JP2001092213A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize superior fixing property and color reproducibility for a toner image. SOLUTION: This device is provided with area-extracting parts 55 to 58 for extracting an overlap area standing for an overlap section of the image area of at least two color component signals, an overlap area density comparing part 63 respectively comparing the density signal of color component signal for pixel in pixels under consideration extracted thereby, with a comparison table being preset for each component signal, and a system controller controlling a quantity of heat at the fixing time of the toner image according to the comparison result. When the comparison result is αexcessive', the excess signal is transmitted to the system controller, and the fixing temperature of the fixing device is changed from 170 deg.C in a normal state to 180 deg.C. As a result, the satisfactory fixation property is obtained even in the case of the overlap image of 80% to 100% M color and 80% to 100% C color, that is poor in fixation property in a normal state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for obtaining a color image by heating and fixing a plurality of color toner images.

[0002]

2. Description of the Related Art FIG. 9 shows a conventional four-color full-color image forming apparatus.

A photosensitive drum 1 as an image carrier is driven by a driving means (not shown) in the direction of arrow R 1, and is uniformly charged by a primary charger 2. Next, the exposure device 3 irradiates the photosensitive drum 1 with a laser beam according to the image signal of the black component signal, and an electrostatic latent image is formed on the photosensitive drum 1. When the photosensitive drum 1 further moves in the direction of the arrow, the electrostatic latent image is transferred to the rotary support 4A and the four developing devices 4a, 4a.
b, 4c, and 4d. Developing devices 4a, 4 supported by rotating support 4A
Among the b, 4c and 4d, the developing device 4a containing the black toner is arranged at the developing position facing the photosensitive drum 1. The electrostatic latent image on the photosensitive drum 1 is developed with a black toner image by the black developing device 4a to which black toner is attached. The transfer unit 5 has three rollers 5b, 5c, 5d and an intermediate transfer belt 5a stretched around the rollers. The intermediate transfer belt 5 a rotates in a direction indicated by an arrow R <b> 5 at substantially the same speed as the photosensitive drum 1, and the toner image formed and carried on the photosensitive drum 1 is surfaced by the primary transfer bias applied to the primary transfer roller 6. (Outer peripheral surface). The toner remaining on the surface of the photosensitive drum 1 without being primarily transferred is removed by the cleaning device 7.

The above image forming processes, ie, primary charging, exposure, development, primary transfer, and cleaning are sequentially performed on the remaining magenta (M), cyan (C), and yellow (Y) other than the above-described black (K). The toner images of four colors are superimposed on the intermediate transfer belt 5a.

Next, the transfer material P is fed at a predetermined timing. At the same time, a secondary transfer bias is applied to the secondary transfer roller 8b, so that the four color toner images on the intermediate transfer belt 5a are secondarily transferred to the transfer material P at once. At this time, the transfer material P
The toner remaining on the intermediate transfer belt 5 a without being transferred to the intermediate transfer belt 5 a is removed by the belt cleaner 9.

The transfer material P after the secondary transfer of the toner image is conveyed toward the fixing device 10 in the direction of arrow Kp. Transfer material P
Is heated and pressed at a fixing nip portion between a fixing roller 11 having a heater 21 inside and a pressure roller 12 also having a heater 22 inside, so that a toner image is fixed on the surface. The surface temperature of the fixing roller 11 is detected by the thermistor 23.

The above-described fixing device 10 will be described in detail.

A fixing device 10 includes a fixing roller 11 that is driven to rotate by a driving motor (not shown), and a fixing roller 11.
The non-fixed toner image on the transfer material P is heated and conveyed by nipping and transferring the transfer material P such as paper at a fixing nip portion N formed between the pressure roller 12 and the pressure roller 12 that rotates following the pressure. Pressure is applied to fix the toner image on the surface of the transfer material P.

The heating operation of the fixing device 10 is performed by heating heaters 21 and 22 such as halogen lamps as heating means disposed inside the fixing roller 11 and the pressure roller 12.
The temperature is detected by a thermistor 23 as a temperature measuring device disposed so as to be in contact with the surface of the fixing roller 11, and is controlled to maintain a predetermined value.

That is, information on the temperature detected by the thermistor 23 is input to a system controller (not shown) for controlling the operation of the entire image forming apparatus, and
The ON / OFF control of the heaters 21 and 22 is performed so that the surface temperature of No. 1 is maintained at the temperature Th required for fixing. In the conventional device shown in FIG.
Although the roller 3 contacts only the fixing roller 11, the roller 3 may contact the pressure roller 12 in some cases.

