JP2017198764A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can reduce a difference in density level occurring at the period of a fixing member with a simple configuration.SOLUTION: An image forming apparatus gradually changes, at the period of a fixing member, image data in the vicinity of the boundary of the period by using a level difference correction coefficient α.SELECTED DRAWING: Figure 11

Description

  The present invention relates to an image forming apparatus.

  Conventionally, a heat roller method has been used as a fixing device of an image forming apparatus, but in recent years, a belt heating type fixing device (hereinafter referred to as a belt fixing device) has been put into practical use from the viewpoint of quick start and energy saving. .

  As an example of a belt heating type fixing device, there is a configuration in which a fixing belt is conveyed in close contact with a pressure roller as a pressure rotating body via a fixing belt as a fixing member, as in Patent Documents 1 and 2. A recording material carrying a toner image is introduced into a fixing nip formed by a heating element and a pressure roller via a fixing belt, and is nipped and conveyed together with the fixing belt. In this nipping conveyance, the fixing device performs a fixing process in which a toner image is fixed on a recording material surface by heat applied from a heating element via a fixing belt and pressure applied at a fixing nip portion.

  The belt fixing device can employ a thin film belt-like member having a small heat capacity as a fixing member. Accordingly, since the heating time (wait time) until the fixing process becomes possible is shortened, the energy required for the fixing process can be greatly reduced, so that energy saving can be achieved.

  By the way, a fixing unit such as a fixing roller or a fixing belt is always fixed at a constant temperature or sufficiently by a heater or the like at the time of fixing so that the toner image formed on the surface of the recording material is sufficiently fixed on the surface of the recording material. The heating is controlled so as to obtain a control temperature capable of obtaining the characteristics.

  However, in a fixing device, particularly a fixing device having a small heat capacity of the fixing member as described above, a color difference may occur in the image after the fixing process in the period of the circumference of the fixing member from the leading edge of the recording material.

  Since the fixing belt applies heat to the toner and the recording material during the fixing process, the temperature of the fixing belt decreases as the recording material passes, so that the amount of heat applied to the toner decreases with the period of the fixing belt.

  When the amount of heat applied to the toner in the fixing nip is different, the temperature and viscosity of the toner in the fixing nip are not uniform. Specifically, the smaller the amount of heat applied to the toner, the lower the temperature of the toner in the fixing nip and the higher the viscosity of the toner. As the toner viscosity increases, the amount of toner deformation applied by the pressure between the fixing belt and the pressure roller at the fixing nip decreases, so the area of the toner covering the recording material decreases, so the image density is reduced. It will decline.

  That is, when the temperature of the fixing belt decreases in the fixing belt cycle, the image density on the recording material decreases in the fixing belt cycle, and a color difference occurs in the fixing belt cycle. In particular, a density (color) level difference due to a temperature level difference that occurs at the boundary of the period becomes conspicuous on the output sample, causing a reduction in image quality.

  As a configuration for preventing the occurrence of such a density step, Patent Document 3 relaxes the temperature step by partially heating or cooling the temperature step portion before coming into contact with the paper to be passed again. A configuration is proposed. However, in the configuration of Patent Document 3, a new member for partial heating or a cooling mechanism is required to relieve the temperature step, and the apparatus becomes large and complicated. Furthermore, since it is necessary to accurately contact or cool the partial heating member with respect to the temperature step portion, it is necessary to accurately control the attachment / detachment timing of the partial heating member and the operation timing of the cooling mechanism. .

JP-A-63-313182 Japanese Patent Laid-Open No. 2-157878 JP 2010-14759 A

  SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide an image forming apparatus that can reduce a density step generated in a cycle of a fixing member with a simple configuration.

In order to achieve the above object, an image forming apparatus according to the present invention includes:
An image forming apparatus including a fixing unit for fixing a toner image on a recording medium to the recording medium,
Control means for controlling the image forming conditions of the output image based on the input image data;
Among the images on the recording medium fixed by the fixing unit,
After the recording medium has entered the fixing member, an image of a portion corresponding to the first turn of the fixing member and an image of a portion corresponding to the second turn,
The image forming condition of the output image based on the input image data is changed three times or more.

  According to the image forming apparatus of the present invention, it is possible to reduce color unevenness due to a density step generated in a cycle of the fixing member with a simple configuration.

