JP2015045744A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of, when performing double-sided printing by digital fixation, preventing the generation of an error when a toner image is transferred on a surface on which printing is performed later.SOLUTION: In an image forming apparatus capable of performing printing on double sides of a recording medium P, a transfer device 30 comprises transfer current control means for controlling the transfer current supplied by transfer means to form a transfer electric field, according to the area divided in the conveyance direction of the recording medium P, a fixation device 60 comprises: a fixation member 62; a pressure member 61; heating means 63 which has a plurality of heating areas in the direction orthogonal to the conveyance direction of the recording medium P, and heats the fixation member 62; heating control means for controlling the plurality of heating areas independently; and further, means for recording information on the temperature in the heating area, and the transfer current control means, when performing double-sided printing, controls the transfer current when a toner image is transferred on a second surface of the recording medium P, on which printing is performed later, on the basis of the temperature information in the heating area when a toner image has been fixed on a first surface on which printing is performed first.

Description

  The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus.

  In an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system, a toner image is formed on an image carrier based on image information, and the toner image is formed on a recording medium such as paper or an OHP sheet. The recording medium that has been transferred and carries the toner image is fed between a fixing member and a pressure member that are heated to a predetermined fixing temperature by a heating means, and the unfixed toner image is transferred onto the recording medium by heat and pressure. Established.

  In recent years, there has been a demand for energy saving, and in a fixing operation that occupies most of the energy used in an image forming apparatus, a method for reducing power consumption by controlling the fixing temperature based on image information of image data. Has been done. Specifically, in a fixing member that is divided into a plurality of heating areas and is heated by a heating unit that is independently controlled for each heating area, the toner image is transferred by associating the image information with each of the heating areas. There is known a method of heating only the formed region.

  In such fixing by the digital heating method (digital fixing), a region where the toner image is transferred (hereinafter also referred to as “image portion”) and a region where the toner image is not transferred (hereinafter referred to as “non-image portion”). In other words, since the fixing temperature is different, the amount of water evaporation of the recording medium at the time of fixing varies depending on the part, and the moisture content differs between the image area and the non-image area in the recording medium after fixing.

Since the resistance value of the recording medium changes depending on the water content, in the transfer device to which the toner image is transferred, the transfer efficiency varies depending on the water content, and the quality of the obtained image also changes. For example, when a filled image is formed, there is a phenomenon that density unevenness occurs due to the presence of a portion having a different resistance value, or a portion where the toner is not transferred but white is generated.
In order to obtain a high-quality toner image, it is necessary to appropriately control the transfer current (transfer bias) according to the moisture content of the recording medium.

  As the control of the transfer bias, a method of stabilizing the charged state of the image carrier by performing control to change the transfer bias in the image transport direction in accordance with the image portion and the non-image portion in the primary transfer portion in the intermediate transfer method. Is known (see, for example, Patent Document 1). Also known is a method of controlling the transfer current according to the moisture content of the recording medium and the paper type (see, for example, Patent Document 2).

  Patent Document 1 discloses a transfer bias for applying a bias to a primary transfer bias roller as an image forming apparatus capable of obtaining both the advantages of pre-transfer exposure (PTL) and the advantages of switching a transfer bias in a non-image portion. A mode having an application unit and an exposure member that removes the charge of the photosensitive member after development and before transfer, and switches between an image forming mode in which the charge of the photosensitive member is removed by the exposure member and an image forming mode in which the charge is not removed An image forming apparatus having a control means as a switching means and a control means as a bias switching means for switching a transfer bias between an image portion and a non-image portion, and does not switch the transfer bias in an image forming mode for removing charges Is disclosed.

  Patent Document 2 discloses an image forming apparatus for always and stably separating transfer paper regardless of the thickness, hardness, type, paper size, time, and environmental conditions of the transfer paper. By controlling the transfer current according to the moisture content and paper type, a stable image without abnormal discharge etc. can be obtained at all times, and the separation current can be changed at the same time to generate separation nail marks and toner due to the charge on the image carrier. Is described in Japanese Patent Application Laid-Open No. 2003-110826.

In the recording medium after the fixing by the digital heating method (digital fixing), a problem arises due to the difference in the water content between the image portion and the non-image portion during double-sided printing.
The toner image is fixed on one surface (first surface), and the difference in water content between the image portion and the non-image portion on the first surface results in the toner on the other surface (second surface). Abnormalities may occur during image transfer, which may cause a reduction in image quality.

  The control of the transfer bias in the image forming apparatus described in Patent Document 1 is a control for changing the transfer bias in the image sub-scanning direction according to the difference between the image portion and the non-image portion. Since the purpose is to stabilize the charged state of the image carrier, the control is based only on the image information of the image to which the transfer bias control is reflected, and the control during duplex printing is not described.

  Further, in the image forming apparatus described in Patent Document 2, it is described that the transfer current is controlled according to the moisture content of the transfer paper and the paper type. However, the purpose is to ensure the separation of the transfer paper. The control during double-sided printing is not described.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of an abnormality when transferring a toner image on a surface to be printed later when performing double-sided printing by digital fixing.

