JP4608968B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a printer or a copier that employs an electrophotographic method, and more specifically, in an image forming apparatus that uses a two-component developer, the toner concentration of the two-component developer is stably controlled over a long period of time. The present invention relates to a toner concentration control technique that can be performed.

JP 2002-162895 A JP 10-39555 A

  Conventionally, in an image forming apparatus such as a printer or a copying machine that employs this type of electrophotographic method, the surface of a photosensitive drum as an image carrier is charged to a predetermined potential by a charger, and then according to image information. The image is exposed to form an electrostatic latent image, and the electrostatic latent image is visualized by a developing device, and the toner image is directly transferred and fixed onto a transfer sheet as a toner image. The image is formed by being transferred and fixed onto a transfer sheet.

  By the way, the image forming operation in such an image forming apparatus is executed in a cycle of image density control for setting image forming conditions, printing, and toner density control after printing a predetermined number of sheets. The increase / decrease of the replenishment toner amount when the toner is replenished is determined.

  In the image density control operation, a first reference toner image is formed, the density of the first reference toner image is detected, and an image formation such as a developing bias and an exposure amount is formed according to a difference from the target value. Configured to change conditions. At this time, when the first reference toner image is developed by the developing device, a developing bias on which an alternating current component used during normal image formation is superimposed is used.

  On the other hand, in the toner density control operation, a second reference toner image is formed, the density of the second reference toner image is detected, and the toner replenishment amount is controlled according to the difference from the target value. It is configured. At this time, when the second reference toner image is developed by the developing device, a developing bias consisting of only a DC component is used. Thus, when developing the second reference toner image, the reason for using the development bias consisting of only the DC component is that the sensitivity of the patch density of the second reference toner image to the toner density of the developer is superimposed on the AC component. This is because the development bias is higher than that applied.

  However, in the above-described toner density control operation, when the second reference toner image is developed, a developing bias consisting only of a direct current component is used. Therefore, the density of the second reference toner image is set to the developing sleeve of the developing device. There is a drawback that it is easily affected by the amount of developer above. For example, when the toner concentration in the developer is sufficiently high, the surface roughness of the developing sleeve of the developing device decreases with time, so that the amount of developer carried on the developing sleeve decreases. When a developing bias consisting of only a direct current component is applied, the density of the second reference toner image becomes thin due to the effect of the decrease in the developer amount, and even though the toner density is high, further toner is added. The replenishment operation will be performed. As a result, when the toner concentration in the developer increases, the amount of carrier in the developer relatively decreases and the magnetic adhesion to the developing sleeve decreases, so that the amount of developer on the developing sleeve further increases. There is a tendency to decrease. As a result, the density of the second reference toner image is further reduced, and there is a risk of a vicious circle in which the operation of replenishing the toner is continued. This causes a problem that the toner is ejected from the developing device. Had.

  Therefore, as a technique for preventing the ejection of toner from the developing device due to such an abnormal increase in toner density, for example, a technique disclosed in Japanese Patent Laid-Open No. 2002-162895 has already been proposed. As a related technique, a technique disclosed in Japanese Patent Laid-Open No. 10-39555 has already been proposed.

  The image forming apparatus according to Japanese Patent Laid-Open No. 2002-162895 forms a plurality of image density adjustment toner images by using at least two different development bias voltages with the same color toner, and the plurality of images. In the image forming apparatus that measures the density of the density adjusting toner image by the density measuring unit and adjusts the toner density of the two-component developing unit according to the measurement result of the density measuring unit, the plurality of image density adjusting toner images Among them, it is confirmed that the toner density of the two-component developing unit has reached a specified value or more from both the measurement result of the first image density adjustment toner image and the measurement result of the second image density adjustment toner image. It comprises so that the control means to detect may be provided.

  In the image forming apparatus according to Japanese Patent Application Laid-Open No. 2002-162895, the two-component developing unit uses both the measurement result of the first image density adjustment toner image and the measurement result of the second image density adjustment toner image. The control means detects that the toner density has reached a specified value or more, and restricts or stops the supply of toner, thereby preventing the toner from being ejected from the developing means.

  In addition, an image forming apparatus according to the above Japanese Patent Laid-Open No. 10-39555 includes a first forming unit that forms a first pattern on a recording material, a first detecting unit that detects the density of the first pattern, First control means for setting the processing content at the time of visible image formation based on the density detected by the first detection means; second formation means for forming a second pattern on the image holder; A second detection unit that detects the density of the second pattern, a second control unit that controls processing at the time of visible image formation based on the density detected by the second detection unit, and a control result by the first control unit And adjusting means for adjusting the second control means based on the above.

  In the image forming apparatus according to Japanese Patent Laid-Open No. 10-39555, the second control unit is adjusted by the adjusting unit based on the control result by the first control unit, so that the toner density target is adjusted according to the change in the developer. The value is changed.

  However, the above prior art has the following problems. That is, in the case of the image forming apparatus according to Japanese Patent Application Laid-Open No. 2002-162895, from both the measurement result of the first image density adjustment toner image and the measurement result of the second image density adjustment toner image, Although the control unit detects that the toner density of the two-component developing unit has reached a specified value or more and restricts or stops the supply of toner, it is possible to prevent the toner from being ejected from the developing unit. In the case of the provided full-color image forming apparatus, there is a problem that the image bias of each color cannot be adjusted by appropriately controlling the developing bias of each developing apparatus.

  Further, in the case of the image forming apparatus according to the above-mentioned JP-A-10-39555, in order to stabilize the image, the state of the developer when the developing condition of the developing device is determined is maintained. The target value for toner density control is determined again by the adjusting means so that the toner density at the time of determining the development conditions is maintained.

  Therefore, in the case of the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 10-39555, the correction is not performed so that the developing bias falls within a certain range, and the image forming apparatus can be freely controlled. Although it can be performed, it has a problem that it cannot be applied to an image forming apparatus in which the control range of the developing bias is limited.

  More specifically, in the image forming apparatus, the charging potentials of a plurality of photosensitive drums may be set to be the same for cost reduction. As described above, when the charging potentials of a plurality of photosensitive drums are set to be the same, the developing bias applied to the developing device is within a certain range with respect to the charging potential of the photosensitive drum due to the cleaning field. Therefore, the developing bias cannot be freely controlled, and the control range of the developing bias is limited.

  Therefore, in the case of the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 10-39555, in order to stabilize the image, the developing bias at the time of forming the visible image is determined based on the detection result of the density of the first pattern. Although it is necessary to determine the processing content, if the control range of the developing bias is limited, the processing content such as the developing bias at the time of forming a visible image is freely determined based on the detection result of the density of the first pattern. In other words, there is a possibility that the balance of development density in a plurality of developing devices may be lost, and the color of process black may change, resulting in a decrease in image reproducibility.

  Accordingly, the present invention has been made to solve the above-described problems of the prior art, and an object of the present invention is to surely prevent the ejection of toner from the developing means and the development bias. Even when the control range is limited, an image forming apparatus capable of stabilizing the density balance of each color, suppressing a change in the color tone of process black, and improving image reproducibility Is to provide.

