JP3900815B2 - Color image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention employs an intermediate transfer system and can form a color image, such as a color printer or a color copier, capable of forming a color image constituted by appropriately combining toner images of at least yellow, magenta, and cyan color components. The present invention relates to a forming apparatus.
[0002]
[Prior art]
In recent years, as a color image forming apparatus such as a color printer and a color copying machine using an electrophotographic system, a type employing an intermediate transfer system is increasing. A color image forming apparatus employing such an intermediate transfer method basically has a plurality of colors (at least three colors of yellow: Y, magenta: M and cyan: C) formed on an image carrier such as a photosensitive drum by an electrophotographic process. However, when a full color image is formed, the toner image of 4 colors (black: K added thereto) is first transferred onto a belt-shaped or drum-shaped intermediate transfer body by a primary transfer device, and then the intermediate transfer body. The toner images of multiple colors transferred onto the recording medium are secondarily transferred to a recording paper such as plain paper, coated paper, or OHP sheet by a secondary transfer device, and the toner image transferred last is fixed to the recording paper P. Thus, a desired color image is formed.
[0003]
[Problems to be solved by the invention]
By the way, in an image forming apparatus employing such an intermediate transfer method, when forming a color image, a maximum of three color (or four color) toner images are superimposed on the intermediate transfer member. However, when the multi-color toner images are collectively transferred to the recording paper P, a part of the lowermost toner image (layer) on the intermediate transfer member is not transferred to the recording paper side. A phenomenon of remaining on the intermediate transfer member, so-called “transfer residue”, may occur. When such a transfer residue occurs, the amount of toner of the color that has not been transferred is reduced as compared with the normal amount, and accordingly, image quality defects such as color variation and color unevenness are more likely to occur. .
[0004]
In general, when a color image is formed so that a yellow (Y) toner image is the uppermost layer on the recording paper P, even if the above-described transfer residue and image quality defects occur due to the yellow toner image, As shown in FIG. 7, such image quality defects tend to be difficult to detect visually. Further, this tendency appears almost the same even if the type of the recording paper P is different.
[0005]
FIG. 7 shows a secondary transfer device when a toner image is formed and primary-transferred in the color order (Y / M / C / K order or C / M / Y / K order) shown in FIG. When the secondary transfer voltage applied to 250 is changed, the secondary transfer efficiency is changed to change the remaining color transfer state of the uppermost color toner image on the recording paper P in a pseudo manner. The occurrence of image quality defects with uneven color is examined, and the results are shown by grade. FIG. 6A shows the results when plain paper is used as the recording paper P, and FIG. 6B shows the results when coated paper (recording paper having a coating layer formed on the plain paper) is used as the recording paper P. Indicates. A smaller grade value indicates that there is no image quality defect and is in a good situation, while a larger value indicates that the image quality defect becomes more conspicuous and is in a worse situation. It should be noted that the grade of the image quality defect has a practically no problem if the numerical value is “2 or less”.
[0006]
For this reason, conventionally, an image forming process (particularly a toner image forming step and a primary transfer step) in which a yellow toner image is the uppermost layer on the recording paper P regardless of the type of paper is executed. Thus, measures are taken so that the image quality defect due to the untransferred image becomes less conspicuous (becomes insensitive to the image quality defect).
[0007]
On the other hand, when the color image is formed so that the yellow toner image is the uppermost layer on the recording paper P, as shown in FIG. 11, a color image in which a plurality of toner images are superimposed is formed. In particular, a phenomenon occurs in which the density in the region from the medium density to the high density stagnate and the density adjustment becomes difficult. FIG. 11 shows three toner concentrations (TMA: g / m ² ) Shows the result of measuring the density obtained with respect to the image input signal (Cin) in the case of using the developer composed of (3), and “YMCK-PK” in FIG. “YMCK-4FC” in the figure shows the result when the image is formed using four colors of toner of YMCK. Such stagnation of density occurs particularly when a black toner image is formed by superimposing three color (Y, M, C) or four color (Y, M, C, K) toner images. To do. In addition, when such density stagnation occurs, the color reproduction range that can be expressed tends to be narrow (see FIG. 9). Each of these problems, for example, when a high-quality color image such as a photographic image is formed using coated paper or the like, for example, insufficient color reproduction range greatly affects the quality of the image. It becomes a serious problem.
