JP2008185638A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce offset staining due to defective fixing of a part of an image colored by using small numbers of toner. <P>SOLUTION: The image forming apparatus U includes: a toner image forming device G for dividing an image d1 into pixels d2a of a fine area, further dividing the pixel d2a into fine pixels d3 of a finer area and forming a toner image Tn in which gradation is displayed by means of a ratio of colored area of colored fine pixels d3a to total fine pixels d3 within the pixels d2a; a transfer device TR transferring the toner image Tn formed by the toner image forming device G onto a medium S; a fixing device F for fixing unfixed toner image Tn on the medium S; a colored fine pixel distribution discrimination means C6d, C6d', C6d" for discriminating whether the colored fine pixels d3a are arranged in a distributed manner; and a fixing condition setting means C7 for setting a fixing condition to a preset fixing condition when it is discriminated that the colored fine pixels d3a are arranged in a distributed manner. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer, a copying machine, a FAX, or a multifunction machine having a plurality of functions.

  In a conventional electrophotographic image forming apparatus, a toner image is formed, transferred to a sheet, and an unfixed toner image on the sheet is fixed to form an image. Fixing may not be sufficiently performed depending on the toner image formed at the time of fixing and the type of paper used. Accordingly, there is known a technique for suitably changing the fixing conditions when it is difficult to fix the toner image on the paper.

In particular, the following conventional techniques (J01) to (J03) are conventionally known as techniques for changing fixing conditions based on toner density (print density) and resolution.
(J01) Techniques described in Patent Document 1 (Japanese Patent Laid-Open No. 4-156490) and Patent Document 2 (Japanese Patent Laid-Open No. 6-161196) In Patent Document 1 and Patent Document 2, a scanner unit comprising a CCD is provided on a recording sheet. A technique for setting the fixing temperature to a high temperature when the toner density is high by reading the toner density of the toner image is described.
(J02) Technology described in Patent Document 3 (Japanese Patent Application Laid-Open No. 1-267582) Patent Document 3 describes that the fixing nip pressure is increased when the toner density is high in a gradation image such as a photographic image or a halftone image. The technology to do is described.
(J03) Technology described in Patent Document 4 (Japanese Patent Laid-Open No. 7-319238) Patent Document 4 describes a halftone mode (a mode with a high resolution such as a photographic mode, a halftone mode, etc.) among image selection modes by user input. In the case where () is selected, a technique is described in which the fixing temperature is increased as compared with the normal mode, or the process speed is decreased to increase the fixing section passage time.

JP-A-6-161196 ("0003" to "0007", FIGS. 3 and 8) JP-A-8-83015 ("0003" to "0010", "0025" to "0037", FIG. 5) JP-A-1-267582 (Specification, page 2, upper left column, line 1 to lower left column, line 15; FIG. 1) JP-A-7-319238 ("0011" to "0013", FIGS. 1 and 3)

(Problems of conventional technology)

FIG. 16 is an explanatory diagram of form image data which is an example of image data.
FIG. 17 is an explanatory diagram of halftone dots that are isolated and colored, FIG. 17A is an explanatory diagram of an arrangement example when there are halftone dots that are isolated and colored in the gradation image portion, and FIG. It is explanatory drawing which expanded and displayed the halftone dot of 17A.
The prior arts (J01) to (J03) are intended to change the fixing conditions so that the toner image is easily fixed on the paper when the toner density and the resolution increase. Therefore, for example, when the form image data 01 as an example of the image data shown in FIG. 16 is transferred to a sheet as a toner image, there are many white areas where the toner is not transferred if the entire sheet is viewed. Therefore, there is a possibility of fixing under the same fixing conditions as normal.

  16 and 17A, the form image data 01 has a gradation image portion 02 expressed by a conventionally known gradation display method (for example, see Japanese Patent Application Laid-Open No. 2003-125201). The gradation image portion 02 is divided by pixels (see broken lines in FIG. 17A) that have a predetermined region and are arranged in a predetermined arrangement pattern without a gap, and each pixel has a halftone dot ( Sub-pixel) 03. In the gradation display method, the halftone dots 03 in each pixel are colored in a predetermined coloring order in accordance with the image density of the gradation image portion 02, so that the gradation of the gradation image portion 02 is changed. Can be expressed.

  Further, as shown in FIG. 17B, the halftone dot 03 has a surface area of about 40 μm (42.333...), For example, at 600 dpi, and is colored by a small number of toners 04. Accordingly, when the form image data 01 is transferred to a sheet as a toner image, the halftone dots 03 shown in FIG. 17A that are isolated are colored adjacently without being isolated. Since the total number of the toners 04 existing in the colored area is smaller than that in the case where the toners are colored, the binding force between the toners 04 on the paper is weak, and the force that the toner 04 is attracted to the paper is also weak. For this reason, at the halftone dots 03 that are colored in isolation, the toner 04 is difficult to be fixed to the paper, and the toner 04 may be separated from the paper and adhere to the heating roll of the fixing device during heat fixing. There is. Therefore, an image having a fixed portion and a non-fixed portion is obtained, and the image quality may be deteriorated. When the job is executed in a state where the toner 04 is attached to the heating roll, the toner 04 attached to the heating roll may be transferred and fixed on another subsequent sheet, so-called offset. Dirt may occur.

  When the resolution in the image forming apparatus is increased, the area of each halftone dot 03 is further reduced. Therefore, the halftone dot 03 is colored with a smaller number of toners 04, and the possibility of the occurrence of the offset stain is further increased. In particular, when the form image data 01 is formed on the sheet, the gradation image portion 02 included in the form image data 01 is colored with a predetermined arrangement pattern by the gradation display method. The halftone dots 03 (see FIG. 17A) are distributed in a band shape along the paper transport direction (see FIG. 16). As a result, the toner is concentrated on the portion of the heating roll where the gradation image portion 02 is heated and fixed, that is, the portion where the toner 04 of the halftone dots 03 distributed in a band shape is fixed (the width portion of the band). The possibility that 04 will adhere becomes high, and the possibility that the offset contamination will occur in the portion becomes even higher.

  In this case, in the prior arts (J01) to (J03), even if the halftone dot 03 (see FIG. 17A) that is isolated and colored exists on the image, if the toner density or resolution is low, the image is fixed. There is a possibility that the fixing conditions are not changed so as to be easy. As a result, the conventional image forming apparatus has a problem that the toner 04 on the halftone dot 03 cannot be reliably fixed, and the offset stain may occur.

In view of the above circumstances, the present invention has the following description (O01) as a technical problem.
(O01) To reduce offset contamination due to poor fixing of a part of an image colored by a small number of toners.

Next, the present invention that solves the above problems will be described. However, in order to easily understand the correspondence between the constituent elements of the present invention and the constituent elements of the embodiments described later, it is implemented immediately after the constituent elements of the present invention. The reference numerals of the constituent elements in the example are enclosed in parentheses ().
The reason why the present invention is described in correspondence with the reference numerals of the embodiments described later is to facilitate understanding of the present invention, and does not limit the scope of the present invention to the embodiments.

(Invention)
In order to solve the above problems, an image forming apparatus (U) according to the present invention includes:
The image (d1) is divided into pixels (d2a) having a minute area, the pixels (d2a) are further divided into minute pixels (d3) having a minute area, and all the pixels (d3) in the pixel (d2a) are divided. A toner image forming apparatus (G) for forming a toner image (Tn) displayed in gradation according to the ratio of the colored area of the colored micropixel (d3a);
A transfer device (TR) for transferring the toner image (Tn) formed by the toner image forming device (G) to a medium (S);
A fixing device (F) for fixing an unfixed toner image (Tn) on the medium (S);
Colored fine pixel distribution discriminating means (C6d, C6d ′, C6d ″) for discriminating whether or not the colored fine pixels (d3a) are dispersedly arranged;
When the colored fine pixel distribution determining means (C6d, C6d ′, C6d ″) determines that the colored fine pixels (d3a) are dispersedly arranged, the colored fine pixels (d3a) are dispersed. Fixing condition setting means (C7) for setting the fixing condition in the fixing device (F) set in advance in correspondence with the arrangement,
It is provided with.

(Operation of the present invention)
In the image forming apparatus (U) of the present invention having the above-described configuration requirements, the toner image forming apparatus (G) divides the image (d1) into pixels (d2a) having a small area, and further subdivides the pixels (d2a). A toner image (Tn) that is divided into fine pixels (d3) having a large area, and gradation is displayed according to the ratio of the colored area of the colored fine pixels (d3a) to all the fine pixels (d3) in the pixel (d2a). Form. The transfer device (TR) transfers the toner image (Tn) formed by the toner image forming device (G) to the medium (S). The fixing device (F) fixes the unfixed toner image (Tn) on the medium (S). Colored fine pixel distribution discriminating means (C6d, C6d ′, C6d ″) discriminates whether or not the colored fine pixels (d3a) are arranged in a dispersed manner. The fixing condition setting means (C7) is the colored fine pixel distribution (C7). When it is determined by the pixel distribution determining means (C6d, C6d ′, C6d ″) that the colored fine pixels (d3a) are arranged in a dispersed manner, the colored fine pixels (d3a) are arranged in a dispersed manner. The fixing conditions are set in advance in the fixing device (F) corresponding to the case where there is a toner image.

  Therefore, in the image forming apparatus (U) of the present invention, even when the colored fine pixels (d3a) are arranged in a dispersed manner and the colored fine pixels (d3a) are colored with a small number of toners, the fixing Since the fixing can be surely performed by the apparatus (F), it is possible to reduce the offset contamination due to the fixing failure.

