Image forming apparatus and image forming method

An image forming apparatus, including: a determining portion to acquire a density value for respective colors of toners, obtain a sum value of the density values of the colors of the toners, obtain a numerical value indicating the number of the density values being values larger than 0 out of the density values corresponding to the colors of the toners, and determine a target temperature on the basis of the sum value and the numerical value, wherein in a case the sum value is a first value and the numerical value is a first number, the determining portion determines a first temperature as the target temperature, in a case the sum value is the first value and the numerical value is a second number that is larger than the first number, the determining portion determines a second temperature that is lower than the first temperature as the target temperature.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus using an electrophotographic system such as printers including a laser printer and an LED printer, digital copiers, and the like, an image forming method, and a program.

Description of the Related Art

In conventional image forming apparatuses using an electrophotographic system, there is a technique for controlling a set temperature of a heating apparatus that heats and melts toner on a recording material in accordance with an amount of image data to be printed. Japanese Patent Application Laid-open No. 2016-4231 discloses a method of dividing image data into areas constituted by 32 dots×32 dots or the like and determining a set temperature on the basis of an image data amount of an area with a largest image data amount out of all areas and a print percentage of an entire image. A fixing process is performed by raising the set temperature when a maximum image data amount is large but by lowering the set temperature when the maximum image data amount is small. Accordingly, fixing at an unnecessarily high set temperature with respect to a toner image is avoided in order to reduce power consumption of the heating apparatus.

SUMMARY OF THE INVENTION

When printing is performed by overlapping toners of a plurality of colors on a recording material as in the case of a color image forming apparatus, even when a sum value of image density of image data is the same, an amount of unfixed toner that is actually laid onto the recording material may differ. Therefore, when a set temperature of a heating apparatus is determined in accordance with a sum value of image density in image data, an excessive amount of heat may be supplied to a recording material and the heating apparatus may end up consuming an excessive amount of power.

An object of the present invention is to reduce power consumption by more suitably controlling a set temperature of a heating apparatus in accordance with the number of colors of toners.

In order to achieve the object described above, an image forming apparatus including:

a fixing portion configured to fix a toner image formed in accordance with image data to a recording material;

a determining portion configured to acquire a density value indicating image density represented by the image data for each of colors of toners constituting the toner image, obtain a sum value of the density values with respect to the colors of the toners, obtain a numerical value indicating the number of the density values being values larger than 0 out of the density values corresponding to the colors of the toners, and determine a target temperature for maintaining a temperature of the fixing portion on the basis of the sum value and the numerical value; and

a control portion configured to control power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature, wherein

in a case the sum value is a first value and the numerical value is a first number, the determining portion determines a first temperature as the target temperature, in a case the sum value is the first value and the numerical value is a second number that is larger than the first number, the determining portion determines a second temperature that is lower than the first temperature as the target temperature.

In order to achieve the object described above, an image forming apparatus including:

a fixing portion configured to fix a toner image formed in accordance with image data to a recording material;

a determining portion configured to acquire a density value indicating image density represented by the image data for each of colors of toners constituting the toner image, calculate a toner bearing amount for the each of the colors of the toners from the density values with respect to the respective colors of the toners, obtain a sum amount of the toner bearing amounts with respect to the respective colors of the toners, and determine a target temperature for maintaining a temperature of the fixing portion on the basis of the sum amount; and

a control portion configured to control power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature.

In order to achieve the object described above, an image forming method, causing a computer included in an image forming apparatus to perform:

a fixing step of fixing a toner image formed in accordance with image data to a recording material using a fixing portion;

a determining step of acquiring a density value indicating image density represented by the image data for respective colors of toners constituting the toner image, obtaining a sum value of the density values with respect to the respective colors of the toners, obtaining a numerical value indicating the number of the density values being values larger than 0 out of the density values corresponding to the respective colors of the toners, and determining a target temperature for maintaining a temperature of the fixing portion on the basis of the sum value and the numerical value; and

a controlling step of controlling power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature, wherein

the determining step includes, in a case the sum value is a first value and the numerical value is a first number, determining a first temperature as the target temperature, in a case the sum value is the first value and the numerical value is a second number that is larger than the first number, determining a second temperature that is lower than the first temperature as the target temperature.

In order to achieve the object described above, an image forming method, causing a computer included in an image forming apparatus to perform:

a fixing step of fixing a toner image formed in accordance with image data to a recording material using a fixing portion;

a determining step of acquiring a density value indicating image density represented by the image data for respective colors of toners constituting the toner image, calculating a toner bearing amount for the respective colors of the toners from the density value with respect to the respective colors of the toners, obtaining a sum amount of the toner bearing amounts with respect to the respective colors of the toners, and determining a target temperature for maintaining a temperature of the fixing portion on the basis of the sum amount; and

a controlling step of controlling power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature.

According to the present invention, power consumption can be reduced by more suitably controlling a set temperature of a heating apparatus in accordance with the number of colors of toners.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, it is to be understood that dimensions, materials, shapes, relative arrangements, and the like of components described in the embodiments are intended to be changed as deemed appropriate in accordance with configurations and various conditions of apparatuses to which the present invention is to be applied and are not intended to limit the scope of the present invention to the embodiments described below.

