Patent Publication Number: US-9841709-B2

Title: Fixing device and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2016-082934 filed on Apr. 18, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a fixing device configured to fix toner to a sheet, and to an image forming apparatus. 
     In a fixing device, a sheet with a toner image formed thereon is passed through a nip portion between a pair of fixing members that are rollers or belts, and the sheet is heated and pressed while it passes through the nip portion. This allows the toner image to be fixed to the sheet. 
     In the above-mentioned fixing device, an offset phenomenon may occur, wherein in the offset phenomenon, a part of the toner transfers to the surface of the fixing member. There is known an image forming apparatus in which, to prevent the offset phenomenon, a corotron is provided between a transfer portion and the fixing portion such that the corotron applies an electric charge to the toner on the sheet. 
     SUMMARY 
     A fixing device according to an aspect of the present disclosure includes a first fixing member, a second fixing member, and a charge application portion. The first fixing member contacts unfixed toner on a sheet conveyed along a conveyance path. The second fixing member forms a nip portion between itself and the first fixing member such that the sheet passes through the nip portion. The charge application portion applies an electric charge to the unfixed toner on the sheet and to an outer circumferential surface of the first fixing member at a region on an upstream side of the nip portion in a movement direction of the outer circumferential surface, by causing a corona discharge to be generated between a discharge electrode and a counter electrode that are disposed apart from the outer circumferential surface of the first fixing member. 
     An image forming apparatus according to another aspect of the present disclosure includes an image generating portion and the fixing device. The image generating portion forms a toner image on a sheet. The fixing device fixes the toner image to the sheet. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram showing a configuration of a fixing portion of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 3  is a diagram showing shapes of a discharge electrode and counter electrodes of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 4  is a diagram showing a configuration of a charge application portion of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 5  is a diagram showing an example of a fixing belt and a toner image that are in a charged state in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 6  is a diagram showing an example of relationship among a current applied to the discharge electrode, a surface potential of the fixing belt, and Zener voltages, in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG. 7  is a diagram showing a configuration of a variation of the charge application portion in the image forming apparatus according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the accompanying drawings for the understanding of the present disclosure. It should be noted that the following embodiment is an example of a specific embodiment of the present disclosure and should not limit the technical scope of the present disclosure. 
     [Configuration of Image Forming Apparatus] 
     First, a description is given of the configuration of an image forming apparatus  1  according to an embodiment of the present disclosure, with reference to  FIG. 1 . As shown in  FIG. 1 , the image forming apparatus  1  includes a sheet cassette  2 , an image generating portion  3 , a fixing portion  4 , and a control portion  5 . The fixing portion  4  or a combination of the fixing portion  4  and the control portion  5  is an example of the fixing device of the present disclosure. It is noted that the present disclosure is not limited to a printer, but is applicable to an arbitrary image forming apparatus such as a copier, a facsimile apparatus, or a multifunction peripheral. 
     The sheet cassette  2  stores sheets such as sheets of recording paper. 
     The image generating portion  3  includes a photoconductor drum, a charger, an exposure device, a developing device, and a transfer device. The image generating portion  3  forms a toner image on a sheet fed from the sheet cassette  2 . 
     The fixing portion  4  fixes the toner image to the sheet by heating and pressing the sheet on which the toner image has been formed. A specific configuration of the fixing portion  4  is described below. 
     The control portion  5  includes control equipment such as CPU, ROM, and RAM. The CPU is a processor that executes various calculation processes. The ROM is a nonvolatile storage portion in which various information such as control programs for causing the CPU to execute various processes are stored in advance. The RAM is a volatile or nonvolatile storage portion that is used as a temporary storage memory (working area) for the various processes executed by the CPU. It is noted that the control portion  5  may include an electronic circuit that realizes the various processes. 
     [Configuration of Fixing Portion] 
     Next, a description is given of the configuration of the fixing portion  4  in the image forming apparatus  1  according to the embodiment of the present disclosure, with reference to  FIG. 2 . 
     The fixing portion  4  includes a fixing unit  10 , an induction heating unit  20 , and a charge application portion  30 . 