Block diagram showing the overall configuration of the image processing system Block diagram showing the software module 1 is an internal configuration diagram of a printer image processing unit according to an embodiment. Figure showing a part of the internal structure of the printer engine Cross section of the image forming part of the printer engine The figure which shows the density | concentration level | step difference correction table which concerns on 1st embodiment. Schematic model diagram of fixing device 1414 Diagram showing temperature-viscosity characteristics Diagram showing the relationship between the step correction coefficient α and chromaticity Diagram showing the relationship between image signal and density step Diagram showing the relationship between image signal and density step The figure showing the effect in Example 1 The figure showing the effect in Example 1 The figure which shows the density | concentration change in Example 2.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Example 1]
In the following, an embodiment in which the present invention is applied to an electrophotographic color copying machine having a plurality of drums will be described. Needless to say.

[System configuration]
FIG. 1 is a block diagram showing the overall configuration of the image processing system according to the present embodiment.

  In FIG. 1, in an image forming apparatus 100, a scanner 101 as an image input device and a printer engine 102 as an image output device are internally connected. The scanner 101 is connected to the device I / F 117 via the scanner image processing unit 118. The printer engine 102 is connected to the device I / F 117 via the printer image processing unit 119. Then, the scanner image processing unit 118 and the printer image processing unit 119 perform control for reading image data and printing output. The image forming apparatus 100 is connected to the LAN 10 or the public line 104 and performs control for inputting / outputting image information and device information via the LAN 10.

  The CPU 105 is a central processing unit for controlling the image forming apparatus 100. A RAM 106 is a system work memory for the CPU 105 to operate, and is also an image memory for temporarily storing input image data. A ROM 107 is a boot ROM that stores a system boot program. An HDD 108 is a hard disk drive, and stores system software for various processes, input image data, and the like.

  The operation unit I / F 109 is an interface to the operation unit 110 having a display screen capable of displaying image data and the like, and outputs operation screen data to the operation unit 110. The operation unit I / F 109 plays a role of transmitting information input by the operator from the operation unit 110 to the CPU 105. The network I / F 111 is realized by a LAN card or the like, for example, and is connected to the LAN 10 to input / output information to / from an external device (not shown). The modem 112 is connected to the public line 104 and inputs / outputs information to / from an external device (not shown). The above units are arranged on the system bus 113.

  The image bus I / F 114 is an interface for connecting the system bus 113 and an image bus 115 that transfers image data at high speed, and is a bus bridge that converts a data structure. The image bus 115 includes a raster image processor (RIP) unit 116, a device I / F 117, a scanner image processing unit 118, an image editing image processing unit 120, an image compression unit 103, an image expansion unit 121, and a color management module (CMM). 130 is connected.

  The RIP unit 116 expands a page description language (PDL) code into image data. The device I / F 117 connects the scanner 101 and the printer engine 102 via the scanner image processing unit 118 and the printer image processing unit 119, and performs synchronous / asynchronous conversion of image data.

  The scanner image processing unit 118 performs various processes such as correction and editing on the image data input from the scanner 101. The image editing image processing unit 120 performs various types of image processing such as rotation of image data, color processing, binary conversion, and multi-value conversion. The image compression unit 103 encodes the image data processed by the RIP unit 116, the scanner image processing unit 118, and the image editing image processing unit 120 by a predetermined compression method when the HDD 108 stores the image data once.

  The image decompression unit 121 compresses the image data compressed by the HDD 108 when necessary, by the image editing image processing unit 120 or by the printer image processing unit 119 for output by the printer engine 102. Decode and decompress the encoded data. The printer image processing unit 119 performs image processing such as γ correction and halftone processing according to the printer engine on the image data to be printed out.

  The CMM 130 is a dedicated hardware module that performs color conversion processing (also referred to as color space conversion processing) on image data based on a profile or calibration data. Here, the profile is information such as a function for converting color image data expressed in a device-dependent color space into a device-independent color space (for example, a Lab color space). The calibration data is data for correcting the color reproduction characteristics of the scanner 101 and the printer engine 102.

Software configuration
Each software module shown in FIG. 2 is stored in the HDD 108 or the like as a storage unit, and mainly operates on the CPU 105. A job control process 201 shown in FIG. 2 controls and controls each software module (not shown) and controls all jobs generated in the image forming apparatus 100 such as copying, printing, scanning, and FAX transmission / reception. The network processing 202 is a module that controls communication with the outside mainly performed through the network I / F 111, and performs communication control with each device on the LAN 10.

