JPH096149A - Color image forming device - Google Patents

Abstract

PURPOSE: To provide a color image forming device capable of keeping the transfer efficiency constant irrespective of transfer position. CONSTITUTION: The device is provided with a photoreceptor 21 on which toner images in plural colors are formed, an intermediate transfer medium 11 which is arranged in contact with the photoreceptor 21 and to which respective color toner images are transferred, and a transfer means 27 which is arranged opposite to the medium 11 and for transferring the toner image to a recording medium 12. Provided that VTR denotes a multiple transfer voltage for forming a color toner image, VT denotes the toner layer potential of the already transferred toner image, VO denotes the electrostatic charging potential of the transfer medium 11, in the case of using toner with negative polarity 25Y, 25M, 25C and 25K, VTR>=240-(VT+VO) [V] is established, and in the case of using toner with positive polarity 25Y, 25M, 25C and 25K, VTR<=-240-(VT+VO) [V] is established.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus.

[0002]

2. Description of the Related Art Conventionally, various color image forming apparatuses have been proposed. Among them, a color image forming apparatus using an electrophotographic method has high definition, high printing speed, and running cost. Has been attracting attention because it is low. By the way, in a color image forming apparatus using an electrophotographic method, at least three color toners of yellow (Y), magenta (M), and cyan (C) are required. Various colors are reproduced by controlling the position of the toner image of each color, the amount of toner, and the like at that time.

Toners of three colors are superposed in a multiple transfer process, and in the multiple transfer process, the toner images formed on the photoconductor are transferred to a recording medium, an intermediate transfer medium or the like. . Therefore, it is ideal that all the toner on the surface of the photoconductor is attached to the recording medium, the intermediate transfer medium, or the like. However, toner that could not be attached to a recording medium, an intermediate transfer medium, or the like remains on the surface of the photoconductor after the multiple transfer process. Transfer efficiency is then used to evaluate the multiple transfer process. The transfer efficiency is represented by the ratio of the amount of toner that can be attached to the amount of toner that has been attached, and the larger the value, the better the multiple transfer process.

However, the multiple transfer process has a problem as shown in FIG. FIG. 2 is a relationship diagram between a conventional transfer voltage and transfer efficiency. The transfer voltage (absolute value) is plotted on the horizontal axis.
[V] is plotted, and the vertical axis is the transfer efficiency [%]. As can be seen from the figure, when the toner image of the first layer is transferred to a recording medium having no toner, the maximum transfer efficiency is about 98%, but the toner image of the second layer is formed on the toner image of the first layer. When the toner image is transferred, the maximum transfer efficiency drops to 95%,
When the toner image of the third layer is further transferred onto the toner image of the layer, the maximum transfer efficiency is reduced to about 75 [%].

Further, the transfer efficiencies of the toner images of the first and second layers are equal, but the transfer efficiency of the toner images of the third layer is extremely low, and the amount of toner adhered to a recording medium, an intermediate transfer medium, etc. Also, in the case of transferring the toner image of the third layer, it is much smaller than that in the case of transferring the toner images of the first and second layers. In this way, the place where the toner image is transferred on the recording medium, the intermediate transfer medium, etc.
Say. ) Causes the transfer efficiency to change and the amount of toner to be attached to differ, so that the density of each color changes depending on the transfer location, resulting in poor color reproducibility.

Therefore, in the multiple transfer process, it is necessary not only to increase the transfer efficiency but also to keep the transfer efficiency constant regardless of the transfer location. Therefore, for example, when transferring toner images in the order of yellow, magenta, and cyan, the transfer voltage applied when transferring the magenta toner image is higher than the transfer voltage applied when transferring the yellow toner image. Then, from the transfer voltage applied when transferring the magenta toner image,
The transfer voltage applied when transferring the cyan toner image is increased (see Japanese Patent Laid-Open No. 5-80634).

