Patent Publication Number: US-8532539-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     This application is based on Japanese Patent Application No. 2009-108106 filed on Apr. 27, 2009, the contents of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus that uses a photoreceptor drum and a development device, and more particularly, to an image forming apparatus that is able to perform a refresh process to refresh toner on a development roller in a time an image is not formed. 
     DESCRIPTION OF THE RELATED ART 
     In a conventional image forming apparatus, when repeating image formation, in a case where especially a print rate (a percentage of a printed area to an area (paper-sheet area) where image formation is possible. Hereinafter, the same as above) is low, because toner that flies from a development roller (toner carrier) to a photoreceptor drum (electrostatic latent image carrier) to be used for development is a little, a small amount of toner particles are refilled into a development device, so that the toner is excessively electrified and decrease in image density and fogging occurs in some cases. Especially, in a case where it is necessary to deal with from images such as a photograph and a graphic image which have a high print rate to images such as a letter and a logotype which have a low print rate, unevenness in the print rate becomes large, and decrease in the image density and fogging become likely to occur. 
     In such a case, by printing a pattern such as a solid image or the like that has a high document print rate, it is possible to fly a large amount of toner from a development roller to a photoreceptor drum side, consume the toner by transferring the toner onto a recording medium and alleviate the disadvantage; however, in a case where the machine is left for a long time without printing a solid pattern, the toner is not consumed and toner particles are fixed on the surface of the development roller by the influence of humidity and the like, so that the development roller does not recover in some cases. 
     To avoid this, conventionally, it is improved in such a way that electrification controllability of toner becomes stable by making toner surface shape, materials and external additives better; however, in the present situation, the toner is excessively electrified and it has not become possible to surely prevent the above phenomena from occurring. 
     To solve the above problems, various methods for forcibly consuming the toner on a development roller; in JP-A-2000-310909, for example, an image forming apparatus is disclosed, in which an average print rate is calculated from the number of paper sheets printed and print rates; in a case the average print rate becomes lower than a predetermined value, a direct-current bias and an alternating-current bias added on the direct-current bias are applied to a development roller, so that the toner on the development roller is forcibly ejected to a photoreceptor side for refreshment. Besides, in JP-A-2000-330379, an image forming apparatus is proposed, in which in a case where an average print rate becomes lower than a predetermined value, the toner on a development roller is forcibly consumed with rotation of the photoreceptor and the development roller stopped. 
     In the image forming apparatuses in JP-A-2000-310909 and JP-A-2000-330379, by performing a refresh process to forcibly eject the toner on the development roller to the photoreceptor side, it is possible to alleviate decrease in the image density and occurrence of fogging; however, it is necessary to eject toner equivalent to one or more turns of the development roller to forcibly consume the toner on the whole circumference of the development roller. Accordingly, there are disadvantages that the amount of unnecessary toner consumption other than consumption for the printing becomes large; and the toner ejected onto the photoreceptor easily adheres to a transfer roller, thereby causing rear-surface dirt of a recording medium. 
     To avoid this, in JP-A-2000-206770 and JP-A-2009-53582, methods are disclosed, in which a toner ejection pattern, a zigzag pattern, a reticulate-point pattern, a line pattern or the like other than a solid pattern is used to efficiently eject deteriorated toner. 
     According to JP-A-2000-206770 and JP-A-2009-53582, it is possible to efficiently consume the toner on a development roller by so-called edge enhancement in which a potential difference on an edge region of the toner ejection pattern becomes higher than that in a solid region. However, because of requirements for increase in image forming efficiency and low running cost of the apparatus in recent years, a method is desired to be developed, in which the amount of unnecessary toner consumption other than the consumption for the printing is further reduced and the rear-surface dirt of a recording medium is surely prevented by alleviating toner adhesion to a transfer roller. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to deal with the above problems, and it is an object of the present invention to provide an image forming apparatus which forcibly ejects mainly excessive-electrified toner on a toner carrier to an image carrier side and effectively forces a refresh process to function without increasing the toner consumption amount compared with the conventional. 
     To achieve the above object, an image forming apparatus according to an aspect of the present invention includes: an image carrier that carries an electrostatic latent image; a development device that includes a toner carrier which is disposed opposite to the image carrier, carries and supplies toner to the image carrier, and develops the electrostatic latent image formed on a surface of the image carrier; and a control means that performs a refresh process in which toner is ejected from a side of the toner carrier to a side of the image carrier in a time an image is not formed, and a toner ejection pattern which is formed by ejecting a line image a plurality of times that has an acute angle to a main scan direction at predetermined intervals in a circumferential direction of the image carrier over a total width of a development region. 
     Still other objects of the present invention and specific advantages derived from the present invention will be more apparent from description of the embodiments described below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing an overall structure of an image forming apparatus according to an embodiment of the present invention. 
         FIG. 2  is a sectional view of a development device used in an image forming apparatus according to an embodiment of the present invention. 
         FIG. 3  is a block diagram showing a control route of an image forming apparatus according to an embodiment of the present invention. 
         FIG. 4  is a timing chart showing an example of ON/OFF timing of an electrification bias, a laser exposure, a development bias and a transfer bias in a refresh process. 
         FIG. 5  is a diagram showing an example of a toner ejection pattern used in an embodiment of the present invention. 
         FIG. 6  is a diagram showing a conventional toner ejection pattern in which a line image is perpendicular to a main scan direction. 