(Embodiment 1 of the present invention)
In the present invention, the image forming apparatus (U) according to the first embodiment of the present invention is
The colored fine pixel distribution discriminating means (C6d, C6d ′, C6d ″) for discriminating whether or not the colored fine pixels (d3a) are arranged in a dispersed manner by judging whether or not the colored fine pixels (d3a) are arranged in isolation. ),
It is provided with.

(Operation of Form 1 of the Present Invention)
In the image forming apparatus (U) according to the first embodiment of the present invention having the above-described structural requirements, the colored fine pixel distribution discriminating means (C6d, C6d ′, C6d ″) is arranged so that the colored fine pixels (d3a) are isolated. By determining whether or not they are arranged, it is determined whether or not they are distributed.
Therefore, in the image forming apparatus (U) according to the first embodiment of the present invention, even when the colored micropixels (d3a) are arranged in isolation and the colored micropixels (d3a) are colored with a small number of toners. Since the toner can be reliably fixed by the fixing device (F), offset contamination due to fixing failure can be reduced.

(Embodiment 2 of the present invention)
The image forming apparatus (U) according to the second embodiment of the present invention is the above-described present invention or the first embodiment of the present invention.
The colored fine pixel distribution discriminating means (C6d) for discriminating whether or not the colored fine pixels (d3a) arranged in a dispersed manner are arranged in a band shape in the medium transport direction,
It is provided with.

(Operation of Embodiment 2 of the present invention)
In the image forming apparatus (U) according to the second embodiment of the present invention having the above-described configuration requirements, the colored fine pixel distribution discriminating means (C6d) is arranged such that the colored fine pixels (d3a) arranged in a dispersed manner are in the medium transport direction. It is determined whether or not it is arranged in a band shape.
Therefore, in the image forming apparatus (U) according to the second embodiment of the present invention, the colored fine pixels (d3a) arranged in a dispersed manner are arranged in a band shape in the medium transport direction, and the colored fine pixels (d3a) are slightly present. In the case where the toner is colored with a large number of toners, when the portion (band width portion) where the toner of the colored fine pixels (d3a) arranged in a band shape in the medium conveyance direction is fixed is defectively fixed, However, since the toner can be reliably fixed by the fixing device (F) in which the fixing conditions are set, offset contamination due to poor fixing can be reduced.

(Embodiment 3 of the present invention)
The image forming apparatus (U) according to the third embodiment of the present invention is the above-described present invention or the first embodiment of the present invention.
The colored fine pixel distribution discriminating means (C6d ′, C6d ″) for discriminating whether or not the colored fine pixels (d3a) arranged in a dispersed manner are partially arranged with respect to the entire image area,
It is provided with.

(Operation of Form 3 of the Present Invention)
In the image forming apparatus (U) according to the third embodiment of the present invention having the above-described structural requirements, the colored fine pixel distribution discriminating means (C6d ′, C6d ″) includes the colored fine pixels (d3a) arranged in a dispersed manner. Then, it is determined whether or not it is partially arranged with respect to the entire image area.
Therefore, in the image forming apparatus (U) according to the third embodiment of the present invention, the colored fine pixels (d3a) arranged in a distributed manner are partially arranged with respect to the entire image region, and the colored fine pixels (d3a) are arranged. When the toner is colored by a small number of toners, the portion of the colored fine pixel (d3a) that is partially arranged to fix the toner (for example, the central portion when arranged at the central portion) is poorly fixed. Then, the partial offset stains are easily noticeable, but the toner can be reliably fixed by the fixing device (F) in which the fixing conditions are set. it can.

(Embodiment 4)
An image forming apparatus (U) according to a fourth embodiment of the present invention is the above-described present invention or any one of the first to third embodiments of the present invention.
The fixing temperature (T0 to T4, Ta, Tb) in the fixing region (Q5) to which the toner image (Tn) is fixed, and the fixing temperature (T0 to T4, Ta, Tb) as the fixing condition, When the colored micropixels (d3a) are arranged in a dispersed manner, the fixing is set at a higher fixing temperature (T1 to T4, Ta) than when the colored micropixels (d3a) are not dispersed. Condition setting means (C7),
It is provided with.

(Operation of Form 4 of the Present Invention)
In the image forming apparatus (U) according to the fourth embodiment of the present invention having the above-described structural requirements, the fixing condition setting unit (C7) is configured so that the fixing temperature (T0) in the fixing region (Q5) where the toner image (Tn) is fixed. To T4, Ta, Tb), and the fixing temperature (T0 to T4, Ta, Tb) as the fixing condition is set when the colored fine pixels (d3a) are arranged in a dispersed manner. The fixing temperature (T1 to T4, Ta) is set higher than that in the case where the fine pixels (d3a) are not dispersed.
Therefore, in the image forming apparatus (U) according to Embodiment 4 of the present invention, the fixing temperature (T0 to T4, Ta, Tb) can be set as the fixing condition. As a result, even when the colored fine pixels (d3a) arranged in a dispersed manner are colored with a small number of toners, the fixing temperature (T0 to T4, Ta, Tb) in the fixing device (F) is changed to the colored. By setting the fixing temperature (T1 to T4, Ta) higher than that in the case where the fine pixels (d3a) are not dispersed, the toner can be reliably fixed, thereby reducing offset contamination due to fixing failure. be able to.

(Embodiment 5)
An image forming apparatus (U) according to a fifth embodiment of the present invention is the above-described present invention or any one of the first to fourth embodiments of the present invention.
The fixing pressure (P0, Pa, Pb) applied to the medium (S) in the fixing region (Q5) where the toner image (Tn) is fixed, and the fixing pressure (P0, Pa as the fixing condition). , Pb) is set to a higher fixing pressure (Pa) when the colored micropixels (d3a) are dispersedly arranged than when the colored micropixels (d3a) are not dispersed. Fixing condition setting means (C7),
It is provided with.

(Operation of Form 5 of the Present Invention)
In the image forming apparatus (U) according to the fifth embodiment of the present invention having the above-described structural requirements, the fixing condition setting unit (C7) is configured such that the medium (S) in the fixing region (Q5) where the toner image (Tn) is fixed. ) And the fixing pressures (P0, Pa, Pb) as the fixing conditions are arranged with the colored micropixels (d3a) dispersed therein. In this case, the fixing pressure (Pa) is set to be higher than that in the case where the colored fine pixels (d3a) are not dispersed.
Therefore, in the image forming apparatus (U) according to Embodiment 5 of the present invention, the fixing pressure (P0, Pa, Pb) can be set as the fixing condition. As a result, even when the colored fine pixels (d3a) arranged in a dispersed manner are colored with a small number of toners, the fixing pressure (P0, Pa, Pb) in the fixing device (F) is changed to the colored fine pixels. By setting the fixing pressure (Pa) to be higher than that in the case where (d3a) is not dispersed, the toner can be fixed reliably, so that offset contamination due to fixing failure can be reduced.

(Embodiment 6 of the present invention)
An image forming apparatus (U) according to a sixth embodiment of the present invention is the above-described present invention or any one of the first to fifth embodiments of the present invention.
The medium transport speed, which is the speed at which the medium (S) is transported through the fixing region (Q5) to which the toner image (Tn) is fixed, and the medium transport speed as the fixing condition is defined as the colored fine pixel. The fixing condition setting means (C7) for setting the medium conveyance speed at a lower speed when (d3a) is arranged in a distributed manner than when the colored fine pixels (d3a) are not dispersed;
It is provided with.

(Operation of Embodiment 6 of the present invention)
In the image forming apparatus (U) according to the sixth embodiment of the present invention having the above-described structural requirements, the fixing condition setting unit (C7) sets the fixing region (Q5) where the toner image (Tn) is fixed to the medium (S). ) Is a medium conveyance speed that is a conveyance speed, and the coloring micropixels (d3a) are arranged when the coloring micropixels (d3a) are dispersedly arranged at the medium conveyance speed as the fixing condition. ) Is set to a medium conveyance speed that is lower than that in the case where the above is not dispersed.
Therefore, in the image forming apparatus (U) according to Embodiment 6 of the present invention, the medium conveyance speed can be set as the fixing condition. As a result, even when the colored fine pixels (d3a) arranged in a dispersed manner are colored with a small number of toners, the colored fine pixels (d3a) disperse the medium conveyance speed in the fixing device (F). Since the toner can be reliably fixed by setting the medium conveyance speed at a lower speed than in the case where the toner is not transferred, offset contamination due to fixing failure can be reduced.

(Embodiment 7 of the present invention)
An image forming apparatus (U) according to Embodiment 7 of the present invention is the image forming apparatus (U) according to any one of Embodiments 4 to 6 of the present invention.
The fixing condition setting means (C7) for setting fixing conditions in the fixing device (F) based on the smoothness of the surface of the medium (S);
It is provided with.

(Operation of Embodiment 7 of the present invention)
In the image forming apparatus (U) according to Embodiment 7 of the present invention having the above-described structural requirements, the fixing condition setting unit (C7) is the fixing apparatus (F) based on the smoothness of the surface of the medium (S). Set the fixing conditions.
Therefore, in the image forming apparatus (U) of the seventh embodiment of the present invention, when the smoothness of the surface of the medium (S) is high, the toner hardly penetrates into the medium (S). It is difficult to fix the unfixed toner image (Tn) of the toner, and in particular, a small number of toners colored by the colored fine pixels (d3a) arranged in a dispersed manner are more difficult to be fixed. The toner can be reliably fixed by the fixing device (F) in which the fixing conditions set in the above are set, so that offset contamination due to fixing failure can be reduced.

