Abstract:
A method and apparatus for producing continuous belts is disclosed. The belts are formed from plastic films and are used as transfer belts in electrographic printers and copiers. The ends of the film are welded together by abutting their front faces and the ends are held together under pressure while being heated by radiation to a temperature to cause welding of the ends.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a method and to a device for manufacturing an endless band of plastic for an intermediate carrier band in an electrographic printer or copier. 
     2. Description of the Related Art 
     Intermediate carrier bands are utilized in electrographic printers or copiers in order to generate latent electrostatic images and/or to offer a carrier for toner images to be transfer-printed. For example, an endless band with a photoconductive layer, for example an OPC band (Organic PhotoConducting material) is employed as intermediate carrier band, this forming a corresponding electrostatic charge image, what is referred to as a latent charge image, by being exposed according to a predefined image pattern. This latent charge image is then inked with toner material in a developer station; later, this toner image is transferred onto paper or some other recording medium and is fixed thereon. 
     An endless intermediate carrier band can also serve as transfer band for collecting toner images and conveying these to a transfer printing location. Given, for example, a multi-color printing, a first toner image of a first color is transferred onto the intermediate carrier band. Subsequently, a second toner image with a second color is transferred onto this first toner image, etc. The multi-colored toner images on the intermediate carrier band superimposed on one another in this way are then conveyed to a transfer printing station and transferred onto the recording medium thereat and fixed. 
     Ends of a plastic film must be connected to one another for manufacturing an endless intermediate carrier band. The weld that thereby arises can be the cause of numerous disruptions. For example, a thickening along the weld leads to increased wear due to circulation of the intermediate carrier band. Moreover, the material properties can have been changed in the region of the weld, so that this region can generally not be used as a photoconductive region or as a region for the acceptance of a toner image. 
     German Patent Document DE 19 832 168 A1 discloses a method and an apparatus for welding thermoplastic synthetics using laser light. The ends of a thermoplastic plastic film are arranged abutting and can be held with the assistance of a retainer elements and a silica glass plate. Laser light is coupled in via the silica glass plate, as a result whereof the ends of the film are welded to one another. Special measures for producing a uniform weld are not disclosed. 
     German Patent Document DE 19 516 726 A1 discloses a method for shaping and closing a folding box, whereby plastic layers for packing a welded to one another upon employment of radiation. The welding process is promoted by applying pressure. 
     German Patent Document DE 3 713 527 A1 discloses the welding of plastic parts whose ends are place flush against one another. The plastic parts are provided with profiles at their ends, so that these profiles can engage in one another. The ends with the profiles are then welded to one another with the assistance of a laser welding device. 
     European Patent Document EP-A-0 705 682 discloses a method for the thermal joining of substrates of polymers, whereby at least one substrate is coated with a medium that absorbs microwaves. The substrates are then welded to one another in a microwave field. 
     Internet information of the EWi WELDNET company with the title “Hot Plate Welding”, obtainable under http:H/www.ewi.org/matjoin/plastics/ttir.html; http://www.ewi.org/matjoin/plastics/hotplate.html; http://www.ewi.org/matjoin/plastics/infrared.html, teaches that plastic parts be firmly joined to one another by means of a butt joint welding. A heating element is thereby placed between the contact surface of the parts to be welded, the surfaces lying opposite one another at their end faces, and the contact surface are heated up to the melting phase. Subsequently, the heating element is removed and the contact surfaces residing opposite one another are pressed against one another. A durable connection between the contact surfaces is provided after cooling. Such a method, however, has the disadvantage that a raised weld projecting out of the surfaces of the parts welded to one another arises at the contact surfaces. 
     An overlap welding method is disclosed under the titles “Through-transmission Infrared Welding TTIR” or, respectively, “Infrared Heating” in the aforementioned Internet information. The ends of plastic parts to be welded are thereby placed on top of one another and heated up to the melting phase by infrared radiation or laser emission from a radiation source that is not in contact with the parts to be welded. One application of this method is the welding of a part that is transparent for the radiation to a part that is impermeable to the radiation, whereby the welding arises in the region of the surfaces of the parts to be welded that lie on top of one another. A further application of this method is the welding of plastic films lying on top of one another at ends with the assistance of a thin intermediate layer. The method is not suitable for the manufacture of endless bands. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and a device for manufacturing an endless band of thermoplastic synthetic whose surfaces allows high usage given low wear. 
