JP3519933B2 - Color image forming equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image bearing member carrying a toner image, charging means for charging the image bearing member, and electrostatic charging by irradiating the image bearing member charged by the charging means with light. Exposure means for forming a latent image, developing means for supplying toner to the image carrier to develop the electrostatic latent image formed by the exposure means to form a toner image of each color, and toner on the image carrier A plurality of transfer means for transferring an image to a sheet are respectively provided, and the plurality of image carriers are sequentially arranged along the sheet conveyance path, and toner images of respective colors are formed on the sheet conveyed through the conveyance path. The present invention relates to a color image forming apparatus that sequentially superimposes a color image to form a color image.

[0002]

2. Description of the Related Art Conventionally, toner images of multiple colors (toner images)
There is a color image forming apparatus that forms a single color image by superimposing sheets of paper on each other. This color image forming apparatus uses a photosensitive drum, which is an image carrier corresponding to each color (black, cyan, magenta, and yellow), as a sheet. Are arranged in parallel along the transport path (transfer belt for electrostatically attracting and transporting paper), and a charging device around each photoconductor drum, a laser unit for exposing, a developing device, a transfer device, and photoconductor cleaning. A device and a charge-removing lamp are arranged to form a photoconductor unit, and toner images of respective colors are sequentially superposed on a sheet conveyed through a conveyance path to form one toner image, so that a single toner image is formed on the sheet. A color image was formed by the transport operation.

According to the above-mentioned color image forming apparatus, the surface of each photoconductor drum is uniformly charged by the charging device corresponding to each color, and the photoconductor drum of each color is exposed by the laser unit so that the surface of the photoconductor drum is exposed. An electrostatic latent image is formed, and the electrostatic latent image is developed by a developing device having toner of each color to form a toner image of each color on the photosensitive drum. Then, the toner image on the photosensitive drum of the first color is transferred onto the paper electrostatically adsorbed on the transfer belt by the transfer device, and then the second color toner image is transferred onto the transferred toner image of the first color. A color image is formed by sequentially superposing the toner images of the third and fourth color photosensitive drums and fusing and fixing the superposed toner images by a fixing device.

In the transfer process of each color described above, by applying a positive potential to the sheet that is electrostatically adsorbed on the transfer belt and conveyed, a negatively charged toner image is transferred from the photosensitive drum onto the sheet. Transferred on paper.

In such a transfer system, the toner images of the second and subsequent color photosensitive drums, which are disposed downstream of the first color photosensitive drum in the conveying direction, are toner images already on the sheet. Because it will have to be transferred onto
When the toner image is transferred, the negative charges flow from the photoconductor drum to the surface of the transfer belt, so that the negative charges are accumulated on the sheet, and the negative charges are accumulated on the sheet each time the transfer operation is repeated for each color. By accumulating the negative charges on the paper in this way, the transfer efficiency of the toner image onto the paper is deteriorated and the image quality is greatly deteriorated.

As means for preventing such deterioration of transfer efficiency, a method of increasing transfer voltage in order and a method of pre-transfer charging the toner upstream of the transfer device for transferring the toner image on the photosensitive drum in the rotation direction of the photosensitive drum. There is a method of increasing the charge amount of the toner of the toner image on the photosensitive drum by providing a pre-transfer charging device.

When this pre-transfer charging device is used, a voltage in which an AC voltage and a DC voltage are superimposed is applied to the pre-transfer charging device so that the surface potentials of the photosensitive drums are made uniform and the charge amounts of the respective toners are made uniform. By doing so, there is a method of improving the transfer efficiency (Japanese Patent Laid-Open No. 63-55569).

[0008]

However, in the case of using the above-mentioned pre-transfer charging device, it is not necessary to perform pre-transfer charging as in the case of the first color where the toner image does not exist on the paper. In addition, since the pre-transfer charging device is provided, the number of pre-transfer charging devices increases, which causes a cost increase, and there is a problem that ozone generation increases due to unnecessary charging operation of the pre-transfer charging device. .
Moreover, the pre-transfer charging device provided corresponding to the photoconductor drums of each color does not form the optimum transfer conditions for the photoconductor drums (toner images) of all the colors, and thus the transfer efficiency cannot be sufficiently improved. There was a problem.

In the case of increasing the transfer voltage in order to improve the transfer efficiency, the larger the transfer voltage, the more positive charges flow into the photosensitive drum. The positive charge that has flowed into this photosensitive drum cannot be neutralized by the neutralizing lamp because the photosensitive drum does not have a photoconductive property with respect to the positive charge.
As it is, negative charging is performed by the charging device (men- ger charger), and the positive charge due to the transfer and the negative charge by the charging device cancel each other, so that there is a problem that the surface potential of the photosensitive drum decreases.

Moreover, the decrease in the potential of the photosensitive drum affects the amount of toner adhered when a toner image is formed and directly affects the image. Further, the higher the transfer voltage is, the more current flows into the photoconductor drum, which may damage the photoconductor drum itself.

