Abstract:
An image forming apparatus includes an image forming device that forms an image on a recording medium with a powdery material, the image being sized so as to extend beyond an edge of the recording medium; a collecting device that collects the material that has extended off the recording medium as a result of the image forming device forming the image; and a restricting unit that controls the image forming device in such a manner that the image forming device reduces an amount of use of the material in a region of the image that extends off the recording medium to a larger degree on an outer side of the image than a side of the image near the recording medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2014-152699 filed Jul. 28, 2014. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to image forming apparatuses. 
         [0004]    (ii) Related Art 
         [0005]    Frameless images, which are images extended up to the edge of a print sheet, are known. 
       SUMMARY 
       [0006]    According to an aspect of the invention, an image forming apparatus includes an image forming device that forms an image on a recording medium with a powdery material, the image being sized so as to extend beyond an edge of the recording medium; a collecting device that collects the material that has extended off the recording medium as a result of the image forming device forming the image; and a restricting unit that controls the image forming device in such a manner that the image forming device reduces an amount of use of the material in a region of the image that extends off the recording medium to a larger degree on an outer side of the image than a side of the image near the recording medium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0008]      FIG. 1  is a schematic diagram of a configuration of a printer, corresponding to a specific first exemplary embodiment of an image forming apparatus; 
           [0009]      FIG. 2  is a functional block diagram illustrating the functions of an image processor at the time of frameless printing; 
           [0010]      FIG. 3  is a schematic diagram illustrating restriction on the amount of use of toner imposed by a toner-amount restricting unit; 
           [0011]      FIG. 4  is a schematic diagram illustrating restriction on the amount of use of “transparent” toner; and 
           [0012]      FIG. 5  is a schematic diagram illustrating restriction on the amount of use of color toner. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A developing device and an image forming apparatus according to exemplary embodiments of the invention are described below referring to the drawings. 
         [0014]      FIG. 1  is a schematic diagram of the configuration of a printer corresponding to an image forming apparatus according to a first exemplary embodiment. 
         [0015]    An image forming apparatus  1  illustrated in  FIG. 1 , is a tandem color printer including image forming units  10 Y,  10 M,  10 C, and  10 K corresponding to yellow (Y), magenta (M), cyan (C), and black (K) arranged parallel with one another. The image forming apparatus  1  prints single-color images as well as full-color images containing four-color toner images. The image forming apparatus  1  also includes an optional image forming unit  10 P arranged parallel with the YMCK image forming units  10 Y,  10 M,  10 C, and  10 K and at the upstream most position of all the image forming units. The optional image forming unit  10 P is used to express a color other than YMCK colors, such as white, gold, or transparent, so that the image is capable of expressing various distinctions including granularity, fine quality, contrast enhancement, or embossing effect. In this embodiment, the optional image forming unit  10 P is used to express “transparency” (and colorlessness). 
         [0016]    The image forming apparatus  1  includes toner cartridges  18 Y,  18 M,  18 C,  18 K, and  18 P that accommodate toner of YMCK colors and the color corresponding to the optional image forming unit  10 P. 
         [0017]    The four image forming units  10 Y,  10 M,  10 C, and  10 K and the optional image forming unit  10 P have the same configuration apart from developers accommodated therein. Thus, the image forming unit  10 Y corresponding to yellow is described as a typical example. When image forming units or their components are collectively described below, alphabets appended to the reference numerals of the image forming units or their components are omitted regardless of color and the image forming units are simply referred to as, for example, image forming units  10 . 
         [0018]    The image forming unit  10 Y includes a photoconductor  11 Y, a charging device  12 Y, an exposing device  13 Y, a developing device  14 Y, a first transfer device  15 Y, and a photoconductor cleaner  16 Y. 
         [0019]    The photoconductor  11 Y rotates in the direction A illustrated in  FIG. 1 . The charging device  12 Y, the exposing device  13 Y, the developing device  14 Y, the first transfer device  15 Y, and the photoconductor cleaner  16 Y are arranged in this order around the photoconductor  11 Y. 
