JP2016051008A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that is inexpensive and excellent in reliability and durability, and that is capable of securely preventing the generation of electrostatic offset.SOLUTION: An image forming apparatus comprises a transfer unit 203 that transfers a toner image on a recording sheet P that passes through a transfer nip part Nt; and a fixation device 23 that makes the recording sheet P having the toner image transferred thereon pass through a fixation nip part Nf to fix the toner image. Between the transfer unit 203 and the fixation device 23 is provided a prefixation conveyance guide 107 that is conductive and guides the recording sheet P between the transfer unit 203 and the fixation device 23. An edge part 107e of the prefixation conveyance guide 107 is disposed higher than the straight line connecting the transfer nip part Nt and the fixation nip part Nf, and the sheet conveyance speed at the fixation nip part Nf is set higher than the sheet conveyance speed at the transfer nip part Nt.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image forming apparatus and an image forming method in which a toner image is transferred onto a recording sheet by electrostatic force and then fixed.

  Conventionally, an electrophotographic apparatus used in a copying machine, a printer, or the like forms an electrostatic latent image by uniformly charging the surface of a photosensitive drum with a charging device and then irradiating the surface of the photosensitive drum with image light. The electrostatic latent image is developed by a developing device to form a toner image. Then, after the toner image is transferred onto the recording paper, the toner image on the recording paper is fixed by heating and pressing with a fixing device.

  The fixing device includes a heating roller (fixing roller) having a built-in lamp inside a rotatable cylindrical portion, and a pressure roller that presses the sheet against the fixing roller. Then, the recording paper carrying the toner image is passed between the fixing roller and the pressure roller (fixing nip portion), and the toner image is melted and fixed on the recording paper by heating and pressing.

  In this fixing, it is known that a phenomenon in which toner electrostatically adheres to the surface of the fixing roller (this phenomenon is called “electrostatic offset”) occurs.

  This electrostatic offset is a phenomenon in which the toner (toner image) carried on the recording paper before the fixing process is electrostatically attracted to the surface of the fixing roller, and the recording paper may be charged during the transfer process. It is the main cause. Specifically, the toner is normally negatively charged, and in order to transfer the toner onto the recording paper, the recording paper is charged positively with a polarity opposite to that of the toner in the transfer process. Thereafter, depending on the charging series of the material of the fixing roller surface, when the recording paper passes through the fixing nip portion, the fixing roller is positively charged by the positive charge of the recording paper. As a result, the negatively charged toner on the recording paper is electrostatically attracted to the surface of the fixing roller by the positive charge of the fixing roller, and electrostatic offset occurs.

  As means for preventing this electrostatic offset, in Patent Document 1 and Patent Document 2, a static eliminating brush is attached to the back surface of the paper guide before fixing, and the static eliminating brush is brought into contact with the back surface of the recording paper to static neutralize the sheet. Fixing devices that suppress the occurrence of electrical offset have been proposed.

JP 2008-96695 A JP 2008-129384 A

However, the fixing devices disclosed in Patent Document 1 and Patent Document 2 have the following problems.
(1) Since the static elimination brush does not have strong hair, it is difficult to make it come into contact with the back surface of the recording paper and the static elimination effect is not sufficient.
(2) Since the static elimination brush is not uniformly planted in the width direction, unevenness in static elimination partially occurs in the width direction of the recording paper.
(3) Depending on the relationship between the conveyance speed of the recording paper in the fixing process and the transfer process on the upstream side, the paper path after biting into the fixing nip changes, and the static elimination brush is recorded reliably and constantly. Difficult to contact the back of the paper.
(4) The static elimination brush is difficult in terms of durability, and the long-term use causes the flocking to wear and the static elimination performance to deteriorate.
(5) When the static elimination brush is soiled with toner or the like, a sufficient static elimination effect cannot be obtained.
(6) The static elimination brush may be peeled off from the paper guide before fixing due to use in a fixing device having a high ambient temperature, long-term use for 4 to 5 years, or use in a high-temperature and high-humidity environment.
(7) An additional charge eliminating brush is required, leading to an increase in the cost of the fixing device.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that is inexpensive, excellent in reliability and durability, and can reliably prevent the occurrence of electrostatic offset. And

  In order to solve the above problems, an image forming apparatus according to the present invention passes a transfer unit that transfers a toner image onto a recording sheet that passes through a transfer nip, and passes the recording sheet onto which the toner image has been transferred to a fixing nip unit. An image forming apparatus including a fixing unit that performs fixing and includes a guide member that has conductivity and guides recording paper between the transfer unit and the fixing unit. At least a part of the guide surface is disposed so as to protrude from a straight line connecting the transfer nip portion and the fixing nip portion, and the sheet conveyance speed at the fixing nip portion is set to the sheet conveyance at the transfer nip portion. The recording paper which is set at a speed higher than the speed and is conveyed between the transfer nip portion and the fixing nip portion is conveyed while being rubbed against the guide surface protruding from the straight line. Features .

  According to the above configuration, the recording paper conveyed between the transfer unit and the fixing unit is set so that the paper conveyance speed at the fixing nip is higher than the paper conveyance speed at the transfer nip. It is conveyed without bending. Further, the guide member is disposed so that the guide surface protrudes from a straight line connecting the transfer nip portion and the fixing nip portion. For this reason, the recording paper back surface is conveyed while being pressed against the guide surface of the protruding portion of the guide member, and an effective charge eliminating effect by the guide member is obtained, so that electrostatic offset at the time of fixing can be effectively prevented.

  In the image forming apparatus, the guide member may be grounded through a high resistance element. The resistance value of the high resistance element is preferably 500 MΩ or more and 5 GΩ or less.

  According to the above configuration, the guide member is grounded via the high resistance element, thereby preventing the transfer omission caused by the transfer current leaking from the guide member via the recording paper, particularly in a high humidity environment. can do. As a result, it is possible to achieve both suppression of electrostatic offset due to charge removal of the recording paper and prevention of transfer omission due to transfer current leakage in a high humidity environment.