[0008]
By the way, the applicant of the present invention has developed an image formation in which a background portion is colored with respect to a character having a high density (for example, an image formation in which a multi-color toner image is superimposed on a halftone solid background portion). When color toner is developed and formed on the carrier, and the toner image is transferred onto the transfer paper by a plurality of times, the character portion is in order to prevent transfer failure (transfer omission) around the character portion. An image forming method is proposed in which color development and transfer are performed after the background portion (Japanese Patent Laid-Open No. 6-250483). According to this image forming method, the character portion and the background portion are separately developed and transferred in the above-described procedure, so that no step is generated between the character portion toner and the background portion toner. Will be prevented. However, simply adopting such an image forming method cannot solve the above-mentioned problems such as image quality defects due to residual transfer, stagnant density, and a decrease in color reproduction range.
[0009]
The present invention has been made in view of the above-described circumstances. When a color image is formed by adopting an intermediate transfer method, the above-described problem of image quality deterioration due to a transfer residue and density stagnation as described above. An object of the present invention is to provide a color image forming apparatus capable of appropriately avoiding the problem of a decrease in the color reproduction range.
[0010]
[Means for Solving the Problems]
The color image forming apparatus of the present invention capable of achieving the above object, as exemplified in FIG. 1, carries one toner image (T) of at least yellow, magenta and cyan color components constituting a color image. An image forming unit (2) that can be formed on the body (1) sequentially or individually on a plurality of image carriers, and the toner images (T) of the color components formed by the image forming unit (2). An intermediate transfer member (3) to be conveyed, and a primary transfer device (4) for transferring the toner image (T) from the image carrier (1) of the image forming unit (2) to the intermediate transfer member (3); An image forming apparatus including a secondary transfer device (5) that collectively transfers a plurality of color toner images transferred to the intermediate transfer member (2) by the primary transfer device (4) onto a recording sheet (P). And using plain paper (Pf) as the recording paper (P) Mode selection means (6) for selecting a plain paper mode for forming a color image and a coated paper mode for forming a color image using coated paper (Pc) as the recording paper (P), and this mode selection When the plain paper mode is selected by the means, the image forming process and the primary transfer process in which the yellow toner image (Ty) becomes the uppermost layer on the plain paper (Pf) are executed, and the mode selection means performs the coated paper. And a process switching means (7) for executing an image forming process and a primary transfer process in which the yellow toner image (Ty) is at least the second layer or less from the uppermost layer on the coated paper (Pc). It is characterized by having.
[0011]
Here, the mode selection means (5) is not particularly limited as long as it has a configuration having the above-described function, but usually, an operator of the image forming apparatus uses a recording sheet used for forming a color image. Color image formation with a configuration with a print condition setting function (operation panel, etc.) that allows arbitrary selection and setting of types (operation panel, etc.), or with an external connection device (personal computer, etc.) connected to the image forming apparatus In this case, a printing condition setting screen by software that can arbitrarily select and set the type of recording paper to be used is provided. Further, when the image forming unit (2) is configured to form a full-color image of four colors, a black toner image is formed in addition to the toner images composed of yellow, magenta, and cyan color components. Is done.
[0012]
According to such a color image forming apparatus, the image formation is performed so that the order (layer position) of the yellow toner image on the recording paper differs between when the plain paper mode is selected and when the coated paper mode is selected. And the primary transfer process is switched.
[0013]
That is, when the plain paper mode is selected, as shown on the left side of FIG. 1b, a color image in which the yellow toner image (Ty) is positioned on the uppermost layer on the plain paper (Pf) is formed. As a result, in the case of plain paper that is used at a relatively low frequency when a high-quality color image such as a photographic image is formed, even if the high-quality color image is formed, the stagnation of the above-described density and color reproduction are possible. Problems such as a decrease in the range become less noticeable. And, when a color image of medium or lower image quality that is not high image quality such as a photographic image is formed, even if an image quality defect such as transfer unevenness occurs due to transfer residue during the secondary transfer, The image quality defect becomes inconspicuous. Accordingly, when the plain paper mode is selected, the image quality defect due to the untransferred image is preferentially avoided, and the versatility of the plain paper is utilized.