     As shown in  FIG. 2 , the fixing unit  10  includes a fixing belt  11  (an example of the first fixing member of the present disclosure), a pressure roller  12  (an example of the second fixing member of the present disclosure), a holding member  13 , a nip forming member  14 , a guide plate  15 , a conveyance guide  17 , and a separation plate  18 , wherein the holding member  13 , the nip forming member  14 , and the guide plate  15  are disposed inside the fixing belt  11 . 
     The fixing belt  11  is formed in a substantially cylindrical shape that is elongated in a width direction perpendicular to the conveyance direction of a sheet S (hereinafter, the direction is merely referred to as a “width direction”). The fixing belt  11  is supported by the holding member  13 , the nip forming member  14 , and the guide plate  15  so as to be capable of rotating around a rotation axis that extends in the width direction. 
     The fixing belt  11  is composed of a base layer, an elastic layer and a release layer, wherein the elastic layer is formed on the base layer, and the release layer is formed to cover the elastic layer. The base layer is, for example, formed by the nickel electrocasting or by performing the plating treatment or the rolling treatment on a metal such as copper. The elastic layer is formed from silicon rubber, for example. The release layer is formed from fluororesin such as PFA, for example. 
     The pressure roller  12  is formed in a substantially cylindrical shape that is elongated in the width direction. The pressure roller  12  is pressed against the fixing belt  11  by a pressure mechanism (not shown), and a nip portion  16  is formed between the fixing belt  11  and the pressure roller  12 . The pressure roller  12  is rotatably supported by a fixing frame (not shown). The pressure roller  12  is rotationally driven by a driving mechanism (not shown). 
     The pressure roller  12  is composed of, for example, a cylindrical core material, an elastic layer and a release layer, wherein the elastic layer is formed on the core material, and the release layer is formed to cover the elastic layer. The core material is formed, for example, by a metal such as stainless steel or aluminum. The elastic layer is formed from silicon rubber or silicon sponge, for example. The release layer is formed from fluororesin such as PFA, for example. 
     The induction heating unit  20  includes induction coils  21 , coil holding portions  22 , arch cores  23   a,  a center core  23   b,  side cores  23   c,  arch core holding portions  24 , and a cover portion  25 , wherein the induction coils  21  are disposed in an arc shape along an outer circumference of the fixing belt  11 , the coil holding portions  22  hold the induction coils  21 , and the arch core holding portions  24  hold the arch cores  23   a.    
     When toner T is fixed to the sheet S, a high-frequency current is applied to the induction coils  21 . This causes the induction coils  21  to generate a magnetic field. An eddy current is then generated in the fixing belt  11  by the act of the magnetic field, and the fixing belt  11  is heated. That is, the fixing belt  11  is heated by the induction coils  21 . In addition, the guide plate  15  is heated by the act of the magnetic field, and the fixing belt  11  is heated also by the guide plate  15 . 
     In addition, when the toner T is fixed to the sheet S, the pressure roller  12  is rotationally driven by a driving mechanism (not shown). This causes the fixing belt  11  that is pressed against the pressure roller  12 , to be rotated following the rotation of the pressure roller  12 . The fixing belt  11  rotated in this way slides over the nip forming member  14 . In this situation, when the sheet S enters the nip portion  16 , the heated fixing belt  11  contacts the unfixed toner T on the sheet S. This causes the toner T to be fused, pressed, and fixed to the sheet S. After it passes through the nip portion  16 , the sheet S is separated from the fixing belt  11  by the separation plate  18 , and is discharged to outside the fixing unit  10 . 
     Meanwhile, in the fixing portion  4  having the above-described configuration, an offset phenomenon may occur, wherein in the offset phenomenon, a part of the toner T transfers to the surface of the fixing belt  11 . It is considered that, to prevent the offset phenomenon, a corotron may be provided between the fixing portion  4  and the transfer device of the image generating portion  3  such that the corotron applies an electric charge to the toner T on the sheet S. However, the ease of occurrence of the offset phenomenon depends not only on the charged state of the toner T on the sheet, but also on the charged state of the fixing belt  11 . Accordingly, only the application of an electric charge to the toner T on the sheet S cannot restrict the offset phenomenon sufficiently. As a result, in the present embodiment, the charge application portion  30  is provided so that the offset phenomenon can be restricted sufficiently. 
