Patent Publication Number: US-7917051-B2

Title: Image forming apparatus

Description:
This application is based on Japanese Patent Application No. 2009-144822 filed on Jun. 18, 2009, 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 more particularly to an image forming apparatus that transfers a toner image to a printing medium. 
     2. Description of Related Art 
     For example, image forming apparatuses disclosed by Japanese Patent Laid-Open Publication No. 2005-258275 (Reference 1) and Japanese Patent Laid-Open Publication No. 2004-272144 (Reference 2) are well known. These image forming apparatuses are structured with an intension of preventing a printing medium from being stained with toner leaking out from a developing device. The image forming apparatuses disclosed by these documents are briefly described in the following. 
     The image forming apparatus disclosed by the Reference 1 has a structure to keep the inside of a developing device in a negative pressure. The image forming apparatus disclosed by the Reference 2 has a structure to prevent occurrence of an air flow from a developing device to the outside thereof. In the image forming apparatuses with the structures, toner leakage from the developing devices is suppressed. 
     Thus, the References 1 and 2 discuss suppression of toner leakage out from a developing device, while not discussing prevention of a printing medium from being stained with toner leaking out from a developing device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus wherein a printing medium can be prevented from being stained with toner leaking from a developing device. 
     An image forming apparatus according to an embodiment of the present invention comprises: a photosensitive drum that rotates in a specified direction; a developing device for forming a toner image on a surface of the photosensitive drum; a transfer member for transferring the toner image from the photosensitive drum to a printing medium passing between the photosensitive drum and the transfer member; a first guide, which is disposed in a position downstream from the developing device and upstream from the transfer device with respect to the specified direction to face the photosensitive drum, for guiding the printing medium to between the transfer member and the photosensitive drum; an air flow generator for generating an air flow in a position that is different from a position where a first air flow along the surface of the photosensitive drum is generated by the rotation of the photosensitive drum; and an air passage, which is disposed upstream from a path of the printing medium and downstream from the developing device with respect to the specified direction, for directing part of the first air flow to the second air flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is an enlarged view of an image forming unit employed in the image forming apparatus shown by  FIG. 1 ; 
         FIG. 3  is an illustration of an image forming unit employed in an image forming apparatus according to a comparative example; 
         FIG. 4  is an illustration showing the dimensions of various parts of the image forming unit employed in the image forming apparatus according to the embodiment of the present invention; 
         FIG. 5  is an illustration showing the result of an analysis of a first model; and 
         FIG. 6  is an illustration showing the result of an analysis of a second model. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Image forming apparatuses according to preferred embodiments of the present invention are described with reference to the drawings. 
     General Structure of the Image Forming Apparatus 
     First, an image forming apparatus according to an embodiment of the present invention is described with reference to the drawings.  FIG. 1  shows an image forming apparatus  10  according to an embodiment.  FIG. 2  is an enlarged view of an image forming unit  14  employed in the image forming apparatus  10 . In the following paragraphs, a front side of a sheet means the side of a sheet on which an image is to be printed, and a reverse side of a sheet means the side of a sheet on which an image is not to be printed. 
     The image forming apparatus  10  is an electrophotographic copying machine or printer. The image forming apparatus  10 , as shown by  FIG. 1 , comprises a sheet feed unit  12 , an image forming unit  14 , a fixing unit  16 , a pair of ejection rollers  18 , an ejected-sheet tray  20  and a fan  22 . The sheet feed unit  12  is stored with a stack of sheets (of a printing medium) and feeds the sheets to the image forming unit  14  one by one with a feed roller. 
     The image forming unit  14  is to form a toner image on a sheet. The image forming unit  14  comprises a pair of timing rollers  24 , guides  25   a  and  25   b , a photosensitive drum  26 , a cleaner  27 , a charger  28 , an optical scanning device  30 , a developing device  32  and a transfer roller  34 . A detailed description of the image forming unit  14  will be given later. 