[0014]
On the other hand, when the coated paper mode is selected, as shown on the right side of FIG. 1b, a color image in which the yellow toner image (Ty) is located on the coated paper (Pc) in at least the second or lower layer. Is formed. As a result, in coated paper frequently used for forming high-quality color images such as photographic images, the yellow toner image is not positioned on the uppermost layer on the coated paper. Although image quality defects may occur, when a high-quality color image is formed, density stagnation does not occur even in an area from medium density to high density, and the color reproduction range is widened. Therefore, when the coated paper mode is selected, the stagnation of density during the formation of a high-quality color image and thus the reduction of the color reproduction range are preferentially avoided, and the high image quality feasibility of the coated paper is utilized.
[0015]
In the color image forming apparatus, the process switching means (7) is configured such that when coated paper is selected by the mode selecting means, a yellow toner image is lower than both magenta and cyan toner images on the coated paper. It is desirable to set so as to execute the image forming process and the primary transfer process. In such a configuration, it is possible to further widen the color reproduction range.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
FIG. 2 shows a main part of the color image forming apparatus according to Embodiment 1 of the present invention.
[0017]
This color image forming apparatus uses, as an image carrier, toner images Ty, Tm, Tc, Tk of respective color components of yellow (Y), magenta (M), cyan (C), and black (K) based on color image information. An electrophotographic image forming unit 10 sequentially formed on the photosensitive drum 11, an intermediate transfer belt 20 as an intermediate transfer member to which each toner image T formed by the image forming unit 10 is transferred, and the intermediate transfer belt The secondary transfer device 15 transfers the toner image T from the photosensitive drum 11 of the image forming unit 10 to the image transfer unit 10, and the secondary transfer device 25 transfers the toner image T on the intermediate transfer belt 20 onto the recording paper P. The image forming apparatus basically includes a fixing device 30 that fixes the toner image on the recording paper P and a paper feeding unit 40 that stores and supplies the recording paper P. A one-dot chain line in the figure indicates a conveyance path of the recording paper P.
[0018]
The image forming unit 10 includes a photosensitive drum 11 that rotates at a predetermined speed in a direction indicated by an arrow A by a rotation driving source (not shown), a charger 12 that uniformly charges the surface of the photosensitive drum 11, and a charger 12. The surface of the photosensitive drum 11 charged in this way is exposed to a laser beam Bm corresponding to image information to form an electrostatic latent image, and the photosensitive drum 11 formed by the latent image forming apparatus 13. Developing devices 14y, 14m, 14c, and 14k for developing the electrostatic latent image with toner of predetermined colors (Y, M, C, and K), the primary transfer device 15 described above, and a photosensitive drum after the primary transfer. The drum cleaning device 16 that removes and removes untransferred toner remaining on the surface 11 and the static eliminator 17 that removes the electric charge on the photosensitive drum 11 after the cleaning constitute the main part.
[0019]
Among these, the photosensitive drum 11 is formed such that a photosensitive layer made of an inorganic photosensitive material or an organic photosensitive material is positioned on the outer peripheral surface of the drum. As the charger 12, a corona discharger such as a scorolon type is used, but a contact charging device such as a charging roll may be used. The latent image forming apparatus 13 is configured to modulate laser light emitted from a light source such as a semiconductor laser or a light emitting diode in accordance with image information and to expose the photosensitive drum 11 via various optical systems. . Further, when the image forming apparatus is a printer, the color image information in the latent image forming apparatus 13 uses image information after being input from an externally connected device such as a personal computer and subjected to image processing. When the image forming apparatus is a copying machine, image information that has been input from a color original reading apparatus and processed has been used. The four developing devices 14y, 14m, 14c, and 14k are arranged at equal intervals along the peripheral surface of the photosensitive drum 11, and each of the above-described yellow, magenta, cyan, and black color toners is accommodated. In addition, the toner is supplied to each developing position of the photosensitive drum 11 through the developing roll 14a. Each of the developing devices 14 is supported so as to be displaced between a developing position and a non-developing position (retracted position). As the primary transfer device 15, a charging roll to which a primary transfer voltage is applied is used, but a corona discharger may be used. An erase lamp was used as the static eliminator 17.
