JP6021390B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a color copying machine or a color printer that obtains a fixed image on a transfer material by transferring a toner image formed on an image carrier by an electrophotographic method onto a transfer material and then fixing it.

  There is a known electrophotographic image forming apparatus including a step of transferring a toner image formed on the surface of an image carrier onto a transfer material such as paper. The color image forming apparatus has a configuration in which a plurality of photosensitive drums are arranged in parallel, toner images are sequentially formed on the photosensitive drums, and the toner images are transferred directly to a transfer material or via an intermediate transfer belt. It has been put into practical use. In such a color image forming apparatus, generally, a full-color image is formed using toners of four colors of yellow (Y), magenta (M), cyan (C), and black (Bk), and a photosensitive drum corresponding to each color is provided. I have. Also, a configuration in which a toner image is formed in order from yellow on one photosensitive drum, sequentially transferred onto an intermediate transfer belt, and a full color image is formed by superimposing toner images of respective colors has been put into practical use. In each of the above configurations, there are multiple transfer processes, and there is a process in which another toner image is transferred from the photosensitive drum to the toner image already transferred onto the intermediate transfer belt. Here, in the second and subsequent transfer processes among the plurality of transfer processes, a photosensitive drum that superimposes and transfers a new toner image is superimposed on the toner image that has already been transferred onto the belt. At that time, the portion of the photosensitive drum where there is no toner image electrostatically acts on the toner image already transferred onto the belt, depending on the potential, and the toner image already transferred onto the belt, etc. May be disturbed. For this reason, weak exposure (micro exposure) is performed on the portion of the photosensitive drum where the toner is not adhered (background portion), thereby stabilizing the potential and reducing this electrostatic effect, thereby causing image disturbance and transfer efficiency. An apparatus for preventing the reduction has been devised (see Patent Document 1).

JP 2000-131899 A

  However, the properties and characteristics of components in the apparatus such as the photosensitive drum vary depending on the amount of use. For this reason, when the exposure amount of the above-described weak exposure (micro exposure) is always set to be constant, the potential of the portion (background portion) on the photosensitive drum where the toner is not attached cannot be appropriately maintained. As a result, the image density, chromaticity, character thickness changes depending on the amount of use of the device, and the image defect such as a so-called fogging image in which the toner adheres to the background portion where the toner is not adhered and is developed. May occur.

  SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to perform image formation with stable image quality while keeping the potential of a background portion to which toner on a photosensitive drum is not attached appropriately.

  In order to achieve the above object, the present invention comprises the following arrangement.

(1) Photoconductor, charging unit for charging the photoconductor, light irradiation unit for irradiating light to the photoconductor charged by the charging unit to form a latent image, and developing by attaching toner to the latent image Developing means, and detecting means for detecting a value relating to the density of the toner image formed by the developing means, wherein the light irradiating means has a toner on the photoconductor according to image data for image formation. in the image forming apparatus is irradiated with light, light is irradiated to the second exposure to the non-image area which does not adhere to the toner of the photoconductor is performed the images formed by the first exposure to the image portion to deposit, a correction means for correcting the second exposure amount of the light irradiation unit, before Symbol light irradiation means is first irradiated with light in the second exposure amount and the first region irradiated with the light in the first exposure form the aligned latent image pattern and a secondary area alternately on the photosensitive member And said detecting means, the latent image pattern detected a toner image obtained by developing by the developing unit, said correction means, characterized in that the correction of the second exposure amount based on the detected value An image forming apparatus.
(2) Photoconductor, charging unit for charging the photoconductor, light irradiation unit for irradiating light to the photoconductor charged by the charging unit to form a latent image, and developing the toner by attaching toner to the latent image Developing means, and detecting means for detecting a value relating to the density of the toner image formed by the developing means, wherein the light irradiating means has a toner on the photoconductor according to image data for image formation. In an image forming apparatus for performing image formation by irradiating light at a first exposure amount to an image portion to which toner is adhered and irradiating light at a second exposure amount to a non-image portion to which toner of the photoreceptor is not adhered, A correction unit that corrects the second exposure amount of the light irradiation unit, wherein the light irradiation unit is configured to irradiate the first region irradiated with the light with the first exposure amount and the second region irradiated with the light with the second exposure amount; The latent image pattern having a portion surrounded by The detection unit detects a toner image obtained by developing the latent image pattern by the development unit, and the correction unit corrects the second exposure amount based on the detected value. An image forming apparatus.
(3) Photoconductor, charging unit for charging the photoconductor, light irradiation unit for irradiating light to the photoconductor charged by the charging unit to form a latent image, and developing the toner by attaching toner to the latent image Developing means, and detecting means for detecting a value relating to the density of the toner image formed by the developing means, wherein the light irradiating means has a toner on the photoconductor according to image data for image formation. In an image forming apparatus for performing image formation by irradiating light at a first exposure amount to an image portion to which toner is adhered and irradiating light at a second exposure amount to a non-image portion to which toner of the photoreceptor is not adhered, The light irradiation unit includes a correction unit that corrects the second exposure amount, and the light irradiation unit includes a first region irradiated with light at the first exposure amount and a second region irradiated with light at the second exposure amount. A latent image pattern with the same ratio to The detecting unit detects a toner image obtained by developing the latent image pattern by the developing unit, and the correcting unit corrects the second exposure amount based on the detected value. An image forming apparatus.

  According to the present invention, it is possible to perform image formation with stable image quality while keeping the potential of the background portion where the toner on the photosensitive drum is not adhered appropriately.

FIG. 1 is a diagram illustrating an image forming system according to a first exemplary embodiment and a configuration diagram of an image forming apparatus. FIG. 1 is a layer configuration diagram of the photosensitive drum of Example 1, and a graph showing an EV curve of the photosensitive drum. The block diagram explaining the exposure of Example 1, The figure explaining an electrostatic latent image Flowchart for explaining the fine exposure correction of the first embodiment The figure which shows the relationship between the fine exposure correction | amendment of Example 1, and a patch image density. The figure which shows the relationship between the number of passing sheets of Example 1, a minute exposure amount, and a background part electric potential. Flowchart for explaining microexposure correction of embodiment 2 Flowchart for explaining microexposure correction of embodiment 3 Graph and table showing results of comparative experiments in Example 3

  Hereinafter, the form for implementing this invention is demonstrated in detail by an Example. However, the constituent elements described in this embodiment are merely examples, and are not intended to limit the scope of the present invention only to them.

[Image forming system diagram]
FIG. 1A is an image forming system diagram showing the connection between the image forming apparatus and the image transmitting apparatus. The image forming apparatus 100 according to the first exemplary embodiment is connected to a personal computer (hereinafter referred to as a PC) 101 serving as an image transmission apparatus that is an external device, for example, via a cable 102 by wire. Note that the image forming apparatus 100 may be configured to be connected to the image transmission apparatus by wireless communication. The image information is transmitted from the PC 101 to the controller 103 via the cable 102, and is transmitted to the printer engine control unit 104 after performing image data processing described later.

[Configuration of Image Forming Apparatus]
FIG. 1B is a cross-sectional view showing the image forming apparatus 100 of this embodiment. As an image forming apparatus of this embodiment, there are four photosensitive drums as photosensitive members, and a recording material as a final image carrier through an intermediate transfer belt (intermediate transfer member) as a first image carrier. An intermediate transfer type tandem type full-color printer for transferring images to the image will be described. Here, an example is used in which the image resolution is 600 dpi (600 pixels per inch) and the image forming speed (photosensitive drum rotational peripheral speed) is 100 mm / s. The image forming apparatus according to the present exemplary embodiment includes an image forming station that is an image forming unit for four colors of yellow (Y), magenta (M), cyan (C), and black (Bk), and includes an intermediate transfer belt 1. A transfer device and a fixing device (fixing device) 3 are provided. The image forming apparatus is not necessarily limited to a four-color image forming apparatus. For example, a six-color image forming apparatus to which light cyan, light magenta, or the like is further added may be used.