[0007]

However, in the above-mentioned conventional color image forming apparatus, the transfer efficiency changes depending on whether or not the toner image is present at the transfer place, and thus changes depending on the number of times the toner image is transferred. Not of. Therefore, even if the transfer voltage is changed for each color toner image, a good effect cannot be obtained. For example, when the toner images are transferred in order of yellow, magenta, and cyan, the yellow toner image is always transferred as the first layer, but the magenta toner image has two layers if there is a yellow toner image at the transfer location. If the yellow toner image is not present at the transfer location, it is transferred as the first layer. This is because the presence / absence of the yellow toner image depends on the output image.

Here, in FIG. 2, considering the case where the transfer voltage is changed during transfer of the yellow toner image and during transfer of the magenta toner image, the first layer is transferred when transferring the yellow toner image. Transfer voltage V ₁ is applied so that the transfer efficiency of the second layer is maximized, and when the magenta toner image is transferred, the transfer voltage V _{2 is} applied so that the transfer efficiency of the second layer is maximized.
Is assumed to have been applied.

In this case, the transfer efficiency at the time of transferring the magenta toner image to the transfer location where the yellow toner image is present is 95% as shown by the point A, but the transfer location where the yellow toner image is not present is 95%. The transfer efficiency when transferring the magenta toner image is 80% as indicated by the point B. Thus, the transfer efficiency varies depending on the transfer location.

Therefore, the output image is analyzed in advance, and a high transfer voltage is applied to the overlapping portion of the toner images,
A method of applying a low transfer voltage to a portion where the toner images do not overlap can be considered. However, in this case, not only a new circuit and mechanism are required to analyze the output image, but also the minimum unit of presence / absence of overlapping toner images is 1 dot. Need to control.

Therefore, not only the manufacturing cost of the color image forming apparatus becomes high, but also the time required for performing analysis and fine control becomes long, and the image output time becomes long accordingly. Further, since the toner images of the respective colors cannot be accurately overlapped, the dots that need to be overlapped do not overlap, or conversely, the dots that do not need to be overlapped overlap, resulting in deterioration of image quality.

An object of the present invention is to solve the problems of the conventional color image forming apparatus described above and to provide a color image forming apparatus capable of keeping the transfer efficiency constant regardless of the transfer location.

[0013]

Therefore, in the color image forming apparatus of the present invention, a photoconductor on which toner images of a plurality of colors are formed, and a toner of each color are provided in contact with the photoconductor. The image forming apparatus includes an intermediate transfer medium to which an image is transferred, and a transfer unit that is disposed so as to face the intermediate transfer medium and transfers a toner image to a recording medium.

Then, the multiple transfer voltage for superposing the toner images of the respective colors on each other and transferring them to the transfer medium to form a color toner image is V _TR, and the toner layer potential of the toner image already transferred is V _When the charge potential of the transfer medium is V _O and the toner of negative polarity is used, V _TR ≧ 240− (V _T + V _O ) [V] and the toner of positive polarity is used. In that case, V _TR ≦ −240− (V _T + V _O ) [V].

[0015]

According to the present invention, as described above, in the color image forming apparatus, the photoconductor on which the toner images of a plurality of colors are formed and the photoconductor are provided in contact with the photoconductor, and the toner images of the respective colors are formed. It has an intermediate transfer medium to be transferred, and a transfer unit which is arranged so as to face the intermediate transfer medium and transfers the toner image to a recording medium.

In this case, first, a toner image of each color is formed on the photoconductor, and the toner image is sequentially transferred to the intermediate transfer medium. Next, the toner image is transferred onto a recording medium by a transfer unit, and the transferred toner image is fixed to form a color image. Then, the toner images of the respective colors are superimposed and transferred to a transfer medium, and a multiple transfer voltage for forming a color toner image is set as V _TR, and a toner layer potential of the already transferred toner image is set as V _T , When the charge potential of the transfer medium is V _O , when toner of negative polarity is used, V _TR ≧ 240− (V _T + V _O ) [V], and when toner of positive polarity is used Is V _TR ≦ −240− (V _T + V _O ) [V].

In this case, since the transfer efficiency can be increased, the toner consumption can be reduced.