     The fixing unit  16  comprises a pair of fixing rollers and practices a heat treatment and a pressure treatment to a sheet traveling between the fixing rollers. Thereby, the toner image formed on the sheet is fixed thereon. The pair of ejection rollers  18  ejects a sheet fed from the fixing unit  16  onto the ejected-sheet tray  20 . Thereby, sheets with images printed thereon are stacked on the ejected-sheet tray  20 . 
     The fan (an air flow generator)  22  cools the inside of the image forming apparatus  10  by exhausting air from the image forming apparatus  10  to the outside. As shown in  FIG. 1 , the fan  22  is disposed in a position downstream from the image forming unit  14  in a sheet feeding direction. The operation of the fan  22  causes an air flow, which will be described later, inside the image forming apparatus  10 . 
     Next, the image forming unit  14  is described in more detail, referring to  FIGS. 1 and 2 . The photosensitive drum  26  is cylindrical and functions as a toner image bearing member. The photosensitive drum  26  is driven by a motor (not shown) to rotate, as shown in  FIG. 2 , in a direction shown by arrow “A” (in the counterclockwise direction). 
     The charger  28  is disposed to face the surface of the photosensitive drum  26  as shown in  FIG. 1  and charges the surface of the photosensitive drum  26 . As shown in  FIG. 1 , the optical scanning device  30  is disposed in a position downstream from the charger  28  in the direction “A” to face the photosensitive drum  26 . The optical scanning device  30  is controlled by a control section (not shown) in accordance with image data outputted from an image reader (scanner) or a computer and emits a beam “B” to the surface of the photosensitive drum  26 . Thereby, an electrostatic latent image is formed on the surface of the photosensitive drum  26 . 
     As shown in  FIG. 1 , the developing device  32  is disposed in a position downstream from the optical scanning device  30  in the direction “A” to face the photosensitive drum  26 . The developing device  32  forms a toner image on the surface of the photosensitive drum  26 . As shown in  FIG. 2 , the developing device  32  comprises a case  40  and a developing roller  42 . Toner is contained in the case  40 . As shown in  FIG. 2 , the case  40  faces the photosensitive drum  26  keeping out of contact with the photosensitive drum  26 , and more particularly, there is a small gap SP 1  between the case  40  and the photosensitive drum  26 . 
     The developing roller  42  is located inside the case  40  as shown in  FIG. 2  and is cylindrical. The developing roller  42  is partly uncovered from the case  40  and therefore faces the surface of the photosensitive drum  26 . The developing roller  42  is driven by a motor (not shown) to rotate in a direction shown by arrow “C” (in the clockwise direction) and supplies toner to the photosensitive drum  26 . Thereby, a toner image in accordance with the electrostatic latent image is formed on the surface of the photosensitive drum  26 . The developing device further comprises a supply roller and other components, but descriptions of these components are omitted. 
     As shown in  FIG. 2 , the transfer roller  34  is disposed in a position downstream from the developing device  32  in the direction “A” to face the photosensitive drum  26 . The transfer roller  34  transfers a toner image to a sheet that is traveling between the transfer roller  34  and the photosensitive drum  26 . The transfer roller  34  is driven by a motor (not shown) to rotate in a direction shown by arrow “D” (in the clockwise direction) as shown in  FIG. 2 . 
     As shown in  FIG. 1 , the cleaner  27  is disposed in a position downstream from the transfer roller  34  in the direction “A” to face the photosensitive drum  26 . The cleaner  27  collects residual toner from the surface of the photosensitive drum  26 . 
