JP2015175951A - image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to eliminate peripheral toner-transfer defect.SOLUTION: An image forming apparatus includes: a plurality of image carriers 112 carrying toner images of different colors; an intermediate transfer body 131 to which a superimposed toner image formed by superimposing the toner images carried on the image carriers 112 is transferred; a secondary transfer body 135 which forms a transfer nip 139 with the intermediate transfer body 131, to transfer the superimposed toner image to a recording medium 20 passing through the transfer nip 139; and drive control means Ctr that performs drive control of the intermediate transfer body 131 or/and the secondary transfer body 135. When a step difference is formed in the superimposed toner image, the drive control means Ctr performs drive control so as to generate a difference in surface velocity between the intermediate transfer body 131 and the recording medium 20.

Description

  The present invention relates to an image forming apparatus.

In the market, image forming apparatuses capable of high value-added expression are constantly demanded. Recently, in addition to yellow (Y), magenta (M), cyan (C), and black (K) colors, white and colorless and transparent An image forming apparatus equipped with a special color toner has been developed.
White toner and colorless and transparent special color toner (hereinafter also referred to as clear toner or special color toner) are assumed to be used for forming a solid image of white toner as a background of a recording medium such as a transparent medium.
In addition, a clear toner full-color image or solid patch image may be formed on an image formed with Y, M, C, and K color toners to make the image glossy. .

By the way, if a solid full-color image formed by superimposing one or more colors of Y, M, C, and K color toners on a background and a solid patch image is formed on it with special color toner, the color of the solid patch peripheral area is obtained. It is known that there is a problem of missing (surrounding white).
This “peripheral white spot” is caused by a difference in the amount of toner formed between the area where the solid patch image of the special color toner is superimposed and the other areas of the entire background solid image. This is because the adhesiveness with the recording medium is insufficient in the peripheral area of the solid patch image, and the toner is not favorably transferred from the intermediate transfer belt to the recording medium.
In addition to the case where the special color toner is used, for example, if a solid color patch image of C color is formed on the entire solid background image by using Y, M color toners, a similar problem may occur.

It is already known that this problem can be improved to some extent by using a superimposed bias in which an AC component is superimposed on a DC component as a secondary transfer bias for forming a transfer electric field at the secondary transfer nip.
For example, in Patent Document 1, toner detachment from the intermediate transfer belt is promoted by using a superimposed bias in which an AC component is superimposed on a DC component as a secondary transfer bias for forming a transfer electric field at the secondary transfer nip. In addition, it is described that peripheral white spots can be improved even when a special color toner is formed on an image carrier.

However, the disclosure of Patent Document 1 also has a problem in that, when the amount of toner in a patch image superimposed on a background solid image is increased, the level difference becomes high and sufficient white-out cannot be improved.
SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that can further improve white spots.

  In order to solve the above-mentioned problems, the invention of claim 1 is characterized in that a transfer nip is formed between an intermediate transfer body to which a superimposed toner image obtained by superimposing a plurality of color toner images is transferred, and the intermediate transfer body. A secondary transfer member that forms and transfers the superimposed toner image to a recording medium that passes through the transfer nip, and drive control means that controls the drive of the intermediate transfer member and the secondary transfer member, The drive control unit performs the drive control so that a speed difference is generated between the surface speeds of the intermediate transfer member and the recording medium when a step is generated in the superimposed toner image.

  With the configuration as described above, according to the present invention, it is possible to realize an image forming apparatus capable of further improving the defect of white spots.

1 is an overall configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a control system of the image forming apparatus according to the present embodiment. FIG. 6 is a diagram for explaining problems that a conventional image forming apparatus has. FIG. 4 is an enlarged schematic diagram showing the behavior of toner in a secondary transfer nip when the image shown in FIG. 3 is formed. FIG. 3 is a diagram illustrating a configuration for improving peripheral white spots in the image forming apparatus according to the present embodiment. The figure explaining the effect by control in this embodiment. FIG. 9 is a graph showing the result of sensory evaluation of peripheral white spots when the speed difference between the intermediate transfer belt surface and the recording medium surface is changed. FIG. 10 is a diagram illustrating a measurement result of a magnification error in the sub-scanning direction when the linear speed of the secondary transfer roller is changed. FIG. 6 is a graph showing evaluation results of peripheral white spots when the amount of spot color toner attached is changed. FIG. 5 is a graph plotting speed differences at which the peripheral white spot evaluation rank becomes a specified value when the amount of spot color toner attached is changed. The graph figure which shows the result of having sensory-evaluated the surrounding white void by changing the experimental environment. 9 is a flowchart for explaining an example of a control process for preventing a white image in the image forming apparatus according to the present embodiment.

Embodiments of the present invention will be described below in detail with reference to the drawings.
FIG. 1 is an overall configuration diagram of an image forming apparatus according to an embodiment of the present invention.
An image forming apparatus 1 shown in FIG. 1 is a color image forming apparatus that forms a color image using a tandem type image forming unit, and includes an image reading unit 10, an image forming unit 11, a paper feeding unit 12, a transfer unit 13, and a fixing unit. 14 and a paper discharge unit 15.

[Image reading unit]
The image reading unit 10 includes a contact glass 101 and a reading sensor 102, and reads an image of a document on the contact glass 101 by the reading sensor 102 to generate image information.
That is, the image reading unit 10 irradiates a document on the contact glass 101 with light, and the reflected light is received by a reading sensor 102 such as a CCD (charge coupled device) or CIS (contact image sensor). The electrical color separation signals for each of the primary colors RGB are read.

[Image forming unit]
The image forming unit 11 forms and outputs a toner image of a special color (S) such as colorless and transparent (clear) or white in addition to four colors of yellow (Y), magenta (M), cyan (C), and black (K). The five image forming units 110 (S, Y, M, C, K) are provided.
The five image forming units 110 (S, Y, M, C, and K) use S, Y, M, C, and K color toners of different colors as image forming materials, but otherwise have the same configuration. It will be replaced when the life is reached.
Each of the image forming units 110 (S, Y, M, C, K) is configured to be detachable from the apparatus main body 2 and constitutes a so-called process cartridge.

Hereinafter, the common configuration will be described using the image forming unit 110K for forming a K-color toner image as an example.
The image forming unit 110K includes a charging device 111K, a photoconductor 112K as an image carrier or a latent image carrier, a developing device 114K, a charge eliminating device 115K, a photoconductor cleaning device 116K, and the like.
These devices are held by a common holding body, and can be exchanged at the same time by being integrally attached to and detached from the device main body 2.
The photoconductor 112K has a drum shape with an outer diameter of 60 mm in which an organic photosensitive layer is formed on the surface of the substrate, and is driven to rotate counterclockwise by a driving unit (not shown).
The charging device 111K generates a discharge between the charging wire and the outer peripheral surface of the photoconductor 112K by applying a charging bias to a charging wire that is a charging electrode of a charging charger (charger), and thereby the surface of the photoconductor 112K. Is uniformly charged.