[0018]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a schematic view of a color image forming apparatus according to an embodiment of the present invention, and FIG. 3 is an enlarged view of a developing device according to the embodiment of the present invention. In the figure, reference numeral 11 denotes an intermediate transfer medium, and the intermediate transfer medium 11 includes a conductive base material 41 and an elastic resistance layer 4 formed on the conductive base material 41.
2 and a coat layer 43 formed on the elastic resistance layer 42 as needed, and the power source 16 is connected to the conductive base material 41.

As the elastic resistance layer 42, an intermediate transfer medium is used.
Carbon was dispersed in order to avoid charging of the body 11 itself.
Things are used. Then, the entire intermediate transfer medium 11
Resistivity is 10¹¹If it is larger than [Ω · cm], it will be released after transfer.
Electricity is large, toner is scattered, and toner image is greatly disturbed.
I get sick. Also, the total resistivity of the intermediate transfer medium 11
Is 10^FourIf it is [Ω · cm] or less, the intermediate transfer medium 11
The charge easily escapes to the member that comes in contact with the transfer voltage
Therefore, the transfer efficiency will be low. Therefore, the intermediate transfer
The total resistivity of the medium 11 is 10^Five-10^Ten[Ω · cm],
Preferably 10 ⁷-10⁹Set in the range of [Ω · cm]
You.

In this embodiment, a cylindrical drum made of aluminum is used as the conductive base material 41, and
Silicone rubber in which carbon was dispersed was molded as an elastic resistance layer 42, and the surface of the silicone rubber was covered with a fluororesin such as PFA or FEP, which has high releasability. A photoconductor drum 21 as a photoconductor is disposed in contact with the intermediate transfer medium 11, and the photoconductor drum 21
A charging device 22, an exposure device 23, and developing devices 24Y, 24M, 24C, and 24K for the respective colors are arranged on the outer circumference of.
Further, the toners 25Y, 25M, 25 of the respective colors are provided in the toner containing layers of the developing devices 24Y, 24M, 24C, 24K.
C and 25K are housed respectively.

Photosensitive drum 2 formed in a cylindrical drum shape
1 comprises a conductive support 44 and a photoconductive layer 45 formed on the conductive support 44, and the conductive support 44 is grounded. As the photoconductor drum 21, any of a selenium photoconductor, an organic photoconductor, a zinc oxide photoconductor, an amorphous silicon photoconductor, and the like can be used.

The charging device 22 is composed of the photosensitive drum 21.
It is arranged so as to face. In this embodiment, as the charging device 22, a contact type charging roller formed by laminating conductive rubber on a metal shaft is used. In addition to the charging roller, a conductive charging blade, a non-contact type corona discharger, or the like can be used. Then, the exposure device 23
Converts the image signal into light and irradiates the photoconductor drum 21 with the light to perform exposure. As the exposure device 23, a combination of a light source such as a laser and an LED array and an image forming optical system can be used. In this embodiment,
An LED array was used as a light source, and a rod lens array was used as an image forming optical system.

Further, the developing devices 24Y, 24M, 24
In C and 24K, each toner carrier 26Y and 26
Toners 25Y and 25 attracted to M, 26C, and 26K
M, 25C, and 25K are conveyed in the direction of arrow C, adhere to the photosensitive drum 21, and develop the electrostatic latent image formed on the photosensitive drum 21. Toner carrier 26Y, 26M, 26
C and 26K are conductive base materials 261Y, 261M and 261.
Elastic resistance layers 262Y, 262M, 262 on C, 261K
The conductive base materials 261Y, 261M, 261C, and 261K are formed by coating C and 262K, and a power source 48 is connected to apply a developing bias.
As the developing devices 24Y, 24M, 24C and 24K, a two-component magnetic brush developing device, a one-component magnetic brush developing device, a one-component non-magnetic developing device or the like can be used.