  In the image forming apparatus, the guide member is connected to a switching unit that switches between a grounded state and a floating state, and the switching unit sets the guide member to a grounded state in a low-humidity environment. Then, it can be set as the structure which makes the said guide member into a floating state.

  According to the above configuration, the toner contamination due to electrostatic offset can be reduced by placing the guide member in a grounded state in a low humidity environment, and the leakage of transfer current can be prevented by placing the guide member in a floating state in a high humidity environment. By doing so, transfer omission can be prevented. As a result, it is possible to achieve both suppression of electrostatic offset due to charge removal of the recording paper and prevention of transfer omission due to transfer current leakage in a high humidity environment.

  In the image forming apparatus, in the projecting region where the guide surface of the guide member projects from the straight line, when the maximum projecting distance between the straight line and the guide surface is a projecting height, the projecting height is 1 mm or more and 3 mm or less. It is preferable that the range is set.

  According to the above configuration, a sufficient sheet neutralization effect can be obtained by setting the protruding height to 1 mm or more, and image defects due to the trailing edge of the sheet can be avoided by setting the protruding height to 3 mm or less. it can.

  In the image forming apparatus, in the projecting region in which the guide surface of the guide member projects from the straight line, when the maximum projecting distance between the straight line and the guide surface is the projecting height, the guide member changes the projecting height. The changing means comprises a changing means that enables the protruding height to be increased in a low-humidity environment, and the guide member is disposed so that the back side of the recording paper contacts the protruding area. Then, it can be set as the structure which arrange | positions the said guide member so that the said protrusion height may be made small and the said protrusion area | region may not exist.

  According to the above configuration, in a low-humidity environment, the toner contamination due to electrostatic offset can be reduced by making the guide member abut against the recording paper and discharging, and in a high-humidity environment, the guide member is separated from the recording paper. Transfer omission due to transfer current leakage can be prevented. As a result, it is possible to achieve both suppression of electrostatic offset due to charge removal of the recording paper and prevention of transfer omission due to transfer current leakage in a high humidity environment.

  Further, in the image forming apparatus, the guide surface of the guide member is subjected to an insulating process except for a region that is rubbed with a recording sheet conveyed between the transfer nip portion and the fixing nip portion. Is preferred.

  According to the configuration described above, it is possible to prevent the guide surface of the guide member from being contaminated with toner and hindering the conveyance of the recording paper or the recording paper from being stained.

  In the image forming apparatus, the recording paper is charged to a charge opposite to that of the toner image after passing through the transfer nip portion.

  In the image forming apparatus, the edge portion on the downstream side in the sheet conveyance direction on the guide surface of the guide member may protrude most far from the straight line.

  According to said structure, the recording paper conveyed can be contact | abutted with a line (edge) with respect to a guide member, and a guide member can be contact | abutted to the recording paper back surface more reliably and stably.

  Furthermore, in order to solve the above problems, the image forming method of the present invention transfers a toner image onto a recording paper passing through the transfer nip portion, and passes the recording paper onto which the toner image has been transferred through the fixing nip portion. An image forming method in which fixing is performed, and the recording paper is conveyed between the transfer nip portion and the fixing nip portion while guiding the recording paper with a conductive guide member, and the guide surface of the guide member At least a portion thereof is arranged so as to protrude from a straight line connecting the transfer nip portion and the fixing nip portion, and the paper conveyance speed at the fixing nip portion is higher than the paper conveyance speed at the transfer nip portion. By setting a high speed, the recording paper conveyed between the transfer nip portion and the fixing nip portion is conveyed while being rubbed against the guide surface protruding from the straight line. It is.

  According to the above configuration, as in the image forming apparatus of the present invention, an effective charge removal effect by the guide member can be obtained, and electrostatic offset during fixing can be effectively prevented.

  In the image forming apparatus and the image forming method of the present invention, in the recording paper conveyed between the transfer portion and the fixing portion, the back surface of the recording paper is reliably pressed against a guide member having a charge eliminating function (conductive). It can be conveyed, and an effective charge removal effect by the guide member can be obtained. For this reason, there is an effect that the electrostatic offset at the time of fixing can be effectively prevented.

1 is an internal configuration diagram showing a configuration of an image forming system according to an embodiment of the present invention. 2 is a cross-sectional view of the periphery of a photoconductor, a transfer unit, and a fixing device of the image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a schematic diagram viewed from the axial direction of a fixing roller of the fixing device in FIG. 2. 6 is a cross-sectional view of the periphery of a photoconductor, a transfer unit, and a fixing device of an image forming apparatus according to Embodiment 2. 6 is a cross-sectional view of the periphery of a photoconductor, a transfer unit, and a fixing device of an image forming apparatus according to Embodiment 3. FIG. FIG. 6 is a cross-sectional view of the periphery of a photoconductor, a transfer unit, and a fixing device of an image forming apparatus according to a fourth embodiment. FIG. 10 is a cross-sectional view of the periphery of a photoconductor, a transfer unit, and a fixing device of an image forming apparatus according to a fifth embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same members are denoted by the same reference numerals. The following embodiments are examples embodying the present invention, and do not limit the technical scope of the present invention.

[Embodiment 1]
FIG. 1 is an internal configuration diagram showing a configuration of an image forming system 1 according to an embodiment of the present invention. In the image forming system 1, a scanner 3, an automatic document conveying device 4, a post-processing device 5, a recording material supply device 6, a relay conveying unit 8, and a double-sided conveying device 10 are connected with an image forming device 2 as a core. And the functionality has been extended. Among these, the image forming apparatus 2 and the post-processing apparatus 5 are placed side by side on the upper surface of the recording material supply apparatus 6. Above these, the scanner 3 and the automatic document feeder 4 are arranged on a system rack 7 so as to be supported.

  When the image forming apparatus 2 functions as a copying machine, it records and outputs an image read by the scanner 3. When the printer functions as a printer, it records and outputs image data from an image processing apparatus such as a personal computer that is an externally connected device.