(E01) Offset contamination due to poor fixing of a part of an image colored with a small number of toners can be reduced.

Next, specific examples (examples) of the embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following examples.
In order to facilitate understanding of the following description, in the drawings, the front-rear direction is the X-axis direction, the left-right direction is the Y-axis direction, the up-down direction is the Z-axis direction, and arrows X, -X, Y, -Y, The direction indicated by Z and -Z or the indicated side is defined as front, rear, right, left, upper, lower, or front, rear, right, left, upper, and lower, respectively.
In the figure, “•” in “○” means an arrow heading from the back of the page to the front, and “×” in “○” is the front of the page. It means an arrow pointing from the back to the back.
In the following description using the drawings, illustrations other than members necessary for the description are omitted as appropriate for easy understanding.

FIG. 1 is an overall explanatory view of an image forming apparatus according to Embodiment 1 of the present invention and is a sectional view.
In FIG. 1, an image forming apparatus U is detachably mounted on a digital copying machine body U1 as an image forming apparatus body having a platen glass (transparent document table) PG on an upper surface thereof, and the platen glass PG. And an automatic document feeder (auto document feeder, ADF) U2.
The automatic document feeder U2 has a document feed tray TG1 on which a plurality of documents Gi to be copied are stacked. Each of the plurality of documents Gi placed on the document feed tray TG1 sequentially passes through a copy position on the platen glass PG (pressure contact position of the platen roll GR1) and is discharged from the document discharge roll GR2 to the document discharge tray TG2. It is comprised so that.

  The copier body U1 includes an image scanner U1a and a printer U1b having the platen glass PG.

(Image scanner U1a)
The image scanner U1a includes an exposure system registration sensor (platen registration sensor) Sp disposed at a platen registration position and an exposure optical system A.
The movement and stop of the exposure optical system A are controlled by the detection signal of the exposure system registration sensor Sp, and always stop at the home position.
In the case of the ADF mode in which copying is performed using the automatic document feeder (auto document feeder) U2, the exposure optical system A is stopped at the home position and sequentially passes through the copy position on the platen glass PG. The document Gi is exposed.
In the platen mode in which the document Gi is manually placed on the platen glass PG and copied, the exposure optical system A exposes and scans the document Gi on the platen glass PG while moving.
The reflected light from the exposed document Gi passes through the exposure optical system A and is converged on a CCD (solid-state imaging device). The CCD converts the reflected document light converged on the imaging surface into an electric signal.

(Printer U1b)
An IPS (image processing system, that is, an image processing unit) converts the read image signal input from the CCD into a digital image write signal and outputs it to the laser drive signal output device DL of the printer U1b.
The laser drive signal output device DL whose operation timing is controlled by the controller C of the printer U1b outputs a laser drive signal corresponding to the input image data to a ROS (latent image writing scanning device).
The image carrier PR disposed below the ROS rotates in the direction of the arrow Ya. The surface of the image carrier PR is charged to, for example, − (minus) 700 V by a charging roll (charger, charge roll) CR in the charging region Q0, and then the ROS (latent image writing device) at a latent image writing position Q1. ) To be exposed and scanned to form an electrostatic latent image of −300 V, for example. The latent image formation by the laser beam L on the image carrier PR is started after a predetermined time has elapsed since a sheet sensor (not shown) detects the leading edge of the sheet. The surface of the image carrier PR on which the electrostatic latent image is formed rotates and moves sequentially through the development area Q2 and the sheet transfer area (image recording position) Q4.

A developing device (developing apparatus) D that develops the electrostatic latent image in the developing area Q2 conveys a developer including a negatively charged toner and a positively charged carrier to the developing area Q2 by the developing roller R0. Then, the electrostatic latent image on the image carrier PR passing through the development area Q2 is developed into a toner image Tn.
A transfer roll (transfer device) TR facing the image carrier PR in the sheet transfer region (image recording position) Q4 is a member that transfers the toner image Tn on the surface of the image carrier PR to the sheet S, and a developing device D A transfer voltage having a polarity opposite to the charging polarity of the developing toner used in 1 is supplied from the power supply circuit E. A power supply having a charging bias applied to the charging roll CR, a developing bias applied to the developing roll R0, a bias such as a transfer bias applied to the transfer roll TR, and a heater power supply for heating a heater of a heating roll of the fixing device F described later The circuit E is controlled by the controller C.

A first paper feed tray TR1 and a second paper feed tray TR2 are arranged vertically below the copier body U1.
A take-out roll (pickup roll) Rp is disposed at the upper end of the right end of the first paper feed tray TR1 and the second paper feed tray TR2, and the sheets taken out by the take-out roll Rp are fed into the paper feed trays TR1, It is conveyed to the paper feed path SH0 on the right side of TR2.
A paper feed member Rs is disposed in the paper feed path SH0, and the paper feed member Rs has a paper feed roll Rs1 and a separation roll (separation member) Rs2 that form a nip portion by a portion that presses against each other. . The sheets conveyed to the nip portion are separated one by one by the sheet feeding member Rs and conveyed to the vertical sheet conveying path SH1 extending in the vertical direction on the downstream side of the sheet feeding path SH0. A transport roll (forward / reverse rotation transport roll) Rb capable of forward and reverse rotation is disposed in the vertical sheet transport path SH1. The sheet S conveyed to the vertical sheet conveyance path SH1 is conveyed to the upper upstream sheet conveyance path SH2 by a conveyance roll Rb that can rotate forward and backward.

The sheet S conveyed to the pre-transfer sheet conveyance path SH2 is conveyed to the registration roll Rr by the conveyance roll Ra. The sheet S conveyed to the registration roll Rr is moved from the pre-transfer sheet guide SG1 to the sheet transfer area in time with the toner image Tn on the image carrier PR moving to the sheet transfer area (image recording position) Q4. It is conveyed to Q4.
The toner image Tn developed on the surface of the image carrier PR is transferred to the sheet S by the transfer roll TR in the sheet transfer region Q4. After the transfer, the surface of the image carrier PR is cleaned by the photoreceptor cleaner CL1 to remove the residual toner, and then the charge is removed by the photoreceptor neutralizer JL and then recharged by the charging roll CR.
A toner image forming apparatus G (PR + CR + ROS + D) is configured by the image carrier PR, the charging roll CR, ROS (latent image writing apparatus), the developing device D, and the like.

A post-transfer sheet conveyance path SH3 for conveying the recorded sheet S on which the toner image Tn is recorded in the sheet transfer area Q4 to the fixing area Q5 is provided on the downstream side of the sheet transfer area Q4 in the sheet conveyance direction. The sheet S on which the toner image has been transferred by the transfer roll TR in the sheet transfer region Q4 is peeled off from the surface of the image carrier PR, and is moved to the fixing region Q5 by the sheet guide SG2 and the sheet transport belt BH in the post-transfer sheet transport path SH3. Be transported. When the sheet S passes through the fixing region Q5, the toner image is heated and fixed by a pair of fixing rolls (heating roll Fh, pressure roll Fp) of the fixing device F, and then discharged through the sheet discharge path SH4. It is conveyed to the paper tray TRh.
In the sheet discharge path SH4, a switching gate (sheet transport direction control member) GT1 is disposed on the downstream side of the fixing device F. The switching gate GT1 switches the conveyance direction of the sheet S that has passed through the fixing device F to either the discharge tray TRh side or the sheet reverse connection path SH5. The sheet reversing connection path SH5 connects the upstream end of the sheet discharge path SH4 (the downstream side portion of the fixing device F) and the vertical sheet transport path SH1.

In the case of duplex copying, the one-side recorded sheet S on which the first-side toner image has been recorded is forward-reversely conveyed by the switching gate GT1 from the sheet reversal connection path SH5 through the mylar gate GT2 to the upper end of the sheet feeding path SH0. After being conveyed by the roll Rb below the sheet feeding path SH0, it is re-transmitted to the upper pre-transfer sheet conveying path SH2 in a state of being switched back and reversed.
The one-side recorded sheet S that has been reversed and retransmitted to the pre-transfer sheet conveyance path SH2 is retransmitted to the sheet transfer area (image recording position) Q4, and a toner image is transferred to the second side.

A sheet feeding device UT is configured by elements indicated by the symbols SH0, Rp, Rs, etc. A sheet conveyance path SH is configured by elements indicated by the symbols SH1 to SH5.
A sheet conveying apparatus US is constituted by the sheet conveying path SH and rollers Ra, Rb, Rr and the like having a sheet conveying function arranged there.

(Description of the control part of Example 1)
FIG. 2 is a block diagram of the control unit of the image forming apparatus according to the first exemplary embodiment of the present invention.
In FIG. 2, the controller C stores an I / O (input / output interface) that performs input / output of signals to / from the outside and adjustment of input / output signal levels, a program and data for performing necessary processing, and the like. ROM (read-only memory), RAM (random access memory) for temporarily storing necessary data, CPU (central processing unit) that performs processing according to the program stored in the ROM, clock oscillator, etc. Various functions can be realized by executing a program stored in the ROM.

(Signal input element connected to controller C)
The controller C receives an output signal from the next signal output element UI (user interface) or the like.
UI: User Interface The user interface UI includes a display unit UI1, a menu key UI2, a start key UI3, a cancel key UI4, a numeric keypad UI5, and the like.

