Image forming apparatus

Abstract

To suppress the occurrence of fixing failure while suppressing cost as compared with the case of controlling a fixing temperature by using a configuration for estimating a medium type by reflectance or transmittance from a medium.SOLUTION: The fixing device includes a fixing means (F) which has a heat generating means (31) generating heat according to a predetermined fixing temperature and fixes an unfixed image held on a medium (S), and a heating means (H) arranged on the upstream side in the conveying direction of the medium (S) with respect to the fixing means (F). The image forming device (U) includes a curvature detection means (21-28) for detecting the curvature of the medium (S) when the front end of the medium (S) reaches the fixing means (F), and a heat generation control means (C3a) for controlling the heat generation means (31) based on the curvature of the medium (S) detected by the curvature detection means (21-28).SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In an image forming apparatus, the techniques described in Patent Documents 1 to 3 below are known in the art for detecting the type of medium on which an image is recorded.

[0003] Patent Document 1 (JP 2019-12174 A) describes a technology in which a media sensor (40) located upstream of the transfer area detects the basis weight from the light reflected from the recording material (P) and controls the fixing temperature. Patent Document 1 describes that two heating devices are arranged as a fixing device, and that the heating device to be used is selected depending on the basis weight so as to minimize the difference in gloss level.

[0004] Patent Document 2 (JP 2019-184778 A) describes a technology in which a media sensor (16) located upstream of the transfer area determines the basis weight from the light reflected from the paper (P) and determines the fixing conditions. Patent Document 2 also describes correcting the basis weight using thermal conductivity and thickness after detection by the media sensor (16).

[0005] Patent Document 3 (JP 2007-199343 A) describes a technology for determining the type of recording medium, including the basis weight of the recording medium, from the conveyance speed and image density of the recording medium immediately after it enters the fixing nip of a fixing device. In Patent Document 3, the conveyance speed of the recording paper passing through the fixing nip is controlled to a target speed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2019-12174 A ("0049"-"0087") [Patent Document 2] JP 2019-184778 A ("0046"-"0078") [Patent Document 3] JP 2007-199343 A ("0054"-"0061") Summary of the Invention [Problem to be solved by the invention]

[0007] The technical objective of the present invention is to reduce costs and prevent the occurrence of poor fixing, compared to controlling the fixing temperature using a configuration that estimates the media type based on the reflectance or transmittance from the media. [Means for solving the problem]

[0008] In order to solve the above technical problem, the image forming apparatus of the invention described in claim 1 comprises: a fixing unit having a heat generating unit that generates heat in accordance with a predetermined fixing temperature and that fixes an unfixed image held on the medium; a curvature detection means disposed upstream of the fixing means in the conveying direction of the medium, the curvature detection means detecting the curvature of the medium when the leading edge of the medium reaches the fixing means; a heat generation control means for controlling the heat generation means based on the curvature of the medium detected by the curvature detection means; The present invention is characterized by the following features.

[0009] The invention described in claim 2 is the image forming apparatus described in claim 1, the curvature detection means having a contact portion that comes into contact with the medium, a curvature detection portion that detects when the curvature of the medium reaches a predetermined small curvature amount, and a large curvature detection portion that detects when the curvature of the medium reaches a large curvature amount that is larger than the small curvature amount; The present invention is characterized by the following features.

[0010] The invention described in claim 3 is the image forming apparatus described in claim 2, the heat generation control means for controlling the heat generation means so that the fixing temperature is higher when the amount of curvature is small than when the amount of curvature is large; The present invention is characterized by the following features.

[0011] The invention described in claim 4 is the image forming apparatus described in claim 1, a conveying unit disposed upstream of the fixing unit in a conveying direction of the medium, the conveying unit conveying the medium toward the fixing unit; a guide means arranged between the fixing means and the transport means in the transport direction of the medium, and guiding the medium toward an entrance of the fixing means, the guide means having a first guide section that can guide the medium in a direction away from an imaginary line connecting the entrance of the fixing means and the transport means, and a second guide section that is arranged downstream of the first guide section in the medium transport direction and approaches the imaginary line to guide the medium toward the entrance of the fixing means; The present invention is characterized by the following features.

[0012] The invention described in claim 5 is the image forming apparatus described in claim 1, the fixing means that rotates in accordance with the transport of the medium; a curvature detection means for detecting a curvature of the medium during the time from when the leading edge of the medium reaches the fixing means until when the fixing means makes one rotation; a heat generation control unit that controls the heat generation unit while the medium whose curvature has been detected is passing through the fixing unit after the curvature detection unit detects the curvature of the medium; The present invention is characterized by the following features.

[0013] The invention described in claim 6 is the image forming apparatus described in claim 5, When an image is formed on a plurality of media, the curvature of the first medium is detected by the curvature detection means, and the result of the detection of the curvature of the first medium is used for subsequent media. It is characterized by: [Effects of the Invention]

[0014] According to the invention described in claim 1, it is possible to reduce costs and prevent the occurrence of poor fixing, compared to controlling the fixing temperature using a configuration that estimates the media type based on the reflectance or transmittance from the media. According to the invention of claim 2, the small curvature amount and the large curvature amount can be detected more reliably than when there is no small curvature detection portion and large curvature contact portion that come into contact with the medium. According to the invention of claim 3, in the case of thick paper or the like with a small amount of curvature, the occurrence of poor fixing due to insufficient fixing temperature can be suppressed compared to when the fixing temperature is not controlled to be high. According to the invention as set forth in claim 4, the first guide portion and the second guide portion can encourage the medium to curve. According to the invention of claim 5, the curvature is detected in the first rotation of the fixing means, when a drop in fixing temperature is unlikely to be a problem, and temperature control can be performed according to the detected curvature from the second rotation onwards. According to the sixth aspect of the present invention, it is possible to reduce redundant processing compared to when detecting curvature for all media. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is an explanatory diagram of the entire image forming apparatus according to the first embodiment. [Figure 2] FIG. 2 is an explanatory diagram of the main part of the image recording unit of the first embodiment. [Figure 3] FIG. 3 is an explanatory view of the main part from the transfer area to the fixing area in the first embodiment. [Figure 4] FIG. 4 is a perspective view of the guide means including the curvature detection means. [Figure 5] 5A is an explanatory diagram of a state in which the curvature detection means is not in contact with the medium, FIG. 5B is an explanatory diagram of a state in which a medium with a small curvature is in contact with the curvature detection means, and FIG. 5C is an explanatory diagram of a state in which a medium with a large curvature is in contact with the curvature detection means. [Figure 6]Figure 6 is an explanatory diagram of the detection timing of the curvature of the medium in Example 1, where Figure 6A is an explanatory diagram of the state before the leading edge of the medium reaches the fixing device, Figure 6B is an explanatory diagram of the state when the leading edge of the plain paper reaches the fixing device, and Figure 6C is an explanatory diagram of the state when the leading edge of the cardboard reaches the fixing device. [Figure 7] FIG. 7 is a graph showing the relationship between the basis weight of the paper and the amount of curvature. DETAILED DESCRIPTION OF THE INVENTION

