JP2019128409A - Image forming apparatus, image forming method, and image forming program - Google Patents

Abstract

To improve image quality by using an organic relationship between a method for halftone processing and an edge effect.SOLUTION: An image forming apparatus comprises: a screen selection unit; a halftone processing unit that executes halftone processing by using a halftone processing method selected by the screen selection unit; developing units that each form, on a developing roller, a toner layer having a thickness according to a toner layer forming potential difference between a magnetic roller and the developing roller, and attach a toner from the toner layer to a photoreceptor on the basis of a developing bias potential that is the potential of the developing roller and an electrostatic latent image; and a calibration processing unit that forms a half patch by using a line screen and adjusts the developing bias potential of the developing unit within an adjustment range set in advance to execute first calibration processing, and when the developing bias potential reaches a potential limiting value within the adjustment range in the first calibration processing, forms a half patch by using a dot screen to execute the first calibration processing.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus, an image forming method, and an image forming program.

  In the technical field of image formation, various techniques for suppressing edge effects have been proposed. The edge effect is a phenomenon in which toner as a developer is concentrated on the edge portion (end portion) of an image area and the density of the image becomes uneven. Specifically, for example, in Patent Document 1, the edge region is detected according to the distance from the boundary of the edge region detected by the edge detection unit and the toner adhesion amount obtained from the image pattern formed at a predetermined timing. A technique has been proposed for controlling the writing exposure amount or the width of a correction value for image data. When adjusting the exposure amount of the edge part, this technology has a large influence on the change in the thickness of characters and lines with respect to the change in the exposure amount in pixels close to the edge boundary, while the exposure amount in pixels far from the edge boundary. This technology was created focusing on the fact that the change in the thickness of the characters and lines is small.

JP, 2015-219407, A

  However, conventionally, the organic relationship between the halftoning method and the edge effect has not been sufficiently studied.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a technique for improving image quality by utilizing an organic relationship between a halftone processing method and an edge effect.

  The image forming apparatus of the present invention includes a screen selection unit that selects any one of a plurality of halftone processing methods including a halftone process that uses a line screen and a halftone process that uses a dot screen, and the screen selection unit. A halftone processing unit that performs halftone processing using the selected halftone processing method, a rotatable photosensitive member, and an exposure that exposes the photosensitive member based on image data to form an electrostatic latent image A toner layer having a thickness corresponding to a toner layer forming potential difference between the magnetic roller and the developing roller is formed on the developing roller. A developing unit for causing toner to adhere from the toner layer to the photosensitive member based on the developing bias potential and the electrostatic latent image; A half patch is formed by using the image forming device, and the developing bias potential of the developing unit is adjusted within a preset adjustment range to execute a first calibration process, and within the adjustment range in the first calibration process. When the potential limit value is reached, a half patch is formed using the dot screen, the first calibration process is executed, and the first calibration process using the dot screen is within the adjustment range. A calibration processing unit that sets a dot area ratio for forming a solid image and executes the second calibration process.

  The image forming method of the present invention includes a screen selection step for selecting one of a plurality of halftone processing methods including halftone processing using a line screen and halftone processing using a dot screen, and the screen selection step. A halftone processing step for performing halftone processing using the selected halftone processing method, a rotatable photoconductor, and exposure for exposing the photoconductor to form an electrostatic latent image based on image data; A toner layer having a thickness corresponding to a toner layer forming potential difference between the magnetic roller and the developing roller is formed on the developing roller using a potential of the developing roller. A developing step of causing toner to adhere to the photosensitive member from the toner layer based on a certain developing bias potential and the electrostatic latent image; A half patch is formed using a screen, and a first biasing process is performed by adjusting a developing bias potential in the developing process within a preset adjustment range. In the first calibration process, the first calibration process is performed. When the potential limit value is reached, a half patch is formed using the dot screen, the first calibration process is executed, and the first calibration process using the dot screen is within the adjustment range. And a calibration processing step of setting the dot area ratio for forming a solid image and executing the second calibration processing.