  The UI processing 203 mainly performs control related to the operation unit 110 and the operation unit I / F 109. The FAX process 204 controls the FAX function. The FAX processing 204 performs FAX reception / transmission via the modem 112.

  The print processing 207 controls the image editing image processing unit 120, the printer image processing unit 119, and the printer engine 102 based on an instruction from the job control processing 201, and performs a specified image printing process. The print process 207 receives image data, image information (image data size, color mode, resolution, etc.), layout information (offset, enlargement / reduction, imposition, etc.), and output paper information (size, print) from the job control process 201. Information).

  The print processing 207 controls the image compression unit 103, the image expansion unit 121, the image editing image processing unit 120, and the printer image processing unit 119 to perform appropriate image processing on the image data. The print processing 207 controls the printer engine 102 to print the designated data on the image data.

  The scan process 210 controls the scanner 101 and the scanner image processing unit 118 based on an instruction from the job control process 201 to read a document on the scanner 101.

  A scan process 210 scans a document on a document table of the scanner 101 and inputs an image as digital data. The color information of the input image is notified to the job control process 201. Further, the scan processing 210 controls the scanner image processing unit 118 to perform appropriate image processing such as image compression on the input image, and then notifies the job control processing 201 of the input image that has been subjected to image processing.

  The color conversion process 209 performs a color conversion process on the instruction image based on an instruction from the job control process 201 and notifies the job control process 201 of the image after the color conversion process.

  The RIP process 211 performs PDL interpretation (interpretation) based on an instruction from the job control process 201, and controls the RIP unit 116 to perform rendering, thereby developing the bitmap image.

[Image data processing flow]
With the configuration as described above, the present image processing system performs operations from receiving a print job from the LAN 10 to printing.

  Next, a processing flow of image data input to the printer image processing unit 119 will be described based on the above configuration. First, as described above, the PDL transmitted from the external device via the LAN 10 is received by the network I / F 111 and input to the RIP unit 116 from the image bus I / F 114. The RIP unit 116 interprets the received PDL and converts it into code data that can be processed by the RIP unit 116. The RIP unit 116 performs rendering based on the converted code data. The page data rendered by the RIP unit 116 is compressed by the subsequent image compression unit 103 and sequentially stored in the HDD 108.

  Next, the compressed data stored in the HDD 108 is read in a printing operation according to an instruction from the job control process 201, and the decompression process of the compressed data is performed in the image decompression unit 121. The image data expanded by the image expansion unit 121 is input to the printer image processing unit 119 via the device I / F 117.

  FIG. 3 shows an internal block diagram of the printer image processing unit 119 in the present embodiment. The color space conversion unit 301 converts image data from a luminance value (RGB, YUV, etc.) to a density value (CMYK, etc.), and corresponds to color components that can be printed by the subsequent printer engine 102. Convert to color space.

  The multi-valued image data converted into density values by the color processing conversion unit 301 is converted by the density step correction unit 302 into signal values obtained by correcting the density step in the same page, which is a feature of the present invention. The density step correction unit 302 has a one-dimensional table that changes the same input / output signal as the γ correction circuit 309 described below, and multiplies the table by a step correction coefficient for step correction in accordance with the position in the page. Make it work.

  In this embodiment, the density value is corrected by using an 8-bit-8-bit conversion table and a step correction coefficient α for reducing the output value in the linear table A having the same input / output shown in FIG. The in-page position information of the image data is input from the position information generation unit 303 to the density step correction unit 302, and the density step correction unit 302 changes the conversion table accordingly. The position information generation unit 303 generates correction position information by adding position information for correcting a density value, which will be described later, to the in-page image position information of the image data input from the device I / F 117, and the density step correction unit 302. Notify Details of the step correction coefficient α in accordance with the position in the page in the density step correction unit 302 will be described later.

  The image data that has been subjected to the density step correction is converted into a signal value for reproducing the density by the printer engine by a γ correction circuit 309 (hereinafter referred to as γLUT). The γLUT is a table that converts input / output signals that are made in accordance with the γ characteristics of the printer engine. In this embodiment, a table stored in advance is set and processed. May be created, set, and used.

