JP2018146954A - Image formation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation apparatus which can improve the image quality by suppressing the void due to the reduction in the transfer efficiency by defining the relation between the peripheral velocity difference between an image carrier and an intermediate transfer belt and the nip width of a transfer nip part as a parameter.SOLUTION: An image formation apparatus includes: an image carrier; an intermediate transfer belt to which a toner image formed on the image carrier is transferred; and a primary transfer member which is arranged so as to contact the surface opposite to the image carrier of the intermediate transfer belt. The image carrier and the intermediate transfer belt contact with each other and constitute a transfer nip part holding and transferring the toner image. The image formation apparatus sets the lower limit value of the deviation amount to the value of at least 3/8, or preferably the half-value of the average peripheral length calculated from the previously-measured toner weight average particle diameter when the peripheral velocity difference is applied to the gap between the peripheral velocity of the intermediate transfer belt and the peripheral velocity of the image carrier and the relative movement amount between the image carrier and the intermediate transfer belt generated in the transfer nip part due to the peripheral velocity difference is defined as the deviation amount of the toner image.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer that forms an image by electrophotography, and more particularly to an image forming apparatus including an intermediate transfer belt that primarily transfers a toner image from an image carrier.

As a conventional image forming apparatus of this type, for example, the one described in Patent Document 1 is known. The image forming apparatus described in Patent Document 1 is in contact with an image carrier, an intermediate transfer belt to which a toner image formed on the image carrier is transferred, and a surface of the intermediate transfer belt opposite to the image carrier. A primary transfer roller arranged in this manner. The intermediate transfer belt is in contact with the image carrier to form a transfer nip portion, and the toner image is transferred from the image carrier to the intermediate transfer belt at the transfer nip portion.
In the transfer nip portion, if the peripheral speeds of the image carrier and the intermediate transfer belt are completely the same, the transfer efficiency is lowered, and so-called hollows are formed in which the central portion of the toner image such as letters and lines is whitened. Are known. Therefore, in Patent Document 1, the peripheral speed difference between the image carrier and the intermediate transfer belt is positively imparted, thereby increasing the transfer efficiency and suppressing the occurrence of voids and improving the image quality. Yes.

Japanese Patent Laying-Open No. 2006-1268

However, as a result of intensive studies on the problem of image quality deterioration (transfer efficiency decrease) due to hollowing out, it has been found that not only the peripheral speed difference but also the nip width of the transfer nip portion is affected.
The present invention uses the relationship between the peripheral speed difference between the image bearing member and the intermediate transfer belt and the nip width of the transfer nip as parameters, so that the image quality can be improved by suppressing voids due to a decrease in transfer efficiency. It is to provide a forming apparatus.

In order to solve the above object, the present invention provides:
An image carrier, an intermediate transfer belt to which a toner image formed on the image carrier is transferred, and a contact member in contact with a surface of the intermediate transfer belt opposite to the image carrier,
In the image forming apparatus that constitutes a transfer nip portion between the intermediate transfer belt and the image carrier by pressing the intermediate transfer belt against the image carrier by the contact member,
A peripheral speed difference is provided between the peripheral speed of the intermediate transfer belt and the peripheral speed of the image carrier, and the relative between the image carrier and the intermediate transfer belt generated in the transfer nip due to the peripheral speed difference. Assuming that the amount of movement is a toner image shift amount, the lower limit value of the shift amount is set to at least three-eighths of the average circumference calculated from the weight average particle diameter of the toner measured in advance. It is characterized by being.
Further, in order to solve the above-mentioned object problem, the present invention
An image carrier, an intermediate transfer belt to which a toner image formed on the image carrier is transferred, and a contact member in contact with a surface of the intermediate transfer belt opposite to the image carrier,
Exposure means for forming a latent image by exposing the image carrier,
In the image forming apparatus that constitutes a transfer nip portion between the intermediate transfer belt and the image carrier by pressing the intermediate transfer belt against the image carrier by the contact member,
A peripheral speed difference is provided between the peripheral speed of the intermediate transfer belt and the peripheral speed of the image carrier, and the relative between the image carrier and the intermediate transfer belt generated in the transfer nip due to the peripheral speed difference. Assuming that the amount of movement of the toner image is the amount of shift of the toner image, the size of the unit dot of the latent image formed on the surface of the image carrier that is the exposed surface by the exposure means = (to the toner image The size of the corresponding unit dot of the latent image in the main scanning direction−shift amount × ½) × 0.9 to (the size of the unit dot of the latent image corresponding to the toner image in the main scanning direction−shift amount × 1/2) × 1.1 range.

  According to the present invention, by using the relationship between the peripheral speed difference between the image carrier and the intermediate transfer belt and the nip width of the transfer nip as a parameter, it is possible to improve image quality by suppressing voids due to a decrease in transfer efficiency. it can.

1 is an overall configuration diagram of an image forming apparatus according to Embodiment 1 of the present invention. The enlarged view of the primary transfer part of FIG. The figure explaining the behavior of the toner in a drum nip part. The graph which shows the relationship between a drum nip width and a peripheral speed difference rate. The figure which shows the force relationship which acts on the primary transfer part of FIG. FIG. 3 is a diagram illustrating a relationship between a load applied to a primary transfer roller in FIG. 2 and a drum nip width. FIG. 6 is a diagram illustrating a configuration of a primary transfer unit according to a second embodiment of the present invention. FIG. 2 is a control block diagram of the photosensitive drum and the intermediate transfer belt in FIG. 1. FIG. 10 is a configuration diagram of an image exposure unit as exposure means according to Embodiment 4 of the present invention. FIG. 10 is a schematic diagram of a size of a unit dot according to a fourth embodiment of the present invention on an image carrier. The graph which shows the relationship between transfer efficiency and a peripheral speed difference rate. FIG. 5 is a schematic diagram showing the diameter of a unit dot in the sub-scanning direction and the expansion / contraction of a toner image.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. That is, it is not intended to limit the scope of the present invention to the following embodiments.
[Embodiment 1]
FIG. 1 is a schematic diagram showing an example of an image forming apparatus to which the present invention is applied.
This image forming apparatus is a color image forming apparatus that employs an intermediate transfer belt 31, and forms yellow, magenta, cyan, and black (hereinafter, Y, M, C, and Bk) images. A plurality of image forming stations 20 are provided. In the following description, the alphabetical symbols a, b, c, and d are added to the end of the symbols constituting the image forming stations in the order of Y color, M color, C color, and Bk color. In the case where no alphabetic code is assigned, the description is common to all the image forming stations 20.
The intermediate transfer belt 31 is an endless belt, which is an intermediate resistance elastic body, and is wound around the secondary transfer counter roller 34 and the belt driving roller 11 that are arranged apart from each other. Assuming that the side moving from the secondary transfer counter roller 34 toward the belt drive roller 11 is the forward movement side, each of the image forming stations 20a, 20b, 20c, and 20d is along the forward movement surface of the intermediate transfer belt 31. , Y color, M color, C color, and Bk color.
Each image forming station 20 includes a drum-shaped image carrier 2 (hereinafter referred to as a toner image) on which a toner image is formed.
A plurality of photosensitive drums 2 are arranged along the moving direction of the intermediate transfer belt 31. The photosensitive drum 2 is incorporated in a process cartridge 32 together with a charging roller 1, a developing device 5, and a drum cleaner 6 which are process means for performing an electrophotographic process, and an image exposure unit 4 is disposed adjacent to each process cartridge 32. ing.
A primary transfer roller 14 as a contact member abuts on the surface of the intermediate transfer belt 31 opposite to the photosensitive drum 2 to form a primary transfer unit 21.

