JP2012027076A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which is capable of increasing secondary transfer efficiency, when transparent toner is used and capable of suppressing the degradation of image quality due to the scattering of toner.SOLUTION: In such a configuration that a transparent toner image made of transparent toner T and a color toner image made of a plurality of color toners Y and M are laminated in this order on an intermediate transfer belt 21 and secondarily transferred to a paper S altogether, a secondary transfer bias voltage is obtained by superimposing an AC on a DC and the transparent toner image comprises an overlapping area 96 where the transparent toner image overlaps with the color toner image and a widened area 95 widened by a predetermined width with respect to the overlapping area 96. The transparent toner image is secondarily transferred to the paper S and then, covers the color toner image and transparent toner Ta constituting the widened area 95 regulates the movements of the color toners Y and C in secondary transfer, to suppress the scattering of the color toners.

Description

  The present invention relates to an image forming apparatus that forms a color toner image and a transparent toner image of different colors.

  In recent years, image forming apparatuses capable of forming color images include, for example, photosensitive drums for different colors of Y (yellow), M (magenta), C (cyan), and K (black) as intermediate transfer belts. The toner images formed on the respective photosensitive drums are transferred in a multiple manner so as to be sequentially superimposed on the intermediate transfer belt, and the multiple transferred toner images are collectively recorded at the secondary transfer position. There is a so-called tandem system that performs secondary transfer.

  In Patent Document 1, in the tandem method, a dedicated photosensitive drum for forming a transparent toner image is arranged along the arrangement direction of the Y to K color photosensitive drums in addition to the colored color toner image. The structure to perform is disclosed. In this configuration, a transparent toner image is further formed on the color toner image so that the transparent toner is embedded in the concave and convex portions formed on the surface of the color toner image composed of Y to K toner particles superimposed on the recording sheet. Is used to increase the glossiness of the color image by making the amount of applied toner uniform throughout the image.

JP 2009-37102 A

As described above, when a transparent toner image is used to improve the glossiness of a color image, a multi-toner layer of Y to K colors and a transparent toner layer are laminated on the intermediate transfer belt, and these laminated toner images are recorded. The secondary transfer needs to be performed collectively on the sheet.
In secondary transfer, it is desired to transfer all of the toner image on the intermediate transfer belt to a recording sheet. However, in practice, a certain percentage (for example, about 5%) of toner particles is not transferred and the intermediate transfer belt is not transferred. The remaining toner is removed by a cleaner.

When a transparent toner is used in addition to the four color toners, the amount of toner to be secondarily transferred is increased as compared with the configuration using only the four color toners. The ratio of the residual toner remaining in the toner tends to increase, and when the ratio of the residual toner increases, the secondary transfer efficiency decreases, leading to deterioration of the image quality of the reproduced image.
A DC voltage is normally used as the bias voltage for secondary transfer. To increase the secondary transfer efficiency, the value of the DC voltage is increased. However, if the value of the DC voltage is increased too much, Discharge occurs at the secondary transfer position, and the secondary transfer efficiency may decrease.

If a transparent toner is to be used in addition to the four-color toner, the DC voltage value for the four-color toner will be insufficient, and the voltage will be further increased. There is a limit.
If a method of superimposing an AC voltage on a DC voltage is used as the bias voltage, the toner particles electrostatically adsorbed on the intermediate transfer belt vibrate due to the AC component, and the toner particles are easily separated from the intermediate transfer belt. Secondary transfer efficiency can be increased.

However, due to the addition of an AC component, when toner particles on the intermediate transfer belt try to move from the intermediate transfer belt to the recording sheet during the secondary transfer, Therefore, so-called toner scattering that moves toward a position deviated from the original position of the toner tends to occur.
When toner splatters, color reproducibility deteriorates in an image having gradation such as a photograph, and the image quality of the reproduced image decreases, such as a thin character in an image of a thin character.

  The present invention has been made in view of the above problems, and can improve the secondary transfer efficiency even when using a transparent toner, and image formation capable of suppressing deterioration in image quality due to toner scattering. The object is to provide a device.