     This is achieved for a method for manufacturing an endless band of plastic for an intermediate carrier band in an electrographic printer or copier, whereby the ends of a thermoplastic plastic film that comprises at least the width of a standard printing format, have their end faces placed abutting one another, the ends of the plastic film are heated by radiation to a temperature required for the welding, and whereby a respective pressing surface is arranged at both sides of the ends, the length of the pressing surface at least corresponding to the width if the plastic film and this pressing the surfaces of the ends against one another such that, when the plastic material of the end faces of the ends residing opposite one another melts, the spacing of the pressing surfaces defined by the thickness of the cold plastic film is preserved. 
     According to the invention, the pressing surfaces have a spacing from one another that is defined by the thickness of the cold plastic film. When the adjoining ends of the plastic film are heated, the molten mass cannot become thicker than prescribed by the spacing of the two pressing surfaces from one another. The cold weld along the entire width of the plastic film therefore has the same thickness as the plastic film itself. The weld therefore has no raised shape and is subject to only low wear even given a high usage of the endless band as intermediate carrier band in a printer or copier. As has been shown in practice, the weld is so uniform that this region can be fully utilized as a functional surface given the function as an intermediate carrier band. As, for example, a photoconductive intermediate carrier band, the region of the weld can be coated with a photoconductive layer within which latent image structures form due to exposure. This region of the weld can likewise be utilized given employment of the endless band as an intermediate carrier band for the transfer of toner images, for example for superimposed toner images as well. It thus follows that an endless intermediate carrier band manufactured in this way can have its surface fully utilized, as a result whereof its overall length can be short and further design advantages derive in the structuring of the printer or copier. 
     The plastic film has at least the width of a standard printing format, i.e. at least the width of a DIN A4 sheet. The pressing surfaces should also be correspondingly designed in terms of their respective length. As a result of the guidance by the pressing surfaces, it is possible to produce a uniform and functional weld along this relatively great width. The force exerted on the pressing surfaces is to be empirically determined. It is dependent on the type of plastic film, on the thickness of the plastic film and on the length of the weld to be produced. 
     According to a further aspect of the invention, a device is recited for manufacturing an endless band of thermoplastic plastic for an intermediate carrier band in an electrographic printer or copier. The advantages obtainable with this device agree with the advantages described for the method. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
       The invention is explained in greater detail below on the basis of exemplary embodiments according to the Figures of the drawings. 
         FIG. 1  is a schematic illustration of a first exemplary embodiment of an inventive device; 
         FIG. 2  is a schematic illustration of a second exemplary embodiment of an inventive device; 
         FIG. 3  is a schematic illustration of a part of an inventive device that can be additionally employed given the exemplary embodiments according to  FIGS. 1 and 2 ; 
         FIG. 4  is a schematic illustration of an inventive device according to  FIG. 1  with two absorption devices, as fourth exemplary embodiment; 
         FIG. 5  shows an absorption device composed of CrNi steel sheet having an absorption layer; and 
         FIG. 6  shows an absorption device with a transparent glass pane, a DLC layer and an anti-adhesion coating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Given the exemplary embodiment according to  FIG. 1 , a thermoplastic plastic film  10  is placed between a transparent mounting element, for example a glass pane  11 , and a transparent counter-mounting element, for example also a glass pane  12 , being placed such that the film ends have their end faces lying exactly blunt against one another. Pressing frames  13  and  14  are provided for fixing the plastic film  10  as well as for securing a smooth, non-raised weld, these exerting a prescribed force F onto the glass panes  11  and  12  and, thus, on to the plastic film  10 . The flat surfaces of the glass panes  11  and  12  lying against the plastic film  10  form planar pressing surfaces  11   a  and  12   a . Alternatively, these pressing surfaces  11   a  and  12   a  can also be concentric, for example cylindrical surfaces. The corresponding glass panes are then elements of generated cylindrical surfaces. In a direction perpendicular to the paper plane, the plastic film has a width of at least the width of a standard printing format, for example DIN A4. The glass panes  11  and  12  have a length that is greater than this width. 
