Abstract:
A device for enhancing the transport of sheets in a printing machine is constructed upstream of the image transfer station to flatten the paper against the transport belt or drum on which it is riding. An air cushion is generated by expelling pressurized air through an array of holes to provide an air bearing for the sheets as it travels in its transport path. The air cushion is designed to flatten the paper against the belt or drum.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to a color or monochrome reprographic printing system, and more particularly concerns apparatus for delivering the paper or other media to a photoconductive surface, offset printing roll, etc. 
     In an electrophotographic printing machine, a photoconductive member (often a photoreceptor belt) is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is treated with toner particles and is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration. 
     Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated in a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. Alternately, a plurality of images may be superimposed on the photoreceptor surface, and transferred simultaneously to the sheet. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material. 
     Surface irregularities in the paper may be occur as the paper (copy media) travels through the machine. Such irregularities may be caused by the various rollers and belts which carry the paper through the printing or other machine processes. For example, irregularities could occur as side one is fused before transferring an image to side two, or because of the characteristics of the paper stock due to manufacturing, humidity, etc. In addition, misalignment, excessive force, etc. may create localized deformities in the copy paper. As a result, air gaps may form between the paper and the photoreceptor belt. Such gaps result in poor transfer of toner from the belt to the paper, which may, in turn, cause deletions or distortions in the printed copy. 
     It is a purpose of this invention to improve the delivery of the paper to the photoreceptive belt and throughout the printing machine to minimize the formation of surface deformatities, such as wrinkles, etc. and thereby improve the engagement of the paper to the photoreceptive belt. It is another purpose of this invention to accomplish the above purpose without adding further media-contacting elements to the print media path. It is a further purpose of this invention to enable self-alignment of the handling means to accommodate media of various thicknesses and to provide means to apply uniform forces to sheets with deep texture. 
     U.S. Pat. No. 5,913,268, which issued in 1999 to the same assignee of this application, discloses a roller element for use in a copy machine. The disclosure of this patent is incorporated herein by reference. The roller described in the &#39;268 is a pneumatic roller having an internal chamber which is selectively connected to a source of fluid under pressure or a vacuum. Through holes in the exterior periphery of the roller, copy paper, which is being handled by the roller, may be selectively subjected to pressure or vacuum. This is said to improve the paper handling performance of the transport system. It is a purpose of this invention to adapt the pneumatic roller concept to enhance the delivery of copy paper to an image transfer station. 
     SUMMARY OF THE INVENTION 
     In the method and apparatus of this invention, a bearing surface of constant shape is constructed to provide a cushion of air between the print medium and the moving supporting surface. The air cushion causes a downward pressure to hold the medium to the transport element, i.e. a belt or drum, as the media is moved towards the image transfer station or other process station of the printing machine. The air bearing surface is positioned just upstream from the image transfer station to minimize distortion of the media as it is transferred to the electrostatic belt or drum. 
     To accomplish this, a housing is constructed just upstream of the image transfer station, adjacent the transport element. The housing contains a plenum for storing air under pressure. The housing has an exterior wall, the outer surface of which faces the path of the copy paper to form a bearing surface. The bearing surface is constructed to conform to the shape of the transport element. For example, a belt would generally require a flat bearing surface and a drum may require a slightly arcuate bearing surface. 
     The plenum is connected to a source of pressurized air through a series of valves. Sensors within the plenum monitor the pressure in the plenum and control the supply of air through the valves. An array of holes are constructed in the exterior wall to allow streams of pressurized air to escape to lubricate and form the bearing surface. The streams of pressurized air form a cushion outside the exterior wall to separate the copy paper from the wall and generate an overall pressure against the paper to encourage its flat engagement with the transport element. A flat, well aligned, presentation of the copy paper to the image transfer station may then be accomplished without supplementary transport elements which rely on physical contact. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention is described in more detail below with reference to the attached drawing in which: 
     FIG. 1 is a schematic diagram of the air bearing transport element of this invention; 
     FIG. 2 is a schematic diagram of a first alternate embodiment of the air bearing transport element of this invention; 
     FIG. 3 is a schematic diagram of a second alternate embodiment of the air bearing transport element of this invention; and 
     FIG. 4 is a schematic diagram of a third alternate embodiment of the supplementary transport element of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the schematic diagram of FIG. 1, an air bearing transport element  1  is depicted as part of a paper transport system of a copy machine. This portion of the transport system could be just upstream of an image transfer station at which paper alignment and flatness is critical. The air bearing transport element  1  could also be useful immediately prior to the heating station at which the paper is heated by the fuser roll. This invention could also be used in printers and other paper handling environments. Although it will be described in the context of handling copy paper in a copier, it would be equally advantageous in the handling of other copy or print media. 
