JP4453766B2 - Image forming apparatus and image forming method - Google Patents

Description

  In the present invention, an electrostatic latent image is formed on the surface of an electrostatic latent image carrier such as a printer, a copying machine, or a facsimile machine, and a developing bias is applied to the developing means to visualize the electrostatic latent image with toner. The present invention relates to an image forming apparatus and an image forming method for forming a toner image.

  As an example of this type of image forming apparatus, a main part of a laser beam printer using negatively charged toner is shown in FIG. In this image forming apparatus, the surface of the photoconductor as the electrostatic latent image carrier 20 is charged to a negative surface potential Vo, and when the surface is irradiated with the light LL from the semiconductor laser 60 as the exposure means, irradiation is performed. A part of the charge of the formed portion is neutralized, and the surface potential changes to Von. In this way, by performing scanning exposure on the photoconductor while turning on / off the light LL corresponding to the image signal, as shown in FIG. 8B, the surface corresponding to the image portion corresponding to the image signal. While the potential of the region becomes Von (≠ Vo), the potential of the surface region corresponding to the non-image portion attenuates from the surface potential Vo immediately after charging to Vd (| Vd | ≦ | Vo |) due to dark decay, and thus the image. An electrostatic latent image corresponding to the signal is formed on the electrostatic latent image carrier 20.

  The electrostatic latent image formed in this way is conveyed to a developing position P facing the developing roller 40 constituting the developing means by the rotation of the electrostatic latent image carrier 20 in the arrow direction. A negatively charged toner is carried on the developing roller 40, and a developing bias Vb is applied to promote toner adhesion to the image portion of the electrostatic latent image carrier 20. As shown in FIG. 8B, the potential of the developing bias Vb is set to a value between the non-image portion potential Vd and the image portion potential Von. The surface of the electrostatic latent image carrier 20 has a lower potential than the developing roller 40, whereas the surface of the electrostatic latent image carrier 20 has a higher potential than the developing roller 40 in the image portion. Therefore, of the negatively charged toner carried on the developing roller 40, the toner at the position facing the image portion is moved to the electrostatic latent image carrier 20 side by the electrostatic force, and is at the position facing the non-image portion. A force in the direction of drawing toward the developing roller 40 is applied to the toner. As described above, by attaching the toner only to the image portion, the electrostatic latent image on the electrostatic latent image carrier 20 is visualized by the toner.

  In the image forming process for forming the toner image in this manner, the exposure energy of the irradiation light LL, the non-image portion potential Vd, the image portion potential Von, and the development bias potential Vb are each greatly increased in the image density of the final toner image. Many techniques have been proposed for optimizing the image density by appropriately adjusting some of these parameters as image density control factors (see, for example, Patent Document 1). ).

Japanese Patent Laid-Open No. 2001-100471

  By the way, in an actual image forming process, a toner image is formed while these parameters are related to each other. Therefore, these parameters cannot always be controlled independently and arbitrarily. In particular, the relative potential relationship between the development bias potential Vb and the non-image portion potential Vd is not only the density of the image, but also the image quality of the obtained toner image and the toner scattering inside the apparatus as described below. Since it also has a great influence on the amount, it is important to set these values appropriately in order to form a toner image with better image quality.

  In this specification, the absolute value of the potential difference between the developing bias potential Vb and the non-image portion potential Vd is referred to as a reverse contrast potential Vr. That is, the reverse contrast potential Vr = | Vb−Vd |.

  Here, in order to examine the influence of the potential relationship, first, consider a case where the reverse contrast potential Vr is reduced by bringing the developing bias potential Vb close to the level of the non-image portion potential Vd. At this time, the potential difference between the image portion potential Von and the developing bias potential Vb, that is, the contrast potential (= | Vb−Von |) increases, and the developing roller 40 to the electrostatic latent image carrier 20 in the image portion. Since toner transfer is promoted, a high image density can be obtained. However, on the other hand, since the potential difference with the developing roller 40 is small in the non-image portion of the electrostatic latent image carrier 20, the action of pulling excess toner back to the developing roller 40 side is weak. Therefore, the amount of toner that is separated from the developing roller 40 and scatters inside the apparatus increases.

  On the other hand, when the absolute value of the non-image portion potential Vd is increased and the reverse contrast potential Vr is increased with the developing bias potential Vb as it is, the amount of toner scattering into the apparatus can be reduced, but the electrostatic latent image carrier 20. Since the negative charge held in the non-image part increases the power to spread the negatively charged toner, the electrostatic latent image shown in FIG. 8 has a particularly narrow area between the non-image parts. Makes it difficult for toner to adhere, resulting in image quality degradation in low-density images with a relatively low dot area ratio, such as faint isolated dots and fine lines, and loss of line width uniformity. Become.