  An electrophotographic process section (image forming section) centered on a photosensitive body (photosensitive drum) 200 is disposed on the substantially left side of the center of the image forming apparatus 2. Around the photoconductor 200, a charging roller 201, an optical scanning unit 22, a developing unit 202, a transfer unit 203, a cleaning unit 204, a static elimination lamp unit (not shown), and the like are arranged. The charging roller 201 uniformly charges the surface of the photoreceptor 200. The optical scanning unit 22 scans the optical image on the uniformly charged photoreceptor 200 and writes an electrostatic latent image. The developing unit 202 reproduces the electrostatic latent image written by the optical scanning unit 22 with a developer. The transfer unit 203 transfers the image reproduced on the photoreceptor 200 onto the recording material. The cleaning unit 204 removes the developer remaining on the photoconductor 200 after the transfer and enables a new image to be recorded on the photoconductor 200. The static elimination lamp unit removes the charge on the surface of the photoreceptor 200.

  A recording material supply unit capable of accommodating a recording material is disposed below the image forming apparatus 2, that is, on the recording material supply device 6 side. The recording materials separated and fed one by one from the recording material supply unit are sequentially supplied between the photoconductor 200 and the transfer unit 203 of the electrophotographic process unit, and the toner image recorded and reproduced on the photoconductor 200 is transferred. Is done. In addition, a recording material supply device 6 including at least one recording material supply unit is optionally disposed on the lower surface of the main body of the image forming apparatus 2. The recording material sent from the recording material supply device 6 is received by the image forming device 2 from the recording material receiving port on the lower surface of the main body of the image forming device 2.

  A fixing device 23 is disposed above the electrophotographic process unit, which sequentially receives the recording material onto which the image has been transferred and heat-fixes the developer transferred onto the recording material. The recording material on which the image is recorded is sent out of the image forming apparatus 2 and transferred to the relay conveyance unit 8 on the upper surface of the main body of the image forming apparatus 2. As described above, the sheet conveyance path (recording material conveyance path) of the image forming apparatus 2 is configured in a substantially vertical shape from the bottom to the top.

  An optical scanning unit 22, an apparatus control unit, an image control unit, a power supply unit, and the like are disposed on the side of the electrophotographic process unit, that is, at a position adjacent to the electrophotographic process unit in a direction substantially perpendicular to the paper conveyance path. Has been.

  The post-processing device 5 introduces a recording material on which an image discharged from the image forming apparatus 2 and the relay conveyance unit 8 is introduced, and performs post-processing on the recording material. Post processing includes stapling processing, punching processing, sorting processing, and the like. Those that have been post-processed are selectively discharged to the lower discharge tray, and those that have not been post-processed are selectively discharged to the upper discharge tray.

  The scanner 3 has an automatic reading mode and a manual reading mode. In the automatic reading mode, a sheet-like document automatically supplied by the automatic document feeder 4 is sequentially exposed and scanned one by one to read a document image. In the manual reading mode, a document image of a book-like document set by a user's manual operation or a sheet-like document that cannot be automatically supplied by the automatic document feeder 4 is read.

  In the automatic reading mode, the automatic document feeder 4 sequentially conveys the document placed on the document set tray toward the document placement table of the scanner 3. Further, in the manual reading mode, the automatic document feeder 4 rotates upward with the rear side of the apparatus as a fulcrum so that a sheet-like document that cannot be automatically supplied can be placed on the document table and read. The front side of the device is opened.

  The relay conveyance unit 8 is a conveyance unit that is mounted on an upper portion of a paper discharge tray provided at the top of the image forming apparatus 2 and introduces a recording material discharged from the image forming apparatus 2 toward the post-processing apparatus 5. . Further, in the middle of the transport path of the relay transport unit 8, another recording material transport path for branching the recording material to a discharge tray formed by the upper surface of the unit and the upper surface of the post-processing device 5 is branched.

  The double-sided conveying device 10 is a conveying device that is attached to the side surface of the image forming apparatus 2 and reverses the recording material when forming images on both sides of the recording material. The recording material that has passed through the double-sided conveyance device 10 enters the paper conveyance path again from before the electrophotographic process unit and passes through the electrophotographic process unit or the like, whereby an image is formed on the back surface.

  By configuring the system by combining peripheral devices as in the image forming system 1, an image forming system desired by the user can be easily realized.

  Next, the transfer unit 203 and the fixing device 23 which are characteristic portions of the present invention will be described in detail with reference to FIGS. 2 is a cross-sectional view of the periphery of the photosensitive member 200, the transfer unit 203, and the fixing device 23 of the image forming apparatus 2 according to the present embodiment, and FIG. 3 is a schematic view viewed from the axial direction of the fixing roller of the fixing device 23. is there.

  The transfer unit 203 includes a transfer roller 2031 as a transfer member, and a high-voltage power supply 2032 connected to the transfer roller 2031. The transfer roller 2031 is rotatably installed by bearings (not shown) provided at both ends in the axial direction. In addition, a gear (not shown) is connected to one end of the transfer roller 2031 at the end in the axial direction. A driving force from a separately installed motor is transmitted to the transfer roller 2031 via a gear, and the transfer roller 2031 is rotationally driven.

The transfer roller 2031 includes a cored bar 2031a that is a main body of the transfer roller 2031 and a conductive elastic layer 2031b formed on the outer peripheral surface of the cored bar 2031a. The transfer roller 2031 is pressed against the photoreceptor 200 with a predetermined load (30 N in the present embodiment) by a spring (not shown), thereby forming a transfer nip portion Nt. For the core metal 2031a, for example, a metal such as iron or stainless steel or an alloy thereof is used. In addition, as the metal core 2031a of the present embodiment, a stainless steel metal bar having a diameter of 8 mm is used. For the conductive elastic layer 2031b, urethane rubber, EPDM (ethylene / propylene / diene) rubber, or the like is used. As the conductive elastic layer 2031b of this embodiment, foamed EPDM rubber having a thickness of 3.5 mm and a volume resistance of 1 × 10 ⁸ Ωcm is used.