  In order to achieve the above object, an image forming apparatus according to the present invention forms a color toner image and a transparent toner image of different colors on each of a plurality of image carriers, and the formed transparent toner images of different colors. An image forming apparatus in which each color toner image is multiplex-transferred onto an intermediate transfer member in this order, and then the toner image multiplex-transferred onto the intermediate transfer member is secondarily transferred onto a recording sheet by a secondary transfer unit, Voltage output means for applying to the secondary transfer means a voltage in which alternating current is superimposed on direct current as a secondary transfer bias, so that the color toner image is covered with the transparent toner image on the recording sheet after the secondary transfer; In addition, the transparent toner image is formed in an extended region obtained by expanding a region overlapping with a formation region of the color toner image to be transferred.

And a setting unit configured to set the extended area to an area including the overlapping area obtained by widening the overlapping area by a predetermined amount, and forming a transparent toner image in the set extended area. To do.
Here, the setting means determines a region where a color toner image of at least one color is formed in one page based on image data for forming the toner images of different colors in units of pages. A first setting means for setting the color toner image area in the one page as an overlapping area of the transparent toner image by the predetermined amount from the outline of the overlapping area. A second setting unit configured to set an area separated from the overlapping area as an enlarged area, and to set an area composed of the overlapping area and the enlarged area as the extended area;

Here, the first setting unit specifies an image area in which a color toner image is formed in the one page for each different color, and performs an OR operation on the specified image area for each color. The color toner image area is set.
In addition, the predetermined amount may be changed to a larger value as the number of color toner layers stacked in the color toner image formation region to be multiplex-transferred onto the intermediate transfer member is increased.

  In this way, while the secondary transfer efficiency is improved by the AC component of the secondary transfer bias voltage, the color toner image is transferred when the toner image on the intermediate transfer member moves from the intermediate transfer member to the recording sheet during the secondary transfer. Toner particles are surrounded by the toner particles constituting the expanded portion of the transparent toner layer, so that the toner particles are not greatly deviated from the trajectory even under the influence of the vibration due to the AC component, and the scattering is difficult to occur. On the other hand, some of the toner particles constituting the expanded portion of the transparent toner may be scattered under the influence of vibration, but since it is transparent, it does not cause a reduction in the reproduced image, thereby suppressing a reduction in image quality.

1 is a diagram illustrating an overall configuration of a printer according to an embodiment. It is a block diagram which shows the structure of the control part with which a printer is equipped. 6 is a flowchart showing the contents of color toner image formation region specifying processing. 6 is a flowchart showing the content of a transparent toner image formation region setting process. It is a schematic diagram which shows notionally the content of each said process. FIG. 4 is a schematic diagram illustrating a state in which toner particles move from an intermediate transfer belt to a sheet during secondary transfer. FIG. 5 is a diagram illustrating an example of conditions for forming a color toner image and a transparent toner image, divided into an example and a comparative example. It is a figure which shows the example of the experimental result when a color toner image and a transparent toner image are formed on the conditions of FIG.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described using a tandem color digital printer (hereinafter simply referred to as “printer”) as an example.
(1) Overall Configuration of Printer FIG. 1 is a diagram illustrating the overall configuration of the printer 100.
As shown in the figure, the printer 100 forms an image by a well-known electrophotographic system, and includes an image processing unit 10, a transfer unit 20, a feeding unit 30, a fixing unit 40, and a control unit 50. When a print job execution instruction is received from an external terminal device (not shown) connected to a network (for example, a LAN), the colored yellow, magenta, cyan, and black colors are received based on the instruction. The color image formation is executed. Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and Y, M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.

The image processing unit 10 includes image forming units 10Y, 10M, 10C, and 10K corresponding to Y to K colors, and an image forming unit 10T for forming a toner image using transparent toner.
Image forming units 10Y to 10K are photosensitive drums 1Y to 1K, chargers 2Y to 2K disposed around the photosensitive drums 1Y to 1K, exposure units 3Y to 3K as an example of image writing means, developing units 4Y to 4K, and photosensitive drums. Cleaners 6Y to 6K for cleaning the toner, and Y to K color toner images are formed on the photosensitive drums 1Y to 1K.