  The present invention provides an image forming program for controlling an image forming apparatus that forms an image on an image forming medium according to image data. The image forming apparatus includes a rotatable photosensitive member, an exposure unit that exposes the photosensitive member based on image data to form an electrostatic latent image, a magnetic roller, and a developing roller, and the magnetic roller A toner layer having a thickness corresponding to a potential difference in toner layer formation between the toner and the developing roller is formed on the developing roller, and the toner is developed based on the developing bias potential that is the potential of the developing roller and the electrostatic latent image. A developing unit that attaches toner to the photoconductor from a layer, and the image forming program is one of a plurality of halftone processing methods including a halftone process using a line screen and a halftone process using a dot screen. A screen selection unit for selecting a halftoning method, and halftone processing for performing halftone processing using the halftone processing method selected by the screen selection unit. A half patch is formed using the line screen and the line screen, the developing bias potential of the developing unit is adjusted within a preset adjustment range, and a first calibration process is performed, and the first calibration process is performed. When the potential limit value in the adjustment range is reached, a half patch is formed using the dot screen, the first calibration processing is executed, and the first calibration processing using the dot screen is performed. When the potential limit value in the adjustment range is reached, the dot area ratio for forming a solid image is set and the image forming apparatus is caused to function as a calibration processing unit for executing the second calibration processing.

  According to the present invention, it is possible to improve the image quality by utilizing the organic relationship between the method of halftone processing and the edge effect.

2 is a block diagram illustrating a functional configuration of the image forming apparatus 1 according to an embodiment of the present disclosure. 1 is a cross-sectional view illustrating an overall configuration of an image forming apparatus 1 according to an embodiment. FIG. 2 is a side cross-sectional view showing the structure of the developing unit 100 according to an embodiment of the present invention. It is a conceptual diagram which shows a mode that a back end accumulation generate | occur | produces in the image development process which concerns on one Embodiment. 4 is a flowchart showing the contents of a half patch calibration processing procedure of the image forming apparatus 1 according to an embodiment. 5 is a flowchart showing the contents of development bias adjustment processing and dot area ratio adjustment processing according to an embodiment. FIG. 2 is an explanatory diagram showing an outline of a line screen and a dot screen that can be used by the image forming apparatus 1 according to an embodiment. 5 is a flowchart showing the contents of an image forming processing procedure of the image forming apparatus 1 according to an embodiment.

  Hereinafter, modes for carrying out the present invention (hereinafter, referred to as “embodiments”) will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a functional configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 includes a control unit 10, an image forming unit 20, a storage unit 40, an image reading unit 50, and a fixing unit 80. The image reading unit 50 reads an image from a document and generates an image data ID that is digital data.

  The image forming unit 20 includes a color conversion processing unit 21, a halftone processing unit 22, a density sensor 28 for calibration, an exposure unit 29, and photosensitive drums (image carriers) 30c to 30k, which are amorphous silicon photosensitive members. , And developing units 100c to 100k and charging units 25c to 25k. The color conversion processing unit 21 converts the color space of the image data ID, which is RGB data, into CMYK data in a color space that can be reproduced by the developing units 100c to 100k. The halftone processing unit 22 performs halftone processing on the CMYK data to generate print data PD as CMYK halftone data. The halftone data represents the formation state of dots formed by each toner of CMYK, and is also called dot data.

  The control unit 10 includes main storage means such as RAM and ROM, and control means such as MPU (Micro Processing Unit) and CPU (Central Processing Unit). The control unit 10 also has a controller function related to interfaces such as various I / O, USB (Universal Serial Bus), bus, and other hardware, and controls the entire image forming apparatus 1. The control unit 10 includes a calibration processing unit 11 and a screen selection unit 12.

  The storage unit 40 is a storage device including a hard disk drive or a flash memory that is a non-temporary recording medium, and stores a control program and data for processing executed by the control unit 10. In the present embodiment, the storage unit 40 further stores calibration data CD1 and screen data CD2. The calibration data CD1 can be configured, for example, as a LUT (Look Up Table). The screen data CD2 is data for using a screen that can be used by the halftone processing unit 22. Details of the calibration data CD1 and the screen data CD2 will be described later.

  FIG. 2 is a cross-sectional view showing the overall configuration of the image forming apparatus 1 according to an embodiment. The image forming apparatus 1 of the present embodiment is a tandem type color printer. In the image forming apparatus 1, photosensitive drums (image carriers) 30 m, 30 c, 30 y and 30 k are arranged in a line in a casing 70 corresponding to each color of magenta, cyan, yellow and black. Developing units 100m, 100c, 100y and 100k are disposed adjacent to the photosensitive drums 30m, 30c, 30y and 30k, respectively.