In this embodiment, the toner is charged to the same negative polarity as that of the toner. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed.
Instead of the charging charger, a method using a charging roller provided in contact with or close to the photosensitive member 112K may be employed.
An electrostatic latent image for K is formed on the surface of the uniformly charged photoreceptor 112K by optical scanning with a laser beam emitted from an exposure device 113 described later.
Of the entire surface of the uniformly charged surface of the photoconductor 112K, the potential of the portion irradiated with the laser light is attenuated, and the potential of the laser irradiated portion is smaller than the potential of the other portion (background portion). Become a statue.
The electrostatic latent image for K is developed by a developing device 114K using K toner described later to become a K toner image. Then, primary transfer is performed on an intermediate transfer belt 131 described later.
The developing device 114K has a container (not shown) in which a two-component developer containing K toner and a carrier is accommodated, and development is performed by the magnetic force of a magnet roller (not shown) inside the developing sleeve provided in the container. The agent is carried on the surface of the developing sleeve.

A developing bias having the same polarity as that of the toner and larger than the electrostatic latent image of the photosensitive member 112K and smaller than the charging potential of the photosensitive member 112K is applied to the developing sleeve. A developing potential from the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 112K.
In addition, a non-developing potential that moves the toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 112K.
By the action of the developing potential and the non-developing potential, the K color toner on the developing sleeve is selectively attached to the electrostatic latent image on the photoconductor 112K and developed, so that a K color toner image is formed on the photoconductor 112K. It is formed.
The neutralization device 115K neutralizes the surface of the photoreceptor 112K after the toner image is primarily transferred to the intermediate transfer belt 131.
The photoreceptor cleaning device 116K includes a cleaning blade and a cleaning brush (not shown), and removes transfer residual toner and the like remaining on the surface of the photoreceptor 112K that has been neutralized by the neutralization device 115K.

In FIG. 1, in the image forming unit 110 (C, M, Y, S) as well as the image forming unit 110K, S, Y, M, A C toner image is formed.
Above the image forming unit 110 (S, Y, M, C, K), an exposure device 113 as a latent image writing unit or an exposure unit is arranged.
The exposure device 113 causes the photoconductor 112 (S, Y, M, C, K) to be emitted by laser light emitted from a laser diode based on image information sent from an external device such as the image reading unit 10 or a personal computer. Light scan.
The exposure device 113 is configured such that a laser beam emitted from a light source is polarized in the main scanning direction by a polygon mirror that is rotationally driven by a polygon motor, and the photosensitive member 112 (S, Y, M, C) is passed through a plurality of optical lenses and mirrors. , K).
Moreover, it may replace with a laser beam and you may employ | adopt the structure which carries out optical writing and irradiation with the LED light emitted from several LED.

[Paper Feeder]
The paper feed unit 12 includes a paper storage unit 121, a paper feed pickup roller 122, a paper feed belt 123, and a registration roller 124, and supplies paper that is an example of a recording medium to the transfer unit 13.
The paper feed pickup roller 122 is provided so as to rotate in order to move a paper (not shown) accommodated in the paper accommodation portion 121 toward the paper feed belt 123.
The paper feed pickup roller 122 provided in this way takes out the uppermost one of the stored paper one by one and places it on the paper feed belt 123.
The paper feed belt 123 conveys the paper taken out by the paper feed pickup roller 122 to the transfer unit 13.
The registration roller 124 feeds the sheet at a timing when a portion where the toner image is formed on the intermediate transfer belt 131 reaches a secondary transfer nip 139 as a transfer nip of the transfer unit 13.

[Transfer section]
The transfer unit 13 is disposed below the image forming unit 110 (S, Y, M, C, K), and includes a driving roller 132, a driven roller 133, an intermediate transfer belt 131, and a primary transfer roller 134 (S, Y, M, C, K).
The transfer unit 13 includes a secondary transfer roller 135, a secondary transfer counter roller 136, a toner adhesion amount sensor 137, and a belt cleaning device 138.
The intermediate transfer belt 131 functions as an endless intermediate transfer member, and a driving roller 132, a driven roller 133, a secondary transfer counter roller 136, and a primary transfer roller 134 (S, Y, and S) disposed inside the loop. M, C, K) etc.
In addition, arrangement | positioning means providing by arrange | positioning or determining a position, and a tension | tensile_strength means hang | hanging in the state with tension | tensile_strength.

The intermediate transfer belt 131 is endlessly moved and traveled in the same direction by the intermediate transfer belt driving roller 132 that is rotated in the clockwise direction in the drawing by the intermediate transfer belt driving motor (FIG. 2), and the photosensitive member 112 (S, Y, Move while touching M, C, K).
The intermediate transfer belt 131 has a thickness of 20 to 200 [μm], preferably about 60 [μm].
Further, the volume resistivity of the intermediate transfer belt 131 is about 1 × 10 ⁶ to 1 × 10 ¹² [Ω · cm], preferably about 1 × 10 ⁹ [Ω · cm] (by Mitsubishi Chemical Hi-Lester UP MCP HT45). , Measured under the condition of an applied voltage of 100 V).

A toner adhesion amount sensor 137 is disposed in the vicinity of the intermediate transfer belt 131 wound around the driving roller 132.
The toner adhesion amount sensor 137 functions as a toner amount detection unit that detects the amount of a specific toner image transferred onto the intermediate transfer belt 131.
The toner adhesion amount sensor 137 includes, for example, a light reflection type photosensor, and detects a specific toner image adhered and formed on the intermediate transfer belt 131, that is, a reflected light amount from a special color toner such as a colorless and transparent clear toner. As a result, the adhesion amount of the special color toner is measured.
The toner adhesion amount sensor 137 may also be used as a toner concentration sensor or the like as a toner concentration detecting means for detecting and measuring a toner concentration that has been generally used from the above function.
In this case, since it is possible to avoid the provision of a new toner amount detection means, it is possible to reduce the number of parts and contribute to cost reduction. The toner adhesion amount sensor 137 may be disposed at a position where the toner image on the photoconductor 112 is detected.