  The same applies to the image forming unit 10T, which includes a photosensitive drum 1T, a charger 2T, an exposure unit 3T, a developing unit 4T, a cleaner 6T, and the like, and forms a transparent toner image with transparent toner on the photosensitive drum 1T. . Here, for the colorless transparent toner, for example, particles formed using a resin used for manufacturing a color toner such as Y color are used. The resin may be substantially transparent, and the material is appropriately selected. Specifically, for example, resins used for general toners such as polyester resins and polyacrylic resins and polymers thereof can be used. The properties of the transparent toner such as the particle size and charging property are set to the same particle size as that of the colored toner or to the particle size and charging property according to the apparatus configuration. Hereinafter, colorless may be referred to as T color.

  The transfer unit 20 includes an endless intermediate transfer belt 21 that circulates in the direction of the arrow, a driving roller 22 that stretches the intermediate transfer belt 21, a driven roller 23, and the intermediate transfer belt 21. Primary transfer rollers 5Y to 5K, 5T arranged to face 1K, 1T, a secondary transfer roller 35 arranged to face the driving roller 22 across the intermediate transfer belt 21, and a secondary transfer bias voltage output unit 36. .

The secondary transfer bias voltage output unit 36 includes a DC power supply 362 and an AC power supply 361 connected in series. The secondary transfer bias voltage output unit 36 outputs a voltage in which an AC is superimposed on a DC as a bias voltage for secondary transfer. Supply to roller 35.
The image forming units 10T, 10Y, 10M, 10C, and 10K arranged below the intermediate transfer belt 21 are in this order from the upstream side in the running direction of the intermediate transfer belt 21 (hereinafter referred to as “upstream side”) to the downstream side in the running direction. (Hereinafter referred to as “downstream side”) with a predetermined interval therebetween. In the present embodiment, since toner having a negative charging characteristic is used, the voltage used for transfer is a positive voltage for both primary transfer and secondary transfer.

The feeding unit 30 is fed to the paper feed cassette 31 that stores the paper S as a recording sheet, the feed roller 32 that feeds the paper S in the paper feed cassette 31 one by one onto the transport path 37, and the transport path 37. And a timing roller pair 33 for taking a timing to send the sheet S to the secondary transfer position 351.
The fixing unit 40 includes a fixing roller and a pressure roller, and heats and presses the sheet S at a predetermined fixing temperature to fix the toner image.

  The control unit 50 converts an image signal from an external terminal device into image data for each pixel for Y to K colors on a page basis, and generates a drive signal for driving the laser diodes of the exposure units 3Y to 3K. . Here, the page indicates the page number of the document to be output included in the print job, and the document image is printed on the paper S in units of one page. Note that one side of one sheet can be regarded as one page area and can be regarded as one page unit.

In addition, the control unit 50 sets a transparent toner image formation region in units of pages so that the transparent toner is superimposed on the Y to K color toner images in order to improve glossiness. A method for setting the formation area of the transparent toner image will be described later.
Then, image data for each pixel for forming an image of the transparent toner in the set transparent toner image formation region is generated, and a drive signal for driving the laser diode of the exposure unit 3T is generated. Hereinafter, the contents common to the formation of the color toner image and the formation of the transparent toner image will be described by omitting the reference symbols Y to T while indicating the fact as Y to T colors.

  The laser diode of the exposure unit 3 is driven for each of the Y to T colors by the drive signal generated in the control unit 50, the laser beam is emitted, and the photosensitive drum 1 is exposed and scanned (image writing). Prior to this exposure scanning, the photosensitive drum 1 is uniformly charged by the charger 2 for each of the Y to T colors, and an electrostatic latent image is formed on the photosensitive drum 1 by laser beam exposure. It is imaged.

  For each of Y to T colors, each electrostatic latent image is developed with toner by the developing device 4, and the developed toner image is intermediated by the action of electrostatic force generated by the electric field generated between the primary transfer roller 5 and the photosensitive drum 1. Primary transfer is performed on the transfer belt 21. At this time, for each of the Y to T colors, the image forming operation is performed by locating each pixel in one page on the XY plane (two-dimensional) coordinates of the X axis (main scanning direction) and the Y axis (sub scanning direction). In this case, the timing is shifted so that the pixels at the same coordinate position are located at the same position on the intermediate transfer belt 21 for each of the Y to T colors.