On the downstream side of the photosensitive drum 21 in the rotation direction of the intermediate transfer medium 11, a transfer roller 27 as a batch transfer means is arranged so as to be able to move forward and backward with respect to the intermediate transfer medium 11. As the batch transfer means, any means that can electrostatically attach the toner may be used, and a corona discharger or a contact type conductive material can be used.

In this embodiment, as the transfer roller 27, a conductive roller formed by laminating a highly elastic resistance material 52 such as rubber or sponge on a metal support 51 is used. Further, the surface of the resistance material 52 may be coated with a protective material or the like, if necessary. A power source 53 is connected to the metal support 51 of the transfer roller 27, and a transfer voltage having a polarity opposite to the polarity of the toner is applied to the metal support 51 during the batch transfer process.

A heating / pressurizing means 28 as a fixing device is disposed downstream of the intermediate transfer medium 11 in the transport direction of the recording medium 12. The heating / pressurizing means 28
Is composed of a heating roller 29 and a pressure roller 30 which are arranged to face each other with the recording medium 12 interposed therebetween. The heating roller 29 is disposed so as to face the front side of the recording medium 12 onto which the toner image has been transferred, and is pressed against the recording medium 12 with a constant pressure. Further, the heating roller 29 is formed by disposing a halogen lamp in a hollow member made of metal. Further, it is also possible to use one in which a heating resistance layer made of Ni-P or the like is laminated on a base material such as glass or ceramic, and a protective layer such as Ta ₂ O ₅ or fluororesin is further laminated thereon. .

On the other hand, the pressure roller 30 is used for the recording medium 1.
The toner image of No. 2 is disposed so as to face the back side on which the toner image is not transferred. Further, the pressure roller 30 is biased toward the heating roller 29 side, and sandwiches the recording medium 12 with the heating roller 29 by a constant pressing force. The pressure roller 30
Is composed of an elastic roller, and is formed by laminating rubber on a metal pipe. In this example, silicone rubber was used as the rubber. Further, a paper feed roller 31 is provided to insert the recording medium 12 between the intermediate transfer medium 11 and the transfer roller 27.

Next, the operation of the color image forming apparatus will be described. As shown in the figure, the intermediate transfer medium 11 and the photoconductor drum 21 are rotated by a driving unit (not shown) in the arrow A direction and the arrow B direction at a constant peripheral speed and at the same peripheral speed. The photoconductor drum 21 is uniformly and evenly charged by the charging device 22 based on the recording start signal. Next, the exposure device 23
Thus, light corresponding to the first color yellow image signal is applied to the photoconductor drum 21 and an electrostatic latent image is formed. Here, since the light is irradiated only to the portion corresponding to the image, the charge in the image portion is erased, and the charge in the non-image portion is left as it is.

A yellow developing device 24Y is disposed in contact with or close to the photosensitive drum 21.
The electrostatic latent image is visualized by 4Y and becomes a toner image. In this embodiment, reversal development is used, and a bias voltage is applied to the toner carrier 26Y by the power supply 48. As a result, the charged toner 25Y on the toner carrier 26Y is attracted and attached to the photoconductor drum 21 by the electrostatic force.

The yellow toner image formed on the photosensitive drum 21 is transferred to the intermediate transfer medium 11 by static electricity due to the transfer voltage of the power supply 16. The intermediate transfer medium 1
When the beginning of the image portion on 1 reaches the beginning of the yellow toner image on the photoconductor drum 21, the yellow toner 25
Y is the photosensitive drum 21 in response to the movement of the intermediate transfer medium 11.
They are sequentially attached to the intermediate transfer medium 11 from above.

Then, when the yellow toner 25Y is attached to the end of the image portion on the intermediate transfer medium 11, the application of the transfer voltage by the power source 16 is temporarily stopped, and the tip of a charge eliminating brush (not shown) is removed. By contacting the non-image portion of the intermediate transfer medium 11, the charge accumulated on the intermediate transfer medium 11 during the transfer of the yellow toner 25Y is removed. When the last part of the yellow toner image is developed on the photoconductor drum 21, a second-color magenta toner image is developed on the photoconductor drum 21 so as to coincide with the beginning of the image portion on the intermediate transfer medium 11. Begin to be. Since the charge removal and the development of the magenta toner image are performed in parallel, there is no time loss due to the charge removal.