The primary transfer rollers 134 (S, Y, M, C, and K) are disposed to face the photoreceptors 112 (S, Y, M, C, and K), respectively, with the intermediate transfer belt 131 interposed therebetween. The belt 131 is driven and rotated to move.
As a result, a primary transfer nip is formed in which the front surface of the intermediate transfer belt 131 and the photosensitive member 112 (S, Y, M, C, K) are in contact (meaning that they are in contact with each other). Is done.
A primary transfer bias is applied to the primary transfer roller 134 (S, Y, M, C, K) by a primary transfer bias power source (not shown).
As a result, the S, Y, M, C, and K color toner images on the photoreceptor 112 (S, Y, M, C, and K) and the primary transfer roller 134 (S, Y, M, C, and K) During this, a primary transfer electric field is formed.
Then, each color toner image is sequentially transferred to the intermediate transfer belt 131.
The S-color toner image formed on the surface of the S-color photoconductor 112S enters the S-color primary transfer nip as the photoconductor 112S rotates.
Then, the image is primarily transferred from the photoreceptor 112S to the intermediate transfer belt 131 by the action of the transfer electric field and nip pressure.

The intermediate transfer belt 131 on which the S-color toner image is primarily transferred in this manner sequentially passes through the primary transfer nips for Y, M, C, and K.
Then, the Y, M, C, and K color toner images on the photoconductor 112 (Y, M, C, and K) are sequentially superimposed and sequentially transferred onto the S color toner image.
By this primary transfer of superposition, a superposition toner image including a color toner image and a special color toner image such as colorless and transparent is formed on the intermediate transfer belt 131.

The primary transfer roller 134 (S, Y, M, C, K) is composed of an elastic roller having a metal core and a conductive sponge layer fixed on the surface, and has an outer diameter of 16 mm. The core metal diameter is 10 mm.
In addition, a voltage of 1000 V is applied to the core of the primary transfer roller 134 (S, Y, M, C, K) in a state where a grounded metal roller with an outer diameter of 30 mm is pressed against the sponge layer with a force of 10 N. The resistance value R of the sponge layer was calculated from the current I flowing at the time.
Specifically, the resistance value R of the sponge layer calculated based on Ohm's law (R = V / I) from the current I flowing when a voltage of 1000 V is applied to the core metal is about 3 × 10 7 Ω. is there.
A primary transfer bias output by a constant current control from a primary transfer bias power source (not shown) is applied to the primary transfer roller 134 (S, Y, M, C, K).
In place of the primary transfer roller 134 (S, Y, M, C, K), a transfer charger, a transfer brush, or the like may be employed.

The secondary transfer roller 135 rotates with the intermediate transfer belt 131 and the recording medium sandwiched between the secondary transfer counter roller 136. Thus, a secondary transfer nip 139 is formed in which the surface of the intermediate transfer belt 131 and the secondary transfer roller 135 abut.
The secondary transfer roller 135 is rotationally driven by a secondary transfer roller drive motor 61 (FIG. 2) and functions as a nip forming member and a transfer member.
Further, the secondary transfer counter roller 136 functions as a nip forming member and a counter member.
While the secondary transfer roller 135 is grounded, a secondary transfer bias is applied to the secondary transfer counter roller 136 by the secondary transfer bias power source 130.

The secondary transfer bias power supply 130 has a DC power supply and an AC power supply, and can output a superimposed bias obtained by superimposing an AC voltage on a DC voltage as a secondary transfer bias.
The output terminal of the secondary transfer bias power supply 130 is connected to the core of the secondary transfer counter roller 136.

The potential of the core metal of the secondary transfer counter roller 136 is substantially the same as the output voltage value from the secondary transfer bias power source 130.
By applying a secondary transfer bias to the secondary transfer counter roller 136, a negative polarity toner is transferred between the secondary transfer counter roller 136 and the secondary transfer roller 135 from the secondary transfer counter roller 136 side. A secondary transfer electric field that is electrostatically moved toward the roller 135 is formed.
Accordingly, the negative polarity toner on the intermediate transfer belt 131 can be moved from the secondary transfer counter roller 136 side to the secondary transfer roller 135 side.
The secondary transfer bias power supply 130 uses a DC component having the same negative polarity as that of the toner, and the time average potential of the superimposed bias is set to the same negative polarity as that of the toner.
Instead of grounding the secondary transfer roller 135 while applying the superimposed bias to the secondary transfer counter roller 136, the core metal of the secondary transfer counter roller 136 is applied while applying the superimposed bias to the secondary transfer roller 135. It may be grounded, in which case the polarity of the DC voltage / DC component is made different.

In the case of using a recording medium having a large surface irregularity, such as embossed paper, the toner is reciprocated by applying the above-described superimposing bias, while relatively moving from the intermediate transfer belt 131 side to the recording medium side. To be transferred onto the recording medium.
As a result, the transferability to the paper recess can be improved, and the transfer rate can be improved and abnormal images such as voids can be improved.
On the other hand, in the case of using a recording medium with small unevenness, such as a normal transfer paper, since a shading pattern that follows the uneven pattern does not appear, sufficient transferability is obtained by applying a secondary transfer bias only with a DC component. be able to.

The secondary transfer counter roller 136 is formed by laminating a resistance layer on a cored bar made of stainless steel or aluminum.
The secondary transfer counter roller 136 has the following characteristics.
That is, the outer diameter is about 24 mm, and the diameter of the cored bar is about 16 mm.

The resistance layer is made of polycarbonate, fluorine rubber, silicon rubber in which conductive particles such as carbon or metal complex are dispersed, rubber such as NBR or EPDM, rubber of NBR / ECO copolymer, polyurethane semiconductive Made of rubber.
Its volume resistance is 10 ⁶ to 10 ¹² Ω, preferably 10 ⁷ to 10 ⁹ Ω.
The rubber hardness (ASKER-C) may be a foam type of 20 to 50 degrees or a rubber type of 30 to 60 degrees. However, since it contacts the secondary transfer roller 135 via the intermediate transfer belt 131, a small contact pressure is obtained. However, a sponge type that does not generate a non-contact portion is desirable.
This is to prevent characters and fine lines from being lost as the contact pressure between the intermediate transfer belt 131 and the secondary transfer counter roller 136 increases, and to prevent this.
Transfer residual toner that has not been transferred to the recording medium remains on the intermediate transfer belt 131 after the secondary transfer that has passed through the secondary transfer nip. This is removed and cleaned from the surface of the intermediate transfer belt 131 by a belt cleaning device 138 having a cleaning blade in contact with the surface of the intermediate transfer belt 131.

[Fixing part]
The fixing unit 14 is a belt fixing method, and is configured by pressing a pressure roller 142 against a fixing belt 141 that is an endless belt.
The fixing belt 141 is wound around a fixing roller 143 and a heating roller 144, and at least one of the rollers is provided with a heat source / heating means (a heater, a lamp, or an electromagnetic induction heating device, etc.) (not shown). ing.
The fixing belt 141 forms a fixing nip between the fixing belt 141 and the pressure roller 142 while being sandwiched and pressed between the fixing roller 143 and the pressure roller 142.
The recording medium sent to the fixing unit 14 is sandwiched between the fixing nips in such a posture that the unfixed toner image carrying surface is in close contact with the fixing belt 141.
As a result, the toner in the toner image is softened by heating or pressurization, so that the toner image is fixed and the recording medium is discharged outside the apparatus.