  The laser beams Lm, Lc, Ly and Lk for respective colors are irradiated (exposed) from the exposure unit 29 to the photosensitive drums 30m, 30c, 30y and 30k. By this irradiation, electrostatic latent images are formed on the photosensitive drums 30m, 30c, 30y and 30k. The developing units 100m, 100c, 100y and 100k cause the toner to adhere to the electrostatic latent images formed on the surfaces of the photosensitive drums 30m, 30c, 30y and 30k while stirring the toner. Thus, the developing process is completed, and toner images of respective colors are formed on the surfaces of the photosensitive drums 30c to 30k.

  The image forming apparatus 1 has an endless intermediate transfer belt 27. The intermediate transfer belt 27 is stretched around a tension roller 24, a drive roller 26a and a driven roller 26b. The intermediate transfer belt 27 is driven to circulate by the rotation of the driving roller 26a.

  For example, a black toner image on the photosensitive drum 30k is primarily transferred to the intermediate transfer belt 27 by sandwiching the intermediate transfer belt 27 between the photosensitive drum 30k and the primary transfer roller 23k, and the intermediate transfer belt 27 being driven to circulate. Ru. The same applies to the three colors of cyan, yellow and magenta.

  A full-color toner image is formed on the surface of the intermediate transfer belt 27 by performing primary transfer so as to be superimposed on each other at a predetermined timing. The calibration density sensor 28 is disposed at a position where the primary transfer is completed and the density of the toner image before the secondary transfer can be measured. Thereafter, the full-color toner image is secondarily transferred to the printing paper P supplied from the paper feed cassette 60 and fixed on the printing paper P as a printing medium by the fixing roller pair 81 of the fixing unit 80.

  FIG. 3 is a side sectional view showing the structure of the developing unit 100 according to an embodiment of the present invention. The developing units 100m, 100c, 100y and 100k have the same configuration, and these are also simply referred to as the developing unit 100. The developing unit 100 includes two agitating and conveying members 141 and 142, a magnetic roller 143, a developing roller (developer carrying member) 144, a developing container 145, and a regulating blade 146.

  The developing container 145 constitutes the outline of the developing unit 100. At the lower part of the developing container 145, a partition part 145b is provided. The partitioning portion 145 b partitions the inside of the developing container 145 into a first transfer chamber 145 a and a second transfer chamber 145 c. The first transfer chamber 145a and the second transfer chamber 145c extend in a column shape in a direction perpendicular to FIG. 3, and contain a two-component developer (also simply referred to as a developer) made of a magnetic carrier and black toner.

  The developing container 145 further holds the magnetic roller 143 and the developing roller 144. The developing container 145 is formed with an opening 147 for exposing the developing roller 144 toward the photosensitive drum 30 (30 k).

  The two agitating and conveying members 141 and 142 are cyclically moved while stirring the developer inside the first conveying chamber 145a and the second conveying chamber 145c, respectively. The stirring and conveying member 142 supplies a positively charged developer to the magnetic roller 143 as a magnetic brush. The magnetic roller 143 includes a nonmagnetic rotating sleeve 143a and a fixed magnet body 143b fixed inside the rotating sleeve 143a. The magnetic roller 143 and the developing roller 144 face each other with a predetermined clearance. The regulation blade 146 adjusts the magnetic brush to a predetermined height.

  The developing roller 144 includes a rotatable nonmagnetic developing sleeve 144a and a developing roller side magnetic pole 144b fixed inside the developing sleeve 144a. A magnetic roller potential Vmag is applied to the magnetic roller 143. A developing bias potential Vslv is applied to the developing roller 144.

  In the present embodiment, in the photosensitive drum 30, the surface potential is set to 20 V, and a developing electric field is formed between the photosensitive drum 30 and the developing roller 144. On the other hand, an alternating bias in which a DC potential of 20 to 80 V as the developing bias potential Vslv and a sine wave potential of a peak-to-peak value of 2000 V at a frequency of 2 kHz are superimposed is applied to the developing roller 144. A DC potential of 200 V is applied to the magnetic roller 143 as a magnetic roller potential Vmag during development, and a DC potential of −200 V is applied during non-development.

