Patent Publication Number: US-8543022-B2

Title: Image forming apparatus

Description:
This application is based on Japanese Patent Application No. 2010-042226 filed on Feb. 26, 2010, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus that forms an image by means of toner. 
     2. Description of Related Art 
     In an image forming apparatus, toner stored in a development unit decreases in volume under its own weight when it is left unstirred for a long period of time. When the volume of the toner decreases as thus described, a charge amount of the toner might become insufficient. It consequently becomes difficult to obtain a sufficient toner adhering amount upon forming a toner image. 
     As an image forming apparatus for solving the above problem, there is known, for example, an image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-62984. In the image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-62984, a stirring roller is rotated at a higher rate than usual when the development unit has been stopped for a predetermined time or longer. This can lead to recovery of the volume of the toner, and to recovery of the charge amount of the toner. 
     However, the image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-62984 may stir the toner unnecessarily. More specifically, the charge amount of the toner does not depend only upon the volume of the toner. Therefore, even when the toner has been left unstirred for a long period of time and the volume of the toner has decreased, a sufficient charge amount may be obtainable. In such a case, rotating the stirring roller is not necessary. However, in the image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-62984, the stirring roller is rotated on only a condition that the development unit has been stopped for a predetermined time or longer. Consequently, the image forming apparatus described in Japanese Patent Laid-Open Publication No. H08-62984 may execute unnecessary stirring. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to an embodiment of the present invention comprises: an image carrier; an electrostatic latent image forming device that forms an electrostatic latent image on the image carrier; a development device having a housing section that stores toner therein, a stirring device for stirring the toner inside the housing section, and a toner carrier that develops the electrostatic latent image into a toner image by imparting the toner to the image carrier; a voltage applying device that applies a development bias voltage between the image carrier and the toner carrier; a control device that makes the electrostatic latent image forming device and the development device form a test toner image on the image carrier; and a sensing device that senses a toner adhering amount of the test toner image; wherein the control device determines whether to make the stirring device stir the toner based upon a relation between the toner adhering amount of the test toner image and the development bias voltage that was applied by the voltage applying device for formation of the test toner image. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       This and other features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a view showing an overall configuration of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of the image forming apparatus; 
         FIG. 3  is a graph showing test results, as well as a graph showing a development characteristic; 
         FIG. 4  is a graph showing test results, as well as a graph showing toner adhering amounts in the case of using toner left unstirred and in the case of using toner not left unstirred; 
         FIG. 5  is a flowchart of operations performed by a control section of the image forming apparatus; and 
         FIG. 6  is a view showing a development unit provided with a replenishment unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Structure of Image Forming Apparatus 
     Hereinafter, an image forming apparatus according to an embodiment of the present invention is described with reference to the drawings.  FIG. 1  is a view showing an overall structure of an image forming apparatus  1  according to the embodiment of the present invention. 
     An image forming apparatus  1  is an electrophotographic color printer of a tandem type, which is configured so as to synthesize an image of four colors, namely, Y (yellow), M (magenta), C (cyan) and K (black). The image forming apparatus  1  has a function of forming an image on paper (print medium) based upon image data read by a scanner, and includes a printing section  2 , a paper feeding section  15 , a fixing unit  20 , a paper discharge tray  21 , a control section  30 , a voltage applying section  32 , and a sensor  34 . 
     The paper feeding section  15  serves to feed paper piece by piece, and includes a paper tray  16  and a paper feeding roller  17 . In the paper tray  16 , a plurality of pieces of paper in a pre-printed state is stacked and placed. The paper feeding roller  17  takes out the paper from the paper tray  16  piece by piece. 