         FIG. 7  is a diagram showing a conventional toner ejection pattern in which a line image is parallel to a main scan direction. 
         FIG. 8  is a flow chart showing a performance procedure of a refresh process in an image forming apparatus according to an embodiment of the present invention. 
         FIG. 9  is a graph showing a relationship between a line width of a line image that constitutes a toner ejection pattern and a toner adhesion amount on a drum in an image forming apparatus according to an embodiment of the present invention. 
         FIG. 10  is a graph showing a relationship between a line width of a line image that constitutes a toner ejection pattern and an adhesion amount of large-diameter toner on a drum in an image forming apparatus according to an embodiment of the present invention. 
         FIG. 11  is a graph showing a relationship between a line width of a line image that constitutes a toner ejection pattern and an adhesion amount of small-diameter toner on a drum in an image forming apparatus according to an embodiment of the present invention. 
         FIG. 12  is a graph showing a relationship between an angle of a line image that constitutes a toner ejection pattern to a main scan direction and a toner adhesion amount on a drum in an image forming apparatus according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the embodiments of the present invention are described with reference to the drawings.  FIG. 1  is a schematic structural view showing a whole structure of an image forming apparatus according to an embodiment of the present invention, in which a right side is represented as a front side of the image forming apparatus. As shown in  FIG. 1 , in a lower portion of a main body of an image forming apparatus  100 , a paper-sheet supply cassette  2  that stores paper sheets loaded is disposed. Above this paper-sheet supply cassette  2 , a paper-sheet carry pathway  4  that extends substantially horizontally from the front of the main body toward the back of the main body, further extends upward to reach a paper-sheet ejection portion  3  that is formed on an upper surface of the main body; and in order from an upstream side along this paper-sheet carry pathway  4 , a pick-up roller  5 , a feed roller  6 , an intermediate carry roller  7 , a pair of resist rollers  8 , an image forming portion  9 , a fixing portion  10  and a pair of ejection rollers  11  are disposed. 
     In the paper-sheet supply cassette  2 , a paper-sheet loading plate  12  that is disposed rotatably with respect to the paper-sheet supply cassette  2  by a rotation pivotal point  12   a  which is disposed at a back-end portion in the paper-sheet carry direction; and a paper sheet as a recording medium loaded on the paper-sheet loading plate  12  is pressurized to the pick-up roller  5 . Besides, in a front side of the paper-sheet supply cassette  2 , a retard roller  13  is disposed to come into tight contact with the feed roller  6 ; in a case where a plurality of paper sheets are carried by the pick-up roller  5  at the same time, the paper sheets are separated by these feed roller  6  and retard roller  13 , so that only the uppermost paper sheet is carried. 
     And, the carry direction of the paper sheets separated by the feed roller  6  and the retard roller  13  is changed toward the back of the apparatus by the intermediate carry roller  7 , so that the paper sheets are carried to the pair of resist rollers  8 , adjusted in timing by the pair of resist rollers  8  and supplied to the image forming portion  9 . 
     The image forming portion  9  forms a predetermined toner image on a paper sheet by using an electro-photographic process and is composed of: a photoreceptor drum  14  that is an image carrier which is supported rotatably clockwise in  FIG. 1 ; an electrification device  15 , a development device  16 , a cleaning device  17  that are disposed around the photoreceptor drum  14 ; a transfer roller  18  as a transfer member that is disposed opposite to the photoreceptor drum  14  with respect to the paper-sheet carry pathway  4 ; and an exposure unit (LSU)  19  that is disposed above the photoreceptor drum  14 . Above the development device  16 , a toner container  20  that supplies toner to the development device  16  is disposed. 
     The electrification device  15  is equipped with an electrically conductive rubber roller  15   a  to which an electrification-bias power supply  52  (see  FIG. 3 ) is connected; this electrically conductive rubber roller  15   a  is disposed to come into contact with the photoreceptor drum  14 . And, when the photoreceptor drum  14  rotates, the electrically conductive rubber roller  15   a  comes into contact with the surface of the photoreceptor drum  14  and is driven to rotate. Here, a predetermined voltage is applied to the electrically conductive roller  15   a , so that the surface of the photoreceptor drum  14  is evenly electrified. 
     Then, an electrostatic latent image based on input image data is formed on the photoreceptor drum  14  by a laser beam from the exposure unit (LSU)  19 ; toner is made adhere to the electrostatic latent image by the development device  16 , so that a toner image is formed on the surface of the photoreceptor drum  14 . And, a paper sheet is supplied from the pair of resist rollers  8  into a nip portion (transfer position) between the photoreceptor drum  14  and the transfer roller  18 , so that the toner image on the photoreceptor drum  14  is transferred to the paper sheet by the transfer roller  18 . 
     The paper sheet to which the toner image is transferred is separated from the photoreceptor drum  14  and carried to the fixing portion  10 . This fixing portion  10  is disposed in a downstream side in the paper-sheet carry direction with respect to the image forming portion  9 ; the paper sheet to which the toner image is transferred at the image forming portion  9  is heated and pressurized by a heat roller  21  and a pressurizing roller  22  that are disposed in the fixing portion  10 , so that the toner image transferred to the paper sheet is fixed. 
     And, the paper sheet on which the image is formed at the image forming portion  9  and the fixing portion  10  is ejected to the paper-sheet ejection portion  3  by the pair of ejection rollers  11 . On the other hand, toner that remains on the surface of the photoreceptor drum  14  is removed by the cleaning device  17 . And, the photoreceptor drum  14  is electrified again by the electrification device  15 ; and thereafter, in the same way, the image formation is performed. Besides, for the purpose of measuring an apparatus installation environment (around-machine environment), an outside-machine temperature and humidity sensor  23  is disposed over the toner container  20 . 
       FIG. 2  is a side sectional view of a development device incorporated in an image forming apparatus according to the embodiment of the present invention. As shown in  FIG. 2 , in the development device  16 , a first spiral member  32 , a second spiral member  33 , a development roller  35  and a limit blade  36  are disposed in a housing  31  that is composed of: a casing  31   a  in which a one-component developer including magnetic toner is stored; and a cover  31   b  that closes the casing  31   a  to prevent the toner stored in the casing  31   a  from leaking to outside 