[0020]
The intermediate transfer belt 20 is stretched around a drive roll 21 that is rotationally driven by a rotational drive source (not shown), a plurality of driven rolls 22 and 23, and a backup roll 24 of a secondary transfer device, and the drive roll 21 and the driven roll. 22 is arranged so as to pass through the transfer position of the photosensitive drum 11 in each of the image forming units 10, and is rotated so as to circulate in the direction of arrow B by the drive roll 21. . This intermediate transfer belt 20 is composed of a belt base material in which an appropriate amount of a conductive material such as carbon black is contained in a synthetic resin such as polyimide, polycarbonate, or polyester, or rubber. A belt having a two-layer structure is used in which a lubricating paint such as resin is applied and the coated surface is a transfer surface. A belt cleaning device 28 that removes and cleans toner remaining on the intermediate transfer belt 20 after the secondary transfer is disposed at a position facing the drive roll 21 of the intermediate transfer belt 20 with respect to the belt 20. It is arranged so as to be able to contact and separate.
[0021]
The secondary transfer device 25 applies a secondary transfer voltage to the secondary transfer roll 26, which is disposed on the opposite side of the intermediate transfer belt 20 from the backup roll 24 so as to be able to contact and separate. It is comprised with the electric power feeding roll 27 arrange | positioned in contact. In the figure, reference numeral 26a denotes a cleaning blade for removing toner and the like adhering to the secondary transfer roll 26. The fixing device 30 is a roll-type fixing device, and rotates in a state where the heating roll 31 and the pressure roll 32 are in pressure contact with each other, and the recording paper P to be fixed is introduced into the pressure contact portion of both rolls. And let it pass.
[0022]
The paper feeding unit 40 is mainly composed of a plurality of paper trays 41a and 41b for stacking and storing a plurality of recording papers P, and a paper feeding device 42 for sending out the recording papers P stored in each paper tray 41 one by one. It is configured. In this example, “plain paper (Pf)” is stored as recording paper P in at least one paper feed tray 41a, and “coated paper (Pc) is used as recording paper P in the other paper feed tray 41b. ) "Can be accommodated. Each paper feed tray 41 is provided with paper type setting switches 43a and 43b for selecting and setting the type of recording paper P to be stored (plain paper, coated paper, etc.). In FIG. 2, reference numeral 51 denotes a pair of registration rolls that sends the recording paper P supplied from the paper feeding unit 40 in accordance with the secondary transfer timing, and 52 denotes a paper transport for transporting the recording paper P after the secondary transfer to the fixing device 30. Indicates belt.
[0023]
In addition, the color image forming apparatus is provided with a control system related to the image forming process as shown in FIG. In the figure, reference numeral 60 is a main control device that comprehensively controls the operation and the like of the entire image forming apparatus system. The main control device 60 is mainly composed of a central processing unit (CPU) and storage means (ROM 61, RAM, etc.). ) And the like.
[0024]
In the main controller 60, “plain paper mode” in which a plain paper Pf is used as the recording paper P and a color image is formed using the coated paper Pc as the recording paper P “ A mode selection means 70 for selecting “coated paper mode” is connected. The mode selection means 70 has a print condition setting function (setting input button, display screen, etc.) that allows the operator of the image forming apparatus to arbitrarily select and set the type of recording paper P used when performing color image formation. It consists of an operation panel provided. In addition, information of the paper type setting switches 43a and 43b in the paper feeding device 40 is input to the main control device 60.
[0025]
On the other hand, the main control device 60 includes an image forming unit controller 62 that comprehensively controls each operation in the image forming unit 10 (including rotation driving of the photosensitive drum and the operation of the image forming device), and the primary transfer device 15. A primary transfer unit controller 63 for controlling the operation (application of a primary transfer voltage, etc.), an intermediate transfer unit controller 64 for controlling each operation related to the intermediate transfer belt 20 (rotation drive of the belt, contact / separation operation of the cleaning device, etc.); Controls the secondary transfer unit controller 65 that controls the operation related to the secondary transfer device 25 (application of a secondary transfer voltage, etc.), and the various operations related to the fixing device 30 (rotation drive of the heating roll, voltage application for heating, etc.) A fixing unit controller 66 for controlling the sheet feeding device 40 and a sheet feeding unit controller 67 for controlling each operation of the sheet feeding device 40 (sheet feeding operation, detection of presence / absence of sheet) are connected. Among them, the image forming unit controller 62 mainly includes a charging unit 62a that controls operations such as voltage application of the charging device 12, a latent image forming unit 62b that controls operations such as exposure of the latent image forming device 13, and four developments. Control is performed by being subdivided into a developing unit 62c for controlling the developing operation and the contacting / separating operation of the apparatus 14.