  The image data corrected by γLUT 309 is subjected to halftone processing by a halftone processing unit 304, and converted into image data in which each color component of one pixel is expressed in binary (1 bit). The halftone processing generally includes a dither method, an error diffusion method, and the like, and either method may be used in this embodiment. The halftone process is not limited to the above method, and other methods may be used.

  The binary image data generated by the conversion processing in the halftone processing unit 304 is separated for each color component of each pixel in the image data via the inter-drum delay memory control unit 305 and temporarily stored in the page buffer memory 306. Stored in

  The data of the corresponding color component is read at the timing when the video data request signal corresponding to each color component transmitted from the printer engine 102 is input. Note that the video data request signal is VREQ_Y, VREQ_M, VREQ_C, and VREQ_K for each color component. This is because the exposure timing of each of the photosensitive drums 1401 to 1404 differs depending on the distance from the upstream side to the downstream side where the photosensitive drums 1401 to 1404 corresponding to the respective color components in the printer engine 102 are arranged. The read timing is also different.

[Printer engine operation]
Next, an operation when the color component data output from the printer image processing unit 119 is input to the printer engine 102 will be described.

  FIG. 4 shows a part of the internal configuration of the printer engine 102. The printer I / F unit 1201 receives color component data sequentially transmitted from the printer image processing unit 119. Also, the printer I / F unit 1201 receives a video data request signal VREQ_ * (* indicates Y / M / C / K) that requests data of each color component when the printer engine 102 is ready for a printing operation. Any one).

  The color component data is input to the pulse width modulation circuit 1203. Then, the pulse width modulation circuit 1203 generates a pulse signal (drive signal) for driving the * laser drive units 1212 to 1215 of the subsequent colors based on the actual color component data. Then, the pulse width modulation circuit 1203 transmits to each laser driving unit 1212 to 1215.

  Each laser driver 1212 to 1215 corresponding to each color component drives a laser exposure apparatus corresponding to each color component based on the pulse signal received from the pulse width modulation circuit 1203.

  FIG. 5 is a cross-sectional view of an image forming portion of the printer engine 102.

  Hereinafter, the yellow image forming portion will be mainly described. However, the image forming portions of magenta, cyan, and black having other color components have the same configuration. In this embodiment, the printer engine 102 is an image forming apparatus using a tandem engine of four colors YMCK, but is not limited thereto.

  The printer engine 102 includes a photosensitive drum 1401 that is an image carrier, a charging roller 1405, a Y laser exposure device 1406, a primary transfer device 1408, a secondary transfer device 1413, a fixing device 1414, and a cleaning device 1415. The Y laser exposure apparatus 1406 is driven by a Y laser drive unit 1212. The primary transfer device 1408 primarily transfers the visualized toner image onto the transfer material 1412. The secondary transfer device 1413 secondarily transfers the toner image formed on the transfer material 1412 onto a recording sheet. The fixing device 1414 fixes the toner image transferred onto the recording paper. The cleaning device 1415 removes transfer residual toner remaining on the transfer material 1412 after the secondary transfer.

  The developing device 1416 includes a developer container, and stores a developer in which toner particles (toner) and magnetic carrier particles (carrier) are mixed as a two-component developer. The A screw 1420 and the B screw 1421 respectively carry the toner particles and mix the magnetic carrier particles. The developing sleeve 1422 is disposed in the vicinity of the photosensitive drum 1401 and rotates so as to be driven by the photosensitive drum 1401 to carry a developer in which toner and a carrier are mixed. The developer carried on the developing sleeve 1422 contacts the photosensitive drum 1401 and the electrostatic latent image on the photosensitive drum 1401 is developed. The printer engine 102 includes a transport unit (not shown) that transports printing paper in addition to the configuration shown in FIG. 14, and the description thereof is omitted in this embodiment.

  In the configuration of the printer engine as described above, when printing yellow, the photosensitive drum 1401 is exposed by the Y laser exposure device 1406 driven by the Y laser driving unit 1212, and an electrostatic latent image is formed on the photosensitive drum 1401. Form. The formed electrostatic latent image is visualized as a toner image by a yellow developer carried on the developing sleeve 1422 in the developing device 1416, and the visualized toner image is transferred to the primary transfer device 1408 on the transfer material 1412. Is transcribed by.