  The fixing device 23 includes a fixing roller 101 as a heating member, a pressure roller 106 as a pressure member, a heater lamp 105 as a heat source for the fixing roller, temperature sensors (thermistors) 103a and 103b, thermostats 104a and 104b, and a cleaning device. 110 is provided. The temperature sensors (thermistors) 103 a and 103 b constitute temperature detection means for detecting the temperature of the fixing roller 101. The thermostats 104a and 104b constitute an abnormal temperature detecting means. In addition, the fixing device 23 includes, as a control system (not shown), a first control circuit that controls the temperature and rotational driving of the fixing roller 101, a second control circuit that controls the feeding of the cleaning web of the cleaning device 110, and a toner image. Is provided with a printing rate measuring circuit for measuring the printing rate.

  The fixing roller 101 is rotatably installed by bearings 102 provided at both ends in the axial direction. Further, a gear (not shown) is connected to one end of the fixing roller 101 at an axial end. Then, a driving force from a separately installed motor is transmitted to the fixing roller 101 via a gear, and the fixing roller 101 is rotationally driven. The pressure roller 106 is provided with a bearing (not shown) at the shaft end so as to be rotatable. The bearing is supported by a pressure holding unit 108 a for pressing the pressure roller 106 to the surface of the fixing roller 101. Further, a spring 108b is provided for applying a load to the pressure holding unit 108a.

  The heater lamp 105 is fixedly disposed inside the fixing roller 101, and includes a main lamp 105a, a sub lamp 105b, and an assist lamp 105c. Further, halogen lamps are used for these lamps. When the heater lamp 105 is energized from the first control circuit, the heater lamp 105 emits light with a predetermined heat distribution, infrared rays are emitted, and the inner peripheral surface of the fixing roller 101 is heated. As shown in FIG. 3, the main lamp 105 a is a central heating heater in which a filament 105 af is provided at the center in the longitudinal direction of the fixing roller 101, and its rated output is 700 W. The sub lamp 105b is a heater for heating an end portion in which filaments 105bf are provided at both ends in the longitudinal direction of the fixing roller 101, and has a rated output of 270W. The assist lamp 105c is a warm-up assist heater in which the filament 105cf is provided over the entire width in the longitudinal direction of the fixing roller 101. The assist lamp 105c is lit only during warm-up and has a rated output of 230W.

  The fixing roller 101 is heated to a predetermined temperature (here, 180 ° C.) by the main lamp 105a, the sub lamp 105b, and the assist lamp 105c, and a recording in which an unfixed toner image T that passes through the fixing nip portion Nf of the fixing device is formed. Heat the paper P. Here, the fixing nip portion Nf is a pressure contact portion between the fixing roller and the pressure roller. The fixing roller 101 includes a cored bar 101a as a main body and a release layer 101b formed on the outer peripheral surface of the cored bar 101a. The release layer 101b is provided to prevent the toner T on the recording paper P from being offset.

  For the core metal 101a, for example, a metal such as iron, stainless steel, aluminum, copper, or an alloy thereof is used. As the core metal 101a of the present embodiment, an iron (STKM) metal core having a diameter of 40 mm and a wall thickness of 0.45 mm is used in order to reduce the heat capacity.

  For the release layer 101b, fluororesins such as PFA (copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether) and PTFE (polytetrafluoroethylene), silicone rubber, fluororubber, and the like are suitable. The release layer 101b of the present embodiment was formed by applying and baking a mixture of PFA and PTFE to a thickness of 22.5 μm through a 7 μm-thick primer layer.

  The pressure roller 106 is configured to have a heat-resistant elastic body layer 106b such as silicone rubber on the outer peripheral surface of a cored bar 106a made of steel, stainless steel, aluminum or the like. On the surface of the heat resistant elastic layer 106 b of the pressure roller 106, a release layer 106 c made of a fluororesin similar to the case of the fixing roller 101 may be formed. The pressure roller 106 in the present embodiment has a diameter of 40 mm, and is provided with a heat-resistant elastic body layer 106b on a stainless steel core 106a and a release layer 106c on the surface thereof. The heat-resistant elastic layer 106b is made of a silicone sponge rubber having a thickness of 8 mm, and the release layer 106c is made of a PFA tube having a thickness of 50 μm. The pressure roller 106 is pressed into contact with the fixing roller 101 by a force of 22 kgf (216 N) by a spring 108 b as a pressure member, whereby a fixing nip portion Nf having a width of about 7.5 mm is formed between the pressure roller 106 and the fixing roller 101. It is formed.

As the PFA tube used for the release layer 106c, it is more preferable to use a conductive PFA tube containing a conductive agent such as carbon. When an insulating PFA tube is used for the release layer 106 c, the pressure roller 106 is charged to approximately −5 kV by frictional charging with the recording paper and the fixing roller 101. As a result, an electric field that repels the toner of the same negative polarity toward the fixing roller 101 acts on the fixing nip portion Nf, and an electrostatic offset with respect to the fixing roller 101 (a phenomenon in which toner adheres electrostatically to the fixing roller). Cause. Therefore, in this embodiment, by using a conductive PFA tube for the release layer 106c, charging of the pressure roller 106 is prevented, and the occurrence of electrostatic offset is suppressed. In order to obtain this antistatic effect, a PFA tube having a volume resistance of 10 ⁵ Ωcm is used in this embodiment.

  Thermistors 103 a and 103 b as temperature detecting means are disposed on the peripheral surface of the fixing roller 101 so as to detect the surface temperature of the fixing roller 101. Based on the temperature data detected by the thermistors 103a and 103b, a first control circuit (not shown) controls energization to the main lamp 105a, the sub lamp 105b, and the assist lamp 105c. Thereby, the temperature of the fixing roller 101 is controlled to be a predetermined temperature.