     The inside of the casing  31   a  is divided by a partition plate  37  that extends in a longitudinal direction (direction perpendicular to paper surface in the drawing) into a first storage room  38  and a second storage room  39 ; the first spiral member  32  is disposed in the first storage room  38 , while the second spiral member  33  is disposed in the second spiral room  39 . The partition plate  37  is not disposed at both end portions in the longitudinal direction of the casing  31   a ; and this portion serves as a pathway (developer delivery portion) where toner moves between the first storage room  38  and the second storage room  39 . 
     The first spiral member  32  and the second spiral member  33  are composed of rotation shafts  32   a ,  33   a  and spiral blades  32   b ,  33   b  that are disposed along outer surfaces of the rotation shafts  32   a ,  33   a , respectively; and rotatably disposed in the casing  31   a  to be substantially parallel to each other. And, a structure is employed in such a way that the first spiral member  32  and the second spiral member  33  rotate in a predetermined direction, thereby circulating and carrying the toner in the first storage room  38  and the second storage room  39 . Besides, to allow toner to be supplied into the casing  31   a  based on a detection result from a toner amount detection sensor (not shown), the cover  31   b  is equipped with a toner supply opening (not shown) through which toner is supplied from the container  20  (see  FIG. 1 ). 
     The development roller  35  is rotatably disposed in the first storage room  38  to be substantially parallel to the first spiral member  32  and the second spiral member  33 . In this development roller  35 , a stationary magnet body  40  that includes a permanent magnet which has a plurality of magnetic poles (here, 3 poles); when the development roller  35  rotates following rotation of the photoreceptor drum  14 , toner is made adhere (carried) to the surface of the development roller  35  by magnetic force of the stationary magnet body  40 , so that a tone layer is formed. And, the toner adhering to the development roller  35  flies to the photoreceptor drum  14  and adheres to a photosensitive layer according to a potential difference between a surface potential of the photoreceptor drum  14  and a development bias applied to the development roller  35  in development region R where the development roller  35  and the photoreceptor drum  14  face each other, so that a toner image is formed on the surface of the photoreceptor drum  14 . 
     The limit blade  36  limits the toner amount that is supplied to the photoreceptor drum  14 , that is, the toner adhesion amount to the development roller  35 ; for example, a magnetic material such as SUS (stainless steel) or the like is used. And, the limit blade  36  is disposed so as to form a predetermined gap (0.2 to 0.3 mm) between its tip end and the development roller  35 ; the toner adhesion amount is limited by the distance between the limit blade  36  and the development roller  35  and a magnetic field that is generated in the gap; and a toner thin layer of dozens of micrometers is formed on the surface of the development roller  35 . 
     Next, a control route of the image forming apparatus according to the present invention is described.  FIG. 3  is a block diagram showing an example of a control route that is used in the image forming apparatus according to the embodiment of the present invention. Here, because various kinds of control of each portion of the apparatus are performed to use the image forming apparatus  100 , the control route of the entire image forming apparatus  100  becomes complicated. Accordingly, here, description is performed focusing on portions of the control route that are necessary for practice of the present invention. 
     The control portion  90  includes: a CPU (Central Processing Unit)  91  as a central operation process device; a ROM (Read Only Memory) that is a storage for read only; a RAM (Random Access Memory)  93  that is a dynamically readable and writable storage; a temporary storage portion  94  that temporarily stores image data and the like; a counter  95 ; and a plurality (here, two) of I/Fs (interfaces)  96  that transmit control signals to each device in the image forming apparatus  100  and receive an input signal from an operation portion  60 . Besides, the control portion  90  is able to be disposed in an arbitrary place in the main body of the apparatus. 
     The ROM  92  stores control programs for the image forming apparatus  100 , numerical values and the like necessary for control, data and the like that are not changed during use of the image forming apparatus  100 . The RAM  93  stores necessary data generated during control of the image forming apparatus  100 , data and the like that are temporarily necessary for control of the image forming apparatus  100 . Besides, the ROM  92  (or RAM  93 ) stores the number of paper sheets printed to determine whether it is necessary to perform a refresh process or not, a reference print rate and a rotation speed of the photoreceptor drum  14  that are used for calculation of a print length (toner ejection length) of a toner ejection pattern, and the like. The counter  95  sums up and counts the number of paper sheets printed. Here, without separately disposing the counter  95 , the RAM  93 , for example, may count the number of times. 
     Besides, the control portion  90  transmits control signals from the CPU  91  to each portion and device of the image forming apparatus  100  via the I/F  96 . Besides, signals that indicate states of each portion and device and input signals are transmitted to the CPU  91  via the I/F  96 . As portions and devices that the control portion  90  controls, there are, for example: the paper-sheet supply cassette  2 ; the fixing portion  10 ; the development device  16 ; the exposure unit  19 ; the toner container  20 ; the outside-machine temperature and humidity sensor  23 ; the image input portion  50 ; the bias control circuit  51 ; the operation portion  60 ; and the like. 
     In a case where the image forming apparatus  100  is a printer shown in  FIG. 1 , the image input portion  50  is a receiving portion that receives image data transmitted from a personal computer and the like. Besides, in a case where the image forming apparatus  100  is a copy machine, the image input portion  50  is an image reading portion that is composed of: a scan optical system that incorporates a scanner lamp which directs light to a document at a time of copy and a mirror which changes a light path of reflected light from the document; a collecting lens that collects the reflected light from the document to form an image; a CCD that converts the image-formed image light into an electrical signal; and the like. An image signal that is input from the image input portion  50  is converted into a digital signal, then, output to the temporary storage portion  94 . 