  In this way, the magenta, cyan, and black color components are similarly developed by the developing devices 1417, 1418, and 1419, and are visualized as toner images on the photosensitive drums 1402, 1403, and 1404, respectively. The visualized toner image is sequentially transferred by the primary transfer devices 1409, 1410, and 1411 in synchronization with the toner image of the color component transferred immediately before, and the four color toner images are transferred onto the transfer material 1412. A final toner image is formed.

  The toner image formed on the transfer material 1412 is secondarily transferred to a recording sheet conveyed synchronously by the secondary transfer device 1413, and the toner image is fixed by the fixing device 1414.

[Fixing device]
Next, the fixing device of this embodiment will be described.

  FIG. 7 is a schematic configuration model diagram of the fixing device 1414.

  The heater unit 60 includes a heater 600, a heater holder 660 having a semicircular arc cross-sectional shape that supports the heater 600, an inverted U-shaped reinforcing sheet metal 670, and a fixing belt 650 made of a cylindrical heat-resistant material. The thermistor 630 for detecting the temperature of the heater 600 is fixedly supported by the heater holder 660, and the fixing belt 650 is loosely fitted to the heater holder 660.

  In the fixing belt 650, a silicone rubber layer (elastic layer) having a thickness of about 300 μm is formed by a ring coating method on a base material in which polyimide is formed in a cylindrical shape having a thickness of 60 μm. Further, a PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer) resin tube having a thickness of 20 μm is coated on the outermost surface layer. The fixing belt 650 of this embodiment has an outer diameter of 30 mm and an outer peripheral length of 94.2 mm.

  The pressure roller 70 has a multilayer structure in which a silicone rubber layer 72 having a thickness of about 3 mm and a PFA resin tube 73 having a thickness of about 40 μm are sequentially laminated on a stainless steel core 71. Both ends of the core of the pressure roller 70 are rotatably received and held between side plates (not shown) on the front side and a front side of the apparatus frame.

  The heater 600 includes a substrate 610 made of ceramic having insulating, heat resistant, and low heat capacity with a direction perpendicular to the conveying direction of the recording material P as a longitudinal direction, and a resistance heating element 620. Further, the heater 600 is fixed to the lower surface of the heater holder 660 and the fixing belt 650 and the substrate 640 are in contact with each other.

  The heater holder 660 is formed of a liquid crystal polymer resin having high heat resistance, and plays a role of holding the heater 600 and guiding the fixing belt 650. In this example, Zenite 7755 (trade name) manufactured by DuPont was used as the liquid crystal polymer. Both end portions of the heater holder 660 are urged in the axial direction of the pressure roller 70 by a pressure mechanism (not shown) at one end side with a force of 156.8 N (16 kgf) and a total pressure of 313.6 N (32 kgf). Therefore, the lower surface of the heater 600 is pressed against the elastic layer of the pressure roller 70 via the fixing belt 650, and a fixing nip portion N having a predetermined width necessary for the fixing process is formed.

  The pressure roller 70 is rotationally driven (counterclockwise) in the conveyance direction of the recording material P by pressure roller driving means (not shown). As a result, the fixing belt 650 closely slides on the surface of the heating element of the heater 600 and rotates around the film guide 660.

  The thermistor 630 has a function of detecting the temperature of the heater 600, and is connected to the CPU 105 via an A / D converter. The CPU 105 samples the output of each thermistor at a predetermined period, and reflects it in the temperature control based on the obtained temperature information. Specifically, the CPU 105 determines the temperature control content of the heater 600 based on the output of the thermistor 630, and controls the energization of the heater 600 by the heater drive circuit unit 51 which is a power supply unit.

  Grease is applied to the inner surface of the fixing belt 650 to reduce wear on the inner surface of the fixing belt 650 due to sliding with the heater 600 and the heater holder 660.

  Next, an operation in which the fixing device performs a fixing process will be described. When the color component data is input to the printer engine 102, the pressure roller 70 is rotationally driven by the CPU 105, and when the fixing belt 650 is driven to rotate, the resistance heating element 620 of the heater 600 is energized. When the temperature of the thermistor 630 reaches the set temperature of 200 ° C., the recording material P carrying the unfixed toner image is introduced into the fixing nip portion N along the entrance guide 23. In the fixing nip N, the toner image carrying surface side of the recording material P is in close contact with the outer surface of the fixing belt 650, and the recording material P moves together with the fixing belt 650.