According to the present invention, by providing the pre-transfer charging means on the image carrier for the second and subsequent colors, which is on the downstream side of the first-color image carrier in the sheet conveying direction, the unnecessary pre-transfer charging means is eliminated. It is intended to suppress the generation of ozone, optimally control the charge amount of the toner on the image carrier to improve the transfer efficiency, and perform good transfer at a lower transfer voltage.

[0012]

In order to achieve the above object, the invention according to claim 1 is characterized in that an image carrier for carrying a toner image, a charging means for charging the image carrier, and a charging means. Exposure means for irradiating the charged image carrier with light to form an electrostatic latent image, and toner for each color developed by supplying toner to the image carrier to develop the electrostatic latent image formed by the exposing means. And a transfer means for transferring the toner image on the image carrier onto the paper, and the plurality of image carriers are sequentially arranged along the paper transport path. In a color image forming apparatus that forms a color image by sequentially superimposing toner images of respective colors on a sheet conveyed through a conveying path, the color image forming apparatus should be arranged in the sheet conveying direction of the image carrier.
Each image bearing located downstream of the image carrier at the most upstream position
Each of the holding members is provided with a pre-transfer charging unit that charges the toner before the transfer of the toner image by the transfer unit.
Do not provide pre-transfer charging means only for the image carrier at the flow position.
Yes .

According to a second aspect of the present invention, each of the image bearing members is
In image formation, the toner on the image carrier at the most upstream position
-Without image formation, the image carrier after that does not form two or more colors.
When forming a color image with a toner image , the first
To clear the pre-transfer charging means of the image bearing member as a color, it
There is provided control means for turning on the pre-transfer charging means of the image carrier of the second color located on the downstream side of the twist .

According to the third aspect of the invention, there is provided control means for controlling the output of the pre-transfer charging means for each color in accordance with each color.

According to a fourth aspect of the present invention, output setting means for setting the outputs of the plurality of pre-transfer charging means are provided so that the charge amount of the image carrier on the downstream side in the sheet conveying direction is sequentially increased. .

According to a fifth aspect of the present invention, there is provided output setting means for setting the outputs of the plurality of pre-transfer charging means in accordance with the environmental conditions or the type of paper.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of an embodiment of a digital color copying machine which is a color image forming apparatus of the present invention will be described with reference to FIGS.

In FIG. 1, reference numeral 1 is a transparent document placing table, and 2
Is an exposure optical system, 3a to 3d are laminated OPC photosensitive drums for respective colors, 4a to 4d are developing devices including a developer including a magnetic carrier for each color and toner, and 5 are electrostatically attracted and conveyed. A black transfer belt in which carbon is dispersed in polyimide, 6a and 6b are size-dependent sheet feeding cassettes, 7a to 7d are charging devices for charging the photoconductor drums 3 of respective colors, and 8a to 8d are photosensitizers of respective colors. A brush-type transfer device for transferring the toner image on the body drum 3 to a sheet, a fixing device 9 for melting and fixing the toner image formed on the sheet, 10a.
10d are laser units for exposing the surface of the photoconductor drum 3 for each color to form an electrostatic latent image, and 11b to 11d are second laser units.
This is a pre-transfer charging device that charges the pre-transfer toner on the photosensitive drums 3b to 3d for the colors after.

The exposure optical system 2 includes a light source lamp 2a for irradiating a document placed on the document table 1 with light, and a zoom lens 2b for reflecting light from the document, for example, as shown by a chain line in the figure. It is provided with a plurality of reflecting mirrors 2d, 2e, 2f which lead to the CCD sensor 2c via the. This CCD sensor 2
1-line CCD is used in c, and RGB sensor is provided in front of CCD sensor 2c to obtain electric signals of each color component.
Color separation filter (not shown) is provided, and the light transmitted through this color separation filter is transmitted to an image processing circuit described later as electrical signal data of each color component.

Around the photosensitive drums 3a to 3d,
As shown in FIG. 2, the charging devices 7a to 7d and the laser units 10a to 10d are arranged along the rotation direction of the photosensitive drum itself.
d, developing devices 4a to 4d, transfer devices 8a to 8d via the transfer belt 5, photoconductor cleaning devices 12a to 12d for cleaning the residual toner on the photoconductor drum 3, and a discharge lamp 13a for removing the electric potential of the photoconductor drum 3. ~ 13d
Are arranged respectively. Further, pre-transfer charging devices 11b to 11d are arranged on the upstream side of the transfer devices 8b to 8d of the photosensitive drums 3b to 3d for the second and subsequent colors.

The transfer belt 5 is stretched around a driving roller 5a and a driven roller 5b, and the transfer belt 5 is rotationally driven by the rotation of the driving roller 5a. Contact the photoconductor drums 3a to 3d of the respective colors, and the transfer devices 8a to 8d of the respective colors via the transfer belt 5 respectively correspond to the photoconductor drums 3a.
Pressed to ~ 3d. A sheet suction roller 14 for electrostatically attracting a sheet to the transfer belt 5 is provided above the drive roller 5a, and a cleaning device 15 for cleaning the transfer belt 5 is provided below the driven roller 5b. It is arranged.

