on the substrate  41 , thereby even the curvature of the substrate  41 . In that example, the fastening element  61  may by a metal ruler saving a plurality of holes. An edge of the substrate  41  is interposed between the first transversal portion  83  of the mounting frame  80  and the ruler  61 .  
         [0052]    One of the advantages of such a construction, compared with conventional mounting frames, is that the substrate, being only fastened at one end, is allowed to vibrate during the print process. That vibration contributes to dislodge toner particles agglomeration from the apertures, ensuring a uniform printing without clogging.