  As described above, it is preferable to increase the reverse contrast potential Vr in order to suppress toner scattering, but it is contrary to the desire to decrease the reverse contrast potential Vr in order to ensure image quality such as uniformity of fine lines. In order to perform image formation with good image quality while suppressing toner scattering inside the apparatus, it is necessary to set parameters such as the developing bias potential Vb so that the reverse contrast potential Vr is always an appropriate value. There is.

  However, in the conventional image forming apparatus in which these parameters are variably controlled for the purpose of adjusting the image density, the above-described problems are not taken into consideration, and therefore, an image having a good image quality is stably formed. There was much room for improvement above.

  The present invention has been made in view of the above problems, and provides an image forming apparatus and an image forming method capable of stably forming a toner image with good image quality while preventing toner scattering into the apparatus. Objective.

In order to achieve the above object, an image forming apparatus according to the present invention irradiates a photosensitive member charged to a predetermined surface potential with a light beam toward the surface of the photosensitive member, and an image portion of the surface of the photosensitive member. An exposure unit that forms an electrostatic latent image on the surface by differentiating the image portion potential of the surface region corresponding to the surface area and the non-image portion potential of the surface region corresponding to the non-image portion, and a given developing bias potential Accordingly, the electrostatic latent image is visualized with toner to form a toner image, and the potential difference between the development bias potential and the non-image portion potential is constant when the toner image is formed. Control means for controlling the development bias potential, and the control means maintains the potential difference between the development bias potential and the non-image portion potential at the same value as when the toner image is formed, and multistages the development bias potential. The development bias potential is optimized as an image density control factor that affects the image density based on the density of the patch image formed at each setting value, and the development bias potential is set to an optimum value. The non-image portion potential is set so that the potential difference between the optimum value of the developing bias potential and the non-image portion potential becomes the same value as when the toner image is formed, and the energy density of the light beam is multistage. As an image density control factor that affects the image density based on the density of an isolated dot line image whose dot area ratio is 20% or less with respect to the entire patch image as a patch image formed with each set value. It is characterized by optimizing the energy density of the light beam.

  In the invention configured as described above, image formation is performed in a state where the potential difference between the developing bias potential and the non-image portion potential, that is, the reverse contrast potential is kept constant, so that the reverse contrast potential is always set to an appropriate value. Therefore, it is possible to form a toner image with good image quality while effectively suppressing toner scattering inside the apparatus. Further, the developing bias potential is optimized based on the density of the patch image formed by changing the developing bias potential while keeping the potential difference between the developing bias potential and the non-image portion potential at the same constant value as at the time of toner image formation. At the same time, the energy density of the light beam is optimized under the development bias potential and the non-image portion potential thus optimized. Therefore, it is possible to further optimize the image density of the toner image while achieving both suppression of toner scattering and good image quality.

Here, the patch image formed when the energy density of the light beam is optimized is an isolated dot line image in which the area ratio of dots with respect to the entire patch image is 20% or less. In such a patch image, since there is no mutual interference between adjacent lines in the electrostatic latent image, the difference in image density due to the magnitude of the exposure energy appears clearly, so that the optimum value of the exposure energy can be obtained with high accuracy. Further, a patch image formed in optimizing the developing bias potential, the area ratio of the dots for the entire patch image may be a Ru image der of 80% or more. In a high-density patch image in which the dot area ratio with respect to the entire patch image is 80% or more, a difference in density due to the magnitude of exposure energy hardly appears .

  A charging unit that applies a predetermined charging bias to charge the surface of the photoconductor to a predetermined initial surface potential, and the control means has a surface potential of the photoconductor after being attenuated from the initial surface potential by dark decay; May be the non-image portion potential, and the charging bias may be controlled so that the potential difference between the non-image portion potential and the developing bias is constant. Since the magnitude of the charging bias, the initial surface potential of the photoreceptor, and the attenuation due to dark decay are known, a desired non-image area potential can be obtained by controlling the charging bias of the charging unit, thereby developing bias potential. And the non-image portion potential can always be kept constant.

  Further, the image forming apparatus further includes density detecting means for detecting an image density using a predetermined toner image formed on the photosensitive member by the developing means or a toner image obtained by transferring the toner image on a transfer medium as a patch image, The control means may optimize the image density control factor based on a detection result of the density detection means. In this way, by adjusting the image density control factor based on the image density of the actually formed patch image, it is possible to adjust the image density in a state close to that during normal image forming operation. The image density of the toner image can be adjusted with higher accuracy.

  The control means obtains the image density of each patch image by the density detection means, and sets the development bias potential by setting the development bias potential closest to the preset target image density to the optimum value. You may optimize. At this time, the reverse contrast potential can be kept constant by changing the non-image portion potential as the development bias potential is changed.

  Further, the control means obtains the image density of each patch image by the density detection means, and sets the energy density of the light beam whose image density is closest to a preset target image density as the optimum value. The energy density of the light beam may be optimized. At this time, the development bias potential is set to the optimum value obtained earlier, and a low-density image is created with high reproducibility by using a patch image formed by setting a non-image potential to keep the reverse contrast potential constant. It becomes possible to do.