  As a result, during development, the time of developing bias potential Vslv <magnetic roller potential Vmag (the potential state in which toner is supplied to developing roller 144) is lengthened, and the time for supplying toner to developing roller 144 is lengthened. At the time of non-development, the time of developing bias potential Vslv> magnetic roller potential Vmag (a potential state in which toner is collected from the developing roller 144) becomes longer, and the time for collecting toner from the developing roller 144 becomes longer.

  Furthermore, by adjusting the magnetic roller potential Vmag applied to the magnetic roller 143 during development and non-development, the toner layer formation potential difference ΔV during development between the development bias potential Vslv and the magnetic roller potential Vmag is changed. Can. Accordingly, a thin toner layer (simply a toner) having a thickness D (see FIG. 4A described later) corresponding to the toner layer forming potential difference ΔV between the developing bias potential Vslv and the magnetic roller potential Vmag is applied to the developing roller 144. Also called a layer) is formed.

  The developing roller 144 attaches toner to the photosensitive drum 30 through a facing portion (developing nip) having a predetermined clearance with the photosensitive drum 30 to form a toner image on the surface of the photosensitive drum 30. . The toner image is formed based on the potential difference between the electrostatic latent image potential on the surface of the photosensitive drum 30 and the developing bias potential Vslv applied to the developing roller 144.

  Amorphous silicon photoconductors have a characteristic that the relative permittivity is about three times higher than that of organic photoconductors (OPC), and the amount of toner that can be held by the photoconductor is larger than the development contrast potential. For this reason, the amorphous silicon photosensitive member can hold more toner than the solid density which is usually used. Therefore, when used in a saturated state, the amorphous silicon photosensitive member is held in excess of the amount required for the solid density. Therefore, in the present embodiment, the amorphous silicon photosensitive member is used in the non-saturated state even in the solid density, and the toner formed on the developing roller 144 is almost entirely developed on the photosensitive member, and the development is completed. Is used to be determined.

  FIG. 4 is a conceptual diagram illustrating how the trailing edge accumulation occurs in the developing process according to the embodiment. FIG. 4A shows a state in which an image is formed at the front end portion and the center portion of the image. FIG. 4B shows that the image is formed at the rear end of the image. In the present specification, the front end portion, the central portion, and the rear end portion are defined as the front end portion, the central portion, and the rear end portion in order from the traveling direction with reference to the traveling direction of the photosensitive drum 30.

  In this embodiment, as shown in FIG. 4A, the photosensitive drum 30 is supplied with toner from the developing sleeve 144a of the developing roller 144 while neutralizing the potential of the latent image. At this time, the developing process is configured to be completed when the toner thin layer formed on the developing sleeve 144a is consumed in the non-saturated state, not in the saturation of the potential. The thickness D of the toner thin layer is set to have a thickness T1 for achieving the maximum density at the time of solid development in image formation.

  As shown in FIG. 4B, the developing sleeve 144a has a peripheral speed Vs and is configured to form an image while overtaking the photosensitive drum 30 at the peripheral speed Vd. For this reason, the surface of the developing sleeve 144a in which the toner is not consumed exists near the rear end of the solid at the time of solid development. The surface on which the toner is not consumed will overtake the rear end of the solid latent image on the amorphous silicon photosensitive member 30.

  At this time, since the photosensitive drum 30 as the amorphous silicon photosensitive member is not saturated, the toner is further developed from the surface of the developing sleeve 144a where the toner is not consumed. By this development, the trailing edge stagnation (thickness T2) as a solid density higher than the density assumed in advance becomes apparent.

  FIG. 4C shows, as an example, the adhesion state (stacked state) of the toner at the time of image formation of the line image TP. In the line image TP, the toner layer is raised at the rear end portion at the time of image formation. This rise of the toner layer is called a rear end reservoir.

  FIG. 5 is a flowchart showing the contents of the half patch calibration process procedure (step S100) of the image forming apparatus 1 according to an embodiment. FIG. 6 is a flowchart showing the contents of the development bias adjustment process and the dot area ratio adjustment process according to an embodiment. FIG. 7 is an explanatory view showing an outline of a line screen and a dot screen which can be used by the image forming apparatus 1 according to an embodiment. In the present embodiment, since the amorphous silicon photosensitive members are adopted for the photosensitive drums 30c to 30k, the image forming apparatus 1 is configured to express a solid by a half patch in order to suppress the problem of the rear end accumulation. There is.