[0016] Next, examples of the present invention will be described with reference to the drawings, but the present invention is not limited to the following examples. To facilitate understanding of the following explanation, in the drawings, the front-to-back direction is defined as the X-axis direction, the left-to-right direction as the Y-axis direction, and the up-down direction as the Z-axis direction, and the directions or sides indicated by arrows X, -X, Y, -Y, Z, and -Z are defined as the front, rear, right, left, upper, and lower, or the front side, rear side, right side, left side, upper side, and lower side, respectively. In addition, in the figures, a circle with a "·" inside it means an arrow pointing from the back to the front of the page, and a circle with an "x" inside it means an arrow pointing from the front to the back of the page. In the following description using the drawings, illustrations of components other than those necessary for the description are omitted as appropriate to facilitate understanding. [Example]

[0017] FIG. 1 is an explanatory diagram of the entire image forming apparatus according to the first embodiment. 1, a copier U as an example of an image forming apparatus according to a first embodiment of the present invention has a printer unit U1 as an example of an image recording device, which is an example of an image recording means. A scanner unit U2 as an example of an image reading device, which is an example of a reading means, is supported above the printer unit U1. An auto feeder U3 as an example of a medium conveying device is supported above the scanner unit U2.

[0018] An original tray TG1, which is an example of a medium storage means, is disposed above the auto feeder U3. The original tray TG1 can store a stack of multiple originals Gi to be copied. Below the original tray TG1, an original discharge tray TG2, which is an example of an original discharge section, is formed. An original transport roller is disposed between the original tray TG1 and the original discharge tray TG2 along the original transport path U3a.

[0019] A platen glass PG, which is an example of a transparent document table, is disposed on the upper surface of the scanner unit U2. In the scanner unit U2 of the first embodiment, a light source unit U2a, which is an example of a light source, is disposed below the platen glass PG. The light source unit U2a of the first embodiment is supported along the lower surface of the platen glass PG so as to be movable in the left-right direction, which is an example of a sub-scanning direction. Light emitted from the light source unit U2a and reflected by the document Gi is reflected by the optical system A and enters the reading element CCD. The light entering the reading element CCD is converted into R, G, and B signals and input to the image processing unit GS.

[0020] FIG. 2 is an explanatory diagram of the main part of the image recording unit of the first embodiment. The image processing unit GS is electrically connected to a writing circuit DL of the printer unit U1, which is electrically connected to exposure devices LHy, LHm, LHc, and LHk as an example of an image forming means. The exposure devices LHy to LHk of the first embodiment are configured, for example, with an LED head in which a plurality of LEDs (Light Emitting Diodes), which are an example of light emitting elements, are arranged on a substrate along the main scanning direction. The exposure devices LHy to LHk can output writing light corresponding to each of the colors yellow (Y), magenta (M), cyan (C), and black (K) in accordance with a signal input from the writing circuit DL. The write circuit DL and the power supply circuit E have their write timing and power supply timing controlled in response to control signals from a control unit C, which is an example of a control means.

[0021] In Fig. 1, photoconductors PRy, PRm, PRc, and PRk, which are an example of image holding means, are arranged above exposure devices LHy to LHk. In Fig. 1 and Fig. 2, writing areas Q1y, Q1m, Q1c, and Q1k are formed by areas on each of photoconductors PRy to PRk where writing light is irradiated. Charging rollers CRy, CRm, CRc, and CRk, which are an example of charging means, are disposed upstream of the writing areas Q1y to Q1k in the rotation direction of the photoconductors PRy to PRk. The charging rollers CRy to CRk in the first embodiment are supported in contact with the photoconductors PRy to PRk so as to be rotatable by the photoconductors PRy to PRk. Developing devices Gy, Gm, Gc, and Gk, which are an example of developing means, are arranged downstream of the writing areas Q1y to Q1k in the rotation direction of the photoconductors PRy to PRk. The areas where each of the photoconductors PRy to PRk and each of the developing devices Gy to Gk face each other form developing areas Q2y, Q2m, Q2c, and Q2k.

[0022] Primary transfer rollers T1y, T1m, T1c, and T1k, which serve as an example of primary transfer means, are disposed downstream of the developing devices Gy-Gk in the direction of rotation of the photosensitive members PRy-PRk. Primary transfer regions Q3y, Q3m, Q3c, and Q3k are formed by regions where the photosensitive members PRy-PRk and the primary transfer rollers T1y-T1k face each other. Photoconductor cleaners CLy, CLm, CLc, and CLk, which are an example of cleaning means, are arranged downstream of the primary transfer rollers T1y to T1k with respect to the rotation direction of the photoconductors PRy to PRk. Dischargers Jy, Jm, Jc, and Jk, which are an example of a discharge means and an example of a discharge device, are arranged downstream of the photoconductor cleaners CLy to CLk in the rotation direction of the photoconductors PRy to PRk.

[0023] The Y photoconductor PRy, charging roller CRy, exposure device LHy, developing device Gy, primary transfer roller T1y, photoconductor cleaner CLy, and static eliminator Jy constitute a Y image forming unit Uy that forms a Y toner image and is an example of a Y visible image forming means in Example 1. Similarly, the M, C, and K image forming units Um, Uc, and Uk are constituted by the photoconductors PRm, PRc, and PRk, charging rollers CRm, CRc, and CRk, exposure devices LHm, LHc, and LHk, developing devices Gm, Gc, and Gk, primary transfer rollers T1m, T1c, and T1k, photoconductor cleaners CLm, CLc, and CLk, and static eliminators Jm, Jc, and Jk.