In order to achieve the above object, the invention described in claim 1
Forming means for forming first and second reference toner images on the image carrier;
Density detecting means for detecting the optical density of the first and second reference toner images;
First calculating means for calculating an image forming condition based on a difference between a density detected by the density detecting means of the first reference toner image and a first target density;
Second calculating means for calculating a replenishment amount of the developer to the two-component developing means based on the difference between the density detected by the density detecting means of the second reference toner image and the second target density. In an image forming apparatus using a two-component developer,
Based on the difference between the density detected by the density detection means of the first reference toner image and the first target density, the value of the image forming condition calculated by the first calculation means exceeds the predetermined range. And a control means for controlling the image forming condition in a direction approaching a predetermined range by changing the second target density based on the value of the image forming condition itself. The image forming apparatus.

  According to a second aspect of the present invention, the first reference toner image is developed with a developing bias in which an AC bias is superimposed on a DC bias, and the second reference toner image is developed only with a DC bias. 2. The image forming apparatus according to claim 1, wherein development is performed by bias.

Furthermore, the invention described in claim 3 includes a plurality of image forming units that form images of different colors, and images of different colors formed by the respective image forming units are transferred to at least one intermediate transfer. An image forming apparatus for forming an image by transferring the image on a recording medium in a superposed state via a body, wherein the control means includes an image of each image forming unit calculated by the first calculating means. calculates the mean value of formation conditions, the mean value itself of the image forming conditions of the respective image forming units, when it exceeds a predetermined range, the average value itself the second depending on the the image forming conditions The image forming apparatus according to claim 1, wherein the image forming condition is controlled in a direction approaching a predetermined range by changing a target density.

According to a fourth aspect of the present invention, a plurality of image forming units for forming images of different colors are provided, and images of different colors formed by the respective image forming units are transferred to at least one intermediate transfer. An image forming apparatus for forming an image by transferring the image on a recording medium in a superposed state via a body, wherein the control means includes an image of each image forming unit calculated by the first calculating means. calculates the average value of the forming condition, based on the average value of the image formation condition of the image forming units, when the average value itself of the image forming condition of said image forming units exceeds a predetermined range, by changing the second target concentration according to the average value itself of the image forming condition, said image forming condition is controlled in a direction toward the predetermined range The image forming apparatus according to claim 1 or 2, symptoms.

  Furthermore, in the invention described in claim 5, when the control unit changes the second target density, the second target is detected based on the detection result of the previous second reference toner image. 5. The image forming apparatus according to claim 1, wherein the density changing range is controlled to be limited to a certain range.

  The invention described in claim 6 is characterized in that the control means determines a predetermined range for determining an average value of the image forming conditions according to environmental conditions. The image forming apparatus according to any one of the above.

  According to the present invention, it is possible to surely prevent the ejection of toner from the developing means, and to stabilize the density balance of each color even when the control range of the developing bias is limited. Thus, it is possible to provide an image forming apparatus capable of suppressing the change in the color of process black and improving the reproducibility of the image.

  Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1
2 shows a tandem type full color printer as an image forming apparatus according to Embodiment 1 of the present invention, and FIG. 3 shows a tandem type full color printer as an image forming apparatus according to Embodiment 1 of the present invention. The image forming unit is shown. In addition, the arrow in FIG. 3 has shown the rotation direction of each rotation member.

  As shown in FIGS. 2 and 3, the full-color printer 01 includes photosensitive drums (image carriers) 11, 12, yellow (Y), magenta (M), cyan (C), and black (K). Image forming units 1, 2, 3, 4 having 13, 14, and charging rolls (contact type charging means) 21, 22, 23, 24 for primary charging contacting these photosensitive drums 11, 12, 13, 14 FIG. 2 is a diagram in which electrostatic latent images are written by irradiating laser beams 31, 32, 33, and 34 corresponding to the respective color images of yellow (Y), magenta (M), cyan (C), and black (K). A laser optical unit (latent image writing means) 03, a developing device (developing means) 41, 42, 43, 44, and two photosensitive drums 11 out of the four photosensitive drums 11, 12, 13, 14. , 12 is in contact with the first primary intermediate transfer drum (intermediate transfer member) 51 and the other two photosensitive drums 13, 14 are in contact with the second primary intermediate transfer drum. A drum (intermediate transfer body) 52, a secondary intermediate transfer drum (intermediate transfer body) 53 that contacts the first and second primary intermediate transfer drums 51 and 52, and a final contact that contacts the secondary intermediate transfer drum 53 The transfer roller (transfer member) 60 constitutes the main part.

  As shown in FIG. 3, the photosensitive drums 11, 12, 13, and 14 are arranged in parallel to each other at a predetermined interval so as to have a common tangential plane M. Further, the first primary intermediate transfer drum 51 and the second primary intermediate transfer drum 52 have respective rotational axes parallel to the rotational axes of the photosensitive drums 11, 12, 13, and 14 and having a predetermined symmetry plane as a boundary. Are arranged so as to have a symmetrical relationship. Further, the secondary intermediate transfer drum 53 is arranged so that the rotation axis thereof is parallel to the photosensitive drums 11, 12, 13, and 14.

  Signals corresponding to the image information for each color are rasterized by an image processing unit (not shown) and input to the laser optical unit 03 shown in FIG. In this laser optical unit 03, the laser beams 31, 32, 33, and 34 corresponding to the respective color images of yellow (Y), magenta (M), cyan (C), and black (K) are modulated in accordance with the image information. Then, the photosensitive drums 11, 12, 13 and 14 of the corresponding colors are irradiated to write an electrostatic latent image.

  Around each of the photosensitive drums 11, 12, 13, and 14, an image forming process for each color is executed by a well-known electrophotographic method. First, as the photosensitive drums 11, 12, 13, and 14, for example, photosensitive drums using an OPC photosensitive member having a diameter of 20 mm are used, and these photosensitive drums 11, 12, 13, and 14 are not shown. It is rotationally driven by the driving means at a rotational speed of 95 mm / sec, for example. As shown in FIG. 3, the surfaces of the photosensitive drums 11, 12, 13, and 14 are applied with a DC voltage of about -800 V to charging rolls 21, 22, 23, and 24 as contact-type charging means. For example, it is charged to about -300V. The contact type charging means includes a roll type, a film type, a brush type, etc., but any type may be used. In this embodiment, a charging roll generally used in an electrophotographic apparatus in recent years is employed. Further, in order to charge the surfaces of the photosensitive drums 11, 12, 13, and 14, in this embodiment, a charging method in which only DC is applied is used, but a charging method in which AC + DC is applied may be used.

  Thereafter, the surface of each of the photosensitive drums 11, 12, 13, and 14 has a yellow (Y), magenta (M), cyan (C), and black (K) color by a laser optical unit 03 as a latent image writing unit. Laser beams 31, 32, 33, and 34 corresponding to the image are irradiated, and an electrostatic latent image corresponding to input image information for each color is formed. When the electrostatic latent image is written by the laser optical unit 03, the surface potential of the image exposure portion of the photosensitive drums 11, 12, 13, and 14 is discharged to about −60 V or less.

  Further, the electrostatic latent images corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) formed on the surfaces of the photosensitive drums 11, 12, 13, and 14 are compatible. Are developed in reverse by the developing devices 41, 42, 43, and 44, and each color of yellow (Y), magenta (M), cyan (C), and black (K) is formed on the photosensitive drums 11, 12, 13, and 14. It is visualized as a toner image.