     Via radiation-conducting fibers  15  and  17  as well as focussing optics  16  and  18 , radiation is supplied from radiation sources (not separately shown) for heating the plastic film  10  beyond the melting point in the region of the adjoining film ends. A weld  19  arises between the film ends as a result thereof. The radiation sources are preferably laser radiation sources, for example instance diode lasers, solid-state lasers, gas lasers or laser diode arrays. Dependent on the absorptivity of the material of the plastic film  10 , a specific part of the radiation is absorbed and converted into heat. The pressing surfaces  11   a  and  12   a  have a spacing from one another that is determined by the thickness of the plastic film  10  in its cold condition. This spacing is preserved when the ends of the plastic film  10  are heated and they melt, i.e. the force F is selected correspondingly high. The molten material then distributes along the bluntly abutting ends with a thickness corresponding to this spacing. 
     As a result of simultaneous irradiation of the plastic film  10  from both sides via the focussing optics  16  and  18 , a uniform weld  19  can be achieved over the entire thickness of the plastic film  10 , which is especially advantageous given film materials with good absorbency. The counter-mounting element  12  is then composed of a material that is transparent for the radiation, for instance glass. This is particularly advantageous given film materials with a small penetration depth of the radiation that is less than half the film thickness. A noteworthy transmission part of the radiation is then no longer present. 
     For improving the quality of the welding process and for compensating film material fluctuations, it is also expedient to measure the temperature in the region of the weld  19 . In a control circuit, the temperature can then be kept constant at a defined value by modifying the radiation capacity. 
     In the exemplary embodiment according to  FIG. 2 , wherein identical elements are provided with the same reference characters, as in the other Figures, a radiation source is provided at only one side of the plastic film  10  as well as the mounting element  11  and the pressing frame  13 , the radiation source supplying radiation for the weld  19  via the radiation conducting fiber  15  and the focussing optics  16 . 
     Given a material of the plastic film  10  that is largely impermeable for the radiation and a transparent counter-mounting element  12 , a check is additionally implemented in this exemplary embodiment as to whether a gap is still present between the ends of the plastic film  10  to be welded. To this end, a radiation detector  20 , for example a photodiode, is arranged at that side of the plastic film  10  facing away from the irradiated side, the radiation detector  20  acquiring a radiation part that potentially passes through an existing gap. The radiation part that passes through is nearly zero only given an exact positioning of the ends of the plastic film. The exact positioning of the ends of the plastic film  10  can be implemented manually or automated, whereby the radiation part that passes through should be minimal. 
     In a further exemplary embodiment, the thickness of the plastic film  10  and the radiation delivered by the radiation sources  15  and  16  are matched such to one another that the optical penetration depth of the radiation is less than or equal to half the thickness of the plastic film  10 . It is thereby assured that sufficient energy can be supplied to the plastic film  10  in order to be able to correctly weld it. 
     For improving the efficiency, one of the mounting elements  11  and  12 , preferably the counter-mounting element  12 , can be fashioned to be reflective at the appertaining pressing surface  12   a . Transmitted radiation is then reflected back into the plastic film  10 . The mounting element  12  can, for example, be fashioned as a mirror or as a polished metal sheet, preferably a copper or aluminum sheet, or can comprise a reflection-coated material. 
     In the above-described exemplary embodiments according to  FIGS. 1 and 2  as well as in the exemplary embodiment according to  FIG. 3  that is yet to be described, the mounting element  11  and the counter-mounting element  12  can be provided with an anti-adhesion coating (not separately shown), for example Teflon or a hydrophobic DLC coating, at the side of the plastic film  10 . A sticking of the plastic film during the welding process is thus avoided. 