[0024] A belt module BM, which is an example of an intermediate transfer device, is disposed above the photoreceptors PRy to PRk. The belt module BM is an example of an image holding means, and has an intermediate transfer belt B, which is an example of an intermediate transfer means. The intermediate transfer belt B is made of an endless belt-shaped member. The intermediate transfer belt B in the first embodiment is rotatably supported by a tension roller Rt as an example of a tensioning means, a walking roller Rw as an example of a deviation correcting means, an idler roller Rf as an example of a driven means, a backup roller T2a as an example of an opposing means in the secondary transfer region, primary transfer rollers T1y to T1k, and a drive roller Rd as an example of a drive member. In the first embodiment, when drive is transmitted to the drive roller Rd, the intermediate transfer belt B rotates. Further, an image detection sensor SN1 as an example of a detection means for detecting an image on the intermediate transfer belt B is disposed downstream of the primary transfer rollers T1y to T1k and between the backup roller T2a, facing the surface of the intermediate transfer belt B.

[0025] A secondary transfer roller T2b, which is an example of a transfer means and an example of a secondary transfer means, is disposed at a position opposite the backup roller T2a across the intermediate transfer belt B. The backup roller T2a and the secondary transfer roller T2b constitute a secondary transfer unit T2 of the first embodiment, which is an example of a transfer device. The area where the secondary transfer roller T2b and the intermediate transfer belt B contact each other constitutes a secondary transfer area Q4. The secondary transfer roller T2b in the first embodiment is configured to be movable between a contact position where it contacts the intermediate transfer belt B and a separation position where it is separated from the intermediate transfer belt B. A belt cleaner CLb, which is an example of a cleaning device for the intermediate transfer member, is disposed downstream of the secondary transfer region Q4 in the rotation direction of the intermediate transfer belt B. The primary transfer rollers T1y to T1k, the intermediate transfer belt B, the secondary transfer device T2, etc. constitute the transfer device T1+T2+B of Example 1. The image forming units Uy to Uk and the transfer device T1+T2+B constitute the image recording units Uy to Uk+T1+T2+B of Example 1.

[0026] 1, four pairs of left and right guide rails GR, which serve as an example of a guide means, are provided below the imaging units Uy to Uk. Each guide rail GR supports paper feed trays TR1, TR2, TR3, and TR4, which serve as an example of a medium storage means, so that they can be moved in and out in the front-rear direction. Each of the paper feed trays TR1 to TR4 stores recording paper S, which serves as an example of a medium. A pickup roller Rp, which is an example of a take-out device, is arranged above and to the left of the paper feed trays TR1 to TR4. A separation roller Rs, which is an example of a separation device, is arranged downstream of the pickup roller Rp in the transport direction of the recording paper S. A paper feed path SH1, which is an example of a medium transport path, is formed downstream of the separation roller Rs in the transport direction of the recording paper S and extends upward. A plurality of transport rollers Ra, which are an example of a transport device, are arranged on the paper feed path SH1.

[0027] A manual feed tray TR0, which serves as an example of a medium storage means, is located in the lower left of the copier U. A pickup roller Rp0 is located in the upper right of the manual feed tray TR0, and a manual feed path SH0 extends from the manual feed tray TR0. The manual feed path SH0 merges with the feed path SH1. In the sheet feed path SH1, a registration roller Rr as an example of a conveyance timing adjustment unit is disposed upstream of the secondary transfer area Q4. A conveyance path SH2 extends from the registration roller Rr toward the secondary transfer area Q4.

[0028] A fixing device F, which is an example of a fixing means, is disposed downstream of the secondary transfer area Q4 in the conveyance direction of the recording paper S. The fixing device F has a heating roller Fh, which is an example of a fixing member for heating, and a pressure roller Fp, which is an example of a fixing member for applying pressure. The contact area between the heating roller Fh and the pressure roller Fp forms a fixing area Q5. A lower paper output tray TRh, which is an example of a medium output unit, is formed on the upper surface of the printer unit U1. In the first embodiment, a finisher U4, which is an example of a post-processing device, is installed in the lower paper output tray TRh. A paper output path SH3, which is an example of a transport path, extends toward the lower paper output tray TRh above the fixing device F. A paper output roller Rh, which is an example of a medium transport means, is disposed at the downstream end of the paper output path SH3.

[0029] An upper discharge tray TRh2, which is an example of a medium discharge section, is disposed above the lower discharge tray TRh. An upper conveyance path SH4 is formed above the fixing device F, branching off from the discharge path SH3 and extending toward the upper discharge tray TRh2. A reversible roller Rb, which serves as an example of a medium transport means, is disposed on the upper transport path SH4. A reversing path SH6, which serves as an example of a medium transport path, branches off from the upper transport path SH4 to the lower left above the branch point of the discharge path SH3 and the upper transport path SH4.

[0030] A gate GT1, which is an example of a switching means, is disposed across the branching portion between the discharge path SH3 and the upper conveying path SH4, and the branching portion between the upper conveying path SH4 and the reverse path SH6. The gate GT1 guides the recording paper S from the fixing device F toward the lower discharge tray TRh, and is supported switchably between a first guide position (second position) where the gate GT1 guides the recording paper S from the fixing device F toward the reverse path SH6 from the upper conveying path SH4, and a second guide position (first position) where the gate GT1 guides the recording paper S from the fixing device F to the upper conveying path SH4. A plurality of conveying rollers Ra are arranged on the reverse path SH6. The downstream end of the reverse path SH6 joins with the paper feed path SH1 on the upstream side of the registration rollers Rr.

[0031] (Explanation of image formation operation) In the copier U of Example 1 having the above-described configuration, when an operator manually places an original Gi on the platen glass PG to make a copy, the light source unit U2a moves left and right from the initial position, and the original Gi on the platen glass PG is scanned while being exposed to light. Furthermore, when the auto feeder U3 is used, when the copy start key is pressed, the originals Gi are automatically transported. When the copy start key is pressed, multiple originals Gi stored in the original tray TG1 are transported sequentially to and pass through the original reading position on the platen glass PG. Each original Gi that passes the reading position on the platen glass PG sequentially is irradiated with light from the light source unit U2a. After passing the reading position, the originals Gi are discharged onto the original discharge tray TG2. The light reflected from the originals Gi is converted into an electrical signal by the reading element CCD.