Each of the image forming stations is unitized as image forming units 10a to 10d, and photosensitive drums (drum-shaped electrophotographic photosensitive members) 11a to 11d are respectively installed so as to be rotatable in the arrow direction (clockwise direction). The symbols a, b, c, and d correspond to Y, M, C, and Bk, respectively, and will be omitted unless necessary. On the outer peripheral surface of the photosensitive drum 11, a charging roller 12 that is a charging device that uniformly charges the surface of the photosensitive drum is disposed. On the downstream side of the charging drum 12 in the rotation direction of the photosensitive drum, a laser exposure unit 13 (light) is an exposure device that emits (irradiates) the laser beam modulated by the printer engine control unit 104 to the photosensitive drum surface and exposes the photosensitive drum surface. Irradiating means) is arranged. Further, on the downstream side of the laser exposure device 13, a developing device that develops the electrostatic latent images of the respective colors on the surface of the photosensitive drum formed by laser exposure using the corresponding yellow, magenta, cyan, and black toners. The developing device 14 is arranged. The developing devices 14a to 14d are filled with yellow toner t _Y , magenta toner t _M , cyan toner t _C , and black toner t _Bk, respectively.

  A primary transfer roller 15 that forms a primary transfer portion together with the photosensitive drum 11 is disposed opposite to a position (transfer position) of the photosensitive drum 11 with the intermediate transfer belt 1 interposed therebetween. A primary transfer power supply 16 is connected to the primary transfer roller 15 and a primary transfer voltage is applied thereto. The intermediate transfer belt 1 is stretched and installed on three rollers, that is, a driving roller 1a, a tension roller 1b, and a secondary transfer counter roller 1c. The intermediate transfer belt 1 passes through the image forming stations 10a to 10d and passes through the photosensitive drums 11a to 11d. Placed in contact. The intermediate transfer belt 1 is driven to rotate in the direction of the arrow (counterclockwise direction) by a driving roller 1a. A drum cleaner 17 is installed on the downstream side of the primary transfer roller 15 of the photosensitive drum 11. A belt cleaner 4 is disposed in contact with the surface of the intermediate transfer belt 1.

  The printer engine control unit 104 controls each unit of the printer engine in accordance with the image information and various instructions received from the controller 103. Note that the printer engine here generally refers to a portion that performs an operation related to image formation except for the controller 103 in the image forming apparatus 100 of FIG.

  The image forming operation of the image forming apparatus configured as described above will be described using the yellow image forming station 10a as an example. The photosensitive drum 11a of the yellow image forming station 10a has a photoconductive layer formed on the surface of an aluminum cylinder. The surface of the photosensitive drum 11a is charged by a charging roller 12a to which a charging voltage is applied from a charging power source 19 in the process of rotating in the direction of the arrow. It is negatively charged uniformly. Next, the controller 103 executes image data processing on the image information sent from the PC 101, converts it into a signal corresponding to the laser exposure amount, and outputs a signal to the laser exposure unit 13a. The laser exposure device 13 a performs image exposure based on a signal corresponding to the laser exposure amount output from the controller 103. As a result, the potential of the toner image portion (image portion) is attenuated from the background portion (non-image portion) to form a potential difference, and an electrostatic latent image corresponding to the yellow image component of the original image is formed on the surface of the photosensitive drum 11a. Is done. In this embodiment, negative voltages are used for charging and development. For convenience, these negative voltages are expressed as high when the absolute value of the voltage is large and low when the absolute value is small. Express.

  In the developing device 14a, a voltage (developing voltage) higher than the toner image portion on the surface of the photosensitive drum and lower than the background portion is supplied from the developing power supply 18 to the toner supply portion. As a result, the negatively charged yellow toner is selectively moved only to the toner image portion of the photosensitive drum 11a, and development corresponding to the electrostatic latent image is performed to be visualized as a yellow toner image. The obtained yellow toner image is transferred onto the intermediate transfer belt 1 (on the intermediate transfer member) by applying a primary transfer voltage from the primary transfer power supply 16 to the primary transfer roller 15a (hereinafter referred to as primary transfer). After the transfer, the toner adhering to the surface of the photosensitive drum 11a (hereinafter referred to as “transfer residual toner”) is removed by the drum cleaner 17a and used for the next image formation.

  The above image forming operation is performed at a predetermined timing in the other image forming stations 10b to 10d, and the toner images on the photosensitive drums 11b to 11d are sequentially superposed on the intermediate transfer belt 1 at the respective primary transfer portions for primary transfer. . In the full color mode, the toner images are sequentially transferred to the intermediate transfer belt 1 in the order of yellow, magenta, cyan, and black. In the mono color mode, for example, a black toner image is formed in the same process as described above. Transcribed. Thereafter, the four color toner images on the intermediate transfer belt 1 are moved to the secondary transfer nip portion as the intermediate transfer belt 1 rotates in the arrow direction. The secondary transfer nip portion is a portion where the secondary transfer roller 2 is in contact with the grounded secondary transfer counter roller 1c with the intermediate transfer belt 1 interposed therebetween. The transfer material P is fed from the cassette 5 and conveyed by the feeding roller 9 at a predetermined timing. The secondary transfer power source 21 applies a secondary transfer voltage to the secondary transfer roller 2 in contact with the transfer material P conveyed to the secondary transfer nip portion. As a result, the toner images are collectively transferred to the transfer material P (hereinafter referred to as secondary transfer). The transfer residual toner adhering to the surface of the intermediate transfer belt 1 after the secondary transfer is removed by the belt cleaner 4 and used for the next image formation.

  The transfer material P carrying the unfixed toner image that has passed through the secondary transfer nip portion is conveyed to the nip portion between the fixing film 30 and the pressure roller 31 of the fixing device 3 and is heated and pressed to obtain the fixed image. Become. Further, the transfer material P that has passed through the fixing device 3 is discharged to a discharge tray (not shown) outside the image forming apparatus. The density sensor 40 detects a value relating to a toner density of a correction image, which will be described later, formed on the intermediate transfer belt 1.

[About photosensitive drum]
The photosensitive drum 11a of the present embodiment will be described with reference to FIG. The photosensitive drum 11a is formed by laminating a charge generation layer 23a and a charge transport layer 24a on a conductive support base 22a, and corresponds to a sheet passing life of 5000 sheets. Here, the paper passing life is a guarantee period in which the photosensitive drum can be used, expressed as the number of sheets. The conductive support base 22a is an aluminum cylinder having an outer diameter of 30 mm and a thickness of 1 mm. The charge generation layer 23a is a phthalocyanine pigment having a thickness of 0.2 μm. The charge transport layer 24a is obtained by blending a polycarbonate having a thickness of 20 μm as a binder resin and an amine compound as a charge transport material.

  The configuration shown in the present embodiment is not particularly limited, and the conductive support base 22a may use a metal such as aluminum or SUS. Examples of the charge generation layer 23a include azo pigments (for example, monoazo, bisazo, trisazo, etc.), metal and metal-free phthalocyanine pigments, indigo pigments (for example, indigo, thioindigo, etc.), and the like. Furthermore, examples of the charge generation layer 23a include quinone pigments (for example, anthanthrone and pyrenequinone), perylene pigments (for example, perylene acid anhydride, perylene acid imide, and the like), squalium dyes, and the like. Furthermore, examples of the charge generation layer 23a include pyrylium, thiapyrylium salts, and triphenylmethane dyes. As the binder resin for the charge transport layer 24a, for example, polyvinyl acetal, polycarbonate, polyarylate, polystyrene, polyester, polyacetate, polymethacrylate, acrylic resin, cellulose resin and the like are preferably used. Examples of the charge transport material include polycyclic aromatic compounds (for example, pyrene and anthracene). Furthermore, examples of the charge transport material include heterocyclic compounds (for example, carbazole, indole, imidazole, oxazole, thiazole, oxadiazole, pyrazole, pyrazoline, thiadiazole, triazole, etc.). Furthermore, examples of the charge transport material include hydrazone compounds (for example, p-diethylaminobenzaldehyde-N, N-diphenylhydrazone, N, N-diphenylhydrazino-3-methylidene-9-ethylcarbazole, etc.). Furthermore, examples of the charge transport material include styryl compounds (for example, α-phenyl-4′-N, N-diaminostilbene, 5- [4- (di-p-tolylamino) benzylidene] -5H-dibenzo [a, d ] And the like. Furthermore, examples of the charge transport material include benzidine compounds and triarylamine compounds.