     The pair of timing rollers  24 , as shown in  FIG. 2 , feeds a sheet fed out of the sheet feed unit  12  to the photosensitive drum  26  and the transfer roller  34 . The guides  25   a  and  25   b  are disposed in positions downstream from the developing device  32  and upstream from the transfer roller  34  with respect to the direction “A”. The guides  25   a  and  25   b  guide a sheet fed by the timing rollers  24  to between the photosensitive drum  26  and the transfer roller  34 . The sheet is fed along a sheet path R 1  between the sheet feed unit  12  and the pair of ejection rollers  18 . The guide  25   a  has a guide wall  52 . As shown in  FIG. 2 , the guide wall  52  and the guide  25   b  face to each other and define a part of the sheet path R 1 . The guide  25   a  (guide wall  52 ) is disposed to face the reverse side of a sheet traveling along the sheet path R 1 , and the guide  25   b  is disposed to face the front side of a sheet traveling along the sheet path R 1 . In other words, the guide  25   b  is located in a position downstream from the developing device  32  and upstream from the guide  25   a  (guide wall  52 ) with respect to the direction “A”. 
     Further, as shown in  FIG. 2 , the downstream edge  50  (with respect to the sheet feeding direction) of the guide  25   a  faces the photosensitive drum  26  keeping a small gap SP 3  from the photosensitive drum  26 . Thereby, the guide  25   a  (the edge  50 ) and the photosensitive drum  26  define a part of the sheet path R 1 . The sheet path R 1  comes between the photosensitive drum  26  and the transfer roller  34  after the gap SP 3 . Also, the downstream edge (with respect to the sheet feeding direction) of the guide  25   b  faces the photosensitive drum  26  keeping a small gap SP 2  from the photosensitive drum  26  so as to keep out of contact with the photosensitive drum  26 . 
     When the image forming apparatus  10  operates for printing, the photosensitive drum  26  rotates in the direction “A”. At this time, an air flow F 1  along the circumference of the photosensitive drum  26  is generated as shown in  FIG. 2 . The developing roller  42  partly projects from the case  40  to the gap SP 1 . Therefore, toner adhering to the surface of the developing roller  42  is carried out from the developing device  32  by the air flow F 1 . 
     As shown in  FIG. 2 , the air flow F 1  goes through the gaps SP 1  to SP 3 . Accordingly, toner adheres to the guides  25   a  and  25   b  while passing through the gaps SP 1  to SP 3 . The sheet path R 1  includes the gap SP 3 , which is relatively narrow, and if toner adheres to the edge  50  of the guide  25   a , the reverse side of a sheet may be stained with the toner. In order to avoid this trouble, as will be described later, the image forming apparatus  10  has a structure to prevent toner from adhering to the edge  50  of the guide  25   a.    
     More specifically, in the image forming apparatus  10 , as shown in  FIG. 2 , there is a space between the guide  25   b  and the case  40 , the space extending substantially in parallel to the sheet path R 1 . Further, the upstream edge (with respect to the sheet feeding direction) of the guide  25   b  faces the case  40  keeping a gap SP 4  from the case  40 . Thereby, a flow passage R 2  is formed by the guide  25   b  and the case  40  in a position upstream from the sheet path R 1  and downstream from the developing device  32  with respect to the direction “A”. The flow passage R 2  connects the circumference of the photosensitive drum  26  with a space upstream from the guides  25   a  and  25   b  with respect to the sheet feeding direction. 
     The operation of the fan  22  generates an air flow F 2  in a position different from the position of the air flow F 1 . More specifically, the air flow F 2  passes through a space upstream from the guides  25   a  and  25   b  with respect to the sheet feeding direction. Because of the air flow F 2 , the air pressure in the space upstream from the guides  25   a  and  25   b  is lower than those in the other spaces (for example, lower than the air pressure in the flow passage R 2 ). Therefore, air flows out of the flow passage R 2  through the gap SP 4 , that is, an air flow F 3  as shown in  FIG. 2  is generated in the flow passage R 2 . Thereby, the air flow F 1  is partly guided to the air flow F 2  via the flow passage R 2 . In the structure described above, the flow rate of the air flow F 1  passing through the gap SP 3  decreases, and toner adhesion to the edge  50  of the guide  25   a  can be suppressed. 