         [0020]    The charging device  12 Y charges the surface of the photoconductor  11 Y. The exposing device  13 Y exposes the surface of the photoconductor  11 Y to light on the basis of an image signal supplied from an image processor  100  to form an electrostatic latent image. The developing device  14 Y develops the latent image on the surface of the photoconductor  11 Y using a developer containing toner into a toner image. The developing device  14 Y is supplied by the toner cartridge  18 Y with an amount of toner suitable for consumption. The first transfer device  15 Y electrostatically transfers the toner image on the photoconductor  11 Y to an intermediate transfer belt  30 . The photoconductor cleaner  16 Y includes a cleaning blade that comes into contact with and rubs against the surface of the photoconductor  11 Y to clean the surface of the photoconductor  11 Y after the transfer. 
         [0021]    The image forming apparatus  1  includes an intermediate transfer belt  30 , a fixing device  60 , a sheet transporting unit  80 , and an image processor  100 . The intermediate transfer belt  30  is wound around belt support rollers  31  to  35  and rotationally moves in the direction of the arrow B so as to pass by the optional image forming unit  10 P, the image forming units  10 Y,  10 M,  10 C, and  10 K, and a second transfer device  50 . Toner images of the respective colors from the optional image forming unit  10 P and the image forming units  10 Y,  10 M,  10 C, and  10 K are transferred to the intermediate transfer belt  30  so as to be superposed on one another. The intermediate transfer belt  30  moves while carrying a color toner image formed of these superposed color toner images. The second transfer device  50  has a structure in which a polyimide tube covers a roller made of expanded rubber. The second transfer device  50  electrostatically transfers the toner image on the intermediate transfer belt  30  to a sheet. 
         [0022]    A combination of the four image forming units  10 Y,  10 M,  10 C, and  10 K, the optional image forming unit  10 P, the intermediate transfer belt  30 , the second transfer device  50 , and the fixing device  60  corresponds to an example of an image forming device according to an exemplary embodiment of the invention and toner corresponds to an example of a powdery material according to an exemplary embodiment of the invention. 
         [0023]    A belt cleaner  70  has a blade, which is brought into contact with the intermediate transfer belt  30  to scrape the toner off the surface of the intermediate transfer belt  30 . The second transfer device  50  also includes a transfer-device cleaner  51  that has a blade, which is brought into contact with the surface of the second transfer device  50  to scrape the toner off the surface. 
         [0024]    The fixing device  60  fixes toner onto a sheet. The fixing device  60  includes a heating roller  61  and a pressing roller  62 . The heating roller  61  includes a heating device installed therein. The heating roller  61  and the pressing roller  62  causes a sheet carrying an unfixed toner image to pass therethrough to fix a toner image onto the sheet while squeezing the sheet from both sides. The fixing device  60  also includes fixing-device cleaners  63  and  64 , which remove toner from the surfaces of the heating roller  61  and the pressing roller  62 , respectively. 
         [0025]    The transfer-device cleaner  51  and the fixing-device cleaners  63  and  64  correspond to examples of a collecting device according to an exemplary embodiment of the present invention. 
         [0026]    The sheet transporting unit  80  transports sheets along a sheet transport path TP along which the sheets pass by the second transfer device  50  and the fixing device  60 . The sheet transporting unit  80  picks up sheets contained in a sheet container SC using a pick-up roller  81 , transports the sheets using transport rollers  82 , and feeds the sheets to the second transfer device  50  using registration rollers  84 . The sheet transporting unit  80  also ejects the sheets to the outside using ejection rollers  86 . 