  In this embodiment, as the developing devices 41, 42, 43, 44, a magnetic brush contact type two-component development method is adopted, but the present invention is not limited to this magnetic brush contact type, and a two-component development method. As a matter of course, the present invention can be sufficiently applied to other development systems.

The developing devices 41, 42, 43, and 44 are filled with two-component developers including yellow (Y), magenta (M), cyan (C), and black (K) toners and carriers, each having a different color. Has been. When the toner or the two-component developer is supplied from the toner cartridges 04Y, 04M, 04C, 04K shown in FIG. 2, these developing devices 41, 42, 43, 44 are shown in FIG. As shown, the auger 404 is sufficiently agitated with the carrier and triboelectrically charged to the negative polarity. Inside the developing roll 401, a magnet roll (not shown) having a plurality of magnetic poles arranged at a predetermined angle is provided in a fixed state. By the paddle 403 that conveys the developer to the developing roll 401, the amount of the developer conveyed to the vicinity of the surface of the developing roll 401 is regulated by the developer amount regulating member 402. In this embodiment, the amount of the developer is 30 to 50 g / m ² , and the charge amount of the toner existing on the developing roll 401 at this time is about −20 to −35 μC / g. is there.

The toner used in the developing devices 41, 42, 43, 44 has a shape factor MLS2 defined by the following equation of 100 to 140, for example, MLS2 = 130, and an average particle size of 3 μm to 10 μm. A so-called “spherical toner” is used, but other toners may of course be used.
MLS2 = {(absolute maximum length of toner particles) × 2)}
/ {(Projection area of toner particles) × π × 1/4 × 100}

  The toner supplied onto the developing roll 401 is in the form of a magnetic brush composed of a carrier and toner by the magnetic force of the magnet roll, and this magnetic brush comes into contact with the photosensitive drums 11, 12, 13, and 14. ing. A developing bias voltage of AC + DC is applied to the developing roll 401 to develop the toner on the developing roll 401 into an electrostatic latent image formed on the photosensitive drums 11, 12, 13, and 14. It is formed. In this embodiment, a developing bias voltage having an AC component of 4 kHz, 1.5 kVpp and a DC component of about -230 V is used. Image forming conditions such as the developing bias voltage are controlled as described later. Is done.

  Next, the yellow (Y), magenta (M), cyan (C), and black (K) toner images formed on the photosensitive drums 11, 12, 13, and 14 are first primary images. The primary transfer is electrostatically performed on the intermediate transfer drum 51 and the second primary intermediate transfer drum 52. The yellow (Y) and magenta (M) color toner images formed on the photoconductive drums 11 and 12 are formed on the first primary intermediate transfer drum 51 and cyan (on the photoconductive drums 13 and 14). The toner images of C) and black (K) are transferred onto the second primary intermediate transfer drum 52, respectively. Therefore, on the first primary intermediate transfer drum 51, the single color image transferred from either the photosensitive drum 11 or 12 and the two color toner images transferred from both the photosensitive drums 11 and 12 are superimposed. A double color image is formed. In addition, similar single-color images and double-color images are also formed on the second primary intermediate transfer drum 52 from the photosensitive drums 13 and 14.

  The surface potential necessary for electrostatically transferring the toner image from the photosensitive drums 11, 12, 13, 14 onto the first and second primary intermediate transfer drums 51, 52 is about +250 to + 500V. It is. This surface potential is set to an optimum value depending on the charged state of the toner, the ambient temperature, and the humidity. As shown in FIG. 2, the ambient temperature and humidity are detected by an environmental sensor 305 that detects the ambient temperature and humidity. As described above, the surface of the first and second primary intermediate transfer drums 51 and 52 when the charge amount of the toner is in the range of −20 to −35 μC / g and is in a normal temperature and humidity environment. The potential is preferably about + 380V.

The first and second primary intermediate transfer drums 51 and 52 used in this embodiment have an outer diameter of 42 mm, for example, and a resistance value of about 10 ⁸ Ω. The first and second primary intermediate transfer drums 51 and 52 are cylindrical rotating bodies having a single layer or a plurality of layers whose surfaces are flexible or elastic, and are generally made of Fe, Al, or the like. A low resistance elastic rubber layer (R = 10 ² to 10 ³ Ω) typified by conductive silicone rubber or the like is provided on a metal pipe as a metal core as described above to a thickness of about 0.1 to 10 mm. Further, the outermost surfaces of the first and second primary intermediate transfer drums 51 and 52 are typically high release layers (R = 10 ⁵⁾ made of fluoro rubber in which fluororesin fine particles are dispersed and having a thickness of 3 to 100 μm. ˜10 ⁹ Ω) and bonded with a silane coupling agent-based adhesive (primer). What is important here is the resistance value and the releasability of the surface, and the resistance value of the high release layer is about R = 10 ⁵ to 10 ⁹ Ω. It is not limited.

  The single-color or double-color toner images formed on the first and second primary intermediate transfer drums 51 and 52 in this way are electrostatically secondary-transferred onto the secondary intermediate transfer drum 53. Accordingly, on the secondary intermediate transfer drum 53, the final toner from a single color image to a quadruple color (process black) image of yellow (Y), magenta (M), cyan (C), and black (K) is obtained. An image will be formed.

  The surface potential necessary for electrostatically transferring the toner image from the first and second primary intermediate transfer drums 51 and 52 onto the secondary intermediate transfer drum 53 is about +600 to +1200 V. This surface potential is the same as when the toner is transferred from the photosensitive drums 11, 12, 13, 14 to the first primary intermediate transfer drum 51 and the second primary intermediate transfer drum 52. Will be set to the optimum value. Further, since what is necessary for the transfer is a potential difference between the first and second primary intermediate transfer drums 51 and 52 and the secondary intermediate transfer drum 53, the first and second primary intermediate transfer drums 51 and 52 have different potentials. It is necessary to set the value according to the surface potential. As described above, the charge amount of the toner is in the range of −20 to −35 μC / g, is in a normal temperature and humidity environment, and the surface potential of the first and second primary intermediate transfer drums 51 and 52 is In the case of about + 380V, the surface potential of the secondary intermediate transfer drum 53 is about + 880V, that is, the potential difference between the first and second primary intermediate transfer drums 51 and 52 and the secondary intermediate transfer drum 53 is It is desirable to set it to about + 500V.

The secondary intermediate transfer drum 53 used in this embodiment is formed to have the same outer diameter as that of the first and second primary intermediate transfer drums 51 and 52, for example, 42 mm, and the resistance value is set to about 10 ¹¹ Ω. . Similarly to the first and second primary intermediate transfer drums 51 and 52, the secondary intermediate transfer drum 53 is a cylindrical rotating body having a single layer or a plurality of layers whose surface is flexible or elastic. In general, a low resistance elastic rubber layer (R = 10 ² to 10 ³ Ω) typified by conductive silicone rubber or the like is formed on a metal pipe as a metal core made of Fe or Al. It is provided at about 0.1-10mm. Further, the outermost surface of the secondary intermediate transfer drum 53 is typically formed by forming fluororubber in which fine particles of fluororesin are dispersed as a high release layer having a thickness of 3 to 100 μm, and a silane coupling agent-based adhesive (Primer). Here, the resistance value of the secondary intermediate transfer drum 53 needs to be set higher than that of the first and second primary intermediate transfer drums 51 and 52. Otherwise, the secondary intermediate transfer drum 53 will charge the first and second primary intermediate transfer drums 51, 52, making it difficult to control the surface potential of the first and second primary intermediate transfer drums 51, 52. Become. The material is not particularly limited as long as the material satisfies such conditions.