     The air flow F 1  comes between the guide  25   a  and the transfer roller  34  after passing through the gap SP 3 . Then, the air flow F 1  joins to the air flow F 2 . The confluent air flows F 1  and F 2  are exhausted from the image forming apparatus  10  by the fan  22 . At this moment, toner is removed from the exhaust air by a filter or the like. 
     Analyses 
     In order to confirm the effects of the image forming apparatus  10 , the inventors carried out computer simulations as described below.  FIG. 3  shows a model of an image forming unit  114  employed in an image forming apparatus according to a comparative example.  FIG. 4  shows the dimensions of various parts of an image forming unit  14  employed in the image forming apparatus  10  according to the present embodiment. 
     More specifically, as a model of the image forming apparatus  10  according to the present embodiment, a model as shown by  FIG. 2 , which will be hereinafter referred to as a first model, was fabricated. As a model of an image forming apparatus  100  according to a comparative example, a model as shown by  FIG. 3 , which will be referred to as a second model, was fabricated. The difference between the first model and the second model is only that the first model has the gap SP 4 . In each of the first model and in the second model, 29 toner particles were put in the narrowest portion between the photosensitive drum  26  and the developing roller  42 , and the loci of the toner particles were figured out. The analyses were carried out under the following conditions (see  FIG. 4 ). 
     The software used for the analyses was FLUENT made by ANSYS Japan Co., Ltd. 
     The toner particles had diameters of 5.0 μm and had densities of 1500 g/m 3 . 
     The components of the models had the following operation speeds: 
     the circumferential speed of the photosensitive drum  26  was 93 mm/s; 
     the circumferential speed of the transfer roller  34  was 93 mm/s; 
     the circumferential speed of the developing roller  42  was 165 mm/s; 
     the circumferential speed of the pair of timing rollers was 93 mm/s; and 
     the air flow rate by the fan was 0.4 m/s. 
     The components of the models had the following dimensions: 
     the width d 1  of the gap SP 1  (the shortest distance between the photosensitive drum  26  and the case  40 ) was 1.0 mm 
     the width d 2  of the gap SP 2  (the shortest distance between the guide  25   b  and the photosensitive drum  26 ) was within the range from 0.5 mm to 1.5 mm; 
     the width d 3  of the gap SP 3  (the shortest distance between the edge  50  and the photosensitive drum  26 ) was 0.9 mm; 
     the width d 4  of the gap SP 4  (the shortest distance between the guide  25   b  and the case  40 ) was 2.0 mm; 
     the distance d 5  between the downstream edge of the guide wall  52  and the downstream edge of the guide  25   b  was 1.5 mm; 
     the distance d 6  between the middle point of the guide wall  52  with respect to the sheet feeding direction and the guide  25   b  was 3.5 mm; 
     the distance d 7  between the upstream edge of the guide wall  52  and the upstream edge of the guide  25   b  was 8.5 mm; 
     the shortest distance d 8  between the guide  25   a  and the transfer roller  34  was 1.2 mm; 
     the shortest distance d 9  between the end of the bent portion of the guide  25   a  and the transfer roller  34  was 5.0 mm; 
     the angle θ 1  of the downstream portion (with respect to the sheet feeding direction) of the guide wall  52  to the guide  25   b  was 14.3 degrees; and 
     the angle θ 2  of the upstream portion (with respect to the sheet feeding direction) of the guide wall  52  to the guide  25   b  was 43 degrees. 
     The first model and the second model were operated under the conditions above, and the following results were obtained.  FIG. 5  shows the results of the analysis of the first model.  FIG. 6  shows the results of the analysis of the second model. 
     The air flow rates at the points shown in  FIG. 5  were as follows: 0.009 m 3 /min at the point P 1 ; 0.005 m 3 /min at the point P 2 ; 0.006 m 3 /min at the point P 3 ; 0.001 m 3 /min at the point P 4 ; 0.025 m 3 /min at the point P 5 ; 0.029 m 3 /min at the point P 6 ; 0.023 m 3 /min at the point P 7 ; 0.058 m 3 /min at the point P 8 ; and 0.004 m 3 /min at the point P 9 . 