Also, when an image is formed on the surface of the recording medium opposite to the surface to which the toner image is transferred, the toner image is fixed and then transported to a recording medium reversing mechanism (not shown) for reversing the recording medium. The recording medium is reversed by the mechanism.
Thereafter, a toner image is formed on the opposite surface in the same manner as the image forming process described above.
The recording medium on which the toner is fixed by the fixing unit 14 is discharged from the image forming apparatus body 2 to the outside of the image forming apparatus 2 via a discharge roller (not shown) that constitutes the discharge unit 15, and a sheet storage unit 151 such as a discharge tray. Is housed in.
The image forming apparatus 1 illustrated in FIG. 1 includes a control unit Ctr that controls operations of the image forming apparatus such as an image reading operation and an image forming operation.

FIG. 2 is a diagram illustrating a control unit included in the image forming apparatus according to the present embodiment.
The control unit Ctr of the image forming apparatus 1 includes a CPU that executes a control program, a ROM that stores the control program, and a RAM as a storage area in which the control program is expanded for execution by the CPU and temporary data is stored. I have.
The image forming apparatus 1 includes an intermediate transfer belt drive motor 51 that rotates an intermediate transfer belt drive roller 132 that drives an intermediate transfer belt 131, and a secondary transfer roller drive motor 61 that drives a secondary transfer roller 135. ing.

The control unit Ctr controls the intermediate transfer belt drive motor 51 and the secondary transfer roller drive motor 61 via the motor drive circuit 50 and the motor drive circuit 60, and the rotational speed (drive) of the intermediate transfer belt 131 and the secondary transfer roller 135. Speed) can be changed.
The control unit Ctr controls the surface speed of the intermediate transfer belt 131 by controlling the rotation speed of the intermediate transfer belt drive motor 51 via the motor drive circuit 50. The control unit Ctr controls the surface speed of the secondary transfer roller 135 by controlling the rotation speed of the secondary transfer roller drive motor 61 via the motor drive circuit 60.
In FIG. 2, only the intermediate transfer belt drive motor 51 and the secondary transfer roller drive motor 61 are shown for explaining the characteristics of the present invention, but the control by the control unit Ctr is not limited to this. The entire function of the forming apparatus may be extended.

FIGS. 3A and 3B are diagrams for explaining the problems of the conventional image forming apparatus. FIG. 3A is a cross-sectional view of a toner layer for explaining the problems of the related art, and FIG. 3B is a plan view of FIG. is there.
In FIG. 3, the reference numerals with parentheses indicate colors.
For example, an entire solid image in which a layer 21 of C (cyan) toner and a layer 22 of M (magenta) toner are superimposed on a white recording medium 20 is used as a background, and a layer 23 of a transparent special color toner is formed thereon. When a solid patch image is formed, the following problems may occur.

As shown in the cross-sectional view of FIG. 3A, the M-color toner layer 22 and the C-color toner layer 21 are discolored around the spot color toner layer 23, and the plan view of FIG. A blank image 25 in which the peripheral area of the special color toner S is blank is generated.
As described above, when a solid full-color image formed by superimposing one, two, or more of Y, M, C, and K colors as a background and a solid patch image formed thereon with special color toner is used, This causes a problem that the color of the image is lost and a white-out image 25 is generated. This phenomenon is called “surrounding white”.

It has been found that this “periphery white spot” occurs due to the cause described below.
A step due to a difference in image thickness (difference in toner adhesion amount) is formed between a region where the solid patch image is overlaid with the special color toner and other regions in the entire background solid image.
This step causes insufficient adhesion to the recording medium in the peripheral area of the solid patch image in the entire area of the entire solid image, and colored toner does not transfer well from the intermediate transfer belt to the recording medium. Omission "occurs.
In particular, the “periphery white spot” becomes conspicuous when the toner is aggregated and deteriorated under a high temperature and high humidity environment and the adhesion force to the intermediate transfer belt 131 is increased.

Conventionally, the following countermeasures have been taken against such problems.
A conventional countermeasure example will be described with reference to FIG.
FIG. 4 is an enlarged schematic view showing the behavior of the toner in the secondary transfer nip when the image shown in FIG. 3 is formed.
In FIG. 2, a toner image primarily transferred from the photoconductors 112S, 112M, and 112C of FIG. 1 is carried on the intermediate transfer belt 131.
The toner image is secondarily transferred onto the recording medium 20 on the intermediate transfer belt 135 by applying a secondary transfer bias to the secondary transfer counter roller 136.

A “full-color solid image” composed of C toner particles 211 corresponding to the C toner layer 21 in FIG. 3A, M toner particles 221 corresponding to the M toner layer 22, and the special color toner layer 23. A “solid patch image” composed of S color toner particles 231 is superimposed on the recording medium 20.
During secondary transfer at the secondary transfer nip 139, the region 30 where the “solid patch image” of the special color toner overlaps in the entire area of the entire solid image is in good contact with the recording medium 20.
On the other hand, in the region 31 where there is no solid color patch image of the special color toner, there is a toner step between the region 30 where the solid patch image overlaps, and a gap 32 is generated between the recording medium 20 and the toner.
Due to the presence of the air gap 32, the adhesiveness with the recording medium 20 is weak in the peripheral area of the solid patch image, causing a transfer defect, and the blank image 25 shown in FIG. 3 is generated.

It is known that this problem can be improved to some extent by using a superimposed bias in which an AC component is superimposed on a DC component as a secondary transfer bias for forming a transfer electric field at the secondary transfer nip ( Patent Document 1).
That is, a secondary transfer bias composed of a superimposed bias in which a positive polarity DC component is superimposed on an AC component is applied.
As a result, part of the C toner particles 211 and the M color M toner particles 221 that are not in close contact with the recording medium 20 are released, and the C toner particles 211 and the M toner particles 221 reciprocate between the recording medium 20. To do.
As a result, toner detachment from the intermediate transfer belt 131 is promoted, and a sufficient amount of toner particles are transferred to the recording medium 20 side on the entire surface including the peripheral area of the solid patch image (areas 30 and 31 in FIG. 4). The omission can be improved.