First Embodiment

Description of Image Forming Apparatus

A configuration of a color image forming apparatus (hereinafter, expressed as an image forming apparatus)1according to a first embodiment will be described with reference toFIG. 1A.FIG. 1Ais a sectional view of the image forming apparatus1according to the first embodiment. The image forming apparatus1includes a paper feeding tray12, a paper feeding roller13, a resist roller pair14, and a registration sensor15. The image forming apparatus1includes an image forming portion constituted by image forming stations10Y,10M,10C, and10K for forming toner images of each of the colors yellow (Y), magenta (M), cyan (C), and black (K) on a recording material (a recording medium)11. In the first embodiment, the image forming stations10Y,10M,10C, and10K are arranged in a single row in a direction intersecting a vertical direction. Each of the image forming stations10Y,10M,10C, and10K has a photosensitive drum22Y,22M,22C, or22K, an injection charger23Y,23M,23C, or23K as primary charging portions, and a scanner portion24Y,24M,24C, or24K as exposing portions. In addition, each of the image forming stations10Y,10M,10C, and10K has a toner cartridge25Y,25M,25C, or25K, developing portions26Y,26M,26C, or26K, and a primary transfer roller27Y,27M,27C, or27K. The image forming apparatus1includes an intermediate transfer belt28, a secondary transfer roller29, a heating apparatus (a fixing apparatus)40, a paper discharge roller pair61, a control portion108, and a video controller109. The video controller109receives image data (image information) and print instruction signals transmitted from an external apparatus such as a personal computer. The control portion108is connected to the video controller109and controls respective portions constituting the image forming apparatus1in accordance with instructions from the video controller109.

The image forming portion forms an electrostatic latent image by exposure light having been lighted on the basis of an exposure time calculated by the control portion108as an image processing portion, and develops the electrostatic latent image to form a monochrome toner image. In addition, the image forming portion superimposes monochrome toner images to form a multicolor toner image, and transfers the multicolor toner image onto the recording material11. The multicolor toner image on the recording material11is fixed to the recording material11by the heating apparatus40.

The photosensitive drums22Y,22M,22C, and22K are constructed by applying an organic photoconductive layer on an outer circumference of an aluminum cylinder, and rotate as a driving force of a drive motor (not illustrated) is transmitted thereto. The drive motor rotates the photosensitive drums22Y,22M,22C, and22K in a clockwise direction in accordance with an image forming operation. The injection chargers23Y,23M,23C, and23K are provided with sleeves23YS,23MS,23CS, and23KS respectively corresponding thereto. The injection chargers23Y,23M,23C, and23K charge the photosensitive drums22Y,22M,22C, and22K. Exposure light is irradiated to the photosensitive drums22Y,22M,22C, and22K from the scanner portions24Y,24M,24C, and24K to selectively expose surfaces of the photosensitive drums22Y,22M,22C, and22K. Accordingly, an electrostatic latent image is formed on the photosensitive drums22Y,22M,22C, and22K.

The developing portions26Y,26M,26C, and26K develop yellow (Y), magenta (M), cyan (C), and black (K) in order to visualize the electrostatic latent images formed on the photosensitive drums22Y,22M,22C, and22K. The developing portions26Y,26M,26C, and26K are provided with sleeves26YS,26MS,26CS, and26KS respectively corresponding thereto. In addition, a power supply (not illustrated) applies a developing bias between the sleeves26YS,26MS,26CS, and26KS and the photosensitive drums22Y,22M,22C, and22K respectively corresponding thereto. During image formation, the photosensitive drums22Y,22M,22C, and22K rotate clockwise, and the developing portions26Y,26M,26C, and26K supply toner to the electrostatic latent images formed on the photosensitive drums22Y,22M,22C, and22K. Accordingly, a toner image of each color (hereinafter, also referred to as a multicolor toner image) is formed on the photosensitive drums22Y,22M,22C, and22K in accordance with image data transmitted from an external apparatus.

The intermediate transfer belt28is in contact with the photosensitive drums22Y,22M,22C, and22K due to a pressing force of the primary transfer rollers27Y,27M,27C, and27K. In addition, a power supply (not illustrated) applies a primary transfer bias between the primary transfer rollers27Y,27M,27C, and27K and the photosensitive drums22Y,22M,22C, and22K respectively corresponding thereto. During image formation, the intermediate transfer belt28and the primary transfer rollers27Y,27M,27C, and27K rotate so as to follow the photosensitive drums22Y,22M,22C, and22K and primarily transfer the toner images on the photosensitive drums22Y,22M,22C, and22K onto the intermediate transfer belt28.

The recording material11housed in the paper feeding tray12is transported by the paper feeding roller13and reaches the resist roller pair14. The registration sensor15detects a leading end or a trailing end of the recording material11. During image formation, the recording material11is transported so as coincide with a timing of detection by the registration sensor15to a timing where the multicolor toner image on the intermediate transfer belt28arrives at the secondary transfer roller29. In this manner, the recording material11arrives at the secondary transfer roller29from the resist roller pair14at an appropriate timing.

The intermediate transfer belt28is sandwiched by a pair of the secondary transfer rollers29. Accordingly, a secondary transfer nip portion N2as a secondary transfer portion is formed between the intermediate transfer belt28and the secondary transfer rollers29. In the secondary transfer nip portion N2, the secondary transfer rollers29come into contact with the intermediate transfer belt28, sandwiches and transports the recording material11, and transfers the multicolor toner image on the intermediate transfer belt28to the recording material11. A power supply (not illustrated) applies a secondary transfer bias between the secondary transfer rollers29and the intermediate transfer belt28. The transport guide30is a guiding member for transporting the recording material11from the secondary transfer nip portion N2to the heating apparatus40.

The heating apparatus40is a fixing portion which sandwiches and transports the recording material11, heats and melts a toner image on the recording material11, and fixes the toner image to the recording material11. The recording material11subjected to a fixing process by the heating apparatus40is transported to the outside of the image forming apparatus1by the paper discharge roller pair61and discharged to a paper discharge tray62. An image forming operation ends as the recording material11is discharged to the paper discharge tray62.