  In addition, the developing unit may visualize the electrostatic latent image by jumping development to form a toner image. Here, “jumping development” is a development method in which toner is caused to fly between the developing means held in a non-contact state and the photosensitive member, and the toner adheres to the electrostatic latent image on the photosensitive member. In this jumping development type image forming apparatus, since the toner is caused to fly between the developing means and the photosensitive member as described above, the toner floats between the two during image formation. Is easier to scatter around. Therefore, keeping the reverse contrast potential constant in such an apparatus is effective in suppressing toner scattering while maintaining image quality. In addition, when an AC component is added to the developing bias in order to improve the flying property of the toner, the potential difference between the average value of the potential and the non-image portion potential may be kept constant.

  The developing unit includes a plurality of developing units corresponding to different toner colors, and the plurality of developing units are sequentially positioned at a predetermined developing position to form a toner image in the toner color. A color image may be formed by the above. In such a color image forming apparatus, when the toner color is switched, the mechanical movement of the developing device is involved, so that the toner scatters easily, and in order to obtain a color image with good color reproducibility, fine dots of each toner color are formed. In such an image forming apparatus, keeping the reverse contrast potential constant is extremely effective for stably forming a toner image with good image quality.

  Further, when the combination of the developing bias potential and the image portion potential can be changed, the maximum value and the minimum value of the potential difference between the developing bias potential and the image portion potential within the variable range. The difference is desirably 30 V or less, more preferably 20 V or less. This is because these potential differences are the above-described contrast potentials, and it is possible to adjust the image density by making this contrast potential variable, but the influence of the change in contrast potential on the image density should be formed. Since it varies depending on the content of the image, that is, the ratio between the image area and the non-image area within a certain area and its arrangement, if the contrast potential is changed in a very large range, the fluctuation of the image density in the fine line image becomes large or gradation reproduction This is because there is a risk of adverse effects such as a decrease in performance.

  Therefore, as described above, by limiting the variable range of the contrast potential to 30 V or less, more preferably 20 V or less, it is possible to suppress fluctuations in image density and deterioration in gradation reproducibility.

  As such an image forming apparatus, there are a type in which an electrostatic latent image (negative latent image) is formed by irradiating a light beam onto an area corresponding to an image portion on a surface of a photoreceptor, and an area corresponding to a non-image portion. There is a type that forms an electrostatic latent image (positive latent image) by irradiating a light beam, but in any apparatus, the energy density of the irradiated light beam can be used as one of the image density control factors. In addition, by keeping the reverse contrast potential constant, it is possible to form a toner image with good image quality while achieving both reproducibility of fine lines and suppression of toner scattering. In the former type, the irradiation area potential corresponds to the image area potential, and the image area potential is controlled by changing the energy density of the light beam, but in the latter type, the irradiation area potential is conversely non-image area. Since it corresponds to the potential, the non-image portion potential is controlled by changing the energy density of the light beam.

Also, the image forming method according to the present invention is such that the surface of the photoconductor is exposed to light by a light beam, and the image area potential of the surface area corresponding to the image area of the photoconductor surface and the non-image area of the surface area corresponding to the non-image area. An electrostatic latent image is formed on the surface by making the image portion potential different from each other, and the development bias potential applied to the developing means is controlled so that the potential difference from the non-image portion potential is constant. An image forming method for forming the toner image according to an image signal by visualizing the electrostatic latent image with toner, and for achieving the above object, the development bias potential and the non-image portion potential While maintaining the potential difference at the same constant value as when forming the toner image, these are changed and set in multiple stages to form a predetermined patch image each time, and based on the measurement result of the image density, A step of optimizing the image bias potential; setting the developing bias potential to an optimum value; and a potential difference between the optimum value of the developing bias potential and the non-image portion potential being the same constant value as when forming the toner image The non-image portion potential is set so that the energy density of the light beam is changed in multiple stages so that the dot area ratio with respect to the entire patch image as a predetermined patch image is 20% or less each time. Forming an isolated dot line image and optimizing the energy density of the light beam based on the measurement result of the image density, and setting the developing bias potential and the energy density of the light beam to optimized values. The toner image is formed by setting.

  In the image forming method configured as described above, the potential difference between the development bias potential and the non-image portion potential, that is, the reverse contrast potential is kept constant. It is possible to stably form a toner image with good image quality while suppressing scattering.

  Furthermore, while maintaining the reverse contrast potential at the same constant value as when forming the toner image, the development bias potential is changed and set in multiple stages to form a patch image, and the development bias potential is optimized based on the image density measurement results. The development bias potential is set to the optimum value obtained in this way, and the patch image is formed by changing the energy density of the light beam in multiple stages while setting the reverse contrast potential to the same constant value as the toner image formation. The energy density of the light beam is optimized based on the density measurement result. In this way, the development bias potential and the energy density of the light beam can be set to optimum values based on the image density of the patch image that is actually formed, so that the desired image density can be stably obtained. It becomes.

  FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus forms a full color image by superposing four color toners of yellow (Y), magenta (M), cyan (C), and black (K), or uses only black (K) toner. This is an apparatus for forming a monochrome image. In this image forming apparatus, when an image signal is given to the main controller 11 of the control unit 1 from an external device such as a host computer, the engine controller 12 controls each part of the engine unit EG according to a command from the main controller 11. Thus, an image corresponding to the image signal is formed on the sheet S.

  In the engine unit EG, the photosensitive member 2 is provided so as to be rotatable in an arrow direction D1 in FIG. A charging unit 3 for charging the surface of the photosensitive member 2 to a predetermined surface potential, a rotary developing unit 4 as a developing unit, and a cleaning unit 5 are arranged around the photosensitive member 2 along the rotational direction D1. Has been. The charging unit 3 is applied with a charging bias from the charging bias generator 121 and uniformly charges the outer peripheral surface of the photoreceptor 2 to the surface potential Vo. For this reason, in this apparatus, the surface potential Vo of the photosensitive member 2 can be controlled by adjusting the charging bias.

  Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 2 charged by the charging unit 3. As shown in FIG. 2, the exposure unit 6 is electrically connected to an image signal switching unit 122, and the exposure power control unit 123 performs exposure in accordance with an image signal given through the image signal switching unit 122. The unit 6 is controlled, and the light beam L is exposed on the photosensitive member 2 to form an electrostatic latent image corresponding to the image signal on the photosensitive member 2. For example, when the image signal switching unit 122 is electrically connected to the patch creation module 125 based on a command from the CPU 124 of the engine controller 12, the patch image signal output from the patch creation module 125 is sent to the exposure power control unit 123. Given, a patch latent image is formed. On the other hand, when the image signal switching unit 122 is electrically connected to the CPU 111 of the main controller 11, the light beam L is applied to the photoconductor 2 in accordance with an image signal given through an interface 112 from an external device such as a host computer. And an electrostatic latent image corresponding to the image signal is formed on the photoreceptor 2. Thus, in this embodiment, the photoconductor 2 functions as an electrostatic latent image carrier.

  In this image forming apparatus, a portion of the surface of the photosensitive member 2 charged to a uniform surface potential Vo is irradiated with the light beam L from the exposure unit 6 on the portion corresponding to the image portion to neutralize a part of the charge of that portion. Thus, an electrostatic latent image is formed. Therefore, the potential of the portion where the potential has been changed by irradiation with the light beam L, that is, the irradiation region potential is applied to the image portion potential Von, and the potential of the portion that has not been irradiated with the light beam L, that is, the charging unit 3. The potential after the initial surface potential Vo of the photoreceptor 2 is attenuated by dark decay corresponds to the non-image portion potential Vd.

  The electrostatic latent image thus formed is developed with toner by the developing unit 4. That is, in this embodiment, as the developing unit 4, a black developing device 4K, a cyan developing device 4C, a magenta developing device 4M, and a yellow developing device 4Y are rotatably provided around the axis. . The developing devices 4K, 4C, 4M, and 4Y are rotationally positioned and selectively contacted and positioned with respect to the photosensitive member 2, and a developing bias is applied by the developing bias generator 126 to select the selected color. The toner is applied to the surface of the photoreceptor 2. As a result, the electrostatic latent image on the photoreceptor 2 is visualized with the selected toner color.

  The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. Further, in the vicinity of the primary transfer region TR1, a patch sensor PS is arranged as the “density detecting means” of the present invention so as to face the surface of the intermediate transfer belt 71. The optical density of the patch image formed on the outer peripheral surface is measured. Further, a cleaning unit 5 is disposed at a position advanced in the circumferential direction (rotation direction D1 in FIG. 1) from the primary transfer region TR1, and the toner remaining on the outer peripheral surface of the photoreceptor 2 after the primary transfer. Scrap off. Further, the surface potential of the photosensitive member 2 is reset by a neutralizing unit (not shown) as necessary.

  The transfer unit 7 includes an intermediate transfer belt 71 spanned by a plurality of rollers, and a drive unit (not shown) that rotationally drives the intermediate transfer belt 71. When a color image is transferred to the sheet S, a color image is formed by superimposing the toner images of the respective colors formed on the photoreceptor 2 on the intermediate transfer belt 71, and a predetermined secondary transfer region TR2. 2, the color image is secondarily transferred onto the sheet S taken out from the cassette 8. Further, the sheet S on which the color image is formed in this way is conveyed via the fixing unit 9 to a discharge tray portion provided on the upper surface portion of the apparatus main body.

  After the secondary transfer, the toner remaining on the intermediate transfer belt 71 is removed from the intermediate transfer belt 71 by a cleaning unit (not shown).