  Next, the final toner image from the single color image to the quadruple color image formed on the secondary intermediate transfer drum 53 is tertiary transferred onto the transfer paper P passing through the paper transport path by the final transfer roll 60. Is done. As shown in FIG. 2, the transfer paper P passes through a registration roller 90 through a paper feeding process from a paper feed cassette 05 and is fed into a nip portion between the secondary intermediate transfer drum 53 and the final transfer roll 60. After this final transfer process, the final toner image formed on the transfer paper P is fixed by heat and pressure by the fixing device 70 and discharged onto a discharge tray T provided on the upper part of the full-color printer 01. A series of image forming processes is completed.

The final transfer roll 60 has, for example, an outer diameter of 20 mm and a resistance value of about 10 ⁸ Ω. The final transfer roll 60 is configured by providing a coating layer made of urethane rubber or the like on a metal shaft, and coating it as necessary. The optimum voltage applied to the final transfer roll 60 varies depending on the ambient temperature, humidity, paper type (resistance value, etc.), and is approximately +1200 to +5000 V. In this embodiment, a constant current method is employed, and a current of about +6 μA is applied in a room temperature and humidity environment to obtain a substantially appropriate transfer voltage (+1600 to +2000 V).

  By the way, in the full-color printer 01 configured as described above, when the toner image passes through the transfer portion of each transfer process in the series of transfer processes as described above, it is (−) charged by Paschen discharge or charge injection. In some cases, a part of the normal polarity toner in the image becomes a (+) charged toner having a reverse polarity. Since the (+) charged toner is not transferred to the next process due to the transfer charge, it flows backward to the upstream side, so that it adheres to and accumulates on the charging rolls 21, 22, 23, 24 having the highest negative potential. The portions of the charging rolls 21, 22, 23, and 24 where the toner adheres are areas where the toner is frequently adhered because the discharge becomes active and the surface potential of the photosensitive drums 11, 12, 13, and 14 tends to increase. The surface potentials of the photoconductive drums 11, 12, 13, and 14 are uneven at portions where the toner is hardly attached and where the toner is not attached. If unevenness occurs in the surface potential of the photoconductive drums 11, 12, 13, and 14, an image is uniformly exposed on the surface of the photoconductive drums 11, 12, 13, and 14 in order to form an electrostatic latent image. However, unevenness occurs in the latent image potential, resulting in a difference in the development amount. Therefore, when trying to develop a halftone image, the unevenness in density becomes conspicuous.

  Therefore, in order to prevent the occurrence of density unevenness due to the toner adhering to the charging rolls 21, 22, 23, 24, in this embodiment, before the printing operation, after the printing operation, every predetermined number of sheets during continuous printing, etc. The following cleaning operation is executed at a predetermined timing.

  In this cleaning operation, the charging rolls 21, 22, 23, 24, the photosensitive drums 11, 12, 13, 14, the first and second primary intermediate transfer drums 51, 52, the secondary intermediate transfer drum 53, and the final transfer roll By applying a voltage with a potential gradient in order so that the final transfer roll 60 has the highest negative potential, it was deposited on the charging rolls 21, 22, 23, and 24 during the printing operation. The (+) charged toner of reverse polarity is sequentially transferred and moved to the final transfer roll 60, and is collected by a cleaning device 80 including a final cleaning member 801 such as a blade provided in contact with the final transfer roll 60. .

  In this embodiment, the surface potential of the charging rolls 21, 22, 23, 24 is 0V, the surface potential of the photosensitive drums 11, 12, 13, 14 is -300V, and the first and second primary intermediate transfer drums 51, The surface potential of 52 is set to -800 V, the surface potential of the secondary intermediate transfer drum 53 is set to -1300 V, and the surface potential of the final transfer roll 60 is set to -2000 V. This potential gradient is obtained by supplying a voltage to the metal part (shaft or pipe) of each member. For example, the first and second primary intermediate transfer drums 51 and 52 or the secondary intermediate transfer drum 53 are used. When a desired surface potential is obtained by the relationship of the resistance values of these members, the above method may be used. In such a negative application cleaning mode, that is, a (+) charged toner recovery mode having a reverse polarity, it is possible to prevent density unevenness due to toner adhering to the charging rolls 21, 22, 23, 24.

  If necessary, the toner is charged to a normal (−) polarity, and the photosensitive drums 11, 12, 13, 14, the first and second primary intermediate transfer drums 51, 52, and the secondary intermediate The toner remaining on the surface of the transfer drum 53 can be removed by a similar method (by reversing only the polarity of the applied voltage).

  The above is the image forming process in the full color printer 01 configured as described above. In the full color printer, when the image printing operation is repeated, yellow (Y) in the developing devices 41, 42, 43, 44, magenta (M), cyan (C), and black (K) toners are gradually consumed, so the density of each color toner image developed by the developing device 41, 42, 43, 44 is measured, It is necessary to replenish toner appropriately. In addition, since the electrophotographic full color printer 01 uses static electricity, the image density is likely to fluctuate due to environmental changes and changes over time. For this reason, it is configured to control the process with respect to environmental fluctuations and changes with time.

  As one of the methods, instead of directly measuring the toner density of each color in the developing devices 41, 42, 43, 44, the image density of the photoconductor, intermediate transfer body, transfer roll to paper, transfer belt, etc. There is a method in which a reference toner image for density detection is formed on the surface of a detection medium, the density is detected by an optical density sensor, and toner replenishment, image forming conditions, and the like are controlled.

  Therefore, in this embodiment, a forming unit for forming the first and second reference toner images on the image carrier, a density detecting unit for detecting the optical density of the first and second reference toner images, A first calculating means for calculating an image forming condition based on a difference between a density detected by the density detecting means for the first reference toner image and a first target density; and a density detecting means for the second reference toner image. An image forming apparatus using a two-component developer, and a second calculating unit that calculates a replenishment amount of developer to the two-component developing unit based on a difference between the detected density by the second target density and the second target density The image forming condition calculated by the first calculation unit based on the difference between the density detected by the density detection unit of the first reference toner image and the first target density exceeds a predetermined range. If the second target density is changed And it is configured to provide a control means for controlled so.

  In this embodiment, the first reference toner image is developed with a development bias in which an AC bias is superimposed on a DC bias, and the second reference toner image is developed with a development bias consisting of only a DC bias. It is configured as follows.

  That is, in this embodiment, on the image density detection medium such as the final transfer roll 60 and the secondary intermediate transfer drum 53, the position in the process direction is shifted to the same position in the axial direction, and the image density detection medium is used. By forming the first and second reference toner images, respectively, the optical density of the first and second reference toner images of each color can be detected by one density detection means.