(Controlled element connected to controller C)
The controller C outputs a control signal for the next controlled element.
IPS: Image Processing System The image processing system IPS converts the read image signal input from the CCD into a digital image write signal and outputs it to the laser drive signal output device DL.
DL: Laser drive signal output device The laser drive signal output device DL outputs a laser drive signal to a ROS (latent image forming device) in accordance with the input image writing signal. In response to the laser drive signal, a laser beam L emitted from a laser diode (not shown) of a ROS (latent image forming apparatus) scans the surface of the image carrier PR, and the surface of the image carrier PR. An electrostatic latent image is formed.

D1: Main motor drive circuit The main motor drive circuit D1 drives the main motor M1, thereby heating the developing roll R0, the image carrier PR, the charging roll CR, the transfer roll TR, and the fixing device F of the developing device D. The roll Fh and the like are driven to rotate.
E: Power Supply Circuit The power supply circuit E includes a developing bias power supply circuit E1, a charging power supply circuit E2, a transfer power supply circuit E3, and a fixing power supply circuit E4.

E1: Power supply circuit for developing bias The power supply circuit for developing bias E1 applies a developing bias to the developing roll R0 of the developing device D.
E2: Charging power supply circuit The charging power supply circuit E2 applies a charging bias to the charging roll CR.
E3: Transfer Power Supply Circuit The transfer power supply circuit E3 applies a transfer bias to the transfer roll TR.
E4: Power Supply Circuit for Fixing The power supply circuit for fixing E4 supplies heating power to the heating roll Fh.

(Function of controller C)
The controller C has a program (function realization means) that realizes a function of executing a process according to an output signal from each signal output element and outputting a control signal to each control element. Next, a program (function realization means) for realizing various functions of the controller C will be described.
C1: Job Control Unit When the print control unit C1 receives print data, the job control unit C1 controls the operation of each member of the printer U and executes a job (image forming operation).
C2: Writing Image Information Storage Unit The writing image information storage unit C2 stores the image data (writing image information) that is the image writing signal to be output to the laser drive signal output device DL. The writing image information storage means C2 of the first embodiment is constituted by a so-called page memory, and image data (dot data) for writing for one page is developed and temporarily stored during job execution.

C3: Paper Type Storage Unit The paper type storage unit C3 stores the type of the sheet S. The sheet type storage unit C3 according to the first exemplary embodiment stores any of plain paper, OHP, colored paper, coated paper, and cardboard as the type of the sheet S stored in each of the paper feed trays TR1 and TR2. In the first embodiment, the type of the sheet S is set by a user input on the user interface UI.
C4: Main motor rotation control means The main motor rotation control means C4 controls the rotation of the image carrier PR, developing device D, developing roll R0, fixing device F, etc. by controlling the operation of the main motor drive circuit D1. .
C5: Power supply circuit control means The power supply circuit control means C5 includes the following means C5a to C5d, and controls the power supply circuit E to control the development bias, charging bias, transfer bias, and heater of the heating roll Fh. Control on-off etc.

C5a: Development Bias Control Unit The development bias control unit C5a controls the operation of the development bias power supply circuit E1 and applies a bias to the development roll R0 of the development unit G.
C5b: Charging Bias Control Unit The charging bias control unit C5b controls the charging bias applied to the charging roll CR by controlling the operation of the charging bias power supply circuit E2.
C5c: Transfer Bias Control Unit The transfer bias control unit C5c controls the operation of the transfer power supply circuit E3 and applies a transfer bias to be applied to the transfer roll TR.
C5d: Fixing power supply control means The fixing power supply control means C5d controls the operation of the fixing power supply circuit E4, and controls the fixing temperature by controlling on / off of the heater of the heating roll Fh.

C6: Fixing Condition Change Determination Unit The fixing condition change determination unit C6 includes a paper type determination unit C6a, a writing image information update determination unit C6b, a writing image information analysis unit C6c, and an isolated colored halftone dot distribution determination unit. C6d, and determines whether or not it is necessary to change the fixing condition when the job is executed.
C6a: Paper Type Discriminating Unit The paper type discriminating unit C6a discriminates the type of the sheet S. The sheet type discriminating means C6a according to the first exemplary embodiment includes the sheet feeding trays TR1 and TR2 to be used and the types of the sheets S stored in the sheet feeding trays TR1 and TR2 stored in the sheet type storing means C3. Based on the information, it is determined whether or not the type of the sheet S is plain paper.
C6b: Write image information update determination means The write image information update determination means C6b determines whether or not the image data for one page temporarily stored in the write image information storage means C2 has been updated during job execution. Is determined.

C6c: Writing image information analysis means The writing image information analysis means C6c analyzes the writing image information (image data) output to the laser drive signal output device DL. First, the writing image information analysis unit C6c according to the first exemplary embodiment uses the image data (dot data) temporarily stored in the writing image information storage unit C2 to convert the image data into the toner image Tn. When transferred to S, the coordinates of dots (colored halftone dots) colored by toner and the coordinates of uncolored dots (non-colored halftone dots) are identified. Then, a colored halftone dot in which all eight surrounding dots (dots in four directions, up and down, left and right, and oblique four directions) are non-colored halftone dots, that is, an isolated colored halftone dot that is an isolated colored halftone dot is detected.

FIG. 3 is an explanatory diagram of form image data which is an example of image data according to the first embodiment of the present invention.
4 is an explanatory diagram of a gradation image portion according to the first embodiment of the present invention, FIG. 4A is an explanatory diagram of an arrangement of halftone dots in which the gradation image portion of FIG. 3 is enlarged, and FIG. 4B is a floor of FIG. 4A. FIG. 4C is an explanatory diagram in which one pixel in the toned image portion is enlarged and displayed, and FIG. 4C is an explanatory diagram in which a colored dot of one dot in one pixel in FIG.
C6d: Isolated colored halftone dot distribution determining means (colored fine pixel distribution determining means)
The isolated colored halftone dot distribution determining means C6d determines whether or not the isolated colored halftone dots are distributed based on the analysis result of the writing image information analyzing means C6c so that the fixing condition needs to be changed. To do. The isolated colored halftone dot distribution determining unit C6d according to the first exemplary embodiment is configured so that the isolated colored halftone dots are arranged in the sheet conveyance direction based on the coordinates of the isolated colored halftone dots detected by the writing image information analyzing unit C6c. Correspondingly, it is determined whether or not it is distributed in a band shape.

Specifically, as shown in FIGS. 3 and 4A, consider a case where the form image data d1 as the image data has a gradation image portion d2. The gradation image portion d2 is divided by pixels d2a having a predetermined region and arranged in a predetermined arrangement pattern without a gap (see the dashed line in FIG. 4A, FIG. 4B), and each pixel d2a is further finer It is divided into halftone dots (fine pixels) d3 having a large area (see FIGS. 4A and 4B). In the gradation display method, the halftone dots in each pixel d2a are colored in a predetermined coloring order according to the image density of the gradation image portion d2, thereby expressing the light and shade of the gradation image portion d2. it can. Such a gradation display method is described in, for example, Japanese Patent Application Laid-Open No. 2003-125201 and the like, and is conventionally known.
As shown in FIG. 4A and FIG. 4B, the pixel d2a has a colored halftone dot d3a which is the colored halftone dot d3 and a non-colored halftone dot d3 which are not colored by the gradation display method. There is a colored halftone dot d3b. When the colored halftone dot (colored fine pixel) d3a is isolated, the writing image information analyzing means C6c detects each as the isolated colored halftone dot. In Example 1, the colored halftone dot d3a detected as the isolated colored halftone dot is colored with a small number of toners d4 as shown in FIG. 4C. The isolated colored halftone dot distribution discriminating means C6d is based on the conveyance direction of the sheets S accommodated in the paper feed trays TR1 and TR2 and the form image data d1 developed in the writing image information storage means C2. Based on the detected coordinates of the isolated colored halftone dots, it is determined whether or not the isolated colored halftone dots are distributed in a band shape along the conveyance direction of the sheet S.

C7: Fixing Condition Setting Unit The fixing condition setting unit C7 includes a fixing temperature storage unit C7a and a fixing temperature setting unit C7b, and sets the fixing condition based on the fixing condition change determination unit C6.
C7a: Fixing temperature storage means The fixing temperature storage means C7a stores the fixing temperature in the heating roll Fh. The fixing temperature storage unit C7a according to the first exemplary embodiment is set to a normal paper fixing temperature T0, which is a normal fixing temperature, and to a temperature higher than the normal paper fixing temperature T0. Fixing temperature Ta for isolated colored dots that can be reliably heated and fixed, and fixing temperature Tb for special paper that can reliably heat and fix the toner image Tn on special paper (OHP, colored paper, coated paper, cardboard) other than plain paper And remember.

C7b: Fixing temperature setting unit The fixing temperature setting unit C7b sets a fixing temperature when the toner image Tn on the sheet S conveyed to the fixing region Q5 is heated and fixed based on the fixing condition change determining unit C6. . The fixing temperature setting unit C7b according to the first exemplary embodiment performs normal job execution, that is, the sheet S is plain paper, and the isolated colored halftone dots are not distributed in a band shape along the conveyance direction of the sheet S. In this case, the fixing temperature is set to the plain paper fixing temperature T0. The fixing temperature setting unit C7b sets the fixing temperature to the isolated coloring when the sheet S is plain paper and the isolated coloring halftone dots are distributed in a band shape along the conveying direction of the sheet S. The halftone dot fixing temperature Ta is set, and when the sheet S is the special paper other than plain paper, the fixing temperature is set to the special paper fixing temperature Tb.
The fixing power source control means C5d controls on / off of the heater of the heating roll Fh based on the fixing temperatures T0, Ta, Tb set by the fixing temperature setting means C7b.