[0032] The image processing unit GS receives the electrical signals output from the reading element CCD. The image processing unit GS converts the electrical signals of the R, G, and B color image read by the reading element CCD into image information of yellow (Y), magenta (M), cyan (C), and black (K) for forming a latent image. The image processing unit GS outputs the converted image information to the writing circuit DL of the printer unit U1. Note that if the image is a single-color image, or a so-called monochrome image, the image processing unit GS outputs only black (K) image information to the writing circuit DL. The writing circuit DL outputs a control signal corresponding to the input image information to the exposure devices LHy to LHk, which then output writing light according to the control signal.

[0033] When image formation starts, each photoconductor PRy-PRk is driven to rotate. A charging voltage is applied to the charging rollers CRy-CRk from the power supply circuit E. Therefore, the surfaces of the photoconductors PRy-PRk are charged by the charging rollers CRy-CRk. An electrostatic latent image is formed on the surface of the charged photoconductors PRy-PRk in the writing areas Q1y-Q1k by the writing light from the exposure devices LHy-LHk. The electrostatic latent image on the photoconductors PRy-PRk is developed into a toner image, an example of an image, by the developing devices Gy-Gk in the developing areas Q2y-Q2k.

[0034] The developed toner images pass through primary transfer areas Q3y to Q3k. A primary transfer voltage of a polarity opposite to the charge polarity of the toner is applied from a power supply circuit E to the primary transfer rollers T1y to T1k. Therefore, the toner images on the photoconductors PRy to PRk are transferred onto the intermediate transfer belt B by the primary transfer rollers T1y to T1k. In the case of a multi-color toner image, the toner image transferred downstream is superimposed on the toner image transferred onto the intermediate transfer belt B in the upstream primary transfer area. After the primary transfer, residues and deposits on the photoconductors PRy to PRk are cleaned by photoconductor cleaners CLy to CLk. After cleaning, the surfaces of the photoconductors PRy to PRk are neutralized by static eliminators Jy to Jk. After neutralization, the surfaces of the photoconductors PRy to PRk are recharged by charging rollers CRy to CRk. The single-color or multi-color toner images transferred onto the intermediate transfer belt B in the primary transfer areas Q3y to Q3k are transported to the secondary transfer area Q4.

[0035] The recording paper S on which an image is recorded is picked up by the pickup roller Rp of the paper feed tray TR1 to TR4 being used. If multiple sheets of recording paper S are picked up by the pickup roller Rp and are stacked, they are separated one by one by the separation roller Rs. The recording paper S separated by the separation roller Rs is transported along paper feed path SH1 by the transport roller Ra. The recording paper S transported along paper feed path SH1 is sent to the registration roller Rr. Recording paper S loaded on the manual feed tray TR0 is also sent to paper feed path SH1 by the pickup roller Rp0 via manual feed path SH0. The registration roller Rr transports the recording paper S to the secondary transfer area Q4 at the same time that the toner image formed on the intermediate transfer belt B is transported to the secondary transfer area Q4. A secondary transfer voltage of a polarity opposite to the charge polarity of the toner is applied to the secondary transfer roller T2b by the power supply circuit E. Therefore, the toner image on the intermediate transfer belt B is transferred from the intermediate transfer belt B to the recording paper S. After the secondary transfer, the intermediate transfer belt B is cleaned by a belt cleaner CLb to remove any foreign matter adhering to the surface thereof.

[0036] The recording paper S onto which the toner image has been secondarily transferred is heated and fixed when passing through a fixing area Q5. If post-processing is to be performed on the recording paper S with the image fixed thereon, the recording paper S is transported to the finisher U4 installed in the lower paper output tray TRh. If post-processing is not to be performed on the recording paper S, the recording paper S is transported to the upper paper output tray TRh2. When the recording paper S is transported to the lower paper output tray TRh, the gate GT1 moves to the first guide position. Therefore, the recording paper S sent out from the fixing device F is transported along the paper output path SH3. The recording paper S transported along the paper output path SH3 is transported by the paper output roller Rh toward the finisher U4 and the lower paper output tray TRh. The finisher U4 performs binding processing, which is an example of post-processing, on the recording sheets S, and then discharges the recording sheets S onto a lower discharge tray TRh.

[0037] When the recording paper S is discharged onto the upper discharge tray TRh2, the gate GT1 moves to the second guide position, and the recording paper S is discharged onto the upper discharge tray TRh2. When double-sided printing is performed on the recording paper S, the gate GT1 moves to the second guide position. Then, when the trailing edge of the recording paper S passes through the gate GT1, the gate GT1 moves to the first guide position and the reversing roller Rb rotates in the reverse direction. Therefore, the recording paper S is guided by the gate GT1 and sent to the reversing path SH6. The recording paper S transported along the reversing path SH6 is sent to the registration roller Rr in an inverted state.

[0038] (Explanation of Curvature Detection Means) FIG. 3 is an explanatory view of the main part from the transfer area to the fixing area in the first embodiment. FIG. 4 is a perspective view of the guide means including the curvature detection means. 5A is an explanatory diagram of a state in which the curvature detection means is not in contact with the medium, FIG. 5B is an explanatory diagram of a state in which a medium with a small curvature is in contact with the curvature detection means, and FIG. 5C is an explanatory diagram of a state in which a medium with a large curvature is in contact with the curvature detection means. In FIG. 3, a paper guide 1, which is an example of a guide means, is disposed between the secondary transfer area Q4 and the fixing area Q5. The paper guide 1 has a post-transfer guide 2, which is an example of a first guide section, and an inlet chute 3, which is an example of a second guide section. The post-transfer guide 2 is disposed downstream of the secondary transfer area Q4 in the paper transport direction. The inlet chute 3 is disposed downstream of the post-transfer guide 2 and upstream of the fixing area Q5. The recording paper S that has passed through the secondary transfer area Q4 is guided by the post-transfer guide 2 and the inlet chute 3 and sent to the fixing area Q5. The post-transfer guide 2 and the inlet chute 3 come into contact with the surface (back surface) of the recording paper S opposite to the surface on which the unfixed image has been transferred.