Further, a cleaning web 113 is disposed above the fixing roller 101. The cleaning web 113 is wound from the supply roller 111 side to the take-up roller 112 side, and is wound by the second control circuit at a predetermined feed amount every time the recording material is fixed. The cleaning web 113 is pressed by the pressing roller 114 and is in contact with the surface of the fixing roller 101 while being wound around the winding roller 112, and cleans the surface of the fixing roller 101 and applies a release agent. The pressing roller 114 is a heat-resistant sponge roller having a diameter of 18 mm. In the present embodiment, silicone oil is used as a release agent, and the cleaning web is impregnated with an amount of 6 g / m ² . The contact width Nc (cleaning nip width) between the cleaning web 113 and the fixing roller 101 is 3 mm.

  A post-transfer conveyance guide 109 and a pre-fixation conveyance guide 107 are disposed between the transfer unit 203 and the fixing device 23. The post-transfer conveyance guide 109 and the pre-fixing conveyance guide 107 form a conveyance path for the recording paper P, and guide the recording paper P to the fixing nip portion Nf of the fixing device 23. The post-transfer conveyance guide 109 is made of insulating resin (ABS in this embodiment), and the pre-fixing conveyance guide 107 is made of metal (stainless steel in this embodiment). The guide surface 107g of the pre-fixing conveyance guide 107 is coated with a fluororesin (insulating property) to prevent the toner surface from being stained except for the edge portion 107e at the rear end of the guide surface.

  Transfer and fixing of the toner image onto the recording paper P in the image forming apparatus 2 described above are performed at a predetermined process speed (255 mm / s in the present embodiment) and a copying speed (56 sheets / min in the present embodiment) as follows. It is performed in the procedure. First, the recording paper P is conveyed to the transfer nip portion Nt, and the toner image T on the photoreceptor 200 is transferred onto the recording paper P by electrostatic force. Thereafter, the recording paper P on which the unfixed toner image T is formed is guided to the post-transfer conveyance guide 109 and the pre-fixation conveyance guide 107, and is conveyed to the fixing nip portion Nf, and is fixed by heat and pressure.

  Next, an electrostatic offset evaluation experiment was performed using the above-described image forming apparatus 2.

<Experiment method>
As an experimental method, a high-printing-rate original (a photographic image-based original with a printing rate of 37% in this experiment) is printed continuously in a low-humidity environment (25 ° C, 10%) where electrostatic offset is likely to occur. did. At this time, the toner image of the document with a high printing rate is electrostatically offset to the fixing roller, and the electrostatically offset toner passes through the cleaning web and moves to the pressure roller between sheets. Thereafter, one sheet of evaluation recording paper (here, a thick paper having a grammage of 300 g / m ^{2 that} can easily remove dirt from the pressure roller) is printed, and toner dirt on the back surface of the evaluation recording paper (toner dirt due to electrostatic offset). ) Level.

  For evaluation of toner contamination, a portable spectral densitometer (model 939) manufactured by x-rite was used. As a specific method, the density of the back surface of the evaluation recording paper before and after printing was measured (average of five locations), and the evaluation was performed based on the difference (density change amount). When the density change amount is greater than 0.009, the level of toner contamination can be clearly recognized with the naked eye and problematic for practical use. In addition, if it is 0.005 or less, it is a level at which there is no problem with the naked eye.

  At the same time, the charge amount of the recording paper P after passing through the fixing nip portion Nf and the charge amount on the surface of the fixing roller 101 were measured using a surface potential meter (MODEL 344 manufactured by Trek Japan Co., Ltd.).

<Experiment 1>
The first experimental conditions and results are shown in Table 1. The following four types of experimental parameters were used in Experiment 1.
(1) Protrusion Height of Pre-Fixing Transport Guide An experiment was conducted by changing the position (projection height) of the pre-fixing conveyance guide 107 in the direction indicated by the arrow A in FIG. Here, the protruding height of the pre-fixing conveyance guide does not mean the height in the vertical direction, but the guide surface of the pre-fixing conveyance guide 107 with respect to the line PP connecting the transfer nip portion Nt and the fixing nip portion Nf. It means the maximum protruding distance. In other words, in the present embodiment, the guide surface 107g of the pre-fixing conveyance guide 107 is disposed so as to intersect the line PP, and a part of the guide surface 107g protrudes from the line PP (the guide surface is opposed to the line PP). Not arranged).

In the configuration shown in FIG. 2, a partial region including the edge portion 107e on the downstream side in the sheet conveyance direction on the guide surface 107g protrudes from the line PP. Since the edge portion 107e is located farthest from the line PP in this protruding region, the distance in the A direction of the edge portion 107e with respect to the line PP becomes the protruding height of the pre-fixing conveyance guide 107. In addition, the numerical value of the height shown in Table 1 indicates the protruding height of the edge portion 107e when the protruding region exists, and the minus indicates a line with the edge portion 107e when the protruding region does not exist. The distance from PP is shown.
(2) Material of conveyance guide before fixing As a material of conveyance guide 107 before fixing, an experiment was conducted using two types of guides of stainless steel and ABS.
(3) Grounding state of conveyance guide before fixing An experiment was conducted with respect to two types of cases where the conveyance guide 107 before fixing is grounded and floating.
(4) Sheet conveyance speed of fixing device The experiment was performed by changing the sheet conveyance speed of the fixing device 23. The numerical values shown in Table 1 represent the ratio of the paper conveyance speed in the fixing device 23 to the paper conveyance speed in the transfer unit 203. For example, if this value is + 6%, it indicates that the sheet conveyance speed in the fixing device 23 is 6% faster than the sheet conveyance speed in the transfer unit 203 (± 0% is constant speed).

  From the comparison results of Comparative Examples 1-1 to 1-2 and Examples 1-1 to 1-3, it can be seen that the toner contamination due to electrostatic offset decreases as the protrusion height increases. The reason is as follows.