(Description of Flowchart of Fixing Condition Setting Process of Embodiment 1)
Next, a processing flow of the image forming apparatus U according to the first exemplary embodiment of the present invention will be described with reference to a flowchart.
FIG. 5 is a flowchart of fixing condition setting processing of the image forming apparatus according to the first exemplary embodiment of the present invention.
The processing of each ST (step) in the flowchart of FIG. 5 is performed according to a program stored in the ROM or the like of the image forming apparatus U. This process is executed in a multitasking manner in parallel with other various processes of the image forming apparatus U.

The flowchart shown in FIG. 5 is started when the image forming apparatus U is turned on.
In ST1 of FIG. 5, it is determined whether or not the job has been started. If yes (Y), the process proceeds to ST2, and if no (N), ST1 is repeated.
In ST2, it is determined whether or not the sheet S is plain paper. If yes (Y), the process proceeds to ST3, and, if no (N), the process proceeds to ST7.
In ST3, it is determined whether or not the image data of the first page is expanded and temporarily stored in the page memory. If yes (Y), the process proceeds to ST4, and, if no (N), ST3 is repeated.
In ST4, it is determined whether or not the isolated colored halftone dots are distributed in a band shape along the conveyance direction of the sheet S. If no (N), the process moves to ST5, and if yes (Y), the process moves to ST6.

In ST5, the fixing temperature is set to the fixing temperature T0 for plain paper. Then, the process proceeds to ST8.
In ST6, the fixing temperature is set to the isolated coloring halftone fixing temperature Ta. Then, the process proceeds to ST8.
In ST7, the fixing temperature is set to the special paper fixing temperature Tb. Then, the process proceeds to ST8.
In ST8, it is determined whether or not the job is finished. If no (N), ST8 is repeated, and if yes (Y), the process returns to ST1.

(Operation of Example 1)
In the image forming apparatus U according to the first embodiment having the above-described configuration, when the job is executed, the sheet S is plain paper and the isolated colored halftone dots are distributed in a band shape along the conveyance direction of the sheet S ( 3 and 4A), the fixing temperature is set to the isolated coloring dot fixing temperature Ta higher than the plain paper fixing temperature T0 (see ST6 in FIG. 5). With the heating roller Fh set to the point fixing temperature Ta, the toner of the isolated colored halftone dot can be reliably heated and fixed. Further, when the sheet S is the special paper (OHP, colored paper, coated paper, thick paper), the fixing temperature Tb for the special paper whose fixing temperature is set higher than the fixing temperature T0 for the plain paper. (See ST7 in FIG. 5), the toner image Tn can be reliably fixed by the heating roller Fh set at the special paper fixing temperature Tb.
As a result, in the image forming apparatus U according to the first embodiment, when there is no isolated colored halftone dot, the image can be fixed as usual, and an image in which the isolated colored halftone dot is arranged in a belt shape can be fixed reliably. It is possible to reduce the possibility of occurrence. In addition, the heat fixing process can be surely executed for the special paper that is harder to fix the toner image Tn than the plain paper.

  Next, the image forming apparatus of the second embodiment will be described. In the description of the second embodiment, the same reference numerals are given to the components corresponding to the components of the first embodiment, and the detailed description thereof will be given. Omitted. The second embodiment is different from the first embodiment in the following points, but is configured in the same manner as the first embodiment in other points.

(Description of Control Unit of Example 2)
FIG. 6 is a block diagram of the control unit of the image forming apparatus according to the second embodiment of the present invention, and corresponds to FIG. 2 according to the first embodiment.
(Function of controller C)
In FIG. 6, the controller C of the second embodiment is such that the isolated colored halftone dot distribution determining means C6d and the fixing temperature setting means C7b of the first embodiment are the same as the isolated colored halftone dot distribution determining means C6d 'and the fixing temperature setting means C7b'. Has been replaced.

C6d ': Isolated colored halftone dot distribution determining means (colored fine pixel distribution determining means)
The isolated colored halftone dot distribution discriminating means C6d ′ determines whether or not the isolated colored halftone dots are distributed to the extent that the fixing condition needs to be changed based on the analysis result of the writing image information analyzing means C6c. Determine. The isolated colored halftone dot distribution discriminating means C6d 'according to the second embodiment distributes the isolated colored halftone dots based on the coordinates of the isolated colored halftone dots detected by the writing image information analyzing means C6c. Is larger than a preset fixing condition change determination area. For example, when the isolated colored halftone dot is concentrated and distributed in the center of the sheet S, or when the isolated colored halftone dot is distributed in a strip shape along the axial direction of the heating roll Fh, When the isolated colored halftone dots are distributed in a strip shape along a direction oblique to the conveying direction of the sheet S, the range in which the isolated colored halftone dots are distributed is wider than the fixing condition change determination area. Determined.

C7b ': Fixing temperature setting means The fixing temperature setting means C7b' determines the fixing temperature when the toner image Tn on the sheet S conveyed to the fixing area Q5 is heated and fixed based on the fixing condition change determining means C6. Set. The fixing temperature setting unit C7b ′ according to the second exemplary embodiment performs the normal job execution, that is, when the sheet S is plain paper and the distribution range of the isolated colored halftone dots is equal to or less than the fixing condition change determination area. Sets the fixing temperature T0 for plain paper. Further, the fixing temperature setting means C7b ′ is configured to fix the isolated coloring halftone fixing temperature when the sheet S is plain paper and the distribution range of the isolated coloring halftone dots is wider than the fixing condition change determination area. Ta is set, and when the sheet S is the special paper other than plain paper, the special paper fixing temperature Tb is set.

(Explanation of Flowchart of Fixing Condition Setting Process of Embodiment 2)
FIG. 7 is a flowchart of the fixing condition setting process of the image forming apparatus according to the second embodiment of the present invention, and corresponds to FIG. 5 according to the first embodiment.
In FIG. 7, in the flowchart of the fixing condition setting process of the image forming apparatus U according to the second embodiment, only ST4 ′ is executed instead of ST4 with respect to the flowchart of the fixing condition setting process according to the first embodiment in FIG. 5. Since other processes ST1 to ST3 and ST5 to ST8 are the same as the corresponding processes in FIG. 5, detailed descriptions of the other processes are omitted.
In ST4 ′ of FIG. 7, it is determined whether or not the range in which the isolated colored halftone dots are distributed is wider than the fixing condition change determination area. If no (N), the process moves to ST5, and if yes (Y), the process moves to ST6.

(Operation of Example 2)
In the image forming apparatus U of Example 2 having the above-described configuration, when the sheet S is plain paper and the range where the isolated colored halftone dots are distributed is wider than the fixing condition change determination area (ST4 ′ in FIG. 6). The fixing temperature is set to the fixing temperature Ta for the isolated colored halftone dot (see ST5 in FIG. 6), and the isolated fixing halftone dot is set by the heating roller Fh set to the fixing temperature Ta for the isolated colored halftone dot. The toner can be reliably heated and fixed.
Therefore, in the image forming apparatus U according to the second embodiment, the isolated colored halftone dots are not only partially distributed along the sheet S in the transport direction but also partially isolated on the image. Even in the case of being used, the heat fixing process can be executed in a state where the fixing temperature is set to the isolated coloring dot fixing temperature Ta. In addition, the image forming apparatus U according to the second embodiment has the same effects as the image forming apparatus U according to the first embodiment.

  Next, the image forming apparatus of the third embodiment will be described. In the description of the third embodiment, the same reference numerals are given to the components corresponding to the components of the first embodiment, and the detailed description thereof will be given. Omitted. The third embodiment is different from the first embodiment in the following points, but is configured in the same manner as the first embodiment in other points.

(Description of Control Unit of Example 3)
FIG. 8 is a block diagram of the control unit of the image forming apparatus according to the third embodiment of the present invention, and corresponds to FIG. 2 according to the first embodiment.
(Function of controller C)
In FIG. 8, a paper smoothness storage unit C3a is newly added to the paper type storage unit C3 of the controller C of the third embodiment. In the fixing condition change determining means C6, the isolated colored halftone dot distribution determining means C6d is replaced with an isolated colored halftone dot distribution determining means C6d ″, and a sheet smoothness determining means C6e is newly added. Further, in the fixing condition setting means C7, the fixing temperature storage means C7a and the fixing temperature setting means C7b are replaced with the fixing temperature storage means C7a ′ and the fixing temperature setting means C7b ″.

C3a: Paper smoothness storage means The paper smoothness storage means C3a stores the smoothness of the sheet S. The paper smoothness storage unit C3a according to the third exemplary embodiment stores in the paper type storage unit C3 that the smoothness of the sheet S is “normal”, “slightly high”, or “high”. Each sheet S is stored in association with each type (plain paper, OHP, colored paper, coated paper, thick paper). In the third embodiment, the smoothness of the sheet S can be set by a user input on the user interface UI.
C6d ″: isolated colored halftone dot distribution determining means (colored fine pixel distribution determining means)
The isolated colored halftone dot distribution range discriminating means C6d ″ discriminates the range in which the isolated colored halftone dots are distributed based on the analysis result of the writing image information analyzing means C6c. The distribution discriminating means C6d ″ has a distribution range of the isolated colored halftone dots wider than the fixing condition change discriminating area based on the coordinates of the isolated colored halftone dots detected by the writing image information analyzing means C6c. In this case, it is determined to be “wide”, “narrow” if it is smaller than the fixing condition change determination area, or “none” if the isolated colored halftone dot does not exist.