  The thickness of the charge generation layer 23a is preferably 0.001 to 5 μm, more preferably 0.05 to 2 μm, and the thickness of the charge transport layer 24a is preferably 5 to 40 μm, more preferably 10 to 30 μm. The charge generation layer 23a preferably contains 10 to 100% by mass, more preferably 40 to 100% by mass of the charge generation material. The charge transport layer 24a preferably contains 20 to 80% by mass, more preferably 30 to 70% by mass of the charge transport material.

FIG. 2B is called an EV curve of the photosensitive drum used in this embodiment. The photosensitive drum 11 whose surface is charged to V ₀ is exposed to laser light having an exposure amount E (μJ / cm ² ). This is a graph showing the potential attenuation in the case of the photosensitivity, and showing the photosensitive characteristics. The horizontal axis of the graph of FIG. 2B is the exposure amount E (μJ / cm ² ), and the vertical axis is the photosensitive drum potential V (V) when the DC voltage Va = −1180 V is applied to the charging roller 12 by the charging power source 19. It is. Although this EV curve differs depending on the photosensitive drum, there is a tendency that a larger potential attenuation can be obtained by increasing the exposure amount E. Further, since recombination of carriers hardly occurs due to a strong electric field in a high potential portion, a large potential attenuation is exhibited even at a low exposure amount. On the other hand, since the generated carriers are likely to recombine in the low potential portion, there is a tendency that the potential attenuation is small even for exposure with a high exposure amount, and a curve as shown in FIG. It is common. The solid line indicates the initial curve of the photosensitive drum 11, and the broken line indicates the EV curve when the life of the photosensitive drum 11 is reached.

[Charging]
The charging roller 12 of this embodiment uses a core bar in which an elastic layer made of nitrile butadiene rubber (NBR) having a thickness of 3 mm is formed, and a surface layer made of polyurethane resin having a thickness of 10 μm is formed thereon. . The volume resistivity of the elastic layer is adjusted to 10 ⁴ Ω · cm, and the volume resistivity of the surface layer is adjusted to 10 ¹¹ Ω · cm. These charging rollers 12 are brought into contact with the photosensitive drum 11 with a pressurizing force of 6 N and are driven to rotate.

The structure shown in the present embodiment is not particularly limited, and the elastic layer is made of a conductive polymer such as polyacetylene, polypyrrole, or polythiophene, carbon, a conductive metal oxide, a metal, an ion conductive material, or the like. You may use the hydrin rubber which disperse | distributed and electroconductively processed. Further, the elastic layer is made of ethylene propylene rubber (EPDM), nitrile rubber (NBR), styrene butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), chloroprene rubber (CR), acrylic rubber ( ACM) may be used. Furthermore, as the material of the elastic layer, rubber materials such as silicone rubber and fluororubber, thermoplastic elastomers such as polystyrene, polyolefin, polyvinyl chloride, polyurethane, polyester, and polyamide may be used. Moreover, as a material of the surface layer, for example, a polyamide resin, a polyurethane resin, a fluororesin, a silicone resin, or a material in which a conductive treatment material is dispersed can be used. The film thickness of the surface layer is preferably in the range of 0.1 to 500 [mu] m, particularly 0.2 to 250 [mu] m, so as not to impair flexibility and durability. The volume resistivity of the elastic layer is preferably in the range of 10 ² to 10 ¹¹ Ω · cm, particularly 10 ² to 10 ¹⁰ Ω · cm in order to obtain good charge, and the volume resistivity of the surface layer is similarly 10 A range of ⁶ to 10 ¹³ Ω · cm is preferable.

In this embodiment, a constant voltage is supplied from the charging power source 19 to the core of the charging roller 12 with a DC voltage V _a = −1180 V. The gap between the charging roller 12 and the surface of the photosensitive drum is z (μm), the dielectric thickness of the photosensitive drum 11 is d (μm), the charged surface potential of the photosensitive drum (the above-described photosensitive drum potential) is V ₀ , air The dielectric constant ratio of the photosensitive drum 11 is k (≈3). Then, the potential difference V _g applied to the minute gap is expressed by Equation 1.
_{V g = (V a -V 0} ) · z / (d / k + z) ( Equation 1)

On the other hand, discharge occurs with respect to the gap z in accordance with Paschen's law, and the gap voltage V _g is approximated by Equation 2 in the region where z ≧ 8 μm.
−V _g = 312 + 6.2 · z (Formula 2)
Therefore, the potential converges to the potential given by Equation 3 at the discharge start point.
V ₀ = V _a + (312 + 6.2 · d / k) + (7737.6 · d / k) ^1/2
≒ -868 + 2 ・ d + 51√d (Formula 3)
From Equation 3, it can be seen that when the film thickness d of the photosensitive drum 11 is worn by use, the absolute value of the surface potential V ₀ of the photosensitive drum 11 increases.

  Further, the charging by the DC voltage used in the present embodiment is superior to the charging by the AC voltage in that no vibration or sound is generated and the amount of discharge is suppressed and the deterioration of the photosensitive drum 11 is suppressed. Yes. However, since charging with a DC voltage is performed in one direction, the leveling action is poor with respect to a portion higher than the convergence potential, and the charged state tends to be uneven. In the transfer portion of the downstream image forming station, the background portion of the station may overlap the toner image portion of the upstream station. In such a case, potential unevenness in the background portion of the downstream image forming station may act on the already transferred toner image and retransfer may occur, resulting in a non-uniform image. Here, retransfer refers to a phenomenon in which a toner image is returned to the photosensitive drum 11 by causing an electric potential difference with the photosensitive drum 11 at the transfer portion to cause discharge and reversing the toner charge. In order to improve such non-uniformity, in this embodiment, minute exposure is performed on the background portion described below.

[Exposure and micro exposure]
The normal exposure amount and the minute exposure amount may be controlled by the pulse width signal 60a shown in FIG. 3A or may be controlled by the luminance signal 61a. Each case will be described below.

(When the exposure amount is controlled by the pulse width signal 60a)
The laser exposure unit 13 will be described with reference to FIG. 3A by taking the laser exposure unit 13a for the yellow photosensitive drum 11a as an example. The laser driver 62a performs exposure driving by controlling the light emission luminance and time of the laser diode 63a by the luminance signal 61a and the pulse width signal 60a generated by the printer engine control unit 104 based on the image data. The emission luminance is determined by the current supplied to the laser diode 63a. On the other hand, the exposure amount is light energy per unit area when laser light irradiated with a certain light emission luminance is irradiated onto the photosensitive drum, and is proportional to the product of the light emission luminance and the irradiation time per pixel.

In this embodiment, an example in which the luminance signal 61a is not active or fixed will be described. The laser driver 62a controls the current supplied to the laser diode 63a so as to obtain a predetermined light emission luminance, and obtains a stable luminance by applying feedback for each scanning. In the exposure for forming an electrostatic latent image on the photosensitive drum 11a, the exposure amount E ₀ (second exposure amount for minute exposure) used for exposure of the background changes the light emission time by the pulse width signal 60a. To be adjusted. Similarly, the exposure amount E _x for normal exposure (first exposure amount for printing) used for exposure of the toner image portion is adjusted by changing the light emission time by the pulse width signal 60a.

Specifically, the pulse width signal 60a is generated from image data of a multi-value signal (0 to 255) of 8 bits = 256 gradations. For example, when the image data is 0 (background portion), the pulse width is PW _MIN (12.5% for one pixel), and when it is 255, the pulse width is 1 pixel (PW ₂₅₅ ) and 1 to 254 For value image data, an intermediate pulse width (PW _x ) between the two is generated. Based on the pulse width signal 60a, the laser driver 62a drives the laser diode 63a, and the laser beam 6a is emitted. The laser light 6a is irradiated on the photosensitive drum 11a as scanning light through a correction optical system 67a including a polygon mirror 64a, a lens 65a, and a folding mirror 66a.