Hardware Configuration of Image Forming Apparatus

FIG. 1Bis a hardware configuration diagram of the image forming apparatus1according to the first embodiment. The image forming apparatus1includes a CPU501, a ROM502, a RAM503, a bus504, an I/O port505, a fixing motor drive circuit506, a fixing motor507, and the heating apparatus40. The heating apparatus40has a fixing film41, a pressure roller45, a heater42, a thermistor Th, a heater circuit508, and a thermistor circuit509. In order to drive the pressure roller45, the CPU501outputs a signal to the fixing motor drive circuit506via the bus504and the I/O port505to drive the fixing motor507. The fixing film41rotates so as to follow a rotation of the pressure roller45. The CPU501acquires a temperature detected by the thermistor Th via the bus504, the I/O port505, and the thermistor circuit509. The CPU501causes the heater42to generate heat via the bus504, the I/O port505, and the heater circuit508in order to perform temperature control.

Functional Configuration of Control Portion

Next, a functional configuration of the control portion108will be described.FIG. 1Cis a functional block diagram of the control portion108according to the first embodiment. As shown inFIG. 1C, the control portion108has a target temperature determining portion601and a power control portion602. The target temperature determining portion601and the power control portion602are realized as the CPU501shown inFIG. 1Bexecutes a program stored in the ROM502. The target temperature determining portion601determines a target temperature (a set temperature of the heating apparatus40) for maintaining the temperature of the heating apparatus40. The power control portion602controls power supplied to the heating apparatus40so that the temperature of the heating apparatus40is maintained at the target temperature.

Description of Configuration of Heating Apparatus

Next, the heating apparatus40will be described with reference toFIG. 2A. The heating apparatus40includes the fixing film41as a fixing member, the heater42as a heating member that comes into contact with an inner surface of the fixing film41, and the pressure roller45as a pressing member. The heater42is held by a holding member43which also has a guiding function for guiding rotation of the fixing film41. A stay44is a member for applying pressure of a pressure spring (not illustrated) to the holding member43toward a side of the pressure roller45to form a fixing nip portion N for heating and fixing a toner image on the recording material11. For example, the stay44is formed by a metal with high rigidity. In this case, total pressure of the pressure spring is 250 N, and a width of the fixing nip portion N in a transport direction of the recording material11(hereinafter, expressed as a recording material transport direction) is set to 9.0 mm. The pressure roller45receives power from a motor (not illustrated) and rotates clockwise. Due to the rotation of the pressure roller45, the fixing film41rotates counterclockwise so as to follow the rotation of the pressure roller45. The recording material11bearing a toner image is heated while being sandwiched and transported in a direction R1at the fixing nip portion N to perform a fixing process of the toner image on the recording material11.

The fixing film41has, for example, an outer diameter of 24 mm and has a base layer made of polyimide resin with a thickness of 60 μm, an elastic layer made of a thermally-conductive rubber layer with a thickness of 200 μm on an outer side of the base layer, and a releasing layer made of a PFA tube with a thickness of 20 μm as an outermost layer. In addition, the pressure roller45has, for example, an outer diameter of 25 mm and has a steel core with an outer diameter of 19 mm, an elastic layer made of silicone rubber with a thickness of 3 mm, and a releasing layer made of a PFA tube with a thickness of 40 μm as an outermost layer. The thermistor Th as a temperature detecting portion of the heater42is installed on a rear surface side of the heater42, and the thermistor Th is connected to the control portion108. During normal use, a driven rotation of the fixing film41starts as a rotation of the pressure roller45starts, and an inner surface temperature of the fixing film41rises as a temperature of the heater42rises. The heater42is controlled by the control portion108as a temperature control portion and a power control portion, and the set temperature (target temperature) of the heating apparatus40is determined and input power to the heater42is controlled so that a surface temperature of the fixing film41reaches a prescribed temperature. In other words, on the basis of a detected temperature of the thermistor Th, the control portion108performs power control of the heater42so that the temperature of the heating apparatus40(the surface temperature of the fixing film41) is maintained at the set temperature. For example, the heater42may be controlled by the control portion108by controlling power supplied to the heater42in accordance with a signal of the thermistor Th. Due to the heater42being controlled in this manner, temperature control of the heating apparatus40is performed by holding the temperature inside the fixing nip portion N (a fixing temperature, a heating temperature) during a heating-fixing operation at a desired temperature (a target temperature). In other words, the heater42is controlled so that the temperature detected by the thermistor Th is maintained at the set temperature of the heating apparatus40. Alternatively, the heater42may be controlled so that the temperature detected by the thermistor Th is kept within an allowable range (a prescribed temperature range) of the set temperature of the heating apparatus40.

The thermistor Th is arranged so as to come into contact with a center position of the heater42in a longitudinal direction of the heater42and a center position of the heater42in a transverse direction of the heater42. The longitudinal direction of the heater42is a direction perpendicular to the recording material transport direction. The transverse direction of the heater42is a direction perpendicular to the longitudinal direction of the heater42and coincides with the recording material transport direction. In the first embodiment, as shown inFIG. 2A, temperature control of the heating apparatus40is performed by bringing the thermistor Th as a temperature detecting portion into contact with a rear surface of the heater42as a heating portion and controlling the heater42. In addition, as shown inFIG. 2B, by using the thermistor Th as a contactless temperature detecting portion that detects infrared rays or visible light rays, the thermistor Th may be arranged in a state where the heater42and the thermistor Th are separated from each other.