  In order to detect the first and second reference toner images on the photosensitive drums 11, 12, 13, and 14, four density detecting means are required for each of the photosensitive drums 11, 12, 13, and 14, respectively. End up. Two density detectors may be used on the first and second primary intermediate transfer drums 51 and 52. If it is on the secondary intermediate transfer drum 53 or the final transfer roll 60, only one density detecting means may be used. Further, when controlling the image density with the first and second reference toner images, the downstream process is preferable because it is closer to the paper. That is, the secondary intermediate transfer drum 53, more preferably the final transfer roll 60, may be used as an image density detection medium for detecting the density of the first and second reference toner images.

  In this embodiment, as shown in FIG. 3, the first and second reference toner images transferred onto the final transfer roll 60 are transferred onto the final transfer roll 60. The density of the image is detected by an optical density sensor 100 as a density detecting means.

  The first reference toner image is a non-image area where no image is formed, and a development bias in which an AC voltage is superimposed on a normal DC voltage under the same charging, exposure, development, and transfer conditions as during image formation. Using a voltage (DC + AC), for each color of yellow (Y), magenta (M), cyan (C), and black (K), an image density Cin of 33%, 10 × 12 mm, as shown in FIG. They are formed on the final transfer roll 60 at intervals of 3 mm. Here, the image density of the first reference toner image 200 is set to Cin 33% when the first reference toner image 200 is formed by the laser optical unit 03 using a line screen. This is because the line screen can be easily formed by performing image exposure with one of the three lines, that is, with an image density of 1/3.

  Further, the second reference toner image 201 is a non-image area, in which no image is formed at this time, under the same charging, exposure, and transfer conditions as those at the time of image formation. FIG. 5 shows a 10 × 12 mm image density Cin of 100% for each color of yellow (Y), magenta (M), cyan (C), and black (K) using a developing bias voltage (DC only) As shown, it is formed on the final transfer roll 60 at intervals of 3 mm.

  Here, the reason for using the developing bias voltage (DC only) consisting only of the DC voltage when forming the second reference toner image 201 is as follows. The embodiment of the present invention is configured such that the image density is corrected with the first reference toner image, and the toner density (TC) is corrected with the second reference toner image. At that time, the amount of toner used for both reference toner images is preferably small (low density) for the following reason. That is, the sensitivity of the optical density sensor 100 is not sensitive where the density is high, and the burden of cleaning the reference toner image after density measurement with the optical density sensor 100 is small and wasted (does not contribute to image formation). This is because it is better to use less.

  By the way, for the purpose of correcting the image density as described above, if the reference toner image is not created with the developing bias when the image is actually output, the first reference toner image has a variation in the density of the reference toner image. The error due to will increase. Also, if a Cin 100% reference toner image is created with the development bias used for image output, the toner consumption will increase, so Cin is reduced to 33% for the purpose of reducing the toner usage as described above. ing.

  On the other hand, the second reference toner image is a reference toner image formed for correcting the toner density (TC), and the reference toner image is most preferably Cin 100% (the toner is applied to the edge portion of the image due to the characteristics of electrophotography). The toner density tends to vary in images other than Cin 100%. Therefore, in order to reduce the amount of toner used, a method of lowering the development bias is adopted. However, if only DC is lowered by AC + DC development, it becomes difficult to develop an image from a certain point. Therefore, if the developing bias AC (Vp-p) is lowered, the developing can be performed again. That is, in order to develop with a low DC bias, it is necessary to lower AC, and ultimately DC development is preferable.

  As shown in FIG. 6, the optical density sensor 100 is arranged at the center of the final transfer roll 60 in the axial direction so as to be positioned on the radial extension line on the outer periphery of the final transfer roll 60. The optical density sensor 100 is attached in a fixed state in the holder 101. A cleaning device 80 including a blade-like final cleaning member 801 is disposed below the final transfer roll 60. In FIG. 6, 802 is a toner collection box, 803 is a support frame of the final transfer roll 60, 804 is a static eliminator provided on the support frame 803, and 805 is a bias plate.

  Further, as shown in FIG. 7, the optical density sensor 100 is a specular reflection type sensor that detects specular reflection light. The optical density sensor 100 has a predetermined incident angle φ with respect to the detection position on the surface of the final transfer roll 60. In order to detect the specularly reflected light that is emitted from the light emitting element 102 to the detection position on the surface of the final transfer roll 60 and is specularly reflected from the detection position. The light receiving element 103 is formed of a phototransistor or the like that is disposed so as to be inclined with respect to the detection position on the surface of the roll 60 by a reflection angle equal to the predetermined incident angle.

  The optical density sensor 100 may be a diffuse reflection type sensor that detects diffused light as shown in FIG. Further, both a specular reflection type sensor and a diffuse reflection type sensor may be used. In this case, the toner density detection accuracy can be further improved by detecting the toner density based on both the specular reflection component and the scattered light component.

  FIG. 9 is a block diagram showing a control circuit of the image forming apparatus according to this embodiment.

  In FIG. 9, reference numeral 300 denotes a first CPU comprising a CPU for controlling the toner density of the developing devices 41, 42, 43, 44 and the image forming conditions such as the developing bias voltage based on the detection result of the optical density sensor 100. And a printer engine control circuit 301 as a second calculation means and control means, 301 generates a developing bias voltage Vbias to be applied to the developing devices 41, 42, 43, 44 based on an output signal from the printer engine control circuit 300. Based on the output signal from the printer engine control circuit 300, the development bias generation circuit 302 is supplied from the color toner boxes 04Y, 04M, 04C, 04K corresponding to the development devices 41, 42, 43, 44, and dispense motors 303Y, 303M, 303C, 303K is a toner supply circuit that supplies toner, 304 is a density determination circuit that determines the density of the test patches 200 and 201 based on the detection result of the optical density sensor 100, and 305 is temperature and humidity The environmental sensor for detecting the boundary condition is indicative respectively.

  In this embodiment, in order to reduce the cost associated with simplification of the power supply circuit, the charging potentials of the photosensitive drums 11, 12, 13, and 14 are all set and controlled to have the same value.

  With the above configuration, the image forming apparatus according to this embodiment can reliably prevent the ejection of toner from the developing unit and the control range of the developing bias is limited as follows. Even in this case, it is possible to stabilize the density balance of each color, to suppress a change in the color of the process black, and to improve the reproducibility of the image.

  In other words, in the full-color printer according to this embodiment, as shown in FIG. 10, before starting the print job, the control circuit 300 first does not transfer the first and second reference toner images. The surface of the final transfer roll 60 is detected by the optical density sensor 100, and the output Vcln of the optical density sensor 100 at this time is stored (step 101). Next, the control circuit 300 uses a developing bias voltage (DC + AC) obtained by superimposing an AC voltage on a normal DC voltage under the same charging, exposure, development, and transfer conditions as in image formation, and uses yellow (Y), magenta ( For each color of M), cyan (C), and black (K), a 10 × 12 mm first reference toner image 200 having an image density Cin of 33% is placed on the final transfer roll 60 as shown in FIG. They are formed at intervals (step 102). As shown in FIG. 11, the conditions for forming the first reference toner image 200 are as described above. The photosensitive drum charging potential Vh is the same as that during image formation, and the photosensitive drum surface potential V33 accompanying image exposure is as follows. It is set to the DC component Vdc of the developing bias voltage obtained by superimposing the AC voltage on the DC voltage.