The printer engine control unit 104 appropriately changes PW _MIN to optimize the minute exposure amount E ₀ in the background portion. The minute exposure amount E ₀ is set so that the average surface potential per image obtained at the time of exposure does not fall below the development potential (about −400 V), and the potential is attenuated to obtain charge uniformity described later. In this embodiment, the printer engine control unit 104 outputs PW _MIN at a value of 12.5% of PW ₂₅₅ for one pixel, so that the initial microexposure E ₀ is set to 0.03 μJ / cm ^2. A background potential attenuation of 100 V of the drum 11 is obtained (see FIG. 2B). Further, the maximum exposure amount E _{255 at} the time of full exposure with the PW ₂₅₅ is an exposure amount in a region where the EV curve in FIG. 2B is almost horizontal so that variations in surface potential due to exposure do not appear easily. It is set to 0.25 μJ / cm ² . Incidentally, the intermediate exposure of small exposure amount _{E 0} and the maximum exposure amount _{E 255} and _{E x.}

With the above settings, the exposure potential of the toner image portion exposed at the maximum exposure amount E ₂₅₅ is attenuated to V _L = −150 V, and the background portion is changed from the charged potential V ₀ (≈−600 V) before exposure to V V. Decrease to _D (≈-500V).

The effect | action by this micro exposure is demonstrated using FIG.3 (b-1)-FIG.3 (b-4). 3 (b-1) and 3 (b-2) are diagrams showing the surface potential of the photosensitive drum 11 before exposure, and are schematic views showing a state in which they are charged to V ₀ and V _D , respectively. In FIG. 3B-1, charging is performed in advance so that the surface potential of the photosensitive drum 11 becomes V ₀ for the minute exposure. As described above, both FIG. 3 (b-1) and FIG. 3 (b-2) are in an inhomogeneous state with a potential portion higher than the convergence potential. 3B-1 and FIG. 3B-2, the photosensitive drum 11 is exposed after exposure when the micro exposure is performed and when the micro exposure is not performed. FIGS. 3 (b-3) and 3 (b-4) show the surface potential. Here, V _T1 is a transfer voltage, and V _th is a discharge threshold. FIG. 3B-4 is a schematic diagram when the pre-exposure potential shown in FIG. 3B-2 is exposed only to the toner image without performing micro exposure. In FIG. 3 (b-4), the potential difference between the high potential portion locally present in the background portion (portion indicated by pixel 2 in the drawing) and the transfer voltage V _T1 exceeds the discharge threshold V _th , Discharge may occur. For example, it is a location described as “discharge generation part” in the pixel 2 in FIG. Under that condition, if there is a toner image formed at the upstream image forming station, charging reversal occurs due to discharge, and a retransfer phenomenon occurs.

On the other hand, FIG. 3 (b-3) for the pre-exposure potential shown in FIG. 3 (b-1), while lowering the V _D by performing a minute exposure to the background portion, in the case of performing the latent image forming a toner image This is a schematic diagram showing the latent image formation performed in this embodiment. The microexposure of the background portion causes a “leveling effect” that improves potential non-uniformity by decreasing the potential of the portion having a higher surface potential according to the EV curve shown in FIG. Due to this leveling effect, the potential difference between the background potential V _D and the transfer voltage V _T1 becomes less than the discharge threshold V _th as shown in the pixel 2 (post-exposure background potential) in FIG. A re-transfer phenomenon is suppressed, and a color image having a uniform density can be obtained.

(When the exposure amount is controlled by the luminance signal 61a)
The luminance signal 61a shown in FIG. 3A is a signal for designating the light emission luminance of the laser diode 63a, and the laser driver 62a controls the current supplied to the laser diode 63a based on this signal, for each scan. Apply feedback to obtain stable brightness. Luminance signal 61a of this embodiment, the laser driver 62a, specifying the light emission luminance A ₀ for small exposure, the light emission luminance A ₁ for normal exposure. In this embodiment, as the exposure amount necessary for forming an electrostatic latent image on the photosensitive drum 11a, the exposure amount E ₀ obtained while the light emission of 1 pixel time emission luminance A ₀ for small exposure, the exposure of the background portion Used for. On the other hand, in the exposure of the toner image portion, the exposure amount E per pixel is controlled by controlling the time during which the light emission luminance A ₀ for fine exposure is maintained from the light emission luminance A ₁ based on the pulse width signal 60a. _x is determined. In the present embodiment, the ratio of the minute exposure amount E ₀ and the maximum exposure amount E ₂₅₅ is equal to the ratio of the light emission luminances A ₀ and A ₁ , and the ratio of the light emission luminance A ₀ to the light emission luminance A ₁ is changed. The minute exposure amount E ₀ can be linearly increased or decreased at a ratio to the maximum exposure amount E ₂₅₅ .

The pulse width signal 60a is generated from image data of a multi-value signal (0 to 255) of 8 bits = 256 gradations. For example, when the image data is 0 and the pulse width is 0 and ₂₅₅ , the full exposure is performed and the pulse width is equivalent to one pixel (PW ₂₅₅ ). For image data having a value of 1 to 254, the intermediate pulse width ( PW _x ) is generated. In accordance with this pulse width signal 60a, the laser driver 62a controls the retention time of the light emission luminance _{A 1.} In exposure pulse width for pixel 0, retention time of the light emission luminance A ₁ is 0, the exposure amount E ₀ of the background portion. In the case of full exposure with a pulse width of PW ₂₅₅ for one pixel, the light emission luminance A ₁ is held for one pixel and becomes the maximum exposure amount E _{255. With} the pulse width PW _x between them, the exposure amounts E ₀ and E It is controlled to ₂₅₅ exposure amount _{E x} between.

[About small exposure correction]
As can be seen from Equation 3 above, the surface potential of the photosensitive drum 11 after charging changes according to the film thickness of the photosensitive drum 11. This film thickness becomes thinner as the photosensitive drum 11 is used. For this reason, if the minute exposure amount is always constant, the background portion of the photosensitive drum 11 is not at an appropriate potential. As a result, the image density, chromaticity, character thickness changes depending on the amount of use of the device, and the image defect such as a so-called fogging image in which the toner adheres to the background portion where the toner is not adhered and is developed. May occur.

  Therefore, the image forming apparatus according to the present embodiment performs control for determining an appropriate minute exposure amount.

  The image forming apparatus of the present embodiment performs control for determining the background portion exposure amount by the control of FIG. The image forming apparatus of this embodiment includes a density sensor 40. The density sensor 40 is installed facing the surface of the intermediate transfer belt 1 as shown in FIG. 1B, and irradiates light onto images formed on the intermediate transfer belt 1 by the primary transfer rollers 15a to 15d. The value of the reflected light amount is output to the printer engine control unit 104. The printer engine control unit 104 converts the reflected light amount value input from the density sensor 40 into a density value, determines an image formation state from the information, and transmits a luminance signal 61 and a pulse width sent to each laser driver 62. The signal 60 is corrected. In this embodiment, the printer engine control unit 104 as a correction unit uses the laser exposure device 13 to perform correction image formation different from normal image formation, and the density sensor 40 controls the density of the formed correction image. By measuring, the optimum correction amount of the minute exposure amount is obtained and corrected. The operation of the background portion exposure amount correction will be described with reference to FIG.

[Background exposure correction]
(Correction image formation)
The correction in this embodiment is performed for all four image forming stations, and the measurement result of the image formed in each image forming station is reflected in the background portion exposure amount of each laser exposure unit 13. It is. Here, the yellow image forming station 10a will be described as an example. The exposure amount is controlled by the luminance signal 61a. Here, in the case of controlling the exposure amount by the pulse width signal 60a performed by switching the laser diode 63a at a high frequency, the switching operation is not performed in the case of controlling the exposure amount by the luminance signal 61a. As a result, when the control is performed using the luminance signal 61a, there is an advantage that high-frequency noise and the like are easily reduced as compared with the case where the control is performed using the pulse width signal 60a.

Photosensitive drum 11a before the exposure process, is charged by the charging voltage _{V a} by the charging roller 12a. When performing the first image forming the photosensitive drum 11a, using the predetermined initial value, the luminance signal 61a, designated as light emission luminance A ₀ for small exposure amount, the light emission luminance A ₁ for normal exposure Then, exposure is performed according to the above-described image forming process. The initial value in this embodiment, so that a small amount of exposure _E 0 = _0.03μJ / cm ^2, emission luminance _{A 0} normal has 12% of the light emission luminance _{A 1} for exposure, the background of the photosensitive drum 11a The part attenuates the potential to V _D ≈−500 V as described above (see FIG. 2B).