However, even when such measures are taken, peripheral white spots cannot be improved, and an abnormal image may be caused depending on the magnitude of the AC component of the secondary transfer bias.
For example, in order to increase glossiness and brightness, and to enhance the image quality after image formation / printing, when the amount of spot color toner attached is set larger than usual, the S color toner particles 231 of the spot color toner layer 23 The number increases.
As a result, the level difference between the area 30 where the spot color toner solid patch image overlaps and the area 31 where the spot color toner solid patch image does not exist increases, and the gap 32 further expands between the recording medium 20 and a transfer failure occurs. It becomes easy to do.
Therefore, it is necessary to increase the AC component of the secondary transfer bias for transferring the toner layer particles in the region 31 where there is no solid patch image of the special color toner, and the dot dust (transfer dust) deteriorates in the halftone image or the like. There is a problem.
On the other hand, in the present embodiment, as shown in FIG. 5, by controlling the drive by the control unit Ctr, the peripheral white spots (white spots 25) are determined by making a difference in the speed between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20. ) Has been improved.

FIG. 5 is a diagram illustrating a configuration for improving peripheral white spots in the image forming apparatus according to the present embodiment.
The toner particles on the intermediate transfer belt 131 and the recording medium 20 have the same configuration as in FIG.
That is, it corresponds to the entire surface solid image composed of the C toner particles 211 corresponding to the C toner layer 21 in FIG. 3A, the M toner particles 221 corresponding to the M toner layer 22, and the special color toner layer 23. A solid patch image composed of S color toner particles 231 is superimposed on the recording medium 20.

As shown in FIG. 5, when the surface speed of the intermediate transfer belt 131 driven by the intermediate transfer belt drive motor 51 is V1, and the surface speed of the recording medium 20 driven by the secondary transfer roller drive motor 61 is V2, the control is performed. The part Ctr controls the speed V1 and the speed V2 to be different.
For example, by performing drive control so that speed V1> speed V2, a force F1 in the same direction as the movement direction of the intermediate transfer belt 131 acts on the surface of the intermediate transfer belt 131 and the interface between the toner layers.
Further, a force F2 in the opposite direction acts on the interface between the recording medium 20 and the toner layer on the secondary transfer roller.
As a result, a shearing force acts on the toner layer existing on the transfer nip 139 (on the intermediate transfer belt 131), the adhesion force between the intermediate transfer belt 131 and the toner layer is reduced, and toner can be easily transferred from the surface of the intermediate transfer belt 131. Detachable.
As a result, peripheral white spots (occurrence of white spots 25) in the toner image transferred to the recording medium 20 can be improved.
If a lubricant such as stearic acid is applied to the surface of the intermediate transfer belt 131, the releasability of the surface of the intermediate transfer belt 131 is improved, and the toner layer is more easily separated from the surface of the intermediate transfer belt 131. The omission can be further improved.

6A and 6B are diagrams for explaining the effect of the control in this embodiment. FIG. 6A is a state of the toner particles transferred to the recording medium 20, and FIG. 6B is a state of the toner layer transferred to the recording medium 20. Indicates the state.
As shown in FIG. 6A, the C toner particles 211 and the M toner particles 221 in the region 31 where there is no solid color patch image of the special color toner in FIG. 4 are transferred to the recording medium 20 beyond the gap 32.
Then, as shown in FIG. 6B, the white spot (white spot image 25) shown in FIG. 3 is improved in the toner layer on the recording medium 20.

Hereinafter, experiments conducted by the present inventors on the image forming apparatus of the present embodiment will be described with reference to FIGS.
The inventors modified a part of “RICOH Pro C751EX” manufactured by Ricoh Co., Ltd. having the same configuration as that of the image forming apparatus 1 according to the present embodiment and used it as a test machine.
The part that has been remodeled is that it has an image forming unit that can form and output toner images of special colors such as colorless and transparent (clear) and white.

The present inventors conducted various printing experiments using this testing machine.
As an image pattern, a blue solid image composed of C color toner and M color toner shown in FIG. 3 was used as a background, and conditions were set for forming a solid patch with special color toner thereon.
Further, a colorless and transparent clear toner was used as the “special color toner”.
As a developer, a toner by a polymerization method having an average particle diameter of 5.2 [μm] is used for C and M colors, and a toner by a pulverization method having an average particle diameter of 6.8 [μm] is used for a special color. did.
The carrier used was a magnetic carrier having an average particle size of 55 [μm] and a surface coated with a resin layer.
The laboratory environment was set to a temperature of 27 ° C. and a relative humidity of 80%. Gloss-coated paper (basis weight 128 g / m ² ) was used as the recording medium.

FIG. 7 is a graph showing the result of sensory evaluation of peripheral white spots when the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 is changed in the experiment using the test machine.
In FIG. 7, the horizontal axis indicates the linear speed adjustment value (%) of the secondary transfer roller 135 with respect to the intermediate transfer belt 131, and the vertical axis indicates the peripheral white spot evaluation rank corresponding to the linear speed adjustment value.
The experiment was conducted by setting the amount of spot color toner on the intermediate transfer belt 131 detected by the toner amount sensor 137 to 0.45 mg / cm ² .

When changing the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20, the speed of the intermediate transfer belt drive motor 51 is kept constant by the control of the control unit Ctr (FIG. 2), and the linear speed of the secondary transfer roller drive motor 61 is increased. The surface speed of the recording medium 20 was changed by adjusting (driving speed).
When the linear speed adjustment value is a positive value, the secondary transfer roller 135 is controlled to increase the linear speed with respect to the intermediate transfer belt 131, and when the linear speed adjustment value is a negative value, the secondary transfer roller 135 is controlled. 135 is controlled late with respect to the intermediate transfer belt 131.
The peripheral blank evaluation rank indicates a result of sensory evaluation by visual observation with a limit sample relating to peripheral blank that is set in advance with a human having a predetermined normal visual acuity as an evaluator.
From the sensory evaluation results, it was confirmed that if the evaluation rank is 3.5 or higher (the absolute value of the linear velocity adjustment value is 0.6 or higher), the white spots on the periphery can be improved to an inconspicuous level.

As shown in FIG. 7, when the linear transfer speed adjustment value of the secondary transfer roller is set to 0% and there is no speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20, the white spot on the periphery is not as good as rank 1. When the linear speed adjustment value of the secondary transfer roller was set to −0.8%, the peripheral white spot was improved to rank 5.
That is, it can be seen that when the amount of spot color toner is fixed, the larger the difference in speed between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20, the better the white spots on the periphery.
Therefore, the control unit Ctr basically performs control so that the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 becomes large in order to improve the peripheral white spots.
Even when the linear speed of the secondary transfer roller 135 was changed to the plus side, that is, even when the linear speed of the surface of the secondary transfer roller 135 was made faster than the surface of the intermediate transfer belt 131, the peripheral blank rank was improved.
In other words, if there is a speed difference between the secondary transfer roller 135 and the intermediate transfer belt 131, it is possible to improve peripheral white spots regardless of which is faster or slower.
Of course, the surface linear velocity of the secondary transfer roller 135 may be fixed and the surface velocity of the intermediate transfer belt 131 may be changed, or the surface velocity of both may be changed, but considering the printing speed and accuracy. It is more desirable to make the linear speed of the secondary transfer roller 135 variable.