     The printing section  2  forms a toner image on paper being fed from the paper feeding section  15 . The printing section  2  includes: image forming sections  22 Y,  22 M,  22 C,  22 K; transfer sections  8 Y,  8 M,  8 C,  8 K; an intermediate transfer belt  11 ; a driving roller  12 , a driven roller  13 , a secondary transfer roller  14 , and a cleaning unit  18 . Further, the image forming sections  22 Y,  22 M,  22 C,  22 K include: photosensitive drums (image carriers)  4 Y,  4 M,  4 C,  4 K; chargers  5 Y,  5 M,  5 C,  5 K; exposure units  6 Y,  6 M,  6 C,  6 K; development units  7 Y,  7 M,  7 C,  7 K; cleaners  9 Y,  9 M,  9 C,  9 K, and erasers  10 Y,  10 M,  10 C,  10 K. It is to be noted that, hereinafter, in the case of collectively naming the photosensitive drums, the chargers, the exposure units, the development units, the transfer units, the cleaners, the erasers and the image forming sections, those are simply described respectively as a photosensitive drum  4 , a charger  5 , an exposure unit  6 , a development unit  7 , a transfer unit  8 , a cleaner  9 , an eraser  10  and an image forming section  22 . In the case of indicating the individual photosensitive drums, chargers, exposure units, development units, transfer units, cleaners, erasers and image forming sections, those are respectively described as the photosensitive drums  4 Y,  4 M,  4 C,  4 K, the chargers  5 Y,  5 M,  5 C,  5 K, the exposure units  6 Y,  6 M,  6 C,  6 K, the development units  7 Y,  7 M,  7 C,  7 K, the transfer units  8 Y,  8 M,  8 C,  8 K, the cleaners  9 Y,  9 M,  9 C,  9 K, the erasers  10 Y,  10 M,  10 C,  10 K and the image forming sections  22 Y,  22 M,  22 C,  22 K. 
     The charger  5  charges the peripheral surface of the photosensitive drum  4 . The exposure unit  6  applies laser by control of the control section  30 . Thereby, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum  4 . That is, the charger  5  and the exposure unit  6  serve as an electrostatic latent image forming device that forms an electrostatic latent image on the peripheral surface of the photosensitive drum  4 . 
     As shown in  FIG. 1 , the development unit  7  includes a development roller  72 , a feeding roller  74 , a stirring roller  76 , and a housing section  78 . In  FIG. 1 , for the sake of simplicity of the drawing, only a development roller  72 Y, a feeding roller  74 Y, a stirring roller  76 Y, and a housing section  78 Y of the development unit  7 Y are provided with reference numerals. The housing section  78  constitutes a body of the development unit  7 , and houses the development roller  72 , the feeding roller  74  and the stirring roller  76 . Also, toner is stored in the housing section  78 . The stirring roller  76  stirs the toner inside the housing section  78  to negatively charge the toner. The feeding roller  74  feeds the negatively charged toner to the development roller  72 . The development roller  72  imparts the toner to the photosensitive drum  4 . Specifically, the voltage applying section  32  applies a negative development bias voltage to the development roller  72  so as to form a development field between the photosensitive drum  4  and the development roller  72 . Since the toner is negatively charged, the toner moves from the development roller  72  to the photosensitive drum  4  under the influence of the development field. Further, since the electrostatic latent image is formed on the photosensitive drum  4 , the toner adheres to the photosensitive drum  4  based upon the electrostatic latent image. A toner image based upon the electrostatic latent image is thereby developed on the photosensitive drum  4 . 
     The intermediate transfer belt  11  is extended between the driving roller  12  and the driven roller  13 , and the toner image formed on the photosensitive drum  4  is primarily transferred. The transfer section  8  is arranged so as to face to the inner peripheral surface of the intermediate transfer belt  11 . The transfer section  8  is impressed with a primary transfer voltage and serves to transfer the toner image formed on the photosensitive drum  4  to the intermediate transfer belt  11  (primary transfer). The cleaner  9  serves to collect toner that remains on the peripheral surface of the photosensitive drum  4  after the primary transfer. The driving roller  12  is rotated by an intermediate transfer belt driving section (not shown in  FIG. 1 ) to drive the intermediate transfer belt  11 . In this manner, the intermediate transfer belt  11  carries the toner image to the secondary transfer roller  14 . The sensor  34  is provided so as to face to the intermediate transfer belt  11 , and senses a toner adhering amount of a test toner image, which will be describer later. 