[0039] In the post-transfer guide 2 of the first embodiment, the front guide surface 2a is formed in a direction that moves away from the back side relative to the imaginary line 4 that connects the secondary transfer area Q4 and the fixing area Q5 as it moves downstream in the paper transport direction. Here, the imaginary line 4 is an imaginary line that connects the downstream side of the secondary transfer device T2 as an example of a transport means, which is the exit of the secondary transfer area Q4, and the upstream side of the rollers Fh and Fp of the fixing device F, which is the entrance of the fixing area Q5. The inlet chute 3 has a guide surface 3a formed on its surface in a direction approaching an imaginary line 4 connecting the secondary transfer area Q4 and the fixing area Q5 as it moves downstream in the paper transport direction. Therefore, the recording paper S is likely to be guided by the post-transfer guide 2 and the inlet chute 3 from the secondary transfer area Q4 to the fixing area Q5 in a curved shape that bulges out toward the rear side.

[0040] The post-transfer guide 2 has a plurality of rib-shaped guide portions 11. The guide portions 11 are arranged in the width direction of the recording paper S at intervals. An actuator 21, which is an example of a movable member, is disposed on the opposite side (inside) of the post-transfer guide 2 from the guide surface 2a (outside). The actuator 21 is disposed between the secondary transfer area Q4 and the fixing area Q5 in the paper transport direction. The actuator 21 is rotatably supported by the bearing portion 2b of the post-transfer guide 2 around a rotation shaft 22.

[0041] The actuator 21 has a contact portion 23 extending radially from the rotation shaft 22. The contact portion 23 is disposed so as to pass through a notch 12, which is an example of a passage portion, formed in the post-transfer guide 2 and enter the transport path of the recording paper S. The contact portion 23 is disposed between the guide portions 11 in the paper width direction. 5A, the outer end of contact portion 23, when not in contact with recording paper S, is located closer to imaginary line 4 than guide surface 2a of guide portion 11. Therefore, contact portion 23 can come into contact with recording paper S being transported along the transport path.

[0042] 4, the rotating shaft 22 extends along the width direction of the recording paper S. A small loop shielding plate 24, which is an example of a first detectable portion, is supported on the rotating shaft 22 at a position offset in the axial direction (paper width direction) from the position of the contact portion 23. In addition, a large loop shielding plate 25, which is an example of a second detectable portion, is supported on the rotating shaft 22 at a position offset in the axial direction (paper width direction) from the contact portion 23 and the small loop shielding plate 24. 4 and 5, the small loop shielding plate 24 and the large loop shielding plate 25 are arranged at positions that are out of phase with each other in the circumferential direction (rotation direction) of the rotating shaft 22.

[0043] Below the rotation shaft 22, a small loop sensor unit 27 as an example of a small curvature detection unit and a large loop sensor unit 28 as an example of a large curvature detection unit are arranged. The small loop sensor unit 27 is disposed at a position corresponding to the small loop shielding plate 24. The small loop sensor unit 27 has a light emitting unit and a light receiving unit disposed on either side of the small loop shielding plate 24, and is capable of detecting whether or not the light from the light emitting unit is shielded by the small loop shielding plate 24. Large loop sensor unit 28 is disposed at a position corresponding to large loop shielding plate 25. Large loop sensor unit 28 has a light-emitting unit and a light-receiving unit disposed on either side of large loop shielding plate 25, and is capable of detecting whether light from the light-emitting unit is shielded by large loop shielding plate 25 or not.

[0044] When contact portion 23 comes into contact with recording paper S, actuator 21 rotates around rotation axis 22. The angle of rotation around rotation axis 22 varies depending on the degree of curvature of the recording paper S that comes into contact. When recording paper S is slightly curved and the angle of rotation is small, actuator 21 moves to the small loop detection position where small loop shielding plate 24 blocks light from small loop sensor unit 27 and large loop shielding plate 25 does not block light from large loop sensor unit 28, as shown in FIG. 5B. When recording paper S is significantly curved and the angle of rotation of actuator 21 is large, actuator 21 moves to the large loop detection position where small loop shielding plate 24 blocks light from small loop sensor unit 27 and large loop shielding plate 25 also blocks light from large loop sensor unit 28, as shown in FIG. 5C. Therefore, the actuator 21 of the first embodiment is movable between the non-contact position shown in Fig. 5A, the small loop detection position shown in Fig. 5B, and the large loop detection position shown in Fig. 5C. In addition, the circumferential positions (phases) of the small loop shielding plate 24 and the large loop shielding plate 25 on the rotation axis 22 are set according to the magnitude of the curvature to be detected (small loop or large loop).

[0045] In the actuator 21 of the first embodiment, a torsion spring (not shown) is attached to the rotation shaft 22 as an example of a return means. The torsion spring exerts an elastic force that moves the actuator 21 to the non-contact position when no external force is acting on the actuator 21. Therefore, when the recording paper S is not in contact with the actuator 21, the actuator 21 is set to automatically return to the non-contact position. Note that it is also possible to adopt a configuration in which a torsion spring is not provided by adjusting the size and position of the contact portion 23 and each of the shielding plates 24, 25 so that the actuator 21 returns to the non-contact position naturally under its own weight depending on the position of the center of gravity of the entire actuator 21. The small loop sensor unit 27 and the large loop sensor unit 28 constitute a sensor unit 27+28 as an example of a detection unit in Example 1. The actuator 21 and the sensor unit 27+28 constitute loop sensors 21-28 as an example of a curvature detection means.

[0046] (Function of control unit C) In FIG. 3, the copier U is controlled by a control unit (controller) C, which is an example of a control means. The control unit C has an input / output interface I / O for inputting and outputting signals from and to the outside. The control unit C also has a read-only memory (ROM) that stores programs and information for performing necessary processing. The control unit C also has a random access memory (RAM) for temporarily storing necessary data. The control unit C also has a central processing unit (CPU) that performs processing according to the programs stored in the ROM or the like. Therefore, the control unit C in Example 1 is configured by a small information processing device, a so-called microcomputer. Therefore, the control unit C can realize various functions by executing programs stored in the ROM or the like. The control unit C of the first embodiment will be described below, but only the contents related to the present invention will be described, and the explanation and illustration of the contents unrelated to the present invention will be omitted.