  In order to solve the above-described problems, an image forming apparatus according to the present invention includes a transfer device that transfers a toner image formed on an image carrier onto a recording medium by applying a transfer electric field controlled at a constant current. And a fixing device that heats and fixes the toner image on the recording medium, and is capable of printing on both sides of the recording medium. The image forming apparatus includes transfer current control means for controlling the supplied transfer current for each region divided in the conveyance direction of the recording medium, and the fixing device includes a fixing member that contacts an unfixed toner image, the fixing member, A pressure member that forms a fixing nip portion therebetween, a heating unit that has a plurality of heating regions in a direction orthogonal to the conveyance direction of the recording medium, and heats the fixing member, and a toner on the recording medium Depending on the position of the image The plurality of heating regions are independently controlled so that the image portion to which the toner image is transferred is heated to a fixing temperature, and the non-image portion to which the toner image is not transferred is heated to a temperature lower than the fixing temperature. Heating means, and further comprising means for recording temperature information of the heating area, wherein the transfer current control means fixes the toner image on the first surface printed on the front side of the recording medium during duplex printing. In the image forming apparatus, the transfer current at the time of transferring the toner image on the second surface to be printed later is controlled based on the temperature information of the heating area at the time.

  According to the image forming apparatus of the present invention, it is possible to provide an image forming apparatus capable of preventing the occurrence of an abnormality when transferring a toner image on a surface to be printed later when performing double-sided printing by digital fixing.

1 is a schematic configuration diagram illustrating an example of an image forming apparatus according to an exemplary embodiment. 1 is a cross-sectional view illustrating an example of a fixing device of an image forming apparatus according to an exemplary embodiment. 2 is a perspective view illustrating an example of a configuration of a main part of a fixing device of the image forming apparatus according to the exemplary embodiment. It is explanatory drawing which shows an example of the image formation pattern which consists of an image part and a non-image part. It is explanatory drawing which shows an example of the image formation pattern which consists of an image part and a non-image part. 1 is a cross-sectional view illustrating an example of a fixing device of an image forming apparatus according to an exemplary embodiment. It is a schematic diagram which shows an example of the heating means of the fixing device of FIG. 1 is a cross-sectional view illustrating an example of a fixing device of an image forming apparatus according to an exemplary embodiment. 6 is a graph illustrating an example of a fixing temperature of a fixing member corresponding to an image portion and a non-image portion. It is explanatory drawing which shows the example of the image of the 1st surface at the time of double-sided printing, and a 2nd surface. It is a graph which shows the relationship between the transfer current at the time of 2nd surface transfer, and a transfer rate. It is a graph which shows the example of the set value of the surface resistance value of a 2nd surface, and the transfer current at the time of transcription | transfer. It is explanatory drawing which shows the example of the image of the 1st surface at the time of double-sided printing, and a 2nd surface. It is a graph which shows the example of the setting value of the transfer current at the time of transfer of the 2nd surface. It is a graph which shows the relationship between the transfer current at the time of 2nd surface transfer, and a transfer rate. It is a graph which shows the relationship between the fixing temperature of the 1st surface in a different environment, and the surface resistance value of a recording medium.

  Hereinafter, an image forming apparatus according to the present invention will be described with reference to the drawings. It should be noted that the present invention is not limited to the embodiments of the examples shown below, and other embodiments, additions, modifications, deletions, and the like can be changed within a range that can be conceived by those skilled in the art. Any aspect is included in the scope of the present invention as long as the operations and effects of the present invention are exhibited.

FIG. 1 shows a schematic configuration of an electrophotographic color printer which is an example of an image forming apparatus according to the present invention.
As shown in FIG. 1, an image forming apparatus 100 includes a transfer device 40 that transfers a toner image formed on an image carrier (photosensitive member) 3 to a recording medium by applying a transfer electric field controlled by constant current, A fixing device 60 that heats and fixes the transferred toner image onto the recording medium P is provided, and printing on both sides of the recording medium P is possible.
The transfer device 40 includes a transfer current control unit that controls a transfer current supplied to the transfer unit to form a transfer electric field for each region divided in the conveyance direction of the recording medium P.
The fixing device 60 includes a fixing member (fixing roller) 62 that contacts an unfixed toner image, a pressure member (pressure roller) 61 that forms a fixing nip SN between the fixing member 62, and a recording medium P A heating unit 63 having a plurality of heating regions in a direction orthogonal to the conveying direction of the toner and heating the fixing member 62, and fixing the image portion on which the toner image is transferred according to the position of the toner image on the recording medium P Heating control means (not shown) for independently controlling the plurality of heating regions so as to heat the non-image portion to which the toner image is not transferred to a temperature lower than the fixing temperature. .
The apparatus further includes means (not shown) for recording temperature information of the heating area.
The transfer current control means transfers the toner image on the second surface to be printed later based on the temperature information of the heating area at the time of fixing the toner image on the first surface printed on the front side of the recording medium P during double-sided printing. Control the transfer current at the time.