  Such a problem of rear end accumulation can be suppressed by expressing a solid with a half patch. In this example, it is assumed that the image forming apparatus 1 expresses a solid by a half patch having a dot area rate of 65% to 90%. In the present embodiment, the solid density by the half patch is calibrated by adjusting the developing bias potential Vslv, which is the applied potential of the developing units 100m, 100c, 100y, and 100k, and the dot area ratio.

  In step S110, the screen selection unit 12 of the control unit 10 executes a line screen selection process. In the line screen selection process, the screen selection unit 12 reads the screen data CD2 from the storage unit 40 and selects the line screen LS (see FIG. 7A). A line screen generally has a feature that color unevenness is less noticeable than a dot screen even when an image is reproduced by a collection of lines and color misregistration occurs. Further, the line screen has a feature that it is possible to make the rear end accumulation less visible as compared with the dot screen by sufficiently reducing the dot area ratio, that is, by increasing the thinning rate.

  In step S120, the calibration processing unit 11 of the control unit 10 executes a development bias adjustment process. In step S121, the calibration processing unit 11 forms a chart having a plurality of half patches on the intermediate transfer belt 27. The halftone processing unit 22 generates halftone data for reproducing a plurality of half patches using the line screen LS. Halftone data represents the formation state of dots formed by CMYK toners.

  The plurality of half patches are formed by stepwise changing the developing bias potential Vslv. Specifically, the calibration processing unit 11 forms on the intermediate transfer belt 27 a chart having a plurality of half patches in which the developing bias potential Vslv is different at a dot area ratio (65% in this example) as an initial value before calibration. Do. The plurality of half patches include those having the development bias potential Vslv having the maximum value.

  A plurality of half patches in which the development bias potential Vslv is changed in stages are used because the toner image has an electrostatic latent image potential on the surface of the photosensitive drum 30 and a development bias potential Vslv applied to the development roller 144. It is because it is formed based on the potential difference. A plurality of half patches are formed for each of CMYK. Hereinafter, a cyan (C) half patch will be described as an example.

  In step S122, the calibration processing unit 11 measures the density of each color (for example, cyan (C)) patch using the calibration density sensor 28. The density of the patch can be measured, for example, the amount of red reflected light which has a complementary color relationship of cyan (C). The same process is performed for MYK.

  In the present embodiment, the calibration concentration sensor 28 emits infrared light from, for example, an LED (not shown), passes through a polarizing filter that transmits only P waves, and irradiates the patches with infrared P waves. The density is detected based on the ratio of P wave and S wave of the reflected light detected by the light receiving element. The calibration density sensor 28 includes a regular reflection method for detecting regular reflection light from the patch and a diffuse reflection method for detecting diffuse reflection light from the patch.

  In step S123, the calibration processing unit 11 executes a developing bias setting process. In the developing bias setting process, the calibration processing unit 11 detects a patch which has reached a preset solid image target density among a plurality of cyan (C) half patches in which the developing bias potential Vslv is changed stepwise. Is present, it is executed by selecting the developing bias potential Vslv corresponding to the patch. Specifically, when there is a half patch in which the ratio of the P wave to the S wave is equal to or less than a preset threshold, the calibration processing unit 11 calibrates the lowest developing bias potential Vslv among the half patches. It is adjusted to the potential of the later development bias.

  In step S130, if the calibration of the half patch using the line screen LS is possible within the adjustment range of the development bias, the calibration processing unit 11 proceeds to step S180 and the half patch calibration indicates the development bias potential. If it is not possible within the adjustment range of Vslv, the process proceeds to step S140. In step S180, the calibration processing unit 11 stores the development bias potential Vslv after calibration in the storage unit 40 as part of the calibration data CD1. Calibration by adjusting the development bias potential Vslv is also referred to as first calibration processing.

  The case where half patch calibration is not possible within the adjustment range of development bias means that there is no patch which has reached a preset solid image target density among a plurality of cyan (C) patches. ing. Normally, any one of the plurality of half patches reaches the solid image target density, but the solid image target density may not be reached in a state where the toner charge amount is increased due to, for example, an environmental change. However, when half-patch calibration is not possible within the adjustment range of the developing bias, the effect of improving the trailing edge accumulation is reduced by increasing the dot area ratio as will be described later.