Next, the copying operation at the time of color copying of the digital color copying machine having the above structure will be briefly described.

First, a light source lamp 2a irradiates an original document placed on the original document table 1 to perform one scan, and reflected light reflected by the original document is reflected by mirrors 2d to 2f at a zoom lens 2d and RGB. An image is formed on the CCD sensor 2c through the color separation filter of R, and at this time, the image data is converted into an electrical signal of each color component of RGB, and the data (R
GB signal) is transmitted. This transmitted data (RG
The B signal) is further converted by the image processing circuit into YMC and K electric signals subjected to blackening processing, and this signal is transmitted to the laser units 10a to 10d for each color.

In response to this signal, the laser units 10a to 10d for the respective colors irradiate the laser beams onto the photosensitive drums 3a to 3d which are uniformly negatively charged by the charging devices 7a to 7d, so that the photosensitive drums 3a to 3d are imaged. Exposed photoconductor drum 3
An electrostatic latent image is formed on a to 3d. After that, the electrostatic latent image is visualized by the developer in each of the developing devices 4a to 4d to form a toner image on the photosensitive drums 3a to 3d.

In the developing devices 4a-4d, there are magnetized rotating electrodes forming a magnetic brush, and a bias voltage is applied to the rotating electrodes during development so that the photosensitive drums 3a-3d are formed.
The negative polarity toner in the developer moves to the surfaces of the photosensitive drums 3a to 3d due to the potential difference between the electrostatic latent image and the electrostatic latent image, and a toner image is formed. This photoconductor drum 3
Photosensitive drums 3b to 3d for the second and subsequent colors of a to 3d
The toner image of each color above is formed by the pre-transfer charging device 11b for each color.
Pre-transfer charging is performed by 11d to adjust the charge amount of the toner.

On the other hand, the paper fed from one of the paper feed cassettes 6a and 6b passes through the paper feed timing roller 16 while passing the paper, and the bias voltage is applied to the paper to electrostatically attract it onto the transfer belt 5. Applied paper suction roller 14
As a result, the sheet that has passed the sheet feed timing roller 16 is electrostatically attracted to the transfer belt 5. The electrostatically attracted sheet is conveyed toward the fixing device 9 by the rotation of the transfer belt 5.

At this time, the toner images formed on the photosensitive drums 3a to 3d are transferred to the transfer devices 8a to 8d below the transfer belt 5 while being in contact with the conveyed paper, and have a polarity opposite to that of the toner. By applying the bias voltage,
This toner image is electrically transferred onto the paper. This operation is sequentially performed for each color with a time difference, and a toner image in which four color toner images are superimposed is formed on the conveyed paper.

After that, the sheet is peeled from the transfer belt 5 by a peeling claw (not shown) provided on the downstream side of the transfer belt 5 in the sheet conveying direction (on the side of the fixing device 9) and is introduced into the fixing device 9, and the fixing device 9 is attached. By melting the toner on the paper, a full-color image is formed, and the full-color image is fixed on the paper.

On the other hand, the residual toner on the photoconductor drums 3a to 3d after the transfer is removed by the photoconductor cleaning devices 12a to 12d.
To be cleaned by the blade. After that, the photoconductor drums 3a to 3d are removed by the charge eliminating lamps 13a to 13d.
The unnecessary charges above are removed. At the same time, the transfer belt 5
The residual toner on the top is also cleaned by the blade of the cleaning device 15. The above is the image forming process during normal color copying by the digital color copying machine of this embodiment.

The digital color copying machine used in this embodiment has the photosensitive drums 3a to 3d, the charging devices 7a to 7d, and the developing device 4 in the order of black, cyan, magenta, and yellow.
a to 4d, photoconductor cleaning devices 12a to 12d, charge removal lamps 13a to 13d, and the like are arranged as photoconductor units. At this time, the optimum transfer voltage for each color is set to the minimum transfer voltage at which the image density is constant.

In such a digital color copying machine, when an image is formed by using all four colors (black, cyan, magenta and yellow), the pre-transfer charging device 11 is used.
FIG. 3 shows the relationship between the transfer voltage and the image density when all the outputs b to 11d are turned off. As shown in FIG. 3, after the second color (cyan), the optimum transfer voltage for the density gradually shifts to the high voltage side (right side in the figure), and the transfer voltage for the fourth color (yellow) at this time is 3. The voltage becomes 5 kV, and there is a risk that the surface potential of the photoconductor drum 3 will decrease and the photoconductor drum 3 itself will be damaged.

Further, as the transfer is performed, the residual charges are accumulated on the sheet, and the accumulated amount of the residual charges is increased. This residual charge acts in the direction of hindering the transfer of the negatively charged toner particles to the sheet, and the transfer efficiency decreases as the residual charge increases. This decrease in transfer efficiency can be prevented by increasing the transfer voltage, but the risk of damaging the photosensitive drum 3 increases due to the increase in the transfer voltage.