     The bias control circuit  51  is connected to an electrification-bias power supply  52 , a development-bias power supply  53  and a transfer-bias power supply  54 ; and operates the power supplies  52  to  54  in response to an output signal from the control portion  90 . These power supplies  52  to  54  respond to control signals from the bias control circuit  51 ; the electrification-bias power supply  52  applies a predetermined bias to the electrically conductive rubber roller  15   a  in the electrification device  15 ; the development-bias power supply  53  applies a predetermined bias to the development roller  35  in the development device  16 ; and the transfer-bias power supply  54  applies a predetermined bias to the transfer roller  18 . 
     The operation portion  60  is equipped with a liquid crystal display portion  61 , a LED  62  that indicates various states and a ten-key pad  63 ; a user operates the operation portion  60  to input a command, thereby performing various kinds of setting and performing various functions such as image formation and the like. The liquid crystal display portion  61  indicates states of the image forming apparatus  100 , displays image formation conditions and the number of copies printed; and as a touch panel, allows various kinds of setting such as setting of functions for both-surface print and black/white reverse and the like, magnification, density and the like. The ten-key pad  63  is used to set the number of copies printed and input a FAX number of the other end of the line in a case where the image forming apparatus  100  has also a FAX function. 
     Besides, the operation portion  60  is equipped with: a start button that a user uses to start image formation; a stop/clear button that is used to stop image formation and the like; a reset button that is used to bring various kinds of setting of the image forming apparatus  100  to default states and the like. 
     The image forming apparatus according to the embodiment of the present invention is structured in such a way that it is possible to perform the refresh process in which the toner on the development roller  35  in the development device  16  is ejected to the photoreceptor drum  14  side in a time a transfer to a recording medium is not performed, for example, a time the image forming apparatus is brought from a power-off state or a sleep mode (energy saving) to a copy start state or a time a predetermined number of prints are performed. 
       FIG. 4  is an example of a timing chart showing ON/OFF timing of an electrification bias, a laser exposure, a development bias and a transfer bias (cleaning bias) in the refresh process.  FIG. 5  is a diagram showing an example of a toner ejection pattern formed on the photoreceptor drum. A performance procedure of the refresh process is described by using  FIGS. 4 and 5 , with reference to  FIGS. 1 to 3  when necessary. Here, description is performed about a case where positive-electrified toner is used. 
     When performance of the refresh process is commanded, as shown in  FIG. 4 , an electrification bias applied to the electrically conductive rubber roller  15   a  of the electrification device  15  is turned on, the surface of the photoreceptor drum  14  is evenly positively electrified. At the same time, a development bias that has a polarity (positive) identical to that of the toner and is applied to the development roller  35  is also turned on. Then, laser exposure by the exposure unit  19  repeats turning on/off a predetermined times during t 1  to attenuate partially the electrification of the surface of the photoreceptor drum  14 . 
     When this laser exposure region (electrification attenuation region) comes to a position to face the development roller  35  as the photoreceptor drum  14  rotates, the toner on the development roller  35  counters the development bias and adheres to the laser exposure region. The on state of the development bias is continued until the photoreceptor drum  14  rotates by a predetermined amount and the laser exposure region completely passes the development roller  35 ; and turned off in a time of t 2  after the electrification bias and the laser exposure are turned off. And, a toner ejection pattern P shown in  FIG. 5  is formed on the photoreceptor drum  14 . 
       FIG. 5  is a diagram showing an example of the toner ejection pattern that is used in the embodiment of the present invention, and shows a state of the photoreceptor drum  14  that is expanded in a plane. The toner ejection pattern P is obtained by repeating development of a line image L, which has a predetermined angle to a main scan direction (the right-to-left direction in the drawing), at predetermined intervals over a width H of a development region of the development roller  35  (see  FIG. 2 ); and the continuation time t 1  of the laser exposure in  FIG. 4  is decided on by an ejection length X in a drum circumference direction (subscan direction) of the toner ejection pattern P. A method for deciding on the ejection length X is described later. 
     In a case where the toner ejection pattern is composed of many line images, it is known that an adhesion amount of deteriorated toner to the photoreceptor drum  14  increases compared with a solid image. This is because force to attract toner becomes strong because of edge enhancement, so that it becomes possible to efficiently move the electrified toner from the development roller  35  to the photoreceptor drum  14 . 
     Besides, in a relationship between a line width of the line image that constitutes the toner ejection pattern and a particle diameter of the toner that adheres to the drum surface, the narrower the line width becomes, the smaller the toner particle diameter becomes. This is because the narrower the line width becomes, the higher the percentage of an edge region that occupies the toner ejection pattern, so that small-diameter toner that is present on the outermost surface of the development roller  35  and has a large amount of electrification becomes to be easily developed onto the edge region that has a large potential difference. Accordingly, it is possible to raise the refresh effect by ejecting mainly the small-diameter toner, which is electrified and adversely affects the development quality, to the photoreceptor drum  14  side. 
     In the embodiment of the present invention, the toner ejection pattern P is composed of the slanted line image L. Here, in comparison of the toner ejection pattern P that is composed of the line image L, a toner ejection pattern P 1  that is composed of a line image L 1  perpendicular to the main scan direction as shown in  FIG. 6 , and a toner ejection pattern P 2  that is composed of a line image L 2  parallel to the main scan direction as shown in  FIG. 7 , the toner adhesion amount (toner ejection amount) per unit area in the toner ejection pattern P is large compared with the toner ejection patterns P 1 , P 2 . Although the reason for this has not been cleared yet, the following reasons are supposed. 
     In the toner ejection pattern P 1 , the toner ejection amount in the shaft direction (main scan direction) of the development roller  35  changes from a place where the line image L 1  is present to a place where the line image L 1  is not present (between lines); and a region where the toner is not ejected depending on an interval of the line image L 1  appears. Besides, because of the same reason, in the toner ejection pattern P 2  as well, the toner ejection amount in the circumferential direction (subscan direction) of the development roller  35  becomes to easily change. 