By the way, if there is a speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20, an image that extends in the conveyance direction (sub-scanning direction) of the recording medium 20 or a contracted image may be formed.
FIG. 8 is a diagram illustrating a measurement result of a magnification error in the sub-scanning direction when the linear velocity of the secondary transfer roller 135 is changed in the experiment of FIG.
As shown in FIG. 8B, the magnification error referred to here is lower than the ideal sub-scanning direction length L0 in the original image data from the sub-scanning direction length L1 of the output image. This is a value calculated by the equation (1).
It represents the ratio of expansion / contraction of the output image with respect to the input image, and it is desirable that the absolute value is small.
Magnification error A = {(L1 / L0) × 100} −100% (1)
As shown in FIG. 8A, when the absolute value of the linear velocity adjustment value of the secondary transfer roller 135 increases, the absolute value of the magnification error in the sub-scanning direction also increases, and it can be seen that the image is stretched.
In order to prevent such image expansion and contraction, it is desirable that the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 is as small as possible.
Therefore, the control unit Ctr determines the linear speed adjustment value of the secondary transfer roller drive motor 61 so that the magnification error is equal to or less than the reference predetermined value while preventing the occurrence of peripheral white spots.

FIG. 9 is a graph showing the evaluation result of the white spots when the amount of spot color toner attached is changed. The experiment was performed with the linear speed adjustment value of the secondary transfer roller 135 set to 0%.
From the experimental results shown in FIG. 9, when no speed difference is given between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20, the evaluation rank of the surrounding white spots changes according to the amount of spot color toner (the amount of adhesion is large). It can be seen that the evaluation rank has decreased.
As described with reference to FIG. 4, when the amount of spot color toner attached is large, the level difference of the toner increases between the area where the spot color toner solid patch images overlap and the area where the spot color toner solid patch images do not exist.

Therefore, it is necessary to increase the speed difference between the intermediate transfer belt 131 and the surface of the recording medium 20 as shown in FIG.
On the other hand, when there is no special color toner on the intermediate transfer belt 131 or when the amount of special color toner attached is small, the toner level difference is small, so that the peripheral white spots do not occur or are within an allowable range. If the adhesion amount is less than or equal to a specified value, the problem of white spots on the periphery can be solved without increasing the linear speed difference between the surface of the intermediate transfer belt 131 and the recording medium 20 (or without giving a speed difference), and The expansion and contraction of the image in the sub-scanning direction can also be suppressed.
Therefore, the control unit Ctr determines the linear speed adjustment value of the secondary transfer roller drive motor 61 in accordance with the amount of spot color toner adhering to the intermediate transfer belt 131.

FIG. 10 is a graph plotting the speed difference at which the peripheral white spot evaluation rank becomes the specified value (3.5) when the amount of spot color toner attached is changed.
In this experiment, the linear speed adjustment value of the secondary transfer roller 135 was changed to the minus side.
As shown in FIG. 10, when the amount of the spot color toner is small, the expansion and contraction of the image in the sub-scanning direction can be suppressed by setting the absolute value of the linear speed adjustment value of the secondary transfer roller to be small.
Further, when the amount of spot color toner attached is large, peripheral white spots can be improved by increasing the absolute value of the linear speed adjustment value of the secondary transfer roller.
When there is a target peripheral blank spot evaluation rank, the control unit Ctr sets the line of the secondary transfer roller 135 so that the magnification error of the image on the recording medium 20 becomes an arbitrary allowable value according to the adhesion amount of the special color toner. Change the speed adjustment value.
By doing so, it is possible to improve peripheral white spots and to suppress the expansion and contraction of the image in the sub-scanning direction.

FIG. 11 is a graph showing the result of sensory evaluation of surrounding white spots by changing the experimental environment in the experiment of FIG.
The image pattern and experimental conditions were the same as those described with reference to FIG. 7, and only the experimental environment was changed to a temperature of 23 ° C. and a relative humidity of 50%.
The adhesion amount of the special color toner is 0.45 mg / cm ² . As shown in the experimental results, the peripheral white spot evaluation rank of the white spot was improved at low temperature and low humidity even though the amount of the spot color toner adhered was the same.
This indicates that when the temperature and humidity are lowered, the toner is less likely to aggregate, and the toner charge amount is increased and transferability is improved, so that “periphery white spots” are less likely to occur.
Therefore, the control unit Ctr may adjust the magnitude of the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 according to the use environment.
That is, the control unit Ctr reduces the speed difference as the temperature and humidity are lower and controls the speed difference as the temperature and humidity are higher.
In this case, the image forming apparatus needs to include a thermometer as a temperature measuring unit and a hygrometer as a humidity measuring unit.

As described above, as the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 increases, the white spot is improved, but conversely, in order to prevent the expansion and contraction of the image, it is desirable that the speed difference is small. The part Ctr appropriately adjusts the balance between prevention of peripheral white spots and prevention of image expansion / contraction.
Further, when the amount of spot color toner is small, even if the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 is reduced, the problem of large white spots does not occur.
Therefore, the control unit Ctr reduces the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 when the amount of the spot color toner on the intermediate transfer belt 131 detected by the toner adhesion amount sensor 137 is small.
When the adhesion amount of the special color toner is large, the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 is increased within a range where the expansion and contraction of the image is allowable.

The control unit Ctr changes the speed difference between the surface of the intermediate transfer belt 131 and the surface of the recording medium 20 in accordance with the amount of the specific toner image so as to achieve both prevention of white spots and prevention of image expansion / contraction. I do.
Thereby, even when a special color toner such as clear or colorless and transparent is used, stable quality image formation can be performed.
In the above-described embodiment, when the clear background or colorless transparent color toner image (solid patch image) is overlaid on the solid background toner image (entire solid image) with toners of Y, M, C, and K colors, peripheral white spots are eliminated. Although the structure to prevent was demonstrated, it is not restricted to it.

For example, the present invention can also be applied to a case where a step occurs in a superimposed toner image by superimposing a C color toner image (peta patch image) on a solid background toner image (entire solid image) of Y and M color toners.
In the above-described embodiment, an example of a tandem type image forming apparatus (FIG. 1) that forms toner images of each color on a plurality of photoconductors 11 has been described. Any image forming apparatus that needs to transfer a superimposed image to the above can be applied as appropriate.
For example, the present invention can also be applied to an image forming apparatus that repeats the process of forming a single color toner image on a single photoconductor 11 and transferring it to an intermediate transfer body for the number of colors.