Therefore, in order to suppress the transfer voltage and improve the transfer efficiency, the optimum toner charge amount is adjusted by the pre-transfer charging devices 11b to 11d, and the toner charge amount is increased toward the photosensitive member unit on the downstream side in the paper transport direction. Needs to be increased. At this time, the polarities of the pre-transfer charging devices 11b to 11d have a positive polarity when the toner charge amount is small, and a negative polarity when the toner charge amount is large, and are controlled so that the transfer efficiency is the highest.

When forming a toner image on the photosensitive drum 3, if the toner charge amount is low, the toner is fogged in a white background area, and if the toner charge amount is high, the image density is lowered. Therefore, when forming a toner image on the photoconductor drums 3a to 3d, the normal toner charge amount for each color is -25 μq.
/ M.

The toner charge amount that maximizes the transfer efficiency is
Depending on the configuration of the transfer device 8 (including, for example, the resistance of the transfer belt, the contact pressure between the transfer belt and the photosensitive drum, the process speed of the transfer process, the type and method of the transfer electrode, etc.), the toner charge amount and the transfer amount are different. The efficiency relationship is shown in FIG.

In FIG. 4, the contact pressure and the process speed are fixed by the structure of the transfer device 8 to form the resistance of the transfer belt (the transfer system is the brush system) A: 10 ⁹ Ω / cm ³ , the structure (the transfer system is (Roller system) B: The relationship between the toner charge amount and the transfer efficiency is shown for 10 ¹⁰ Ω / cm ³ .

In configuration A, the toner charge amount that maximizes the transfer efficiency is smaller than the charge amount during development. Therefore, the pre-transfer charging device is output by the positive electrode to control the toner charge amount so that the transfer efficiency is increased. To do. On the other hand, in the case of the configuration B, the toner charge amount that maximizes the transfer efficiency is larger than the charge amount at the time of development. Therefore, the pre-transfer charging device is output with the negative electrode, and the toner charge amount is controlled so that the transfer efficiency is increased. .

That is, when the toner charge amount that maximizes the transfer efficiency is smaller than the charge amount at the time of developing on the photosensitive drums 3a to 3d, the positive polarity is output, and when the toner charge amount is large, the negative polarity is output to the pre-transfer charging device. It

As shown in FIG. 5, the toner of each color has a larger charge amount as the output current of the pre-transfer charging devices 11b to 11d increases, but the output of the pre-transfer charging devices 11b to 11d for each color increases. The charging characteristics for current are different. This is because the toners of the respective colors are composed of different pigments and the like, and therefore have different electrical characteristics.

By performing the pre-transfer charging by using the pre-transfer charging devices 11b to 11d, the transfer when the transfer amount of the toner images on the photosensitive drums 3b to 3d of the second and subsequent colors is adjusted and transferred. 6 and 7 show the relationship between the voltage and the image density.
Is shown in. At this time, in FIG. 6, the pre-transfer charging device 11b is used.
The outputs of ˜11d are equal to −100 μA for cyan, magenta, and yellow, respectively.

In FIG. 7, the output currents of the pre-transfer charging devices 11b to 11d are cyan as shown in Table 1 based on FIGS. 4 and 5 in consideration of the decrease in transfer efficiency due to the residual charge on the paper. Pre-transfer charging device 11b: -100 μA,
The magenta pre-transfer charging device 11c is set to −170 μA, and the yellow pre-transfer charging device 11d is set to −350 μA.

[0042]

[Table 1]

Therefore, when the output currents of the pre-transfer charging devices 11b to 11d are the same for the respective colors, the transfer voltage for obtaining a better toner density in the photoconductor units for the second and subsequent colors is required as compared with the conventional one. Therefore, the transfer efficiency can be improved.

However, the pre-transfer charging devices 11b-1
When the output current of 1d is the same, as shown in FIG. 6, the improvement of transfer efficiency (setting of the optimum toner charge amount) is not optimal, and the toner image on the photosensitive drum 3 on the downstream side in the paper transport direction is not. In order to sequentially increase the toner charge amount of each of the pre-transfer charging devices 11b to 11 based on the charging characteristics of each toner.
By setting (adjusting) the output current of d as shown in Table 1 above, it is possible to realize the optimum transfer efficiency for each color as shown in FIG. As a result, it is possible to suppress an increase in the transfer voltage, and it is possible to obtain sufficient transfer efficiency in the transfer area where the photosensitive drum 3d is not affected even in the fourth color.

In the digital color copying machine having the above structure, a case where printing is performed only with the first color of black will be described with reference to FIG. 1. Laser light is emitted from the black laser unit 10a and uniformly charged by the charging device 7a. The charged photoconductor drum 3a is imagewise exposed to form an electrostatic latent image.

The black photosensitive drum 3a is not provided with the pre-transfer charging device 11, and the toner image on the photosensitive drum 3a comes into contact with the paper without being pre-charged by the rotation of the photosensitive drum 3a.

The residual charges on the paper are accumulated on the paper by the negative charges flowing from the photoconductor drum 3a to the surface of the transfer belt 5 when the transfer is performed from the photoconductor drum 3a to the paper. And the transfer efficiency of the toner image will be reduced when the next transfer is performed.
The black toner image on the photoconductor drum 3a, which is the first color, is always transferred to the first layer where the toner image does not exist on the paper, and thus the negative residue accumulated on the paper by the transfer. There is no charge and it is not affected by the negative residual charge.