     The secondary transfer roller  14  is opposed to the intermediate transfer belt  11  so as to form a nip section N. The secondary transfer roller  14  transfers the toner image that has been carried by the intermediate transfer belt  11  to paper that has been delivered from the paper feeding section  15  and is passing through the nip section N (secondary transfer). The cleaning unit  18  removes toner that remains on the intermediate transfer belt  11  after the secondary transfer of the toner image to the paper. 
     The paper with the toner image transferred thereto is delivered to the fixing unit  20 . The fixing unit  20  performs a heat treatment and a pressure treatment on the paper to fix the toner image to the paper. In the paper discharge tray  21 , printed paper is placed. 
     Next, a control configuration of the image forming apparatus  1  is described with reference to the drawings.  FIG. 2  is a block diagram of the image forming apparatus  1 .  FIG. 2  illustrates the charger  5 , the exposure unit  6 , the development unit  7 , the transfer unit  8 , the control section  30 , the voltage applying section  32 , and the sensor  34 . As shown in  FIG. 2 , the charger  5 , the exposure unit  6 , the development unit  7 , the transfer unit  8 , the voltage applying section  32  and the sensor  34  are connected to the control section  30  through a bus. Further, the control section  30  controls operations of the charger  5 , the exposure unit  6 , the development unit  7 , the transfer unit  8 , the voltage applying section  32  and the sensor  34 , and is configured by a CPU, for example. 
     Test 
     Next, tests performed by the present inventors are described. The present inventors performed the following tests in order to ensure that a sufficient toner adhering amount on paper is not obtained when toner is left unstirred for a long period of time. Specifically, using toner left unstirred and toner not left unstirred for a long period of time, the present inventors made the image forming apparatus  1  print toner images of a solid pattern on paper, while changing the development bias voltage. Next, the present inventors checked a transmission density of the paper with the toner image printed thereon. The transmission density indicates a degree of interception of light incident to the paper by the toner image. When the transmission density is high, it indicates that a transmitted amount of light is small, and a large amount of toner has adhered. When the transmission density is low, it indicates that a transmitted amount of light is large, and a small amount of toner has adhered. That is, the transmission density is synonymous with the toner adhering amount.  FIG. 3  is a graph showing test results, and the curves in the graph show development characteristics. In the graph, the horizontal axis indicates the development bias voltage, and the vertical axis indicates the transmission density. Further, the solid lines indicate results of the tests performed by using the toner left unstirred, and the dotted lines indicate results of the tests performed by using the toner not left unstirred. 
     According to  FIG. 3 , it is found that in the case of using the toner left unstirred, as compared with the case of using the toner not left unstirred, an equivalent transmission density cannot be obtained unless a development bias voltage with a large absolute value is applied. In other words, this means that the charge amount of the toner is insufficient in the case of the toner left unstirred, and the toner does not sufficiently adhere to paper unless a larger development bias voltage than usual is applied. 
     Next, the present inventors performed the following tests for ensuring whether or not the toner always insufficiently adheres to paper in the case of using the toner left unstirred. Specifically, using the toner left unstirred and the toner not left unstirred, the present inventors made the image forming apparatus  1  perform stabilizing operation, to form a toner image of a test pattern as the solid pattern on the intermediate transfer belt  11 . Then, the sensor  34  was used to sense a toner adhering amount of the test pattern formed on the intermediate transfer belt  11 .  FIG. 4  is a graph showing test results, that is, a graph showing toner adhering amounts in the case of using the toner left unstirred and in the case of using the toner not left unstirred. 
     According to  FIG. 4 , it is found that in the case of using the toner not left unstirred, the toner adhering amount distributes between 4.5 to 5.1 g/m 2 . On the other hand, it is found that in the case of using the toner left unstirred, the toner adhering amount distributes between 3.8 to 5.1 g/m 2 . That is, it is found that the toner adhering amount has a wide distribution in the case of using the toner left unstirred as compared with the case of using the toner not left unstirred. This means that, even when the toner is left unstirred for a long period of time, the charge amount of the toner does not always decreases. It is thus found that, even after the toner has been left unstirred for a long period of time, it is not always necessary to stir the toner with the stirring roller  76 . Thereat, the present inventors invented the image forming apparatus  1  that performs operations described below. 