     The transport element  1  is positioned adjacent to the path of copy paper  2  at a point where copy paper  2  is being transported into or through image transfer station  3  on transport belt  4 . As shown in FIG. 1, air bearing  1  is positioned partially over support roller  5  to engage the paper prior to contact with the belt  4 . In this case belt  4  is a photoreceptor element and contains the toner image  30 . Transport belt  4  is mounted for movement on driven rollers or pulleys  5  and  6 , as shown by arrows  7  and  8 . Transport element  1  is constructed of a housing  9  in which a plenum  10  is defined by side walls  11  and  12  and exterior wall  13 . An air diffuser  14  acts as a cover to seal the plenum and a bracket to support fan  16  which is driven by motor  15 . The housing is resiliently supported by the frame of the copy machine  17  through springs  18  and  19  or other resilient elements which are capable of reducing vibration and allowing alignment of the housing  9  with transport belt  2 . 
     Pressurized air may be supplied by a fan  15 , as shown, or other suitable blower or compressor. The source of pressurized air charges plenum  10  during operation of the copy machine. An array of openings  20  are constructed in the exterior wall  13  of the housing  9  to allow air to escape from plenum  10  and coat the exterior surface  21  to form a bearing surface  22 . Bearing surface  22  spreads between the paper  2  and the exterior surface  21  of housing  9  to force the paper  2  towards belt  4 . This action tends to flatten the paper  2 , as it makes initial contact to the photoreceptor or transfer roll of an offset press, as shown in FIG.  1 . Such action will also tend to hold it more firmly to the support surface of belt  4  as it moves towards the image transfer station  3 . The image formed by the toner on the photoreceptor belt is generally shielded from infiltrating air currents originating at the air bearing surface  22  by means of a skimmer. In addition paper  2  will block air from blowing directly on the toner image. An air current of significant velocity is thereby avoided that might dislodge the toner which is electrostatically bonded to the photoreceptor. 
     Exterior surface  21  may be shaped to conform to the opposing surface of transport element  4 . This would be advantageous in the case of the transport element  4  being a drum instead of the belt as shown. 
     An appropriate control system for the air supply may include a release valve  24  and a sensor  25  to prevent the accumulation of excessive pressure. In addition the pressurized air supply could be located remote from the housing  9  and communicate through a flexible hose. Air flow would be controlled by a valve which would be opened and closed by signals from the main copier process controller or by feeding back to a voltage controllable pressure source. 
     In a first alternate embodiment, as shown in FIG. 2, a device, i.e. corotron  23 , for electrostatically charging the copy medium to assist in the transfer of toner is mounted in housing  9 . 
     In a second alternate embodiment, at least a portion of the array of holes  20  are canted forward at an angle x of about 45° to the direction of paper travel. This generates components of force which tenter or stretch the sheet, thereby, tending to further flatten any buckles. Upstream of the position where the sheet  2  contacts the photoreceptor, the canted holes  20  assist sheet transport without the need to use driver rollers. 
     Another alternate embodiment is shown in FIG.  4 . In this system a double surface of air is generated by positioning a second air bearing plenum  26  under plenum  10  and transporting the paper between the two bearing surfaces. A fan  28  or other source of pressurized air supplies air to plenum  26 . Holes  27  in plenum  26  allow air to escape and form a support surface for sheet  2 . The dual air bearing surface is positioned before sheet  2  makes contact with the photoreceptor. This configuration provides enhanced flattening of sheet  2  while fully supporting sheet  2  without physical contact. This is particularly useful when a previously toned image exists on the top side of sheet  2 .