Abstract:
An electrophotographic printing device with a developer unit and a photoconductor. The photoconductor is either directly connected to a substrate to be printed in the region of a transfer zone, or is connected by an intermediate circuit of one or several transfer media. At least one charger is provided for the substrate and the substrate may be transported through the transfer zone by a transport device. According to this invention, an effective transfer of toner to the substrate surface can be achieved with such an arrangement, even with a poor electrically-conducting and thick-walled, sheet-like substrate, whereby a charger is arranged as the primary charger in the transport direction and a secondary charger is arranged in the region after the transfer zone and both primary and secondary chargers affect the surface of the substrate to be printed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electrophotographic printing device, having a developer unit and a photoconductor, wherein the photoconductor is connected directly or with the interposition of one or several transfer media with a substrate to be imprinted, located in a transfer zone, wherein at least one charger is assigned to the substrate, and wherein the substrate can be conveyed through the transfer zone by a conveying device. 
     2. Discussion of Related Art 
     A printing device is known from German Patent Reference DE 198 49 500 A1. There, a developer unit is used, in which a toner is stored. A photoconductor drum is assigned to the developer unit. The photoconductor drum can be activated on its surface by an exposure device, so that a toner application becomes possible. The photoconductor drum is in contact with a transfer roller via a contact line. The toner is transferred from the photoconductor drum to the transfer roller using coronas. The transfer roller rolls off on the surface of a substrate which is to be imprinted. In the process the toner is transferred to the substrate surface using a corona arranged on the underside of the substrate. Two transfer processes of the toner image take place with this arrangement. The first transfer process occurs during the transfer from the photoconductor drum to the transfer roller, the second transfer process occurs during the transfer of the toner to the substrate. No complete transfer of the toner takes place during each of the transfer processes. However, the greatest possible transfer of the toner should be attempted, so that clear print images with sharp contours can be generated. In this connection the design and arrangement of the corona in the area of the second transfer process is of importance. It is necessary to assure that the surface of the substrate to be imprinted is sufficiently electrostatically charged. With flat substrates of greater wall thickness in particular, insufficient charging occurs when the substrate is of a material which does not conduct electricity well. 
     SUMMARY OF THE INVENTION 
     It is one object of this invention to provide an electrophotographic printing device of the type mentioned above but which has an effective transfer of the toner to the substrate, regardless of the thickness of the material of the substrate and its chemical properties. 
     This object is achieved with a charger arranged on the side facing the surface of the substrate to be imprinted and directly acts on this surface to be imprinted. 
     A dependable charge is achieved because, in a reversal of the prior art, the charger no longer act on the underside of the substrate, but directly on its surface to be coated. The charge can then be applied regardless of the consistency of the substrate. 
     In accordance with one embodiment of this invention, one charger, as the primary charger, is arranged upstream of the transfer zone in the conveying direction, and a secondary charger downstream of the transfer zone, and that the primary and secondary chargers act on the surface of the substrate to be imprinted. 
     With this arrangement, the substrate is first conducted to the primary charger. Its surface to be imprinted can then be charged. Thereafter the substrate is conducted through the transfer zone. During this, toner is applied to the surface to be imprinted. During continued conveyance the substrate leaves the primary charger. Depending on the size of the substrate and of the print image, during this the toner transfer to the substrate is not yet finished. In this case the secondary charger prevents a drop in the charge by recharging the substrate. It is thus possible to assure an even and effective transfer of the toner material throughout the entire coating process. 
     It is possible for the effects of the primary and/or secondary charger to take place with or without contact. For example, a charging brush can glide over the surface to be imprinted, or a charging roller can roll off on it. Particularly good charging results can be achieved in the course of the contactless charge method when using a primary, or a secondary charging corona. Charging spray heads with piezo-effect charging generators can also be employed as contactless chargers. In accordance with a preferred embodiment variation of this invention, the primary and/or secondary coronas are designed as flat coronas, which cover the entire width extending transversely with respect to the conveying direction of the surface of the substrate to be imprinted, and at least also partially over the surface in the conveying direction. 
     With this arrangement it is possible to charge large surfaces of the substrate, which makes a rapid charge application possible. In this way it is also possible to apply high substrate feeding speeds. 
     In one embodiment of this invention, the primary charging corona and/or the secondary charging corona have a corona wire holder in which several corona wires, which are arranged next to each other, are held under tension, and the corona wires are connected to a uniform electrical potential. Because all corona wires have a uniform electrical potential, it is possible to generate an even voltage image. Thus the corona wire holders are installed in a housing and are electrically insulated against it, the housing is connected with an electrical counter-potential, and the housing shields the photoconductor and/or the transfer medium against the corona wires. The housing prevents the corona wires from affecting the charge image on the image drum, or on the transfer roller. 