     Operation of Image Forming Apparatus 
     Hereinafter, the operation of the image forming apparatus  1  is described with reference to the drawings.  FIG. 5  is a flowchart of operations performed by the control section  30  of the image forming apparatus  1 . 
     The control section  30  checks whether or not a predetermined time has elapsed since the control section  30  made the stirring roller  76  stir the toner most recently (Step S 1 ). The predetermined time is the minimum time during which the toner is left unstirred to cause a decrease in volume of the toner and insufficient charge amount of the toner. An example of the predetermined time is the order of twelve hours. When the predetermined time has elapsed, the process goes to Step S 2 . When the predetermined time has not elapsed, the process is completed. 
     When the predetermined time has elapsed, the control section  30  makes the printing section  2  execute the stabilizing operation (Step S 2 ). Specifically, the control section  30  makes the charger  5 , the exposure unit  6  and the development unit  7  form the toner image of the test pattern on the peripheral surface of the photosensitive drum  4 . Further, the control section  30  makes the transfer section  8  transfer the test pattern from the photosensitive drum  4  to the intermediate transfer belt  11 . The control section  30  then makes the intermediate transfer belt driving section drive the intermediate transfer belt  11 . Moreover, the control section  30  obtains a toner adhering amount of the test pattern which is outputted from the sensor  34 . It should be noted that in the stabilizing operation, the control section  30  makes the voltage applying section  32  apply a plurality of different development bias voltages to obtain a plurality of different toner adhering amounts. That is, in the stabilizing operation, the control section  30  obtains a development characteristic curve (as shown in  FIG. 3 ) showing the relation between the toner density of the test image and the development bias voltage applied by the voltage applying section  32  for formation of the test toner image. 
     Next, referring to the development characteristic curves shown in  FIG. 3 , which were obtained in Step S 2 , the control section  30  checks whether or not to execute stirring of the toner (Step S 3 ). Specifically, with respect to the development characteristic curve obtained at Step S 3 , the control section  30  checks whether or not an average inclination of the characteristic curve, wherein absolute values of the, development bias voltage and toner adhering amounts are spotted in the x axis and in the y axis, respectively, is larger than a predetermined value. When the average inclination is larger than the predetermined value, the control section  30  determines that the toner can be sufficiently charged and determines to keep the stirring roller  76  from stirring the toner. On the other hand, when the average inclination is not larger than the predetermined value, the control section  30  determines that the toner cannot be sufficiently charged and determines to make the stirring roller  76  stir the toner. 
     It is to be noted that for example, a least-square method can be adopted to calculate the average inclination. Further, as shown in  FIG. 3 , the transmission density is saturated when the absolute value of the development bias voltage is very large. Hence in the case of calculating the average inclination, it is desirable to use values within a range where the transmission density is not saturated (e.g., −100 to −200 V). Moreover, the predetermined value here is the maximum inclination that can be regarded to prevent achievements of sufficient charge amount of the toner and a sufficient toner adhering amount. For example, in  FIG. 3 , the predetermined value is a value between average inclinations of the test results indicated by the dotted lines and average inclinations of the test results indicated by the solid lines. 
     In the case of executing stirring, the process goes to Step S 4 . In the case of not executing stirring, the process is completed. 
     In the case of executing stirring, the control section  30  makes the stirring roller  76  execute stirring (Step S 4 ). Further, as in Step S 2 , the control section  30  executes the stabilizing operation (Step S 5 ). Then, the process is completed. 
     Effect 
     The image forming apparatus  1  performs the stabilizing operation when toner has been left unstirred for a long period of time. Further, the image forming apparatus  1  checks whether or not a sufficient charge amount of the toner can be obtained, and stirs the toner only when the sufficient charge amount of the toner cannot be obtained. That is, even when the toner has been left unstirred for a long period of time, the image forming apparatus  1  does not stir the toner if a sufficient charge amount of the toner can be obtained. Therefore, in the image forming apparatus  1 , unnecessary stirring of toner will not be executed. 