  In step S140, the screen selection unit 12 executes a dot screen selection process. In the dot screen selection process, the screen selection unit 12 selects a dot screen DS (see FIG. 7B). The dot screen DS generally reproduces an image with a collection of points, has better developability than the line screen LS, and can have a higher density than the line screen LS under the same development conditions as the line screen LS. have.

  That is, the dot screen DS has a feature that it can reach the solid image target density with a low developing bias as compared with the line screen LS. As described above, the dot screen DS can reach the solid image target density within the adjustment range of the developing bias even when the line screen LS does not reach the solid image target density in a state where the toner charge amount is increased due to environmental fluctuations. It has a possibility. The halftone processing unit 22 generates halftone data for reproducing a half patch using the dot screen DS.

  In step S150, the calibration processing unit 11 executes development bias adjustment processing. The development bias adjustment process in step S150 is the same process as the development bias adjustment process in step S120 except that the half patch is formed using the dot screen DS instead of the line screen LS.

  In step S160, if the calibration of the half patch is possible within the adjustment range of the development bias, the calibration processing unit 11 proceeds to step S180, and the calibration of the half patch is within the adjustment range of the development bias potential Vslv. If not possible, the process proceeds to step S170. In step S180, the calibration processing unit 11 stores the development bias potential Vslv after calibration in the storage unit 40 as part of the calibration data CD1. Calibration by adjusting the development bias potential Vslv is also referred to as a first calibration process when using the dot screen DS.

  In step S170, the calibration processing unit 11 executes dot area ratio adjustment processing. In step S171, the calibration processing unit 11 sets the developing bias potential Vslv to the maximum value and starts an operation mode in which the dot area ratio is adjusted and calibrated. The maximum value of the developing bias potential Vslv is also referred to as a potential limit value, and is set from the viewpoint of, for example, the output limit of the developing bias and an adverse effect (fogging etc.) on the image.

  In step S <b> 172, the calibration processing unit 11 forms on the intermediate transfer belt 27 a chart having a plurality of half patches in which the dot area ratio is changed in stages. In this example, the dot area rate is changed stepwise in the dot area rate ranging from 66% to 90%.

  In step S173, the calibration processing unit 11 measures the density of the cyan (C) patch using the calibration density sensor 28. The calibration concentration sensor 28 measures the ratio of the P wave and the S wave. The same process is performed for MYK.

  In step S174, the calibration processing unit 11 sets a dot area rate. Specifically, when there is a half patch whose ratio between the P wave and the S wave is equal to or less than a preset threshold, the calibration processing unit 11 has the lowest dot area ratio of the half patches. The dot area rate of is acquired as calibration data. Calibration by setting the dot area rate is also referred to as a second calibration process. The calibration processing unit 11 stores the dot area ratio after calibration in the storage unit 40 as a part of the calibration data CD1.

  In step S190, the screen selection unit 12 executes a screen flag setting process. In the screen flag setting process, the screen selection unit 12 sets a flag indicating which of the line screen LS and the dot screen DS the calibration is completed. Specifically, when the calibration is completed in the first calibration process using the line screen LS, the screen selection unit 12 sets a flag indicating the line screen LS and uses the dot screen DS to set the first. When the calibration is completed in the first calibration process or the second calibration process, a flag indicating the dot screen DS is set.

  FIG. 8 is a flowchart showing the contents of an image forming process procedure (step S200) of the image forming apparatus 1 according to an embodiment. In step S210, the screen selection unit 12 determines a screen flag. If the screen flag indicates the line screen LS, the screen selection unit 12 proceeds to step S220. If the screen flag indicates the dot screen DS, the screen selection unit 12 proceeds to step S230.

  In step S220, the screen selection unit 12 selects the line screen LS. Accordingly, the halftone processing unit 22 can use the line screen LS capable of reproducing an image in which color unevenness is less noticeable. In step S230, the screen selection unit 12 selects a dot screen DS. Thereby, the halftone processing unit 22 can use a dot screen DS that achieves a solid image target density with a low dot area ratio (including the initial value) and has a high effect of suppressing rear end accumulation.

  In step S240, the halftone processing unit 22 executes dot data generation processing. In the dot data generation processing, the halftone processing unit 22 performs halftone processing using either the line screen LS or the dot screen DS selected based on the screen flag, and forms dots of CMYK toners. Dot data representing the state is generated.