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112, and reference numeral 127 denotes an arithmetic program executed by the CPU 124. A memory (storage means) for storing calculation results in the CPU 124, control data for controlling the engine unit EG, and the like.

  Next, an image density control factor optimization process executed in the image forming apparatus configured as described above will be described. In this image forming apparatus, exposure energy and development bias are used as image density control factors that affect the image density of the toner image by executing the optimization processing shown in FIG. 3 at an appropriate timing, for example, when the apparatus power is turned on. Is adjusted to adjust the image density of each color toner image.

  FIG. 3 is a flowchart showing an optimization process of an image density control factor in the image forming apparatus of FIG. In this optimization process, for each toner color, a patch image is formed while changing and setting the image density control factor in multiple stages, and the development bias potential Vb and the exposure energy E are optimized based on the image density of the patch image. ing. First, one of the toner colors, for example, black (K) is selected and set as a patch creation color (step S1), and the developing unit 4K is positioned at a developing position facing the photosensitive member 2. Next, development bias optimization processing (step S2) and exposure energy optimization processing (step S3), which will be described later, are sequentially performed to obtain the optimum values of the development bias potential Vb and the exposure energy E for the black color. When the optimum values of the developing bias potential Vb and the exposure energy E are obtained for one toner color in this way, the other developing units 4C, 4M, and 4Y are successively positioned at the developing position, and the developing bias potential Vb is similarly set for each toner color. Then, the optimum value of the exposure energy E is obtained, and if the optimum value is obtained for all the toner colors (step S4), the optimization process is terminated.

  In the image density control factor optimization process in this image forming apparatus, first, a high density image, that is, an image with a high dot area ratio relative to the entire patch image, for example, a solid image or an image with an area ratio of 80% or more is patched. After forming as an image and obtaining the optimum value of the developing bias potential Vb based on the image density, a low density patch image is formed and the optimum value of the exposure energy E is obtained based on the image density. The reason is as follows.

  That is, the image portion potential Von of the photoreceptor 2 depends on the exposure energy E. First, when the entire surface of the photosensitive member 2 or a relatively wide range is irradiated with the light beam L, the exposure energy E is a potential saturation region (exposure) of the light attenuation characteristic curve of the photosensitive member 2, so-called PIDC (Photo Induced Discharge Curve). If the change rate of the surface potential of the photosensitive member 2 with respect to the change of the energy E is large enough to enter the region), the image portion potential Von becomes substantially constant even if the exposure energy E is changed. Therefore, when image formation is performed with the same developing bias potential Vb, the contrast potential is substantially constant, so that the image density is also substantially constant. On the other hand, when the area to be exposed is small and spot-like, such as a low-density patch image, even if the range in which the exposure energy E is changed is within the above-described potential saturation area of the PIDC, the exposure energy E is As it increases, the width of the latent image corresponding to the image portion increases. Therefore, even when image formation is performed under the same development bias condition, the image density changes depending on the exposure energy E.

  Therefore, in this image forming apparatus, an optimum value of the developing bias Vb is first obtained using a high density patch image in which the difference in density due to the magnitude of the exposure energy E is relatively small, and then the exposure energy E is obtained using the low density patch image. The optimum value is obtained. In addition, as described later, since the low density patch image is formed in a state where the potential difference between the developing bias potential Vb and the non-image portion potential Vd, that is, the reverse contrast potential Vr is held at an appropriate value, such a low density is formed. An image can be created with high reproducibility, so that the exposure energy E can be optimized with higher accuracy.

  As described above, in this embodiment, the image portion potential Von can be changed depending on the magnitude of the exposure energy E, but the variable range is that when a relatively wide range of the surface of the photoreceptor 2 is irradiated. It is desirable to set the fluctuation of the image portion potential Von to be 30V or less, more preferably 20V or less. By doing so, the variation of the contrast potential accompanying the change of the exposure energy E can be suppressed to 30 V or less, or 20 V or less, and the variation of the image density can be minimized.

  Next, the development bias optimization process shown in step S2 of FIG. 3 will be described in detail with reference to FIG. FIG. 4 is a flowchart showing development bias optimization processing in the image forming apparatus of FIG. In this development bias optimization processing, the exposure energy E is fixed at, for example, the maximum value, and the development bias potential Vb and the surface potential of the photosensitive member 2, that is, the non-image portion potential Vd, are maintained in multiple stages while keeping the potential difference constant. The patch image is formed by changing the setting.