Therefore, even if the pre-transfer charging device 11 is not provided on the black photosensitive drum 3a for the first color, the transfer from the photosensitive drum 3a onto the sheet can be performed well. As a result, the power consumption of the copying machine can be reduced, the amount of ozone generated can be suppressed, the number of parts can be reduced, and the cost can be reduced.

Next, when printing is performed in the three-color full-color (cyan, magenta, yellow) mode by the instruction of the operation panel (not shown), the laser beam is not output from the black laser unit 10a, and the photoconductor drum 3a is not output. No electrostatic latent image is formed without imagewise exposure.

For each color other than black, the photosensitive drum 3b is irradiated with laser light from the laser units 10b to 10d and uniformly charged by the charging devices 7b to 7d.
~ 3d is imagewise exposed to form an electrostatic latent image. After this,
The electrostatic latent image is made visible by the developer in the developing devices 4b to 4d, and toner images are formed on the photosensitive drums 3b to 3d.

That is, since the cyan photosensitive member unit is transferred to the first color, the pre-transfer charging device 1
When 1b is turned off, the cyan toner image on the photosensitive drum 3b is not pre-transfer charged and the toner image charge amount is not adjusted. The toner amounts of the magenta and yellow toner images on the photoconductor drums 3c and 3d are adjusted by the corresponding pre-transfer charging devices 11c and 11d, respectively.

The paper fed from the paper feed cassette 6a or 6b is electrostatically adsorbed on the transfer belt 5,
It is conveyed toward the fixing device 9. By this conveyance, the transfer device 8 to which the bias voltage is applied when the toner images formed on the photoconductor drums 3b to 3d are in contact with the sheet.
By b to 8d, the toner images are electrically transferred onto the paper by sequentially superimposing them for each color.

At this time, since the cyan toner image is transferred to the first color, there is no residual charge on the paper and the transfer can be performed without increasing the toner charge amount. Even if the pre-charging device 11b is turned off, sufficient transfer efficiency can be obtained. Further, this makes it possible to reduce the amount of ozone generated and at the same time reduce the power consumption of the copying machine.

Next, when printing is performed in a two-color mono-color mode (two colors are selected from three colors other than black, for example, two colors, magenta and yellow are selected) mode according to an instruction from an operation panel (not shown). In the magenta photoconductor unit for the color, the toner image formed on the photoconductor drum 3c has the pre-transfer charging device 11c turned off. Therefore, the pre-transfer charging is not performed, and the transfer device is applied to the sheet conveyed by the transfer belt 5. Transfer is electrically performed by the transfer device 8c to which a bias voltage is applied to 8c.

At this time, in the photoconductor unit that does not perform printing, image exposure is not performed by the corresponding laser units 10a and 10b, and the corresponding developing units 4a and 4b.
b does not form a toner image, the transfer device 8a,
The transfer process is not performed without applying a bias voltage to 8b.

Then, in the photoconductor unit for transferring to the second color, the output currents of the pre-transfer charging devices 11c and 11d for each color are set according to the transfer characteristics of FIG. 4 and the charging characteristics of FIG.

At this time, the output current of the pre-transfer charging device 11c of the yellow photoconductor unit 3c for the second color is shown in Table 1.
Is set to −140 μA and the toner charge amount of the toner image on the photoconductor unit 3c is increased, and then the transfer device 8c to which a bias voltage is applied electrically changes the toner image to a magenta toner image on the paper. Transfer overlaid on top.

As described above, in the photoconductor unit to be transferred to the first color, there is no residual charge on the paper, so it is not necessary to increase the toner charge amount, and the pre-transfer charging device 11c in the photoconductor unit is turned off. Even if it does, sufficient transfer efficiency can be obtained.

As a result, the amount of ozone generated can be reduced, and at the same time, the power consumption of the copying machine can be reduced. Further, in the photoconductor unit that is transferred to the second color, the pre-transfer charging device 11 is used depending on the charging property of the toner.
By setting (adjusting) the output current of d, the transfer voltage can be suppressed and the optimum transfer condition can be set.

When two colors of magenta and cyan are selected, the magenta photosensitive drum 3c of the second color is selected.
By setting the output current of the pre-transfer charging device 11c to −120 μA as shown in Table 1, it is possible to obtain the same effect as when the two colors of magenta and yellow are selected.

Next, when printing is carried out in the four-color full-color mode using all four colors in accordance with an instruction from the operation panel (not shown), the photosensitive drums 3a to 3d are moved along the sheet conveying direction as shown in FIG. In the arranged one, the transfer efficiency is gradually reduced as the negative residual charge accumulated on the paper by the transfer process on the upstream side increases toward the downstream side in the paper transport direction. Therefore, when the four color toner images are overlaid on the paper, the transfer efficiency can be improved by sequentially increasing the toner charge amount of the toner images on the downstream side.