         [0027]    The image processor  100  processes data of an original image captured from a device such as an external personal computer or a scanner to generate image signals for respective colors and transfers the respective image signals to the image forming units  10  of the corresponding colors. For generating image signals corresponding to respective colors, the image processor  100  executes screen processing to reproduce a halftone image with dots. Upon receipt of an instruction from a user through a user interface, not illustrated, the image processor  100  executes processing such as removal of a background color or frameless printing for forming an image so sized as to fully expand throughout the entire sheet. 
         [0028]    In the frameless printing, the image forming unit  10  forms a toner image having a size slightly larger than the sheet size. Thus, the edge portion of the toner image extends off the sheet and the toner in the edge portion is later collected by the transfer-device cleaner  51  or the fixing-device cleaners  63  and  64 . The amount of toner collected in the frameless printing is larger than the amount of toner collected in normal printing other than the frameless printing. The amount of toner collectable by collecting devices such as the transfer-device cleaner  51  or the fixing-device cleaners  63  and  64  is limited. When the amount of the collected toner arrives at the upper limit, the life of the image forming apparatus  1  is exhausted and the image forming apparatus  1  requires recovery operations such as maintenance. In other words, frameless printing shortens the life of the image forming apparatus  1  compared to normal printing. Moreover, if the amount of toner that extends off a sheet is large, some amount of toner may adhere to the edge of the sheet due to melting while being fixed and cause matter protruding from the edge of the sheet, impairing the appearance of the edge. For the purposes of preventing shortening of the apparatus life caused by frameless printing or for other purposes, the image forming apparatus  1  according to the embodiment reduces the amount of collected toner. Now, the function of reducing the amount of collected toner is described below. 
         [0029]      FIG. 2  is a functional block diagram illustrating the functions of an image processor at the time of frameless printing. 
         [0030]    As a functional structure at the time of frameless printing, the image processor  100  includes an enlarging/shrinking unit  110  and a toner-amount restricting unit  120 . 
         [0031]    The enlarging/shrinking unit  110  converts an original image to an image having a size slightly larger than a sheet size by enlarging or reducing the size of the original image. Here, the “size slightly larger than a sheet size” in this embodiment is a size that allows for misalignment of the toner image and the sheet that may be caused by, for example, sheet transport errors or toner-image transfer errors occurring in the sheet transporting unit  80 . This size is the minimum size that allows the edge portion of the sheet to be prevented from remaining white even when the toner image and the sheet are misaligned with each other. 
         [0032]    The toner-amount restricting unit  120  restricts the amount of use of toner at an outer edge portion of the toner image. The toner-amount restricting unit  120  corresponds to an example of a restricting unit according to an exemplary embodiment of the invention. 
         [0033]      FIG. 3  is a schematic diagram illustrating restriction of the amount of use of toner imposed by the toner-amount restricting unit. 
         [0034]    In  FIG. 3 , for convenience of illustration, probable misalignment of a toner image T and a sheet P is exaggeratingly drawn. 
         [0035]    As described above, the size of the toner image T is larger than the size of the sheet P so that the entire surface of the sheet T is covered by the toner image T even in the case where the toner image T is misaligned with respect to the sheet P. In other words, the size of the toner image T is restricted to such a degree that the edge of the toner image T is positioned just within the edge of the sheet P when the toner image T and the sheet P have maximum misalignment. 
         [0036]    An outer edge portion of the toner image T includes a toner-amount restricted region R, in which a toner image is formed at a density calculated by multiplying the image density of the original image by a toner use rate. The toner use rate in a region inward of the toner-amount restricted region R is 100%, which means that the amount of use of toner is not restricted at all in this region. In other words, the toner image is formed at the image density corresponding to the data of the original image. 