Such temperature control is performed to secure the amount of heat required for fixing. However, the amount of heat required for fixing is required to be larger as the amount of toner of the toner image to be fixed is larger. . In particular, a color image forming apparatus that performs multiple transfer of toner images of a plurality of colors onto one transfer material P and fixes the toner images requires a large amount of heat.

Therefore, in an electrophotographic color printer or a digital color copying machine, an image is read as a digital quantity, so that the control is performed by using the accumulated value of the image data as a control amount and varying the fixing speed in accordance with the image. A method has been proposed.

[0013]

However, the amount of the color toner often differs depending on the color, depending on the characteristics of the pigment or dye used as a colorant. In particular, pigments are widely used as toner colorants because of their excellent light and water resistance.However, when added to toner resins, pigments have a larger particle size than dyes and act as fillers in the resin. If the amount is increased, the softening temperature of the toner increases, and the fixing property may be deteriorated. However, in actuality, since priority is given to satisfying each desired color tone, saturation, coloring power, charging characteristics, etc., rather than satisfying sufficient fixing properties, M (magenta), C (cyan), Y (yellow),
For K (black), the amount of the pigment added varies for each toner color, and as a result, the fixability differs slightly for each color.

As described above, in a four-color full-color image forming apparatus, multiple transfer of toner images of a plurality of colors is performed on a transfer material. , And the fixability differs depending on the combination of the color and the load amount. In a color image forming apparatus using a color toner, since color reproduction is obtained by using the subtractive color mixing action, if the fixing property is different, the toner cannot be sufficiently melted and the toner color partially remains. In such a case, the color reproducibility is poor and the image quality is impaired. Furthermore, when the melting of the toner is insufficient and the binding property with the transfer material cannot be obtained,
The toner is peeled off from the transfer material, thereby deteriorating the image quality.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a color image forming apparatus capable of realizing good fixability and color reproducibility of a toner image. It is.

[0016]

According to a first aspect of the present invention, there is provided an image forming apparatus comprising: a step of forming an electrostatic latent image on a surface of an image carrier based on a color component signal; The steps of adhering toner to the image and developing it as a toner image and the step of transferring the toner image to an intermediate transfer member are repeated for a plurality of color toners so that a plurality of color toner images are superimposed on the intermediate transfer member. After that, in a color image forming apparatus for transferring a plurality of color toner images on the intermediate transfer body to a transfer material and heating the transfer material to fix the toner image, at least two color component signal image areas are formed. An overlapping area extracting means for extracting an overlapping area which is an overlapping portion, and a density signal of each color component signal at the pixel of interest extracted by the overlapping area extracting means are preset for each component signal. Comparing means for comparing the degree table,
Control means for controlling the amount of heat at the time of fixing the toner image in accordance with the comparison result of the comparison means.

According to a second aspect of the present invention, there is provided a step of forming an electrostatic latent image on the surface of an image carrier based on a color component signal, and a step of adhering toner to the electrostatic latent image and developing it as a toner image. And the step of transferring the toner image to an intermediate transfer member is repeated for toners of a plurality of colors so that toner images of a plurality of colors are superimposed on the intermediate transfer member. A color image forming apparatus for transferring an image to a transfer material and fixing the toner image by heating the transfer material, wherein a boundary extracting means having a predetermined density change based on the color component signal; and at least two color components An overlapping region extracting unit for extracting an overlapping region that is an overlapping portion of the image region of the signal; and a pixel of interest extracted by the boundary extracting unit and the overlapping region extracted by the overlapping region extracting unit. Determination means for determines whether to,
A comparison unit that compares a color component signal of the target pixel determined by the determination unit to be coincident with the overlap area with a density table that is set in advance for each component signal; and a comparison result of the comparison unit. Control means for controlling the amount of heat at the time of fixing the toner image in accordance with

According to a third aspect of the present invention, in the image forming apparatus of the first or second aspect, the control of the amount of heat at the time of fixing is performed by adjusting a fixing temperature.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 FIG. 1 shows an example of an image forming apparatus according to the present invention. The image forming apparatus shown in FIG.
It is an electrophotographic four-color full-color laser printer, and FIG. 1 is a longitudinal sectional view showing a schematic configuration thereof. Members having the same configuration and operation as those of the image forming apparatus shown in FIG. 9 are denoted by the same reference numerals, and description thereof will be omitted as appropriate.