When the head of the image portion on the intermediate transfer medium 11 reaches the head of the magenta toner image, a primary transfer voltage is applied to the intermediate transfer medium 11, and the intermediate transfer medium 1
The magenta toner image is transferred to No. 1 and is superimposed on the yellow toner image. Similarly, cyan and black toner images are also transferred to the intermediate transfer medium 11, and when toner images of all colors are transferred to the intermediate transfer medium 11, a color toner image is formed on the intermediate transfer medium 11. It

When the head of the image portion on the intermediate transfer medium 11 approaches the transfer roller 27, the transfer roller 27
Is brought into pressure contact with the intermediate transfer medium 11 by a movable means (not shown), the recording medium 12 is fed by the paper feed roller 31, and is conveyed between the intermediate transfer medium 11 and the transfer roller 27. A transfer voltage having a polarity opposite to that of the color toner image formed on the intermediate transfer medium 11 is applied to the transfer roller 27 by a power source 53. Therefore, the color toner images formed on the intermediate transfer medium 11 are collectively transferred to the recording medium 12.

When the color toner image on the intermediate transfer medium 11 is transferred to the recording medium 12, the transfer roller 27 is separated from the intermediate transfer medium 11 and the application of the transfer voltage by the power source 53 is stopped. Next, the recording medium 12 to which the color toner images have been transferred is heated and pressed by the heating / pressurizing means 28. That is, when the recording medium 12 is inserted between the heating roller 29 and the pressure roller 30, the toner 25Y, 25 of the color toner image on the recording medium 12 is inserted.
Heat is transferred from the heating roller 29 to the M, 25C, and 25K to be heated and melted. Then, the toner 25Y, 25 on the recording medium 12 that has passed through the heating / pressurizing means 28
M, 25C, and 25K are naturally cooled and return to the solid state again. In this way, the color toner image is recorded on the recording medium 1.
It is fixed at 2.

Then, the recording medium 12 is discharged to a stacker (not shown) arranged outside the color image forming apparatus. By the way, in the color image forming apparatus having the above-described structure, in the multiple transfer process of superposing the toner images of the respective colors on the intermediate transfer medium 11, the transfer efficiency at the time of transferring the toner images is already attached to the intermediate transfer medium 11. It is related to the potentials of the toners 25Y, 25M, 25C and 25K.

That is, the charged toner 25Y,
A transfer electric field is applied to 25M, 25C, and 25K to generate static electricity, and the toner image can be transferred by the electrostatic force. However, when the toner image is already transferred to the intermediate transfer medium 11, the toner image is transferred. Toner 25Y, 25
M, 25C, and 25K form an electric field in a direction that hinders the transfer electric field. Therefore, the transfer electric field for transferring the next toner image is weakened, the electrostatic force is reduced, and the transfer efficiency is reduced accordingly.

Toners 25Y, 25M, 25C, 2
The strength of the electric field formed by 5K itself is proportional to the magnitude of the surface potential of the toner layer formed on the intermediate transfer medium 11.
The surface potential of the toner layer formed on the intermediate transfer medium 11 and the transfer efficiency when the toner layer is formed on the toner layer will be described. FIG. 4 is a conceptual diagram of the surface potential in the embodiment of the present invention, and FIG. 5 is a relationship diagram between the surface potential and the transfer efficiency in the embodiment of the present invention. In addition,
In FIG. 5, the horizontal axis represents the surface potential (absolute value) V _D [V] on the intermediate transfer medium 11, and the vertical axis represents the transfer efficiency [%].

In the figure, 11 is an intermediate transfer medium,
The intermediate transfer medium 11 includes a conductive base material 41, an elastic resistance layer 42 formed on the conductive base material 41, and a coat layer 43 formed on the elastic resistance layer 42 as necessary. And the power source 16 is connected to the conductive base material 41.
Here, assuming that one or more toner layers 61 are already formed on the intermediate transfer medium 11, the primary transfer voltage is applied between the photosensitive drum 21 (FIG. 1) and the intermediate transfer medium 11. V _TR1 is applied, and the surface potential V _D on the intermediate transfer medium 11 is measured by a non-contact surface electrometer.