Next, color image formation performed by the image forming apparatus will be described.
The photosensitive drum 2 is rotationally driven at a predetermined peripheral speed in the direction of the arrow. For example, the Y-color image forming station 20a will be described. The image exposure unit 4a performs image exposure on the photosensitive drum 2a uniformly charged by the charging roller 1a. As a result, an electrostatic latent image corresponding to the Y color component image of the target color image is formed on the photosensitive drum 2a, and then the electrostatic latent image is developed at the developing position by the developing device 5a to form a toner image. It is visualized on the photosensitive drum 2a.
The Y color toner image formed on the photosensitive drum 2a is transferred to the intermediate transfer belt 31 by applying a voltage of reverse polarity to the primary transfer roller 14 in the primary transfer portion 21a. Further, residual toner on the photosensitive drum 2a is removed by the drum cleaner 6a.
The toner image formation on the photosensitive drum 2a at the image forming station 20a and the transfer process of the toner image onto the intermediate transfer belt 31 are also performed at the C, M, and Bk image forming stations 20b, 20c, and 20d. . As a result, the toner images of the respective colors are transferred onto the intermediate transfer belt 31 to form a four-color full-color toner image.
On the other hand, a transfer material S is fed from a transfer material container 37 disposed below the intermediate transfer belt 31 by a paper feed roller 38, and sent to the secondary transfer unit 22 by a registration roller pair 39 at a predetermined timing. In this secondary transfer portion 22, a four-color full-color toner image is collectively transferred onto a transfer material by a secondary transfer roller 35 as a secondary transfer member, and is fused and fixed by a fixing device 18 to form a color print image. The Residual toner on the intermediate transfer belt 31 is removed by a belt cleaner 33.

Next, the primary transfer unit 21 of the image forming apparatus will be described.
FIG. 2 is an enlarged view of the primary transfer unit 21.
In this embodiment, the primary transfer roller 14 has a configuration in which an elastic body 14b having rubber-like elasticity is wound around a metal core 14a. In the primary transfer unit 21, the primary transfer roller 14 is disposed so as to face the photosensitive drum 2 with the intermediate transfer belt 31 interposed therebetween, and the intermediate transfer belt 31 is pressed against the photosensitive drum 2 by the primary transfer roller 14 to be narrowed. I have it. The intermediate transfer belt 31 is wound around the photosensitive drum 2 by a predetermined length and is in contact with each other, and this contact area becomes the drum nip portion 15 constituting the transfer nip portion.
If the contact width in the conveyance direction at the drum nip portion 15 is the drum nip width Ld, the intermediate transfer belt 31 is wound around and contacts the photosensitive drum 2 by the drum nip width Ld. The drum nip width Ld is the length of the partial arc of the outer circumferential circle having a circular cross section perpendicular to the central axis of the photosensitive drum 2, and the center angle corresponding to the drum nip width Ld is hereinafter referred to as a winding angle θ.
The photosensitive drum 2 is rotated at a predetermined peripheral speed Vd (process speed), the intermediate transfer belt 31 is rotated at a predetermined peripheral speed Vb, and the toner images TI are sequentially transferred to the intermediate transfer belt 31 at the drum nip 15. It will be done. The primary transfer roller 14 rotates with the intermediate transfer belt 31. The peripheral speeds Vd and Vb of the photosensitive drum 2 and the intermediate transfer belt 31 are the speeds of the drum surface and the belt surface, respectively.

Next, a mechanism for improving transfer efficiency by imparting a peripheral speed difference to the drum nip 15 which is a premise of the present invention will be described with reference to FIG.
FIG. 3A schematically shows the inside of the drum nip portion 15, and is a diagram illustrating the behavior of the toner Tn when a peripheral speed difference is applied between the photosensitive drum 2 and the intermediate transfer belt 31.
In the drum nip 15, the photosensitive drum 2 rotates at a peripheral speed of Vd, and the intermediate transfer belt 31 rotates at a peripheral speed Vb, so that a peripheral speed difference Vd−Vb (hereinafter also referred to as ΔV) is given. In the first embodiment, Vd and Vb include

Vd <Vb (Formula 1)

Thus, the primary transfer configuration is such that the peripheral speed Vb of the intermediate transfer belt 31 is greater than the peripheral speed Vd of the photosensitive drum 2.
The lowermost toner adhering to the latent image forming portion on the photosensitive drum 2 by the development process has a contact point with the photosensitive drum 2. In many cases, the toner having a contact point with the photosensitive drum 2 is in a stable state on each surface with the contact point being a portion having a larger adhesion force to the photosensitive drum 2. The toner tends to adhere at a point with a large adhesion force that varies depending on the surface shape and surface charge state. In addition, it is difficult to transfer the toner adhered at a point with a large adhesion force, and in order to increase the primary transfer efficiency, a transfer condition that expresses a force greater than the adhesion force is required.
First, when the toner Tn reaches the drum nip portion 15, the toner Tn rotates like a bearing due to the difference in peripheral speed, and moves from the state A to the state B. Along with this movement, the contact point Pt between the toner Tn and the photosensitive drum 2 moves to the point Pt ′. Therefore, the contact Pt having a large adhesion force that has been in contact with the photosensitive drum 2 before entering the drum nip portion 15 is separated from the photosensitive drum 2, and the adhesion force between the toner Tn and the photosensitive drum 2 is reduced.
If there is a difference in peripheral speed, the contact point Pt is separated from the photosensitive drum 2, so the magnitude relationship between the peripheral speed Vd of the photosensitive drum 2 and the peripheral speed Vb of the intermediate transfer belt 31 does not need to follow Formula 1 and may be reversed. Needless to say, an effect occurs.
As described above, by providing a peripheral speed difference between the photosensitive drum 2 and the intermediate transfer belt 31, the adhesion force between the toner Tn and the photosensitive drum 2 is reduced, and the toner Tn is easily peeled off from the photosensitive drum 2. The effect of improving the primary transfer efficiency appears.

<Defining the shift amount as a special parameter>
In the present invention, the relative movement amount between the photosensitive drum 2 and the intermediate transfer belt 31 generated in the drum nip portion (transfer nip portion) due to the peripheral speed difference is not simply set based on the peripheral speed difference. It is defined as the amount of toner image shift. Then, the nip width and peripheral speed difference ΔV of the drum nip portion 15 are set so that the shift amount falls within a preset range, and the balance between transfer efficiency and image quality deterioration is optimized.
Hereinafter, the relationship between the “shift amount” of the toner image, which is a specific parameter defined in the present invention, and the image extension will be described with reference to FIG. FIG. 3B is a schematic diagram in which the toner Tn of FIG. 3A is a sphere for simplification of description.
The shift amount of the toner image is a relative movement amount of the photosensitive drum 2 and the intermediate transfer belt 31 generated in the drum nip portion 15 due to the peripheral speed difference, and is defined as follows in this embodiment.

Shift amount (S) = Differential speed ratio (R) × Drum nip width (Ld) (Equation 2)

Here, the peripheral speed difference rate of Expression 2 is expressed as the ratio of the peripheral speed difference | Vd−Vb | between the peripheral speed Vd and the peripheral speed Vb of the intermediate transfer belt 31 with respect to the peripheral speed Vd of the photosensitive drum 2. Defined in

Peripheral speed difference ratio (R) = | Vd−Vb | / Vd (formula 3)

The peripheral speed difference rate R indicates a relative peripheral speed difference between the photosensitive drum 2 and the intermediate transfer belt 31, and is not particularly limited to Equation 3.
Table 1 shows the shift amount S when the drum nip portion 15 and the peripheral speed difference rate R are changed. As shown in Table 1, the shift amount S is a parameter that increases as the peripheral speed difference rate R increases or the drum nip width Ld increases.

Next, the expansion of the toner image in the drum nip portion 15 when a difference in peripheral speed between the photosensitive drum 2 and the intermediate transfer belt 31 is described.
As shown in FIG. 3B, when the toner Tn is rotated from the state A to the state B by the distance D, the toner Tn moves the distance D, so that the position Ptd on the photosensitive drum 2 is Move to distance Ptd 'by distance D. Then, the point Ptb on the intermediate transfer belt 31 moves to the point Ptb ′ by the distance D in the direction opposite to the movement of the photosensitive drum 2. Therefore, the relative moving distance between the photosensitive drum 2 and the intermediate transfer belt 31 is D × 2. That is, with respect to the relative moving distance of the photosensitive drum 2 and the intermediate transfer belt 31, that is, the “shift amount”, the toner Tn moves in the drum nip portion 15 by a half value of its circumference, and the image is extended. . For the toner Tn, the point A0 that contacts the photosensitive drum 2 at the position Ptd moves to A0 ′.

Next, the range definition of “shift amount”, which is a numerical specification specific to the first embodiment, will be described with reference to FIG.
In FIG. 4, the horizontal axis represents the drum nip width Ld [mm], and the vertical axis represents the peripheral speed difference ratio R [%] expressed as a percentage. The graph is an equal shift amount curve in which the shift amount S is fixed at three levels of 10.05 μm, 42.33 μm, and 84.67 μm, and the relationship between the peripheral speed difference ratio R and the drum nip width Ld at these three levels is drawn. When the shift amount S is fixed, the peripheral speed difference rate R and the drum nip width Ld have an inversely proportional relationship.
The lower limit value of the shift amount S is defined on the condition that desired transfer efficiency can be obtained.
Table 1 shows the result of measuring the reflectance corresponding to the reflection density. That is, a solid image is printed at the М color station, and the residual toner image after the transfer on the C color photosensitive drum after the solid image is printed (after transfer) at the downstream C color station is taped to a reflection densitometer ( (Existence) Tokyo Electric Decoration: It is the result of having measured the reflectance by model number TC-6DS.
Round off the decimal point of the reflectivity and use 6% and 8% as thresholds. If it is 6% or less, “○”, if it is greater than 6% and less than 8%, “Δ”, 8% "X" if larger
And the reflectance of the residual toner image after the transfer of the secondary solid color was evaluated.

When the results of Table 2 are drawn with ◯, Δ, and X on the graph of FIG. 4, the boundary of the transfer residual toner reflectance of 6% is approximately the curve of the shift amount of 10.05 μm of FIG. The boundary at the reflectance of 8% is approximately a curve with a shift amount of 7.5 μm. In order to set the reflectance of the transfer residual toner to 6% or less, it is necessary to use the upper right side region (region where the shift amount is large) in the graph rather than the curve of the shift amount of 10.05 μm. Further, in order to set the reflectance of the transfer residual toner to 8% or less, it is necessary to use the upper right region (region where the shift amount is large) in the graph rather than the curve of the shift amount of 7.5 μm.
Therefore, if the shift amount is set to 7.5 μm or more, the reflectance of the transfer residual toner is 8% or less, and if the shift amount is set to 10.05 μm or more, the reflectivity of the transfer residual toner is 6% or less. Will be satisfied.
In this study, toner having a weight average particle diameter (D4) of 6.4 μm is used, and when the toner is assumed to be a sphere (FIG. 3B), the peripheral length of the toner is 20.11 μm. The shift amount of 7.5 μm at which acceptable transferability is obtained is a value that is approximately three-eighth of the circumference of the toner, and an arc length that is three times the octant. Further, the shift amount of 10.05 μm at which a suitable transfer property can be obtained is approximately half the toner peripheral length. By rotating approximately half a circle, the initial contact point of the toner Tn with the photosensitive drum 2 moves to the intermediate transfer belt 31 side, ensuring a sufficient distance from the photosensitive drum 2 to reduce the adhesive force, and more transfer. Efficiency is expected to improve.
As described above, in Embodiment 1, the value of three-eighths of the average circumference calculated from the weight average particle diameter of the toner used is set as the lower limit value. More preferably, if the half value is set as the lower limit value of the shift amount, the transfer efficiency can be further improved. Although the allowable threshold value of the residual toner reflectivity is 8%, it is not necessary to be limited to this, and it may be more than three-eighth of the average circumference calculated from the weight average particle diameter of the toner used. For example, the lower limit of the amount by which the desired transfer efficiency is shifted to the target may be specified.

<Method for Measuring Toner Weight Average Particle Size (D4)>
The weight average particle diameter (D4) of the toner can be calculated as follows.
As a measuring device, a precise particle size distribution measuring device “Coulter Counter Multisizer 3” (registered trademark, manufactured by Beckman Coulter, Inc.) using a pore electrical resistance method provided with an aperture tube of 100 μm is used. The attached dedicated software “Multisizer 3 Version 3.51” (manufactured by Beckman Coulter, Inc.) is used for setting the measurement conditions and analyzing the measurement data. The measurement is performed with 25,000 effective measurement channels.
As the electrolytic aqueous solution used for the measurement, special grade sodium chloride is dissolved in ion-exchanged water so as to have a concentration of about 1% by mass, for example, “ISOTON II” (manufactured by Beckman Coulter, Inc.) can be used.
Prior to measurement and analysis, the dedicated software is set as follows.
On the “Change Standard Measurement Method (SOM)” screen of the dedicated software, set the total count in the control mode to 50000 particles, set the number of measurements once, and set the Kd value to “standard particles 10.0 μm” (Beckman Coulter, Inc.) Set the value obtained using By pressing the “Threshold / Noise Level Measurement Button”, the threshold and noise level are automatically set. In addition, the current is set to 1600 μA, the gain is set to 2, the electrolyte is set to ISOTON II, and the “aperture tube flush after measurement” is checked.
In the “Pulse to particle size conversion setting” screen of the dedicated software, the bin interval is set to logarithmic particle size, the particle size bin is set to 256 particle size bin, and the particle size range is set to 2 μm to 60 μm.
The specific measurement method is as follows.
(1) About 200 ml of the electrolytic solution is put in a glass 250 ml round bottom beaker dedicated to Multisizer 3 and set on a sample stand, and the stirrer rod is stirred counterclockwise at 24 rpm. Then, dirt and bubbles in the aperture tube are removed by the “aperture flush” function of the dedicated software.
(2) About 30 ml of the electrolytic aqueous solution is put into a glass 100 ml flat bottom beaker. In this, "Contaminone N" (nonionic surfactant, anionic surfactant, 10% by weight aqueous solution of neutral detergent for pH7 precision measuring instrument cleaning, made by organic builder, manufactured by Wako Pure Chemical Industries, Ltd. About 0.3 ml of a diluted solution obtained by diluting 3) with ion-exchanged water is added.
(3) Two oscillators with an oscillation frequency of 50 kHz are incorporated with the phase shifted by 180 degrees, and an ultrasonic disperser “Ultrasonic Dispersion System Tetora 150” (manufactured by Nikka Ki Bios) having an electrical output of 120 W is prepared. About 3.3 l of ion-exchanged water is placed in the water tank of the ultrasonic disperser, and about 2 ml of Contaminone N is added to the water tank.
(4) The beaker of (2) is set in the beaker fixing hole of the ultrasonic disperser, and the ultrasonic disperser is operated. And the height position of a beaker is adjusted so that the resonance state of the liquid level of the electrolyte solution in a beaker may become the maximum.
(5) In a state where the electrolytic aqueous solution in the beaker of (4) is irradiated with ultrasonic waves, about 10 mg of toner is added to the electrolytic aqueous solution little by little and dispersed. Then, the ultrasonic dispersion process is continued for another 60 seconds. In the ultrasonic dispersion, the temperature of the water tank is appropriately adjusted so as to be 10 ° C. or higher and 40 ° C. or lower.
(6) To the round bottom beaker (1) installed in the sample stand, the electrolytic solution (5) in which the toner is dispersed is dropped using a pipette, and the measurement concentration is adjusted to about 5%. . Measurement is performed until the number of measured particles reaches 50,000.
(7) The measurement data is analyzed with the dedicated software attached to the apparatus, and the weight average particle diameter (D4) and the number average particle diameter (D1) are calculated. The “average diameter” on the “analysis / volume statistics (arithmetic average)” screen when the graph / volume% is set by the dedicated software is the weight average particle diameter (D4). The “average diameter” on the “analysis / number statistical value (arithmetic average)” screen when the graph / number% is set in the dedicated software is the number average particle diameter (D1).

Next, the definition of the upper limit value of the shift amount S will be described. The upper limit of the shift amount is defined from the image stretch.
In order to guarantee the resolving power which is the specification of the image forming apparatus, the maximum value of the image expansion is determined. For example, if a 600 dpi specification is to be guaranteed, it is necessary to suppress image expansion within the resolution of 42.33 μm. Therefore, the shift amount is 84.67 μm, which is a double value of the shift amount, and may be used in the lower left region in the graph rather than the curve of the shift amount 84.67 μm in FIG. Further, if the 1200 dpi specification is guaranteed, it is necessary to suppress the image expansion within the resolution of 21.17 μm. Therefore, the shift amount is 42.33 μm, which is a double value of the shift amount, and may be used in the lower left region in the graph rather than the curve of the shift amount 42.33 μm in FIG.
That is, the resolution in the moving direction of the intermediate transfer belt 31 of the image forming apparatus is based on the upper limit of the shift amount, and the shift amount S is less than or equal to twice the resolution in the sub-scanning direction parallel to the moving direction of the intermediate transfer belt 31. Set.

As described above, in the first exemplary embodiment, the shift amount S is set to a value that is three-eighths of the average circumference calculated from the weight average particle diameter of the toner measured in advance, preferably half or more, and the intermediate transfer belt 31. Is set to a value equal to or less than twice the resolution in the sub-scanning direction parallel to the moving direction. Within this range, the drum nip width Ld and the peripheral speed difference ΔV of the drum nip portion 15 are set. For example, the peripheral speed Vb of the intermediate transfer belt is determined by adding ΔV to the peripheral speed Vd of the photosensitive drum.
As a method of giving the peripheral speed difference, a method in which the photosensitive drum 2 and the intermediate transfer belt 31 have independent drive systems, a common drive system in the photosensitive drum 2 and the intermediate transfer belt 31, and a mechanical ratio such as a gear ratio are used. There is a way to attach. The former has the freedom to make the peripheral speed difference variable.
FIG. 8A shows an example of a configuration in which the photosensitive drum 2 and the intermediate transfer belt 31 have independent drive systems.
The photosensitive drum 2 is driven by a drum driving motor Md via a transmission mechanism 110 such as a gear, and the intermediate transfer belt 31 is driven by a belt driving motor Mb via a transmission mechanism 120 such as a gear. The The rotational speeds of the motors Md and Mb are set by the control unit 100 having a CPU corresponding to the target peripheral speed Vd of the photosensitive drum 2 and the peripheral speed Vb of the intermediate transfer belt 31. Since the peripheral speeds Vd and Vb can be set freely, the peripheral speed difference ΔV can be set freely within a preset shift amount range. The peripheral speeds of the photosensitive drum 2 and the intermediate transfer belt 31 are sequentially detected by the speed sensors 101 and 102 and fed back to the control unit 100 to be controlled so as to maintain the set value.
FIG. 8B is a schematic diagram of an example in which a drive system is shared.
In the drive system, power is transmitted from a common motor Mo as a drive source to the photosensitive drum 2 and the belt drive roller 11 of the intermediate transfer belt 31.
In this case, the peripheral speed Vd of the photosensitive drum 2 is determined by the rotational speed of the motor Mo, the gear ratio between the motor gear 131 and the drum driving gear 132, and the outer diameter of the photosensitive drum 2. The peripheral speed Vb of the intermediate transfer belt 31 is determined by the rotational speed of the motor Mo, the gear ratio between the motor gear 131 and the roller drive gear 133, the diameter of the belt drive roller 11, and the thickness of the intermediate transfer belt 31. Therefore, by changing the gear ratio, the peripheral speed and speed transmission ratio of the photosensitive drum 2 are changed, and a predetermined peripheral speed difference is given to the peripheral speed Vd of the photosensitive drum 2 and the peripheral speed Vb of the intermediate transfer belt 31. it can.
As a method for verifying the peripheral speed difference, the surface speeds of the photosensitive drum 2 and the intermediate transfer belt 31 can be actually measured and compared with a speedometer such as a laser Doppler method.

Next, a method for calculating the drum nip portion 15 will be described with reference to FIGS. 5 and 6.
FIG. 5 shows the force relationship acting on the primary transfer portion 21.
The drum nip portion 15 is formed by pressing the intermediate transfer belt 31 by the primary transfer roller 14 that is an elastic body against the rigid photosensitive drum 2. Since tension is applied to the intermediate transfer belt 31, a non-contact region g in which the intermediate transfer belt 31 cannot be pushed into the photosensitive drum 2 by the primary transfer roller 14 is generated as shown in FIG. Therefore, the nip width of the drum nip 15 tends to be smaller than the nip portion 16 (hereinafter referred to as the roller nip portion 16) where the primary transfer roller 14 and the intermediate transfer belt 31 are in contact with each other. A point Pm is an end of the drum nip portion 15, and a drum tangent 2T at the point Pm is indicated by a broken line.
FIG. 5B shows that the tension force Ft of the intermediate transfer belt 31 acting on the end point Pm of the drum nip 15 is changed to a force Ftx in the drum tangent 2T direction and a force Fty perpendicular to the drum tangent 2T at the point Pm. It is the schematic diagram which decomposed | disassembled. The force Fty perpendicular to the drum tangent 2T is one of the forces that crush the primary transfer roller 14.
On the other hand, the bending stress Fb of the intermediate transfer belt 31 (not shown) becomes an impeding force of the pressing force that presses the intermediate transfer belt 31 against the photosensitive drum 2 by the primary transfer roller 14. When the pushing force for pushing up the intermediate transfer belt 31 by the primary transfer roller 14 is Fr, the point Pm

Fr = Ft · tan β + Fb (Equation 4)

Thus, the portion where the sum of the pushing force of the primary transfer roller 14 on the left side and the inhibition force on the right side balances becomes the end of the drum nip. Here, Fr is expressed by Equation 5 below.

Fr = E · ε (Equation 5)

In the equation, E is the Young's modulus of the rubber constituting the elastic body 14 b of the primary transfer roller 14, and ε is the strain of the primary transfer roller 14. Needless to say, if the right side of Expression 4 is smaller than the left side of Expression 4, the pushing force by the primary transfer roller 14 is superior to the inhibition force, and the drum nip portion 15 is formed.
Accordingly, the position of the point Pm on the photosensitive drum 2 is derived from the transfer pressure of the primary transfer roller 14, the physical property value of the rubber of the elastic body 14 b, the tension of the intermediate transfer belt 31, and the physical property value of the intermediate transfer belt 31. Ld can be calculated.
For the verification of the drum nip 15, Dassault Systèmes' structural analysis software “Abacus / Standard Ver. 6.91” was used.
Table 3 shows parameters for calculating the drum nip width and specific values thereof. The parameters include the outer diameter of the photosensitive drum 2, the outer diameter of the primary transfer roller 14, the longitudinal length of the primary transfer roller 14, the rubber thickness of the elastic body 14b of the primary transfer roller 14, and the rubber of the elastic body 14b of the primary transfer roller 14. There is a Young's modulus. In addition, there are a belt tension, a belt Young's modulus, a belt thickness of the intermediate transfer belt 31, a roller load of the primary transfer roller 14, and a load direction with respect to the intermediate transfer belt 31.

FIG. 6 shows the analysis results of the nip widths at the drum nip portion 15 and the roller nip portion 16 when the weight applied to the primary transfer roller 14 is changed by 200 gf from 200 gf to 800 gf with these parameters.
According to FIG. 6, the nip widths of the drum nip portion 15 and the roller nip 16 both increase as the roller load increases, but the increase amount of the nip width of the drum nip portion 15 is smaller than the increase amount of the roller nip portion 16. For example, when the load is 400 gf, the nip width of the drum nip portion 15 is 0.32 mm.
As a method for verifying the drum nip width Ld of the drum nip 15, the width of the color material transferred onto the photosensitive drum 2 by contacting and separating from the photosensitive drum 2 with a color material such as toner placed on the intermediate transfer belt 31. Can be obtained by measuring In this case, it should be noted that the width of the color material transferred tends to increase depending on the amount and thickness of the color material.
As described above, according to the first exemplary embodiment, by defining the “shift amount” as a parameter related to the peripheral speed difference among the drum nip portion 15, the photosensitive drum 2, and the intermediate transfer belt 31, transfer efficiency (middle loss) Prevention) and image quality (image elongation) can both be achieved.

Next, another embodiment of the present invention will be described. In the following description, only differences from the first embodiment will be described, and the same components are denoted by the same reference numerals and description thereof is omitted.
[Embodiment 2]
7A and 7B show the primary transfer unit 221 according to the second embodiment of the present invention. FIG. 7A is a cross-sectional view of an image forming station, and FIG. 7B is a diagram showing the relationship between the two image forming stations.
As the primary transfer member, as in the first embodiment, a primary transfer roller 14 in which an elastic body 14b such as rubber is wound around a metal core 14a is generally used. However, in the second embodiment, only a rigid metal is used. This is constituted by the metal roller 214. In the configuration using the metal roller 214, the primary transfer unit 221 is arranged with the metal roller 214 offset with respect to the photosensitive drum 2 on the upstream side or the downstream side in the transport direction of the intermediate transfer belt 31. By adopting the offset arrangement, the intermediate transfer belt 31 cannot be held between the photosensitive drum 2 and the metal roller 214, and the intermediate transfer belt 31 is wound around the photosensitive drum 2 to form the drum nip portion 15.
In order to ensure a desired transfer pressure at the drum nip 15, the amount of penetration of the metal roller 214 into the photosensitive drum 2 side tends to increase. Therefore, the winding amount around the photosensitive drum 2 becomes larger than the primary transfer roller 14 using the elastic body 14b such as rubber, and the nip width of the drum nip portion 15 becomes larger. If the circumferential speed difference is set in this situation, the shift amount increases even if the circumferential speed difference rate is set to be small, and the risk of image damage increases.
The definition of the shift amount, the effect of the shift amount on the transfer efficiency, and the image elongation are the same even when the metal roller 214 is used, and the specified range of the shift amount is the same as that of the first embodiment. In the configuration of the metal roller 214, the drum nip portion 15 can be measured by measuring the position of the member three-dimensionally.

A method for calculating the drum nip 15 will be described below.
As shown in FIG. 7A, the metal roller 214 is arranged on the downstream side in the rotation direction of the intermediate transfer belt 31 and is placed on the photosensitive drum 2 side by a pressure member (not shown) to secure a desired transfer pressure. It is lifted and enters the photosensitive drum 2 side with the penetration amount Dt.

Drum nip width (Ld) = Drum circumference x (θ / 360 °) (Equation 6)

Here, from FIG.

θ = tan ⁻¹ ((Dt + Bt) / D1) + tan ⁻¹ ((Dt + Bt) / D2) (Equation 7)

The winding angle θ [°] is the winding angle at which the surface of the intermediate transfer belt 31 contacts the surface of the photosensitive drum 2, and Bt is the thickness of the intermediate transfer belt 31. D1 and D2 are both the distance between the metal roller 214 and the photosensitive drum 2, and D1 and D2 are used as two-level values in order to distinguish the difference in distance from the metal roller 214 of the adjacent station. In the case of the second embodiment, the winding angle θ is represented by the sum of the winding angle θ1 by the metal roller 214 at the station itself and the winding angle θ2 by the metal roller 214 at another station adjacent to the photosensitive drum 2.
Here, the winding angle θ may be approximated to the angle at which the surface of the intermediate transfer belt 31 is wound around the photosensitive drum 2 and is not limited to Equation 7. The metal roller 214 has been described as an example of the primary transfer member. However, the metal roller 214 is not limited to the metal roller 214 as long as the intermediate transfer belt 31 is wound around the photosensitive drum 2 with a certain amount of penetration with respect to the photosensitive drum 2. .
If the intermediate transfer belt 31 is wound around the photosensitive drum 2 without having a primary transfer function, the winding angle θ may be similarly treated.
As described above, even in the configuration in which the intermediate transfer belt 31 is wound around the photosensitive drum 2 and the drum nip width is increased as in the second embodiment, by defining the “shift amount”, transfer efficiency and image quality degradation are reduced. The balance can be optimized.

[Embodiment 3]
Next, a third embodiment of the present invention will be described.
The upper limit value of the shift amount in the first embodiment is determined based on the image stretch that guarantees the resolution, which is the specification of the image forming apparatus. However, in the case of a low resolution image forming apparatus, the upper limit value of the shift amount becomes large, and the image quality of the printed matter is increased. This is not preferable because the deterioration of the level becomes visible.
In the third embodiment, the upper limit value of the shift amount is defined based on the result of a subjective evaluation experiment in which presence / absence of image degradation is subjectively determined.
The evaluation pattern is a Mincho “Den” character, and 6pt single-color black characters and single-color characters were evaluated. The evaluation environment was a D50 light source, and a subjective evaluation experiment was conducted on 10 people. The drum nip width Ld is four levels of 0.75, 1.25, 1.75, 2.25 [mm], and the peripheral speed difference ratio R is 1.0, 1.5, 2.0, 3.0 [%]. The sample is printed under 16 conditions with a combination of the four levels.
Test results are averaged for each sample, ○ “degradation is not detectable”, △ “degradation is detectable but acceptable”, × “degradation is not acceptable but can be visually recognized (letters can be read)”, XX “Classification is not possible” (characters cannot be read).
Table 4 shows the results of 6pt single color black characters, and Table 5 shows the results of 6pt single color characters.

By comparing with Table 1, it can be seen that the character quality deteriorates as the image expands in the sub-scanning direction as the shift amount increases. Further, when Table 4 and Table 5 are compared, it can be seen that the deterioration of the extracted character is confirmed in comparison with the black character, and the extracted character is sensitive to the deterioration with respect to the peripheral speed difference. When collating with Table 1, it can be seen that the allowable “◯” and “Δ” for both the black characters and the blank characters are 30 μm or less. Therefore, the threshold value is set to 30 μm, and the shift amount exceeding the threshold value is out of the allowable range of deterioration of character quality.
As described above, from the viewpoint of maintaining character quality, it is desirable that the upper limit of the shift amount is 30 μm.
As a specific effect of the third embodiment, by defining the upper limit of the “shift amount” based on subjective judgment, it becomes possible to suppress deterioration in image quality within a practically acceptable level, and transfer efficiency and practical use Therefore, it is possible to construct a primary transfer configuration in consideration of a good image quality balance.

[Embodiment 4]
Next, a fourth embodiment of the present invention will be described. In the first to third embodiments, the shift amount is defined within a range in which the character quality is not deteriorated. The fourth embodiment relates to a mechanism for setting a spot shape of a laser beam in order to form a latent image having an aspect ratio that cancels the shift amount in advance. FIG. 9 shows an image exposure unit 4 as exposure means according to Embodiment 4 of the present invention.
FIG. 9A shows a main scanning section, and FIG. 9B shows a sub-scanning section.

In the present embodiment, laser light (light beam) 418 emitted from the light source 401 is incident on the coupling lens 403 after the light beam diameter in the main scanning direction is limited by the main scanning diaphragm 402. The light beam that has passed through the coupling lens 403 is converted into substantially parallel light and enters the anamorphic lens 404. The anamorphic lens 404 condenses the light beam on the deflector (polygon mirror) 405 in the sub-scan section, and forms a line image that is long in the main scanning direction.
The light beam condensed on the deflector 405 is reflected by a deflecting surface 405 (hereinafter referred to as a reflecting surface 405a) 405a. The light beam reflected by the reflecting surface 405 a is limited in the sub-scanning direction by the sub-scanning stop 408, shaped into a substantially circular shape, and then passes through the imaging lens 406 and enters the surface of the photosensitive drum 2. A light beam is imaged on the photosensitive drum 2 by the imaging lens 406 to form a predetermined spot-like image (hereinafter referred to as a spot). By rotating the deflector 405 at a constant angular velocity in the direction of arrow A by a driving unit (not shown), the spot moves on the photosensitive drum 2 in the main scanning direction, and an electrostatic latent image is formed on the photosensitive drum 2.

The main scanning direction is a direction parallel to the surface of the photosensitive drum 2 and orthogonal to the moving direction of the surface of the photosensitive drum 2. The sub-scanning direction is a direction orthogonal to the main scanning direction and the optical axis of the light beam. The spot diameter in the main scanning direction (main scanning spot diameter) is defined as follows. That is, the light amount profile obtained by integrating the static spot profile formed on the surface of the photosensitive drum 2 as the scanning surface in the sub-scanning direction is sliced at a position of, for example, 13.5% of the maximum value. It is defined as the width when Further, the spot diameter in the sub-scanning direction (sub-scanning spot diameter) is defined as follows. That is, the light amount profile obtained by integrating the static spot profile formed on the surface of the photosensitive drum 2 as the scanning surface in the main scanning direction is sliced at a position of, for example, 13.5% of the maximum value. It is defined as the width when The stationary spot diameter is measured by installing a CCD camera at the position of the photosensitive drum 2. In this embodiment, a CCD camera “TAKEEX-NC300” is used. Measurement is performed by deflector 40.
The spot profile of the laser beam is acquired by causing the light source 401 to emit light in a state where the angle of 5 is adjusted so that the laser beam 418 enters the CCD camera. Since the spot diameter does not depend on the amount of light, the light emission intensity at the time of measurement may be any intensity.

  Next, the unit dot shape of the latent image written on the photosensitive drum 2 by this spot will be described. The size of the unit dot of the latent image in the main scanning direction is the maximum value of the dynamic spot profile formed on the surface of the photosensitive drum when the laser beam is emitted for a unit time while scanning in the main scanning direction. For example, the width when sliced at a position of 13.5% is defined. The size of the unit dot of the latent image in the sub-scanning direction is the light amount profile obtained by integrating the static spot profile formed on the surface of the photosensitive drum 2 that is the surface to be scanned in the main scanning direction. For example, the width when sliced at a position of 13.5% with respect to the maximum value is defined. The unit dot diameter is measured while scanning the laser beam 418 in the main scanning direction by rotating the deflector 405 at a constant angular velocity in the direction of arrow A by a driving unit (not shown). The same. This unit dot is the minimum unit for forming an image, and all images are obtained by combining the unit dots. That is, all image quality including character quality is determined by this unit dot shape. Note that the size of the unit dot in the sub-scanning direction is determined by the static profile of the spot, that is, the above-described sub-scanning spot diameter. On the other hand, since the size in the main scanning direction is the laser scanning direction, it is larger than the static main scanning spot diameter.

Usually, the main / sub-spot diameter of the laser beam is set to such a size that the toner image visualized by developing toner on the unit dots of the latent image has a resolution that is assured by the image forming apparatus. Is desirable. That is, if a 600 dpi specification is guaranteed, the main scanning stop 402 and the sub-scanning stop 408 are set so that the unit dot diameter after development is about 42 μm main / sub.
And the spot diameter may be set. If the 1200 dpi specification is guaranteed, the spot diameter may be set by adjusting the main scanning stop 402 and the sub-scanning stop 408 to be about 21 μm.
Here, in order to measure the size of the visualized toner image of the unit dot formed on the photosensitive drum 2, a laser microscope OneShotVR300 manufactured by Keyence Corporation is used, and the toner image is captured with an 80 × lens. To calculate the main scanning diameter and the sub-scanning diameter, the attached analysis application is used, and the dot size is measured using the two-point measurement function in the analysis mode. At this time, in order to eliminate measurement variation by the measurer, the boundary around the dot is extracted in the edge automatic extraction mode of the auxiliary function, and the measurement is performed. In addition, the measurement variation is reduced by measuring dots at a total of three or more positions at the center and both ends of the photosensitive drum 2 and averaging the measurement results. The toner image on the ITB after the primary transfer and the toner image on the paper after the fixing can be measured by the same method.

  By doing so, it is possible to form on the drum a faithful toner image that does not expand and contract with respect to the original image.

However, when a faithful toner image that does not expand and contract with respect to the original image is formed on the drum, the image extends in the sub-scanning direction in the primary transfer process when a peripheral speed difference is provided,
The horizontal line becomes thicker than the vertical line. Therefore, in this embodiment, by adjusting the main scanning diaphragm 402 and the sub scanning diaphragm 408, the unit dot diameter of the latent image on the surface of the photosensitive drum 2 (image carrier surface) that is the surface to be scanned (surface to be exposed) is as follows. The size is limited to the size expressed by the following formula.

The size of the unit dots of the latent image formed on the photosensitive drum 2 in the sub-scanning direction = the size of the unit dots of the latent image corresponding to the toner image having no expansion / contraction with respect to the original image− Shift amount × 1/2 (Expression 8)

That is, as shown in FIG. 10, the aspect ratio of the unit dots of the latent image on the surface to be scanned is changed, and the size in the sub-scanning direction is reduced in advance by a size that takes into account the image expansion due to the peripheral speed difference. Then, a latent image is formed on the photosensitive drum, which is contracted by the amount of image expansion in the sub-scanning direction in accordance with the aspect ratio of the unit dot, and the toner image developed thereon is the same. The toner image contracted in the sub-scanning direction on the drum is then expanded in the sub-scanning direction according to the drum nip width and the peripheral speed difference in the primary transfer process, and the expansion and contraction with respect to the original image becomes plus or minus zero after the primary transfer.
By doing as described above, even when image expansion occurs in the sub-scanning direction due to the difference in peripheral speed, it is possible to obtain high-quality image quality that is faithful to the original image by canceling the expansion.

In this embodiment, the unit dot diameter is limited so that the vertical and horizontal sizes are the same. However, according to the applicant's study, the aspect ratio of dots is within a range of about 10% from the viewpoint of character quality. What is necessary is just to set a unit dot diameter in the range as shown by the following formula | equation 9.

Size in the sub-scanning direction of unit dots of the latent image formed on the photosensitive drum 2 = (size in the main scanning direction of unit dots of the latent image corresponding to the toner image having a size without expansion / contraction with respect to the original image) −shift amount × ½) × 0.9
~ (Size of unit dot of latent image corresponding to toner image having size without expansion / contraction with respect to original image−shift amount × ½) × 1.1 (Equation 9)

As described above, by adjusting the main scanning stop 402 and the sub-scanning stop 408, the unit dot diameter of the latent image can be set as in the fourth embodiment, so that image expansion can be prevented and transfer efficiency can be reduced. It is possible to construct a primary transfer configuration that balances practical image quality balance.

[Embodiment 5]
Next, a fifth embodiment of the present invention will be described.
In the first embodiment, from the viewpoint of image expansion, the upper limit value of the shift amount is defined as twice the resolution in the sub-scanning direction, and in the third embodiment, it is defined as 30 μm, and the image expansion is within an allowable range.
On the other hand, from the viewpoint of transfer efficiency, a larger peripheral speed difference rate is required to obtain a desired transfer property, and the shift amount may exceed the above-described upper limit value. FIG. 11 shows the relationship between the peripheral speed difference rate and the transfer efficiency. In the figure, the horizontal axis indicates the peripheral speed difference rate, the vertical axis indicates the transfer efficiency, and the higher the peripheral speed difference rate, the better the transfer efficiency.
The present embodiment relates to a configuration in which the shape of a unit dot is limited in advance and the image extension is within an allowable range when the shift amount exceeds the above-described upper limit value.

First, in the light of the first embodiment, a method for setting the upper limit value of the shift amount to a value equal to or larger than twice the resolution in the sub-scanning direction will be described.
FIG. 12 is a schematic diagram showing the relationship between the unit dot diameter and the expansion / contraction of the toner image.
As shown in FIG. 12, the net extension amount from the original image is smaller than the actual image extension amount (shift amount × ½) by the reduction of the diameter of the unit dot in the sub-scanning direction.
In other words, even when the actual image stretch exceeds the resolution in the sub-scanning direction, the net image stretch amount from the original image can be suppressed below the resolution in the sub-scanning direction by reducing the unit dot by an appropriate size. Will be able to. Specifically, the diameter of the unit dot in the sub-scanning direction may be set as shown in Equation 10 below.

The size of the unit dots of the latent image formed on the photosensitive drum 2 in the sub-scanning direction = (the size of the unit dots of the latent image corresponding to the toner image without expansion / contraction with respect to the original image) − ( Shift amount x1 / 2 -Resolution in the sub-scanning direction) ... (Equation 10)

At this time, the shift amount is calculated from Equation 10.

Shift amount = (resolution in the sub-scanning direction + size in the main scanning direction of the unit dot of the latent image corresponding to the toner image without expansion / contraction with respect to the original image−the unit dot of the latent image formed on the photosensitive drum 2 Size in the sub-scanning direction) × 2 (Expression 11)

It becomes.
As a result, for example, if the diameter of the unit dot in the sub-scanning direction is made 10 μm smaller than the diameter in the main scanning direction, the upper limit value of the shift amount can be extended to double the resolution in the sub-scanning direction + 20 μm. Further, if the diameter in the sub-scanning direction is reduced by 15 μm, the upper limit value can be expanded to a double value of the resolution in the sub-scanning direction +30 μm.
By doing as described above, the upper limit value of the shift amount can be set to at least twice the resolution in the sub-scanning direction.

Next, a method for setting the upper limit of the shift amount to 30 μm or more will be described.
In this case as well, based on the same concept, the diameter of the unit dot in the sub-scanning direction may be reduced so that the net image expansion amount is 15 μm or less. Specifically, the following equation 12 is set.

Size of the latent image unit dots formed on the photosensitive drum 2 in the sub-scanning direction = size of the latent image unit dots in the main scanning direction corresponding to the toner image without expansion / contraction with respect to the original image− (shift amount) × 1 / 2-15 μm) (Formula 12)

At this time, the shift amount is calculated from Equation 12.

Shift amount = 30 μm + (size of the unit dot of the latent image corresponding to the toner image without expansion / contraction with respect to the original image−size of the unit dot of the latent image formed on the photosensitive drum 2 in the sub-scanning direction) ) X 2 (Equation 13)

It becomes.
As a result, for example, if the diameter of the unit dot in the sub-scanning direction is made 10 μm smaller than the diameter in the main scanning direction, the upper limit of the shift amount is expanded to 50 μm, and if the diameter in the sub-scanning direction is reduced by 15 μm, the upper limit is expanded to 60 μm. can do.

By doing in the above manner, the upper limit value of the shift amount can be set to 30 μm or more.
Even when the upper limit value of the shift amount is expanded after reducing the diameter of the unit dot in the sub-scanning direction as in this embodiment, the net image extension amount with respect to the original image is equivalent to that of the third embodiment. Therefore, the character quality can be maintained at the same level as that of the third embodiment.
That is, as an effect peculiar to the present embodiment, even when a large shift amount is necessary to obtain a desired transfer property, the character quality is impaired by adjusting the aspect ratio of the unit dots to an appropriate size in advance. In addition, the shift amount can be increased.

  DESCRIPTION OF SYMBOLS 2 ... Photosensitive drum (image carrier), 14 ... Primary transfer roller (contact member), 214 ... Metal roller (contact member), 15 ... Drum nip part (transfer nip part), 31 ... Intermediate transfer belt, Vb ... Intermediate transfer belt Peripheral speed, Vd: photosensitive drum peripheral speed, | Vb−Vd | ... peripheral speed difference, Ld: drum nip width (nip nip width), S: shift amount, R: peripheral speed difference ratio, 4: image Exposure unit (exposure means), 402 ... main scanning stop, 408 ... sub-scanning stop

Claims (15)

An image carrier, an intermediate transfer belt to which a toner image formed on the image carrier is transferred, and a contact member in contact with a surface of the intermediate transfer belt opposite to the image carrier,
In the image forming apparatus that constitutes a transfer nip portion between the intermediate transfer belt and the image carrier by pressing the intermediate transfer belt against the image carrier by the contact member,
A peripheral speed difference is provided between the peripheral speed of the intermediate transfer belt and the peripheral speed of the image carrier, and the relative between the image carrier and the intermediate transfer belt generated in the transfer nip due to the peripheral speed difference. The lower limit value of the shift amount is set to at least three-eighths of the average circumference calculated from the weight average particle diameter of the toner measured in advance. An image forming apparatus.   2. The lower limit value of the shift amount is a value determined so that a reflection density of a residual toner image remaining on an image carrier after transfer to the intermediate transfer belt is smaller than a predetermined threshold value. The image forming apparatus described.   The image forming apparatus according to claim 1, wherein an upper limit value of the shift amount is set to be not more than twice a resolution of the toner image in a moving direction of the image carrier. Exposure means for forming a latent image by exposing the image carrier, further comprising
The upper limit value of the shift amount is the resolution of the toner image in the moving direction of the image carrier × 2 + (the main unit dots of the latent image formed on the surface of the image carrier that is the exposed surface by the exposure unit) The size in the scanning direction-the size in the sub-scanning direction of the unit dots of the latent image formed on the surface of the image carrier that is the exposed surface by the exposure unit.times.2. The image forming apparatus according to 1 or 2.   3. The image forming apparatus according to claim 1, wherein the upper limit value of the shift amount is a value determined based on a result of an evaluation experiment based on visual recognition on the elongation of a toner image that has been previously tested.   The image forming apparatus according to claim 1, wherein an upper limit value of the shift amount is 30 μm. Exposure means for forming a latent image by exposing the image carrier, further comprising
The upper limit of the shift amount is 30 μm + (the size of the unit dot of the latent image formed on the surface of the image carrier that is the surface to be exposed by the exposure unit−the surface to be exposed by the exposure unit) 6. The image according to claim 1, wherein a size of a unit dot of the latent image formed on the surface of the image carrier is 2 × 2. Forming equipment.   8. The image formation according to claim 1, wherein the peripheral speed difference is controlled by a control unit that controls rotation of each drive source of a drive system that drives the image carrier and the intermediate transfer belt. 9. apparatus.   The image formation according to claim 1, wherein the peripheral speed difference is mechanically set by changing a speed transmission ratio from a common drive source to the image carrier and the intermediate transfer belt. apparatus.   The image forming apparatus according to claim 1, wherein the primary transfer member is disposed to face the image carrier with the intermediate transfer belt interposed therebetween.   The image forming apparatus according to claim 10, wherein the primary transfer member is a roller having an elastic body.   The image forming apparatus according to claim 1, wherein the primary transfer member is disposed at a position away from the image carrier on the upstream side or the downstream side.   The image forming apparatus according to claim 12, wherein the primary transfer member is a metal roller. A plurality of the image carriers on which toner images are formed are arranged along the moving direction of the intermediate transfer belt,
A plurality of toner images formed on the image carrier are transferred onto the intermediate transfer belt at a transfer nip portion with the intermediate transfer belt, and further transferred onto a transfer material by a secondary transfer member. The image forming apparatus according to claim 1, wherein the image forming apparatus is configured as described above. An image carrier, an intermediate transfer belt to which a toner image formed on the image carrier is transferred, and a contact member in contact with a surface of the intermediate transfer belt opposite to the image carrier,
Exposure means for forming a latent image by exposing the image carrier,
In the image forming apparatus that constitutes a transfer nip portion between the intermediate transfer belt and the image carrier by pressing the intermediate transfer belt against the image carrier by the contact member,
A peripheral speed difference is provided between the peripheral speed of the intermediate transfer belt and the peripheral speed of the image carrier, and the relative between the image carrier and the intermediate transfer belt generated in the transfer nip due to the peripheral speed difference. Assuming that the amount of movement of the toner image is the amount of shift of the toner image, the size of the unit dot of the latent image formed on the surface of the image carrier that is the exposed surface by the exposure means = (to the toner image The size of the corresponding unit dot of the latent image in the main scanning direction−shift amount × ½) × 0.9 to (the size of the unit dot of the latent image corresponding to the toner image in the main scanning direction−shift amount × An image forming apparatus characterized by being set in a range of 1/2) × 1.1.

Images