For example, when Y to K and T color images are present at the same position in the same page, the Y to K color toner images and the transparent toner image are at the same position on the intermediate transfer belt 21. It will be transferred in superposition. The position where the toner image on the photosensitive drum 1 is primarily transferred to the intermediate transfer belt 21 for each of the Y to T colors is the primary transfer position for that color.
The Y to T color toner images primarily transferred onto the intermediate transfer belt 21 move to the secondary transfer position 351 as the intermediate transfer belt 21 travels.

In accordance with the timing of the image forming operation, the paper S is conveyed from the feeding unit 30 via the timing roller pair 33. The paper S includes the intermediate transfer belt 21 that circulates and the sheet S. It is sandwiched and conveyed between the secondary transfer roller 35 that is in pressure contact.
A secondary transfer bias voltage (a voltage in which an alternating current is superimposed on a direct current) is applied to the secondary transfer roller 35 from the secondary transfer bias voltage output unit 36. An electric field for secondary transfer is generated between the secondary transfer roller 35 and the driving roller 22 via the transfer belt 21. The toner images on the intermediate transfer belt 21 are collectively transferred onto the sheet S by the action of electrostatic force generated by the electric field generated between the secondary transfer roller 35 and the drive roller 22. The position where the toner image on the intermediate transfer belt 21 is secondarily transferred to the paper S is the secondary transfer position 351.

The sheet S that has passed the secondary transfer position 351 is conveyed to the fixing unit 40, where the toner image is heated and pressurized and fixed on the sheet S, and then discharged outside the apparatus via the discharge roller pair 38. In the storage tray 39.
(2) Configuration of Control Unit 50 FIG. 2 is a block diagram showing the configuration of the control unit 50.

As shown in the figure, the control unit 50 includes a communication interface (I / F) unit 51, a CPU 52, a ROM 53, an image processing unit 54, an image memory 55, a color toner image formation region specifying unit 56, and a transparent component as main components. A toner image forming area setting unit 57 and the like are provided.
The communication I / F unit 51 is an interface for connecting to a LAN, such as a LAN card or a LAN board, receives print job data from the outside, and sends the received data to the image processing unit 54.

The image processing unit 54 converts the image signal included in the print job data from the communication I / F unit 51 into image data of reproduction colors Y to K in units of pages, and outputs the image data to the image memory 55. Data is stored for each reproduction color.
The ROM 53 stores a control program related to the image forming operation.
The CPU 52 reads a necessary program from the ROM 53 and controls the operations of the image processing unit 10, the transfer unit 20, the feeding unit 30, etc. in a unified manner and performs a smooth printing operation. In addition, the color toner image forming area specifying unit 56 is instructed to execute a color toner image forming area specifying process for specifying the forming area of the color toner image to be formed on the page for each page.

Further, the transparent toner image forming area setting unit 57 is instructed to set a transparent toner image forming area for the color toner image forming area specified by the color toner image forming area specifying unit 56. Is executed.
FIG. 3 is a flowchart showing the contents of the color toner image forming area specifying process, FIG. 4 is a flowchart showing the contents of the transparent toner image forming area setting process, and FIG. 5 conceptually shows the contents of each process. It is a schematic diagram shown. Here, the processing of FIGS. 3 and 4 is repeatedly executed for each page in page units. In FIG. 5, an area for one page when the image data is expanded in the memory area is indicated by a broken line P. Note that the memory area may be an area in the image memory 55 or an area of an internal memory (not shown) including another RAM.

  As shown in FIG. 3, in the color toner image formation area specifying process, based on the Y color image data stored in the image memory 55, the Y color image area to be formed on the nth page, for example, the first page is selected. Specify (step S1). Specifically, among all the pixels in the first page for the Y color, an image area (toner image of the toner image) excluding a pixel whose gradation value is a value (for example, zero) indicating a non-image area (a portion where no toner image is formed). A method of specifying a pixel of a portion to be formed) by a coordinate position is used. Other methods may be used as long as the pixels constituting the image area can be determined.