[0047] The control unit C of the first embodiment has the following functional means (functional modules, program modules) C1 to C3. The curvature determination means C1 determines the magnitude of curvature based on the measurement results of the loop sensors 21 to 28. The curvature determination means C1 of the first embodiment determines that the curvature (loop) is small when only the small loop sensor unit 27 detects the small loop shielding plate 24. Furthermore, the curvature determination means C1 of the first embodiment determines that the curvature (loop) is large when the small loop sensor unit 27 and the large loop sensor unit 28 detect the small loop shielding plate 24 and the large loop shielding plate 25. The curvature determination unit C1 in the first embodiment detects the curvature when the leading edge of the recording paper S reaches the fixing area Q5. In the first embodiment, the curvature is detected 50 ms after the predicted arrival time, rather than immediately after the time when the leading edge of the recording paper S reaches the fixing area Q5 (the predicted arrival time). This is because, in an actual configuration, there are individual differences in the size of the recording paper S and errors in the conveying speed, and so 50 ms is set as an example to allow for some leeway and margin. Note that the value of 50 ms can be changed as desired depending on the design, specifications, etc.

[0048] Figure 6 is an explanatory diagram of the detection timing of the curvature of the medium in Example 1, where Figure 6A is an explanatory diagram of the state before the leading edge of the medium reaches the fixing device, Figure 6B is an explanatory diagram of the state when the leading edge of the plain paper reaches the fixing device, and Figure 6C is an explanatory diagram of the state when the leading edge of the cardboard reaches the fixing device. FIG. 7 is a graph showing the relationship between the basis weight of the paper and the amount of curvature. 6A, when the leading edge of the recording paper S passes through the secondary transfer area Q4, it is transported downstream along the post-transfer guide 2 and comes into contact with the actuator 21. At this time, it is possible to detect that the leading edge of the recording paper S has passed the position of the actuator 21. Regardless of the type or basis weight of the recording paper S, when the detection results of the loop sensors 21 to 28 change from the non-contact position to either the small loop detection position or the large loop detection position, it is possible to determine that the leading edge of the recording paper S has passed the position of the actuator 21. The curvature determination unit C1 determines the magnitude of curvature according to the time when the leading edge of the recording paper S reaches the fixing area Q5 after passing the position of the actuator 21, based on the transport speed of the recording paper S. In the first embodiment, the curvature is detected 50 ms after the predicted arrival time. This corresponds to a position approximately 10 to 20 mm from the leading edge of the recording paper S, although this differs depending on errors in the transport speed, the size and shape of the medium, etc.

[0049] 6C and 7, when the medium type is recording paper S with a high basis weight such as cardboard, or high paper rigidity (so-called stiff paper) such as coated paper or OHP sheet, the leading edge of the recording paper S is likely to be in a small curvature state when it reaches the fixing area Q5, as shown in Fig. 6C. Therefore, as shown in Fig. 6C, when the amount of curvature of the recording paper S reaches the small curvature amount that rotates the actuator 21 to the small curvature position, the curvature determination unit C1 determines that the curvature of the recording paper S is small. 6B and 7, when the recording paper S is a medium with a small basis weight and low paper rigidity (so-called weak paper stiffness), such as thin paper or plain paper, as the leading edge reaches the fixing area Q5, the recording paper S is likely to be in a state of being curved significantly along the post-transfer guide 2 and the inlet chute 3, as shown in Fig. 6B. Therefore, when the amount of curvature of the recording paper S is large and reaches a large curvature amount that rotates the actuator 21 beyond the small curvature position to the large curvature position, the curvature determination unit C1 determines that the curvature of the recording paper S is large.

[0050] In Figures 6B and 6C, in the copier U of Example 1, when the medium type is plain paper or the like, the copier is likely to assume a posture or curved state that is far away from the imaginary line 4, and when the medium type is cardboard or the like, the copier is likely to assume a posture or curved state that is close to the imaginary line 4. Furthermore, when images are formed on multiple sheets of recording paper S in one image forming operation (job), the curvature determination means C1 of the first embodiment detects the curvature of the first recording paper S, and assumes that the subsequent recording papers S are similar, and uses the curvature determination result of the first recording paper S. Note that curvature detection can be performed on all recording papers S, or can be performed periodically, such as every 10 sheets.

[0051] The medium type discrimination unit C2 discriminates the type (medium type) of the recording paper S. The medium type discrimination unit C2 first discriminates the medium type of the recording paper S being used based on the medium type input via the user interface UI and the medium types pre-registered in each of the paper feed trays TR1 to TR4. Then, after the recording paper S is fed out, it discriminates whether there is any inconsistency in the medium type based on the discrimination result of the curvature discrimination unit C1. For example, if the medium type registered in the paper feed trays TR1 to TR4 is plain paper but the discrimination result of the curvature discrimination unit C1 is slight curvature, it determines that there is any inconsistency in the medium type. In this case, the medium type discrimination unit C2 of the first embodiment prioritizes the discrimination result of the curvature discrimination unit C1 and determines that the medium type is cardboard. For example, if the medium type registered in the paper feed trays TR1 to TR4 is plain paper but the discrimination result of the curvature discrimination unit C1 is large curvature, it determines that there is no inconsistency in the medium type. In this case, the medium type discrimination unit C2 of the first embodiment gives priority to the medium types registered in the paper feed trays TR1 to TR4.

[0052] The fixing control means C3 has a heat generation control means C3a and controls the fixing device F. When image formation is performed, the fixing control means C3 controls the conveying speed, fixing temperature, and fixing pressure according to the type of medium, in accordance with the discrimination result of the medium type discrimination means C2. The heat generation control means C3a controls the heater 31, which is an example of a heat generation means built into the heating roller Fh. The heat generation control means C3a controls the on / off of the heater 31 so that the temperature in the fixing area Q5 becomes a fixing temperature according to the type of medium. In the first embodiment, when the amount of curvature is small, that is, when the type of medium is cardboard or the like, the fixing temperature is set and controlled to be higher than when the amount of curvature is large.