By increasing the protruding height, the pre-fixing conveyance guide 107 and the back surface of the recording paper P come into contact with each other, and the recording paper P positively charged by the transfer unit 203 is discharged. On the other hand, the electrostatic offset is caused by the fixing roller 101 being positively charged by the recording paper P. For this reason, the positive potential of the fixing roller 101 also decreases as the potential of the recording paper P decreases due to static elimination. As a result, an electrostatic offset to the fixing roller 101 is less likely to occur. At this time, the higher the protrusion height, the lower the potential of the recording paper P and the lower the surface potential of the fixing roller 101. In other words, the higher the protrusion height, the greater the static elimination effect on the recording paper P and the greater the electrostatic offset prevention effect. Note that the guide surface 107g of the pre-fixing conveyance guide 107 is insulated except for the edge portion 107e. Fluorine resin coating is applied. From this, it is understood that the charge removal of the recording paper P is performed through the edge portion 107e. This can be explained from the results of Comparative Examples 1-3 and 1-4. That is, in an experiment using an insulating ABS resin as the conveyance guide 107 before fixing (Comparative Example 1-3) and an experiment in which the conveyance guide before fixing is floated without being grounded (Comparative Example 1-4), The potential of the recording paper P is +350 to +400 V, which is the highest among the eight types of experiments, and the potential of the fixing roller 101 is also the highest, +120 to +140 V. The toner contamination is the worst, 0.09 to 0.01.

  Further, from the results of Example 1-1 and Comparative Example 1-5, it can be seen that the toner contamination due to the electrostatic offset can be further reduced by increasing the conveyance speed of the fixing device 23 with respect to the transfer unit 203. The reason is as follows.

  When the paper conveyance speed of the fixing device 23 is the same as or slower than the paper conveyance speed of the transfer unit 203, the recording paper P is bent between the transfer unit 203 and the fixing device 23. When the recording paper P is bent, the contact state between the back surface of the recording paper P and the edge portion 107e becomes unstable, and the static elimination effect is reduced. On the other hand, when the paper conveyance speed of the fixing device 23 is made faster than the paper conveyance speed of the transfer unit 203, the recording paper P is given tension after the leading edge of the recording paper P is bitten by the fixing nip portion Nf. It is conveyed in the state. As a result, the back surface of the recording paper P is conveyed while being pressed against the edge portion 107e by tension, and the back surface of the recording paper P and the edge portion 107e come into contact with each other more reliably, so that the static elimination effect is maintained. it can.

  As described above, in the image forming apparatus 2 according to the first embodiment, the edge portion 107e of the pre-fixing conveyance guide 107 is disposed so as to protrude with respect to the line PP, and the sheet conveyance speed at the fixing nip portion Nf is the transfer rate. The speed is set to be faster than the paper conveyance speed at the nip portion Nt. As a result, the recording paper P can be conveyed while being rubbed (pressed) against the edge 107e, and an effective static elimination effect by the pre-fixing conveyance guide 107 can be obtained. For this reason, electrostatic offset at the time of fixing can be effectively prevented.

  Further, since the back surface of the recording paper P always rubs against a narrow area called the edge portion 107e of the pre-fixing conveyance guide 107, toner contamination does not collect on the edge portion 107e, and the pre-fixing conveyance guide 107 is made of toner. There is no worry that the static elimination performance will deteriorate due to contamination.

  Further, from the results in Table 1, it is more preferable that the protrusion height is 1.0 mm or more because the toner contamination (density change amount) is a problem-free level of 0.005 or less. Further, the relationship between the protrusion height and the charge removal effect is saturated at a certain height. That is, in the example of Table 1, there is no difference in the charge removal effect (and toner contamination) between Example 1-3 (projection height 3.0 mm) and Comparative Example 1-2 (projection height 3.5 mm). However, when the protruding height is 3.5 mm or more, the problem of “sheet trailing edge splashing” occurs. “Paper trailing edge splashing” is a phenomenon in which the trailing edge of the recording paper P bounces upward after passing through the transfer nip portion Nt and touches the photoreceptor 200 or the like to disturb the toner image. From the above, the protrusion height is preferably in the range of 1.0 mm to 3.0 mm.

[Embodiment 2]
Next, an image forming apparatus according to Embodiment 2 will be described with reference to FIG. The image forming apparatus according to the second embodiment is the same as the image forming apparatus according to the first embodiment except for the pre-fixing conveyance guide 107, and thus the description thereof is omitted here. The image forming apparatus according to the second embodiment is different from the first embodiment in that the pre-fixing conveyance guide 107 is grounded via a high resistance element 1071.

<Experiment 2>
Next, an experiment (Experiment 2) using the image forming apparatus of the second embodiment will be described with reference to Table 2. Since the experimental method of Experiment 2 is exactly the same as that of Experiment 1, the description thereof is omitted here. The experimental parameters in Experiment 2 are the following two types.
(1) Environmental conditions As environmental conditions, it experimented by two patterns, high-humidity (NH) conditions (25 degreeC, 85%) and low-humidity (NL) conditions (25 degreeC, 10%).
(2) Resistance value of the high-resistance element When the resistance value of the high-resistance element is four kinds of elements of 50 MΩ, 500 MΩ, 5 GΩ, and 50 GΩ, the pre-fixing conveyance guide 107 is grounded without using the high-resistance element. Experiments were conducted with the case.

  In Examples 2-1 to 2-2 and Comparative Examples 2-1 to 2-2, even in a low humidity environment where electrostatic offset is likely to occur, the effect of reducing toner contamination due to electrostatic offset (the density change amount is 0.1%). 009 or less). However, in Comparative Examples 2-1 to 2-2, a problem of “transfer omission” occurs in which the toner image T cannot be properly transferred from the photoreceptor 200 in a high humidity environment. “Transfer missing” occurs when the recording paper P absorbs moisture in a high-humidity environment, the paper resistance decreases, and the transfer current leaks from the pre-fixing conveyance guide 107 via the recording paper P.

  The image forming apparatus according to the second embodiment attempts to prevent “transfer omission” by grounding the pre-fixing conveyance guide 107 via the high resistance element 1071.