A configuration of the heater42will be described with reference to the schematic views ofFIGS. 3A and 3B.FIG. 3Ais a sectional view of the heater42. An aluminum nitride base material401of the heater42is constituted by an aluminum nitride substrate that is a ceramic substrate with a thickness of 0.6 mm. For example, a longitudinal width of the aluminum nitride base material401is 260 mm and a transverse width (a paper-passing direction) thereof is 9 mm A sliding glass layer404with a thickness of 15 μm is provided on a front surface side of the heater42which comes into contact with the fixing film41. The sliding glass layer404comes into contact with the fixing film41via a fluorine grease (not illustrated) and exhibits favorable slidability. In addition, a resistance heating layer402with a thickness of 10 μm and protective glass403with a thickness of 50 μm are provided on a rear surface side of the heater42. The resistance heating layer402is formed by applying a conductive paste containing a silver-palladium (Ag/Pd) alloy on the aluminum nitride base material401by screen printing.FIG. 3Bis a schematic view of the heater42when viewed from the rear surface side of the heater42. The resistance heating layer402is formed in a band shape along the longitudinal direction of the heater42. A dotted line inFIG. 3Bdenotes the protective glass403. Due to the protective glass403covering the resistance heating layer402and a conductive portion406, insulation properties of the resistance heating layer402and the conductive portion406are secured. In addition, in the heater42, the resistance heating layer402generates heat when electrode portions405A and405B are energized by an external power supply. In this case, in the longitudinal direction of the heater42, a heated region A that is heated by the resistance heating layer402is, for example, 220 mm In the first embodiment, power-supply voltage of the external power supply is 120 V and resistance of the heater42is set to 10Ω. In order to measure power (to be described later), the external power supply is connected to cables (not illustrated) for feeding power to the electrode portions405A and405B via a power meter WT310 manufactured by Yokogawa Test & Measurement Corporation.

<Description of Temperature Control of Heating Apparatus>

Temperature control of the heating apparatus40on the basis of an image density value and the number of colors of toners which is a feature of the first embodiment will now be described in detail with reference to the flow chart inFIG. 4. In the first embodiment, a method will be described of extracting a maximum sum image density value Dsum_max and a toner coefficient E indicating the number of colors of toners constituting a toner image from image data received by the video controller109and reflecting the maximum sum image density value Dsum_max and the toner coefficient E on a set temperature T of the heating apparatus40. The maximum sum image density value Dsum_max will be described later. InFIG. 4, printing is started as the image forming apparatus1receives a print job (S501). The video controller109as an image data detecting portion receives image data (S502). The control portion108calculates the maximum sum image density value Dsum_max of the recording material11to pass through the heating apparatus40next from the image data and extracts the toner coefficient E (S503). The toner coefficient is an example of the numerical value.

The maximum sum image density value Dsum_max will now be described.FIG. 5Ais a schematic view for illustrating an image density value of each recording material11. The longitudinal direction which is a print surface side of each recording material11and which is perpendicular to the recording material transport direction is adopted as X coordinates, the recording material transport direction is adopted as Y coordinates, and a left end of the X coordinates and a distal end of the Y coordinates are adopted as a coordinate origin (0, 0), whereby each pixel on X-Y coordinates at an image resolution of 600 dpi has image density. In this case, image density of 16 gradations can be expressed per pixel. With four pixels in all directions (a total of 16 pixels) on the X-Y coordinates as one pixel block, image density of 256 gradations (gradation data: 0 to 255) can be expressed within one pixel block and the image density is defined as an image density value of 0 to 100%. In other words, an image density value is a density value indicating image density expressed by image data and is a value indicating image density of image data (image information) as a percentage.

FIG. 6Ashows image data in a case where image density value: 0% and gradation data: 0, and shows a state where toner is unused.FIG. 6Eshows image data in a case where image density value: 100% and gradation data: 255, whereby image density value: 100% represents an upper limit value of the image density value of each color and a maximum image density (O.D.) in this case is approximately 1.4 (O.D.) for each color.FIGS. 6B to 6Dindicate image data in cases where image density value: 25%, 50%, and 75% and gradation data: 63, 127, and 191. The halftones inFIGS. 6B to 6Dare indicated by numerical values having been linearly interpolated with respect to image density. Image density (O.D.) is a measurement value obtained by measuring an output image from the image forming apparatus1according to the first embodiment using X-rite504as a spectral densitometer. In the first embodiment, while an image density value has a linear relationship with respect to image density as shown in FIG.5B, this is not restrictive and, for example, an image density value may have a linear relationship with respect to color difference (ΔE). In addition, high white paper GF-0081, A4 size, manufactured by Canon Inc. was used as the recording material11and a 100% image pattern (30 mm×30 mm) of each color such as that shown inFIG. 5Cwas created at center of the A4-size recording material11. Image creation was performed using YMCK color mode of Photoshop CS4 manufactured by Adobe Inc. In addition, a toner bearing amount (toner laid-on level) per unit area on the recording material11is approximately 0.45 (mg/cm2) at an image density value of 100% for all colors. This numerical value is a measurement value obtained by performing a weight measurement of unfixed toner when toner is present in an unfixed state on the recording material11in a section from the secondary transfer nip portion N2to the heating apparatus40.

The control portion108acquires a sum density value Dsum of each point on the X-Y coordinates. The sum density value Dsum is an image density value of each point on the X-Y coordinates. The sum density value Dsum is a sum value of image density values of the four YMCK colors in each pixel block in one page of the recording material11and is calculated using expression (1) below.
Dsum(x,y)=DY(x,y)+DM(x,y)+DC(x,y)+DK(x,y)  (1)

In expression (1), DY(x, y), DM(x, y), DC(x, y), and DK(x, y) denote image density values of the respective YMCK colors at each point on the X-Y coordinates. In this manner, the control portion108acquires an image density value for each color of toners constituting a toner image and calculates a sum value of image density values (a sum density value Dsum) for each color of toners constituting the toner image.

The maximum sum image density value Dsum_max represents a maximum value (a maximum amount) of sum density values Dsum(x, y) of the respective pixel blocks in one page of the recording material11. The toner coefficient E represents the number of colors constituting a pixel block indicating a maximum value out of the sum density values Dsum(x, y) of the respective pixel blocks in one page of the recording material11. The toner coefficient E is a numerical value indicating the number of image density values that are image density values larger than 0% out of the image density values corresponding to each color of toners constituting a toner image. In addition, in the first embodiment, the video controller109adjusts the maximum sum image density value Dsum_max to be within a range of 0% to 300%.