FIG. 12 is a flowchart illustrating an example of a control process for preventing a blank image in the image forming apparatus according to the present embodiment.
Here, an example in which control is performed so that the blank evaluation rank is always 3.5 is shown.
In step S101, the control unit Ctr analyzes an image read by the reading unit 10 or an input image input from an external device such as a PC.

Next, in step S102, the control unit Ctr determines whether the superimposed toner image based on the read image and the input image includes a spot color toner image as a result of the analysis in step S101.
When the superimposed toner image does not include the special color toner image (No in step S102), the control unit Ctr controls so that no surface speed difference is generated between the intermediate transfer belt 131 and the secondary transfer roller 135 in step S103. .
When the superimposed toner image includes the special color toner image (Yes in Step S102), the control unit Ctr analyzes the output of the toner adhesion amount sensor 137 in Step S104.
If it is determined from the output of the toner adhesion amount sensor 137 that the spot color toner adhesion amount is smaller than the predetermined reference value, the control unit Ctr proceeds to step S103 and determines the surface speed difference between the intermediate transfer belt 131 and the secondary transfer roller 135. Don't make it happen.
When it is determined from the output of the toner adhesion amount sensor 137 that the spot color toner adhesion amount is larger than the reference value, the control unit Ctr determines that the magnification error of the image on the recording medium 20 is within the allowable range in step S106. The secondary transfer roller linear speed adjustment value is determined based on the relationship between the specific toner adhesion amount and the secondary transfer roller linear speed adjustment value.
The relationship between the specific toner adhering amount and the secondary transfer roller linear speed adjustment value in which the magnification error of the image on the recording medium 20 is within an allowable range is preliminarily set as a control table Ctr based on the experimental result of FIG. Stored in a ROM or the like.

Based on the secondary transfer roller linear velocity adjustment value determined in step S106, the control unit Ctr performs drive control of the secondary transfer roller 135 in step S107 to adjust the linear velocity.
However, as described above, the degree of occurrence of white spots (peripheral white spot evaluation rank) varies depending on temperature, humidity, and the like.
Therefore, a plurality of tables defining the relationship between the specific toner adhesion amount and the secondary transfer roller linear speed adjustment value in which the magnification error of the image on the recording medium 20 is within an allowable range are defined for each environmental state such as temperature and humidity. Alternatively, the table may be read according to the installation environment of the image forming apparatus.

In addition, the ROM defines a table that defines the relationship between the spot color toner adhering amount and the secondary transfer roller linear speed adjustment value in which the blank spot evaluation rank is a value other than (higher) than 3.5 and the magnification error is within an allowable range. It may be prepared and can be appropriately selected by a user operation according to the required print quality.
Further, the spot color toner adhesion amount may be calculated by analyzing an image read by the reading unit 10 or an input image input from an external device such as a PC, instead of calculating from the output result of the toner adhesion amount sensor 137. .
In step S103 described above, the intermediate transfer belt 131 and the secondary transfer roller 135 and the secondary transfer roller 135 are controlled so as not to cause a surface speed difference. It may be controlled so that a predetermined surface speed difference is generated between the first and second surfaces. In this case, the predetermined surface speed difference is made smaller than the surface speed difference when it is determined that the spot color toner adhesion amount is larger than the reference value.

[First invention]
A transfer nip 139 is formed between the intermediate transfer member 131 and the intermediate transfer member 131 onto which the superimposed toner image is transferred by superimposing a plurality of color toner images, and is superimposed on the recording medium 20 passing through the transfer nip 139. A secondary transfer body 135 that transfers the toner image, and a drive control means Ctr that controls the drive of the intermediate transfer body 131 and / or the secondary transfer body 135. The drive control means Ctr has a step difference in the superimposed toner image. When it occurs, drive control is performed so that a speed difference is generated between the surface speeds of the intermediate transfer member 131 and the recording medium 20.
Accordingly, a shearing force is generated in the toner image on the intermediate transfer member 131, so that the toner layer is easily separated from the surface of the intermediate transfer belt 131, and white spots on the recording medium 20 can be improved.
[Second invention]
The drive control means Ctr controls the recording drive by generating a speed difference when a stepped portion is generated in the superimposed toner image by superimposing the patch image of the other color toner image on the solid background image of one or more color toner images. Do.
As a result, even when a step is generated in the superimposed toner image by superimposing a patch image of a toner image of another color on the solid background image, a shearing force is generated in the toner image on the intermediate transfer member 131, thereby causing the intermediate transfer belt. The toner layer is easily separated from the surface of 131, and the white spots on the recording medium 20 can be improved.

[Third invention]
The other color toner image is a special color toner image using a white or colorless and transparent special color toner different from the four colors of yellow, magenta, cyan, and black.
When a patch image is superimposed on a solid background image with white or colorless and transparent special color toner, even if a step is generated in the superimposed toner image, a shearing force is generated in the toner image on the intermediate transfer member 131, thereby causing an intermediate transfer belt. The toner layer is easily separated from the surface of 131, and the white spots on the recording medium 20 can be improved.
[Fourth Invention]
A detection unit 137 that detects the toner amount of the patch image is provided, and the drive control unit Ctr performs drive control so as to change the speed difference according to the toner amount detected by the detection unit 137.
As a result, the “periphery of white spots” can be prevented while preventing the expansion and contraction of the image according to the level of the step generated in the superimposed toner image.

[Fifth Invention]
The drive control means Ctr performs drive control so that the speed difference increases as the toner amount of the detected patch image increases. As a result, even when the amount of the specific toner image is large, it is possible to prevent the “periphery white spot” problem.
[Sixth Invention]
The drive control unit Ctr controls the drive control based on the detected toner amount of the patch image so that a magnification error due to the speed difference in the superimposed image transferred to the recording medium becomes a predetermined allowable value. I do. This makes it possible to prevent white spots and prevent image expansion and contraction.
[Seventh Invention]
The drive control means Ctr performs drive control so as to change the speed difference according to the use environment. As a result, an optimal image can be obtained in accordance with the toner state depending on the use environment.