  The image forming unit 30 includes four image forming units 1Y, 1C, 1M, and 1K for generating yellow, magenta, cyan, and black (hereinafter referred to as Y, C, M, and K) toner images. . These image forming units 1 use different colors of Y toner, C toner, M toner, and K toner as image forming substances for forming an image, but the other configurations are the same.

  The image forming unit 1Y for generating a Y toner image will be described as an example. The image forming unit 1Y includes a photoreceptor unit 2Y and a developing device 7Y. The photosensitive unit 2Y and the developing device 7Y are detachably attached to the printer main body integrally as the image forming unit 1Y. However, in a state where it is detached from the printer main body, the developing device 7Y can be attached to and detached from a photosensitive unit (not shown).

An optical writing device 20 is disposed below the image forming units 1Y, 1C, 1M, and 1K in the drawing.
The optical writing device 20 serving as the latent image forming means applies the laser light L to the photoreceptors 3Y, 3C, 3M, and 3K that are image carriers of the image forming units 1Y, 1C, 1M, and 1K based on the image information. Irradiate. Thereby, electrostatic latent images for Y, C, M, and K are formed on the photoreceptors 3Y, 3C, 3M, and 3K.
The optical writing device 20 irradiates the photoreceptors 3Y, 3C, 3M, and 3K through a plurality of optical lenses and mirrors while deflecting the laser light L emitted from the light source by the polygon mirror 21 that is rotationally driven by a motor. To do. In addition, it can replace with the structure which has the polygon mirror 21 shown in this embodiment, and can also be set as the structure which performs the optical scanning by an LED array.

A first paper feed cassette 31 and a second paper feed cassette 32 are arranged below the optical writing device 20 so as to overlap in the vertical direction. A plurality of recording media (recording paper) P are accommodated in the first paper feeding cassette 31 and the second paper feeding cassette 32, and the first paper feeding roller 31a is included in the uppermost recording media P. The second paper feed rollers 32a are in contact with each other.
When the first paper feed roller 31a is driven to rotate counterclockwise in the figure by a driving means (not shown), the uppermost recording medium P in the first paper feed cassette 31 is located on the right side of the cassette in the figure. The paper is discharged toward the paper feed path 33 arranged to extend in the vertical direction. Further, when the second paper feed roller 32 a is driven to rotate counterclockwise in the figure by a drive means (not shown), the uppermost recording medium P in the second paper feed cassette 32 is fed to the paper feed path 33. It is discharged towards.

A plurality of conveyance roller pairs 34 are disposed in the paper feed path 33. The recording medium P sent to the paper feed path 33 is conveyed from the lower side to the upper side in the figure while being sandwiched between the rollers of the conveyance roller pair 34.
A registration roller pair 35 is disposed above the paper feed path 33. When the recording medium P sent from the conveying roller pair 34 is sandwiched between the rollers, the registration roller pair 35 temporarily stops the rotation of the rollers, and the recording medium P is moved to a secondary transfer nip described later at an appropriate timing. Send it out.

A transfer device 40 is disposed above the image forming unit 30 in the drawing.
In the transfer device 40, the toner image formed on the photoreceptor 3 by the image forming unit 30 is primarily transferred onto the intermediate transfer belt 41, and the toner image on the intermediate transfer belt 41 is transferred to the secondary transfer roller 50 and the secondary transfer backup. Secondary transfer is performed on the recording medium P by a transfer electric field (secondary transfer electric field) controlled between the roller 46 and constant current, and by the influence of the nip pressure. This will be described in detail below.

The transfer device 40 includes an intermediate transfer belt 41, a belt cleaning unit 42, a first bracket 43, a second bracket 44, and the like.
The intermediate transfer belt 41 is stretched by eight rollers including four primary transfer rollers 45Y, 45C, 45M, and 45K, a secondary transfer backup roller 46, a drive roller 47, an auxiliary roller 48, and a tension roller 49. By the rotational drive of 47, it moves endlessly counterclockwise in the figure.
The four primary transfer rollers 45Y, 45C, 45M, and 45K sandwich the intermediate transfer belt 41 that moves endlessly with the photoreceptors 3Y, 3C, 3M, and 3K to form a primary transfer nip.

  A transfer bias having a polarity opposite to that of the toner (for example, plus) is applied to the back surface (loop inner peripheral surface) of the intermediate transfer belt 41. The intermediate transfer belt 41 sequentially passes through the primary transfer nips for the colors Y, C, M, and K as the endless movement of the intermediate transfer belt 41, and the surface of the intermediate transfer belt 41 is placed on the photoreceptors 3Y, 3C, 3M, and 3K. The Y toner image, the C toner image, the M toner image, and the K toner image are primarily transferred so as to be superimposed. As a result, a four-color superimposed toner image (hereinafter referred to as a four-color toner image) is formed on the intermediate transfer belt 41.