[0053] The heat generation control means C3a basically controls the heater 31 at a fixing temperature according to the type of medium set by the medium type discrimination means C2 based on the registration information of the paper feed trays TR1 to TR4, etc. If the medium type discrimination means C2 determines that there is a discrepancy in the medium type, the heater 31 is controlled at a fixing temperature according to the medium type based on the detection results of the loop sensors 21 to 28. In other words, the heater 31 is controlled based on the curvature of the medium detected by the loop sensors 21 to 28.

[0054] If the medium type discrimination unit C2 determines that there is a discrepancy in the medium type, the heater 31 is controlled to change the fixing temperature while the recording paper S passes through the fixing area Q5. In the first embodiment, when the recording paper S comes into contact with the surface of the heating roller Fh heated by the heater 31, the recording paper S is fixed by the heat of the heating roller Fh. Therefore, the temperature of the contact surface of the heating roller Fh that comes into contact with the recording paper S drops. If the fixing temperature setting is incompatible with the type of medium, the temperature on the second rotation will be too low or too high. The heating roller Fh in commercially available copiers typically has a circumference of approximately 70 to 90 mm (diameter 24 to 30 mm). As mentioned above, curvature detection is performed 10 to 20 mm from the leading edge of the recording paper S. Therefore, in the first embodiment, the curvature of the recording paper S is detected between the time the leading edge of the recording paper S reaches the fixing area Q5 and the time the heating roller Fh completes one rotation. Since the circumference of the heating roller Fh is 70 to 90 mm, even if the curvature is detected at a position 10 to 20 mm from the leading edge of the recording paper S and the fixing temperature is switched to a fixing temperature suitable for the type of medium, there will be enough time for the second rotation.

[0055] (Function of Example 1) In the copying machine U of the first embodiment having the above-described configuration, the curvature of the recording paper S that has passed through the secondary transfer area Q4 when its leading edge reaches the fixing area Q5 is detected by the loop sensors 21 to 28. Then, based on the detection results of the loop sensors 21 to 28, the type of the recording paper S is identified and the fixing temperature is controlled. The technology for identifying the type of medium using reflected light described in Patent Documents 1 and 2 has the problem that the sensor is expensive. Also, with colored paper, coated paper, etc., false detection is likely to occur, resulting in low detection accuracy. Furthermore, because it is a non-contact method, there is also the problem of low reliability. In contrast to these, the loop sensors 21 to 28 of Example 1 can distinguish the type of medium with a simple configuration consisting of a rotating actuator 21 and sensor units 27+28 that only detect the passage and blocking of light. Also, the actuator 21 detects by contacting the recording paper S, so even colored paper and the like can be detected more accurately and reliably than with conventional technology.

[0056] In the configuration described in Patent Document 3, when the recording medium enters the fixing nip, the recording medium collides with the roller, causing the conveying speed to fluctuate suddenly and taking a long time to stabilize. This poses a problem of taking a long time to identify the type of recording medium. Therefore, as described in Patent Document 3, it becomes necessary to not form an image on the first recording medium, but to form images on the second and subsequent recording media. In contrast, in the first embodiment, the type of recording paper S is determined based on the curvature of the recording paper S, which allows for faster detection compared to Patent Document 3. Furthermore, the type of medium can be identified and estimated within one rotation of the heating roller Fh. Therefore, the type of medium can be identified during the first rotation of the heating roller Fh, when a drop in fixing temperature is less likely to be a problem, and temperature control of the fixing temperature becomes possible before the second rotation. Therefore, even in an image forming operation in which images are formed on multiple sheets of recording paper S, image formation with poor fixing suppressed from the first sheet is possible. In the copying machine U of the first embodiment, during an image forming operation in which images are formed on multiple sheets of recording paper S, curvature is detected on the first (first) sheet of recording paper S, the type of medium is estimated, and the result of the first sheet of recording paper S is used for subsequent sheets (second and subsequent sheets). This reduces the detection and discrimination processes that are likely to be unnecessary, and reduces the processing load.

[0057] In particular, in the copier U of the first embodiment, the small amount of curvature and the large amount of curvature are detected by the actuator 21 that comes into contact with the recording paper S. Therefore, the difference in the amount of curvature can be detected more reliably with a contact-type configuration than with a non-contact type detection method. When the amount of curvature is small, the fixing temperature is set higher than when the amount of curvature is large, and the occurrence of fixing failure due to insufficient fixing temperature on cardboard or the like is suppressed.

[0058] Furthermore, in the copier U of Example 1, the post-transfer guide 2 guides the recording paper S in a direction away from the imaginary line 4, and then the inlet chute 3 guides the recording paper S in a direction toward the imaginary line 4. Therefore, the paper guide 1 having the post-transfer guide 2 and the inlet chute 3 encourages the curvature of the recording paper S. Therefore, it is easier to determine the amount of curvature compared to a paper guide 1 that does not encourage curvature.

[0059] (Example of change) Although the embodiments of the present invention have been described above in detail, the present invention is not limited to the above embodiments and various modifications can be made within the scope of the gist of the present invention as set forth in the claims. Modifications (H01) to (H06) of the present invention are exemplified below. (H01) In the above embodiment, a copier U was used as an example of an image forming device, but this is not limited to this and it is also possible to configure it as, for example, a printer, a fax machine, or a multifunction device having multiple or all of these functions.

[0060] (H02) In the above embodiment, the copier U is configured to use four color developers, but the present invention is not limited to this and can be applied to, for example, a monochrome image forming apparatus or a multi-color image forming apparatus with three or less colors or five or more colors. Also, the present invention is not limited to this and can be applied to, for example, a configuration having an intermediate transfer belt B. For example, the present invention is also applicable to a configuration in which a direct transfer is performed from a photosensitive member to a recording sheet S. (H03) In the above embodiment, the mode of detecting two levels of bending amount, small bending amount and large bending amount, has been exemplified, but it is not limited to this. It is also possible to add a shielding plate or a sensor unit to make it possible to distinguish three or more levels.