The image forming apparatus shown in FIG. 1 includes a drum-type electrophotographic photosensitive member (hereinafter, referred to as “photosensitive drum”) 1 as an image carrier. As the photosensitive drum 1, for example, an O.D.
Those provided with a photosensitive layer such as PC (organic optical semiconductor) can be used. The photosensitive drum 1 is moved 12 times in the direction of arrow R1.
It is driven to rotate at a peripheral speed (process speed) of 0 mm / sec.

The surface of the photosensitive drum 1, first, by a charging roller 2 as a charging device, the dark portion potential V _D -700 V
Is uniformly charged so that Next, the first (black)
ON / O by the laser scanner 3 according to the image information of
Scanning exposure is performed by the FF-controlled laser beam. In the exposed portion, the bright portion potential _VL becomes -100 V, and a first (black) electrostatic latent image is formed.

The electrostatic latent image thus formed is developed (visualized) by the developing device 4. The developing device 4 includes a first toner containing a black toner as the first color toner.
Developing device 4a, a second developing device 4b containing magenta toner as the second color toner, a third developing device 4c containing cyan toner as the third color toner, and yellow as the fourth color toner A fourth developing device 4d in which toner is included, and a rotary support 4A on which these are mounted and rotatable in the direction of arrow R4 are provided. This rotary support 4A
Is rotated in the direction of the arrow R4, so that the developing devices to be used for development are sequentially arranged at the developing position facing the photosensitive drum 1. The above-described first electrostatic latent image is a first electrostatic latent image including a black toner as a first color toner.
Is developed as a black toner image by the developing device 4a. As a development method, a jumping development method or the like is used, and image exposure and reversal development are often used in combination.

The developed first toner image is transferred to transfer means 5
Is primarily transferred to an intermediate transfer belt (intermediate transfer body) 5a which constitutes a part of. The intermediate transfer belt 5a has, for example, a thickness of 5 mm.
It is made of a resin such as PVdF, PET, polycarbonate, polyethylene, or silicone prepared to have a volume of 0 to 200 μm and a volume resistivity of 10 ⁸ to 10 ¹⁴ Ω · cm. Also,
Urethane rubber, hydrin rubber, NBR, EP adjusted to a thickness of 0.3 to 2 mm and a volume resistivity of 10 ⁴ to 10 ⁸ Ω · cm
A rubber or resin prepared to have a thickness of 2 to 100 μm and a volume resistivity of 10 ⁸ to 10 ¹⁴ Ω · cm may be provided as a surface layer on an elastic base layer such as DM. The intermediate transfer belt 5a has a circumference set to 441 mm, and the roller 5
b, 5c and 5d. Intermediate transfer belt 5
“a” is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force by a primary transfer roller 6 disposed inside thereof, and forms a primary transfer nip portion T ₁ with the photosensitive drum 1. The intermediate transfer belt 5a is driven to rotate in the forward direction (arrow R5 direction) with respect to the rotational direction of the photosensitive drum 1 (arrow R1 direction) at substantially the same speed as the peripheral speed of the photosensitive drum 1. The toner image formed on the surface of the photosensitive drum 1 is
A voltage (primary transfer bias) having a polarity opposite to that of the toner is applied to the above-described primary transfer roller 6 by a primary transfer bias application power supply (not shown).
The primary transfer is electrostatically performed on the surface of the intermediate transfer belt 5. The primary transfer residual toner slightly remaining on the surface of the photosensitive drum 1 after the primary transfer has been completed is removed by the cleaning device 7.

Subsequently, the above-described series of image forming steps, ie, primary charging, exposure, development, primary transfer, and cleaning are repeated for the remaining three colors, and each time, the second toner image developed with magenta toner, cyan The third toner image developed with the toner and the fourth toner image developed with the yellow toner are sequentially transferred to the intermediate transfer belt 5.
It is primarily transferred to the surface and superimposed. As described above, the intermediate transfer belt 5 is charged during the repetition of the primary transfer process. However, when the toner images on the photosensitive drum 1 are sequentially superimposedly transferred onto the intermediate transfer belt 5, the primary transfer bias value is reduced by the residual charge. In consideration of this, each color has been modified. Normally, the primary transfer bias values are +140 V for the first color, +550 V for the second color, +620 V for the third color, and +620 V for the fourth color.
680V, and the primary transfer roller 6a
Is applied to

Thereafter, the secondary transfer roller 9 which has been separated from the surface of the intermediate transfer belt 5 is pressed against the surface of the intermediate transfer belt 5 with a predetermined pressing force.
A secondary transfer nip T2 is formed between the second transfer nip T1 and the _second transfer nip T2. The secondary transfer roller 8 is driven to rotate in the direction of arrow R8, and a voltage (secondary transfer bias) having a polarity opposite to the charging polarity of the toner is applied by a secondary transfer bias application power supply (not shown). By the secondary transfer bias + 2000V is applied in the present embodiment, the transfer material P surface conveyed at a predetermined timing to the secondary transfer nip T _2, superimposed on the intermediate transfer belt 5 surface 4 The color toner images are collectively secondary-transferred.