The secondary transfer backup roller 46 sandwiches the intermediate transfer belt 41 with the secondary transfer roller 50 disposed outside the loop of the intermediate transfer belt 41 to form a secondary transfer nip.
The registration roller pair 35 sends the recording medium P sandwiched between the rollers toward the secondary transfer nip at a timing synchronized with the four-color toner image on the intermediate transfer belt 41.
The four-color toner image on the intermediate transfer belt 41 has a transfer electric field (secondary transfer electric field) formed between the secondary transfer roller 50 and the secondary transfer backup roller 46 to which a secondary transfer bias is applied, and a nip. Under the influence of the pressure, secondary transfer is performed collectively on the recording paper P in the secondary transfer nip. Then, combined with the white color of the recording medium P, a full color toner image is obtained.

The transfer device 40 controls the transfer current supplied to the secondary transfer roller 50 serving as transfer means for forming a transfer electric field (secondary transfer electric field) for each region divided in the conveyance direction of the recording medium P. A transfer current control means (not shown) is provided.
The set value of the current flowing through the secondary transfer roller 50 while the recording medium P is passing can be changed by the transfer current control means.

After the recording medium P passes through the secondary transfer nip, the transfer residual toner that is not transferred to the recording medium P and remains on the intermediate transfer belt 41 is cleaned by the belt cleaning unit 42.
The belt cleaning unit 42 contacts the front surface of the intermediate transfer belt 41 with the cleaning blade 42a, and scrapes off and removes the transfer residual toner on the belt.

  A fixing device 60 is disposed above the secondary transfer nip in the drawing. Details of the fixing device 60 will be described later.

A paper discharge unit 70 is disposed on the downstream side of the fixing device 60 and at the end of the paper feed path 33.
The paper discharge unit 70 includes a pair of rollers that discharge the recording medium P from the paper discharge port 71 toward the paper discharge tray 72.

  The double-sided conveyance unit 80 is used when printing on both sides of the recording medium P. After the recording medium P is transported in the paper discharge direction by the paper discharge section 70, the paper discharge section 70 is reversed and the recording medium P is reversed, while passing through the transport roller pairs 81, 82, 83, and the registration roller pair The sheet is conveyed to the sheet feeding path 33 on the upstream side of the line 35.

In addition to the first paper feed cassette 31 and the second paper feed cassette 32, a manual feed tray 84 that can supply the recording medium P is provided.
The recording medium P mounted on the manual feed tray 84 is fed out one by one from the uppermost one by the paper feed roller 85 and the pair of transport rollers 86, and sent out to the paper feed path 33. The roller on the driving side of the conveyance roller 86 is shared with the conveyance roller pair 83.

An example of the configuration of the fixing device 60 is shown in FIGS.
As shown in FIG. 2, the fixing device 60 includes a fixing roller 62 that is a fixing member that contacts an unfixed toner image and a pressure roller that is a pressure member that forms a fixing nip portion SN between the fixing roller 62 and the fixing roller 62. 61 and a thermal heater 63 as a heating means. The thermal heater 63 and the power source 64 constitute external heating means.
The pressure roller 61 is pressed against the fixing roller 62 by an urging means (not shown). The thermal heater 63 is pressed against the surface of the fixing roller 62 by an urging means (not shown).

As shown in FIG. 3, the thermal heater 63 has a plurality of heating regions 163 in a direction orthogonal to the conveyance direction of the recording medium P. In the present embodiment, the heating means 63 is composed of seven heaters 63a to 63g and has an example having corresponding heating regions 163a to 163g.
Each of the heaters 63a to 63g is independently controlled by the heating control unit 67 and can heat the fixing roller 62, and fixes the image portion to which the toner image is transferred according to the position of the toner image on the recording medium P. The temperature is controlled so that the non-image portion on which the toner image is not transferred is heated to a temperature lower than the fixing temperature of the image portion.

A thermistor 65 as a temperature detecting means for detecting the surface temperature and a thermistor 66 as a heating member temperature detecting means for detecting the temperature of the heating means 63 at a position downstream of the nip SN of the fixing roller 62 and upstream of the heating means 63; A power source 64 that supplies power to the heating unit 63 and a heating control unit 67 that controls the power source 64 based on detection information of the thermistors 65 and 66 are provided.
The heating control unit 67 includes, for example, a CPU, a ROM, a RAM, an I / O interface, and the like, and is included in a control unit configured by a known microcomputer having a storage unit, a calculation unit, and the like.

  The fixing roller 62 has an aluminum core bar 62a having an outer diameter of 40 mm and a thickness of 1 mm, and a heat insulating layer 62b covered on the surface of the core bar 62a. The heat insulating layer 62b is made of silicon rubber and has a thickness of 3 mm. The heat insulating layer 62b may be formed of foamed silicon rubber with less heat escape in order to further enhance its heat insulating function.