     In accordance with another embodiment of this invention, the corona wires are designed as individual wires, which have a spring element on one of their ends, by which the corona wire is suspended from a first corona wire holder, and the other end of the corona wire is fastened on an oppositely located corona wire holder. With this it is possible to assure that all corona wires are uniformly suspended. Thus their sagging does not extend in different lengths, which would generate a non-uniform charge image on the substrate surface. 
     However, at least two of the corona wires which are arranged next to each other are also formed by a continuous piece of wire, which is respectively reversed at the corona wire holders, and the corona wires are uniformly prestressed. 
     For assuring a continuously uniform toner transfer, the primary and the secondary charging coronas charge the substrate with a potential of the same sign, wherein the size of the potential on the surface of the substrate does not differ by more than 50% from the larger potential value. 
     A rapid surface charge can be achieved if the primary, as well as the secondary corona, are each assigned their own power supply unit. This can be further improved if several power supply units, each of which supplies a group of corona wires with a voltage, are assigned to each of the primary and/or secondary coronas. 
     Typically the voltage potential lies between 1 and 10 kV. In this case it is particularly advantageous if the voltage of the primary and the secondary coronas can be adjusted separately from each other. 
     To assure that while passing through the transfer zone the substrate is always charged by at least one charging corona, the distance of the primary charging corona from the secondary charging corona is less in the conveying direction than the extension in this direction of the surface of the substrate to be imprinted. 
     To prevent the substrate from being discharged via the conveying device the substrate is placed on the conveying device with the interposition of an insulator. The interposed layer has an insulated plastic material, which is highly resistant to disruptive discharge (for example polyimide, polyamide, epoxy resin, laminated paper, bakelite). Layers of a ceramic material (for example Al 2 O 3 ) or thin glass are also conceivable. 
     BRIEF DESCRIPTION OF THE DRAWING 
     This invention is explained in greater detail in view of an exemplary embodiment represented in the drawing which shows a lateral sectional view a device for the electrostatic imprinting of substrates, in particular plate-shaped ones. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The substrate  30  is placed on a conveying device  25  with an insulator  17  interposed. For example, the conveying device  25  can be a linearly displaceable table or a conveyor belt. A primary charging corona  16  and a secondary charging corona  18  are assigned as chargers or charging means to the substrate  30  and provide the surface of the substrate  30  with a charge. 
     The primary and secondary charging coronas  16  and  18  are substantially similarly constructed, wherein the primary charging corona is of a larger size. The primary and secondary charging coronas  16  and  18  are designed as flat coronas, but it will be understood that the coronas could be in the form of a charging brush, charging spray head, or charging roller, as indicated in the alternative by dashed lines to each of boxes  16 . 6  and  18 . 6 . Each one has a corona wire holder  16 . 1 ,  18 . 1 . The corona wire holder essentially has two combs, which extend parallel in respect to each other and between which the corona wires  16 . 2 ,  18 . 2  are suspended. In this case the ends of the corona wires  16 . 2 ,  18 . 2  are suspended on the teeth of the corona wire holders  16 . 1 ,  18 . 1 . Each corona wire  16 . 2 ,  18 . 2  has a spring element  16 . 5 ,  18 . 5  at one of its ends. A loop is provided at the other end. The corona wires  16 . 2 ,  18 . 2  can be suspended by means of the loop from a comb of the corona wire holders  16 . 1 ,  18 . 1 . The end of the corona wires  16 . 2 ,  18 . 2  having a spring element  16 . 5 ,  18 . 5  can be suspended from the oppositely located comb. In the process a tension of the corona wires  16 . 2 ,  18 . 2  in the corona wire holders  16 . 1 ,  18 . 1  is achieved by means of the spring element. Since an identical spring element is assigned to each corona wire  16 . 2 ,  18 . 2 , the tensile stress in each one of the individual corona wires  16 . 2 ,  18 . 2  is identical. It is achieved by means of this that the corona wires  16 . 2 ,  18 . 2  are uniformly tightly stretched. As can be seen in the drawings, the primary charging corona  16  is divided at the center of the corona wire holders  16 . 1 ,  18 . 1 . An insulation is provided here. In this way two sections of corona wires  16 . 2 ,  18 . 2  are formed. At least one power supply unit is indicated as assigned to each one of these sections, which supplies the corona wires  16 . 2 ,  18 . 2  with electrical current. A power supply unit is also assigned to the secondary charging corona  18 . The corona wire holder  16 . 1 ,  8 . 1  has been placed into a housing  16 . 3 ,  18 . 3 . The housing  16 . 3 ,  18 . 3  has a cover section, around which a lateral wall  16 . 4  is placed, which protrudes in the direction toward the substrate  30 . 