[0061] (H04) In the above embodiment, the post-transfer guide 2 and the inlet chute 3 are preferably shaped to encourage the curvature as exemplified in the embodiment, but are not limited to this. They can be arbitrarily changed depending on the design and specifications of the transport path for the recording paper S. (H05) In the above embodiment, it is possible to control the fixing temperature from the first sheet of recording paper S, but it is also possible to configure the system so that no image is formed on the first sheet, only the type of medium is determined, and image formation is performed on the second sheet and thereafter. (H06) In the above embodiment, an example was given of a mode in which the type of recording paper S is estimated for each image forming operation (job), but this is not limiting. It is also possible to store the most recent estimation results for paper fed from each of the multiple paper feed trays TR1 to TR4 as paper settings for each paper feed tray TR1 to TR4, and then read those settings from the beginning for the next job. This allows for more stable control.

[0062] (Addendum) (((1))) a fixing unit having a heat generating unit that generates heat in accordance with a predetermined fixing temperature and that fixes an unfixed image held on the medium; a curvature detection means disposed upstream of the fixing means in the conveying direction of the medium, the curvature detection means detecting the curvature of the medium when the leading edge of the medium reaches the fixing means; a heat generation control means for controlling the heat generation means based on the curvature of the medium detected by the curvature detection means; An image forming apparatus comprising: (((2))) the curvature detection means having a contact portion that comes into contact with the medium, a curvature detection portion that detects when the curvature of the medium reaches a predetermined small curvature amount, and a large curvature detection portion that detects when the curvature of the medium reaches a large curvature amount that is larger than the small curvature amount; The image forming apparatus according to (((1))) is characterized by comprising: (((3))) the heat generation control means for controlling the heat generation means so that the fixing temperature is higher when the amount of curvature is small than when the amount of curvature is large; The image forming apparatus according to (((2))) is characterized by comprising: (((4))) a conveying unit disposed upstream of the fixing unit in a conveying direction of the medium, the conveying unit conveying the medium toward the fixing unit; a guide means arranged between the fixing means and the transport means in the transport direction of the medium, and guiding the medium toward an entrance of the fixing means, the guide means having a first guide section that can guide the medium in a direction away from an imaginary line connecting the entrance of the fixing means and the transport means, and a second guide section that is arranged downstream of the first guide section in the medium transport direction and approaches the imaginary line to guide the medium toward the entrance of the fixing means; The image forming apparatus according to any one of ((1))) to ((3))) is provided with: (((5))) the fixing means that rotates in accordance with the transport of the medium; a curvature detection means for detecting a curvature of the medium during the time from when the leading edge of the medium reaches the fixing means until when the fixing means makes one rotation; a heat generation control unit that controls the heat generation unit while the medium whose curvature has been detected is passing through the fixing unit after the curvature detection unit detects the curvature of the medium; The image forming apparatus according to any one of ((1))) to ((4))) is provided with: (((6))) When an image is formed on a plurality of media, the curvature of the first medium is detected by the curvature detection means, and the result of the detection of the curvature of the first medium is used for subsequent media. The image forming apparatus according to (((5))) is characterized in that

[0063] According to the image forming device of (((1))), it is possible to reduce costs and prevent poor fixing, compared to when the fixing temperature is controlled using a configuration that estimates the medium type based on the reflectance or transmittance from the medium. According to the image forming apparatus of (((2))), the small curvature amount and the large curvature amount can be detected more reliably than in an image forming apparatus that does not have a small curvature detection portion and a large curvature contact portion that come into contact with the medium. According to the image forming apparatus of (((3))), when using thick paper or the like with a small amount of curvature, the occurrence of poor fixing due to insufficient fixing temperature can be suppressed compared to when the fixing temperature is not controlled to be high. According to the image forming device of (((4))), the first guide section and the second guide section can encourage the medium to bend. According to the image forming apparatus of (((5))), the curvature is detected during the first rotation of the fixing means, where a drop in fixing temperature is unlikely to be a problem, and temperature control can be performed according to the detected curvature from the second rotation onwards. According to the image forming apparatus of (((6))), it is possible to reduce redundant processing compared to when detecting curvature for all media. [Explanation of symbols]

[0064] 1...Guidance means, 2...first guide portion, 3...second guide portion, 4...imaginary line, 21 to 28...curvature detection means, 23...contact part, 27...Small curvature detection unit, 28...Large curvature detection unit 31...heat generating means, C3a...heat generation control means, F...fixing means, S...media, T2...Transportation means, U...Image forming device.

Claims

1. a fixing unit having a heat generating unit that generates heat in accordance with a predetermined fixing temperature and that fixes an unfixed image held on the medium; a curvature detection means disposed upstream of the fixing means in the conveying direction of the medium, the curvature detection means detecting the curvature of the medium when the leading edge of the medium reaches the fixing means; a heat generation control means for controlling the heat generation means based on the curvature of the medium detected by the curvature detection means; An image forming apparatus comprising:

2. the curvature detection means having a contact portion that comes into contact with the medium, a curvature detection portion that detects when the curvature of the medium reaches a predetermined small curvature amount, and a large curvature detection portion that detects when the curvature of the medium reaches a large curvature amount that is larger than the small curvature amount; 2. The image forming apparatus according to claim 1, further comprising:

3. the heat generation control means for controlling the heat generation means so that the fixing temperature is higher when the amount of curvature is small than when the amount of curvature is large; 3. The image forming apparatus according to claim 2, further comprising:

4. a conveying unit disposed upstream of the fixing unit in a conveying direction of the medium, the conveying unit conveying the medium toward the fixing unit; a guide means disposed between the fixing means and the transport means in the transport direction of the medium and configured to guide the medium toward an entrance of the fixing means, the guide means having a first guide section capable of guiding the medium in a direction away from an imaginary line connecting the entrance of the fixing means and the transport means, and a second guide section disposed downstream of the first guide section in the medium transport direction and approaching the imaginary line to guide the medium toward the entrance of the fixing means; 2. The image forming apparatus according to claim 1, further comprising:

5. the fixing means that rotates in accordance with the transport of the medium; a curvature detection means for detecting a curvature of the medium during the time from when the leading edge of the medium reaches the fixing means until when the fixing means makes one rotation; a heat generation control unit that controls the heat generation unit while the medium whose curvature has been detected is passing through the fixing unit after the curvature detection unit detects the curvature of the medium; 2. The image forming apparatus according to claim 1, further comprising:

6. When an image is formed on a plurality of media, the curvature of the first medium is detected by the curvature detection means, and the result of the detection of the curvature of the first medium is used for subsequent media.

6. The image forming apparatus according to claim 5,

Images