     Other Embodiments 
     The image forming apparatus  1  is not restricted to the apparatus shown in the foregoing embodiment, but can be modified within the range of its gist. The determination made by the control section  30  at Step S 3  may be, for example, based upon a condition other than the average inclination of a development characteristic. Specifically, the determination may be made based upon whether or not the toner adhering amount obtained with a predetermined development bias voltage applied by the voltage applying section  32  is larger than a predetermined value. In this case, when the toner adhering amount is larger than the predetermined toner adhering amount, the control section  30  determines that stirring is unnecessary. When the toner adhering amount is not larger than the predetermined toner adhering amount, the control section  30  determines that stirring is necessary. It should be noted that the predetermined development bias voltage is a value set within the range (e.g., −100 to −200 V) of the development bias voltage where the transmission density is not saturated in  FIG. 3 . Further, the predetermined toner adhering amount is the maximum toner adhering amount with which it can be regarded that the charge amount of the toner is insufficient and a sufficient toner adhering amount cannot be obtained. 
     Incidentally, in the image forming apparatus  1 , when the toner is left unstirred for a long period of time, the volume of the toner decreases, and accordingly, in this state, the amount of toner stored inside the development unit  7  is larger than it appears. When toner is replenished to the development unit  7  in this state, the toner inside the development unit  7  becomes larger than the capacity. 
     Therefore, in the image forming apparatus  1 , a toner replenishing operation is performed under a control method peculiar to this image forming apparatus  1 . Hereinafter, the replenishment of the toner of the image forming apparatus  1  is described with reference to the drawings.  FIG. 6  is a view showing the development unit  7  provided with a replenishment unit  100 . 
     As shown in  FIG. 6 , the replenishment unit  100  is disposed above the development unit  7 . Further, the development unit  7  is provided with a light-emitting element  80 , a light-receiving element  82 , and light guides  84 ,  86 . The light-emitting element  80  emits light. The light guides  84 ,  86  are disposed on a predetermined level in the housing section  78 . The light guide  84  guides light emitted by the light-emitting element  80  to the housing section  78 . The light guided to the housing section  78  passes through the inside of the housing section  78  and is incident to the light guide  86 . The light guide  86  guides the light that has passed through the inside of the housing section  78  to the light-receiving element  82 . The light-receiving element  82  senses whether or not to have received light, and outputs a sensing result toward the control section  30 . Based upon the sensing result of the light-receiving element  82 , the control section  30  determines whether or not a sufficient volume of toner is stored in the housing section  78 . Specifically, when obtaining a sensing result from the light-receiving element  82  which indicates reception of light, the control section  30  determines that the volume of the toner inside the housing section  78  has decreased to be smaller than a predetermined value. On the other hand, when obtaining a sensing result from the light-receiving element  82  which indicates non-reception of light, the control section  30  determines that the volume of the toner inside the housing section  78  has not decreased to be smaller than the predetermined value. 
     Herein, when the control section  30  determines that the volume of the toner inside the housing section  78  has decreased to be smaller than the predetermined value, the control section  30  next checks whether or not a predetermined time (e.g., twelve hours) or longer has elapsed since the control section  30  made the stirring roller  76  stir the toner most recently. Then, when the control section  30  determines that the predetermined time or longer has elapsed, the control section  30  makes the stirring roller  76  stir the toner while keeping the replenishment unit  100  from replenishing toner. Then, after the stirring of the toner, the control section  30  again checks whether the volume of the toner inside the housing section  78  has decreased to be smaller than the predetermined value. Then, when the control section  30  determines that the volume of the toner has not decreased to be smaller than the predetermined value, the control section  30  keeps the replenishment unit  100  from replenishing toner. This prevents the toner inside the development unit  7  from becoming larger than the capacity. 
     The present invention is useful for an image forming apparatus and is capable of obtaining a sufficient charge amount of toner without executing unnecessary stirring. 
     Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.