[0026]
In the main control device 60, various control programs for the entire system are stored in a ROM (read only memory) 61 of the storage means, and each control operation is executed in accordance with those control programs and other detection information. It has come to be. Also, in this example, as one of the control programs, yellow toner as shown in FIG. 4B is obtained when the plain paper mode and the coated paper mode are selected on the operation panel 70 as mode selection means. A control program for switching the image forming and primary transfer processes so that the order (layer position) of the images on the recording paper P is different is stored. Thereby, the main controller 60 functions as a process switching means. That is, the main control device 60 performs the basic image forming / primary transfer process of a single color as shown in FIG. 4a when the plain paper mode is selected on the operation panel 70 as shown in FIG. Is selected, the control is performed to switch the order of the image formation / primary transfer process of yellow, magenta, and cyan.
[0027]
Next, the operation of this color image forming apparatus will be described.
[0028]
First, in this image forming apparatus, basically, as shown in FIG. 4 a, in the image forming unit 10, the photosensitive drum 11 that rotates in the direction of the arrow A is uniformly set to a desired charging potential by the charger 12. After being charged (charging process), a laser beam Bm corresponding to the image data color-separated from the latent image forming device 13 is scanned and exposed on the charged surface, whereby an electrostatic latent image having a desired latent image potential is formed. After that, the developing device 14 containing the toner of the color corresponding to the latent image is moved to the developing position, and the toner supplied from the developing device 14 via the developing roll 14a is used. Developed (development process). Subsequently, the one-color toner image T formed on the photosensitive drum 11 is transferred to the surface of the intermediate transfer belt 20 by the electrostatic transfer action of the primary transfer device 15 in the transfer portion (primary transfer step). ). After the primary transfer, the untransferred toner adhering to the surface of the photosensitive drum 11 is removed by the drum cleaning device 16, and then neutralized by the static eliminator 17 so that the surface potential becomes zero potential. In preparation for the toner image forming process.
[0029]
In this image forming apparatus, when a single color image is formed, the one-color toner image T primarily transferred onto the intermediate transfer belt 20 as described above is immediately conveyed to the secondary transfer apparatus 25 to be transferred onto the recording paper P. The process proceeds to the next transfer operation step. When a color image is formed by superimposing a plurality of color toner images, the formation (image formation) of each color component on the photosensitive drum 11 as described above and its The primary transfer process for each toner image is repeated for the number of colors. Thus, since one color toner image is primarily transferred every rotation on the intermediate transfer belt 20, the toner images of a plurality of colors are finally transferred so as to be superimposed. Note that the secondary transfer roll 26 and the belt cleaning device 28 are displaced away from the intermediate transfer belt 20 while image formation and primary transfer of each color toner image are being performed during color image formation. Yes.
[0030]
The toner image T primarily transferred to the intermediate transfer belt 20 in this way is conveyed to a secondary transfer position where the secondary transfer device 25 is located as the intermediate transfer belt 20 rotates. In accordance with this timing, the secondary transfer roll 26 and the belt cleaning device 28 are displaced to a position where they contact the intermediate transfer belt 20, and a predetermined recording paper P fed from the paper supply unit 40 is moved to the registration roll pair. 51 is fed between the intermediate transfer belt 20 and the secondary transfer roll 26 at the secondary transfer position. As a result, the toner image T on the intermediate transfer belt 20 is transferred to the recording paper P under the electrostatic transfer action of the secondary transfer device 25 as shown in FIG. 4B (secondary transfer step). Next, the recording paper P onto which the toner image T has been secondarily transferred is peeled off from the intermediate transfer belt 20 and then sent to the fixing device 30 by the paper transport belt 52, and is heated and pressurized by the heating roll 31 and the pressure roll 32. Fixing (fixing process). As described above, the basic color image forming process is completed.