C6e: Paper smoothness determining means The paper smoothness determining means C6e determines the smoothness of the sheet S. In the sheet smoothness storage unit C6e according to the third exemplary embodiment, the smoothness of the sheet S associated with the type of the sheet S determined by the sheet type determination unit C6a is “normal”, “slightly high”, It is determined that it is one of “high”.
C7a ′: Fixing Temperature Storage Unit The fixing temperature storage unit C7a ′ stores the fixing temperature in the heating roll Fh. The fixing temperature storage means C7a ′ of the third embodiment is also used when the fixing temperature T0 is set higher than the fixing temperature T0 for plain paper and higher than the fixing temperature T0 for plain paper, and the smoothness of the sheet S is different. A first fixing temperature T1 for smoothness capable of heat-fixing the toner image Tn, a second fixing temperature T2 for smoothness, a third fixing temperature T3 for smoothness, a fourth fixing temperature T4 for smoothness, and plain paper The fixing temperature Tb for special paper that can reliably heat and fix the toner image Tn on special paper (OHP, colored paper, coated paper, thick paper) other than the above is stored. In the third embodiment, the fixing temperature T0 for plain paper, the first fixing temperature T1 for smoothness, the second fixing temperature T2 for smoothness, the third fixing temperature T3 for smoothness, the first fixing temperature T3 for smoothness. 4. The fixing temperature T4 and the special paper fixing temperature Tb are set so as to increase in this order (T0 <T1 <T2 <T3 <T4 <Tb).

FIG. 9 is an explanatory diagram of a fixing temperature setting table according to the third embodiment of the present invention.
C7b ″: Fixing temperature setting means The fixing temperature setting means C7b ″ determines the fixing temperature when the toner image Tn on the sheet S conveyed to the fixing area Q5 is heated and fixed based on the fixing condition change determining means C6. Set. In the fixing temperature setting unit C7b ″ according to the third exemplary embodiment, the type of the sheet S determined by the sheet type determining unit C6a and the isolated colored halftone dot determined by the isolated colored halftone distribution determining unit C6d ″. The distribution range (“wide”, “narrow”, “none”) and the smoothness (“high”, “slightly high”, “normal”) of the sheet S determined by the paper smoothness determining means C6e Based on the above, appropriate fixing temperatures T0 to T4 and Tb are set.

  Specifically, when the fixing temperature setting unit C7b ″ of Example 3 determines that the sheet S is a special paper (OHP, colored paper, coated paper, thick paper) other than plain paper, When the paper fixing temperature Tb is set and it is determined that the sheet S is plain paper, the smoothness correspondence is set by setting any of the fixing temperatures T0 to T4 according to the fixing temperature setting table shown in FIG. In other words, when the distribution range of the isolated colored halftone dots is “wide” and the smoothness of the sheet S is “high”, the smoothness-corresponding fourth fixing temperature T4 is set. If it is “slightly high”, the smoothness-corresponding third fixing temperature T3 is set. If it is “normal”, the smoothness-corresponding second fixing temperature T2 is set. When the distribution range of the isolated colored halftone dots is “narrow”, the smoothness-corresponding third fixing temperature T3 is set if the smoothness of the sheet S is “high”, and if “slightly high”. The smoothness-corresponding second fixing temperature T2 is set. If “normal”, the fixing temperature T0 for plain paper is set. When the distribution range of the isolated coloring halftone dots is “None”, if the smoothness of the sheet S is “High”, the second fixing temperature T2 for smoothness is set, and “Slightly high” is set. For example, the first fixing temperature T1 corresponding to the smoothness is set. If “normal”, the fixing temperature T0 for plain paper is set.

(Description of Flowchart of Fixing Condition Setting Process of Embodiment 3)
FIG. 10 is a flowchart of the fixing condition setting process of the image forming apparatus according to the third embodiment of the invention, and corresponds to FIG. 5 according to the first embodiment.
In FIG. 10, in the flowchart of the fixing condition setting process of the image forming apparatus U of the third embodiment, ST9 is simply executed instead of ST4 to ST6 with respect to the flowchart of the fixing condition setting process of the first embodiment in FIG. 5. Since other processes ST1 to ST3, ST7, and ST8 are the same as the corresponding processes in FIG. 5, detailed descriptions of the other processes are omitted.
In ST9 of FIG. 10, the smoothness (“high”, “slightly high”, “normal”) of the sheet S and the range in which the isolated colored halftone dots are distributed on the image (“wide”, “narrow”, “none”) ”), And based on the determination result and the fixing temperature setting table (see FIG. 9), the smoothness-corresponding fixing temperature setting process for setting appropriate fixing temperatures T0 to T4 is executed. Then, the process proceeds to ST8.

(Operation of Example 3)
In Example 3, when the smoothness of the sheet S to be fed is high, for example, when paper fibers are clogged or when processing to reduce the roughness of the paper surface is performed, the toner is The toner image Tn is difficult to be fixed because it is difficult to penetrate the paper. Accordingly, the image forming apparatus U according to the third embodiment having the above configuration is set to a temperature higher than the plain paper fixing temperature T0 when the smoothness of the sheet S is determined to be higher than “normal”. Since the fixing temperature corresponding to the smoothness T1 to T4 is set, the toner image Tn can be reliably heated and fixed. In the fixing temperature setting process for smoothness according to the third embodiment, the smoothness of the sheet S is divided into three levels (“high”, “slightly high”, “normal”), and the isolated colored halftone dots on the image. The fixing temperatures T0 to T4 divided into five stages can be set according to the three stages (“wide”, “narrow”, and “none”) of the distribution range (see ST9 in FIG. 10).
As a result, the image forming apparatus U of Example 3 is set to an appropriate fixing temperature even when the smoothness of the sheet S is high and the toner is difficult to fix and the isolated colored halftone dot is present on the image. In this state, the heat fixing process can be executed, and the offset contamination can be reduced. In addition, the image forming apparatus U according to the third embodiment has the same effects as the image forming apparatus U according to the first and second embodiments.

  Next, the image forming apparatus according to the fourth embodiment will be described. In the description of the fourth embodiment, the same reference numerals are given to the components corresponding to the components of the first and second embodiments, and the details thereof will be described. Description is omitted. The fourth embodiment is different from the first and second embodiments in the following points, but is configured in the same manner as the first and second embodiments in other points.

(Description of fixing device F)
FIG. 11 is an enlarged perspective view of the fixing device shown in FIG.
12 is a detailed explanatory view of the fixing device, and is a cross-sectional view taken along line XII-XII in FIG.
FIG. 13 is a diagram showing the relationship of main components with the case of the fixing device omitted.
11 to 13, the fixing device F includes the heating roll Fh and the pressure roll Fp, and bearings are provided at both front and rear ends (X-axis direction both ends) of the rolls Fh and Fp. Fhb, Fhb and Fpb, Fpb are attached.
11 to 13, the fixing device F includes vertical substrates 2F and 2R provided at both front and rear ends of a horizontal base 1, and a lower guide substrate (pressure rotating member) supported by the vertical substrates 2F and 2R. Guide member) 3F, 3R, upper guide substrate (heating rotation member guide member) 4F, 4R, and left end (-Y end) are rotation shafts 5Fa (see FIG. 11), 5Ra (see FIG. 12). Elevating levers 5F and 5R (see FIG. 11) supported so as to be rotatable around.

Guide grooves 6 and 6 for guiding the bearings Fpb and Fpb mounted on both ends of the pressure roll Fp in the vertical direction are formed in the lower guide substrates 3F and 3R. Guide grooves 7 and 7 for guiding the bearings Fhb and Fhb mounted on both ends of the heating roll Fh in the vertical direction are formed in the upper guide substrates 4F and 4R.
The pair of front and rear guide grooves 7 and 7 are narrowly formed at the lower ends thereof to restrict the downward movement of the bearings Fhb and Fhb. The guide grooves 7 and 7 support the bearings Fhb and Fhb so as to be movable within a range of a height difference of about 2 mm in the vertical direction.

The pair of front and rear guide grooves 6 and 6 are open at the top, and the lower ends of the bearings Fpb and Fpb guided by the guide grooves 6 and 6 are supported by the elevating levers 5F and 5R. Cam followers 8, 8 are rotatably supported at the free ends of the lift levers 5F, 5R, and the cam followers 8, 8 are supported by the lift lever rotating cam 9, which is rotatably supported by the vertical substrates 2F, 2R. The position in the vertical direction can be adjusted by rotating 9. The elevating lever rotating cams 9, 9 are integrally connected by a shaft (not shown) extending in the front-rear direction, for fixing nip pressure control for elevating the pressure roll disposed on the rear side of the vertical substrate 2R. The motor M2 (see FIG. 14) rotates at the same time.
When the lift lever rotating cams 9 and 9 are in the state shown in FIGS. 11 to 13, the cam followers 8 and 8 and the lift levers 5F and 5R are moved to the raised positions. In this state, the heating roll Fh has a low pressure contact force and has risen to the upper end position of the guide grooves 7, 7, and the pressure roll Fp has also risen to a position in pressure contact with the heating roll Fh. Yes. When rotating clockwise from the rotational position shown in FIGS. 11 to 13 (the rotational position where the fixing nip pressure (fixing pressure) becomes weak), the fixing nip pressure becomes medium and strong.