     In contrast, because the line image L that constitutes the toner ejection pattern P has an angle to the main scan direction, the toner ejection amount becomes unlikely to change in both of the shaft direction and the circumferential direction of the development roller  35 . Accordingly, it is possible to eject more evenly the toner on the development roller  35  for the toner ejection pattern P compared with the toner ejection patterns P 1 , P 2 ; and even for the same-area pattern represented by X×H, it is thought that the toner adhesion amount per unit area becomes large. Besides, difference in states of the electrostatic latent image potential due to the inclination of the line image and influence of the development system are also supposed. 
     Here, to refresh the whole circumference of the development roller  35 , it is necessary that the length X in the drum circumference direction (subscan direction) of the toner ejection pattern P is set longer than the outer circumferential length of the development roller  35 . Besides, it is sufficient to suitably set the angle, line width and line interval of the line image L in accordance with the specification of the image forming apparatus; however, it is preferable to set the angle of the line image L at 10° or larger to 60° or smaller and the line width at 2 mm or smaller so as to efficiently remove excessive-electrified toner. Besides, it is sufficient to set the line interval in such a way that the toner on the development roller  35  is able to be evenly ejected. 
     Thereafter, the toner ejection pattern P passes through the nip portion between the photoreceptor drum  14  and the transfer roller  18  as the photoreceptor drum  14  rotates and is finally collected from the drum surface by the cleaning device  17 . To prevent adhesion of the toner ejection pattern P, a transfer bias (transfer reverse bias) that has a polarity (positive) identical to that of the toner is applied to the transfer roller  18 . The on state of the transfer reverse bias is continued until the photoreceptor drum  14  rotates by a predetermined amount and the toner ejection pattern P completely passes the transfer roller  18 ; and turned off in a time of t 3  after the development bias is turned off. 
     Here, the because the toner ejection pattern P is pressurized to the transfer roller  18 , part of the toner ejection pattern P physically adheres to the transfer roller  18  regardless of the application of the transfer reverse bias. In the toner that adheres to the transfer roller  18 , not only positive-electrified toner but also reverse-electrified (negative electrified) toner are present. 
     Because of this, not only a cleaning bias (transfer positive bias) that has a polarity (negative) reverse to the toner is applied to the transfer roller  18  but also the electrification bias is turned on again to positive-electrify the drum surface. According to this, the reverse-electrified toner that adheres to the transfer roller  18  counters the negative cleaning bias, is attracted to the positive potential on the drum surface and moves to the photoreceptor drum  14  side. Here, the on time of the electrification bias is equal to the application time t 4  of the cleaning bias that has the polarity reverse to the toner; and the on timing of the electrification bias is earlier by a time of t 6  than the application timing of the cleaning bias that has the polarity reverse to the toner. 
     Then, not only a cleaning bias (transfer reverse bias) that has a polarity (positive) identical to that of the toner is applied to the transfer roller  18  but also the electrification bias is turned off again. According to this, this time, the positive-electrified toner that adheres to the transfer roller  18  counters the positive cleaning bias, is attracted to the drum surface (0 V) and moves to the photoreceptor drum  14  side. Here, the off time of the electrification bias is equal to the application time t 5  of the cleaning bias that has the polarity identical to the toner; and the off timing of the electrification bias is earlier by a time of t 6  than the application timing of the cleaning bias that has the polarity identical to the toner. 
     In  FIG. 4 , the cleaning bias that has the polarity identical to the toner and the cleaning bias that has the polarity reverse to the toner are each applied one time; and two cycles each of which includes one turning on and one turning off of the electrification bias are repeated at the same period as the cleaning bias, so that the positive-electrified toner and the reverse-electrified toner on the transfer roller  18  are removed. Here, to perform cleaning of the whole outer circumferential surface of the transfer roller  18 , it is preferable that the application time (electrification bias on time) t 4  of the cleaning bias that has the polarity reverse to the toner and the application time (electrification bias off time) t 5  of the cleaning bias that has the polarity identical to the toner are set at times or longer each of which is required for one rotation of the transfer roller  18 . Here, because the toner ejection pattern P is composed of the line image L in the present invention, it is possible to alleviate adhesion of the toner to the transfer roller  18  compared with a solid image. 
     Next, a method for deciding on the ejection length X of the toner ejection pattern P is described. Regardless of user&#39;s use conditions, to set the optimum toner ejection amount depending on the print rate of an image that is printed, an ejection length set table that defines the ejection length X for every print rate is used. The ejection length set table is stored in the ROM  92  (or the RAM  93 ). 
     An example of the ejection length set table is shown in a table 1. In the table 1, the average print rate is ranked into a plurality of levels (here, 5 levels), and an ejection length (mm/paper sheet) for one sheet of image is allocated to each print level. For example, when it is assumed that the number of paper sheet printed to perform the refresh process is A, the control portion  90  calculates a print rate bn for every image based on a digital signal in the temporary storage portion  94 , and further calculates an accumulated print rate Σ bn that is obtained by summing up the print rates bn. And, the accumulated print rate Σ bn is divided by the number A of paper sheets printed that is counted by the counter  95 , so that the average print rate B (%) for the number A of paper sheets printed is calculated. The ejection length for one sheet of image that corresponds to the calculated average print rate B is read out from the ejection length set table and multiplied by the number A of paper sheets printed for the performance of the refresh process, so that the ejection length X (mm) in a performance time of the refresh process is calculated. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Average Print 
                 Ejection Length 
               