A good heat conductive layer 62c made of nickel is formed on the heat insulating layer 62b of the fixing roller 62. The good heat conductive layer 62c is not limited to nickel, but is an iron-based alloy such as stainless steel, aluminum, copper, or the like. It is sufficient that the thermal conductivity of the metal system, graphite sheet, etc. is at least as high as that of the heat insulating layer 62b.
By forming the good heat conductive layer 62 c on the fixing roller 62, local temperature unevenness of the surface temperature of the fixing roller 62 due to uneven heat generation of the thermal heater 63 is reduced. In addition, the temperature of the region slightly larger than the region heated by the thermal heater 63 is raised by the function of the good heat conductive layer 62c, so that there is an advantage that a slight deviation from the image can be compensated.
In other words, there is an advantage that the degree of freedom in design is large in setting the size and interval of each heater constituting the thermal heater 63.

  In addition, in order to improve the durability of the fixing roller 62 and ensure the releasability, a release layer having a thickness of 5 to 30 μm made of a fluorine-based resin such as PFA or PTFE may be formed on the surface of the heat insulating layer 62b. .

The pressure roller 61 includes an iron core bar 61a having an outer diameter of 40 mm and a thickness of 2 mm, and an elastic layer 61b covered on the surface of the core bar 61a.
The elastic layer 61b is made of silicon rubber and has a thickness of 5 mm. It is desirable to form a fluororesin layer having a thickness of about 40 μm on the surface of the elastic layer 61b in order to improve the releasability.

  In this embodiment, the heating means 63 is brought into contact with the surface of the fixing roller 62 and heated. However, the external heating means may be constituted by a coil and an inverter and may be a non-contact heating method based on the IH method. The IH system may be configured to arrange a large number of heating coils, or may be configured to control a heating region and a heating amount by arranging a large number of members that cancel magnetic flux.

Below, an example of the control of the heating area by the heating control means 67 will be described.
The heating control unit 67 heats the image portion to which the toner image has been transferred to the fixing temperature in accordance with the information on the image formed on the recording medium P (the position of the toner image), and the toner image has not been transferred. The heating rate of the heating area is changed so that the non-image area is heated to a temperature lower than the fixing temperature.
FIG. 4 shows an example of an image formation pattern composed of an image portion and a non-image portion. FIG. 4A shows an example in which an image part a (a1), a non-image part b, and an image part a (a2) are formed on the recording medium P in order from the leading end side in the transport direction indicated by the arrow. It is a thing. FIG. 4B shows an example in which the image part a and the non-image part b are formed on the recording medium P in order from the leading end side in the transport direction indicated by the arrow.
In the image part a, it is necessary to fix an unfixed toner image. However, in the non-image part b, since there is no toner image to be fixed, it is not necessary to heat at least for fixing.

When the image information of the patterns shown in FIGS. 4A and 4B is input from the image processing apparatus (not shown) to the heating control unit 67, the heating control unit 67 corresponds to the non-image portion b ( The heating region 163 is controlled so that the temperature of the portion of the fixing roller 62 that contacts (is in contact with) the temperature of the portion of the fixing roller 62 that corresponds to (contacts) the image portion a.
For example, in the case of the pattern of FIG. 4A, in the portion of the fixing roller 62 corresponding to the image portion a1 on the front end side, electric power for heating to the fixing temperature is supplied to the entire heating region 163, The power supplied at the portion of the fixing roller 62 corresponding to the non-image portion b is reduced, and the power at which the fixing temperature is obtained is supplied again at the portion of the fixing roller 62 corresponding to the image portion a2 on the rear end side.

  The electric power that is actually supplied is input so as to preliminarily heat the portion before the image portion a is conveyed to the nip portion (region indicated by the hatched portion W in the drawing). This preheating area W is an area mainly provided for the heat generation length in the circumferential direction of the heating means 63 and the temperature raising time required by the heating element. The preheating region W is preferably as small as possible from the viewpoint of energy saving.

FIG. 5 is another example of an image forming pattern including an image portion and a non-image portion. FIG. 5A shows an example in which the image portion c and the non-image portion d are formed on the recording medium P in a direction perpendicular to the conveyance direction indicated by the arrow (longitudinal direction of the fixing member 62). It is. FIG. 5B shows an example in which an image portion a and a region e in which the image portion c and the non-image portion d are mixed are formed in order from the leading end side in the transport direction.
Similar to the control described with reference to FIG. 4, the heating control unit 67 is configured such that the temperature of the portion of the fixing roller 62 corresponding to the non-image portion d is lower than the temperature of the portion of the fixing roller 62 corresponding to the image portions a and c. The heating region 163 is controlled so as to be.

At the portion of the fixing roller 62 corresponding to the non-image portion, the power supply may be completely stopped (turned off). However, the rise in the next heating may be delayed due to a decrease in temperature. Therefore, in the non-image area, as shown in the graph of FIG. 9, the temperature is lower than the first target temperature, which is the temperature of the fixing roller 62 corresponding to the image area, but at least heated and heated. It is preferable to be controlled so as to maintain a certain second target temperature.
When the control as shown in FIG. 9 is performed, power is supplied to the portion of the fixing roller 62 corresponding to the non-image portion, but the power supplied in the area indicated by S1 is smaller than that in the area S2. Energy saving can be realized.