  From the results shown in Table 2, when the resistance value is 0 or 50 MΩ (Comparative Examples 2-1 to 2-2), the resistance value is too low to suppress the “transfer omission” and eliminate the “transfer omission”. I can't. On the other hand, when the resistance value is 500 MΩ or 5 GΩ (Examples 2-1 to 2-2), leakage of the transfer current can be prevented by the high resistance element 1071, and the problem of “transfer omission” can be solved. . Further, when the resistance value is 505 GΩ (Comparative Example 2-3), the resistance value is too high and the charge removal effect of the recording paper P is too small, and the level at which toner contamination can be clearly recognized with the naked eye in a low humidity environment (density change). The amount becomes 0.01). As described above, the resistance value of the high resistance element 1071 is preferably in the range of 500 MΩ to 5 GΩ.

[Embodiment 3]
Next, an image forming apparatus according to Embodiment 3 will be described with reference to FIG. The image forming apparatus according to the third embodiment is the same as the image forming apparatus according to the first embodiment except for the pre-fixing conveyance guide 107, and thus the description thereof is omitted here. The image forming apparatus according to the third embodiment is different from the first embodiment in that the pre-fixing conveyance guide 107 is grounded via a switching element 1072.

<Experiment 3>
Next, an experiment (experiment 3) using the image forming apparatus of the third embodiment will be described with reference to Table 3. Since the experimental method of Experiment 3 is exactly the same as that of Experiment 1, the description thereof is omitted here. The experimental parameters in Experiment 3 are the following two types.
(1) Environmental conditions As environmental conditions, it experimented by two patterns, high humidity (NH) conditions (25 degreeC, 85%) and low humidity (NL) conditions (25 degreeC, 10%).
(2) Grounding state of conveyance guide before fixing Experiments were performed with two types of grounding and floating as the grounding state of the conveyance guide before fixing.

  In Comparative Example 3-1, an effect (a density change amount of 0.009 or less) can be obtained in a low-humidity environment in which electrostatic offset is likely to occur. However, in Comparative Example 3-1, “transfer omission” occurs in a high humidity environment.

  On the other hand, in Comparative Example 3-2, by causing the pre-fixing conveyance guide 107 to be in a floating state, this transfer current leakage can be prevented, and “transfer omission” can be eliminated. However, in this case, the effect of removing the charge of the recording paper P cannot be obtained, and the toner contamination is at a level (density change amount is 0.01) that can be clearly recognized with the naked eye in a low humidity environment.

  Therefore, in the image forming apparatus according to the third embodiment, as shown in the third embodiment, the switching element 1072 is turned on to ground the pre-fixing conveyance guide 107 in a low humidity environment, and the switching element 1072 in a high humidity environment. And the pre-fixing conveyance guide 107 is in a floating state. This makes it possible to achieve both prevention of “toner smear” in a low humidity environment and prevention of “transfer omission” in a high humidity environment.

  In the image forming apparatus according to the third embodiment, a humidity sensor is provided at an arbitrary position in the image forming apparatus in order to switch the switching element 1072 between ON and OFF. That is, when the humidity detected by the humidity sensor falls below a predetermined threshold, it is determined that the humidity environment is low, and the switching element 1072 is turned on. When the humidity is above the predetermined threshold, the humidity environment is high. Control to turn off the switching element 1072 based on the determination may be performed. The determination and the switching control of the switching element 1072 may be performed by, for example, a first control circuit. Further, the location of the humidity sensor is not particularly limited, but it is preferable to arrange the humidity sensor in the vicinity of the recording material supply unit that stores the paper before printing. This is because the moisture absorption amount of the paper is considered to largely depend on the humidity at the location where the paper before printing is stored for a long time.

[Embodiment 4]
Next, an image forming apparatus according to Embodiment 4 will be described with reference to FIG. Note that the image forming apparatus according to the fourth embodiment is exactly the same as the image forming apparatus according to the first embodiment except for the pre-fixing conveyance guide 107, and the description thereof is omitted here. The image forming apparatus according to the fourth embodiment is different from the first embodiment in that the pre-fixing conveyance guide 107 is configured to be rotatable in the direction indicated by the arrow B around the rotation fulcrum 107s.

  Specifically, with respect to the rotation fulcrum 107s, a solenoid 1073 and a spring 1074 are connected to one end of the pre-fixing conveyance guide 107 opposite to the guide surface 107g. When the solenoid 1073 is OFF, the pre-fixing conveyance guide 107 is pulled by the spring 1074, and the edge portion 107e moves to a position 2.0 mm higher than the line PP shown in FIG. 6 (projection height 2.0 mm). When the solenoid 1073 is ON, the pre-fixing conveyance guide 107 is pulled by the solenoid 1073, and the edge portion 107e moves to a position 0.5 mm lower than the line PP (projection height −0.5 mm). The arrangement location of the solenoid 1073 and the spring 1074 may be the reverse of the configuration of FIG.

<Experiment 4>
Next, an experiment (Experiment 4) using the image forming apparatus of the fourth embodiment will be described with reference to Table 4. Since the experimental method of Experiment 4 is exactly the same as that of Experiment 1, the description thereof is omitted here. The experimental parameters in Experiment 4 are the following two types.
(1) Environmental conditions As environmental conditions, it experimented by two patterns, high humidity (NH) conditions (25 degreeC, 85%) and low humidity (NL) conditions (25 degreeC, 10%).
(2) Protrusion Height of Pre-Fixing Transport Guide There are two types of protrusion height of the pre-fixing conveyance guide 107: a position 2.0 mm higher than the line PP (+2.0 mm) and a position 0.5 mm lower (−0.5 mm). The experiment was conducted.

  In Comparative Example 4-1 (projection height of 2.0 mm), an effect of reducing toner contamination due to electrostatic offset (density change amount of 0.009 or less) is obtained in a low-humidity environment where electrostatic offset is likely to occur. Yes. However, in Comparative Example 4-1, “transfer omission” occurs in a high humidity environment.

  On the other hand, in Comparative Example 4-2, by setting the projection height to -0.5 mm, this transfer current leakage can be prevented and "transfer omission" can be eliminated. However, in this case, since the static elimination effect of the recording paper P cannot be obtained stably, the toner contamination is at a level (density change amount is 0.01) that can be clearly recognized with the naked eye in a low humidity environment.