Image data includes a plurality of regions (pixel blocks). The control portion108determines a prescribed region of which the sum density value Dsum(x, y) is a maximum value out of the plurality of regions of image data. On the basis of the sum density value Dsum(x, y) or, in other words, the maximum sum image density value Dsum_max and the toner coefficient E in the determined prescribed region, the control portion108determines the set temperature T using expression (2) below (S504).
T=200+Dsum_max×0.4/√E(2)

In this case, expression (2) is a controlling expression indicating a relationship among the maximum sum image density value Dsum_max, the toner coefficient E indicating the number of colors of toners constituting a toner image, and the set temperature T. Expression (2) is based on a relationship of a toner bearing amount on the recording material11with respect to the image density value of each color shown inFIGS. 7A and 7B. When the image density value of each color of toners constituting a toner image is lower than a reference value (for example, 10%), the control portion108does not include the number of colors of toners with respect to the image density values lower than the reference value in the toner coefficient E. In other words, when the image density value of each color of toners constituting a toner image is lower than a reference value (for example, 10%), the control portion108obtains the toner coefficient E by excluding the image density values lower than the reference value. Alternatively, when the image density value of each color of toners constituting a toner image is lower than a reference value (for example, 10%), the control portion108may include the number of colors of toners with respect to the image density values lower than the reference value in the toner coefficient E.

FIG. 7Ais a graph showing a relationship of a toner bearing amount (a bearing amount of unfixed toner) with respect to an image density value DY in the image forming apparatus1according to the first embodiment. As shown inFIG. 7A, the relationship between the image density value DY and the unfixed toner amount on the recording material11per unit area is non-linear. Although the image density value is generally linear with respect to optical density (O.D.) or color difference (ΔE) relative to chromaticity of a reference color, the toner bearing amount on the recording material11may not be linear with respect to optical density and color difference and may have a non-linear relationship. As shown inFIG. 7A, in a region where the image density value is small (around 0 to 30%), an increment in the toner bearing amount with respect to the image density value is small. On the other hand, in a region where the image density value is large (around 70 to 100%), an increment in the toner bearing amount with respect to the image density value is large. Tendencies of the image density values DM, DC, and DK with respect to the toner bearing amount are similar to a tendency of the image density value DY shown inFIG. 7A.FIG. 7Bis a graph showing a relationship between the maximum sum image density value Dsum_max and a sum toner bearing amount.FIG. 7Bshows cases where a ratio of the respective colors of toners is (D1) Y:M=1:1, (D2) Y:M:C=1:1:1, and (D3) Y:M:C:K=1:1:1:1. As shown inFIG. 7B, when the maximum sum image density values Dsum_max of (D1) to (D3) are the same, the larger the number of colors of toners constituting a toner image, the smaller the sum toner bearing amount.

FIG. 8is a table showing an example of temperature control parameters according to the first embodiment.FIG. 8shows the maximum sum image density value Dsum_max, an image density value of each YMCK color, a sum toner bearing amount, the toner coefficient E, and set temperatures T and T0.FIG. 8shows image density values of the respective YMCK colors in cases where the maximum sum image density value Dsum_max is 50%, 100%, 150%, 200%, 250%, and 300%. The set temperature T is the target temperature (control temperature) of the heating apparatus40calculated using expression (2) above. The set temperature T0will be described later.

FIG. 9Ais a graph showing a relationship between the maximum sum image density value Dsum_max and the set temperature T extracted fromFIG. 8. As shown inFIG. 9A, while the set temperature T rises as the maximum sum image density value Dsum_max increases, when Dsum_max is the same value, the larger the number of colors of toners constituting a toner image, the lower the set temperature T. A case where the maximum sum image density value Dsum_max as a sum of image density values is 200% (A-1to A-3inside a bold frame A inFIG. 8) will now be described. In the case of (A-1) inFIG. 8, the toner coefficient E as the number of colors of toners constituting a toner image is “2” and the set temperature T is “257° C.”. In the case of (A-2) inFIG. 8, the toner coefficient E is “3” and the set temperature T is “246° C.”. In the case of (A-3) inFIG. 8, the toner coefficient E is “4” and the set temperature T is “240° C.”. As shown in (A-1) to (A-3) inFIG. 8, the larger the toner coefficient E, the lower the set temperature T. When the maximum sum image density value Dsum_max is a prescribed value (for example, “200%”) and the toner coefficient E is a first number (for example, “2”), the control portion108determines a first temperature (for example, “257° C.”) as the set temperature T. When the maximum sum image density value Dsum_max is the prescribed value and the toner coefficient E is a second number (for example, “3” or “4”) that is larger than the first number, the control portion108determines a second temperature (for example, “246° C.” or “240° C.”) that is lower than the first temperature as the set temperature T.

FIG. 9Bis a graph showing a relationship between the sum toner bearing amount and the set temperature T extracted fromFIG. 8.FIG. 9Bshows that, even when the number of colors of toners (the toner coefficient E) and the maximum sum image density value Dsum_max differ, a set temperature T in accordance with the sum toner bearing amount can be adjusted. The set temperature T can be adjusted in this manner because using expression (2) above for determining the set temperature T enables the effect of both the maximum sum image density value Dsum_max and the number of colors of toners (the toner coefficient E) with respect to the set temperature T can be sufficiently taken into consideration.