In this embodiment, a patch image is formed on the photosensitive drum 11 (on the photosensitive member) by exposing a background portion with a minute exposure amount E ₀ and exposing a toner image with a maximum exposure amount E ₂₅₅ (0.25 μJ / cm ² ). To do. Specifically, a 1-pixel width line extending in the main scanning direction by the maximum exposure amount E _{255 and} a 1-pixel width line extending in the main scanning direction by the minute exposure amount E ₀ are alternately repeated and arranged next to each other. The latent image pattern is formed. In the present invention, not only the area (first area) exposed on the photosensitive drum 11 with the normal exposure amount (first exposure amount for printing) (the maximum exposure amount in this embodiment), but also the minute exposure amount. The area (second area) exposed at (second exposure amount for fine exposure) is also called a latent image pattern.

Then, the developing unit 14 attaches toner to the latent image pattern and develops it to form a patch image PY (first image) as a toner image. For example, as shown in the patch image (a) in FIG. 5, the patch image PY has a toner image on a portion exposed with a normal exposure amount (first exposure amount for printing) (maximum exposure amount in this embodiment). Is formed. Further, the patch image PY is exposed with a minute exposure amount (second exposure amount for minute exposure) and has no toner image formed (shown in white), for example, as in the patch image (a) of FIG. Part) (second region). That is, the patch image PY of the present embodiment is exposed with a portion exposed with a normal exposure amount (for example, the maximum exposure amount E ₂₅₅ in this embodiment) (a black line portion of the patch image) and with a minute exposure amount E ₀ . Part (white line part of the patch image). A toner image as a patch image PY, which is a correction image formed on the photosensitive drum, is transferred onto the intermediate transfer belt 1 by a transfer process.

In this embodiment, when an image is formed for the first time on the photosensitive drum 11a, the patch image PY is formed in this way, and the reflection density of the patch image PY on the intermediate transfer belt 1 is a density sensor which is an image detection means. Measure at 40. Since the density sensor 40 measures the amount of reflected light that enters the aperture having a diameter of about 3 mm, the maximum exposure amount line and the minute exposure amount line of the patch image PY are present in the spot where the reflected light amount on the belt is measured. It has a configuration including a plurality. Accordingly, the amount of reflected light measured by the density sensor 40 reflects the average toner density when the patch image PY is viewed in a wide field of view. Then, the value detected by the density sensor 40 is converted into a density (also referred to as reflection density) D ₀ by the printer engine control unit 104 that also serves as an image analysis unit, and stored in, for example, a memory (not shown).

Thereafter, correction for the yellow image forming station performs the initial concentration D ₀ as a reference, for example every usage 500 pages to the photosensitive drum 11 is exchanged (every predetermined number of sheets). The same applies to image forming stations of other colors.

Correction image forming when performing correction by the printer engine control unit 104 changes the setting of the light emission luminance _{A 0,} the laser exposure device 13, for example, five levels (in this embodiment, for example 0.03~0.07μJ The exposure is performed while exposing the background portion with a minute exposure amount E ₀ of 0.01 μJ / cm ² at / cm ² . That is, assuming that the n-level exposure amount is B (n) μJ / cm ² , for example, B (1) = 0.03 μJ / cm ² , B (2) = 0.04 μJ / cm ² , B (3) = 0.05 μJ / cm ² , B (4) = 0.06 μJ / cm ² , B (5) = 0.07 μJ / cm ^{2, and the} like. The image formation for correction is preferably performed during one rotation of the photosensitive drum, and there is an advantage that the charging state is uniform even if the charging device has a low charging capability and does not saturate after one charge. The time required for this can also be shortened.

(Background exposure correction control)
The background portion exposure correction control process will be described with reference to the flowchart of FIG. The printer engine control unit 104 sets the counter n to 1 in step (hereinafter referred to as S) 202. In S203, the printer engine control unit 104 sets B (1) = 0.03 μJ / cm ² for the minute exposure amount E ₀ , for example, when n = 1. In step S <b> 204, the printer engine control unit 104 forms a correction image with the small exposure amount E ₀ set in step S <b> 203 and forms a patch image PY on the intermediate transfer belt 1. Note that the correction image formed in S204 is a 7 mm square patch image formed by repeating the pattern of the horizontal line of one pixel width by the maximum exposure amount and the one pixel horizontal line by the minute exposure amount.

Thereafter, in step S205, the printer engine control unit 104 increments the counter n (n = n + 1). In step S206, the printer engine control unit 104 determines whether n is greater than 5. If n is less than 5, the process returns to step S203. If n is greater than 5, the process proceeds to step S207. . As described above, the printer engine control unit 104 continuously forms the patch images PY on the intermediate transfer belt 1 when the five-level minute exposure amount E ₀ is used for the background portion exposure. That is, the printer engine control unit 104, a minute amount of exposure E ₀ to form a plurality of different patch image PY. These image formations for correction are performed almost simultaneously on the other image forming stations, and a series of patch images are formed on the intermediate transfer belt 1 so that these images do not overlap. A series of patch images on the intermediate transfer belt 1 is conveyed along with the rotation of the intermediate transfer belt 1 and reaches the density sensor 40.

In step S <b> 207, the printer engine control unit 104 detects (measures) the reflected light amount R (n) of the patch image PY formed on the intermediate transfer belt 1 by the density sensor 40. In step S208, the printer engine control unit 104 converts the reflected light amount R (n) detected by the density sensor 40 into an image density D (n), and the image density D (n) with respect to the minute exposure amount B (n) of the background portion. Is first-order approximated. The printer engine control unit 104 in S209 from this relationship, consistent with the initial concentration D _0, or, determine the exposure amount becomes close image density, it sets the exposure amount determined as a minute exposure E _0. That is, the printer engine control unit 104, respectively to detect the concentration of a plurality of patch images PY by the density sensor 40, based on a plurality of detection results, to correct the minute exposure E _0.

[Relationship between patch image density and background potential]
The principle of correcting the minute exposure amount of the background portion using the correction image formation in this embodiment will be described with reference to FIG. In FIG. 5, the horizontal axis represents the microexposure amount E (μJ / cm ² ), the vertical axis represents the reflection density, and the white square mark indicates the microexposure amount B (n) ( The points where the reflection density D (n) obtained for n = 1 to 5) are plotted are shown. Further, the extrapolated line in FIG. 5 indicates a straight line obtained by linear approximation by the printer engine control unit 104 based on the relationship between the minute exposure dose B (n) and the reflection density D (n) in S208 in FIG. .

Since the density sensor 40 measures the amount of reflected light that enters the aperture having a diameter of about 3 mm, the measured amount of reflected light is proportional to the average density of the toner image viewed from a wide field of view. The patch image PY in the present embodiment is a horizontal line image composed of lines in the main scanning direction as shown in FIG. Therefore, the density is high when the line width developed with toner is large as in the patch image (c), and the density is low when the line width developed with toner is small as in the patch image (a). Since the latent image potential of the line portion on the photosensitive drum is attenuated by a sufficient exposure amount E ₂₅₅ , it becomes substantially constant according to the EV curve of FIG. In this state, to form a patch image PY while changing the potential of the background portion by changing the small exposure amount E ₀ as the processing of S203~S206 in Fig. 4, it is developed with toner higher potential of the background portion The line width tends to be narrow. This is considered to be due to the fact that the potential difference between the background portion and the toner image portion widens and the action of the electric lines of force existing between the latent images becomes stronger, and the action of bringing the toner to the center becomes stronger. That is, the toner density of the patch image reflects the potential of the background portion. In order to keep the potential of the background portion constant during the period of use, the extrapolated line of the reflection density of the obtained image is the initial density D. _A minute exposure amount E ₀ equal to ₀ may be used for exposure of the background portion. That is, the printer engine control unit 104, a straight line detection result based on first-order approximation of the density sensor 40 determined in S208 of FIG. 4, the initial concentration D ₀ Metropolitan stored in advance, the amount of the photosensitive drum 11 Regardless of this, a minute exposure amount E ₀ capable of stable image formation is obtained.