     In order to assure a qualitatively high-grade weld given the device according to  FIG. 2  with an irradiation from only one side, the plastic film  10  can also be turned over, so that an irradiation from both sides ensues successively. 
     In the exemplary embodiment according to  FIG. 3 , wherein identical elements are provided with the same reference characters as in  FIGS. 1 and 2 , an additional clamping of the plastic film  10  is provided with the assistance of a rigidly seated clamp element  30  and a movable seated clamp element  31 . The motion of these clamp element  30  and  31  for pressing the end of the plastic film  10  together is indicated with an arrow A. In other exemplary embodiments, both the clamp element  30  as well as the clamp element  31  can be movable seated. The quality, particularly the strength of the weld, can be improved by pressing together with the assistance of the clamp elements  30 ,  31 . 
     The inventive device with the plastic film  10  fixed in it can be moved past under the focussed radiation in a direction perpendicular to the plane of the drawing with a linear table. It is likewise possible to move the radiation across the film region that is to be welded and that is fixed in the inventive device. However, the radiation source, i.e. the radiation-conducting fiber  15  or, respectively,  17  as well as the focussing optics  16  or, respectively,  18  must then be fashioned movable. Further, the radiation can be deflected, for example with a galvanometer mirror, or can be expanded, for example with a linear optics. In the latter case, a simultaneous welding over the entire film width is possible. 
     In the exemplary embodiment according to  FIG. 4 , wherein identical elements are provided with the same reference characters as in  FIG. 1 , a respective absorption device  40  and  42  is additionally introduced between plastic film  10  and mounting element  11  and  12 . The absorption device  40  is located directly between the mounting element  11  and the plastic film  10  and forms the first pressing surface  11   a . The absorption device  42  lies directly between the counter-mounting element  12  and the plastic film  10  and forms the second pressing surface  12   a . That side of the absorption device  40  and  42  facing toward the radiation source  16  or, respectively,  18  respectively absorbs the emitted radiant energy and converts it into heat that is transmitted onto the ends of the plastic film  10  residing opposite one another and effects the welding. In this exemplary embodiment, thus, arbitrary thermoplastic material can be employed regardless of the respective absorptivity, for example completely transparent plastic film. 
     In a further exemplary embodiment according to  FIG. 5 , the absorption device  40  and  42  is composed of a thin metal sheet, for example CrNi sheet steel, that is arranged between the plastic film  10  and the mounting element  11  and  12 . That side of the metal sheet  52  facing toward the irradiated side can be roughened for improved absorption or can be provided with an absorbent coating  50 , particularly with black chromium or stove enamel. 
     In the exemplary embodiment according to  FIG. 6 , the absorption device  40  and  42  is composed of an absorbent layer  62 , particularly a hydrophobic DLC layer or a hard-aggregate layer, preferably respectively approximately 0.2-3 μm thick, on a transparent glass pane  60  serving as carrier. The glass pane  60  simultaneously assumes the function of the mounting element  11  or  12  (see  FIG. 4 ). The absorbent layer  62  can be additionally provided with an anti-adhesion layer  64 , particularly a DLC coating, Teflon or silicone, preferably having a thickness of approximately 0.5-3 μm, at its side facing away from the radiation. A sticking of the plastic film  10  during the welding process is thus avoided. 
     The inventive method and the inventive device can be generally applied for all thermoplastics. The employment of polyester, polycarbonate or polyamide is especially beneficial, potentially with absorbent additives for balancing the penetration depth of the radiation to be absorbed. Lampblack-filled polyamide or polycarbonate have thereby proven beneficial. The film thickness lies in the range from 50 to 200 μm. 
     The inventive method and the inventive device serve for the manufacture of endless photoconductor bands, what are referred to as OPC bands (organic photoconducting material), as well as transfer bands for electrophotographic devices. The weld is very uniform and has the same thickness as the plastic film. As a result thereof, it is also possible to employ the region of the weld as a latent image carrier or as a toner image carrier. An endless band manufactured in this way can therefore have a short length and the wear in the region of the weld is reduced.