Let us now return to the flow chart inFIG. 4to continue the description of temperature control of the heating apparatus40. The control portion108controls power supplied to the heating apparatus40so that the temperature of the heating apparatus40is maintained at the set temperature T. By passing the recording material11through the heating apparatus40, unfixed toner is fixed to the recording material11(S505). The control portion108determines whether or not the recording material11is a last recording material11in the print job (S506). When the recording material11is a last recording material11, the print operation is ended (S507). When the recording material11is not a last recording material11, the job is continued, the process returns to S502, and processes of S502to S506are repeated until the control portion108determines that the recording material11is the last recording material11. In the first embodiment, the temperature control of the heating apparatus40is performed according to the flow shown inFIG. 4.

The following comparative experiment was performed in order to confirm an effect of performing temperature control of the heating apparatus40on the basis of the image density value and the number of colors of toners according to the first embodiment. Conditions of the comparative experiment included recording material transportation speed: 300 mm/sec, print speed (throughput): 60 ppm, recording material11: OCE Red Label paper (basis weight 80 g/m2), A4 size, manufactured by Canon Inc., and the number of passed sheets: 110 sheets.FIG. 10is a diagram showing an image pattern used when performing the comparative experiment. As shown inFIG. 10, a high-printing rate image as a pattern B is printed in addition to a low-printing rate halftone image (Bk: 5%) as a pattern A with respect to the recording material11used in the comparative experiment. Image creation is performed using YMCK color mode of Photoshop CS4 manufactured by Adobe Inc. The pattern B printed on the recording material11varies for each experimental condition. Confirmation of the effect of the comparative experiment is performed by comparing power consumption and fixability of the heating apparatus40with respect to 101st to 110th printed sheets. Although the comparative experiment focuses on the 101st to 110th printed sheets after the heating apparatus40has been sufficiently warmed up, the effect of the first embodiment is not limited to the 101st to 110th printed sheets.

FIGS. 11A to 11Care tables showing a result of the comparative experiment, andFIG. 11Ashows an experimental result in a case where temperature control of the heating apparatus40was performed on the basis of the image density value and the number of colors of toners according to the first embodiment. In this case, a film surface temperature is a surface temperature of the fixing film41which comes into contact with the recording material11when the thermistor Th is controlled on the basis of each set temperature T in the 101st to 110th printed sheets. A thermocouple (ST-13E-010-GW1-W) manufactured by Anritsu Meter Co., Ltd. is used to measure the surface temperature of the fixing film41. In conditions A to C inFIGS. 11A to 11C, although the maximum sum image density value Dsum_max is the same, the sum toner bearing amount differs. In the first embodiment, with respect to the conditions A to C, the set temperature T is controlled in accordance with the sum toner bearing amount and the film surface temperature also varies in accordance with the set temperature T. As a result, fixability is favorable (Good) under the conditions A to C and, at the same time, a reduction in power consumption can be achieved under the conditions B and C having a low sum toner bearing amount.

Next, a case where the temperature control according to the comparative example is performed will be described. The set temperature T0in the temperature control according to the comparative example is obtained by expression (3) below.
T0=230.5+Dsum_max/8  (3)

In other words, the set temperature T0is determined solely based on the maximum sum image density value Dsum_max.FIG. 9Cis a graph showing a relationship between the maximum sum image density value Dsum_max and the set temperature T0extracted fromFIG. 8.FIG. 9Cshows that the set temperature T rises in accordance with the maximum sum image density value Dsum_max regardless of the number of colors of toners constituting a toner image. In addition,FIG. 9Dis a graph showing a relationship between the sum toner bearing amount and the set temperature T0extracted fromFIG. 8.FIG. 9Dshows that the set temperature T0is not appropriately determined when a difference in the sum toner bearing amount is created due to a difference in the number of colors of toners.

FIG. 11Bis a table showing an experimental result when performing the temperature control according to a first comparative example. In the first comparative example, temperature control is performed according to the condition A corresponding to a case where the sum toner bearing amount is high and the set temperature T0is set to 256° C. In the first comparative example, since temperature control is performed according to the condition A corresponding to a case where the sum toner bearing amount is high, fixability is favorable (Good) under any of the conditions A to C and power consumption is more or less the same under the conditions A to C. Since temperature control is performed at the same set temperature T0under the conditions B and C which correspond to a case where the sum toner bearing amount is low, although fixability is secured, excess power is being supplied to the heating apparatus40.

FIG. 11Cis a table showing an experimental result when performing the temperature control according to a second comparative example. In the second comparative example, temperature control is performed according to the condition C corresponding to a case where the sum toner bearing amount is low and the set temperature T0is set to 240° C. Therefore, the set temperature T0according to the second comparative example is lower than the set temperature T0according to the first comparative example by 16° C. In the second comparative example, since temperature control is performed according to the condition C corresponding to a case where the sum toner bearing amount is low, although a reduction in power consumption is achieved under the conditions A to C, fixability under the conditions A and B has not been secured.

In the first embodiment, the set temperature T is determined by extracting the maximum sum image density value Dsum_max and the toner coefficient E from image data. When the maximum sum image density value Dsum_max is a same prescribed value, the larger the toner coefficient (the number of colors), the lower the set temperature T. Accordingly, the set temperature T can be appropriately determined in accordance with an actual toner bearing amount on the recording material11. As a result, since excess heat can be prevented from being imparted to the recording material11, power consumption can be suppressed and, at the same time, stable fixability can be secured.

In addition, in the first embodiment, when an image density value related to a prescribed color is lower than a reference value (for example, 10%), since the toner bearing amount is a minute amount, the prescribed color is not included in the toner coefficient E used to calculate the set temperature T. However, when the toner bearing amount is high despite the image density value being low, the prescribed color may be included in the toner coefficient E, and when the toner bearing amount is low despite the image density value being high, the prescribed color may not be included in the toner coefficient E. For example, the reference value may be changed as deemed appropriate in accordance with properties of the image forming apparatus1.