Other examples of the configuration of the fixing device 60 are shown in FIGS.
The fixing device 60 shown in FIG. 6 uses a belt-type fixing member 162.
As shown in FIG. 7, the heating means 63 is a thermal head or a ceramic heater in which a resistance heating element is formed on a plate-like substrate, and is disposed inside the film to increase the temperature of the film or belt with the heat. The unfixed toner image on the recording medium P conveyed to the nip portion SN is heated and fixed.
The plate-like heating means 63 has a heating region 163 that is divided into a plurality of lengths in the direction perpendicular to the paper conveyance direction, and each of the heating regions 163a to 163j can be controlled independently.

The fixing belt 162 has a SUS base 162a having an outer diameter of 40 mm and a thickness of 40 μm, and an elastic layer 162b coated on the surface of the base 162a. The elastic layer 162b is made of silicon rubber and has a thickness of 100 μm.
On the surface of the fixing belt 162, a release layer 162 c having a thickness of 5 to 50 μm is formed by a fluorine-based resin such as PFA or PTFE in order to enhance durability and secure release properties.
The fixing belt base 162a may be made of polyimide.

  Inside the fixing belt 162, there is a support member 69. A pressing member 68 and a heating means 63 are installed at the nip portion SN, and connected to an external member (not shown) to support the fixing belt 162.

  The fixing device 60 shown in FIG. 8 is a configuration example in which the heating unit 63 functions as a pressing member and is disposed at the nip portion SN.

Moreover, it is good also as a structure which heats the belt and film structure shown in FIG. 6 with an external heating member as shown in FIG.
As described above, each heating region 163 of the heating unit 63 is independently heated and is controlled by the heating control unit 67 according to the image information.

Hereinafter, transfer current control at the time of transferring a toner image on a surface (second surface) printed after double-sided printing in the present embodiment will be described.
The image forming apparatus 100 includes means for detecting temperature information of the heating area 163 and recording the temperature information. Specifically, the control unit captures and stores the temperature information transmitted from the image forming apparatus, and controls the transfer current control unit based on the temperature information.

In the image forming apparatus 100 of the present embodiment, the operation when performing double-sided printing will be described below.
First, the case where the images shown in FIG. 10 are printed on the first surface and the second surface will be described.
An image part P1 and a non-image part P2 partitioned at a position (AA) indicated by A in the sheet passing direction are formed on the first surface printed first. The heating region is controlled so that the temperature of the fixing roller corresponding to the image portion P1 is T1, and the temperature of the fixing roller corresponding to the non-image portion P2 is T2, which is lower than T1, and fixing is performed. I do.

The recording medium P after image fixing on the first surface has different moisture content and different surface resistance values in the image area P1 and the non-image area P2. As a result, the transfer rate with respect to the transfer current of the second surface which is the back surface of the first surface is also different as shown in FIG. In FIG. 11, the value of the second surface (P3) in which the first surface corresponds to the image portion (P1) is “□”, and the value of the second surface (P4) in which the first surface corresponds to the non-image portion (P2). Is indicated by “◆”.
As shown in FIG. 11, P3 corresponding to the image portion P1 heated at a higher temperature has a transfer rate peak at a transfer current having a higher value than P4. Therefore, by controlling the transfer current at the time of transferring the toner image on the second surface based on the temperature information of the heating area on the first surface, it is possible to eliminate the variation in the transfer rate.

FIG. 12 shows an example of controlling the transfer current during transfer of the second surface when the image portion is formed on the entire surface as shown in FIG.
Up to position A in the sheet passing direction (area P3), the fixing temperature of the first surface is high, the moisture content of the recording medium P is low, and the surface resistance is high, but after position A (area P4). The fixing temperature of the first surface is low, the moisture content of the recording medium P is high, and the surface resistance gradually decreases. In accordance with the change in the resistance value, the transfer current is changed stepwise and the optimum transfer current value is always set, so that the toner image can be transferred at the optimum transfer rate.

  The transfer current control means sets a transfer current for performing transfer of the second surface at an optimal transfer rate from the temperature information of the heating area at the time of fixing the first surface. A method for calculating the set value is as follows. For example, the control unit holds a transfer current calculation table that associates the temperature information of the heating area during fixing of the first surface and the set value of the transfer current of the second surface with the temperature information. These methods may be used to obtain an optimum transfer current value, or the control unit may use a predetermined function for calculation.

Next, a case where the image shown in FIG. 13 is printed on both sides will be described.
On the first surface to be printed first, the image portion P5 is in the region up to the position (BB) indicated by B in the sheet passing direction, and the image portion P6a is in the region up to the position (CC) indicated by C. And the area | region P6 where the non-image part P6b coexists, and the non-image part P7 are formed.
The heating region is set so that the temperature of the fixing roller portion corresponding to the image portions P5 and P6a is T1, and the temperature of the fixing roller portion corresponding to the non-image portions P6b and P7 is T2 lower than T1. Control and fix.