[0031]
In this color image forming apparatus, when a full-color image of four colors is formed, the image formation and primary transfer of the four color toner images of yellow, magenta, cyan, and black constituting the color image are repeated. At this time, when the “plain paper mode” is selected on the operation panel 70 as the mode selection means and when the “coated paper mode” is selected, toner image formation and primary transfer processes (processes) of the respective colors are performed. ) Are executed with appropriate switching as shown in FIG. 4b.
[0032]
That is, when “plain paper mode” is selected, as shown in the upper left part of FIG. 4B, image formation and primary transfer of toner images of each color are executed in the order of yellow, magenta, cyan, and black. . That is, each time the photosensitive drum 11 rotates, yellow, magenta, cyan, and black toner images Ty, Tm, Tc, and Tk are formed in this order and are primarily transferred to the intermediate transfer belt 20.
[0033]
As a result, the yellow toner image Ty, the magenta toner image Tm, the cyan toner image Tc, and the black toner image Tk are primarily transferred onto the intermediate transfer belt 20 in this order as shown in FIG. 5A. Then, the multiple toner images T primarily transferred in this way are collectively transferred to the plain paper Pf supplied from the paper feeding device 40, thereby being transferred onto the plain paper Pf as shown in FIG. 5b. From the bottom, the multiple toner images stacked in the order of the black toner image Tk, the cyan toner image Tc, the magenta toner image Tm, and the yellow toner image Ty are transferred. As a result, a full color image in which the yellow toner image Ty is located in the uppermost layer is formed on the plain paper Pf.
[0034]
On the other hand, when the “coated paper mode” is selected, as shown in the lower left part of FIG. 4B, image formation and primary transfer of toner images of the respective colors are executed in the order of cyan, magenta, yellow, and black. . That is, each time the photosensitive drum 11 rotates, cyan, magenta, yellow, and black toner images Tc, Tm, Ty, and Tk are formed in this order and are primarily transferred to the intermediate transfer belt 20.
[0035]
As a result, the cyan toner image Tc, the magenta toner image Tm, the yellow toner image Ty, and the black toner image Tk are primarily transferred onto the intermediate transfer belt 20 in this order as shown in FIG. 6A. Then, the multiple toner image T primarily transferred in this way is collectively transferred onto the coated paper Pc supplied from the paper feeding device 40, and as shown in FIG. The multiple toner images stacked in the order of the black toner image Tk, the yellow toner image Ty, the magenta toner image Tm, and the cyan toner image Tc are secondarily transferred. As a result, a full-color image in which the yellow toner image Ty is positioned at the lowest layer among the three colors (yellow, magenta, and cyan) is formed on the coated paper Pc.
[0036]
As described above, FIG. 7 shows color fluctuations and color unevenness when the transfer remaining state of the uppermost color toner image on the recording paper P is changed in a pseudo manner by changing the secondary transfer voltage. The occurrence of image quality defects is examined and the results are shown by grade. As is clear from the results of FIG. 7, in both cases of plain paper and coated paper, toner images are formed and primary transfer is performed in the order of Y / M / C / K, and the recording paper P is printed. When the color image is formed so that the yellow Y toner image Ty is the uppermost layer, the toner image is formed and the primary transfer is performed in order of C / M / Y / K on the recording paper P. It can be seen that color fluctuations and image quality defects such as color unevenness tend to be less noticeable than when color images are formed so that the yellow Y toner image Ty is the lowest layer among the three colors.
[0037]
FIG. 8 shows the result of measuring the relationship between the image input signal and the density when the coated paper mode is selected in this color image forming apparatus, and is the same as in FIG. As is clear from FIG. 8, when a color image is formed so that the yellow toner image Ty is the lowest layer on the coated paper by forming and primary transferring the toner images in the CMYK color order (FIG. 6b), It can be seen that the density in the region from the medium density to the high density is less likely to stagnate and the density can be adjusted well.