  In the nipping and conveying process at the fixing nip portion N, the heat generated by the heater 600 is applied to the recording material P via the fixing belt 650, and the unfixed toner image is further applied by the pressure between the fixing belt 650 and the pressure roller 70. Is melt-fixed. The recording material P that has passed through the fixing nip N is separated from the fixing belt 650, passes through the conveyance path 21, and is sent out onto the paper discharge tray 23 by the paper discharge roller 22. Then, the recording paper is discharged by the printer engine 102, and the printing operation is finished.

[Setting of level difference correction coefficient α]
Next, density signal value conversion for reducing the level difference in the fixing belt cycle using the level difference correction coefficient α performed by the density level difference correction unit 302, which is a feature of this embodiment, will be described. The signal value conversion method described in this embodiment is an example, and the present invention is not limited to this.

  In the following, regarding the conversion method for correcting the color difference between the first and second laps of the fixing belt having a large change, the density of the first lap is the same as that of the second lap. Although the correction method will be described, it is also effective when the color of the second round is matched with the color of the first round. Furthermore, even when the color difference that causes a problem occurs in the third and subsequent rounds, the effect can be obtained by performing the same processing.

  As described above, in order to reduce the level difference caused by the phenomenon of different colors in the first and second laps of the fixing belt, the color of the first lap of the fixing belt is matched with the color of the second lap. In order to correct the density in this way, the CMYK signal value is output with a step correction coefficient α. Here, when the input image data is converted into a CMYK signal value, and the CMYK signal value is INPUT and the step correction coefficient is α, the corrected CMYK signal value OUTPUT for outputting an image is expressed by the following equation. Can be represented.

OUTPUT = INPUT × α
Next, calculation of the step correction coefficient α used in this embodiment will be described. The step correction coefficient α varies depending on the toner, paper type, speed, etc. used. In this embodiment, α is calculated using the following toner, but the present invention is not limited to this. The paper type is 80 g / m2 high-quality paper, P.I. S. Was 200 mm / s.

  In this embodiment, a toner having a binder of polyester having temperature-viscosity characteristics (hereinafter referred to as melt viscosity characteristics) as shown in FIG. 8 is used, but the present invention is not limited to this. Here, a flow tester CFT-500D (manufactured by Shimadzu Corporation) was used for calculation of the melt viscosity characteristics used in this example. The measurement conditions were set as follows.

(Measurement condition)
・ Test mode: Temperature rising method ・ Die hole diameter: 1 [mm]
-Die length: 1 [mm]
・ Test load: 10 [kg]
・ Cylinder pressure: 9.807E + 5 [Pa]
Temperature increase rate: 4 [° C./min. ]
・ Preheating time: 300 [s]
A method for calculating the step correction coefficient for the toner, paper type, and speed as described above will be described. First, a method of determining the step correction coefficient α ′ corresponding to density conversion that eliminates the color difference between the first and second rotations of the fixing belt necessary for calculating α will be described below.

α ′ is determined by the following procedure.
1. An image is formed on the CMYK input signal value without using the step correction coefficient α.
2. Image formation is performed on a value obtained by multiplying the CMYK input signal value of the entire page by the step correction coefficient α.
The chromaticity of the portion corresponding to the second round of the belt of the output image of 3.1 and 2. Α is calculated so that the chromaticity of the portion corresponding to the first round of the belt of the output image becomes the same, and is α ′.

  As an example, the case where the CMYK input signal value is (255, 0, 0, 0) will be described with reference to FIG. FIG. 9A is a graph in which the ab projection plane of the Lab chromaticity coordinate system is plotted. FIG. 9B is a graph plotting LC of the Lab chromaticity coordinate system. Here, C is represented by the following equation.

C = (a ^ 2 + b ^ 2) ^ 0.5
The input signal value is an example, and the correction coefficient calculation method is not limited to this. First, an image of (C, M, Y, K) = (255, 0, 0, 0) is output as a signal value. Of the chromaticity of the output image at this time, the chromaticity of the portion corresponding to the first round of the belt is indicated by Δ in FIG. 9, and the chromaticity of the portion corresponding to the second round of the belt is indicated by □ in FIG. Show.

  Next, an image obtained by converting the input signal value is output using 0.98, 0.96, and 0.94 as the step correction coefficient α. Specifically, the relationship between the used step correction coefficient α and the converted input signal value is as shown in Table 1. Note that the change range of the step correction coefficient α is not limited.