  The image forming apparatus 100 of the present embodiment can set different fixing temperatures in a direction (main scanning direction) orthogonal to the conveyance direction of the recording medium P by performing digital fixing. In the fixing of the region P6 shown in FIG. 13, by setting the image part P6a and the non-image part P6b to different fixing temperatures, a part having a different moisture content in the main scanning direction is generated after fixing. As a result, there are portions having different surface resistances in the main scanning direction, which causes transfer unevenness during transfer of the second surface.

FIG. 14 shows an example of controlling the transfer current during the second surface transfer when the image portion is formed on the entire surface as shown in FIG.
Up to position B in the sheet passing direction (area P8), the moisture content of the recording medium P is low and the surface resistance is high, the moisture content after position C (area P10) is high, and the surface resistance gradually decreases. The amount of water and the surface resistance between position B and position C (region P9) are between these values.
In accordance with the change in the moisture content, the toner image can be transferred at the optimum transfer rate by changing the transfer current stepwise in the region P9 and always setting the optimum transfer current value.
The transfer current in the region P9 is different from both the transfer current in the region P8 where the first surface is an image portion and the transfer current in the region P10 where the first surface is a non-image portion. This is a value set as a transfer current value in a case where a portion and a non-image portion are mixed. For example, when the first surface shown in FIG. 11 is an image portion and the first surface is a non-image portion, it is possible to suppress the occurrence of transfer unevenness by setting a transfer current value with the same transfer rate. it can.

On the other hand, when the constant current control is used for the transfer bias control, the relationship between the transfer current and the transfer rate as shown in FIG.
From the change in the transfer rate shown in FIG. 15, when the image area in the main scanning direction is small with respect to the transfer current of the second surface determined by the temperature information of the heating area at the time of fixing the first surface, a higher transfer current value is obtained. It can be seen that it is preferable to perform correction.
That is, the transfer current control unit corrects the transfer current at the time of transferring the toner image on the second surface to be printed later, according to the area of the toner image in the main scanning direction (direction orthogonal to the transport direction) of the first surface. As a result, a transfer with a higher transfer rate can be realized, and an occurrence of abnormality during the transfer can be prevented.

  Further, the relationship between the temperature of the heating area at the time of fixing the first surface and the optimum transfer current at the time of transferring the second surface differs depending on the type of the recording medium P. Therefore, the transfer current control means can ensure good transferability by correcting the transfer current according to the type of the recording medium P.

In general, the surface resistance value and moisture content differ depending on the type of recording medium P. Therefore, when image formation with higher image quality is required, the transfer current setting value is set according to the type of recording medium P. It is preferable to correct.
In general, as the recording medium P has a higher moisture content before fixing, the difference in moisture content after passing through different fixing temperature regions tends to increase, and as a result, the difference in surface resistance value also increases. Further, the magnitude of the change in the moisture content after fixing also varies depending on the thickness of the recording medium P.
Therefore, it is preferable to set an optimal transfer current value by changing the transfer current control of the second surface relative to the fixing temperature control of the first surface depending on the type of the recording medium P.

  When considering the type of the recording medium P, for example, a value capable of specifying the type such as the moisture content, surface resistance, and thickness of the recording medium is detected in the vicinity of the first paper feeding cassette 31 and the second paper feeding cassette 32. It is possible to set by a method of providing a mechanism for acquiring the obtained value to the control means or a method of selecting by a user from pre-specified types.

Regarding the relationship between the transfer current and the transfer rate, FIG. 11 shows the value of the second surface corresponding to the image portion of the first surface and the value of the second surface corresponding to the non-image portion of the first surface. The difference in the value of the transfer current indicating a high transfer rate becomes large in the type of recording medium P having a high moisture content, and becomes small in the type of recording medium P having a low moisture content.
Therefore, the transfer current value on the second surface corresponding to the non-image portion on the first surface as shown in FIG. 12 is made lower than the transfer current value on the second surface corresponding to the image portion on the first surface. When the control is performed, it is preferable that the amount of decrease is large for the type of recording medium P having a high moisture content and small for the type of recording medium P having a low moisture content.

Further, the moisture content of the recording medium P varies depending on the environment around the apparatus in which the image forming apparatus of the present embodiment is installed and the environment (temperature, humidity, etc.) inside the apparatus. Therefore, as described above, the magnitude of the difference in the value of the transfer current showing a good transfer rate during the transfer of the second surface also varies depending on the use environment.
Therefore, it is preferable that the transfer current control unit corrects the transfer current according to the use environment of the apparatus. In the present embodiment, the image forming apparatus includes an environmental condition detection unit that detects at least humidity.