[0038]
FIG. 9 shows a color reproduction range in this color image forming apparatus. ^* b ^* FIG. 4 is a chromaticity diagram of a color system, showing a case where toner images are formed and primary transferred on coated paper in the order of YMCK and a case where toner images are formed and primary transferred in the order of CMYK. ing. In the figure, the solid line connects the maximum values of the color regions (G: green, C, B, M, R: red, Y) when toner images are formed and primary transfer is performed in the order of YMCK. The dotted lines are lines connecting the maximum values of the respective color regions when toner images are formed and primary transfer is performed in the order of CMYK. As is apparent from the results of FIG. 9, when the toner image formation and primary transfer are performed in the order of CMYK as in the coated paper mode selection, the toner image formation and primary transfer are performed in the order of YMCK. It can be seen that the color reproduction range in the color region of green and red (inside the circle surrounded by the alternate long and short dash line in the figure) is wider than that in the case of performing.
[0039]
From the above, in this color image forming apparatus, when a high-quality color image such as a photographic image is formed on plain paper, the density as described above is due to the difficulty in improving the image quality due to the paper quality. In the first place, the problem of stagnation and a decrease in the color reproduction range is not conspicuous in the first place, and when a color image of medium or lower quality that is not high quality is formed, the color image in which the image quality defect due to the transfer residue as described above is not noticeable. Is obtained. On the other hand, when a high-quality color image is formed on coated paper, the stagnation of density as described above is reduced, the color reproduction range is widened, and a high-quality color image with excellent color reproducibility can be obtained.
[0040]
Therefore, according to this color image forming apparatus, when the plain paper mode is selected, it is possible to satisfactorily form a color image with a medium or lower image quality so that image quality defects such as color unevenness are not noticeable. When coated paper mode is selected, high-quality color images such as photographic images can be well formed with excellent color reproducibility. Therefore, the versatility of plain paper and the high image quality of coated paper Both feasibility can be achieved.
[0041]
[Other embodiments]
In the first embodiment, when the coated paper mode is selected, toner image formation and primary transfer are performed so that the yellow toner image Ty is the lowest layer (FIG. 6b) among the three colors on the coated paper Pk. The toner image may be formed and primary transferred so that the yellow toner image Ty is the second to third layer of the three colors on the coated paper Pk. In this case, the uppermost layer on the coated paper Pk is either the magenta toner image Tm or the cyan toner image Tc.
[0042]
As described above, regarding the color reproduction range when the color image is formed so that the yellow toner image is the second layer, when the magenta toner image Tm is the uppermost layer, the yellow toner image Ty is the uppermost layer. Compared to FIG. ^* b ^* The characteristics of the green region G in the color system chromaticity diagram are not changed, but the characteristics of the red R region show values in the middle (between the solid line and the dotted line). Similarly, when the cyan toner image Tc is the uppermost layer, a in FIG. ^* b ^* The characteristic of the red R region in the chromaticity diagram of the color system does not change, but the characteristic of the green region G shows an intermediate value (between the solid line and the dotted line). In any case, the color reproduction range is wider than the color reproduction range (in the line surrounded by the solid line in FIG. 9) in the case of performing toner image formation and primary transfer in the order of YMCK. .
[0043]
In the first embodiment, the case where the image forming unit is configured to sequentially form toner images of four colors Y, M, C, and K on one photosensitive drum 11 is described. In the present invention, as illustrated in FIG. 10, four image forming units 10Y, 10M, 10M, 10M, 10M, 4M, 4M, 4M, 10M, 10M, and 10K, which respectively form toner images of four colors Y, M, C, and K on one photosensitive drum 11, respectively. It is also possible to apply 10C and 10K. In FIG. 10, parts that are the same as those in the image forming apparatus of FIG.
[0044]
Also in the color image forming apparatus including such an image forming unit, the toner image forming process and the primary transfer process are performed when the plain paper mode is selected and when the coated paper mode is selected, as in the first embodiment. Configure to switch. However, in this color image forming apparatus, the image forming units 10Y, 10M, 10C, and 10K are particularly arranged in this order along the traveling direction of the rotation direction B of the intermediate transfer belt 20 as shown in FIG. In this case, when the plain paper mode is selected, it is possible to perform toner image formation and primary transfer in the YMCK color order at a time by one pass of the intermediate transfer belt 20, but when the coated paper mode is selected, For toner image formation and primary transfer in CMYK color order, the intermediate transfer belt 20 is rotated once for primary transfer of the cyan toner image Yc and for primary transfer of the magenta toner image Tm. It is necessary to rotate the intermediate transfer belt 20 only twice more than when the mode is selected.