Therefore, in order to increase the toner charge amount of the toner images on the photosensitive drums 3a to 3d in the order of black, cyan, magenta, and yellow for printing in the four-color full-color mode, the pre-transfer charging devices 11b to 11d.
As shown in Table 1, the cyan output pre-charging device 11b: -100 μA, magenta pre-transfer charging device 1
1c: −170 μA, yellow pre-transfer charging device 11
d: Set to −350 μA, increase the toner charge amount of the toner image on the downstream side in the paper conveyance direction, and sequentially transfer the four color toner images.

As a result, the transfer voltage for obtaining the required charge amount of toner for each color can be suppressed to a low level as shown in FIG. 7, and optimum transfer conditions can be realized for each color.
Further, it is possible to suppress damage to the photoconductor drum 3 itself and a decrease in surface potential of each of the photoconductor drums 3a to 3d.

Next, a second embodiment of the digital color copying machine of the present invention will be described with reference to FIGS. 8 and 9 and Table 2.

In the second embodiment, the same high-voltage transformer 17 is used as a common power source for the pre-transfer charging devices 11b to 11d, and the pre-transfer charging devices 11b to 11d are connected via resistors R1, R2 and R3, respectively. It is connected to the high voltage transformer 17. The resistance values of the resistors R1, R2 and R3 are R in order to supply the same voltage to the pre-transfer charging devices 11b to 11d.
1 = R2 = R3 = 9 MΩ is set.

Then, as shown in Table 2, the case of the pre-transfer charging device 11 for the second and subsequent colors (L-shaped, thickness 0.3
mm) length X x width Y, the case of the pre-transfer charging device 11b is set to 10 mm x 10 mm, the case of the pre-transfer charging device 11c is set to 9 mm x 9 mm, and the case of the pre-transfer charging device 11d is set to 8 mm x 8 mm. Are formed.

[0067]

[Table 2]

The pre-transfer charging device 11 has different impedances due to different case sizes.
The output currents of the pre-transfer charging devices 11b to 11d are different even if the same voltage is supplied, and as shown in Table 1, the output current of the cyan pre-transfer charging device 11b for the second color is -100.
μA, Magenta third color pre-transfer charging device 11c
Output current is −170 μA, and the output current of the fourth color yellow pre-transfer charging device 11d is −350 μA. By this output current, the second and subsequent cyan toner images after cyan are pre-transfer charged, and each color is separated. The charge amount of the toner is adjusted and the toner images are sequentially superimposed and transferred.

As a result, the transfer voltage for obtaining the necessary charge amount of toner for each color can be suppressed to a low level as shown in FIG. 7, and the increase in the transfer voltage of the fourth color yellow toner image can be minimized. It is possible to suppress the transfer voltage, and it is possible to suppress the transfer voltage to a region having no influence on the photosensitive drum surface potential.

As described above, in the second embodiment, the pre-transfer charging devices 11b to 11d have a simple structure in which the shapes (dimensions) of the cases of the pre-transfer charging devices 11b to 11d are changed.
The output current of d can be adjusted, and the toner charge amount of the photoconductor unit on the downstream side in the paper transport direction can be increased. Therefore, one high-voltage transformer can be used as a common power source for the pre-transfer charging devices 11b to 11d, and the number of parts can be reduced to reduce the cost.

Next, a third embodiment of the digital color copying machine of the present invention will be described with reference to FIG.

Also in the third embodiment, the same high-voltage transformer 17 is provided in each pre-transfer charging device 1 as in the second embodiment.
1b to 11d are used as a common power source, and the pre-transfer charging devices 11b to 11d are connected to the high voltage transformer 17 via resistors R1, R2 and R3, respectively. And this resistance R
The resistance values of 1, R2, R3 and R4 are R1 = 20 MΩ, R2
= 12 MΩ, R3 = 6 MΩ, and the case shapes of the pre-transfer charging devices 11b to 11d are the same (10 mm × 1
0 mm), each pre-transfer charging device 11
As shown in Table 1, the output current of the cyan pre-transfer charging device 11b for the second color is −100 μA and the magenta pre-transfer charging device 11c for the third color is changed as shown in Table 1. Output current of −170 μA, the output current of the yellow pre-transfer charging device 11d for the fourth color is −350 μA, and this output current pre-transfer charges the toner image of cyan and the subsequent colors of the second color. The charge amount of the toner is adjusted and the toner images are sequentially superimposed and transferred.

As a result, the transfer voltage for obtaining the required charge amount of toner for each color can be suppressed to a low level as shown in FIG. 7, and the rise of the transfer voltage of the fourth color yellow toner image can be minimized. It is possible to suppress the transfer voltage, and it is possible to suppress the transfer voltage to a region having no influence on the photosensitive drum surface potential.

As described above, in the third embodiment, the resistors R1, R2 connected to the pre-transfer charging devices 11b to 11d are connected.
By changing the resistance value of R3, each pre-transfer charging device 11b to 11d
By changing the voltage supplied to the pre-transfer charging device 11
By adjusting the output currents b to 11d, the toner charge amount of the photoconductor unit on the downstream side in the paper transport direction can be increased. Therefore, one high-voltage transformer can be used as a common power source for the pre-transfer charging devices 11b to 11d, and the number of parts can be reduced to reduce the cost.