The transfer material P after the secondary transfer of the toner image is indicated by an arrow K
The sheet is conveyed to the fixing device 10 in the p direction, where it is heated and pressed to be fixed as a permanent image, and then discharged outside the apparatus (outside the image forming apparatus main body).

After the secondary transfer of the toner image, the intermediate transfer belt 5 has a secondary transfer residual toner slightly remaining on the surface thereof.
It is removed by a cleaning roller 9 brought into contact with the surface of the intermediate transfer belt 5 at a predetermined timing by a driving unit (not shown). This cleaning roller 9
For example, a thickness of 2 to 6 mm and a volume resistivity of 10 ⁴ to
On an elastic base layer such as rubber or sponge prepared to 10 ⁶ Ω · cm, a thickness of 10 to 300 μm and a volume resistivity of 10 ⁶ to 10
A surface layer of rubber, resin or the like adjusted to ¹² Ω · cm is formed, and the toner image on the surface of the photosensitive drum 1 is transferred to the intermediate transfer belt 5.
When the primary transfer is sequentially performed on the surface of the intermediate transfer belt 5a, the toner image on the surface of the intermediate transfer belt 5a is separated from the surface of the intermediate transfer belt 5a. , High-voltage power supply (not shown)
Applies a bias. As a result, the secondary transfer residual toner remaining on the surface of the intermediate transfer belt 5a is charged to a polarity opposite to the original charging polarity, and the first toner of the first color in the subsequent image forming process is continuously performed. at the same time the toner image is primarily transferred from the photosensitive drum 1 surface to the intermediate transfer belt 5a surface, in the primary transfer nip T _1, reverse transcribed from the intermediate transfer belt 5a surface to the photosensitive drum 1 surface, the intermediate transfer belt 5a The surface cleaning process is completed.

Next, the fixing device 10 according to the present invention will be described in detail with reference to FIGS. In this embodiment, the peripheral speed of the fixing roller 11 is set to 120 mm / sec. The fixing roller 11 had a diameter of 40 mm and a two-layer structure. The lower layer is an HTV silicone rubber elastic layer having a thickness of 2.1 mm and a rubber hardness of 3 mm (JIS-A). It is desirable to add an additive such as red iron or silica to improve the thermal conductivity. The surface release layer is made of 20 μm thick FEP or PTFE, C
It is formed as a fluorine resin tube or a coating layer of PTFE, PFA or the like. The pressure roller 12 is also 40 mm in diameter
In a two-layer configuration. The silicone rubber elastic layer below the pressure roller 12 has a thickness of 1.9 mm, and forms a surface release layer similarly to the fixing roller 11. The fixing roller 11 is a bearing 1
The pressure roller 12 is rotatably supported by a fixing frame 14 as a swing member via a bearing 15.
The pressing frame 16 is swingably supported by a support shaft 17 fixed to the fixing frame 14, and the pressing roller 12 is pressed against the fixing roller 11 by a pressing force of 40 kgf by a pressing spring 18 as pressing means. Have been biased to be. Driving force is applied to the fixing roller 11 by driving means (not shown), and the fixing roller 11 is driven and rotated in a state where the pressing roller 12 is pressed against the fixing roller 11 to form a fixing nip portion N therebetween. That is, the pressure roller 12 is configured to rotate following the fixing roller 11. The nip width of the fixing nip N was 9.5 mm at the center.
The core metal of the fixing roller 11 is a hollow cylindrical body, and has a halogen heater 21 as a heat source inside. Pressure roller 1
2 also has a built-in halogen heater 22.

In FIG. 2, reference numeral 59 denotes a cam as a pressure releasing means which is driven to rotate by a driving means (not shown). The cams are fixed to both sides of a cam shaft 20 rotatably supported by the fixing frame 14, respectively. The camshaft 20 rotates together with the camshaft 20. Then, at the time of non-image formation, the cam 19 is rotated to release the pressure, and the contact portion between the fixing roller 11 and the pressure roller 12 is prevented from being deformed.