FIG. 5A shows an example in which one Y-color image Ty exists on the first page, and an area indicating the image Ty is specified as the Y-color image area 91y, and an area other than the area 91y is shown. An example is shown in which 90y is a non-image area in Y color.
The pixels identified as the pixels constituting the image area 91y are exposed to the photosensitive drum 1Y, and become Y-colored toner images by development, and the pixels not identified are not exposed. , Pixels constituting a non-image area 90y where a Y-color toner image is not formed.

Returning to FIG. 3, in step S <b> 2, the M image area to be formed on the first page is specified based on the M color image data stored in the image memory 55. This specifying method is the same as in the case of Y color.
In step S <b> 3, based on the C color image data stored in the image memory 55, the C color image region to be formed on the first page is specified. This specifying method is the same as in the case of Y color. FIG. 5B shows an example in which a region 91c indicating a C color image Tc is specified as a C color image region. Note that the area 90c other than the area 91c is a non-image area in the C color.

In step S4, based on the K color image data stored in the image memory 55, the K color image region to be formed on the first page is specified. This specifying method is the same as in the case of Y color.
In step S5, an area obtained by ORing the Y to K color image areas specified in steps S1 to S4 on the nth page, here the first page, is set as a color toner image forming area, and this processing is performed. Exit. Here, taking the logical sum means specifying a pixel that belongs to an image area of at least one of the colors Y to K among all the pixels on the first page, and the coordinate position of each specified pixel. Is memorized.

  FIG. 5C shows a region 92 (regions 91y and 91c partially overlap each other) obtained by adding the Y-color image region 91y of FIG. 5A and the C-color image region 91c of FIG. 5B. In this example, the color toner image forming area 92 obtained by calculating the logical sum of the areas 91y and 91c is set. The color toner image forming area 92 is an area composed of pixels belonging to at least one image area of Y to K among all the pixels in one page of the first page document. Indicates a non-image area (non-image area of the first page) where none of the images of Y to K colors exist on the first page. If the area where the color toner image should be formed can be set, the method is not limited to the above method. For example, the non-image area is determined based on the density of each pixel, and pixels that do not belong to the determined non-image area are detected. The method of setting to can also be used.

In the figure, although only one rectangular color toner image forming area 92 exists in one page, a plurality of different areas exist apart from each other, or a rectangular color toner image forming area 92 exists in a rectangular shape. The shape of the image to be formed is not limited, and various closed shapes such as a circular shape, a dot shape, and a line shape indicating the outline in the case of a character or ruled line image can be specified.
In the transparent toner image formation region setting process, as shown in FIG. 4, contour pixels constituting the contour of the color toner image formation region on the nth page, in the above example, the first page are obtained (step 11). In step S5 described above, the coordinate position of each pixel in the color toner image forming area is stored, so that a contour pixel is detected based on the coordinate position of each pixel.

  A method for detecting a contour pixel is, for example, a method for detecting a pixel of interest as a contour pixel when the arrangement of pixels arranged around the pixel of interest matches a pattern for detecting a contour image (Japanese Patent No. 2836992). ) Etc. can be used. Other detection methods may be used. FIG. 5D is a diagram showing a pixel row composed of the contour pixels 94 of the color toner image forming region 92, and shows an example in which the contour is the same as the contour of the color toner image forming region 92.

  Returning to FIG. 4, in step S <b> 12, in the first page of the non-image region 93, a widened region configured by a pixel group existing within a width corresponding to the size of two pixels from the contour pixel 94 is set. . Here, the widened area is the first neighborhood of the pixels existing in the non-image area 93 of the first page when each of the outline pixels 94 constituting the outline of the color toner image forming area 92 is the target pixel in order. Pixels (pixels adjacent to the pixel of interest and existing in the first pixel around the pixel of interest) and second neighboring pixels (pixels existing in the second pixel around the pixel of interest and around the first neighboring pixel 1) An area of a pixel group consisting of pixels existing in the pixel) is shown.