In this case, for the surface potential V _D on the intermediate transfer medium 11, the primary transfer voltage V _TR1 applied between the ground and the surface of the conductive substrate 41 of the intermediate transfer medium 11, the conductive substrate 41 between the surface of the coat layer 43 and the surface of the coat layer 43, that is, the charge potential V _O of the intermediate transfer medium 11 itself formed in the elastic resistance layer 42 and the coat layer 43, and the toner layer potential V _T of the toner layer 61. Is involved.

The surface potential V _D can be expressed by the equation (1). V _D = V _TR1 + V _O + V _T (1) When the toner layer 61 is not formed, the formula (1) is used.
The inner toner layer potential V _T becomes 0 [V]. Further, in this embodiment, since the silicone rubber in which carbon is dispersed is molded as the elastic resistance layer 42 so that the intermediate transfer medium 11 itself is not charged, the charging potential V _O is also changed. It becomes almost 0 [V].

By the way, as shown in FIG. 5, the absolute value of the surface potential V _D on the intermediate transfer medium 11 is always 240 [V].
With the above, the transfer efficiency can always be 80% or more. However, in order to make the absolute value of the surface potential V _D on the intermediate transfer medium 11 always 240 [V] or more,
Since the surface potential V _D must be constantly monitored,
A sensor, a control circuit, etc. are required, and the cost becomes high.

Therefore, in this embodiment, the formula (2) is satisfied in order to make the surface potential V _D of the formula (1) above 240 [V]. V _D = V _TR1 + V _T + V _O ≧ 240 [V] ∴V _TR1 ≧ 240 − (V _T + V _O ) [V] (2) In the formula (2), the influence on the primary transfer voltage V _TR1 is large. The variable is the toner layer potential V _T. When the toners 25Y, 25M, 25C and 25K charged with negative polarity are used, the toner layer potential V _T becomes a negative potential, so the toner layer potential V _T is the lowest (highest in the negative direction). In this case, the primary transfer voltage V _TR1 becomes the highest. Then, the toner layer potential V _T becomes lower (absolute value becomes larger) as the toner layers 61 are piled up, for example, 1
What was -50 [V] in the case of only a layer becomes -200 [V] when there are two layers. Therefore, the toner layer potential V _T
Is the lowest in the portion where all the toner layers 61 other than the last-formed toner layer 61 overlap.

For example, consider a case where toners 25Y, 25M, and 25C that are charged to a negative polarity and have the same charge amount are used in the order of yellow, magenta, and cyan. When the yellow toner image is transferred (primary transfer) from the photosensitive drum 21 to the intermediate transfer medium 11, the toner layer 61 is not formed on the intermediate transfer medium 11, and the intermediate transfer medium 11 itself is also charged. Absent. Therefore,
The toner layer potential V _T and the charging potential V _O are both 0 [V].
Therefore, when the primary transfer voltage V _TR1 of 240 [V] or more is applied, the formula (2) can be satisfied, and the transfer efficiency can always be 80 [%] or more.

Next, when the magenta toner image is transferred, a portion where the yellow toner layer 61 is formed and a portion where the toner layer 61 is not formed are mixed on the intermediate transfer medium 11. Then, in the portion where the toner layer 61 is not formed, the toner layer potential V _T and the charging potential V _T
Each of _O becomes 0 [V], and when the primary transfer voltage V _TR1 of 240 [V] or more is applied, the formula (2) can be satisfied, and the transfer efficiency can always be 80 [%] or more. it can.