FIG. 16 shows the variation of the surface resistance of the recording medium P with respect to the fixing temperature of the first surface due to the difference in the usage environment of the apparatus.
As shown in FIG. 16, the surface resistance value of the recording medium P after fixing is when the first surface is an image portion (fixing temperature T1) and when the first surface is a non-image portion (fixing temperature T2). Although the difference is different, the magnitude D of the difference differs depending on whether the use environment of the apparatus is high humidity (D1) or low humidity (D2).
Therefore, when the usage environment is high humidity, the difference D1 in the surface resistance value is large, so that the correction for increasing the difference in the set value of the transfer current can be performed. In this way, it is possible to more reliably prevent the occurrence of an abnormal image due to the difference in the transfer rate of the second surface depending on whether the corresponding first surface is an image portion or a non-image portion.

The moisture content varies depending on the storage condition of the recording medium P. If the surface resistance is different during sheet feeding, the surface resistance after passing through the fixing of the first surface is also different.
Further, the influence of the fixing temperature on the first surface is greatly influenced at a lower fixing temperature. Therefore, the surface resistance of the recording medium P at the time of paper feeding is detected, and when the moisture content of the recording medium P is large, the transfer current of the second surface to the non-image portion of the first surface is smaller, and the recording medium P In a state where the moisture content is small, the transfer current value reflecting the fluctuation of the surface resistance can be set more accurately by increasing the transfer current of the second surface with respect to the non-image portion of the first surface.

In the present embodiment, a resistance detection unit that detects the resistance of the surface of the recording medium P may be provided, and the transfer current control unit may correct the transfer current based on the detection result of the resistance detection unit.
The resistance detecting means is not particularly limited, and examples thereof include a means for obtaining resistance from a current value that flows by applying a constant voltage.

As described above, according to the image forming apparatus of the present invention, when the recording medium is printed on both sides, the transfer current control unit transfers the toner image on the second surface based on the temperature information of the heating area at the time of fixing the toner image on the first surface. Since the current transfer current is controlled, a suitable transfer current for the transfer of the second surface can be set easily and reliably. Further, by detecting the surface resistance, moisture content, etc. of the second surface of the recording medium with a separately provided sensor or the like and performing correction based on the detected information, it is possible to perform more accurate control.
By setting an optimal transfer current at the time of transferring the second surface in double-sided printing, it is possible to prevent the occurrence of an abnormality at the time of toner image transfer, and a high-quality image can be formed.

3 Image carrier (photoconductor)
DESCRIPTION OF SYMBOLS 20 Optical writer 30 Image forming part 31 1st paper feed cassette 32 2nd paper feed cassette 33 Paper feed path 34,81,83 Pair of conveyance rollers 35 Registration roller pair 40 Transfer device 41 Intermediate transfer belt 50 Secondary transfer roller 46 Secondary transfer backup roller 42 Belt cleaning unit 60 Fixing device 61 Pressure member (pressure roller)
62 Fixing member (fixing roller)
63 Heating means (thermal heater)
64 Power supply 65, 66 Thermistor 67 Heating control means 69 Support member 68 Pressing member 70 Paper discharge part 71 Paper discharge port 72 Paper discharge tray 80 Double-sided conveyance part 100 Image forming apparatus 162 Fixing member (fixing belt)
163 Heating area

JP 2010-61013 A Japanese Patent Laid-Open No. 9-80936

Claims (5)

A transfer device that transfers a toner image formed on an image carrier onto a recording medium by applying a transfer electric field controlled at a constant current, and a fixing device that heats and fixes the transferred toner image onto the recording medium. An image forming apparatus capable of printing on both sides of the recording medium,
The transfer apparatus includes a transfer current control unit that controls a transfer current supplied to the transfer unit to form a transfer electric field for each region divided in the conveyance direction of the recording medium,
The fixing device includes a fixing member that is in contact with an unfixed toner image, a pressure member that forms a fixing nip portion between the fixing member, and a plurality of heating regions in a direction orthogonal to a conveyance direction of the recording medium. And a heating means for heating the fixing member, and the image portion to which the toner image is transferred is heated to a fixing temperature according to the position of the toner image on the recording medium, and the toner image is not transferred. Heating control means for independently controlling a plurality of the heating regions so as to heat the image portion to a temperature lower than the fixing temperature,
Means for recording temperature information of the heating region;
The transfer current control means transfers the toner image on the second surface to be printed later based on the temperature information of the heating area at the time of fixing the toner image on the first surface printed on the front side of the recording medium during double-sided printing. An image forming apparatus that controls a transfer current at the time.   In the double-sided printing, the transfer current control unit is configured to print the second surface toner to be printed later according to the area of the toner image in the direction orthogonal to the transport direction of the first surface printed first on the recording medium. The image forming apparatus according to claim 1, wherein a set value of a transfer current at the time of image transfer is corrected.   The image forming apparatus according to claim 1, wherein the transfer current control unit corrects a set value of the transfer current according to a type of the recording medium.   The image forming apparatus according to claim 1, wherein the transfer current control unit corrects a set value of the transfer current according to a use environment of the apparatus. Having resistance detection means for detecting the resistance of the surface of the recording medium;
5. The image forming apparatus according to claim 1, wherein the transfer current control unit corrects a set value of the transfer current based on a detection result of the resistance detection unit.