In addition, while one image forming station each is arranged in the image forming apparatus1with respect to each toner color of four colors (YMCK) in the first embodiment, a plurality of image forming stations may be arranged in the image forming apparatus1for one toner color. In other words, at least two of a plurality of image forming stations may form a toner image with toners of a same color. For example, two of four image forming stations may be image forming stations of the K toner color and two of four image forming stations may be image forming stations of the M toner color. When the four image density values are all equal to or higher than the reference value, the toner coefficient E is 4. In other words, when different image forming stations having toner of a same color are arranged in the image forming apparatus1, each of the different image forming stations having the toner of a same color is an object of calculation of the toner coefficient. The control portion108increases the number of the toner coefficient E in accordance with the number of the plurality of image forming stations that form the toner image with toner of a same color and, on the basis of the maximum sum image density value Dsum_max and the toner coefficient E, determines the set temperature T using expression (2) above.

First Modification

As a first modification of the first embodiment, a method of changing the set temperature T in stages according to the maximum sum image density value Dsum_max and the toner coefficient E will be described.FIG. 12Ashows, in stages, a reference temperature T1in accordance with the maximum sum image density value Dsum_max and shows that the maximum sum image density value Dsum_max is divided in a prescribed range. In addition,FIG. 12Bshows, in stages, an adjusted temperature T2in accordance with the toner coefficient E and shows that the adjusted temperature T2rises as the toner coefficient E increases. The set temperature T according to the first modification is determined by subtracting the adjusted temperature T2from the reference temperature T1(T=T1−T2).FIG. 13is a table showing an example of temperature control parameters according to the first modification.FIG. 13shows the maximum sum image density value Dsum_max, an image density value of each YMCK color, a sum toner bearing amount, the toner coefficient E, the reference temperature T1, the adjusted temperature T2, and the set temperature T.

FIG. 14Ais a graph showing a relationship between the maximum sum image density value Dsum_max and the set temperature T extracted fromFIG. 13. As shown inFIG. 14A, the set temperature T rises in stages in accordance with the maximum sum image density value Dsum_max and the set temperature T drops in stages as the number of colors of toners constituting a toner image increases.FIG. 14Bis a graph showing a relationship between the sum toner bearing amount and the set temperature T extracted fromFIG. 13.FIG. 14Bshows that, even when the number of colors of toners (the toner coefficient E) and the maximum sum image density value Dsum_max differ, a set temperature T in accordance with the sum toner bearing amount can be adjusted. By determining the set temperature T in stages in accordance with the maximum sum image density value Dsum_max or the toner coefficient E as in the first modification, calculation processes can be simplified. A configuration of the first embodiment or the first modification may be selected in accordance with performance of the control portion108.

Second Embodiment

In a second embodiment, a method of deriving the set temperature T which differs from the first embodiment will be described. Otherwise, the configuration of the image forming apparatus1and the configuration of the heating apparatus40are the same and descriptions thereof will be omitted.

Description of Temperature Control of Heating Apparatus

Temperature control of the heating apparatus40on the basis of toner amount information according to the second embodiment will now be described with reference to the flow chart inFIG. 15. In the second embodiment, a method will be described of calculating a maximum sum toner bearing amount Wsum_max representing a largest sum toner amount of the recording material11from image data received by the video controller109and determining the set temperature T of the heating apparatus40. Printing is started as the image forming apparatus1receives a print job (S601). The video controller109receives image data (S602). The control portion108calculates the maximum sum toner bearing amount Wsum_max of the recording material11to pass through the heating apparatus40next from the image data (S603). The maximum sum toner bearing amount Wsum_max will now be described. Image data of each recording material11is similar to contents described with reference toFIG. 5Ain the first embodiment, and each pixel on X-Y coordinates at an image resolution of 600 dpi has image density. In addition, DY(x, y), DM(x, y), DC(x, y), and DK(x, y) described below are similar to the first embodiment.

FIG. 16is a graph showing a relationship of a toner bearing amount WY on the recording material11relative to an image density value DY for the Y color acquired in advance according to the second embodiment. Based on the relationship shown inFIG. 16, the toner bearing amount WY can be calculated from the image density value DY using expression (4).
WY=0.45×(0.958×(DY)2+0.0422×DY)  (4)

Next, the control portion108acquires a sum toner bearing amount Wsum (a sum amount of toner bearing amounts) of the recording material11at each point on the X-Y coordinates. The sum toner bearing amount Wsum is a sum amount of toner bearing amounts of the four YMCK colors in each pixel block in one page of the recording material11and is calculated using expression (5) below.
Wsum(x,y)=WY(x,y)+WM(x,y)+WC(x,y)+WK(x,y)  (5)

In expression (5), WY(x, y), WM(x, y), WC(x, y), and WK(x, y) denote toner bearing amounts of the respective YMCK colors on the recording material11at each point on the X-Y coordinates. Each of WY(x, y), WM(x, y), WC(x, y), and WK(x, y) is calculated from each of DY(x, y), DM(x, y), DC(x, y), and DK(x, y) using expression (4). In a similar manner to the first embodiment, the control portion108acquires an image density value for each color of toners constituting a toner image. The control portion108calculates a toner bearing amount of each color of toners constituting a toner image from the image density value for each color of toners constituting the toner image. In this case, since a relationship of the toner bearing amounts WM, WC, and WK on the recording material11with respect to image density values DM, DC, and DK in the MCK colors is similar to the relationship of the toner bearing amount WY on the recording material11with respect to the image density value DY, the toner bearing amounts WM, WC, and WK can be calculated using expression (4) in a similar manner to the Y color.