  Therefore, in the image forming apparatus according to the fourth embodiment, the solenoid 1073 is turned off to set the protruding height to 2.0 mm in a low humidity environment, and the solenoid is turned off in a high humidity environment to raise the protruding height of the conveyance guide before fixing. The thickness is set to -0.5 mm. This makes it possible to achieve both prevention of “toner smear” in a low humidity environment and prevention of “transfer omission” in a high humidity environment.

  In the image forming apparatus according to the fourth embodiment, a humidity sensor is provided at an arbitrary location in the image forming apparatus, as in the third embodiment, in order to switch the solenoid 1073 on and off. That is, when the humidity detected by the humidity sensor falls below a predetermined threshold, it is determined that the humidity is low, and the solenoid 1073 is turned off. When the humidity is higher than the predetermined threshold, it is determined that the humidity is high. Then, the solenoid 1073 may be turned on. The determination and the switching control of the solenoid 1073 may be performed by, for example, a first control circuit.

[Embodiment 5]
In the first to fourth embodiments, when the distance of the edge portion 107e with respect to the line PP is the protruding height of the pre-fixing conveyance guide 107, that is, when the recording paper to be conveyed is rubbed against the edge portion 107e and is neutralized. Explained. However, the present invention is not limited to the above configuration.

  For example, as shown in FIG. 7, the guide surface 107g of the pre-fixing conveyance guide 107 may have a curved surface, and a part of the convex surface of the guide surface 107g may be a protruding region. In this case, the distance between the vertex 107h of the convex surface (the place farthest from the line PP) and the line PP is the protruding height of the pre-fixing conveyance guide 107, and the conveyed recording paper is rubbed against the vertex 107h and discharged. The

  In the configuration shown in FIG. 7, the guide surface 107g of the pre-fixing conveyance guide 107 may be coated with a fluororesin (insulating property) except for the vicinity of the apex 107h. FIG. 7 shows a modification of the image forming apparatus shown in Embodiment 1 (see FIG. 2). In the image forming apparatus shown in Embodiments 2 to 4 (see FIGS. 4 to 6), FIG. Can also be a similar modification.

  As described above, according to the present invention, even when a thin heating roller (fixing roller) or a low-temperature fixing toner with a short melting time is used, there is no unevenness in image quality or fixing property, and the fixing device has a short warm-up time. And an image forming apparatus can be realized.

  In each of the above-described embodiments, the case of a monochrome multifunction peripheral has been described. However, it goes without saying that the present invention is not limited to a monochrome multifunction peripheral and can be applied to a color multifunction peripheral.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Image forming system 2 Image forming apparatus 23 Fixing apparatus (fixing part)
107 Pre-fixing conveyance guide (guide member)
107e Edge portion 1071 High resistance element 1072 Switching element (switching means)
1073 Solenoid (change means)
1074 Spring (change means)
200 photoconductor (transfer section)
203 Transfer unit (transfer section)
P Recording paper Nt Transfer nip Nf Fixing nip

Claims (10)

An image forming apparatus comprising: a transfer unit that transfers a toner image onto a recording sheet that passes through a transfer nip; and a fixing unit that fixes the recording sheet on which the toner image has been transferred through a fixing nip. ,
A conductive member having a guide member for guiding recording paper between the transfer portion and the fixing portion;
At least a part of the guide surface of the guide member is disposed so as to protrude from a straight line connecting the transfer nip portion and the fixing nip portion,
The paper transport speed at the fixing nip is set to be higher than the paper transport speed at the transfer nip, and the recording paper transported between the transfer nip and the fixing nip is The image forming apparatus is conveyed while being rubbed against the guide surface protruding from the straight line. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the guide member is grounded via a high resistance element. The image forming apparatus according to claim 2,
The resistance value of the high resistance element is 500 MΩ or more and 5 GΩ or less. The image forming apparatus according to claim 1,
The guide member is connected to switching means for switching between a ground state and a floating state,
The image forming apparatus according to claim 1, wherein the switching unit places the guide member in a grounded state in a low humidity environment and places the guide member in a floating state in a high humidity environment. The image forming apparatus according to any one of claims 1 to 4, wherein:
In the protruding region where the guide surface of the guide member protrudes from the straight line, when the maximum protruding distance between the straight line and the guide surface is the protruding height,
The image forming apparatus, wherein the protruding height is set in a range of 1 mm to 3 mm. The image forming apparatus according to claim 1,
In the protruding region where the guide surface of the guide member protrudes from the straight line, when the maximum protruding distance between the straight line and the guide surface is the protruding height,
The guide member includes a changing means for changing the protruding height,
The changing means arranges the guide member such that the protruding height is increased in a low humidity environment and the back surface of the recording paper is in contact with the protruding area, and the protruding height is decreased in a high humidity environment. An image forming apparatus, wherein the guide member is arranged so that no protruding region exists. The image forming apparatus according to any one of claims 1 to 6,
The image forming apparatus according to claim 1, wherein the guide surface of the guide member is subjected to an insulating process except for a region that slides on a recording sheet conveyed between the transfer nip portion and the fixing nip portion. The image forming apparatus according to any one of claims 1 to 7,
The image forming apparatus, wherein the recording sheet is charged with a charge opposite to that of the toner image after passing through the transfer nip portion. The image forming apparatus according to any one of claims 1 to 8,
An image forming apparatus, wherein an edge portion of the guide surface of the guide member on the downstream side in the sheet conveyance direction protrudes most away from the straight line. An image forming method in which a toner image is transferred to a recording paper passing through a transfer nip portion, and the recording paper on which the toner image is transferred is passed through a fixing nip portion to perform fixing.
While guiding the recording paper with a conductive guide member, the recording paper is conveyed between the transfer nip portion and the fixing nip portion,
At least a part of the guide surface of the guide member is disposed so as to protrude from a straight line connecting the transfer nip portion and the fixing nip portion,
The recording paper conveyed between the transfer nip and the fixing nip by setting the paper conveyance speed at the fixing nip to be higher than the paper conveyance at the transfer nip. Is conveyed while being rubbed against the guide surface protruding from the straight line.

Images