               
                   
                 Rate (%) 
                 (mm/paper sheet) 
               
               
                   
                   
               
             
            
               
                   
                 B ≦ 0.5 
                 5 
               
               
                   
                 0.5 &lt; B ≦ 1.0 
                 4 
               
               
                   
                 1.0 &lt; B ≦ 1.5 
                 3 
               
               
                   
                 1.5 &lt; B ≦ 2.0 
                 2 
               
               
                   
                 2.0 &lt; B 
                 0 
               
               
                   
                   
               
            
           
         
       
     
     Besides, the ejection length X may be obtained by calculation. When it is assumed that the reference print rate (threshold value of the print rate that requires the refresh) is C (%), because a print rate difference C−B between the reference print rate C and the average print rate B is an ejection amount (for consumption shortage) necessary for one sheet of image, (C−B)×A (%) that is obtained by the ejection amount for one sheet of image by the number A of paper sheets printed is a toner ejection amount W (%) that is necessary in a performance time of the refresh process. When it is assumed that the toner ejection amount (%/mm) per unit length of the tone ejection pattern P is W 1 , the toner ejection length X is calculated by X=W/W 1 . Here, it is possible to suitably set the number A of paper sheets printed and the reference print rate C (%) depending on use&#39;s use conditions, toner characteristics and measurement values from the outside-machine temperature and humidity sensor  23  and the like. 
     Incidentally, to raise the image density in a time of image formation, usually, the linear speed of the development roller  35  in a time of image formation is set higher than the linear speed of the photoreceptor drum  14 , so that the toner amount per unit time supplied to a vicinity region (development nip portion) between the development roller  35  and the photoreceptor drum  14  is raised. Accordingly, in a case where the toner is forcibly ejected with the development roller  35  rotated at the same linear speed as the linear speed in a time of image formation, only the toner that is easily developed and present on the surface layer of the toner thin layers formed on the development roller  35  is forcibly ejected to the photoreceptor drum  14  side; and the toner on the lower layers remains on the development roller  35 , so that a sufficient refresh effect is not obtained. 
     To avoid this, it is preferable that the rotation speed (linear speed) of the development roller  35  in a performance time of the refresh process is set lower than the speed in a time of image formation. According to such setting, the time during which the electric field between the drum and the development roller acts on the toner thin layers on the development roller  35  that passes through the development region R (see  FIG. 2 ) becomes long; and it is possible to eject not only the toner that is easily developed and present on the surface layer of the toner thin layers formed on the development roller  35  but also the toner on the lower layers, so that it is possible to obtain a sufficient refresh effect without prolonging the performance time of the refresh process and applying a high-voltage development bias. 
       FIG. 8  is a flow chart showing an example of the refresh process performed in the image forming apparatus according to the embodiment of the present invention. A performance procedure of the refresh process is described by following the steps in  FIG. 8 , with reference to  FIGS. 1 to 5  when necessary. 
     First, when the image formation process is started by operation of the operation portion  60  or a personal computer or the like by a user, the number n of paper sheets printed is counted by the counter  95  (the step S 1 ). The control portion  90  calculates the print rate bn for every image based on a digital signal in the temporary storage portion  94 , and further calculates the accumulated print late Σ bn that is obtained by summing up the print rates bn. Then, it is determined whether or not the number n of paper sheets printed reaches the predetermined number A of paper sheets (the step S 2 ). When n=A, the control portion  90  calculates the average print rate B by Σ bn/A (the step S 3 ). 
     Then, the control portion  90  determines whether or not the calculated average print rate B exceeds the reference print rate (here, 1.0%) (the step S 4 ). When the average print rate B exceeds 1.0%, the toner ejection amount (C−B)×A (%) is calculated by multiplying the print rate difference C×B between the average print rate B and the reference print rate C (%), and further the ejection length X of the toner ejection pattern P is calculated (the step S 5 ). 
     And, the refresh process is performed by following the timing chart in  FIG. 4  to eject the toner ejection pattern P onto the photoreceptor drum  14  (the step S 6 ). After the toner ejection is completed, a cleaning bias is applied to the transfer roller  18  to perform cleaning of the transfer roller  18  (the step S 7 ). After the refresh process is completed, the count number n in the counter  95  is reset to 0 (the step S 8 ), and the process returns to the step S 1  again. On the other hand, the average print rate B is 1.0% or lower in the step S 4 , the refresh process is not performed, the count number n in the counter  95  is reset to 0 (the step S 8 ) and the process returns to the step S 1  again. 