  In step S250, the image forming unit 20 forms dots on a printing medium (image forming medium) based on the dot data and outputs the printed matter.

  As described above, in the image forming apparatus 1 according to the embodiment, when the calibration of the half patch using the line screen LS is possible within the adjustment range of the developing bias, the color unevenness is generated using the line screen LS. It is possible to reproduce an inconspicuous image. On the other hand, in the image forming apparatus 1, even if the half patch calibration using the line screen LS is not possible within the development bias adjustment range, the half patch calibration using the dot screen DS is within the development bias adjustment range. There is a possibility that can be realized.

  Further, the image forming apparatus 1 has a dot area ratio lower than that of the half patch using the line screen LS even when the half patch using the dot screen DS cannot be calibrated within the adjustment range of the developing bias. Since the solid image target density can be achieved with a large amount of thinning (that is, a large thinning amount), it is possible to suppress the trailing edge accumulation as compared with the case of using the line screen LS.

  The present invention can be implemented not only in the above embodiment but also in the following modifications.

  Variation 1: In the above embodiment, when calibration of the half patch using the line screen LS is not possible within the adjustment range of the development bias, the dot screen is not performed without performing the second calibration using the line screen LS. A half patch using DS is being calibrated. However, the second calibration process using the line screen LS may be executed even when the half patch calibration using the line screen LS is not possible within the adjustment range of the developing bias.

  Thereby, the screen selection unit can select both the line screen and the dot screen. Therefore, the screen selection unit, for example, can form a color image by selecting the line screen on which the second calibration process has been executed and reproducing an image with less noticeable color unevenness, and does not include the color image region. For the image formation of a monochrome image, it is possible to reproduce an image in which the trailing edge is suppressed by selecting a dot screen.

  However, when the second calibration process is performed on the line screen, there arises a trade-off problem between the occurrence of rear end accumulation due to the use of the line screen and the conspicuous color unevenness. Therefore, the screen selection unit may separately set an upper limit of the dot area ratio for forming a solid image, for example, and may select a line screen for image formation of a color image below the upper limit. As a result, it is possible to realize image reproduction in which color unevenness is less noticeable while suppressing the occurrence of excessive trailing edge accumulation.

  Further, in the image formation of the color image, the screen selection unit selects the line screen on which the second calibration process has been executed when the ratio of the color image area to the entire image exceeds a preset threshold value. You may do so. Note that the threshold value may include zero.

  Modification 2: In the above embodiment, the screen selection unit selects the screen without considering the image property related to the occurrence of the trailing edge accumulation, but selects the screen using the property of the image. May be That is, the screen selection unit may analyze the image data to estimate the degree of occurrence of the trailing edge accumulation and select either the line screen or the dot screen based on the estimation.

  Specifically, the screen selection unit may count specific pixels where rear end accumulation is likely to occur, and may select a screen according to the number and density of specific pixels. For specific pixels where rear end accumulation is likely to occur, the rear end pixel of the solid image is detected using a contour detection filter such as a differential filter or a Sobel filter, and is set in advance upstream of the rear end pixel in the sub-scanning direction. Pixels forming a solid image can be formed by the number of pixels or more. The screen selection unit may select the screen using the ratio of the area of the color image to the entire image and the number and density of specific pixels.

  Modification 3: In the above embodiment, the screen selection unit selects either halftone processing using a line screen or halftone processing using a dot screen. For example, error diffusion is applied to the halftone processing method. You may choose. The screen selection unit may select any one of a plurality of halftone processing methods including a halftone process using a line screen and a halftone process using a dot screen.

  Modification 4: In the above embodiment, an amorphous silicon photoconductor is used, but the present invention is not limited to the use of an amorphous silicon photoconductor. The present invention is generally applicable to an image forming apparatus that reproduces a solid density with a non-saturated photoconductor.