On the other hand, as shown in FIG. 3, the temperature control of the roller is performed by a thermistor 23 as a temperature measuring device which is brought into contact with the pressure roller 12 near the center of the paper passing area. The surface temperature of the roller is detected by the thermistor 23, and the halogen heater 2 is set so that the roller surface temperature becomes a predetermined value.
The ON / OFF control of each of the first and the second is performed. The thermistor 23 is made of PTFE, CPTFE, PFA
And the like, thereby improving the releasability of the toner. Further, as shown in FIG. 1, the thermistor 23 may be disposed so as to contact the fixing roller 11 side.

Next, the toner used in the present embodiment will be described in detail. As the colorant of the toner, known ones can be used, for example, carbon black; I. Pigment Red 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 1
9, 21, 22, 23, 30, 31, 32, 37, 3
8, 39, 40, 41, 48, 49, 50, 51, 5
2, 53, 54, 55, 57, 58, 60, 63, 6
4, 68, 81, 83, 87, 88, 89, 90, 11
2, 114, 122, 123, 163, 202, 20
6, 207, 209; I. Pigment Violet 19; I. Butt Red 1, 2, 10, 13, 1
5, 23, 29, 35; I. Solvent Red 1,
3, 8, 23, 24, 25, 27, 30, 40, 81,
82, 83, 84, 100, 109, 121; I.
Disperse Red 9; I. Solvent violet 8, 13, 14, 21, 27; I. Oil-soluble dyes such as Disperse Violet 1; I. Basic Red 1, 2, 9, 12, 13, 14, 15, 17, 1
8, 22, 23, 24, 27, 29, 32, 34, 3
5, 36, 37, 38, 39, 40; I. Basic violet 1, 3, 7, 10, 14, 15, 21,
Basic dyes such as 25, 26, 27 and 28; I. Pigment blue 2, 3, 15, 16, 17; I. Bat blue 6; I. Acid blue 45; I.
Pigment Yellow 1, 2, 3, 4, 5, 6, 7, 1
0, 11, 12, 13, 14, 15, 16, 17, 2
3, 65, 73, 83; I. Bat Yellow 1,
3, 20 and the like, and these can be used alone or in combination. In the present embodiment, in order to satisfy desired color tone, saturation, light fastness, water fastness and the like, C.I. I. Pigment Red 1
22 and C.I. I. Pigment Violet 19, C
(Cyan) colorant as C.I. I. Pigment Blue 15, Y (yellow) colorant as C.I. I. Pigment Yellow 17, carbon (black) is used as a K (black) colorant.

Although the desired color tone and saturation can be obtained with these coloring agents, the number of parts added to the toner is different due to the different coloring power of the pigment, and the fixing property is different as described above. There is a melt index as a means for examining the fixability of the toner. Melt index is based on Japanese Industrial Standards' thermoplastic plastic flow test method "JIS K7
210-1976 ", and was measured by manual cutting under the following measurement conditions and measured for 10 minutes (10 min.
)), The value converted to the extrusion weight g of the toner composition is used.

Measurement temperature: 135 ° C. Load: 7.16 kg Sample filling amount: 4 g The melt index of the toner used in the present embodiment is:
2.0g / 10min for color M, 2.5g / 10min for color C
, Y color was 5.0 g / min, and K color was 9.0 g / min. The melt index value indicates the easiness of melting of the toner when heated and pressed, and the larger the numerical value is, the easier the toner is to be melted, indicating that the toner is more easily fixed in the fixing step by the heating and pressing fixing device 10. . As described above, since the melt index value differs for each color, the fixability differs depending on the form of color superimposition at the time of secondary color generation. Therefore, in the present embodiment, since the melt index values of the M color and the C color are both small, for example, the color overlap of the M color and the C color and the color overlap of the C color and the Y color are the same. It can be seen that the color superposition of the color C and the color C is inferior in fixability.

Therefore, in the present embodiment, an area where the colors overlap (overlap area) is extracted, and particularly when the M color and the C color overlap, the fixing temperature is increased to secure the fixability.

The image processing section 32 shown in FIG. 1 will be described.
First, an image signal input from a data supply source such as a host computer is captured by an image input unit 31 including a communication controller and a reception buffer. The fetched image signal is output to the developing unit 4 in the image processing unit 32 shown in FIG.
At 1 the developed color is separated. The developing unit 41 has image memories 41m, 41c, 41y, and 41k for four colors, and converts an input image signal into four colors of magenta (M), cyan (C), yellow (Y), and black (K). And develops them on the corresponding image memory.