The widened area is set by setting the first and second neighboring pixels in the non-image area 93 with respect to the contour pixel 94 to pixels corresponding to the coordinate positions of the contour pixel 94 in the first and second neighborhoods. This is done by specifying the coordinate position.
FIG. 5E shows an example of the widened area 95 with respect to the color toner image forming area 92. As shown in FIG. 5 (e), the widened area 95 is annular so as to surround a rectangle (corresponding to the shape of the color toner image forming area 92) composed of the pixel rows of the contour pixels 94. Is a size corresponding to two pixels. Since the widened area 95 is a part of the non-image area 93 of the first page, a color image of any color from Y to K is not formed.

  Returning to FIG. 4, in step S <b> 13, the transparent toner formation region on the first page is the same as the color toner image formation region 92 on the first page in the same coordinate position (the color image region overlaps after the primary transfer). Corresponding to the region: overlapping region) 96 (FIG. 5E) and the expanded region 97 (FIG. 5E) composed of the widened region 95 are set, and the process is terminated. In this sense, the extended region 97 corresponds to a region including the overlapping region 96 formed by expanding the overlapping region 96 by a predetermined amount (a width corresponding to two pixels).

  When executing the image formation for the first page, the CPU 52 reads the image data stored in the image memory 55 for the colors Y to K, and drives the exposure units 3Y to 3K based on the read image data. However, image data for each pixel for the transparent toner image is generated so that the transparent toner image is formed in the extended region 97 set by the transparent toner image formation region setting unit 57, and exposure is performed based on the generated image data. The unit 3T is driven.

The image forming operation for each of these Y to T colors is performed so that the pixels located at the same coordinate position within one page for each Y to T color are primarily transferred to the same position on the intermediate transfer belt 21 as described above. The intermediate transfer belt 21 is shifted by a predetermined time in the order of T, Y, M, C, and K colors, which are the order of arrangement of the intermediate transfer belt 21 from the upstream side to the downstream side.
Here, for the predetermined time, for example, the peripheral speed of the Y to T color photosensitive drums 1 and the intermediate transfer belt 21 is V, and the distance from the exposure position on the peripheral surface of each photosensitive drum to the primary transfer position in the circumferential direction. L, if the distance in the belt running direction on the intermediate transfer belt 21 between one primary transfer position and the other primary transfer position of two adjacent photoconductor drums is Q, the start of T color image formation (photoconductor Y-color image formation (exposure scan of the photosensitive drum 1Y) is started with a delay of (Q / V) seconds from the reference, and the M-color image formation from the reference ( (2Q / V) second is started, C color image formation is started (3Q / V) second from the reference, and K color image formation is started (4Q / V) second from the reference. Can be configured.

FIG. 5 (f) shows that, for the first page, after the transparent toner image 98 is primarily transferred onto the intermediate transfer belt 21, the color toner image 99 composed of Y and C images Ty and Tc is transparent by the primary transfer. 3 is a diagram showing a state in which the toner image is stacked on the toner image 98 in a plan view. FIG.
In this way, when the transparent toner image 98 is formed wider than the color toner image 99, and the transparent toner image 98 and the color toner image 99 are laminated on the intermediate transfer belt 21, the color toner is seen in plan view. The reason why the layer composed only of the transparent toner image 98 (corresponding to the widened area 95) is provided around the image 99 is to suppress the image quality deterioration due to toner scattering during the secondary transfer.

  That is, the secondary transfer efficiency can be improved by changing the secondary transfer bias from a DC voltage to a voltage in which an AC is superimposed on the DC as described above, but the AC component is included in the secondary transfer bias. The toner particles of the toner image formed on the intermediate transfer belt 21 vibrate, so that among the toner particles that move from the intermediate transfer belt 21 toward the paper S by electrostatic force during secondary transfer, The number of toner particles that deviate from the orbit and go in the other direction increases, and when the toner particles scatter to the surroundings, the image quality deteriorates.

  In the central portion of the image area, a large number of toner particles are concentrated, so that the movement of the toner particles that come into contact with the adjacent toner particles and deviate from the original trajectory is easily restricted, and scattering is unlikely to occur. On the other hand, in the outline of the image area, adjacent toner particles exist inside the outline, but in two dimensions, the toner particles are outside the outline (outside of the image area: non-image area). Since it does not exist, it is not restricted in the direction toward the outside with respect to the toner particles constituting the contour, and it becomes free and tends to scatter off the original trajectory.