     The air flow rates at the points shown in  FIG. 6  were as follows: 0.008 m 3 /min at the point P 11 ; 0.006 m 3 /min at the point P 12 ; 0.002 m 3 /min at the point P 13 ; 0.026 m 3 /min at the point P 14 ; 0.028 m 3 /min at the point P 15 ; 0.024 m 3 /min at the point P 16 ; and 0.059 m 3 /min at the point P 17 . 
     As is apparent from  FIG. 6 , since the second model did not have the gap SP 4 , the air flow F 1  did not come into the flow passage R 2  and came into the gaps SP 2  and SP 3 . In this second model shown by  FIG. 6 , also, since the gap SP 2  was relatively narrow, part of the air flow F 1  leaked out into the sheet path R 1  between the guides  25   a  and  25   b . These results show that in the second model shown by  FIG. 6 , a large volume of toner may adhere to the guide wall  52  and to the edge  50 . 
     On the other hand, as is apparent from  FIG. 5 , since the first model had the gap SP 4 , part of the air flow F 1  (that is, the air flow F 3 ) passed through the passage R 2  and joined to the air flow F 2 . Therefore, in the first model shown by  FIG. 5 , the volume of the air flow F 1  that came into the gaps SP 2  and SP 3  was small, compared with the second model shown by  FIG. 6 . Consequently, adhesion of a large volume of toner to the edge  50  could be suppressed. Further, since the volume of the air flow F 1  coming into the gap SP 3  was small, almost no volume of air leaked from the air flow F 1  into the sheet path R 1  between the guides  25   a  and  25   b . Consequently, in the first model, toner was prevented from adhering to the guide wall  25 . 
     Thus, as in the first model, by making the gap SP 4 , toner adhesion to the guide wall  52  and to the edge  50  can be suppressed. Thereby, as will be described later, stains on the reverse sides of sheets can be prevented. In the second model, in the portions of the guide wall  52  and the edge  50  where sheets of a frequently used size (for example, A4-sized sheets) pass through, toner is often wiped away by these sheets. Accordingly, the volume of toner adhering to the reverse side of such a sheet is small, and the sheet is hardly stained with toner. 
     However, outside of the portions of the guide wall  52  and the edge  50  where sheets of a frequently used size pass through, toner is hardly wiped away and is deposited thereon. Accordingly, when a sheet larger than the frequently used size (for example, an A3-sized sheet) is subjected to printing, the deposited toner adheres to the reverse side of the sheet. Thus, in the second model, sheets of a size larger than the frequently used size are more likely to be stained with toner on the reverse sides when they are subjected to printing than sheets of the frequently used size. 
     In the first model, on the other hand, because of the gap SP 4 , the volume of the air flow F 1  coming into the gaps SP 2  and SP 3  is small, and almost no volume of air leaks from the air flow F 1  to the sheet path R 1  between the guides  25   a  and  25   b . Thereby, in the first model, toner adhesion to the guide wall  52  and to the edge  50  can be suppressed. Consequently, in the fist model, sheets are less likely to be stained with toner on the reverse sides than in the second model. 
     Other Embodiments 
     In the image forming apparatus  10 , the area of the portion where the flow passage R 2  faces the air flow F 2  (that is, the area of the gap SP 4 ) is preferably larger than the area of a section of the portion between the guide  25   b  and the photosensitive drum  26  taken in a direction perpendicular to the arrow “A” (that is, the area of the gap SP 1 ). In this state, part of the air flow F 1  is directed to the air flow F 2  via the flow passage R 2  effectively. Thereby, toner adhesion to the guide  25   b  and to the edge  50  can be suppressed more effectively. 
     There may be provided a changer for changing the area of the portion where the flow passage R 2  faces the air flow F 2 . It is preferred that the changer operates in accordance with the sheet size. 
     The developing device  32  is a device for one-component toner. However, the developing device  32  may be a device for two-component toner. 
     Although the present invention has been described with reference to the embodiment 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.