An outer wall 11 and an inner wall 12 are supported by the upper guide boards 4F and 4R, and a suction duct 13 is formed between the outer wall 11 and the inner wall 12. The suction duct 3 is sucked by a fan (not shown) provided on the rear side (−X side) of the fixing device F, and discharges high-temperature air around the fixing device.
A cylindrical oil application member 14 for applying sheet peeling oil to the surface of the heating roll Fh is supported inside the inner wall 12, and an oil wiping member 15 is used to wipe off the oil on the surface of the heating roll Fh. Is supported. The oil wiping member 15 presses the wiping sheet 15a to the heating roll Fh, the wiping sheet 15a for wiping the oil, the feeding reel 15b around which the wiping sheet 15a is wound, the winding reel 15c for winding the wiping sheet 15a. A pressure contact roll 15d.

  Sheet guides 17a and 17b are arranged on the upstream side in the sheet conveying direction of the pressure contact area Q5 of the heating roll Fh and the pressure roll Fp. On the downstream side of the fixing area Q5, sheet peeling claws 18a and 18b for peeling the sheet S from the surfaces of the heating roll Fh and the pressure roll Fp are arranged. Guide rollers 19a and 19b are arranged on the downstream side of the sheet peeling claws 18a and 18b in the sheet conveying direction with a space therebetween in the vertical direction and are rotatably supported. The guide rollers 19a and 19b are freely rotatable rolls and have a function of guiding the sheet S that has passed through the fixing region Q5 to the downstream side in the transport direction, but do not have a transport function. Further, even if the image forming surface of the high-temperature recorded sheet S that has passed through the fixing region Q5 comes into contact with the guide roller 19a, the image forming surface is not rubbed with a large force. It does not occur. Then, the sheet S conveyed downstream through the guide rollers 19a and 19b is conveyed to either the sheet discharge path SH4 or the sheet reverse connection path SH5 by the switching gate GT1.

(Description of Control Unit of Example 4)
FIG. 14 is a block diagram of the control unit of the image forming apparatus according to the fourth embodiment of the present invention, and corresponds to FIG. 6 according to the second embodiment.
(Controlled element connected to controller C)
In FIG. 14, in the controller C of the fourth embodiment, a process of outputting a control signal to the next controlled element is added.
D2: Fixing nip pressure control motor drive circuit The fixing nip pressure control motor drive circuit D2 drives the fixing nip pressure control motor M2 to rotationally drive the lift lever rotating cams 9, 9.

(Function of controller C)
In FIG. 14, the fixing condition setting means C7 of the controller C according to the fourth embodiment is configured such that the fixing temperature and the fixing nip pressure can be set as the fixing conditions. The fixing nip pressure storage means C7c and the fixing nip pressure setting are set. Means C7d. Further, a fixing nip pressure control means C8 is newly added to the controller C of the fourth embodiment.
C7c: Fixing nip pressure storage means The fixing nip pressure storage means C7c stores the fixing nip pressure in the fixing region Q5. The fixing nip pressure set value storage means C7c of the fourth embodiment is set to a plain paper fixing nip pressure P0 which is a normal fixing nip pressure and a pressure contact force stronger than the plain paper fixing nip pressure P0. The isolated colored halftone dot fixing nip pressure Pa that can reliably press and fix the toner of the isolated colored halftone dot, and the toner image Tn on special paper (OHP, colored paper, coated paper, thick paper) other than plain paper The fixing nip pressure Pb for special paper that can be fixed by pressure is stored.

C7d: Fixing nip pressure setting means The fixing nip pressure setting means C7d is a fixing nip for pressure-fixing the toner image Tn on the sheet S conveyed to the fixing area Q5 based on the fixing condition change determining means C6. Set the pressure. The fixing temperature setting unit C7d according to the fourth exemplary embodiment performs normal job execution, that is, when the sheet S is plain paper and the range where the isolated colored halftone dots are distributed is equal to or less than the fixing condition change determination area. The fixing nip pressure is set to the plain paper fixing nip pressure P0. The fixing nip pressure setting means C7d sets the fixing nip pressure when the sheet S is plain paper and the distribution range of the isolated colored halftone dots is wider than the fixing condition change determination area. The point fixing nip pressure Pa is set, and when the sheet S is the special paper other than plain paper, the fixing nip pressure is set to the special paper fixing nip pressure Pb.

C8: Fixing nip pressure control means The fixing nip pressure control means C8 controls the driving of the fixing nip pressure control motor M2, and is a fixing force that is a pressure contact force when the pressure roll Fp presses the heating roll Fh. Control nip pressure. The fixing nip pressure control means C8 according to the fourth exemplary embodiment rotates the lift lever rotating cams 9 and 9 by driving the fixing nip pressure control motor M2 before the sheet S enters the fixing region Q5. By controlling the position and raising / lowering the elevating levers 5F, 5R and the pressure roll Fp, control is performed so that any of the fixing nip pressures P0, Pa, Pb set by the fixing nip pressure setting means C7d is obtained. To do.

(Description of Flowchart of Fixing Condition Setting Process of Embodiment 4)
FIG. 15 is a flowchart of the fixing condition setting process of the image forming apparatus according to the fourth embodiment of the invention, and corresponds to FIG. 7 according to the second embodiment.
In FIG. 15, in the flowchart of the fixing condition setting process of the image forming apparatus U according to the fourth embodiment, ST5 ′ to ST7 ′ are replaced with ST5 ′ to ST7 ′ instead of the flowchart of the fixing condition setting process according to the second embodiment of FIG. The other processes ST1 to ST3, ST4 ', and ST8 are the same as the corresponding processes in FIG. 7, and detailed descriptions of the other processes are omitted.

In ST5 ′ of FIG. 15, the following (1) and (2) are executed, and the process proceeds to ST8.
(1) The fixing temperature is set to the plain paper fixing temperature T0.
(2) The fixing nip pressure is set to the fixing nip pressure P0 for plain paper.
In ST6 ′, the following (1) and (2) are executed, and the process proceeds to ST8.
(1) The fixing temperature is set to the isolated coloring halftone fixing temperature Ta.
(2) The fixing nip pressure is set to the isolated colored halftone fixing nip pressure Pa.
In ST7 ', the following (1) and (2) are executed, and the process proceeds to ST8.
(1) The fixing temperature is set to the special paper fixing temperature Tb.
(2) The fixing nip pressure is set to the special paper fixing nip pressure Pb.

(Operation of Example 4)
In the image forming apparatus U of Example 4 having the above-described configuration, when the sheet S is plain paper and the range where the isolated colored halftone dots are distributed is wider than the fixing condition change determination area (ST4 ′ in FIG. 15). The fixing temperature is set to the isolated colored halftone fixing temperature Ta, and the fixing nip pressure is set to the isolated fixing halftone fixing nip pressure Pa having a pressing force stronger than the plain paper fixing nip pressure P0. As a result (see ST6 'in FIG. 15), the toner of the isolated colored halftone dot can be reliably fixed, and the offset contamination can be reduced. When the sheet S is the special paper (OHP, colored paper, coated paper, thick paper), the fixing temperature is set to the special paper fixing temperature Tb, and the fixing nip pressure is set to the plain paper. Since the special paper fixing nip pressure Pb is set to a pressure greater than the fixing nip pressure P0 (see ST7 'in FIG. 15), the toner image Tn can be reliably fixed.
Therefore, when it is determined that the toner on the sheet S is difficult to fix, the image forming apparatus U of Embodiment 4 can not only set the fixing temperature to a high temperature but also set the fixing nip pressure to a strong pressing force. As a result, since both the fixing temperature and the fixing nip pressure can be optimized, the fixing process can be executed more reliably. In addition, the image forming apparatus U according to the fourth embodiment has the same effects as the image forming apparatus U according to the first and second embodiments.

(Example of change)
As mentioned above, although the Example of this invention was explained in full detail, this invention is not limited to the said Example, A various change is performed within the range of the summary of this invention described in the claim. It is possible. Modification examples (H01) to (H011) of the present invention are exemplified below.
(H01) The present invention is not limited to the image forming apparatus U as a complex machine in which a copying machine and a printer are integrated. For example, an image forming apparatus to which a function such as FAX is added, a copying machine, a printer, The present invention can also be applied to an image forming apparatus that is not a multifunction machine such as FAX. Further, the present invention is not limited to a monochrome image forming apparatus, and can be applied to a color image forming apparatus.
(H02) In the above-described embodiment, the image forming apparatus U exemplifies one that takes out the sheets S, which are cut sheets, one by one. However, the present invention is not limited to this, for example, continuous paper with feed holes, pinless continuous paper, etc. It is also possible to apply to an image forming apparatus using

(H03) In the above embodiment, the image forming apparatus U that does not have a function as a network printer is illustrated. However, a network to which a host computer is connected and receives image data, printer control command data, etc. (received data) The present invention can also be applied to an image forming apparatus as a printer. In this case, the writing image information analyzing means C6c of the above embodiment analyzes the bitmap (raster image) expanded and temporarily stored in the page memory, but the vector data (vector) included in the received data is analyzed. Image) can also be analyzed. In addition, when the received data includes data designating the type, smoothness, etc. of the sheet S, it is possible to omit a configuration in which each condition can be set by user input in the user interface UI.