  Therefore, in the present embodiment, a widened region 95 made up of a colorless transparent toner region is provided around the colored color image region, and the color toner particles constituting the contour of the color image region (corresponding to the contour pixel 94) are provided. On the other hand, by creating a state in which transparent toner particles for regulating movement in the direction toward the outside of the color image area exist around the color image area, it is difficult for the color toner particles to scatter and suppresses deterioration in image quality. can do. Note that the transparent toner particles formed around the color image area are areas corresponding to the outline of the color image area, so that toner scattering is likely to occur during secondary transfer. Even if the splattering occurs, it does not reach a level where the image quality is lowered by human eyes.

FIG. 6 is a schematic diagram showing how toner particles move from the intermediate transfer belt 21 to the paper S during secondary transfer. The transparent toner image 98 and the color toner image 99 shown in FIG. 5 are laminated on the intermediate transfer belt 21. An example of the state is shown.
In FIG. 6A, a transparent toner image composed of T-color (transparent) toner particles T is first primarily transferred to the intermediate transfer belt 21, and FIG. A toner image composed of toner particles Y is primarily transferred. FIG. 6C shows a toner image composed of C color toner particles C on the Y toner image, and FIG. These toner images are secondarily transferred from the intermediate transfer belt 21 onto the paper S.

As shown in FIG. 6D, after the secondary transfer, the C color toner image, the Y color toner image, and the T color toner image are stacked in this order on the paper S, and the color C color and Y color toners are stacked. The image is covered with a colorless transparent toner image. Thereby, the transparent toner layer becomes the uppermost layer of the formed image on the paper S, and the glossiness is improved.
Further, in the contour portion of the transparent toner image, an example is shown in which some of the transparent toner particles T are scattered. The transparent toner particles T move from the original position on the sheet S during the secondary transfer, in the example of FIGS. 6C and 6D, from the position of FIG. The transparent toner particles constituting the contour of the transparent toner particles, as described above, are partially transferred by the AC component of the secondary transfer bias voltage. It goes to the outside without going to the position directly below, and this is scattered.

The scattered transparent toner particles Ta are not provided for glossiness because they are out of the color toner image (not stacked on the color toner image), but they serve to suppress deterioration in image quality by preventing the color toner particles from scattering. Have
FIG. 7 shows the color toner image and the transparent toner image formed by changing the conditions such as the size of the color toner image and the transparent toner image, the DC / AC of the secondary transfer bias voltage, and the voltage value by experiment. FIG. 8 is a diagram showing an example of each condition separately for an example and a comparative example, and FIG. 8 is a diagram showing an example of the experimental results.

As shown in FIG. 7, in Examples 1 to 5 and Comparative Examples 1 to 8, a solid image is a toner image having a uniform density over an entire area of a predetermined size (for example, 100 × 100 dots). The formed image is shown, and the dot image is an image in which dot-like images are scattered.
In both the examples and the comparative examples, as solid images, an example in which a transparent toner image, an M-color toner image, and a C-color toner image are multiple transferred is shown, and the size of the M-color and C-color toner images is the same In this embodiment, the transparent toner image has two widened areas (corresponding to the widened area 95 described above) than the M and C toner images. Although it is larger than the toner image, in the comparative example, the widened area is not set, and the transparent toner image is the same size as the M and C color toner images.

  As in the case of the solid image, the dot image has a widened area corresponding to two transparent toner images than the M and C color toner images in the embodiment, and no widened area is set in the comparative example. The transparent toner image and the M and C color toner images are examples of the same size. In the dot image section, transparent (5 dots) indicates that the transparent image has a size of 5 × 5 pixels. Similarly, M (3 dots) indicates that an M color image is 3 × 3. This indicates that the pixel size. There is a difference of 2 dots between transparency (5 dots) and M (3 dots), and this difference corresponds to the width of two pixels in the widened area 95. Here, since the resolution is 600 dpi, the size of one dot (corresponding to one pixel) indicates a size of about 42 μm (main scanning direction) × about 42 μm (sub-scanning direction).