  Thereafter, in the first reference toner image 200 of each color of yellow (Y), magenta (M), cyan (C), and black (K) formed on the final transfer roll 60, FIG. As shown in FIG. 12B, the optical density sensor 100 detects 16 points at a pitch of 0.5 mm to obtain the average value Vpatch (step 103). Here, as shown in FIG. 12A, a specular reflection type optical density sensor 100 is used. Then, a ratio Vpatch / Vcln obtained by dividing the average value of each color by the value of the surface of the final transfer roll 60 is used as a substitute value for density (step 104). Here, the reason why the ratio with the value on the surface of the final transfer roll 60 is taken is to correct the variation in the reflectance on the surface of the final transfer roll 60 and the variation in the amount of emitted LED light.

  As a result, Vpatch / Vcln is compared with a predetermined value for each color (step 105), and when the obtained density of the test patch is lower than the predetermined density, the absolute value of DC of the developing bias is increased according to the difference. Thus, as shown in FIG. 11B, developability is improved (step 106). By doing so, as shown in FIG. 11B, the electric field in the developing direction formed between the developing roll of the developing device and the image portion of the electrostatic latent image on the photosensitive drum is increased, and development is performed. The amount of toner increases and the image density increases.

  On the contrary, when the obtained density of the test patch is higher than the predetermined density, the absolute value of the DC of the developing bias is lowered according to the difference, and the developability is lowered as shown in FIG. Step 108). By doing so, as shown in FIG. 11C, the electric field in the developing direction formed between the developing roll of the developing device and the image portion of the electrostatic latent image on the photosensitive drum is reduced, and the developing is performed. As a result, the toner amount decreases and the image density decreases. At this time, the adjustment amount of the developing bias supply voltage is determined by the detected density of the first reference toner image.

  Although the supply voltage to the developing bias is adjusted here, any voltage may be used as long as the image density or gradation is adjusted. The charging potential of the photosensitive drum, the exposure conditions, and the gradation characteristics of the image may be adjusted, or the peripheral speed ratio between the developing roll and the photosensitive drum may be adjusted. Moreover, you may combine these.

  Next, at a predetermined timing such as after the print job is completed, the control circuit 300 is different from the image formation in the predetermined charging, exposure, and transfer conditions, and a development bias voltage (DC only) consisting of only a DC voltage is different. FIG. 5 shows a second reference toner image 201 of 10 × 12 mm with an image density Cin of 100% for each color of yellow (Y), magenta (M), cyan (C), and black (K). And formed on the final transfer roll 60 at intervals of 3 mm.

  Thereafter, in the second reference toner image 201 of each color of yellow (Y), magenta (M), cyan (C), and black (K) formed on the final transfer roll 60, FIG. As shown in FIG. 12B, the optical density sensor 100 detects 16 points at a pitch of 0.5 mm to obtain the average value Vpatch. Here, as shown in FIG. 12A, a specular reflection type optical density sensor 100 is used. Then, a ratio Vpatch / Vcln obtained by dividing the average value of each color by the value of the surface of the final transfer roll 60 is used as a substitute value of density. Here, the reason why the ratio with the value on the surface of the final transfer roll 60 is taken is to correct the variation in the reflectance on the surface of the final transfer roll 60 and the variation in the amount of emitted LED light.

  As a result, the control circuit 300 compares Vpatch / Vcln with a predetermined target value for each color, and in the developing devices 41, 42, 43, 44 based on the measured density of the second reference toner image 201. Then, as shown in FIG. 9, the amount of rotation of the dispense motors 303Y, 303M, 303C, and 303K is controlled.

  Incidentally, the control circuit 300 executes the following control when controlling the rotation amount of the dispense motors 303Y, 303M, 303C, and 303K based on the measured density of the second reference toner image 201. .

  FIG. 1 is a flowchart showing a control operation of the image forming apparatus according to the present embodiment. This flowchart can be roughly divided into three blocks.

  The first block is a target potential range when the average value Vave of the development bias Vdc of each color controlled based on the first reference toner image as described above exceeds a preset target potential range. This is for changing the TC correction value, which is a correction value added to the target value in the toner density control, so as to be within. In the second block, the development bias Vdc of each color is targeted with respect to the average value Vave of the development bias of each color according to the charging characteristics and development characteristics of the developer in the development devices 41, 42, 43, and 44 of each color. This is for changing the toner density correction value so as to fall within the potential range. Further, the third block checks the toner density correction value changed in the first and second blocks, and makes final corrections such as adjusting the toner density correction value to be within a certain range. It is for applying.

  In the first block, as shown in FIG. 1, the image forming condition (development bias and exposure amount) of each color calculated by the control circuit 300 based on the difference between the optical density of the first reference toner image and the target density. The average value Vave of the development bias is compared with the target potential range (steps 211 and 212).

  That is, the control circuit 300 forms a first reference toner image 200 of each color of yellow (Y), magenta (M), cyan (C), and black (K) on the final transfer roll 60 as shown in FIG. Then, the density of the first reference toner image 200 is detected by the optical density sensor 100. The control circuit 300 calculates the DC components VdcY, VdcM, VdcC, and VdcK of the development bias for each color based on the difference between the optical density of the first reference toner image and the target density, and develops each color. An average value Vave (= (VdcY + VdcM + VdcC + VdcK) / 4) of the DC components VdcY, VdcM, VdcC, and VdcK of the bias is calculated. Further, the control circuit 300 compares the average value Vave of the DC components VdcY, VdcM, VdcC, and VdcK of the developing bias with the target potential range (steps 211 and 212).

  As shown in FIG. 12, the range of the target potential of the DC component of the developing bias varies depending on the relative humidity indicating the environmental condition, and has a range between an upper limit value and a lower limit value with respect to the target value set as the target potential. Is set to In FIG. 12, the center plot shows the target potential, the upper plot shows the upper limit value, and the lower plot shows the lower limit value. The DC component of the developing bias is controlled by being replaced with a digital value, for example, as shown on the right vertical axis in FIG.

  In this embodiment, as shown in FIGS. 2 and 9, the relative humidity in the printer is detected by the environmental sensor 305, and the target of the DC component of the developing bias is determined according to the relative humidity detected by the environmental sensor 305. The range of potential is determined as shown in FIG.

  Note that the developing bias has a negative polarity, but here, in order to simplify the description, the developing bias is described in terms of the absolute value. Therefore, increasing the developing bias means increasing the magnitude of the developing bias, and decreasing the developing bias means decreasing the magnitude of the developing bias.

  First, as shown in FIG. 1, the control circuit 300 has an average value of development bias, “Vaverage” within a predetermined range, that is, SAD_AVE_TGT (%) + TARGET_H, which is the upper limit value of the target potential, and the target potential. Is equal to or higher than SAD_AVE_TGT (%) + TARGET_L, which is a lower limit value of toner, TC correction value (Y), TC correction value (M), TC correction value (C), and TC correction value (K) that are toner density correction values. ) Is used, and a predetermined value determined in advance is used.

  Further, the control circuit 300 determines that the average value of the developing bias exceeds the SAD_AVE_TGT (%) + TARGET_H that is the upper limit value of the target potential, and the TC correction value that is the correction value of the toner density of all colors. (Y), the TC correction value (M), the TC correction value (C), and the TC correction value (K) are subtracted from the correction step number SAD2TC_ALL_MINUS, which is a preset value, to increase the toner density (charging the toner). The developing property changes as the toner density increases, and as a result, the average value of the developing bias is lowered (step 213).