The analyzing section 42 is constituted by an overlapping area extracting means and a comparing means. The analyzing section 42 is an area where the image areas of at least two colors are overlapped, and the signal level of each color is a certain level or a certain level or more. In addition to detecting an overlap area, the density data of the image color in the overlap area is compared with each other. If the density data is larger than a predetermined density ratio, an excess signal is supplied to the system controller (control means) 34. In parallel, the image signal of each color supplied from the developing unit 41 is supplied to the signal converting unit 43. The signal conversion unit 43 sequentially converts the input drawing signal into an analog drawing signal and supplies the analog drawing signal to the laser driver 33. As shown in FIG. 5, the above-described overlapping area extracting means mainly includes a part of the shift registers 51 to 54 and the area extracting sections 55 to 58.
And a part of the overlapping area density comparing unit 63. The comparing means is mainly constituted by an overlapping area density comparing section.

FIG. 5 is a block diagram showing a configuration example of the analysis unit 42. The analysis unit 42 corresponds to the multi-valued image signal (M, C, Y, K) of each color, and the number of shift registers 51 to 54, the area extraction units 55 to 58, and the density corresponding to the bit width of each image signal. It has detection units 59 to 62. Each of the area extracting units 55 to 58 is provided with a corresponding shift register 51 to 5.
At 4, an area having an image forming signal is extracted based on the serial / parallel-converted image signal (parallel output signal). Each of the density detectors 59 to 62 detects density data in the corresponding shift register 51 to 54 based on the image signal. In the present embodiment, the image density signal is 8
Sent by a bit signal, the maximum density FFh is set to 100%,
The processing was performed with the minimum density of 00h being 0%.

The overlapping area density comparing section 63 first determines whether or not each of the image signals overlaps in the area detected by the area extracting sections 55 to 58. Compare the density data of the overlapping image colors. When it is determined that the density ratio is larger than the predetermined density ratio, an excess signal is output to the system controller 34.
Specifically, the overlapping area density comparing section 63 has a comparison table (density table) 64 shown in FIG. 6, and compares and determines the density data of each color. For example, if the density level of the M color is 9
When the area is 0%, when the C color exceeds 90% and the areas overlap, an excess signal is output, and when the C color is 90% or less, a determination process is performed such that no excess signal is output. The comparison table 64 is referred to by rounding up the first digit. For example, when the density data is 85%, the part of the density data 90% is referred to.

When the analyzing section 42 determines that the density data in the overlapping area is larger than the threshold value defined in the comparison table 64, an excess signal is supplied to the system controller 34. The system controller 34 to which the excess signal has been supplied sends a control signal to the heater control unit 35 (see FIG. 1) to heat the heaters 21 and 22. The heating state is monitored by a thermistor 23 arranged in contact with the fixing roller, and monitored by a system controller 34.

In this embodiment, when the excess signal is supplied to the system controller 34, the fixing temperature is controlled to 180 ° C. with respect to the normal fixing temperature of 170 ° C., so that the M color is 80% to 100%. And C color 80% -100%
, Good fixability was obtained even in the overlapped image. It is preferable that the timing of this control be determined during the pre-multi-rotation, which is a stage before image formation, and that the fixing device 10 be controlled to a control temperature at least during the image forming process.

As a comparative example, a fixing temperature of 17 in a normal state
When a color image was formed at 0 ° C. while varying the density data of M and C colors, the toner color remained when the total density data of M and C colors exceeded 180%. Insufficient fixing occurred, and when the total density data was 200%, incomplete fixing such that a part of the image peeled off occurred.

Note that the black (K) image signal may be a target for the extraction of the overlapping area. However, the black image signal may generally constitute a character alone or be used as an outline. Because of the large number, the overlapping area need not be extracted. Further, as described above, the K color toner was not targeted for the extraction of the overlapping area because its melt index value was larger than that of the other colors and the fixability was good.

As described above, in a color image forming apparatus using toner, based on an input image signal, the presence / absence of an overlapping area where colors overlap and the manner in which toner colors overlap in that area are determined by the following method. By controlling the fixing temperature,
Good fixing performance was obtained.

In the present embodiment, the melt index value increases in the order of C color <M color <Y color <K color, and the comparison table 64 optimal for the toner is prepared and described. The value of the melt index differs depending on the agent and the amount added, and the order of colors may also differ. In such a case, it is needless to say that the prepared comparison table and the fixing control temperature may be optimized in accordance with the gist of the present invention.