         [0037]    The toner use rate in the toner-amount restricted region R linearly changes from 100% at the innermost edge of the toner-amount restricted region R to 0% at the outermost edge of the toner-amount restricted region R. In other words, the restriction on the amount of use of toner is increasingly tightened toward the outer edge of the toner-amount restricted region R. Thus, the amount of toner that has failed to be transferred to the sheet P among the toner forming the entire toner image T and that is then collected is reduced, and the reduction of the amount of toner enables minimization of shortening of the apparatus life. The toner that has failed to be transferred to the sheet P is reworded as toner left off the sheet P or toner falling out of the sheet P and includes toner that has been transferred to a portion out of the sheet at the transfer operation and toner that has flowed out of the sheet as a result of melting at the fixing operation. 
         [0038]    In the exemplary embodiment, the restriction on the amount of use of toner is linearly tightened in view of the image density. Thus, the color or other characteristics of the image inside the toner-amount restricted region R is natural and the appearance of the image is not impaired. 
         [0039]    On the bottom and the right of  FIG. 3 , cross-sectional diagrams schematically expressing the restriction on the amount of use using the thickness of the toner image T are shown. The cross-sectional diagram taken along the direction of the arrow in  FIG. 3  in which the sheet is transported is shown on the right and the cross-sectional diagram taken along the direction perpendicular to the direction in which the sheet is transported is shown on the bottom. The thickness of the toner image T illustrated in the cross-sectional diagrams is zero at both ends of the toner image T and largest at the position corresponding to the innermost edge of the toner-amount restricted region R. The thickness of an actual toner image is also dependent on the image density. The thickness of the toner image T illustrated in  FIG. 3  corresponds to the thickness in the case of a solid image. 
         [0040]    In this embodiment, the toner-amount restricted region R includes an outer region ROUT extending off the sheet P and an inner region RIN extending within the sheet P. Since the restriction on the amount of use of toner is imposed also on the inner region RIN, the amount of toner collected after having failed to be transferred to the sheet P is reduced compared to the case where restriction on the amount of use of toner is only imposed on the outer region ROUT. Thus, shortening of the apparatus life is minimized. 
         [0041]    The restriction on the amount of use in the inner region RIN, which is located on the leading side of the sheet P in the direction in which the sheet P is transported, facilitates separation of the sheet P when the sheet P passes through the fixing device  60  illustrated in  FIG. 1 . 
         [0042]    In this embodiment, restriction on the amount of use of toner is tighter in first regions R 1 , extending on both sides of the edges of the sheet P in the transportation direction, than in second regions R 2 , extending on both sides of the edges of the sheet P in the width direction perpendicular to the transportation direction. Specifically, the distance between the innermost edge of each first region R 1  and the edge of the sheet P in the first region R 1  is longer than the distance between the innermost edge of each second region R 2  and the edge of the sheet P in the second region R 2 . Thus, each first region R 1  is larger than each second region R 2  and restriction on the amount of use of toner is tighter in the first regions R 1  than in the second regions R 2 . This is because, since the degree of misalignment of the sheet P and the toner image T in the transportation direction is larger than that in the width direction, the toner image T is sized so as to extend off the sheet P to a larger degree in the transportation direction that in the width direction, so that the amount of toner that fails to be transferred to the sheet P is larger in a portion extending in the transportation direction than in a portion extending in the width direction. For convenience of illustration,  FIG. 3  only shows one of the first regions R 1  and one of the second regions R 2  of the sheet P, but actually, the first regions R 1  are located on opposing sides of the sheet P and the second regions R 2  are located on opposing sides of the sheet P. 
         [0043]    The image forming apparatus according to an exemplary embodiment of the invention may be structured so as to, for example, restrict only the amount of use of “transparent” (and colorless) toner, corresponding to the optional image forming unit  10 P, but not to restrict the amount of use of YMCK color toners. However, the image forming apparatus  1  according to the exemplary embodiment restricts both amounts of use of colorless and color toner. Nevertheless, the degree of restriction imposed on colorless toner and that on color toner are different from each other. 
         [0044]      FIG. 4  is a schematic diagram illustrating the restriction on the amount of use of “transparent” toner and  FIG. 5  is a schematic diagram illustrating the restriction on the amount of use of color toner. 