Reference Signs List 1 image forming apparatus 10 control unit 11 calibration processing unit 12 image processing unit 20 image forming unit 21 color conversion processing unit 28 calibration density sensor 29 exposure unit 40 storage unit 50 image reading unit 60 sheet feeding cassette 70 housing

Claims (7)

A screen selection unit for selecting one of a plurality of halftone processing methods including halftone processing using a line screen and halftone processing using a dot screen;
A halftone processing unit that performs halftone processing using the halftone processing method selected by the screen selection unit;
The photosensitive member includes a rotatable photosensitive member, an exposure unit that exposes the photosensitive member based on image data to form an electrostatic latent image, a magnetic roller, and a developing roller, and the magnetic roller and the developing roller The toner layer is formed on the developing roller in a thickness corresponding to the toner layer forming potential difference between the toner layer and the photosensitive member based on the developing bias potential which is the potential of the developing roller and the electrostatic latent image. A developing unit for attaching toner;
A half patch is formed using the line screen, the developing bias potential of the developing unit is adjusted within a preset adjustment range, and a first calibration process is executed, and the adjustment is performed in the first calibration process. When the potential limit value is within the range, a half patch is formed using the dot screen, the first calibration process is executed, and the adjustment is performed in the first calibration process using the dot screen. A calibration processing unit configured to execute a second calibration process by setting a dot area ratio for forming a solid image when the potential limit value in the range is reached;
An image forming apparatus comprising: The image forming apparatus according to claim 1,
When the developing bias potential is set within the adjustment range in the first calibration process using the line screen, the screen selection unit selects the line screen and uses the line screen. An image forming apparatus for selecting the dot screen when the development bias potential becomes a potential limit value within the adjustment range in the calibration process of An image forming apparatus according to claim 1 or 2, wherein
The calibration processing unit further includes a second calibration using the line screen when the development bias potential becomes a potential limit value within the adjustment range in the first calibration processing using the line screen. Execute the process,
The screen selection unit is an image forming apparatus that selects a line screen on which the second calibration process has been performed when a ratio of a color image area to the entire image exceeds a preset threshold. The image forming apparatus according to any one of claims 1 to 3, wherein
The screen selection unit is an image forming apparatus that analyzes the image data, estimates a degree of occurrence of a trailing edge accumulation, and selects either the line screen or the dot screen based on the estimation. The image forming apparatus according to any one of claims 1 to 4, wherein
The photoconductor is an amorphous silicon photoconductor,
The image forming apparatus, wherein the amorphous silicon photosensitive member forms the solid image in a non-saturated state. A screen selection step of selecting any of a plurality of halftoning methods including halftoning using line screens and halftoning using dot screens;
Performing a halftoning process using the halftoning method selected by the screen selection process;
A rotatable photosensitive member, an exposure unit that exposes the photosensitive member based on image data to form an electrostatic latent image, a magnetic roller, and a developing roller, and between the magnetic roller and the developing roller The toner layer having a thickness corresponding to the toner layer formation potential difference is formed on the developing roller, and the toner is transferred from the toner layer to the photosensitive member based on the developing bias potential which is the potential of the developing roller and the electrostatic latent image. A development process for adhering,
A half patch is formed using the line screen, a development bias potential in the development process is adjusted within a preset adjustment range, and a first calibration process is executed, and the adjustment is performed in the first calibration process. When the potential limit value is within the range, a half patch is formed using the dot screen, the first calibration process is executed, and the adjustment is performed in the first calibration process using the dot screen. A calibration process step of executing a second calibration process by setting a dot area ratio for forming a solid image when the potential limit value in the range is reached;
An image forming method comprising: An image forming program for controlling an image forming apparatus for forming an image on an image forming medium according to image data, comprising:
The image forming apparatus includes a rotatable photosensitive member, an exposure unit that exposes the photosensitive member based on image data to form an electrostatic latent image, a magnetic roller, and a developing roller, and the magnetic roller Forming on the developing roller a toner layer having a thickness corresponding to the toner layer forming potential difference between the toner and the developing roller, and based on the electrostatic latent image and the developing bias potential which is the potential of the developing roller A developing unit for adhering toner from the layer to the photoreceptor,
The image forming program includes:
A screen selection unit to select one of a plurality of halftoning methods, including halftoning using a line screen and halftoning using a dot screen;
A halftone processing unit that performs halftone processing using the halftone processing method selected by the screen selection unit; and a half patch is formed using the line screen and the developing bias potential of the developing unit is set in advance. The first calibration process is executed by adjusting within the set adjustment range, and when the potential limit value is within the adjustment range in the first calibration process, a half patch is used using the dot screen. To perform the first calibration process, and the dot area for forming a solid image when the potential limit value within the adjustment range is reached in the first calibration process using the dot screen An image forming program which causes the image forming apparatus to function as a calibration processing unit that sets a rate and executes a second calibration process.