In this embodiment, when an excessive signal is output from the analysis unit 42 to the system controller 34, the fixing temperature is raised by 10 ° C. uniformly to secure the fixing performance. The excess signal
Unnecessary consumption of the heater current can also be prevented by using a level signal of several stages by several bit signals and controlling the heater temperature adjustment stepwise according to the output level of the level signal.

The configuration of the analysis unit 42 is not limited to the configuration shown in FIG. 5, but is an area where at least two color image areas overlap, and the signal level of each color is a constant level or a constant level. In addition to detecting the overlapping area as the above area, comparing the density data of the image colors in the overlapping area, and supplying an excess signal to the system controller (control means) 34 when the density data is larger than a predetermined density ratio. Of course, other configurations may be used.

<Embodiment 2> FIG. 7 shows a block diagram of an analysis unit 42 according to Embodiment 2. Note that the image forming apparatus according to the present embodiment is the same as in the first embodiment, and a detailed description thereof will be omitted.

In the present embodiment, in the analyzing section 42 of the image processing section 32, an edge detecting section (boundary extracting means) 6 for each color.
5 to 68, characterized by processing.

In an image forming apparatus using an electrophotographic method, when there is a non-image portion adjacent to an image portion, a phenomenon that the amount of loaded toner at an edge portion which is a boundary of the image portion increases, a so-called edge effect occurs. Has been known for some time. This phenomenon is affected by the contrast of the electrostatic latent image on the photosensitive drum 1, and becomes more conspicuous as the contrast increases, that is, as the image density increases.

Therefore, in the edge portion, the amount of loaded toner is larger than that in the central portion of the image. If such an edge portion is present at the time of color superposition, the amount of toner at the edge portion is increased and the fixability is impaired. Failure may occur.

The analysis unit 42 in the image processing unit 32 shown in FIG.
In order to solve the above-mentioned problem, the edge detection units 65 to 65
68 to detect the presence or absence of an edge portion.

Each of the edge detectors 65 to 68 detects an edge portion based on the image signal subjected to serial / parallel conversion in the corresponding shift register 51 to 54. The edge portion is detected by a known image density V of the target pixel n.
n, and the image density V (n-
1) and the image density V (n) of the (n + 1) pixel immediately after one pixel
+1) and a decrease ΔV between Vn and V (n−1).
(N−1) and the decrease Δ (n +) between Vn and V (n + 1).
1) is calculated, and when the difference between Δ (n−1) and Δ (n + 1) is equal to or greater than a predetermined value, the n-th pixel is determined to be an edge.

The overlapping area density comparing section (judging means, comparing means) 69 receives the overlapping areas of the colors and the density data of these colors described in the first embodiment, and additionally, When it is determined by the edge detectors 65 to 68 that the input signal is an edge portion, the comparison data is referred to, and a match signal is output to the system controller 34 according to the determination result.
Is output to

Since the M color and the C color according to the present embodiment are harder to fix than the Y color, they are easily affected by the fixing property at the edge portion, and the overlapping area other than the edge portion when the M color is 100%. In the case of C, good fixability was obtained at a control temperature of 170 ° C. for up to 80% of C color and 100% of Y color.
In the case of the edge portion at 0%, up to 70% of the C color and 95% of the Y color can be fixed at the control temperature of 170 ° C., and the range of the density data of the C color and the Y color at which good fixing can be performed becomes small. Was.

FIG. 8 shows a comparison table 70 in the case where each of the colors is judged to be an edge by the edge detectors 65 to 68. Comparison table 70 shown in FIG.
Is set based on the fixing property when the fixing device 10 is controlled at 170 ° C. in a normal state.

If it is determined that the density data of the color overlapping the color forming the edge portion is larger than the comparison table 70 shown in FIG. The system controller 34 that has been output to the controller 21 and supplied with the excess signal sends a control signal to the heater control unit 35 to heat the heater 21,
Heat 22. The heating state is monitored by a thermistor 23 arranged so as to be in contact with the fixing roller 21, and is monitored by a system controller 34.

In the present embodiment, when an excessive signal is supplied to the system controller 34, the fixing temperature is controlled to 180 ° C. with respect to the normal fixing temperature of 170 ° C., so that the overlapping area including the edge portion is controlled. Also, good fixability was obtained.

In this embodiment, when the density data for forming the edge portion is 80% or less for the M color, 80% or less for the C color, and less than 100% for the Y color, good fixing performance can be obtained even at the normal control temperature. Was done.