     According to the above control, in the refresh process, not only the electrified small-diameter toner on the development roller  35  is efficiently removed but also the toner adhesion to the transfer roller  18  is alleviated, so that it is possible to obtain a high-convenient image forming apparatus that is able to reduce the performance time of the refresh process and alleviate unnecessary toner consumption other than the consumption for the printing operation. Besides, by use of the average print rate B of printed images, a toner, amount for consumption shortage depending on an actual print rate is calculated, and based on this, the toner ejection length X of the toner ejection pattern P is decide on, so that it is possible to set the toner ejection amount in a performance time of the refresh process at the optimum amount. 
     Here, in the above embodiment, the ejection length X is obtained by calculation that uses the number A of paper sheets printed, the average print rate B and the reference print rate C; however, the ejection length X may be decide on by use of the ejection length set table shown in the table 1. 
     Besides, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, in the above embodiments, although the control portion  90  calculates the average print rate, a calculation portion that performs the calculation of the average print rate may be disposed separately from the control portion  90 . Besides, of course, the present invention is applicable to not only a monochrome printer shown in  FIG. 1  but also to various image forming apparatuses that include a development device such as a monochrome copy machine, a tandem-type or rotary-type color copy machine, a color printer, or a facsimile machine, a laser printer or the like. 
     Example 1 
     By use of the image forming apparatus according to the embodiment of the present invention shown in  FIG. 1 , a relationship between the line width of the line image that constitutes the toner ejection pattern shown in  FIG. 5  and the amount and particle diameter of toner that adheres onto the photoreceptor drum is examined. As conditions of a test machine, the circumferential speed (system linear speed) of the photoreceptor drum is set at 306 mm/sec.; as for the drum surface potential, the white-portion potential (dark potential) is set at 270 V, and the image-portion potential (light potential) is set at 20 V. As for the developer, positive-electrified electromagnetic toner that has an average particle diameter of 5.0 to 9.0 μm is used. 
     Besides, the direct-current voltage (Vdc) applied to the development roller is set at 175 V; the Vpp, frequency and Duty of the alternating voltage (Vac) are set at 1.7 kV, 2.8 kHz and 57%, respectively. 
     As apparent from  FIG. 9 , it is confirmed that as the line width of the line image becomes narrower, the amount of toner that adheres onto the drum increases; especially, the adhesion amount considerably increases when the line width is 2 mm or smaller. Besides, as apparent form  FIGS. 10 and 11 , it is confirmed that as the line width of the line image becomes narrower, the percentage of large-diameter toner (&gt;10.8 μm) that adheres onto the drum decreases and the percentage of small-diameter toner (&lt;4.3 μm) increases. 
     As the reason for this, it is thought that as the line width of the line image becomes narrower, the percentage of an edge region to a solid region becomes large, which advantageously influences the development of excessive-electrified small-diameter toner that is easily developed onto the edge region that has a large potential difference. 
     Example 2 
     A relationship between the angle of the line image that constitutes the toner ejection pattern and the amount of toner that adheres onto the photoreceptor drum is examined. The conditions of a test machine are the same as those in the example 1; a square toner ejection pattern, which includes 33 straight lines that each have a line width of 0.3 mm and a length of 25 mm and are drawn at equal intervals (about 0.5 mm) in a width of 25 mm, is rotated by a predetermined angle to the main scan direction, made adhere to the photoreceptor drum, and the toner adhesion amount per unit area is measured. The result is shown in  FIG. 12 . 
     As apparent from  FIG. 12 , as the angle of the line image that constitutes the toner ejection pattern to the main scan direction becomes larger from 0° (horizontal), the toner adhesion amount gradually increases from 6.5 g/m 2  and reaches 7.0 g/m 2  at about 45°. Besides, when the angle of the line image exceeds 45°, the toner adhesion amount gradually decreases and reaches 6.0 g/m 2  (the minimum value) at 90°. Besides, it is confirmed that when the angle of the line image is set at 10° or larger to 60° or smaller, the toner adhesion amount per unit area becomes 6.8 to 7.0 g/m 2  and it is possible to efficiently eject the toner on the development roller. 
     The present invention is summed up from the above embodiments as follows. Specifically, the image forming apparatus according to an embodiment of the present invention is an image forming apparatus that includes: an image carrier that carries an electrostatic latent image; a development device that includes a toner carrier which is disposed opposite to the image carrier, carries and supplies toner to the image carrier, and develops the electrostatic latent image formed on a surface of the image carrier; and a control means that performs a refresh process in which toner is ejected from the toner carrier to the image carrier in a time an image is not formed, and a toner ejection pattern which is formed by ejecting a line image a plurality of times that has an acute angle to a main scan direction at predetermined intervals in a circumferential direction of the image carrier over a total width of a development region. 