However, in the portion where the yellow toner layer 61 is formed, assuming that the toner layer potential V _T of the yellow toner layer 61 is, for example, −50 [V], primary transfer of 290 [V] or more is performed. When the voltage V _TR1 is applied, the formula (2) can be satisfied, and the transfer efficiency can always be 80% or more. Therefore, when a magenta toner image is transferred, if a primary transfer voltage V _TR1 of 290 [V] or more is applied, the transfer efficiency is always 80 [%] regardless of the presence or absence of the yellow toner layer 61.
The above can be done.

When the cyan toner image is transferred, the portion where the yellow and magenta toner layer 61 is formed, the portion where the yellow or magenta toner layer 61 is formed, and the toner layer are formed on the intermediate transfer medium 11. The part where 61 is not formed is mixed. Then, the toner layer 6
In the portion where 1 is not formed, the toner layer potential V
_{Both T} and the charging potential V _O become 0 [V], and 240
When the primary transfer voltage V _TR1 of [V] or more is applied, the formula (2) can be satisfied, and the transfer efficiency is always 80 [%].
The above can be done.

Further, the yellow or magenta toner layer 6
When the toner layer potential V _T of the yellow or magenta toner layer 61 is set to, for example, −50 [V] in the portion where 1 is formed, when a primary transfer voltage V _TR1 of 290 [V] or more is applied. (2) can be satisfied, and the transfer efficiency can always be 80% or more.

At the portion where the yellow and magenta toner layers 61 are formed, considering that the toner layer potential V _T is approximately proportional to the square of the layer thickness, the toner layers in the yellow and magenta toner layers 61 are considered. The potential V _T
For example, when the voltage is −200 [V], the expression (2) can be satisfied when the primary transfer voltage V _TR1 of 440 [V] or more is applied, and the transfer efficiency can always be 80 [%] or more. .

Therefore, when the cyan toner image is transferred, if the primary transfer voltage V _TR1 of 440 [V] or more is applied, the transfer efficiency is always 80 [% regardless of the presence of the yellow and magenta toner layers 61. ] You can do more than that. Thus, the conditions of the last one in all portions where the toner layer are overlapped in a color other than the toner layer cyan is transferred color primary transfer voltage V _TR1 is comprehensive conditions of the primary transfer voltage V _TR1 at other portions . Therefore, when the toner layer potential V _T when the toner layers of all colors other than the toner layer of the last transfer color are superposed is measured in advance, and the primary transfer voltage V _TR1 is set based on the toner layer potential V _T. , Without arranging sensors, control circuits, etc.
A transfer efficiency of 80% or more can always be obtained.

In this embodiment, four color toners 25Y, 25 of yellow, magenta, cyan and black are used.
Since M, 25C, and 25K are used, for example, if the black toner image is transferred last, yellow,
Intermediate transfer medium 11 in the order of magenta and cyan toner layers
Transfer efficiency of 80 [%] or more is always obtained by superposing on top, measuring the toner layer potential V _T at that time, and calculating the required primary transfer voltage V _TR1 from the formula (2). You can

Further, the surface potential V _D of the intermediate transfer medium 11
Is set to 320 [V] or more and the primary transfer voltage V _TR1 is set as described above, so that the transfer efficiency is 90%.
It can be more than [%]. Therefore, toner 2
The consumption of 5Y, 25M, 25C and 25K can be reduced. As a means for controlling the surface potential V _D on the intermediate transfer medium 11, a method of increasing the primary transfer voltage V _TR1 and a method of _decreasing the toner layer potential V _T can be considered.

[0052] In a method of increasing the primary transfer voltage V _TR1, upon increasing the primary transfer voltage V _TR1, weaken the discharge generated transfer electric field as shown in the discharge curve of Basshen. Therefore, the primary transfer voltage V
_TR1 cannot be raised. Also, the toner layer potential V
_As for the method of lowering _T , the method of thinning the toner layer of each color, the toner of each color 25Y, 25M, 25C, 25K
It is conceivable to reduce the charge amount of the toner, increase the toner filling rate, and remove the toner charge after each color of the multiple transfer process.