As described above, the edge portion is detected from the image data, the threshold value of the overlapping color is set in accordance with the density data of the image forming the edge portion, and it is determined whether or not the threshold value is exceeded. By increasing the fixing temperature when the temperature is equal to or higher than the threshold value, it is possible to improve the fixability of a color image including an edge portion.

In each of the above embodiments,
Although the case where the image data is 8-bit multi-valued data has been described, the present invention is not limited to this example, and can be applied to image data represented by a bit number other than 8 bits. It is.

The configuration of the analyzing section 42 is not limited to the configuration shown in FIG. 7, and it goes without saying that another configuration may be used as long as the same action can be obtained. .

In the first and second embodiments, the electrophotographic image forming apparatus has been described. However, the present invention is not limited to this. For example, the present invention is applied to an electrostatic recording type image forming apparatus. The same can be applied.

[0064]

As described above, according to the present invention,
Extracting an overlapping area that is an overlapping portion of the image areas of at least two color component signals, appropriately changing a method of comparing density data according to a color forming the extracted overlapping area, and controlling a fixing heat amount according to the comparison result. Thus, a stable color image can be fixed while maintaining good color reproducibility. Further, by controlling the amount of fixing heat in accordance with the color constituting the edge portion of the image, a stable color image can be fixed even at the edge portion.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a first embodiment.

FIG. 2 is a longitudinal sectional view illustrating a schematic configuration of a fixing device according to the first embodiment.

FIG. 3 is a diagram illustrating the arrangement position of a thermistor.

FIG. 4 is a block diagram illustrating a configuration of an image processing unit according to the first embodiment.

FIG. 5 is a block diagram showing a configuration of an analysis unit according to the first embodiment.

FIG. 6 is a diagram showing a comparison table of density data according to the first embodiment.

FIG. 7 is a block diagram illustrating a configuration of an analysis unit according to the second embodiment.

FIG. 8 is a diagram showing a comparison table of density data according to the second embodiment.

FIG. 9 is a longitudinal sectional view showing a schematic configuration of a conventional image forming apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Image carrier (photosensitive drum) 2 Primary transfer roller 3 Exposure device 4 Developing device 5 Intermediate transfer member (intermediate transfer belt) 10 Fixing device 34 Control means (system controller) 55-58 Overlap area extraction means (area extraction part) 63 Comparison means (overlap area density comparison unit) 64 Density table (comparison table) 65-68 Boundary extraction means (edge detection unit) 69 Comparison means, judgment means (overlap area density comparison unit) P Transfer material

Claims (3)

[Claims] A step of forming an electrostatic latent image on the surface of an image carrier based on a color component signal; a step of attaching toner to the electrostatic latent image to develop the toner image as a toner image; The step of transferring to the transfer body is repeated for toners of a plurality of colors to overlap the toner images of a plurality of colors on the intermediate transfer body, and thereafter, the toner images of the plurality of colors on the intermediate transfer body are transferred to a transfer material. A color image forming apparatus that heats the transfer material to fix the toner image, wherein: an overlapping area extracting unit that extracts an overlapping area that is an overlapping portion of image areas of at least two color component signals; Comparing means for comparing the density signal of each color component signal at the pixel of interest extracted with the density table preset for each component signal; and And control means for controlling the amount of heat at the time of fixing the toner image. 2. A step of forming an electrostatic latent image on the surface of an image carrier based on a color component signal, a step of attaching toner to the electrostatic latent image and developing it as a toner image, The step of transferring to the transfer body is repeated for toners of a plurality of colors to overlap the toner images of a plurality of colors on the intermediate transfer body, and thereafter, the toner images of the plurality of colors on the intermediate transfer body are transferred to a transfer material. A color image forming apparatus that heats the transfer material to fix the toner image, a boundary extracting unit having a predetermined density change based on the color component signal, and an overlapping portion of at least two color component signal image areas. And an overlapping area extracting unit for extracting an overlapping region, and determining whether or not the target pixel extracted by the boundary extracting unit matches the overlapping region extracted by the overlapping region extracting unit. Judging means; comparing means for comparing a color component signal at the target pixel, which is determined by the judging means to be coincident with the overlapping area, with a density table preset for each component signal; Control means for controlling the amount of heat at the time of fixing the toner image in accordance with the comparison result of the means. 3. The color image forming apparatus according to claim 1, wherein the control of the amount of heat at the time of fixing is performed by adjusting a fixing temperature.