FIG. 6 shows the microexposure amount E (μJ / cm2) (white square mark) and the potential of the background portion of the photosensitive drum 11 (background portion) when the apparatus is used while correcting the microexposure amount by the method described above. This represents the transition of the potential VD (V)) (x mark), and the horizontal axis represents the number of sheets passed (sheets). According to the present embodiment, by performing the feedback correction using the correction image described above, it is possible to derive an appropriate minute exposure amount even when it is necessary to change the correction range of the minute exposure amount according to the usage amount. . Therefore, regardless of the usage amount of the photosensitive drum 11, the background portion potential VD is kept constant (VD = −600 V) with high accuracy, and the reversal fog (a phenomenon in which the reversal toner is developed by the fog removal voltage) increases. Can be suppressed.

  Also, in the feedback correction of this embodiment, since the relationship between the photosensitive drum film thickness d and the charging potential according to Equation 3 is maintained, the background exposure amount corresponding to the usage amount of the photosensitive drum is inevitably shown in FIG. As shown in FIG. As a result, it is possible to secure a large leveling effect against the reduction in the uniformity of the charging potential due to the charging roller stains and the like that occur with use, and thus the generation of a spot image can be suppressed.

  In this embodiment, a line having a width of one pixel is combined when forming a pattern as a patch image, but the present invention is not limited to this. For example, two pixel width lines or three pixel width lines may be alternately formed according to the characteristics of the developing device 14. Further, the width of the line exposed at the maximum exposure amount and the line exposed at the minute exposure amount may be different. Further, a pattern combining lines extending in the sub-scanning direction or a pattern combining lines inclined with respect to the main scanning direction may be used. Further, the image affected by the potential of the background portion may be an image in which the region exposed with the minute exposure amount in the patch image PY is formed so as to surround the region exposed with the normal exposure amount, or the maximum exposure amount. A pattern in which exposed dots and dots exposed with a minute exposure amount are alternately arranged may be used. In this embodiment, as shown in the patch image (a) in FIG. 5 and the like, the patch image is formed so that the ratio of the area exposed with the minute exposure amount and the area exposed with the normal exposure amount is substantially equal. However, it is sufficient that the ratio is such that density changes appear in a plurality of patch images. Further, instead of the region exposed at the maximum exposure amount, a pattern of the region exposed at an exposure amount equal to or higher than the level to which the toner adheres may be used.

In this way, by changing the minute exposure amount E ₀ and changing the potential of the background portion, an area exposed with an exposure amount (constant) that is equal to or higher than the level to which the toner adheres (an area formed with the maximum exposure amount E ₂₅₅ ) and Any patch image using a pattern in which the potential difference from the potential of the background portion changes and appears as a change in toner density of the patch image may be used.

Although the formation of the patch image with small exposure amount E ₀ of 5 levels in this embodiment, may be formed a patch image with small exposure amount E ₀ of a plurality of levels.

  As described above, according to the present exemplary embodiment, it is possible to appropriately form the image with stable image quality while keeping the potential of the background portion to which the toner on the photosensitive drum is not attached appropriately.

In contrast to the image forming apparatus of the first embodiment, the image forming apparatus of the second embodiment corrects the normal exposure amount E ₂₅₅ prior to the correction of the minute exposure amount E _{0 in} the control as the background portion exposure amount determining means. Is the same except that The image forming apparatus of the present embodiment also includes four color image forming units of yellow (Y), magenta (M), cyan (C), and black (K), and further includes an intermediate transfer belt 1 and a fixing device. It shall have a vessel. The density sensor 40 faces the surface of the intermediate transfer belt 1, and outputs the value of the amount of reflected light of the image formed on the intermediate transfer belt 1 to the printer engine control unit 104. The printer engine control unit 104 determines the image forming state from this value, and corrects the luminance signal 61 and the pulse width signal 60 sent to each laser driver 62. In this embodiment, the exposure amount is controlled by the luminance signal 61a.

  In this embodiment, an image for correction is formed separately from normal image formation, and the density of the image for correction is measured to obtain the optimum fine exposure amount and normal exposure amount correction amount. The operation of the background portion exposure amount correction will be described with reference to FIG.

[Exposure correction]
The correction in this embodiment is performed for all four image forming stations, and the measurement results of the images formed in each image forming station are used as the background portion exposure amount and the toner image portion of each laser exposure unit 13. This is reflected in the exposure amount. Here, the yellow image forming station 10a will be described as an example. Photosensitive drum 11a before the exposure process, is charged by the charging voltage _{V a} by the charging roller 12a. When performing the first image forming the photosensitive drum 11a, using the predetermined initial value, the luminance signal 61a, to specify a light emission luminance A ₀ for small exposure, the light emission luminance A ₁ for normal exposure Then, exposure is performed according to the above-described image forming process. In this embodiment, the initial values are set as the light emission luminance A ₀ for the normal exposure so that the minute exposure amount E ₀ = 0.03 μJ / cm ² and the maximum exposure amount E ₂₅₅ = 0.25 μJ / cm ^2. It is 12% of a _1. Then, the toner image portion of the photosensitive drum 11a is attenuated to V _L ≈−150V, and the background portion is similarly attenuated to V _D ≈−500V.

A 7mm square patch image PY and a 7mm square patch image PX (second latent image pattern) are formed by repeating a pattern of 1 pixel horizontal line / 1 pixel horizontal space while exposing the background portion with a minute exposure amount. Then, they are transferred onto the intermediate transfer belt 1 by a transfer process. The patch image PX is a patch image including only a portion exposed with a normal exposure amount (in this embodiment, a maximum exposure amount E ₂₅₅ ). The reflected light amount of the patch image PY and the patch image PX on the intermediate transfer belt 1 at the concentration sensor 40, the printer engine control unit 104, converted and stored as initial concentration D ₀ and initial concentration F _0. Thereafter, the correction for the yellow image forming station is performed, for example, every 500 pages of usage until the photosensitive drum 11 is replaced with reference to the initial density D ₀ and the initial density F ₀ .

In step S302, the printer engine control unit 104 sets the counter n to 1. In steps S303 to S306, the printer engine control unit 104 sets the patch image PX to the maximum exposure of 5 levels C (n) while keeping the background portion at the current minute exposure amount E ₀ (for example, 0.03 μJ / cm ² ). An image is formed using the quantity E ₂₅₅ . In this embodiment, image formation is performed at five levels of 0.05 μJ / cm ² , for example, at 0.25 to 0.45 μJ / cm ² . That is, C (1) = 0.25 μJ / cm ² , C (2) = 0.30 μJ / cm ² , C (3) = 0.35 μJ / cm ² , C (4) = 0.40 μJ / cm ² , C (5) = 0.45 μJ / cm ² . Specifically, in step S303, the printer engine control unit 104 sets C (1) = 0.25 μJ / cm ² as the maximum exposure amount E ₂₅₅ , for example, when n = 1.

In step S <b> 304, the printer engine control unit 104 forms a correction image with the maximum exposure amount E ₂₅₅ set in step S <b> 303, and forms a patch image PX (solid pattern patch) on the intermediate transfer belt 1. In step S305, the printer engine control unit 104 increments the counter n (n = n + 1). In step S306, the printer engine control unit 104 determines whether n is greater than 5. If n is less than 5, the process returns to step S303. If n is greater than 5, the process proceeds to step S307. . As described above, the printer engine control unit 104 performs the correction image formation on the other image forming stations almost simultaneously, and a series of patches so that the images do not overlap on the intermediate transfer belt 1. An image is formed.

In step S <b> 307, the printer engine control unit 104 measures the reflected light amount S (n) of the patch image PX formed on the intermediate transfer belt 1 using the density sensor 40. In step S308, the printer engine control unit 104 converts the reflected light amount S (n) measured by the density sensor 40 into an image density F (n), and the relationship between the image density F (n) and the maximum exposure amount C (n). First order approximation. In step S <b> 309, the printer engine control unit 104 obtains an exposure amount that matches or is close to the initial density F ₀ from the obtained linear approximation line, and sets the obtained exposure amount as the maximum exposure amount E ₂₅₅ .

Next, in S310 to S314, the patch image PY is continuously formed in the same manner on the intermediate transfer belt 1 while the background portion is exposed with a minute exposure amount of 5 levels. In this embodiment, like in Example 1, and 0.01μJ / cm ² increments 0.03~0.07μJ / cm ^2. Note that the processing of S310 to S314 is the same as the processing of S202 to S206 described with reference to FIG. Here, in step S312, the printer engine control unit 104 forms a patch image PY. Moreover, since the process of S315-S317 is the same as the process of S207-S209 demonstrated in FIG. 4 of Example 1, description is abbreviate | omitted.