(H04) In the embodiment, the fixing conditions are set according to the conditions of the type of the sheet S, the smoothness, and the distribution range of the isolated colored halftone dots. However, the conditions for changing the fixing conditions are limited to this. For example, the sheet size of the sheet S can be added to the condition. The fixing conditions are not limited to the fixing temperature and the fixing nip pressure. For example, the fixing condition is set to a speed at which the sheet S passes through the fixing region Q5 (medium transport speed), and the medium transport speed is set to a lower speed. It is also possible to adopt a configuration that facilitates fixing.
(H05) In the embodiment, when the type of the sheet S is special paper other than plain paper (OHP, colored paper, coated paper, thick paper), the fixing temperature is uniformly set to the special paper fixing temperature Tb. However, the present invention is not limited to this. For example, when the isolated colored halftone dot exists on the image, the fixing temperature is higher than the fixing temperature Tb for the special paper according to the range in which the isolated colored halftone dot is distributed. It is also possible to set to. It is also possible to set the fixing temperature for each type of the sheet S (OHP fixing temperature, colored paper fixing temperature, coated paper fixing temperature, thick paper fixing temperature, etc.).

(H06) In the first, second, and fourth embodiments, when the isolated colored halftone dots are distributed in a band shape along the conveyance direction of the sheet S, or the range where the isolated colored halftone dots are distributed is the fixing condition change. If the area is larger than the discrimination area, the fixing temperature is set to the fixing temperature Ta for the isolated colored halftone dot. If it is determined that at least one isolated colored halftone dot exists, the fixing temperature is set for the isolated colored halftone dot. It is also possible to set the fixing temperature Ta. Furthermore, in the above embodiment, the fixing conditions are changed by the isolated colored halftone dots, but the present invention is not limited to this. For example, even when a plurality of adjacent halftone dots (fine pixels) are colored, the resolution is low, When the total number of toners to be colored is small, it is possible to suitably change the fixing conditions by treating them in the same manner as the isolated colored halftone dots in the above-described embodiment.
(H07) In the third embodiment, the fixing temperature setting table shown in FIG. 9 indicates any one of the fixing temperatures T0 to T4 according to the range in which the isolated colored halftone dots are distributed and the smoothness of the sheet S. Although it is set, the present invention is not limited to this, and it is possible to set a more complicated or simple fixing temperature by combining addition / deletion of conditions, changing each set value, and the like. Further, not only the fixing temperature setting table but also a fixing nip pressure setting table capable of setting the fixing nip pressure, or a fixing condition setting table integrated with these can be replaced.

(H08) In the third embodiment, the fixing temperature setting table shown in FIG. 9 is applied only when the type of the sheet S is plain paper, but is not limited thereto, and is also applied to special paper. It is possible.
(H09) In the fourth embodiment, the fixing condition setting unit C7 sets the fixing temperature and the fixing nip pressure, but is not limited thereto, and omits the fixing temperature storage unit Ca and the fixing temperature setting unit Cb. It is also possible to change only the fixing nip pressure without changing the setting of the fixing temperature.
(H010) In the fourth embodiment, the fixing nip pressure is adjusted by adjusting the amount of rotation of the lifting lever rotating cams 9 and 9. However, the present invention is not limited to this. It is also possible to adjust the expansion / contraction state of the possible pressure contact spring.
(H011) In the above embodiment, the writing image information analyzing means C6c of the fixing condition change determining means C6 analyzes one page of the image data first developed and temporarily stored in a page memory. However, the present invention is not limited to this. For example, the image data can be analyzed for each page to determine whether or not the fixing condition needs to be changed. In this case, it is possible to set fixing conditions for each page.

FIG. 1 is an overall explanatory view of an image forming apparatus according to Embodiment 1 of the present invention and is a sectional view. FIG. 2 is a block diagram of the control unit of the image forming apparatus according to the first exemplary embodiment of the present invention. FIG. 3 is an explanatory diagram of form image data which is an example of image data according to the first embodiment of the present invention. 4 is an explanatory diagram of a gradation image portion according to the first embodiment of the present invention, FIG. 4A is an explanatory diagram of an arrangement of halftone dots in which the gradation image portion of FIG. 3 is enlarged, and FIG. 4B is a floor of FIG. 4A. FIG. 4C is an explanatory diagram in which one pixel in the toned image portion is enlarged and displayed, and FIG. 4C is an explanatory diagram in which a colored dot of one dot in one pixel in FIG. 4B is enlarged and displayed. FIG. 5 is a flowchart of fixing condition setting processing of the image forming apparatus according to the first exemplary embodiment of the present invention. FIG. 6 is a block diagram of the control unit of the image forming apparatus according to the second embodiment of the present invention, and corresponds to FIG. 2 according to the first embodiment. FIG. 7 is a flowchart of the fixing condition setting process of the image forming apparatus according to the second embodiment of the present invention, and corresponds to FIG. 5 according to the first embodiment. FIG. 8 is a block diagram of the control unit of the image forming apparatus according to the third embodiment of the present invention, and corresponds to FIG. 2 according to the first embodiment. FIG. 9 is an explanatory diagram of the fixing temperature setting table for plain paper according to the third embodiment of the present invention. FIG. 10 is a flowchart of the fixing condition setting process of the image forming apparatus according to the third embodiment of the invention, and corresponds to FIG. 5 according to the first embodiment. FIG. 11 is an enlarged perspective view of the fixing device shown in FIG. 12 is a detailed explanatory view of the fixing device, and is a cross-sectional view taken along line XII-XII in FIG. FIG. 13 is a diagram showing the relationship of main components with the case of the fixing device omitted. FIG. 14 is a block diagram of the control unit of the image forming apparatus according to the fourth embodiment of the present invention, and corresponds to FIG. 6 according to the second embodiment. FIG. 15 is a flowchart of the fixing condition setting process of the image forming apparatus according to the fourth embodiment of the invention, and corresponds to FIG. 7 according to the second embodiment. FIG. 16 is an explanatory diagram of form image data which is an example of image data. FIG. 17 is an explanatory diagram of halftone dots that are isolated and colored, FIG. 17A is an explanatory diagram of an arrangement example when there are halftone dots that are isolated and colored in the gradation image portion, and FIG. It is explanatory drawing which expanded and displayed the halftone dot of 17A.

Explanation of symbols

C6d, C6d ′, C6d ″... Colored fine pixel distribution discrimination means,
C7: fixing condition setting means,
d1 ... image,
d2a ... pixel,
d3: Fine pixel,
d3a: colored fine pixels,
F: Fixing device,
G: Toner image forming apparatus,
P0, Pa, Pb: fixing pressure,
Pa: Fixing pressure higher than that in the case where colored fine pixels are not dispersed,
Q5 ... Fixing area,
S ... medium,
T0 to T4, Ta, Tb: fixing temperature,
T1 to T4, Ta: a fixing temperature higher than that in the case where colored fine pixels are not dispersed,
Tn: Toner image,
TR ... Transfer,
U: Image forming apparatus.

Claims (8)

A toner image in which an image is divided into pixels with a small area, the pixel is further divided into fine pixels with a small area, and gradation is displayed according to the ratio of the colored area of the colored fine pixels to all the fine pixels in the pixel. A toner image forming apparatus for forming
A transfer device for transferring the toner image formed by the toner image forming apparatus to a medium;
A fixing device for fixing an unfixed toner image on the medium;
Colored fine pixel distribution determining means for determining whether or not the colored fine pixels are dispersedly arranged;
When the colored fine pixel distribution determining means determines that the colored fine pixels are dispersedly arranged, the fixing that is set in advance corresponding to the case where the colored fine pixels are dispersedly arranged Fixing condition setting means for setting the fixing condition in the apparatus;
An image forming apparatus comprising: The colored fine pixel distribution discriminating means for discriminating whether or not the colored fine pixels are arranged in a dispersed manner by discriminating whether or not the colored fine pixels are arranged in isolation.
The image forming apparatus according to claim 1, further comprising: The colored fine pixel distribution discriminating means for discriminating whether or not the colored fine pixels arranged in a dispersed manner are arranged in a strip shape in the medium conveying direction;
The image forming apparatus according to claim 1, further comprising: The colored fine pixel distribution discriminating means for discriminating whether or not the colored fine pixels arranged in a distributed manner are partially arranged with respect to the entire image region;
The image forming apparatus according to claim 1, further comprising: The fixing temperature in the fixing region where the toner image is fixed, and the fixing fine temperature as the fixing condition is not dispersed when the colored fine pixels are dispersed. The fixing condition setting means for setting the fixing temperature at a temperature higher than the case;
The image forming apparatus according to claim 1, further comprising: The fixing fine pressure applied to the medium in the fixing region where the toner image is fixed, and the fixing fine pressure as the fixing condition is set when the colored fine pixels are arranged in a dispersed manner. The fixing condition setting means for setting the fixing pressure to be higher than that when the pixels are not dispersed;
The image forming apparatus according to claim 1, further comprising: The medium conveyance speed, which is the speed at which the medium is conveyed through the fixing area where the toner image is fixed, and the medium fine speed as the fixing condition is arranged in a dispersed manner. In addition, the fixing condition setting means for setting the medium conveyance speed at a lower speed than when the colored fine pixels are not dispersed,
The image forming apparatus according to claim 1, further comprising: The fixing condition setting means for setting a fixing condition in the fixing device based on the smoothness of the medium surface;
The image forming apparatus according to claim 5, further comprising: