JP2024039447A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To appropriately correct periodic image density unevenness occurring on a rotating body even when the periodic image density unevenness is different depending on the type of a recording material.

SOLUTION: An image forming apparatus has a plurality of pieces of image forming means comprising an image carrier, electrifying means, exposure means, developing means, first transfer means, second transfer means, and fixing means. The image forming apparatus comprises: phase detection means for detecting the phase of at least one image forming means of the plurality of pieces of image forming means; timing generation device that informs that the image forming means reaches a specific phase based on the phase detection means; and density reading means for reading the density of the formed image on a transfer material. The image forming apparatus causes the exposure means to output a phase marking of the image forming means on the image carrier at the timing when the image forming means reaches the specific phase, and detects sub-scanning density unevenness and the phase marking on the transfer material to execute density correction.

SELECTED DRAWING: Figure 7

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to an image forming apparatus that forms a toner image on a recording material.

In recent years, electrophotographic image forming apparatuses have begun to spread in the printing industry, and the demand for high-speed output and high image quality is rapidly increasing. Among the requirements for high image quality, there is a strong demand for uniformity of image density within a page, and it is important to suppress unevenness of image density within a page as much as possible.

There are various causes of density unevenness, but it is thought to be caused by insufficient processing precision such as eccentricity of the charging roller, photoreceptor drum, and developing sleeve.

As a technique for correcting image density unevenness in the image process direction as described above (hereinafter referred to as sub-scanning density unevenness correction), for example, Patent Document 1 discloses a technique for correcting density unevenness in the sub-scanning density unevenness detected on an intermediate transfer member. By constructing a signal, sub-scanning density unevenness is corrected. Specifically, this method uses a rotational position detection sensor that detects the rotational position of a rotating body involved in image formation, and a density detection sensor that detects the density of the image. In this method, density unevenness on the intermediate transfer member detected using a density detection sensor is separated by the period of the rotating body involved in image formation, and a latent image is formed on the photoreceptor drum in order to suppress the detected density unevenness. By correcting the amount of light from the light source for this purpose, sub-scanning density unevenness is corrected.

Japanese Patent Application Publication No. 2013-195586

It is known that since the rigidity, electrical resistance, surface condition, etc. of recording materials differ depending on the type thereof, the transferability in the next transfer step and the way the toner melts and spreads in the fixing step vary depending on the recording material. That is, when correcting sub-scanning density unevenness, the optimum correction amount differs depending on the recording material. Therefore, with the method of reading the periodic image density on the intermediate transfer body and determining the correction amount as in the prior literature, it is not possible to take into account the influence of the secondary transfer process and the fixing process on the sub-scanning density unevenness, and the sub-scanning density unevenness cannot be considered. I can't fully correct it.

An object of the present invention is to provide an image forming apparatus that determines the amount of correction by reading the periodic image density formed on the recording material, and can perform highly accurate correction for each recording material for sub-scanning density unevenness that occurs. It is about providing.

In order to achieve the above object, an image forming apparatus according to claim 1 includes an image bearing member, a charging means for charging the image bearing member, and a charging means for forming a latent image on the charged image bearing member. an exposure means, a developing means for making the toner visible on the latent image, a first transfer means for transferring the developed toner image, and a toner image transferred to the transfer means onto a transfer material. An image forming apparatus comprising: a second transfer means for transferring; and a fixing means for fixing the toner image transferred to the transfer material; A rotating member, at least one rotating member phase detection means for detecting a phase of the rotating member, and at least one rotating member timing generation device that notifies that the rotating member has reached a specific phase based on the rotating member phase detection means. and a density reading means for reading the density of the formed image on paper. It is characterized by carrying out correction.

In the image forming apparatus according to claim 2, in the image forming apparatus according to claim 1, the at least one rotating member is the image carrier, the charging means, and the developing means. It is characterized by

The image forming apparatus according to the present invention determines the correction amount by reading the periodic image density formed on the recording material. As a result, it is possible to perform highly accurate correction for each recording material for sub-scanning density unevenness that occurs.

1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. FIG. 3 is a diagram showing a relationship between potentials in a developing section according to the present embodiment. 3 is a diagram showing a rotational phase detection configuration according to Example 1. FIG. FIG. 3 is a diagram showing a rotational phase detection signal according to the first embodiment. FIG. 3 is a control block diagram according to the first embodiment. FIG. 2 is a block diagram illustrating characteristic parts of the present embodiment inside a CPU 301 according to the first embodiment. 3 is a flowchart of control according to the first embodiment. 2 is a diagram schematically showing a test pattern image according to Example 1. FIG. 3 is a diagram showing the relationship between development contrast and image density according to Example 1. FIG. 3 is a diagram showing the relationship between the output of the laser diode and the potential on the drum according to Example 1. FIG. 3 is a diagram showing the relationship between the detected density on the intermediate transfer body and the detected density on the recording material according to Example 1. FIG.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. The present invention will be described in more detail with reference to Examples. Although these Examples are examples of preferred embodiments of the present invention, the present invention is not limited to the configurations of these Examples.

[overview]
FIG. 1 is a schematic cross-sectional view of an image forming apparatus 200 in this embodiment. The image forming apparatus 200 is a four-color full-color printer using an electrophotographic process. Although only the image forming apparatus 200 is shown in FIG. 1, a configuration may be adopted in which other apparatuses are appropriately added within the same housing. Examples of the above-mentioned configurations include a copying machine, a multifunction device, a facsimile, and the like.

The image forming apparatus 200 forms a toner image on a sheet-like recording material based on a print signal transmitted from an external host device and outputs the toner image. The recording material is a recording medium (hereinafter referred to as paper) that can form a toner image, and includes, for example, plain paper, coated paper, OHT, label paper, and the like.

When the image forming apparatus 200 receives a print signal, the document information is converted into an image signal separated into yellow, magenta, cyan, and black, and the photoreceptor corresponding to each color is charged to a predetermined potential. A latent image is formed by exposure based on the image signal. After the latent image is developed as a toner image, each toner image is transferred onto an intermediate transfer belt in a superimposed manner, and then transferred all at once onto a sheet of paper. The paper onto which the toner image has been transferred is subjected to a fixing process using heat and pressure, and then ejected from the image forming apparatus to obtain a printout.

Further, the image forming apparatus 200 includes an operation unit U, and is capable of executing various procedures. The operation unit U is a user interface that allows the device to give instructions to the user and the user to execute commands for operating the device, and is, for example, a combination of a display and operation buttons, or a touch panel that displays virtual operators. The procedure performed by the operation unit U includes, for example, performing sub-scanning density unevenness correction, which will be described later.

[Image formation process]
The image forming apparatus 200 is a tandem intermediate transfer image forming apparatus in which image forming units Pa to Pd are arranged along the intermediate transfer belt 7. The intermediate transfer belt 7 is an endless belt stretched by a plurality of rollers including a drive roller, a tension roller, and a secondary transfer inner roller, and is conveyed in the R7 direction.

The image forming portions Pa to Pd form toner images of yellow, magenta, cyan, and black, respectively, and only the colors of the toners used are different. In the following explanation, yellow will be taken as an example.

In the image forming section Pa, a charging device 2a, an exposure device 3a, a developing device 4a, an intermediate transfer belt 7, a primary transfer section T1a, and a drum cleaning device 6a are arranged around the photosensitive drum 1a.

The photosensitive drum 1a has a photosensitive layer formed on a grounded cylindrical conductor element tube, and is driven to rotate clockwise around its axis.

The charging device 2a is a roller-shaped device having an elastic layer formed around a conductive central axis, and forms a nip between the charging device 2a and the photoconductor drum 1a by being pressurized against the photoconductor drum 1a. It rotates while being driven. At this time, a charging bias is applied to the central axis from the charging high-voltage power supply, thereby charging the surface of the photosensitive drum 1a to a predetermined potential.

The exposure device 3a is a laser scanner that scans and exposes light emitted from a laser diode in the axial direction of the photoreceptor drum 1a via a polygon mirror and an fθ optical system. Laser irradiation is modulated by a drive signal generated based on the image signal, and image information is written onto the photoreceptor drum 1a. That is, a potential drop occurs in the exposed portion of the photosensitive drum 1a, and an electrostatic latent image corresponding to image information is formed.

The developing device 4a includes an agitating conveyance section filled with a two-component developer consisting of a magnetic carrier and a non-magnetic toner, a conductive developing sleeve disposed around a fixedly arranged magnetic roller, and a predetermined amount of the developing agent from the developing sleeve. It consists of regulating members arranged with gaps. The developer is stirred and transported so that the toner is charged to a predetermined charge, carried on the developing sleeve by the magnetic force of the magnet roller and rotation of the developing sleeve, and adjusted to a predetermined thickness by a regulating member, and then transferred to the photoreceptor drum. 1a. At this time, a developing bias is applied to the developing sleeve from the developing high-voltage power supply, and the toner is moved from the developing sleeve to the photosensitive drum 1a by the driving force generated by the electrostatic latent image formed on the photosensitive drum 1a and the developing bias. The electrostatic latent image is developed as a toner image. Note that in this embodiment, toner of negative polarity is used.

In the primary transfer portion T1a, the primary transfer roller is pressed against the photosensitive drum 1a with the intermediate transfer belt 7 in between, thereby forming a primary transfer nip. The primary transfer roller is a roller-shaped roller with an elastic layer formed around a conductive central axis. By applying a bias having a polarity opposite to that of the toner to the primary transfer roller, the toner image on the photosensitive drum 1a is transferred to the intermediate transfer belt 7. At this time, the toner remaining on the photoreceptor drum 1a without being transferred is collected by the drum cleaning device 6a, and the photoreceptor drum 1a is again used for image formation.

In the image forming portions Pb to Pd, magenta, cyan, and black toner images are formed similarly to the image forming portion Pa. The image forming sections Pb to Pd are substantially the same as the image forming section Pa except for the color of the toner used, so a detailed explanation will be omitted.

The intermediate transfer belt 7 is driven to rotate at a surface speed substantially equal to that of the photoreceptor drum, and the toner images formed at the image forming portions Pa to Pd are superimposed and transferred onto the intermediate transfer belt 7 so as to be aligned.

Paper sheets S on which images are to be formed are stacked and stored in a paper feed cassette 60, and are frictionally separated by a pair of paper feed rollers 61 and fed and transported one by one in accordance with the timing of image formation by image forming units Pa to Pd. be done. The paper is supplied to the registration rollers 62 via a conveyance path, and after correcting the skew, the paper is supplied to the secondary transfer section T2 after adjusting the timing.

In the secondary transfer section T2, the secondary transfer outer roller 9 is pressurized against the secondary transfer inner roller 8 with the intermediate transfer belt 7 in between, thereby forming a secondary transfer nip and rotating in a driven manner. The paper supplied to section T2 is nipped and conveyed through the secondary transfer nip. At this time, a bias having a polarity opposite to that of the toner is applied to the secondary transfer outer roller 9, so that the toner image on the intermediate transfer belt 7 is transferred onto the paper. Toner remaining on the intermediate transfer belt 7 without being transferred at this time is collected by the belt cleaning device 11, and the intermediate transfer belt 7 is used again for image formation.

The fixing device 13 includes a pair of rollers each having a built-in heater, and uses heat and pressure to melt and fix the toner image on the paper. The sheet with the toner image fixed thereon is discharged onto a paper discharge tray 63 outside the machine.

Furthermore, the reader unit 250 has an image reading function and can capture the set paper as image data. The reader unit 250 is a reading device that reads a document or a test chart.

A test chart is a sheet on which a plurality of pattern images are formed. The light source 23 irradiates light onto the original 21 placed on the original platen glass 22. The optical system 24 guides the reflected light from the original 21 to the CCD sensor 25 to form an image.

CCD sensor 25 is an abbreviation for charge coupled device. The CCD sensor 25 generates red, green, and blue color component signals.

The reader image processing unit 1280 performs image processing (eg, shading correction, etc.) on the color component signals obtained by the CCD sensor 25 to generate image data.

[Developing device]
FIG. 2 shows the relationship between the potential of the photosensitive drum and the developing bias in the developing section when the sub-scanning density unevenness correction is not performed. Let the potentials of the non-exposed part and the exposed part of the photosensitive drum be Vd and Vl, respectively. Further, the DC component of the developing bias applied to the developing sleeve is assumed to be Vdc. In this embodiment, in addition to the above-mentioned DC component, an AC component is superimposed in order to improve the developability. As the AC component, for example, one having a frequency of 1.4 kHz and a peak-to-peak voltage of 1.5 kV can be used.

The magnetic carrier of the two-component developer in this example uses a core mainly made of ferrite coated with silicone resin, has a volume resistivity of approximately 1E13 Ωcm, and has a particle size (volume average particle size: laser diffraction particle size). Using a distribution measuring device HEROS (manufactured by JEOL Ltd.), the range of 0.5 to 350 μm was divided into 32 logarithms, and the volume (50% median diameter is taken as the volume average particle size) was approximately 40 μm. , non-magnetic toner is a powder made by dispersing a coloring agent, a charge control agent, etc. in a resin mainly made of polyester, and has a volume average particle size of about 6 μm. The magnetic carrier is triboelectrically charged to a negative polarity.Furthermore, the magnetic carrier is triboelectrically charged to a positive polarity.

[Phase detection]
The photosensitive drums 1a to 1d, the charging rollers 2a to 2d, and the developing sleeves of the developing devices 4a to 4d are provided with phase detection means for detecting rotational phases. FIG. 3 shows a configuration example of a phase detection device 50 including a photointerrupter 51 as a phase detection means for detecting the rotational position of the photoreceptor drum 1.

As shown in FIG. 3, a shaft 53 that forms the center of rotation of the photosensitive drum 1 is connected to a shaft that is the output shaft of a drive motor 54 via a coupling (not shown), and is rotated by the drive of the drive motor 54. Driven. The drum rotational phase detection device 50 includes, in addition to a photointerrupter 51, a light shielding member 52 that is provided integrally with a shaft 53 and rotates as the shaft 53 rotates. The light shielding member 52 is detected by the photointerrupter 51 when the photoreceptor drum 1 reaches a predetermined rotational position as the photoreceptor drum 1 rotates. Thereby, the photointerrupter 51 detects the rotational phase of the photoreceptor drum. The charging roller and the developing sleeve have substantially the same configuration, and the rotational phase of the charging roller and the developing sleeve is detected.

In the example shown in FIG. 3, a direct drive system directly connected to the drive motor is used to drive the photoreceptor drum 1, but a speed reduction mechanism may be included between power transmission from the drive motor 54. The same applies to the driving of the developing sleeve. As described above, the charging roller 2 of this embodiment is brought into pressure contact with the surface of the photoreceptor drum 1 with a predetermined pressing force and rotates as the photoreceptor drum 1 rotates, so it does not have a drive motor.

FIG. 4 shows an example of the output of the photointerrupter 51. It can be seen that when the light shielding member 52 rotating in synchronization with the photoreceptor drum 1 passes the photointerrupter 51, the output drops to approximately 0V. By detecting the falling edge of the signal at this time, the rotational phase of the photosensitive drum 1 is calculated. If the rotational phase at the falling timing of the signal is set to zero, the phase advances by 2π in the drum cycle, and based on this, the rotational phase of the photosensitive drum at any timing during rotational driving can be calculated. In this embodiment, the timing at which the output of the photo-interrupter 51 becomes 0V as it passes through the shielding member 52 is set as the home position.

[Control block diagram]
FIG. 5 is a schematic control block diagram of the image forming apparatus 200. The CPU 301 has a function of generating various command signals and executing arithmetic processing in order to operate various sensors, motors, etc. of the image forming apparatus 200 in accordance with the electrophotographic process. Furthermore, the CPU 301 includes a built-in memory for storing data. The image data generation section 89 has a function of converting various image data into laser control signals and sending control signals to the laser drive sections 303a to 303d.

The laser driving units 303a to 303d have a function of driving the laser elements of the laser scanners 3a to 3d based on signals sent from the image data generation unit 302, and controlling the lighting and light intensity of the lasers. The sensor drive circuit 305 has a function of controlling the drive current inside the rotating body phase detection sensor 70 according to a command signal from the CPU 301. The rotating body phase detection sensor detection circuit 305 sends the received light voltage signal from the rotating body phase detection sensor 70 to the CPU 301 .

The motor control unit 91 is electrically connected to each drive motor (not shown), and has a function of controlling drive timing and drive speed. The high voltage control section 92 has a function of controlling the output of biases necessary for the image forming process, such as charging bias, developing bias, and transfer bias.

Further, the CPU 301 is electrically connected to the paper feed cassette 60, an I/F section 85, and a timer 90, and further connected to the operation section 140 through the I/F section 85. The CPU 301 can perform image formation using the recording material S stored in the paper feed cassette 60. Further, the operation unit U accepts operations by the user, and is configured by, for example, a liquid crystal touch panel. Note that the operation unit 140 may be an external terminal such as a personal computer connected to the image forming apparatus.

Further, the CPU 301 is electrically connected to a reader 250 having an image reading function. A manuscript or a test chart is read by the reader 250, image data is generated by a reader image processing unit 1280 in the reader 250, and is sent to the CPU 301.

Further, the CPU 301 is electrically connected to the controller 87 and the image processing section 84, and the image information 88 is sent to the CPU 301 through the controller 87. The CPU 301 forms an image by processing the received image information 88 in the image processing unit 84.

Further, FIG. 6 partially shows a block diagram illustrating in detail the characteristic parts of the functions in this embodiment configured inside the CPU 301.

The rotating body timing generation unit 3011 has a function of sending a voltage signal to the image data generation unit 89 at a timing when the rotating body phase reaches the home position based on the light reception voltage signal outputted by the rotating body phase detection sensor detection circuit 306. .

The density conversion unit 3012 has the function of converting the luminance information of the image read by the reader into density. In this embodiment, the density corresponding to the phase in the sub-scanning direction is detected from the output test chart.

The sub-scanning density unevenness detection unit 3013 detects the amplitude and phase of the periodic component of the sub-scanning density unevenness and the phase of the rotating body based on the density corresponding to the phase in the sub-scanning direction detected by the density conversion unit 3012. A method for detecting the amplitude and phase of the periodic component of the sub-scanning density unevenness and the phase of the rotating body will be explained in detail in [Sub-scanning density unevenness correction].

The correction parameter determination unit 3014 has the function of determining the correction output of the laser diode based on the sub-scanning density unevenness detected by the sub-scanning density unevenness detection unit 3013 and the rotating body phase. The method of constructing the corrected output will be explained in detail in [Sub-scanning density unevenness correction].

The parameter storage unit 3015 has a function of storing sub-scanning density unevenness correction parameters determined based on the correction output of the laser diode constructed by the correction parameter determination unit 3014. It also has the role of passing the determined sub-scanning density unevenness correction parameters to the rotating body timing generation section 3011 and the image data generation section 89.

[Sub-scan density unevenness correction]
FIG. 7 is a flowchart for performing sub-scan density unevenness correction control in this embodiment.

The sub-scanning density unevenness correction control in this embodiment corrects the sub-scanning density unevenness by correcting the image forming process conditions based on the detection results of the output test chart. Specifically, the phases of the charging roller, developing sleeve, and photosensitive drum are marked on the photosensitive drum along with a test pattern using an exposure device, and a test chart is output. The test chart is read by a reader and the conditions for modulating the laser power so as to cancel out the density unevenness are determined. By executing the above process for each paper type, sub-scanning density unevenness correction parameters are determined for each paper type.

The flow of the sub-scanning density unevenness detection sequence will be explained below.

First, when periodic density unevenness correction control is instructed (S101), the adjustment mode is activated (S102). A user or a service person issues an instruction to implement the sub-scanning density unevenness correction control from the operation unit U, but a control execution signal may be automatically generated when the paper used is changed, etc. Outputs a test chart in which the phases of the charging roller, developing sleeve, and photosensitive drum are written. FIG. 8 shows an outline of the test chart. The test chart is a band-shaped image of a single color and a single tone extending in the sub-scanning direction. A method of outputting a marking patch including phase information of a charging roller, a developing sleeve, and a photosensitive drum will be explained. The home positions of the charging roller, developing sleeve, and photosensitive drum are detected by the phase detection device including the above-mentioned photointerrupter (S103), and the home positions are detected by the charging roller timing generating means, the developing sleeve timing generating means, and the photosensitive drum timing generating means. By instructing the exposure device to output a marking patch at a certain timing, the phases of the charging roller, developing sleeve, and photosensitive drum are output on the test chart (S104).

After the output of the test chart is completed (S105), the test chart is read by the reader and converted into image density (S106). After that, the amplitude and phase of the periodic components of the charging roller, developing sleeve, and photosensitive drum are detected from the belt-shaped patch extending in the sub-scanning direction of the single color and single gradation of the read test chart (S107), and the amplitude and phase of the periodic component of the charging roller, developing sleeve, and photosensitive drum are detected from the marking patch. Detects phase information of the sleeve and photosensitive drum. (S108) Specifically, periodic components of the charging roller, developing sleeve, and photosensitive drum are extracted from the amplitude and phase spectrum of each frequency component obtained by Fourier transforming the detected density profile. (S109) In the image forming apparatus in this embodiment, the process speed is 240 mm/s, the developing sleeve diameter is 20 mm, and the peripheral speed ratio of the developing sleeve to the photosensitive drum is 180%, so the developing sleeve cycle is It is 145ms. The amplitude D and phase Φ of the extracted developing sleeve periodic component are stored in memory.

Furthermore, a corrected output ΔLPW of the laser diode is constructed (S110). Here, the correction output means to modulate the output of the laser diode with an opposite phase so as to generate a development contrast corresponding to the amplitude of the density unevenness, in order to cancel out the sub-scanning density unevenness of the charging roller, developing sleeve, and photosensitive drum cycle. It is to let. A specific method for constructing a corrected output will be described below.

The charging roller period correction output ΔLPW1=A1×cos(ω1t+θ1) will be explained. First, a development contrast difference V1 corresponding to the amplitude D1 is calculated from the amplitude D1 of the charging roller periodic component extracted in the sub-scanning density unevenness detection sequence and the slope of the development gamma characteristic shown in FIG. Thereafter, an LPW difference A1 corresponding to the development contrast difference V1 is calculated from the relationship between the LPW and the potential on the drum shown in FIG. The angular velocity ω1 is an angular velocity corresponding to the period of the charging roller. The phase θ1 can be written as θ1 = Φ1 - ω1 × Δt1, where Δt1 is the time it takes for the charged part of the photoreceptor drum to reach the laser irradiation part, and the process speed S and the distance ds1 from the charged part to the laser irradiation part are It can be expressed as Δt1=ds1/S. The values of Va, ω1, and θ1 obtained through a series of operations are stored in the memory.

The correction output ΔLPW2=A2×cos(ω2t+θ2) of the developing sleeve period will be explained. Regarding the LPW difference A2, the process is the same as the method of constructing the correction output of the charging roller period, so the explanation will be omitted. The angular velocity ω2 is an angular velocity corresponding to the developing sleeve period. The phase θ2 can be written as θ2 = Φ2 - ω2 × Δt2, where Δt2 is the time it takes for the laser irradiation section of the photoreceptor drum to reach the development section, and the process speed S and the distance ds2 from the laser irradiation section to the development section are It can be expressed as Δt2=ds2/S. The values of Vb, ω2, and θ2 obtained through a series of operations are stored in the memory.

Regarding the photosensitive drum period correction output ΔLPW3=A3×cos(ω3t+θ3), the process is the same as the method of constructing the charging roller period correction output, and therefore will be omitted.

Thereafter, the correction output is determined by adding together the correction output ΔLPW1 of the charging roller, the correction output ΔLPW2 of the developing sleeve, and the correction output ΔLPW3 of the photosensitive drum. That is, the corrected output ΔLPW is expressed by the following formula.
ΔLPW = ΔLPW1 + ΔLPW2 + ΔLPW3... (Formula 1)
ΔLPW1 = A1・cos(ω1t+θ1) (Formula 2)
ΔLPW2 = A2・cos(ω2t+θ2) (Formula 3)
ΔLPW3 = A3・cos(ω3t+θ3) (Formula 4)
The last determined value of the correction parameter is stored in the parameter storage unit 3015 (S111).

[Operation after sub-scanning density unevenness correction control]
When performing an image forming operation at a timing after the sub-scanning density unevenness correction control, the home positions of the charging roller, developing sleeve, and photosensitive drum are detected, and then the exposure process modulated at the image forming timing is executed. That is, exposure is performed with an output of LPW+ΔLPW.

[Effect of the invention]
The effects of the sub-scan density unevenness correction control according to this embodiment will be explained below.

FIG. 11 shows the density profile before performing sub-scanning density unevenness correction. The transferability in the secondary transfer process and the way the toner melts and spreads in the fixing process vary depending on the type of recording material, so even if the density on the intermediate transfer body is the same, the density after fixing on the recording material will differ depending on the recording material. Depends on the type. The effects of this example are shown in Table 1 together with comparative examples. Here, the effects of this example will be explained by comparing the cases where recording material A, in which the toner melts and spreads in a small way in the fixing process, and recording material B, in which the toner melts and spreads in a large way in the fixing process, are used. do. The density unevenness is determined by measuring the sub-scanning density unevenness from a test chart for sub-scanning density unevenness correction read by the reader 250, calculating ΔD on the recording material, which is an index of density fluctuation on the recording material, and converting it into a scalar amount. It is calculated by The density variation ΔD on the recording material is determined by the average of the absolute values of the differences from the average value. Comparative Example 1 is a case where sub-scanning density unevenness correction is not performed, and Comparative Example 2 is a case where density unevenness is detected on the intermediate transfer body and sub-scanning density unevenness correction is performed.

Comparative Example 1, Comparative Example 2, and the present example both show a case where the image density unevenness is 0.20 and a case where the image density unevenness is 0.40 when sub-scanning density unevenness correction control is not performed. In each case, the results of density unevenness before and after sub-scanning density unevenness correction are shown for each type of recording material.

Although density unevenness is improved in Comparative Example 2 compared to Comparative Example 1, in recording material B, where the toner melts and spreads more rapidly during the fixing process, the clear dot structure on the intermediate transfer body does not show that the toner on the recording material is Since the density unevenness cannot be predicted accurately, in Comparative Example 2, the density unevenness of recording material B could not be completely corrected, and 0.20 of the density unevenness remained after correction. On the other hand, in this embodiment, density unevenness on the recording material is directly detected and sub-scanning density unevenness correction is performed, so that density unevenness can be improved regardless of the type of recording material.

As explained above, by executing the correction process for each type of recording material to modulate the output of the laser diode based on the periodic fluctuation information extracted from the on-paper density information of the test chart, it is possible to First, it is possible to reduce sub-scanning density unevenness.

In this embodiment, a method has been shown in which density unevenness is corrected by extracting the charging roller periodic component, developing sleeve periodic component, and photoreceptor drum periodic component from the on-paper density results of the test chart and modulating the output of the laser diode. Other methods may be used as long as the modulation amplitude is controlled to be below a predetermined threshold when modulating the image forming conditions based on the periodic variation information extracted from the on-paper density information of the test chart. . For example, the developing sleeve period component may be extracted from the on-paper density result of the test chart and the developing bias may be modulated. Furthermore, in this embodiment, the sub-scanning density unevenness correction is performed using a reader that detects the density of the test chart, but it is also possible to detect the density of the test chart on paper using a sensor placed downstream of the fixing device. You can.

200 Image forming apparatus Pa, Pb, Pc, Pd Image forming section 7 Intermediate transfer belt 301 CPU
50 Rotational phase detection device

Claims (2)

an image carrier on which a toner image is formed;
Charging means for charging the image carrier;
exposure means for forming a latent image on the charged image carrier;
a developing means for visualizing the toner in the latent image;
a first transfer means for transferring the visualized toner image;
a second transfer means for transferring the toner image transferred to the transfer means to a transfer material;
a fixing means for fixing the toner image transferred to the transfer material;
A plurality of image forming means comprising: a density reading means for reading the density of the formed image on the transfer material;
A phase detection means for detecting the phase of at least one image forming means among the plurality of image forming means, and a timing for notifying that the image forming means has reached a specific phase based on the phase detecting means. comprising a generation device and a density correction means for correcting periodic sub-scanning density unevenness,
The exposure means forms a phase marking of the image forming means on the image carrier at a timing when the image forming means has a specific phase, and eliminates sub-scanning density unevenness and the phase marking on the transfer material. An image forming apparatus characterized in that the density correction means performs density correction based on the detection result. Claim 1, wherein the at least one image forming means on which a phase marking is formed using the exposure means is the image carrier, the charging means, and the developing means. The image forming apparatus described in .