  First, the developing bias potential Vb is set to the minimum value in the variable range (step S21). At this time, as the development bias potential Vb is changed by the development bias generator 126, the charging unit 3 controls the surface potential of the photoreceptor 2 so that the reverse contrast potential Vr is always maintained at a constant value. More specifically, as described above, the non-image portion potential Vd of the photosensitive member 2 is a value obtained by darkening the initial surface potential Vo immediately after charging during the rotational movement of the photosensitive member 2 to the developing position. However, since the relationship between the magnitude of the charging bias and the surface potential Vo of the photoreceptor 2 and the attenuation amount due to dark decay are known, the desired non-image portion potential Vd is obtained by controlling the charging bias of the charging unit 3. Thus, the reverse contrast potential Vr can always be kept constant.

  Under this condition, a high-density patch image, for example, a toner image corresponding to a solid image is formed (step S22). Next, the development bias potential Vb and the non-image portion potential Vd are increased by one step to form a patch image again, and this is repeated until the development bias potential Vb reaches the maximum value in the variable range (step S23). , S24). For the patch image at each developing bias formed in this way and transferred to the intermediate transfer belt 71, the image density of each patch image is measured by the patch sensor PS (step S25). Then, a developing bias whose image density is closest to a preset target density, for example, optical density OD = 1.2 is obtained, and the value is set as the optimum developing bias (step S26).

  After obtaining the optimum developing bias in this way, the exposure energy optimizing process (step S3 shown in FIG. 3) shown in FIG. 5 is subsequently performed. FIG. 5 is a flowchart showing exposure energy optimization processing in the image forming apparatus of FIG. In this exposure energy optimization process, the optimum value obtained previously is used as the developing bias potential Vb, and the non-image portion potential Vd is set so that the reverse contrast potential Vr is constant (step S31). Then, while increasing the exposure energy E by one step from the minimum value of the variable range, a patch image is formed with each exposure energy (steps S32 to S35).

  As the patch image used at this time, for example, a 1-on-5-off 1-dot line image LI shown in FIG. 6 can be used. This is because, as described above, in an image having a small exposed area and a spot shape, the magnitude of the exposure energy E has a great influence on the image quality, and in particular, isolation without mutual interference between adjacent lines as shown in FIG. Since the difference appears clearly in the dot line image LI, the optimum value of the exposure energy E can be accurately obtained by using such an image as a patch image. A preferable low-density patch image is an image having a dot area ratio of 20% or less, more preferably 16.7% or less, relative to the entire image.

  As described above, the value of the reverse contrast potential Vr has a great influence on the image quality of such a thin line image. Therefore, in a conventional image forming apparatus that does not consider the reverse contrast potential Vr, it is difficult to accurately adjust the image density in the low density image by controlling the exposure energy E. On the other hand, in this embodiment, the line image LI is formed while maintaining the above-described reverse contrast potential Vr at an appropriate value, and a higher value is obtained by obtaining the optimum value of the exposure energy E based on the image density. It is possible to optimize the exposure energy E with accuracy.

  After forming a patch image at each exposure energy in this way, the image density of each patch image is measured by the patch sensor PS (step S36), and the image density is a preset target density, for example, optical density OD = 0.35. The exposure energy E that is closest to is obtained, and the value is set as the optimum exposure energy (step S37).

  The optimum development bias value and optimum exposure energy value for each toner color thus obtained are stored in the memory 127, and these values are used as needed when the toner image is formed with each toner color in the subsequent image forming process. Reading, developing bias potential Vb and exposure energy E are set.

  The patch images thus formed on the intermediate transfer belt 71 are removed from the surface of the intermediate transfer belt 71 by a cleaning unit (not shown) after the image density is measured by the patch sensor PS.

  As described above, in the image density control factor optimization process of this embodiment, the patch image is formed while the inverse contrast potential Vr is always kept constant, and various image density control factors such as the developing bias potential Vb are used. Even when changed to the above, it is possible to achieve both suppression of toner scattering and ensuring of image quality. Therefore, even when a high density patch image with a large amount of toner transfer is formed, toner scattering is reduced, and since the low density patch image is formed in a state where fine dots can be satisfactorily formed, this patch image is formed. The accuracy of the exposure energy optimization process performed using can be improved.

  Since the image forming is performed while using the development bias potential Vb and the exposure energy E optimized in this way and the reverse contrast potential Vr is kept constant, this image forming apparatus uses fine lines and minute lines. It is possible to stably form a toner image with excellent image quality without causing problems such as blurring or non-uniform line width even in an image composed of dots, and at the same time, it is effective for toner scattering inside the device. Is suppressed.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the development bias potential Vb is optimized using a high-density patch image, and then the exposure energy E is optimized using a low-density patch image. In order to increase the density, the low-density patch image may be formed after the exposure energy optimization process and fine adjustment of the developing bias potential Vb may be performed again. In order to simplify the apparatus configuration and reduce the cost, the exposure energy E may be a fixed value and only the development bias potential Vb may be used as an image density control factor.

  Even in these cases, a low-density patch image is formed while keeping the reverse contrast potential Vr constant, and optimization processing is performed based on the image density, thereby effectively suppressing toner scattering inside the apparatus. A toner image with excellent image quality can be stably formed.