     In the present embodiment, it is supposed that unfixed toner T on the sheet S has been positively charged. On the other hand, fluororesin such as PFA that is used in the release layer of the fixing belt  11 , is likely to be negatively charged. Thus in this state, the offset phenomenon is likely to occur, wherein in the offset phenomenon, the positively charged toner T transfers to the negatively charged surface of the fixing belt  11 . In view of this, in the present embodiment, the charge application portion  30  applies an electric charge of the same polarity as that of the toner T on the sheet S (namely, the positive polarity), to the surface of the fixing belt  11 . With such a configuration of the present embodiment where the surface of the fixing belt  11  is charged to the same polarity as that of the toner T, the repulsion of the charges is utilized, and the transfer of the toner T to the surface of the fixing belt  11  is effectively restricted. 
     [Configuration of Charge Application Portion] 
     The charge application portion  30  includes a discharge electrode  31 , a pair of counter electrodes  32  and  33 , an electrode holding portion  34 , and a bolt  35 . The counter electrodes  32  and  33  face each other across the discharge electrode  31 . The electrode holding portion  34  holds these electrodes. The bolt  35  fixes the electrode holding portion  34  to the coil holding portions  22  of the induction heating unit  20  in a detachable manner. 
     The discharge electrode  31  and the counter electrodes  32  and  33  are disposed apart from the outer circumferential surface of the fixing belt  11 . As shown in  FIG. 3 , the discharge electrode  31  is a plate-like electrode made of stainless steel that is thin in plate thickness (approximately 0.1 mm), wherein saw teeth are formed along the width direction at an edge portion facing the fixing belt  11 . The counter electrodes  32  and  33  are plate-like electrodes made of stainless steel, and are disposed apart by a predetermined distance from pointed end portions  31   a  of teeth formed on the discharge electrode  31 . 
       FIG. 4  shows a configuration of the charge application portion  30 . A high-voltage power source  36  is connected to the discharge electrode  31 . When the fixing portion  4  performs a fixing operation, the control portion  5  performs a control to apply a high voltage to the discharge electrode  31 . When a high voltage is applied to the discharge electrode  31 , a corona discharge is continuously generated between the pointed end portions  31   a  of the teeth formed on the discharge electrode  31  and the counter electrodes  32  and  33 . Positive ions generated by the corona discharge move in a direction of going away from the pointed end portions  31   a  of the discharge electrode  31 , and the surface of the fixing belt  11  is positively charged by part of the positive ions. As a result, as shown in  FIG. 5 , the surface of the fixing belt  11  that had been negatively charged is positively charged by the positive ions coming from the charge application portion  30 , and while keeping that state, the surface of the fixing belt  11  moves toward the nip portion  16 . This allows the toner T on the sheet S and the surface of the fixing belt  11  to have the same polarity in the nip portion  16 , which makes it possible to restrict the transfer of the toner T to the surface of the fixing belt  11 . 
     It is noted that another part of the positive ions generated by the corona discharge passes through a gap between the fixing belt  11  and the counter electrode  33  and reaches the surface of the sheet S that is moving toward the nip portion  16 . As a result, the toner T on the surface of the sheet S is further positively charged by the positive ions coming from the charge application portion  30 , and while keeping that state, the sheet S moves toward the nip portion  16 . This makes it possible to further restrict the transfer of the toner T to the surface of the fixing belt  11 . Here, the charge application portion  30  is disposed to face a region on the outer circumferential surface of the fixing belt  11  that is on the upstream side of the nip portion  16  in the movement direction of the outer circumferential surface (namely, a region lower than the one-dot chain line shown in  FIG. 5 ) so that the positive ions generated by the corona discharge can more easily reach the toner T on the sheet S that is moving toward the nip portion  16 . It is noted that the direction in which the pointed end portions  31   a  of the discharge electrode  31  extend (the direction indicated by the arrow A in  FIG. 5 ) may be slightly inclined toward a conveyance path preceding the nip portion  16 , with respect to a direction parallel to the conveyance path (the direction indicated by the arrow B in  FIG. 5 ) so that the positive ions generated by the corona discharge can more easily reach the toner T on the sheet S. It is noted that in the present embodiment, since the main role of the charge application portion  30  is to positively charge the surface of the fixing belt  11  that has been negatively charged, the distance between the pointed end portions  31   a  and the fixing belt  11  is shorter than the distance between the pointed end portions  31   a  and the conveyance path. 