  Of the chromaticity of the image in which the input signal value is converted by applying the step correction coefficient α, the chromaticity of the portion corresponding to the first round of the belt is indicated by x mark, ◯ mark, and ◆ mark in FIG.

  From FIG. 9, the chromaticity of the portion corresponding to the second round of the belt when the signal value is not converted is 0. 0 as the coefficient α out of the chromaticity of the portion corresponding to the first round of the rotating body output by applying the coefficient α. It can be seen that it is closest to the chromaticity point when 96 is used. Therefore, 0.96 is used as α ′ in this case.

  In determining the step correction coefficient α ′ corresponding to the density conversion that eliminates the color difference between the first and second rounds of the fixing belt, the closest point is used as the coefficient α in the above, but the coefficient α is changed. Alternatively, a method of calculating an appropriate α ′ by performing linear approximation using the plotted points may be used.

  Next, a method for setting α used in accordance with the position in the page will be described. Since the density change due to the temperature factor of the fixing unit described above occurs at the boundary between the first and second rotations of the fixing belt, the density value of the image data is corrected according to the position. The relative relationship with the position in the image data does not necessarily match. This is because the image position on the paper is not fixed due to the deviation of the paper transport timing and the image formation timing. The paper transport timing is shifted because of a change in the coefficient of friction with the paper feed roller due to the physical properties of the paper and the tolerances of each part. In the present embodiment, the maximum is about 4 mm.

  Therefore, if the step difference correction coefficient α is changed in the density difference between the first and second laps at the belt one-round position on the image data, that is, α ′ is used as the step correction coefficient for one belt lap from the leading edge of the image data. If it is output, the step cannot be reduced when it deviates from the actual density step position.

  The positional deviation and density step will be described with reference to FIG.

  (a1) shows the relationship between the transport direction position and the density signal when the step correction coefficient α is set to 1 in the entire area when a uniform density signal is output in the transport direction, and the density signal value is the transport direction position. Does not change. The relationship between the transport direction position and the image density when the image data shown in (a1) is output to the recording paper is shown in (a2).

  Due to the above-described temperature difference in the fixing unit, the image density is high on the first round of the fixing belt and low on the second round. In order to correct the density step shown in (a2), as shown in (b1), if α is α ′, the portion corresponding to the first rotation of the fixing belt is α ′, and the portion corresponding to the second rotation is 1. The output image density becomes uniform as shown in (b2) when the position where the toner image is switched and the boundary position between the first and second rounds of the fixing belt are aligned. However, when the position is shifted, as shown in (b3), two density steps are generated at the boundary position in accordance with the position shift width, and a streak-like color difference is generated.

  In order to avoid this problem, the present invention is characterized in that the step correction coefficient α is changed stepwise in the front and rear to the boundary position of the first and second fixing belts in the page in the image data.

  A case where the step correction coefficient α is changed stepwise will be described with reference to FIG.

  (c1) shows the transport direction position and density signal when α is changed from α ′ to 1 evenly stepwise before and after the boundary position. When the image data shown in (c1) is output to the recording paper, the case where the boundary position is aligned and the case where the boundary positions are aligned are as shown in (c2) and (c3), respectively, and compared with (b3) in FIG. The density change in the vicinity of the boundary position becomes smaller and gentler than the density in the vicinity of the boundary position, making it difficult to visually recognize the color difference.

  In the present embodiment, at the position in the transport direction, 2.5 mm before and after the position corresponding to the boundary between the first and second belts on the image data when the paper transport and image formation timing are the design center values. Α was changed evenly in the region. Specifically, α was changed as shown in Table 2. As a result, even if the density correction position and the actual density change position are deviated, the difference in color tone between the first and second rounds of the fixing belt is suppressed, and the step at the boundary is reduced to make it inconspicuous.

[effect]
Next, the influence on the color difference correction at the portions corresponding to the first and second laps of the fixing belt by performing the signal value conversion as described in the present embodiment will be described.

  In the following, a change in the chromaticity of Blue (hereinafter referred to as B) with (C, M, Y, K) = (255, 255, 0, 0) as the signal value will be described as an example of the correction effect. Has the same effect.