  In the above-described embodiment, the 1-on-5-off 1-dot line image LI is used as the low-density patch image. However, the low-density patch image is not limited to this, and various other half-tone images can be used. A tone image or a line image can be used. For example, an image made up of a plurality of isolated dots spaced apart from each other may be used, or a line image having an off-number different from the above may be used. However, if extremely low density images are used, there may be a problem with the measurement sensitivity of the patch sensor PS. If an image with a higher density than necessary is used, the image quality at low density can be checked appropriately. Therefore, it is necessary to appropriately set the patch image to be formed according to the specifications of the apparatus and the characteristics of the toner.

  In the optimization process of the image density control factor in the above-described embodiment, the development bias potential Vb or the exposure energy E as the image density control factor is increased step by step from the minimum value to form a patch image, The value when the image density is closest to the target density is the optimum value. However, the present invention is not limited to this. For example, patch image formation is performed while decreasing these image density control factors step by step from the maximum value. Alternatively, for example, the optimum value of the image density control factor may be calculated based on the change in the image density for each patch image. Further, the development bias optimization processing of this embodiment is performed with the exposure energy E fixed at the maximum value, but other than this, for example, the median value of the exposure energy E may be used.

  Further, the above-described embodiment is an image forming apparatus using a contact developing method in which development is performed in a state where the developing devices 4Y, 4M, 4C, and 4K are in contact with the photosensitive member 2. The present invention can also be applied to an image forming apparatus using a non-contact development method such as jumping development, which is spaced apart from the body. In such a non-contact development method, the toner is more easily scattered than the above-described contact development method device because the development is performed by flying the toner between the developing unit and the photosensitive member at the development position. ing. Therefore, the effect of preventing toner scattering by applying the present invention to such an apparatus becomes more remarkable.

  In the image forming apparatus of the jumping development system, as shown in FIG. 7, a developing bias applied to each developing device is one in which an alternating current component is superimposed on a direct current component in order to improve the flying property of the toner and increase the developing efficiency ( It is preferable to use an alternating voltage (FIG. (B)) in which the positive and negative duty ratios are changed. When such a development bias is used, the average value of the development bias (“Vmean” shown in each figure) may be considered as the “development bias potential” described above.

  The above-described embodiment is an image forming apparatus that forms a full-color image using four color toners of yellow (Y), magenta (M), cyan (C), and black (K). The present invention can also be applied to an apparatus that forms a monochrome image using only black toner. Since such a monochrome image forming apparatus is considered to be frequently used mainly for forming an image composed of fine lines such as characters, the present invention is applied to such an apparatus to improve the image quality of the fine lines. It is extremely effective.

  In the above embodiment, the toner image formed on the photosensitive member 2 is transferred to the intermediate transfer belt 71. However, a transfer medium other than the intermediate transfer belt (transfer drum, transfer belt, transfer sheet, intermediate transfer drum, intermediate transfer belt) The present invention can also be applied to an image forming apparatus that forms an image by transferring a toner image onto a transfer sheet, a reflection type or a transmission type recording sheet. Further, a patch sensor may be provided at a position where the image density of the toner image formed on the photoconductor can be detected, and the image density of the patch image on the photoconductor may be measured by this patch sensor.

  In the above-described embodiment, the image forming apparatus includes the rotary developing unit 4 in which the developing devices 4Y, 4M, 4C, and 4K corresponding to the respective toner colors are arranged at the center of the axis. The present invention can also be applied to a so-called tandem type image forming apparatus in which the developing units of the respective toner colors are arranged in a line along the transfer medium conveyance direction.

  The above-described embodiment is an image forming apparatus that uses a negatively charged toner and forms an electrostatic latent image on the photoreceptor 2 by irradiating the light beam L to a position corresponding to the image portion. However, the present invention is not limited to this, and the present invention can also be applied to an apparatus using positively charged toner and an apparatus that forms an electrostatic latent image by irradiating light to a non-image portion. .

  As described above, according to the present invention, since the image formation is performed with the potential difference between the developing bias potential and the non-image portion potential, that is, the reverse contrast potential kept constant, the reverse contrast potential is always set to an appropriate value. Therefore, it is possible to form a toner image with good image quality while effectively suppressing toner scattering inside the apparatus. In addition, when forming a patch image while changing the development bias potential or the energy density of the light beam, the reverse contrast potential is maintained at the same constant value as when a normal toner image is formed. The accuracy of optimization of the developing bias potential and the energy density of the light beam can be improved.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus in FIG. 1. 3 is a flowchart illustrating an image density control factor optimization process in the image forming apparatus of FIG. 1. 2 is a flowchart showing a developing bias optimization process in the image forming apparatus of FIG. 3 is a flowchart showing exposure energy optimization processing in the image forming apparatus of FIG. 1. It is a figure which shows a line patch image. It is a figure which shows the example of the developing bias waveform in a jumping development system. It is a principle diagram of a laser beam printer.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Control unit (control means), 2 ... Photoconductor, 3 ... Charging unit, 4 ... Development unit (development means), 6 ... Exposure unit (exposure means), 71 ... Intermediate transfer belt (transfer medium), LI ... Line Patch image, PS ... Patch sensor (density detection means), Vb ... Development bias potential, Vd ... Non-image portion potential, Von ... Image portion potential, Vr ... Inverse contrast potential