The maximum sum toner bearing amount Wsum_max represents a maximum value (a maximum amount) of sum toner bearing amounts Wsum(x, y) of the respective pixel blocks in one page of the recording material11. A toner image includes a plurality of regions (pixel blocks). The control portion108determines a prescribed region of which the sum toner bearing amount Wsum(x, y) is a maximum value out of the plurality of regions of the toner image. On the basis of the sum toner bearing amount Wsum(x, y) or, in other words, the maximum sum toner bearing amount Wsum_max in the determined prescribed region, the control portion108determines the set temperature T using expression (6) below (S604).
T=212.9−(17.994×(Wsum_max)2−64.066×Wsum_max)  (6)

When the toner bearing amount of each color of toners constituting a toner image is lower than a reference value, the control portion108does not include the toner bearing amount that is lower than the reference value in the maximum sum toner bearing amount Wsum_max. Alternatively, when the toner bearing amount of each color of toners constituting a toner image is lower than a reference value, the control portion108may include the toner bearing amount that is lower than the reference value in the maximum sum toner bearing amount Wsum_max.

FIG. 17is a table showing an example of temperature control parameters according to the second embodiment.FIG. 17shows the maximum sum image density value Dsum_max, an image density value of each YMCK color, the maximum sum toner bearing amount Wsum_max, the toner coefficient E, and the set temperature T.FIG. 18Ais a graph showing a relationship between the maximum sum toner bearing amount Wsum_max and the set temperature T extracted fromFIG. 17.FIG. 18Ashows that, even when the number of colors of toners (the toner coefficient E) and the image density value of each YMCK color differ, since the set temperature T is determined on the basis of the maximum sum toner bearing amount Wsum_max, the set temperature T in accordance with an unfixed toner amount on the recording material11can be adjusted.

In addition,FIG. 18Bis a graph showing, as a reference, a relationship among the set temperature T obtained in the second embodiment, the maximum sum image density value Dsum_max, and the toner coefficient E (the number of colors). As shown inFIG. 18B, while the set temperature T rises as the maximum sum image density value Dsum_max increases, when Dsum_max is the same value, the larger the number of colors (the toner coefficient E) of toners constituting a toner image, the lower the set temperature T. A case where the maximum sum image density value Dsum_max as a sum of image density values is 200% (B-1to B-3inside a bold frame B inFIG. 17) will now be described. In the case of (B-1) inFIG. 17, the toner coefficient E as the number of colors of toners constituting a toner image is “2” and the set temperature T is “256° C.”. In the case of (B-2) inFIG. 17, the toner coefficient E is “3” and the set temperature T is “245° C.”. In the case of (B-3) inFIG. 17, the toner coefficient E is “4” and the set temperature T is “239° C.”. As shown in (B-1) to (B-3) inFIG. 17, the larger the toner coefficient E, the lower the set temperature T. When the maximum sum image density value Dsum_max is a prescribed value (for example, “200%”) and the toner coefficient E is a first number (for example, “2”), a first temperature (for example, “256° C.”) is determined as the set temperature T. When the maximum sum image density value Dsum_max is the prescribed value and the toner coefficient E is a second number (for example, “3” or “4”) that is larger than the first number, a second temperature (for example, “245° C.” or “239° C.”) that is lower than the first temperature is determined as the set temperature T.

Let us now return to the flow chart inFIG. 15to continue the description of temperature control of the heating apparatus40. The control portion108controls power supplied to the heating apparatus40so that the temperature of the heating apparatus40is maintained at the set temperature T. By passing the recording material11through the heating apparatus40, unfixed toner is fixed to the recording material11(S605). The control portion108determines whether or not the recording material11is a last recording material11in the print job (S606). When the recording material11is a last recording material11, the print operation is ended (S607). When the recording material11is not a last recording material11, the job is continued, the process returns to S602, and processes of S602to S606are repeated until the last recording material11is processed. In the second embodiment, the temperature control of the heating apparatus40is performed according to the flow shown inFIG. 15.

In addition, while one image forming station each is arranged in the image forming apparatus1with respect to each toner color of four colors (YMCK) in the second embodiment, a plurality of image forming stations may be arranged in the image forming apparatus1for one toner color. In other words, at least two of a plurality of image forming stations may form a toner image of a same color. When calculating the sum toner bearing amount Wsum using expression (5) above, the control portion108calculates the sum toner bearing amount Wsum by multiplying a toner bearing amount of a same color by the number of the plurality of image forming stations that form a toner image using toner of the same color.

FIG. 19is a table showing a result of a comparative experiment performed by a similar method to the first embodiment. Results of comparative examples 1 and 2 as comparison objects are similar toFIGS. 11B and 11C. In conditions A to C inFIG. 19, although the maximum sum image density value Dsum_max is the same, the sum toner bearing amount differs. In the second embodiment, the set temperature T is determined in accordance with the sum toner bearing amount and the film surface temperature also varies in accordance with the set temperature T. As a result, fixability is favorable (Good) and, at the same time, a reduction in power consumption can be achieved under the conditions B and C having a low sum toner bearing amount.

In the second embodiment, a toner bearing amount in each pixel block of image data on the recording material11is calculated and the set temperature T is determined in accordance with a maximum sum toner bearing amount thereof. On the other hand, in the first embodiment, the set temperature T is determined with respect to each pixel block of image data from a relationship between a maximum sum image density value and a toner coefficient (the number of colors). The first embodiment has an advantage in that the absence of a calculation process of a toner bearing amount enables processing by the CPU to be simplified while the second embodiment enables the set temperature T to be determined in accordance with a toner bearing amount. Therefore, the second embodiment has an advantage in that the set temperature T can be adjusted more accurately in accordance with a pixel block with a high toner bearing amount. Whichever is suitable between the first and second embodiments may be selected in consideration of a calculation load on the CPU and fixing performance that is required of the heating apparatus40.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. This application claims the benefit of Japanese Patent Application No. 2018-242510, filed on Dec. 26, 2018, which is hereby incorporated by reference herein in its entirety.