Further, the toners 25Y, 25M, 25C and 25 attached to the surface of the photosensitive drum 21 before the transfer.
K and the toners 25Y, 25M, 25C, and 25K remaining on the surface of the photoconductor drum 21 after the transfer are peeled off with a tape, and the peeled toners 25Y and 2Y are removed.
From the optical densities of 5M, 25C, and 25K, the toner amount W ₁ before transfer and the residual toner amount W ₂ are calculated, and the following formula (3) is used.
The transfer efficiency η can be calculated by

Η = (1−W ₂ / W ₁ ) × 100 [%] (3) In the present embodiment, when the toners 25Y, 25M, 25C and 25K charged with negative polarity are used However, the toner 2 charged to the positive polarity is
When using 5Y, 25M, 25C and 25K, the condition is that the surface potential V _D is -240 [V] or less. Therefore, the expression (4) is satisfied in order to keep the surface potential V _D at -240 [V] or less.

V _D = V _TR1 + V _T + V _O ≦ −240 [V] ∴V _TR1 ≦ −240- (V _T + V _O ) [V] (4) Further, when using four or more color toners Also, by setting the primary transfer voltage V _TR1 under the conditions as described above, it is possible to always obtain a constant transfer efficiency. In this embodiment, the toner images are superimposed on the intermediate transfer medium 11 to form a color toner image, but the toner images are superimposed on the recording medium to form a color toner image. You can also

The present invention is not limited to the above-mentioned embodiments, but can be variously modified within the scope of the present invention, and these are not excluded from the scope of the present invention.

[0057]

As described in detail above, according to the present invention, in a color image forming apparatus, a photoconductor on which toner images of a plurality of colors are formed and a photoconductor are provided in contact with the photoconductor. An intermediate transfer medium to which the toner images of the respective colors are transferred, and a transfer unit which is disposed so as to face the intermediate transfer medium and transfers the toner image to the recording medium.

Then, the multiple transfer voltage for forming the color toner image by superposing the toner images of the respective colors on the transfer medium is set to V _TR, and the toner layer potential of the toner image already transferred is set to V _{TR. When} the charge potential of the transfer medium is V _O and the toner of negative polarity is used, V _TR ≧ 240− (V _T + V _O ) [V] and the toner of positive polarity is used. In that case, V _TR ≦ −240− (V _T + V _O ) [V].

In this case, since the transfer efficiency can be increased, the toner consumption amount can be reduced. Further, since the transfer efficiency can be increased even in the portion where the toner layers are overlapped, the transfer efficiency can be made constant regardless of the transfer location.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a color image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a relationship diagram between a conventional transfer voltage and transfer efficiency.

FIG. 3 is an enlarged view of the developing device according to the embodiment of the present invention.

FIG. 4 is a conceptual diagram of a surface potential in an example of the present invention.

FIG. 5 is a relationship diagram between surface potential and transfer efficiency in the example of the present invention.

[Explanation of symbols]

 11 Intermediate Transfer Medium 12 Recording Medium 21 Photosensitive Drum 27 Transfer Roller 61 Toner Layer 25Y, 25M, 25C, 25K Toner

Claims (3)

[Claims] 1. An (a) photoconductor on which toner images of a plurality of colors are formed, and (b) an intermediate transfer medium which is arranged in contact with the photoconductors and onto which the toner images of the respective colors are transferred, c) a transfer unit which is arranged so as to face the intermediate transfer medium and transfers the toner image onto the recording medium, and (d) the toner images of the respective colors are superposed and transferred onto the transfer medium to form a color toner image. When the multiple transfer voltage for forming the toner image is V _TR , the toner layer potential of the toner image already transferred is V _T, and the charging potential of the transfer medium is V _O , the toner of negative polarity is used. In this case, V _TR ≧ 240− (V _T + V _O ) [V], and when using a toner of positive polarity, V _TR ≦ −240− (V _T + V _O ) [V] And a color image forming apparatus. 2. The color image forming apparatus according to claim 1, wherein the transfer medium is an intermediate transfer medium. 3. The transfer medium is a recording medium.
3. The color image forming apparatus according to 1.