Claims (10)

A photoreceptor charged to a predetermined surface potential;
By irradiating the photosensitive member surface with a light beam, the image portion potential of the surface region corresponding to the image portion of the photosensitive member surface is different from the non-image portion potential of the surface region corresponding to the non-image portion. Exposure means for forming an electrostatic latent image on the surface by
Developing means for developing the electrostatic latent image with toner to form a toner image in accordance with a given developing bias potential;
Control means for controlling the potential difference between the developing bias potential and the non-image portion potential to be constant when forming the toner image;
The control means includes
While maintaining the potential difference between the developing bias potential and the non-image portion potential at the same value as when the toner image is formed, the developing bias potential is changed and set in multiple stages, and the density of the patch image formed at each set value is set. Optimize the development bias potential as an image density control factor that affects the image density based on
The developing bias potential is set to an optimum value, and the non-image portion potential is set so that the potential difference between the optimum value of the developing bias potential and the non-image portion potential becomes the same value as when the toner image is formed. Then, the energy density of the light beam is changed and set in multiple stages, and based on the density of an isolated dot line image in which the area ratio of dots with respect to the entire patch image as a patch image formed with each set value is 20% or less An image forming apparatus characterized by optimizing the energy density of the light beam as an image density control factor that affects image density. The developing bias potential patch image formed in optimizing the image forming apparatus according to 請 Motomeko 1 area ratio of the dot Ru image der of 80% or more to the whole patch image. A charging unit that applies a predetermined charging bias to charge the surface of the photoreceptor to a predetermined initial surface potential;
The control means uses the surface potential of the photoconductor after attenuated from the initial surface potential by dark decay as the non-image portion potential, so that the potential difference between the non-image portion potential and the developing bias is constant. The image forming apparatus according to claim 1, wherein the charging bias is controlled. A density detecting means for detecting a predetermined toner image formed on the photosensitive member by the developing means or a toner image formed by transferring the toner image onto a transfer medium as a patch image;
4. The image forming apparatus according to claim 1, wherein the control unit optimizes the image density control factor based on a detection result of the density detection unit.   The control means obtains the image density of each patch image by the density detection means, and sets the development bias potential by setting the development bias potential closest to the preset target image density to the optimum value. The image forming apparatus according to claim 4 to be optimized.   The control means obtains the image density of each patch image by the density detection means, and sets the energy density of the light beam that is closest to the preset target image density as the optimum value, thereby making the light beam The image forming apparatus according to claim 4, wherein the energy density of the image forming apparatus is optimized.   The developing means includes a plurality of developing units corresponding to different toner colors, and the plurality of developing units are sequentially positioned at a predetermined developing position to form a toner image in the toner color. The image forming apparatus according to claim 1, wherein the image forming apparatus forms an image.   The combination of the developing bias potential and the image portion potential can be changed within a predetermined range, and the maximum value and the minimum value of the potential difference between the developing bias potential and the image portion potential within the range are The image forming apparatus according to claim 1, wherein the difference is 30 V or less.   The combination of the developing bias potential and the image portion potential can be changed within a predetermined range, and the maximum value and the minimum value of the potential difference between the developing bias potential and the image portion potential within the range are The image forming apparatus according to claim 1, wherein the difference is 20 V or less. The photosensitive member surface is exposed by a light beam, and the image portion potential of the surface region corresponding to the image portion of the photosensitive member surface is made different from the non-image portion potential of the surface region corresponding to the non-image portion. While forming an electrostatic latent image on the surface and controlling the developing bias potential applied to the developing means so that the potential difference from the non-image portion potential is constant, the developing means develops the electrostatic latent image with toner. In an image forming method for forming a toner image according to an image signal by forming an image,
While maintaining the potential difference between the developing bias potential and the non-image portion potential at the same constant value as when forming the toner image, these are changed and set in multiple stages to form a predetermined patch image each time. Optimizing the development bias potential based on the image density measurement results;
The developing bias potential is set to an optimum value, and the non-image portion potential is set so that a potential difference between the optimum value of the developing bias potential and the non-image portion potential is the same constant value as when the toner image is formed. By setting the energy density of the light beam in multiple stages , an isolated dot line image having a dot area ratio of 20% or less with respect to the entire patch image as a predetermined patch image is formed each time. A step of optimizing the energy density of the light beam based on a density measurement result, and setting the developing bias potential and the energy density of the light beam to optimized values to form the toner image. An image forming method.

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