  Further, when the average value of the development bias is less than SAD_AVE_TGT (%) + TARGET_L, the control circuit 300 sets a correction value for the toner density of all colors in advance. The correction step number SAD2TC_ALL_PLUS is added to correct the toner density so that the toner density decreases (the toner charge amount increases), and developability decreases as the toner charge amount increases. As a result, the average value Vave of the developing bias is reduced. Control in the upward direction (step 214). The correction step numbers SAD2TC_ALL_MINUS and SAD2TC_ALL_PLUS are both set to about “2”, for example, but may be other values.

  Next, the second block will be described. In this second block, the developing devices 41, 42 of the respective colors are limited due to the limitation of the devices in which the charging potentials of the photosensitive drums 11, 12, 13, 14 are set in common. , 43, and 44 must be limited within a certain range (for example, within ± 20 ·) with respect to the average value Vave of the developing bias. Therefore, depending on the state of the developer, the developing bias calculated under the image forming conditions may exceed this range.

  However, since the control range of the development bias is limited as described above, the calculated development bias needs to be suppressed within a predetermined range. This means that the toner density is not within a desirable range, and as it is, there is a possibility that even if the developing bias is corrected, the color balance is not guaranteed.

  Therefore, in the control circuit 300, it is necessary to adjust the toner density of the developer of the corresponding color so that the calculated development bias falls within a predetermined range and the balance of the development bias is maintained.

  First, the control circuit 300 determines that the calculated development bias Vdc (color) of each color is within a predetermined range, that is, a value equal to or higher than Level-MIN (color), which is the lower limit value of the target potential, and the upper limit value of the target potential. If it is equal to or less than the value of Verbage-MAX (color), the TC correction value (color), which is the correction value of the determined toner density, is used without change (steps 221 and 222).

  In addition, the control circuit 300 uses the correction value for the toner density when the development bias Vdc (color) of any one of the colors is less than the average value of the target potential, ie, Average-MIN (color). As a certain TC correction value (color), a preset correction step number SAD2TC_COLOR_PLUS is added to correct the toner density to decrease (the toner charge amount increases), and as a result, the development bias value of the corresponding color increases. The direction is controlled (step 223).

  Further, the control circuit 300 corrects the toner density when the development bias Vdc (color) of any one of the colors exceeds the upper limit value of the target potential, Level-MAX (color). As a TC correction value (color), a preset correction step number SAD2TC_COLOR_MINUS is subtracted, and as a result, the toner density is increased (the toner charge amount is decreased), and as a result, the developing bias of the corresponding color is corrected. Control in the downward direction (step 224).

In addition, the charging characteristics of the developers used in the developing devices 41, 42, 43, and 44 for the respective colors may be different. The developer used in this embodiment has the following characteristics.
Charging characteristics Y <C, K <M
Image density (SAD) of unit toner amount M, K <Y <C

  This means that the density of magenta and black developers is very difficult to obtain, and that of yellow and cyan is very easy to produce. For this reason, when magenta and black are low with respect to the average value Vave of the developing bias, the toner density is controlled to be higher and toner ejection is expected to occur. On the other hand, when yellow and cyan are higher than the average value Vave of the developing bias, the toner density is controlled to be lower and an auger mark may appear. Therefore, it is desirable to control the development potential of each color in a balanced manner with respect to the average value Vave of the development bias.

  Therefore, in this embodiment, as shown in FIG. 14, it is very important to set a target range of developing bias in consideration of developer characteristics. The toner density control is performed so that the yellow and cyan development biases are in 13 levels within the range of −18 to 0 V with respect to the average value Vave, and the magenta and black development biases are in 13 levels within the range of 0 V to +18 V. By adjusting the TC correction value in, it is possible to constitute an image forming apparatus in which an excessive toner density is not generated and an auger mark is not generated even when the environment changes.

  Furthermore, in this embodiment, as an added function, if the developing bias falls below the lower limit value Average-MIN (color), it can be determined that the toner density is too high, and therefore, it is replenished in the subsequent printing sequence. By setting the toner amount to be reduced as much as possible and not to replenish the toner, the development bias is corrected within a predetermined range as quickly as possible. Is obtained (step 225).

  In the above-described embodiment, the yellow and cyan development biases are controlled within the range of -18 to 0 V, and the magenta and black development biases are controlled within the range of 0 V to + 18 ·, depending on the development characteristics of the developer. Thus, it is possible to control within a predetermined range while maintaining the balance of the development characteristics of the developers of the respective colors.

  Next, the third block will be described. In this third block, the toner density target value to be compared (= initial toner density target value + TC correction value) is adjusted by adjusting the toner density correction value. There is a case where the reading value of the second reference toner image is largely deviated. In particular, when the correction value of the toner density is greatly changed in order to reduce the toner density, the toner density does not change because the toner is hardly consumed if the image printed thereafter is low density. . For this reason, there is a possibility that the toner density correction value may be further changed.

  Therefore, in order to prevent this, first, upper and lower limit values are set in the toner density correction value itself.

  In the control circuit 300, when the TC correction value (Color) of the toner density determined as described above is less than −40 which is the lower limit value of the correction value, it is set in advance as a correction value of the toner density of the color. When the toner density correction value -40 (0xD8) is set (steps 311 and 313), and the toner density TC correction value (Color) exceeds 112, which is the upper limit value of the correction value, As a correction value for the toner density of the color, 112 (0x70) which is a preset correction value for the toner density is set (steps 312 and 314).

  Furthermore, in this embodiment, in order to prevent the target value of the toner density to be compared at the next toner density control from being too far from the reading value of the previous toner patch by more than a predetermined step, Processing 322 is performed.

  That is, in the control circuit 300, the TC correction value (Color) of the toner density is less than the value TCPach reading value−TGT value−32 obtained by subtracting 32 from the difference between the reading value of the previous reference toner image and the target value. In this case, TCPach reading value−TGT value−32 is adopted as the correction value TC correction value of the toner density of the color (steps 321 and 323).

  Further, in the control circuit 300, when the TC correction value (Color) of the toner density exceeds the value TCPach reading value−TGT value + 32 obtained by adding 32 to the difference between the reading value of the previous reference toner image and the target value. The TCPach reading value−TGT value + 32 is adopted as the TC correction value of the toner density of the color (steps 322 and 324).

  Thereafter, the control circuit 300 corrects the target value for comparing the detected density of the second reference toner image based on the TC correction value which is the correction value of the toner density determined as described above.

  Thus, in the above-described embodiment, as described above, the control circuit 300 compares the detected density of the first reference toner image 200 with the target density, and calculates the correction value of the DC voltage Vdc of the developing bias. At the same time, an average value Average of the DC voltages Vdc of the developing bias for each color is calculated. In the control circuit 300, as shown in FIG. 1, the average value of the development bias is within a predetermined range, that is, SAD_AVE_TGT (%) + TARGET_H which is the upper limit value of the target potential, and the target potential When the lower limit value SAD_AVE_TGT (%) + TARGET_L is not less than TC correction value (Y), TC correction value (M), TC correction value (C), and TC correction value (K), which are correction values of toner density. Do not change.