         [0045]      FIG. 4  schematically illustrates the state of a toner layer TP of “transparent” toner formed on the sheet P viewed from one side of the sheet P.  FIG. 5  schematically illustrates the state where a toner layer TKCMY of color toner formed on the sheet P viewed from one side of the sheet P. Actually, the toner layer TKCMY of color toner and the toner layer TP of “transparent” toner are superposed. However, for convenience of illustration, the toner layers TP and TKCMY are separately illustrated in  FIGS. 4 and 5 . 
         [0046]    Both of the toner layer TP of “transparent” toner and the toner layer TKCMY of color toner are so sized as to extend beyond the edge of the sheet P. The outer edges of these toner layers TP and TKCMY are positioned at the outer edge of the range covering the maximum misalignment of the sheet P and the toner layers TP and TKCMY. 
         [0047]    The degree of restriction on the amount of “transparent” toner in the toner layer TP is continuously tightened from a portion inward of the edge of the sheet P to the outer edge of the toner layer TP. Since the “transparent” toner is used for the purpose of exerting a glossy effect on the image, the “transparent” toner is formed in a solid form so as to cover the entire image. Thus, reduction of the amount of collected “transparent” toner contributes to minimization of shortening of the apparatus life to a large degree. In addition, the restriction on the amount of “transparent” toner from the portion inward of the edge of the sheet P negligibly affects the appearance of the image. 
         [0048]    On the other hand, the degree of restriction on the amount of color toner in the toner layer TKCMY in the exemplary embodiment is continuously tightened from the edge of the sheet P to the outer edge of the toner layer TKCMY. Since the toner layer TKCMY of color toner naturally carries a visible toner image itself, excessively restricting the amount of color toner may affect the appearance of the image. Thus, in the exemplary embodiment, the degree of restriction on the amount of color toner in the toner layer TKCMY is made smaller than the degree of restriction on the amount of “transparent” toner in the toner layer TP. Such restriction contributes to minimization of shortening of the apparatus life and minimizes an adverse effect on the appearance of the image. 
         [0049]    The exemplary embodiment has described, as a preferable example, an example in which the degree of restriction on the amount of use of color toner is different from the degree of restriction on the amount of use of “transparent” toner. However, an image forming apparatus according an exemplary embodiment of the invention may restrict the amount of use of color toner and the amount of use of “transparent” toner to the same degree. 
         [0050]    This exemplary embodiment has described, as a preferable example, an example in which the degree of restriction on the amount of use of toner in the first region R 1  illustrated in  FIG. 3  is different from the degree of restriction on the amount of use of toner in the second region R 2  illustrated in  FIG. 3 . However, an image forming apparatus according to an exemplary embodiment of the invention may restrict the amount of use of toner in the first region R 1  and the amount of use of toner in the second region R 2  to the same degree. 
         [0051]    The exemplary embodiment has described, as a preferable example, an example in which the amount of use of toner is restricted in both of the inner region RIN and the outer region ROUT illustrated in  FIG. 3 . However, an image forming apparatus according to an exemplary embodiment of the invention may restrict the amount of use of toner only in the outer region ROUT. 
         [0052]    The description has provided a printer as an example of the image forming apparatus according to the exemplary embodiment. However, an image forming apparatus according to an exemplary embodiment of the invention may be, for example, a copying machine or a multifunctional machine. 
         [0053]    The description has provided a tandem color printer as an example of the image forming apparatus according to the exemplary embodiment. However, an image forming apparatus of the invention may be a revolver color printer or a black-and-white printer. 
         [0054]    The description has provided an indirect transfer apparatus including an intermediate transfer belt as an example of the image forming apparatus according to the exemplary embodiment. However, an image forming apparatus according to an exemplary embodiment of the invention may be a direct transfer apparatus that directly transfers a toner image from image forming units of different colors onto a sheet. 
         [0055]    The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.