     According to this structure, because the toner ejection pattern is composed of the line image that is inclined to the main scan direction, unevenness of the toner ejection amount becomes unlikely to occur in both of the shaft direction and the circumferential direction of the toner carrier, so that it is possible to eject more evenly the toner on the toner carrier. Besides, because the toner ejection pattern is composed of the line image, small-diameter excessive-electrified toner is ejected efficiently by edge enhancement and the toner adhesion to the transfer member is alleviated compared with a case where the toner ejection pattern is composed of a solid image, so that it is possible to prevent rear-surface dirt of a recording medium. 
     Besides, in the image forming apparatus having the above structure according to an embodiment of the present invention, a line width of the line image is 2 mm or smaller. 
     According to this structure, because the percentage of an edge portion in the toner ejection pattern increases, it is possible to efficiently remove small-diameter excessive-electrified toner from the toner carrier by edge enhancement. 
     Besides, in the image forming apparatus having the above structure according to an embodiment of the present invention, an angle of the line image to the main scan direction is 10° or lager to 60° or smaller. 
     According to this structure, because the toner adhesion amount per unit area to the image carrier increases, it is possible to eject more efficiently the toner on the toner carrier to raise the refresh effect. 
     Besides, the image forming apparatus having the above structure according to an embodiment of the present invention includes a control means that controls an ejection length of the line image in a subscan direction in a performance time of the refresh process based on a print rate of an image printed. 
     According to this structure, because a toner amount for consumption shortage depending on an actual print rate is calculated, and a toner ejection length of the toner ejection pattern is decide on, so that it is possible to set the toner ejection amount in a performance time of the refresh process at the optimum amount. 
     Besides, in the image forming apparatus having the above structure according to an embodiment of the present invention, a linear speed of the toner carrier during the refresh process is made slower than a linear speed in a time an image is formed. 
     According to this structure, it becomes possible to eject not only the toner on the surface layer of the toner carrier but also the toner on the lower layers to the image carrier side, so that it is possible to effectively refresh the surface of the toner carrier. 
     Besides, the image forming apparatus having the above structure according to an embodiment of the present invention includes a transfer member that transfers the toner image on the toner carrier that is formed by the development device onto a recording medium, wherein a bias that has a polarity identical to the toner is continuously applied to the transfer member until the toner ejection pattern formed on the image carrier passes the transfer member. 
     According to this structure, it is possible to effectively alleviate the toner ejection pattern adhering to the transfer member when the toner ejection pattern formed on the image carrier passes the transfer member. 
     Besides, the image forming apparatus having the above structure according to an embodiment of the present invention includes an electrification device that electrifies a surface of the image carrier, wherein after the toner ejection pattern formed on the image carrier passes the transfer member, an electrification bias applied to the electrification device is turned on, and a bias that has a polarity reverse to the toner is applied to the transfer member, then, the electrification bias applied to the electrification device is turned off and a bias that has a polarity identical to the toner is applied to the transfer member. 
     According to this structure, it is possible to make positive-electrified toner and reverse-electrified toner that adhere to the transfer member during the refresh process move to the image carrier side, so that it is possible to alleviate more effectively the toner adhering to the transfer member. 
     Besides, in the image forming apparatus having the above structure according to an embodiment of the present invention, the bias that has the polarity reverse to the toner and is applied to the transfer member after the toner ejection pattern passes, and the bias that has the polarity identical to the toner and is applied to the transfer member after the toner ejection pattern passes are applied for a time or longer that is required for one rotation of the transfer member. 
     According to this structure, it is possible to surely make the positive-electrified toner and reverse-electrified toner that adhere to the whole region of the outer circumferential surface of the transfer member move to the image carrier side. 
     The embodiments of the present invention are applicable to image forming apparatuses that have a refresh process to refresh toner on a toner carrier in a time an image is not formed. By use of the present invention, it is possible to efficiently eject excessive-electrified toner on a toner carrier to raise a refresh effect without increasing a toner ejection amount and it is possible to alleviate rear-surface dirt of a recording medium caused by toner adhesion to a transfer member, so that it is possible to provide an image forming apparatus in which disadvantages such as density drop, fogging and the like do not occur and a high-quality image is able to be formed.