Next, a fourth embodiment of the digital color copying machine of the present invention will be described.

The toner used in this embodiment changes its charge amount depending on temperature and humidity. Under high humidity conditions, the developer adsorbs moisture, so that the resistance decreases and the toner charge amount decreases. The temperature of each color toner (5 ° C,
Changes in the charge amount with respect to humidity at 20 ° C. and 35 ° C. are shown in FIGS.

Therefore, in the fourth embodiment, the ambient temperature and humidity are detected by a temperature / humidity sensor (not shown),
An output setting unit (not shown) controls the magnitude of the output current of the pre-transfer charging devices 11b to 11d for each color according to the temperature and humidity characteristics of the charge amount of each color toner shown in FIGS.
After being set and adjusted to the optimum toner charge amount, the toner images of respective colors are sequentially superposed on the sheet and transferred.

In the fourth embodiment, when printing is performed in the four-color full-color mode under the conditions of the temperature of 20 ° C. and the humidity of 50%, the second color cyan pre-transfer charging device 11b.
Output current is −100 μA, and the output current of the magenta pre-transfer charging device 11c for the third color is −170 μA.
The output current of the pre-transfer charging device 11d for yellow, which is the color, is set to -350 μA, and the toner image after cyan, which is the second color, is pre-charged by this output current, and the charge amount of the toner is adjusted for each color. Transfer the images by sequentially superimposing them.

When printing is performed in the four-color full-color mode under the conditions of the temperature of 20 ° C. and the humidity of 70%, the output current of the second color cyan pre-transfer charging device 11b is set to −1.
Magenta pre-transfer charging device 1 for 10 μA and third color
The output current of 1c is −185 μA, the output current of the pre-transfer charging device 11d for the fourth color yellow is −370 μA, and the pre-transfer charging of the cyan and subsequent toner images of the second color is performed by this output current. Separately, the charge amount of the toner is adjusted and the toner images are sequentially superimposed and transferred.

As described above, by controlling the output currents of the pre-transfer charging devices 11b to 11d according to the surrounding environment detected by the temperature and humidity sensor, the chargeability of the toner is stabilized regardless of the surrounding environment. The transfer efficiency can be kept uniform.

Next, a fifth embodiment of the digital color copying machine of the present invention will be described.

When an OHP film is used as a paper for printing, the OHP film has a higher impedance and a larger thickness than the paper, and therefore the toner image on the photosensitive drum 3 is transferred to the OHP film. The voltage is set higher than that for paper, but the pre-transfer charging device 1
By setting the output currents of 1b to 11d to be also larger than those of paper and setting the toner charge amount to be higher, it is possible to suppress the transfer voltage at which the surface potential of the photosensitive drum 3 is not lowered and the photosensitive drum 3 is not damaged. it can.

Therefore, in the fifth embodiment, the transmissivity sensor (not shown) determines whether the type of paper is paper or OHP film, and the pre-transfer charging devices 11b to 11d are classified by color according to the determined type of paper. The magnitude of the output current is controlled and set by an output setting unit (not shown).

In the fifth embodiment, when printing is performed on the OHP film in the four-color mode, the output current of each of the pre-transfer charging devices 11b to 11d is higher than that when paper is used, that is, the second color is used. The output current of the cyan pre-transfer charging device 11b is −200 μA, and the output current of the magenta pre-transfer charging device 11c is −29.
Pre-transfer charging device 11 for 0 μA and fourth color yellow
The output current of d is set to −500 μA, and the charge amount of the toner images by the pre-transfer charging devices 11b to 11d for each color is set to the optimum toner image charge amount, and then the toner images for each color are sequentially superimposed and transferred.

As described above, when the OHP film is used as the paper, the pre-transfer charging devices 11b to 11d perform the pre-transfer charging before transferring the toner images of the respective colors.
The transfer efficiency can be improved.

[0086]

According to the image forming apparatus of the first aspect,
The pre-transfer charging means is not provided on the image carrier for the second and subsequent colors without providing the pre-transfer charging means on the image carrier for the first color at the most upstream position where there is no residual charge due to transfer on the paper. Since it is provided, it is possible to reduce unnecessary pre-transfer charging means, improve the transfer efficiency of the toner image of the second and subsequent colors with the minimum number of parts, reduce the number of parts, and reduce the cost. Can be planned. Further, since the pre-transfer charging means is not provided in the first color image carrier, the amount of ozone generated during pre-transfer charging by the pre-transfer charging means can be minimized.

[0087] According to the image forming apparatus according to claim 2, most
When the toner image is not formed by the image carrier at the upstream position, the pre-transfer charging means of the first color image carrier on the downstream side is turned off to become the second color. Since the pre-transfer charging means of the image carrier is turned on, it is possible to suppress generation of ozone during pre-transfer charging by the pre-transfer charging means for the first color, and it is possible to reduce power consumption.

According to the image forming apparatus of the third aspect, the output of the pre-transfer charging means is controlled according to each color.
Pre-transfer charging can be performed according to the charging characteristics of the toners composed of different pigments, and the transfer efficiency of the toner image of each color can be improved.