  When signal value conversion is not performed, the B chromaticity of the portion corresponding to the first rotation of the fixing belt is (L1, a1, b1), and the B chromaticity of the portion corresponding to the second rotation of the fixing belt is (L0, a0, b0) If the chromaticity of the portion corresponding to the first round of the fixing belt when signal value conversion is performed is (L2, a1, b1), the respective chromaticities are as shown in Table 3 below. As a comparative example, a case where 0.98 is used as the correction coefficient α is also shown.

  Also, Table 3 above is shown on the chromaticity coordinates as shown in FIG. FIG. 12A is a graph in which the ab projection plane of the Lab chromaticity coordinate system is plotted. FIG. 12B is a graph plotting LC of the Lab chromaticity coordinate system.

  As can be seen from Table 3 and FIG. 12, it is understood that the chromaticity difference between the first and second rounds is reduced by performing the signal value conversion as in this embodiment. In terms of the chromaticity coordinate difference between the two points, the chromaticity difference ΔE with respect to the chromaticity in the second round is as shown in FIG. ΔE can be expressed by a three-dimensional distance (Equation 1) between two points (L1, a1, b1) (L2, a2, b2) in the L * a * b * color space defined by CIE.

ΔE ＝ √ ((L1−L2) ^ 2 + (a1−a2) ^ 2 + (b1-b2) ^ 2) ・ ・ ・ (Formula 1)
As can be seen from FIG. 13, by using the configuration of this embodiment, the color difference between the first and second rotations of the fixing belt is suppressed from ΔE = 1.90 to ΔE = 0.82. Is possible. Further, the color difference of the boundary portion is a feature of the present invention, in the case where the step correction coefficient α is not changed stepwise, ΔE = 1.90 remains in the region when the step position is shifted. By changing α stepwise, an effect of about 50% of the effect of the difference between the first and second rounds appears in that region, and the color difference can be reduced.

[Example 2]
In the present embodiment, a case will be described in which the step correction coefficient α at the in-page position is changed not only in the paper transport direction described in the first embodiment but also in a position orthogonal to the paper transport direction. Parts similar to those in the first embodiment are omitted, and only different parts are described.

In this embodiment, the image forming area in the direction orthogonal to the paper conveyance direction is divided into 30 parts, and the positions corresponding to the boundaries of the first and second rounds of the fixing belt on the image data in each area are changed to the paper conveyance direction position. It was. Specifically, if the 30 divided regions are X1, X2, X3, ... X30,
X1, X4, X7,... X28 are +1.0 mm, X2, X5, X8,... X29 are ± 0.0 mm, X3, X6, X9,. The method of change is not limited to this. As a result, as shown in FIG. 12, in Example 1, the density change at the boundary portion appears as a single straight line as shown in (a), whereas in this example, it is discontinuous as shown in (b). Since a density change occurs at the position, even a similar density change becomes more difficult to visually recognize and the image quality is improved.

100 image forming apparatus, 102 printer engine, 105 CPU,
119 printer image processing unit, 302 density step correction unit, 309 γ correction circuit,
1414 Fixing Device

Claims (4)

An image forming apparatus including a fixing unit for fixing a toner image on a recording medium to the recording medium,
Control means for controlling the image forming conditions of the output image based on the input image data;
Among the images on the recording medium fixed by the fixing unit,
After the recording medium has entered the fixing member, an image of a portion corresponding to the first turn of the fixing member and an image of a portion corresponding to the second turn,
An image forming apparatus, wherein an image forming condition of an output image based on input image data is changed three times or more. An image forming apparatus including a fixing unit for fixing a toner image on a recording medium to the recording medium,
Control means for controlling the image forming conditions of the output image based on the input image data;
Among the images on the recording medium fixed by the fixing unit,
After the recording medium has entered the fixing member, an image of a portion corresponding to the first turn of the fixing member and an image of a portion corresponding to the second turn,
In the image forming apparatus for changing the image forming condition of the output image based on the input image data, the change position of the image forming condition of the output image is changed in the carrying direction according to the position in the direction perpendicular to the carrying direction. Image forming apparatus. The image forming apparatus according to claim 1, wherein the image forming condition of the output image based on the input image data is γ correction means (LUT). The image forming condition is changed so that an output image based on the input image data is thinner in a portion corresponding to the first turn of the fixing member than in a portion corresponding to the second turn. The image forming apparatus according to any one of claims 1 to 3.