  Further, in the control circuit 300, as shown in FIG. 1, the average value of the developing bias exceeds SAD_AVE_TGT (%) + TARGET_H which is the upper limit value of the target potential, or SAD_AVE_TGT (which is the lower limit value of the target potential). %) + TARGET_L, a preset negative correction step number SAD2TC_ALL_MINUS or a preset positive correction step number SAD2TC_ALL_PLUS is set as a correction value for the toner density of all colors. Is increased (decreasing the toner charge amount) or the toner density decreasing (toner charge amount increasing). As a result, the average value of the developing bias is controlled to decrease or increase.

  Further, in any one of the developing devices 41, 42, 43, 44, when the DC component Vdc of the developing bias is less than the lower limit value Average-MIN (color) or exceeds the upper limit value Average-MAX (color) By adding a preset value SAD2TC_COLOR_PLUS from the correction value of the corresponding color or subtracting SAD2TC_COLOR_MINUS, the toner replenishment amount at the time of image formation decreases as a result, or the toner replenishment pauses. Alternatively, the toner amount is controlled to increase, and as a result, the developing bias of the corresponding color is controlled to increase or decrease.

  Further, when the toner density correction value determined as described above falls below a preset lower limit value −40, the toner density correction value is set to −40 and the preset upper and lower values are set. When the value exceeds 112, 112 is set as the toner density correction value.

  Further, the TC correction value (Color) of the toner density determined as described above is a value obtained by subtracting 32 from the difference between the reading value of the previous toner patch and the target value, and less than TCPach reading value−TGT value−32. Or the value obtained by adding 32 to the difference between the reading value of the previous toner patch and the target value exceeds the TCPach reading value−TGT value + 32, the TCPach correction value TC correction value for the toner density of the corresponding color The reading value−TGT value−32 or the TCPach reading value−TGT value + 32 is adopted.

  As described above, when the toner density correction value determined as described above exceeds a predetermined range, the correction value is limited to a predetermined value in advance, so that the second reference toner image is detected. It is possible to prevent the target density compared with the density from being changed excessively.

  Therefore, according to the above-described embodiment, it is possible to reliably prevent the ejection of toner from the developing devices 41, 42, 43, 44, and even when the control range of the developing bias is limited. Thus, the density balance of each color can be stabilized, the change in the color of process black can be suppressed, and the reproducibility of the image can be improved.

Experimental Example Next, the present inventors performed control as shown in FIG. 1 under various environments using four printers manufactured by Fuji Xerox Co., Ltd. and product name “DocuPrintC1616”. An experiment was conducted to confirm how the DC component of the developing bias of each color developing device 41, 42, 43, 44 changes.

  FIG. 15 is a graph showing the results of the above experimental example.

  As is apparent from FIG. 15, when the present invention is applied, in any printer, the DC component of the developing bias of each color developing device 41, 42, 43, 44 is between the upper limit value and the lower limit value. It is possible to reliably prevent toner from being ejected from the developing devices 41, 42, 43, 44, etc., and even when the development bias control range is limited, the density balance of each color It has been found that the color of the process black can be suppressed, and the reproducibility of the image can be improved.

  In FIG. 15, high temperature H / high humidity H is a temperature of 28 ° C., relative humidity 85%, medium temperature N / medium humidity N is temperature 22 ° C., relative humidity 55%, low temperature L / low humidity L is temperature 10 ° C. Relative humidity 15%, medium temperature N / low humidity L, temperature 20 ° C., relative humidity 10%, low temperature L / high humidity H environment, temperature 10 ° C., relative humidity 85%, high temperature H / low humidity L environment, temperature 28 Each shows an environment at a temperature of 30 ° C. and a relative humidity of 30%.

FIG. 1 is a flowchart showing a control operation in a tandem full-color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a configuration diagram showing a tandem type full-color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 3 is a block diagram showing an image forming unit of a tandem type full color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 4 is a schematic diagram showing a first reference toner image. FIG. 5 is a schematic diagram showing a second reference toner image. FIG. 6 is a block diagram showing the concentration detecting means. FIG. 7 is a schematic configuration diagram showing the concentration detecting means. FIG. 8 is a schematic configuration diagram showing the concentration detecting means. FIG. 9 is a block diagram showing a control circuit of a tandem type full color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 10 is a flowchart showing the density detection operation of the first and second reference toner images. FIG. 11 is an explanatory diagram showing a developing bias control operation in the developing device. FIG. 12 is an explanatory diagram showing the density detection operation of the first and second reference toner images in the density detection means. FIG. 13 is a graph showing the target potential of the developing bias in the developing device. FIG. 14 is an explanatory diagram showing a developing bias control operation in the developing device. FIG. 15 is a graph showing experimental results in the tandem type full-color printer as the image forming apparatus according to Embodiment 1 of the present invention.

Explanation of symbols

  100: optical density sensor, 200: first reference toner image, 2010: second reference toner image, 300: control circuit.

Claims (6)

Forming means for forming first and second reference toner images on the image carrier;
Density detecting means for detecting the optical density of the first and second reference toner images;
First calculating means for calculating an image forming condition based on a difference between a density detected by the density detecting means of the first reference toner image and a first target density;
Second calculating means for calculating a replenishment amount of the developer to the two-component developing means based on the difference between the density detected by the density detecting means of the second reference toner image and the second target density. In an image forming apparatus using a two-component developer,
Based on the difference between the density detected by the density detection means of the first reference toner image and the first target density, the value of the image forming condition calculated by the first calculation means exceeds the predetermined range. And a control means for controlling the image forming condition in a direction approaching a predetermined range by changing the second target density based on the value of the image forming condition itself. Image forming apparatus. 2. The first reference toner image is developed with a development bias in which an AC bias is superimposed on a DC bias, and the second reference toner image is developed with a development bias consisting of only a DC bias. The image forming apparatus described in 1. A plurality of image forming units that form images of different colors, and images of different colors formed by the respective image forming units are superimposed on a recording medium via at least one intermediate transfer member In the image forming apparatus for forming an image by transferring the image, the control unit calculates an average value of the image forming conditions of each image forming unit calculated by the first calculating unit, and When the average value itself of the image forming conditions of the image forming unit exceeds a predetermined range, the second target density is changed according to the average value itself of the image forming conditions, thereby the image forming conditions The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled in a direction approaching a predetermined range. A plurality of image forming units that form images of different colors, and images of different colors formed by the respective image forming units are superimposed on a recording medium via at least one intermediate transfer member In the image forming apparatus for forming an image by transferring the image, the control unit calculates an average value of the image forming conditions of each image forming unit calculated by the first calculating unit, and based on the average value of the image forming conditions of the image forming unit, when the average value itself of the image forming condition of said image forming units exceeds a predetermined range, in response to said average value itself of the image forming condition 3. The image forming condition is controlled in a direction approaching a predetermined range by changing a second target density. Image forming apparatus. When the second target density is changed, the control unit limits the change range of the second target density to a certain range based on the detection result of the previous second reference toner image. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled as follows. The image forming apparatus according to claim 1, wherein the control unit determines a predetermined range for determining an average value of the image forming conditions according to environmental conditions.

Images