     The primary and the secondary charging coronas  16  and  18  are arranged opposite the substrate surface  30  to be imprinted. Thus they can act directly on the surface of the substrate  30 . A transfer medium  22  of an electrophotographic unit is arranged in the area between the primary and the secondary charging coronas  16  and  18 . In the present embodiment, the transfer medium  22  is embodied as a cylinder body. However, it can also be designed as an endlessly rotating belt. The transfer medium  22  is in contact with the substrate  30  in the area of a contact zone  24 . A charging corona  23  is arranged in the transfer medium  22 . The charging corona  23  charges the surface of the transfer medium  22 , wherein the charge has a polarity opposite to the charge of the substrate. 
     However, with an appropriate design of the photoconductor  20 , the transfer medium  22  can be omitted. 
     The electrophotographic unit also has a developer unit  10 , which is constructed in a known manner. A toner, for example a ceramic toner or a thermoplastic or duromeric plastic toner, is stored in the developer unit  10 . The developer unit  10  has a developer drum  15 , by which the toner is conducted to a photoconductor  20 . The photoconductor  20  is embodied to be cylinder-shaped and is in a linear engagement with the transfer medium  22  in a contact zone  21 . 
     An exposure device  11  is provided above the photoconductor  20 , which exposes a photosensitive layer of the photoconductor in a known manner. A latent electrostatic charge image is created by this. Because of this charge image it is possible to apply toner particles from the developer drum  15  to the outer conductor layer of the photoconductor  20  by means of electrostatic actions. The toner particles are transferred to the transfer medium  22  in the area of the contact zone  21 . Toner remnants, which possibly still adhere to the photoconductor  20 , are removed by a cleaning unit  14 , which follows the contact zone  21 . A discharge light  13  following the cleaning unit  14  discharges the photosensitive coating of the photoconductor. Then this photosensitive layer is returned to a uniform charge structure by means of a charging corona  12 , so that it can again be provided with an electrostatic charge image by the exposure device  11 . In the course of the printing operation the substrate  30  is evenly linearly displaced by means of the conveying device  25 . In the process, the transfer medium  22  rolls off either passively or in a driven manner on the surface of the substrate  30  to be imprinted. In the course of this the toner on the transfer medium  22  is transferred to the substrate  30  in the transfer zone  24 . This transfer takes place in particular because the primary and the secondary charging coronas cause the charging of the entire surface of the substrate surface. As already mentioned above, this charge is polarized opposite to the charge on the transfer medium  22 , so that a dependable toner transfer of high effectiveness can take place. 
     As shown in the drawing, the distance in the conveying direction between the primary and the secondary charging coronas  16  and  18  is selected to be less than the extent of the substrate in this direction. Thus the substrate  30  is continuously charged during its entire passage through the transfer zone  24 . When the substrate  30  leaves the charging area of the primary corona  16 , it is in contact with the charging area of the secondary charging corona  18 . 
     Some examples are shown in what follows, which describe the preferred applications of the above described device in greater detail: 
     1. Imprinting of plate-shaped glass, glass-ceramic or ceramic materials with ceramic toners for decorating purposes. Following imprinting, as a rule the toner is pre-fixed and is subsequently fired at temperatures between 500 and 1000° Celsius. Examples of use are: decorated glass-ceramic cooktops, decorated glass-ceramic layered stove tiles, decorated glass products, such as stove front plates, control panels, glass for shower enclosures, signs made of glass, glass doors, glass tiles, glass in furniture, decorated ceramic articles, such as tiles, etc. 
     2. Imprinting of plate-shaped plastic materials, or glass or glass-ceramic materials with thermoplastic and/or thermoset plastic toners for decorative purposes. Following imprinting, as a rule the toner is pre-fixed and is subsequently fired at temperatures between 120 and 200° Celsius, preferably 150 to 180° Celsius. Examples of use are: decorated plastic surfaces made of thermoplastic or thermoset plastic materials such as, for example, plastic surfaces in the field of furniture or small household devices, tabletops, front panels, or glass materials such as, for example, signs. 
     3. Imprinting of glass, glass-ceramic or plastic surfaces for a specific modification of the surface properties, for example for imprinting electrically conductive surfaces, for surface hardening, or the like. As a rule this is also followed by heating processes for firing, tempering, or the like. 
     It is thus possible to effectively imprint plate-shaped materials in particular. Slight unevenesses of the substrate surface as a result of processing are compensated by the arrangement in accordance with this invention. For compensating surface uneveness it is also possible to provide the transfer medium with a flexible coating placed on the surface of the substrate. The surface of the photoconductor  20  can have a flexible coating, in the same way. In that case the photoconductor  20  can be placed directly on the surface of the substrate  23  without using a transfer medium  22 . 
     With charging from the side to be imprinted, a toner transfer takes place independently to a large extent of the substrate material and of the substrate thickness. It is then possible, if desired, to provide an individual adaptation to the substrate material and to the material thickness by adapting the corona voltage.