According to the image forming apparatus of the fourth aspect, the outputs of the plurality of pre-transfer charging means are set so as to sequentially increase the charge amount of the image carrier on the downstream side in the sheet conveying direction. It is possible to reliably transfer the toner image on the image carrier on the downstream side in the transport direction, where the residual charge on the paper is large, without increasing the transfer voltage, and it is possible to improve the transfer efficiency of each color. Moreover, the power for the plurality of pre-transfer charging means can be supplied from one power supply, and the number of parts can be reduced.

According to the image forming apparatus of the fifth aspect, the outputs of the plurality of pre-transfer charging means are set in accordance with the environmental conditions or the type of paper, respectively. Therefore, regardless of the environmental conditions or the type of paper. The transfer efficiency of the toner image of each color can be stabilized.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a digital color copying machine which is an image forming apparatus of the present invention.

FIG. 2 is an enlarged sectional view of a main part of FIG.

FIG. 3 is an explanatory diagram showing a relationship between a transfer voltage and an image density when all outputs of a pre-transfer charging unit of each color of the digital color copying machine are turned off.

FIG. 4 is an explanatory diagram showing a relationship between a charge amount of toner used in a digital color copying machine and a transfer efficiency.

FIG. 5 is an explanatory diagram showing charging characteristics of toners of respective colors used in a digital color copying machine.

FIG. 6 is an explanatory diagram showing the relationship between the transfer voltage and the image density when the outputs of the pre-transfer charging means for each color of the digital color copying machine are all the same output.

FIG. 7 is an explanatory diagram showing the relationship between the transfer voltage and the image density when the output of the pre-transfer charging unit for each color of the digital color copying machine is set according to the arrangement position in the paper conveyance direction.

FIG. 8 is a cross-sectional view showing a main part of a second embodiment and a third embodiment of a digital color copying machine which is an image forming apparatus of the present invention.

FIG. 9 is an enlarged sectional view of an essential part of a second embodiment of a digital color copying machine which is an image forming apparatus of the present invention.

FIG. 10 is an explanatory diagram showing charging characteristics of a black toner used in a digital color copying machine with respect to humidity.

FIG. 11 is an explanatory diagram showing charging characteristics of cyan toner used in a digital color copying machine with respect to humidity.

FIG. 12 is an explanatory diagram showing the charging characteristics of magenta toner used in a digital color copying machine with respect to humidity.

FIG. 13 is an explanatory diagram showing the charging characteristics of yellow toner used in a digital color copying machine with respect to humidity.

[Explanation of symbols]

3 photoconductor drum 4 Developing device 5 Transfer belt 7 Charging device 8 Transfer device 9 Fixing device 10 laser unit 11 Pre-transfer charging device

─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Tomoe Matsuoka 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture Sharp Corporation (72) Yasutaka Maeda 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Sharp Corporation (56) References JP 10-20683 (JP, A) JP 7-306564 (JP, A) JP 9-90705 (JP, A) JP 3-192280 (JP, A) JP-A-9-281763 (JP, A) JP-A-6-95476 (JP, A) JP-A-7-306555 (JP, A) JP-A-9-319272 (JP, A) JP-A-8-15947 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G03G 15/01-15/01 117

Claims (5)

(57) [Claims] 1. An image carrier for carrying a toner image, a charging unit for charging the image carrier, and an exposure for irradiating the image carrier charged by the charging unit with light to form an electrostatic latent image. Means, developing means for supplying toner to the image carrier and developing the electrostatic latent image formed by the exposing means to form a toner image of each color, and transferring the toner image on the image carrier to a sheet. A plurality of transfer means are provided, respectively, and the plurality of image carriers are sequentially arranged along the paper conveyance path, and toner images of respective colors are sequentially superposed on the paper conveyed through the conveyance path to form a color image. In a color image forming apparatus that forms an image, the most upstream position of the image carrier in the paper transport direction
Each image carrier located downstream of the image carrier is provided with a pre-transfer charging unit for charging the toner before the transfer of the toner image by the transfer unit. Bearer
A color image forming apparatus characterized in that a pre-transfer charging unit is not provided only for a holder . 2. An image formation by each of the image carriers
The toner image on the most upstream image carrier.
However, in the case where a color image is formed by a toner image of two or more colors on the subsequent image carrier, the pre-transfer charging unit of the first image carrier, which is the first color, is turned off, and the downstream thereof is Located on the side
2. The color image forming apparatus according to claim 1, further comprising control means for turning on the pre-transfer charging means for the second color image carrier. 3. The color image forming apparatus according to claim 1, further comprising control means for controlling the output of the pre-transfer charging means for each color according to each color. 4. An output setting means for setting the output of each of the plurality of pre-transfer charging means so as to sequentially increase the charge amount of the image carrier on the downstream side in the sheet conveying direction. 1. The color image forming apparatus described in 1. 5. The color image forming apparatus according to claim 1, further comprising output setting means for setting the outputs of the plurality of pre-transfer charging means in accordance with environmental conditions or paper types.