[0045]
In the first embodiment, the case where the intermediate transfer belt 20 having a belt shape is used as the intermediate transfer member has been described. However, in the present invention, an intermediate transfer drum having a drum shape may be used. The effect is almost the same as that of the intermediate transfer belt. Further, at least one of the plain paper and the coated paper is not limited to the configuration in which the paper is fed from the storage tray 41 of the paper feed unit 40 as in the first embodiment, and the paper is fed from a manual feed tray or the like. It may be configured.
[0046]
【The invention's effect】
As described above, according to the present invention, the order (layer position) of the yellow toner image on the recording paper is different between when the plain paper mode is selected and when the coated paper mode is selected. Since the image and primary transfer processes are switched as appropriate, when the intermediate transfer method is used to form a color image, the above-mentioned problems of image quality deterioration due to residual transfer, density stagnation, and color reproduction range decrease. This problem can be avoided appropriately and in a balanced manner. As a result, it is possible to provide a color image forming apparatus that can achieve both the versatility of plain paper and the high image quality realization of coated paper.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a basic configuration of the present invention.
FIG. 2 is a schematic diagram illustrating a basic configuration of a color image forming apparatus according to a first embodiment.
FIG. 3 is a block diagram showing a configuration of a control system related to an image forming process.
FIG. 4 is an explanatory diagram showing a basic image forming and primary transfer process flow for a single color toner image, and a color image forming process when a plain paper mode is selected and when a coated paper mode is selected.
FIG. 5 is a main part explanatory view showing a layer state of a multiple toner image on an intermediate transfer belt and plain paper when the plain paper mode is selected.
FIG. 6 is a main part explanatory diagram illustrating a layer state of a multiple toner image on an intermediate transfer belt and coated paper when a coated paper mode is selected.
FIG. 7 is a graph showing the relationship between the secondary transfer voltage (transfer residual) and the occurrence state of image quality defects when plain paper and coated paper are used.
FIG. 8 is a graph showing density measurement results for an image input signal when a coated paper mode is selected.
FIG. 9A illustrates a color reproduction range depending on a difference in image forming order of toner images. ^* b ^* A chromaticity diagram of a color system.
FIG. 10 is a schematic diagram showing a basic vegetable configuration of a color image forming apparatus (a tinedem type in which a plurality of image forming units are arranged on an intermediate transfer belt) according to another embodiment.
FIG. 11 is a graph showing density measurement results for an image input signal when toner images are formed and primary transfer is performed in the order of YMCK.
[Explanation of symbols]
10 (2) ... Image forming unit, 11 (1) ... Photosensitive drum (image carrier), 20 (3) ... Intermediate transfer belt (intermediate transfer body), 15 (4) ... Primary transfer device, 25 (5) ... Secondary transfer device, 60 (7) ... main control device (process switching means), P ... recording paper, Pf ... plain paper, Pk ... coated paper, T ... toner image, Ty ... yellow toner image, Ym ... magenta toner image , Tc: Cyan toner image.

Claims (2)

An image forming unit capable of forming each toner image of color components of at least yellow, magenta and cyan constituting a color image sequentially on one image carrier or individually on a plurality of image carriers, and the image forming unit. An intermediate transfer member that carries and conveys the toner images of the color components, a primary transfer device that transfers the toner image from the image carrier of the image forming unit to the intermediate transfer member, and the intermediate transfer member by the primary transfer device. In an image forming apparatus comprising a secondary transfer device that collectively transfers a plurality of color toner images transferred to a transfer body onto a recording sheet,
Mode selection means for selecting a plain paper mode for forming a color image using plain paper as the recording paper and a coated paper mode for forming a color image using coated paper as the recording paper, and this mode When the plain paper mode is selected by the selection means, the image forming process and the primary transfer process in which the yellow toner image is the uppermost layer on the plain paper are executed, and when the coated paper is selected by the mode selection means Comprises a process switching means for executing an image forming process and a primary transfer process in which the yellow toner image is at least the second layer or less from the uppermost layer on the coated paper. When the process switching means selects the coated paper by the mode selection means, the image forming process and the primary transfer process in which the yellow toner image is below the magenta and cyan toner images on the coated paper are executed. The image forming apparatus according to claim 1, wherein the image forming apparatus is set so as to.

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