As described above, in the exposure amount correction of this embodiment, the correction image formation with the maximum exposure amount E ₂₅₅ is performed prior to the correction image formation with the minute exposure amount E ₀ . The patch image PX which is a solid pattern is not easily affected by the potential of the background portion of the photosensitive drum 11, so even if the use of the photosensitive drum 11 progresses and the film thickness wears, the toner image portion potential by the maximum exposure amount E _255. V _L dominates. Accordingly, the maximum exposure amount E ₂₅₅ obtained by the processing of S302 to S309 in FIG. 7 is set to the maximum exposure amount that attenuates the potential to V _L (≈−150 V) at the initial stage of the photosensitive drum regardless of the degree of wear of the photosensitive drum 11. Is done. Subsequently, by correcting the microexposure amount for the background portion, an appropriate microexposure amount can be set even when _VL changes due to a change in the photosensitive drum 11 with time or an environmental change.

  As described above, according to the present exemplary embodiment, it is possible to appropriately form the image with stable image quality while keeping the potential of the background portion to which the toner on the photosensitive drum is not attached appropriately.

[About small exposure correction]
In the third embodiment, as background portion exposure amount determination means, for example, background portion exposure amount optimization control using the integrated rotational speed of the photosensitive drum 11 is provided. By this control, the above-described minute exposure amount E ₀ of the background portion is corrected according to the film thickness d (μm) of the charge transport layer of the photosensitive drum 11. According to Equation 3, when the thickness d of the dielectric layer of the photosensitive drum 11 decreases, the charging potential V ₀ increases. Since the decrease in the thickness of the charge transport layer means the decrease in the thickness of the dielectric layer, the role of this correction is to suppress the increase in the background potential V _D by suppressing the increase in the charging potential V ₀ due to this change in film thickness. It is intended to suppress and keep constant. Rise in the background portion potential V _D will be raised to the developing potential, a fog removal voltage. On the other hand, since the fog removal voltage is equivalent to the development voltage for the reverse toner (toner charged with reverse polarity), the reverse fog increases. Therefore, in this embodiment by maintaining a constant background portion potential V _D, it is set to suppress the inversion head configuration, even if the progress in the use of the photosensitive drum 11.

Using the yellow image forming station as an example, the minute exposure amount correction will be specifically described with reference to FIG. In this embodiment, it is assumed that the minute exposure amount is controlled by a pulse width signal 60a shown in FIG. The image forming unit 10a includes a storage member (not shown) that stores the accumulated rotational speed of the photosensitive drum 11a. In step (hereinafter referred to as “S”) 101, the printer engine control unit 104 reads the accumulated rotational speed of the photosensitive drum 11 a stored in the storage member (denoted as LOAD in the drawing). The printer engine control unit 104 in S102 determines the small exposure amount _{E 0} of the background area exposure based on the integrated number of rotations read in S101. In step S103, the printer engine control unit 104 performs image formation. In step S104, the printer engine control unit 104 measures (counts) the number of rotations of the photosensitive drum 11a rotated in a series of image forming operations (simply referred to as a drum in the figure). In step S105, the printer engine control unit 104 determines whether or not the print job (denoted as JOB in the drawing) has ended. If it is determined that the print job has not ended, the process returns to step S103, and the print job is If it is determined that the process has been completed, the process proceeds to S106. In step S106, the printer engine control unit 104 adds the rotational speed of the photosensitive drum 11a to the cumulative rotational speed at the end of image formation, and updates the cumulative rotational speed of the storage member in the image forming unit in S107 (denoted as SAVE in the figure). ).

The correction of the minute exposure amount is calculated based on the EV curve in FIG. As described above, when the film thickness of the charge transport layer 24a of the photosensitive drum 11a in the initial state is 20 μm, the exposure amount at the background portion exposure is set to 0.03 μJ / cm ² (in FIG. 2B). Solid curve). The broken line curve in FIG. 2 (b) shows the charging potential V ₀ and EV curve on the photosensitive drum 11a at the end of its life. The thickness of the charge transport layer 24a is reduced to 10 μm, so that the charging potential V ₀ increases. Is seen. Further, from the graph of FIG. 2B, when the photosensitive drum 11a reaches the end of its life, the charged potential V ₀ (= −686 V) is maintained at the background potential V _D of −500 V, which is the same as the initial value. It can be seen that it is optimal to set the exposure amount to 0.06 μJ / cm ² .

  Wear of the charge transport layer 24a of the photosensitive drum 11a is promoted by discharge erosion or the like in the drum cleaner 17a or the charging unit that contacts the photosensitive drum 11a. Therefore, the amount of wear of the photosensitive drum 11a may be considered to be proportional to the integrated rotational speed. In the present embodiment, the accumulated number of rotations is associated with the film thickness d (μm) of the charge transport layer 24a based on a preliminary experiment result that the charge transport layer 24a is worn by 1 μm at 15000 revolutions (equivalent to 500 page printing). .

The logic calculation of the above, each cumulative revolution number 15,000 rpm to increase the _{PW MIN,} increasing the exposure amount _{E 0} of 0.003μJ / cm ² background section, 0.06μJ / cm from the initial 0.03μJ / cm ^{2 The} background portion exposure amount is determined by linear transition to ² . This control regardless of the thickness of the charge transport layer 24a of the photosensitive drum 11a, a configuration that maintains a substantially constant background portion potential _{V D} to -500 V.

Here, relative to the variation of the background portion potential V _D by abrasion of the photosensitive drum 11, a method of correcting and changing the charging voltage V _a. By reducing the current V _a = −1180 V toward, for example, −1094 V according to the amount of use, fluctuations in the background portion potential V _D can be suppressed. However, since the charging device deteriorates with use, the charging capability of the charging device itself or the charge uniformizing capability is reduced. Accordingly, when corrected by lowering the charging voltage V _a, smaller potential difference between the developing voltage in low charge capacity portion, a problem that fogging phenomenon increase was found to occur. The charging device using the charging roller 12 used in this embodiment has the advantage of suppressing generation of ozone and the like because charging is performed with a minimum discharge, but the surface is easily soiled and causes insufficient charging as usage progresses. Sometimes. In addition, even when a non-contact corona charger is used, the wire is soiled and a charging failure occurs similarly. With respect to such a phenomenon, the superiority of the configuration shown in the present embodiment was verified, and the results will be described below.

[Comparison experiment]
Regard correction for variations in the background portion potential V _D, Comparative Example 1 If not performed, the case of correcting the charging voltage V _a and Comparative Example 2 were carried out comparative experiments with the image forming apparatus of the present embodiment. When full-color image formation by passing plain recording paper (basis weight 68 g / m ² ) is repeated and an image with a toner image ratio of 1% is printed from the beginning of the photosensitive drum 11 to the end of its life, the image is checked every 1000 sheets. The fogging amount and the image uniformity were evaluated. The amount of fogging is measured by measuring the optical reflectance of the background on the recording paper with an optical reflectance measuring machine and subtracting it from the reflectance of only the recording paper to obtain the amount of change in reflectance. 10 or more points were measured and taken as the average value. Image uniformity was evaluated as “X” when visually noticeable, “△” when visually noticeable, and “◯” when no sign was observed, focusing on the occurrence of local density change on the recording paper. .

FIG. 9A shows the transition of the fogging amount (%). In Comparative Example 1 (white squares, broken lines), the charging potential V ₀ increases with an increase in the number of sheets to be passed, so that the reversal fog due to the increase in the potential difference between the background portion potential V _D and the developing potential is greatly increased. Also, Comparative Example 2 (white triangles, dashed line) Although reversal fog in does not increase, the charging roller 12 is soiled with the local head with the lower part of the background portion potential V _D occurs, increasing the total head weight There is a tendency.