     It is noted that in the present embodiment, as shown in  FIG. 4 , the counter electrodes  32  and  33  are grounded via a Zener diode  37  to enhance the effect of positively charging the surface of the fixing belt  11 .  FIG. 6  is a graph plotting measurement results of the surface potential of the fixing belt  11  against a current applied to the discharge electrode  31  for different Zener voltages of the Zener diode  37 . As apparent from  FIG. 6 , the larger the current applied to the discharge electrode  31  is, the higher the surface potential of the fixing belt  11  is. Also, the higher the Zener voltage of the Zener diode  37  is, the higher the surface potential of the fixing belt  11  is. This is because, as the Zener voltage of the Zener diode  37  is raised, the potential of the counter electrodes  32  and  33  is raised, and as a result, in the positive ions generated by the corona discharge, the ratio of the positive ions moving toward the counter electrodes  32  and  33  decreases, and the ratio of the positive ions moving toward the fixing belt  11  increases. As a result, as the Zener voltage is made higher, the effect of positively charging the surface of the fixing belt  11  is enhanced. However, if the Zener voltage is excessively increased, the surface of the conveyance guide  17  may be positively charged during an interval in which no sheet S passes before the conveyance guide  17 , and the positively charged conveyance guide  17  may attract a negatively charged sheet S. In view of this, the Zener diode  37  with 150 V Zener voltage is used in the present embodiment. 
     It is noted that as another method to enhance the effect of positively charging the surface of the fixing belt  11 , the distance between the pointed end portions  31   a  of the discharge electrode  31  and the counter electrodes  32  and  33  may be increased. However, to increase the distance between the pointed end portions  31   a  of the discharge electrode  31  and the counter electrodes  32  and  33 , a wider installation space is required. On the other hand, the Zener diode  37  adopted in the present embodiment enhances the effect of positively charging the surface of the fixing belt  11  without requiring a wider installation space. 
     As described above, according to the present embodiment, with the configuration where the charge application portion  30  applies an electric charge to the surface of the fixing belt  11  and to the toner T on the sheet S, it is possible to restrict the offset phenomenon sufficiently. 
     It is noted that according to the present embodiment, it is supposed that the toner T on the sheet S has been positively charged. However, in a case where the toner T on the sheet S has been negatively charged, a negative voltage may be applied to the discharge electrode  31  of the charge application portion  30  so that the surface of the fixing belt  11  can be negatively charged. In this case, too, as in the present embodiment, the surface of the fixing belt  11  is charged to the same polarity as that of the toner T, and the transfer of the toner T to the surface of the fixing belt  11  can be effectively restricted, utilizing the repulsion of the charges. 
     In addition, in the present embodiment, a plate-like electrode made of stainless steel on which saw teeth are formed is used as the discharge electrode  31  of the charge application portion  30 . However, the discharge electrode of the present disclosure is not limited to this, but may be an electrode of an arbitrary shape including a plurality of pointed end portions  31   a  that are arranged side-by-side along the width direction perpendicular to the sheet S conveyance direction. Furthermore, a tungsten wire may be used as the discharge electrode  31 . However, using a tungsten wire as the discharge electrode  31  has a defect that a large amount of ozone is generated. As a result, to restrict the amount of ozone to be generated, it is preferable to use an electrode that includes a plurality of pointed end portions  31   a  such as saw teeth. 
     In addition, in the present embodiment, the pair of counter electrodes  32  and  33  that face each other across the discharge electrode  31  are provided. However, to further enhance the effect of positively charging the toner T on the sheet S, only the counter electrode  32  that is farther away from the conveyance path preceding the nip portion  16  than the discharge electrode  31  may be provided, as shown in  FIG. 7 . With this configuration, it becomes easy for the positive ions generated by the corona discharge to move toward the toner T on the sheet S. However, if a priority is given to stable generation of corona discharge, the pair of counter electrodes  32  and  33  that face each other across the discharge electrode  31  may be provided, as in the present embodiment. 
     In addition, in the present embodiment, the induction heating unit  20  is used to heat the fixing belt  11 . However, the present disclosure is not limited to this, but is applicable to, for example, a configuration where the fixing roller is heated by a halogen heater. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.