[Eighth Invention]
A transfer nip 139 is formed between the intermediate transfer member 131 to which the toner image is transferred and the intermediate transfer member 131, and the secondary transfer is performed to transfer the toner image on the intermediate transfer member 131 to a recording medium passing through the transfer nip 139. And a drive control means Ctr for controlling the driving of the intermediate transfer body 131 and the secondary transfer body 135. The drive control means Ctr includes a color toner and a special color toner other than the color toner. Drive control so that the surface speed difference between the intermediate transfer member 131 and the secondary transfer member 135 is greater than the surface speed difference between the intermediate transfer member and the secondary transfer member when the toner image is composed only of colored toner. I do.
As a result, a shearing force is generated in the toner image on the intermediate transfer member 131, so that the toner layer is easily separated from the surface of the intermediate transfer belt 131, and even when the toner image on the intermediate transfer member contains colored toner. The white spots on the recording medium 20 can be improved.
[Ninth Invention]
The drive control unit Ctr performs drive control so that the larger the amount of spot color toner on the intermediate transfer body 131 is, the larger the surface speed difference between the intermediate transfer body 131 and the secondary transfer body 135 is.
As a result, the “periphery of white spots” can be prevented while preventing the expansion and contraction of the image according to the level of the step generated in the superimposed toner image.

[Tenth Invention]
The special color toner is a white or colorless and transparent toner.
When a patch image is superimposed on a solid background image with white or colorless and transparent special color toner, even if a step is generated in the superimposed toner image, a shearing force is generated in the toner image on the intermediate transfer member 131, thereby causing an intermediate transfer belt. The toner layer is easily separated from the surface of 131, and the white spots on the recording medium 20 can be improved.
[Eleventh invention]
A transfer nip 139 is formed between the intermediate transfer belt 131 to which the toner image is transferred, the drive roller 132 that drives the intermediate transfer belt 131, the first drive motor 51 that drives the drive roller 132, and the intermediate transfer belt 131. A secondary transfer roller 139 that transfers the toner image on the intermediate transfer belt 131 to a recording medium that passes through the transfer nip 139, a second drive motor 61 that drives the secondary transfer roller 139, a first drive motor 51, and And a control unit Ctr for controlling the rotation speed of the second drive motor 61. The control unit Ctr transfers the toner image including the color toner and the special color toner other than the color toner to the recording medium. The intermediate transfer belt 1 is used to transfer the difference in surface speed between the toner and the secondary transfer roller 135 to a recording medium from a toner image composed only of colored toner. 1 to greater than the surface speed difference between the secondary transfer roller 135, and controls the rotational speed of the first drive motor 51 and the second driving motor 61.
As a result, a shearing force is generated in the toner image on the intermediate transfer member 131, so that the toner layer is easily separated from the surface of the intermediate transfer belt 131, and even when the toner image on the intermediate transfer member contains colored toner. The white spots on the recording medium 20 can be improved.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus, 2 Image forming apparatus main body, 10 Image reading part, 11 Image forming part, 12 Paper feeding part, 13 Transfer part, 14 Fixing part, 15 Paper discharge part, 20 Recording medium, 21, 22 Toner layer, 23 Special color toner layer, 25 white image, 131 intermediate transfer belt, 132 intermediate transfer belt drive roller, 135 secondary transfer roller, 137 toner adhesion amount sensor, 139 secondary transfer nip, 211 C toner particles, 221 M toner particles

JP 2013-125053 A

Claims (13)

An intermediate transfer body onto which a superimposed toner image obtained by superimposing a plurality of color toner images is transferred;
A secondary transfer member that forms a transfer nip with the intermediate transfer member and transfers the superimposed toner image onto a recording medium that passes through the transfer nip;
Drive control means for performing drive control of the intermediate transfer body and the secondary transfer body,
The drive control means performs the drive control so that a speed difference is generated between the surface speeds of the intermediate transfer member and the recording medium when a step is generated in the superimposed toner image. . The image forming apparatus according to claim 1.
The image forming apparatus, wherein the drive control unit generates the speed difference by changing a drive speed of the secondary transfer member. The image forming apparatus according to claim 1, wherein
The drive control means causes the speed difference when the stepped portion is generated in the superimposed toner image by superimposing a patch image of a toner image of another color on a solid background image of a toner image of one or more colors. An image forming apparatus that performs the drive control. The image forming apparatus according to claim 3.
2. The image forming apparatus according to claim 1, wherein the other color toner image is a special color toner image made of white or colorless and transparent special color toners different from the four colors of yellow, magenta, cyan, and black. The image forming apparatus according to claim 3 or 4,
Detecting means for detecting the toner amount of the patch image;
The image forming apparatus according to claim 1, wherein the drive control unit performs the drive control so that the speed difference is changed according to the toner amount detected by the detection unit. The image forming apparatus according to claim 5.
The image forming apparatus according to claim 1, wherein the drive control unit performs the drive control such that the speed difference increases as the toner amount of the detected patch image increases. The image forming apparatus according to claim 5.
The drive control means includes
The drive control based on the detected toner amount of the patch image is performed so that a magnification error caused by the speed difference in the superimposed image transferred to a recording medium becomes a predetermined allowable value. Image forming apparatus. The image forming apparatus according to any one of claims 1 to 7,
The image forming apparatus, wherein the drive control unit performs the drive control so as to change the speed difference according to a use environment. An intermediate transfer member to which a toner image is transferred;
A secondary transfer member that forms a transfer nip with the intermediate transfer member and transfers a toner image on the intermediate transfer member to a recording medium passing through the transfer nip;
Drive control means for performing drive control of the intermediate transfer body and the secondary transfer body,
The drive control means calculates a surface speed difference between the intermediate transfer member and the secondary transfer member when the toner image includes a colored toner and a special color toner other than the colored toner. An image forming apparatus, wherein the drive control is performed so as to be larger than a difference in surface speed between the intermediate transfer member and the secondary transfer member.   The drive control means performs the drive control so that the larger the adhesion amount of the special color toner on the intermediate transfer body, the larger the surface speed difference between the intermediate transfer body and the secondary transfer body. The image forming apparatus according to claim 9.   The image forming apparatus according to claim 9, wherein the colored toner includes at least one toner of yellow, magenta, cyan, and black.   The image forming apparatus according to claim 9, wherein the special color toner is a white or colorless and transparent toner. An intermediate transfer belt onto which the toner image is transferred;
A driving roller for running the intermediate transfer belt;
A first drive motor for driving the drive roller;
A secondary transfer roller that forms a transfer nip with the intermediate transfer belt and transfers a toner image on the intermediate transfer belt to a recording medium that passes through the transfer nip;
A second drive motor for driving the secondary transfer roller;
Control means for controlling the number of rotations of the first drive motor and the second drive motor,
The control means determines a surface speed difference between the intermediate transfer belt and the secondary transfer roller when transferring a toner image including a colored toner and a special color toner other than the colored toner to the recording medium from only the colored toner. The rotational speeds of the first drive motor and the second drive motor are controlled so as to be larger than the surface speed difference between the intermediate transfer belt and the secondary transfer roller when transferring the toner image to the recording medium. An image forming apparatus.

Images