FIG. 9B shows a transition of image uniformity. In Comparative Example 2, increasing the charging roller 12 contamination later sheet passing life, spots image (background portion potential V _D of the charging roller cycle is lower than the developing voltage, the background portion from being developed in part phenomenon ) Occurs. Since the contamination of the charging roller 12 is considered equivalent to the high-resistance coating on the surface, the partial pressure of the minute gap is reduced to inhibit discharge. This tendency becomes more pronounced lowering the charging voltage V _a, it may lead to an increase in correction of the background portion potential V _D of Comparative Example 2, more noticeable visually compared to "head", "spot image" It shows that there is.

According to this embodiment, it is possible to ensure a sufficient leveling effect by keeping constant the background portion potential V _D not only suppress the increase of the reversal fog, increasing the background area exposure amount E _0. And further, according to the present embodiment, the charging roller contamination without causing the uniformity decrease in charge potential by the like, it is possible to form the background portion potential V _D. Therefore, it can be said that it is possible to apply with the rise and uniformity decrease in the background portion potential V _D with the progress of the use degree, effective measures.

Further, since the background portion potential V _D is kept constant in each image forming station, there is an advantage that increase in fog can be suppressed even when a voltage is supplied from the same power source to each developing device.

  As described above, according to the present exemplary embodiment, it is possible to appropriately form the image with stable image quality while keeping the potential of the background portion to which the toner on the photosensitive drum is not attached appropriately.

In contrast to the image forming apparatus of the third embodiment, the image forming apparatus of the fourth embodiment uses a luminance signal 61a shown in FIG. 3A as an output destination of the exposure amount data determined by the background portion exposure amount determining means. This is the same except that the minute exposure dose E ₀ is controlled. The image forming apparatus of this embodiment also includes four color image forming units of yellow (Y), magenta (M), cyan (C), and black (K), and further includes an intermediate transfer belt as an intermediate transfer member. It shall have a transfer device and a fixing device.

[About small exposure correction]
In the present embodiment, as in the first embodiment, the minute exposure amount E ₀ of the background portion is corrected according to the film thickness d (μm) of the charge transport layers of the photosensitive drums 11a to 11d. In the correction of the minute exposure amount, the exposure amount is increased by 0.003 μJ / cm ² every 15,000 rotations of the photosensitive drum 11a, and is linearly changed from the initial 0.03 μJ / cm ² to 0.06 μJ / cm ² . As a result, the background portion potential V _D is kept substantially constant at −500 V regardless of the film thickness d (μm) of the charge transport layer of the photosensitive drum 11 a.

In this embodiment, the light exposure luminance A ₀ for fine exposure specified by the luminance signal 61a dominates the fine exposure amount E ₀ of the background portion. The printer engine control unit 104, in accordance with the accumulated number of rotations of the photosensitive drum 11a, to increase the light emission luminance A ₀ minute exposure E ₀ by raising the. In Example 3, the _{PW MIN} using pulse width signal 60a, and produces a small amount of exposure _{E 0} by the laser diode 63a is the switching operation at a high frequency. On the other hand, in this embodiment, the switching operation in the minute exposure of the background portion is not accompanied. As a result, there is an advantage that high-frequency noise and the like can be easily reduced.

  As described above, according to the present exemplary embodiment, it is possible to appropriately form the image with stable image quality while keeping the potential of the background portion to which the toner on the photosensitive drum is not attached appropriately.

  In the image forming apparatuses according to the first to fourth embodiments described above, the toner image as the patch image PY is transferred onto the intermediate transfer body 1 and is detected by the density sensor 40, but is not limited thereto. In other words, instead of the intermediate transfer body 1, it may be configured to transfer on the electrostatic conveyance belt (ETB) and detect this. Further, the density sensor 40 may be disposed so as to face the circumferential surface of the photosensitive drum 11 and the density of the patch image PY may be detected without being transferred to the intermediate transfer body 1 or the like. Further, the image forming apparatus 100 including the laser exposure unit 13 as the light irradiation unit has been described, but the laser exposure unit 13 is not limited thereto. That is, a plurality of light emitting units (LEDs or the like) arranged in the axial direction of the photosensitive drum 11 are provided, each of the plurality of light emitting units emits light according to image data, and the surface of the photosensitive drum 11 is rotated by the rotation of the photosensitive drum 11. You may use the light irradiation means which scans only to a moving direction.

13 Laser exposure device 40 Density sensor 104 Printer engine control unit

Claims (9)

A photoreceptor,
Charging means for charging the photoreceptor;
Light irradiating means for irradiating the photosensitive member charged by the charging means to form a latent image; and
Developing means for developing the toner by attaching toner to the latent image;
Detecting means for detecting a value relating to the density of the toner image formed by the developing means,
The light irradiating means irradiates light at a first exposure amount to an image portion to which the toner of the photoconductor is attached in accordance with image data for image formation, and a non-image portion that does not attach the toner of the photoconductor in the image forming apparatus is irradiated with light, it performs the images formed by the second exposure amount to,
A correction means for correcting the second exposure amount of the light irradiation means;
Before Symbol light irradiation means forms the first exposure amount latent image pattern alternating a second region irradiated with the light in the second exposure amount and the first region irradiated with the light to the photosensitive member ,
It said detecting means detected a toner image obtained by developing by the developing unit to the latent image pattern,
The image forming apparatus , wherein the correction unit corrects the second exposure amount based on the detected value. A photoreceptor,
Charging means for charging the photoreceptor;
Light irradiating means for irradiating the photosensitive member charged by the charging means to form a latent image; and
Developing means for developing the toner by attaching toner to the latent image;
Detecting means for detecting a value relating to the density of the toner image formed by the developing means,
The light irradiating means irradiates light at a first exposure amount to an image portion to which the toner of the photoconductor is attached in accordance with image data for image formation, and a non-image portion that does not attach the toner of the photoconductor In an image forming apparatus that irradiates light with a second exposure amount to form an image,
A correction means for correcting the second exposure amount of the light irradiation means;
The light irradiation means forms a latent image pattern on the photoconductor having a portion surrounded by a second region irradiated with light at the second exposure amount in a first region irradiated with light at the first exposure amount,
The detecting means detects a toner image obtained by developing the latent image pattern by the developing means;
The image forming apparatus, wherein the correction unit corrects the second exposure amount based on the detected value. The correcting means forms a plurality of latent image patterns with different second exposure amounts, and based on the values detected by the detecting means for toner images corresponding to the plurality of latent image patterns, respectively. the image forming apparatus according to claim 1 or 2, characterized in that the correction of the secondary exposure. The latent image pattern, the image forming apparatus according to any one of claims 1 to 3, characterized in that the ratio between the first region and the second region are equal. The correction means further corrects the first exposure amount,
The correcting means causes the light irradiating means to form a second latent image pattern consisting only of the first region irradiated with light at the first exposure amount on the photoconductor, and to form the second latent image pattern. the toner image obtained by developing the developing means is detected by said detecting means, based on the detected value, any one of claims 1 to 4, characterized in that the correction of the first exposure amount 1 The image forming apparatus described in the item. The correction means forms a plurality of the second latent image patterns with different first exposure amounts, and based on values detected by the detection means for toner images corresponding to the plurality of second latent image patterns, respectively. The image forming apparatus according to claim 5 , wherein the first exposure amount is corrected. Said correcting means, after performing the correction of the first exposure, the image forming apparatus according to claim 5 or 6, characterized in that the correction of the second exposure. It said correction means, the image forming apparatus according to any one of claims 1 to 7, characterized in that the correction each time an image is formed on a recording material a predetermined number. A photoreceptor,
Charging means for charging the photoreceptor;
Light irradiating means for irradiating the photosensitive member charged by the charging means to form a latent image; and
Developing means for developing the toner by attaching toner to the latent image;
Detecting means for detecting a value relating to the density of the toner image formed by the developing means,
The light irradiating means irradiates light at a first exposure amount to an image portion to which the toner of the photoconductor is attached in accordance with image data for image formation, and a non-image portion that does not attach the toner of the photoconductor In an image forming apparatus that irradiates light with a second exposure amount to form an image,
A correction means for correcting the second exposure amount of the light irradiation means;
The light irradiating unit forms a latent image pattern on the photoconductor in which the ratio of the first region irradiated with light at the first exposure amount and the second region irradiated with light at the second exposure amount is equal,
The detecting means detects a toner image obtained by developing the latent image pattern by the developing means;
The image forming apparatus, wherein the correction unit corrects the second exposure amount based on the detected value.

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