In Example 5, the transparent image is the entire surface of one page, and the widened area is not two pixels. In the fifth embodiment, the transparent toner image formation region setting unit 57 sets the entire page as the transparent toner image formation region regardless of the outline of the color toner image existing in the page.
In each example, the secondary transfer bias voltage changes between direct current and alternating current, a rectangular wave, and a sine wave, and also has a voltage value of Vpp (peak-to-peak when alternating current), Vdv (direct current voltage value), duty (in rectangular wave). An example is shown in which the ratio between the application time on the high voltage side and the application time on the low voltage side in one cycle is changed. The AC cycle was 2000 (Hz).

  In FIG. 8, density unevenness and line scattering items are provided, density unevenness indicates secondary transfer efficiency, and line scattering indicates toner scattering. Here, the density unevenness and the secondary transfer efficiency are such that when the secondary transfer efficiency is lowered, the toner particles that should originally be secondarily transferred are not transferred to the paper S, but the toner particles that form a color image on the paper S. Therefore, the reduction in the number of color toner particles appears as density unevenness. In addition, when the density unevenness is large, unevenness in gloss occurs, and thus the density unevenness is also related to the gloss.

Density unevenness is evaluated using a measuring instrument for measuring the density, and the degree of image unevenness occurring in the formed image of the paper S after printing is evaluated in five stages (1 to 5 is best, 5 is the best as the value decreases). And 1 is the lowest).
For line scattering, a formed image of the paper S after printing was visually evaluated and classified into five levels 1 to 5 (the meaning of the numerical value is the same as the density unevenness). This visual evaluation is based on the amount of deformation when the shape of the reconstructed image is sharpened or shrunk due to splattering or not straight with respect to an image that was circular or linear if there was no splattering. A method of evaluating the degree of the above with reference to a limit sample prepared in advance was taken.

As shown in FIG. 8, Examples 1 to 5 all had the highest numerical value 5 for each item of density unevenness and line scattering, whereas in the comparative example, for example, density unevenness is Comparative Example 1. 3 is 2 and the line scattering is 4 or less over Comparative Examples 1 to 8, particularly 1 and so on in Comparative Examples 5 and 6.
In Comparative Examples 1 and 2, the evaluation of the density unevenness is low (the density unevenness is large). In Comparative Examples 1 and 2, the secondary transfer bias voltage is only DC, and compared to the example in which AC is superimposed. This is presumably because the secondary transfer efficiency was lowered.

  In Comparative Examples 1 to 8, the evaluation of the line scattering is generally low. In the Comparative Examples, the transparent images are all the same size as the color image, and there is no widened region as in the Examples. Therefore, it is presumed that the movement of the toner particles constituting the outline of the color image to the outside during the secondary transfer cannot be restricted, and the color toner particles are scattered more. In particular, in Comparative Examples 5 and 6, the evaluation of toner scattering is 1 as a numerical value.

  In Comparative Examples 5 and 6, the Vpp of the AC component is 800 V, and Vpp represents the potential difference between the maximum value and the minimum value in AC, and the larger the potential difference, the easier the toner particle vibration becomes. This is presumed to be because the amount of vibration is increased, and scattering is likely to occur. Comparative Examples 3 and 4 have the same conditions as Comparative Examples 5 and 6 except for Vpp. However, Vpp is smaller than Comparative Examples 5 and 6, and Vpp is small. Presumably better than Examples 5 and 6. From this, it can be said that Vpp is preferably large in terms of transfer efficiency but small in terms of scattering.

In Examples 1 to 5, since the secondary transfer bias voltage includes an alternating current component, the secondary transfer efficiency can be improved, density unevenness is suppressed, and line scattering is caused by the widened region of the transparent toner image. It is presumed that the scattering of the color toner particles is suppressed.
As described above, the AC component is included in the secondary transfer bias voltage, so that the secondary transfer efficiency is improved and the toner scattering can be suppressed by the widened area of the transparent toner. It is possible to suppress a decrease in color reproducibility in an image having the same, and to suppress a decrease in image quality of the reproduced image, such as a thin character in a thin character image.

In the embodiment, the secondary transfer bias voltage of any one of the above-described Examples 1 to 5 is used, but the value of the secondary transfer bias voltage (DC voltage Vdv, AC component Vpp, duty, cycle) and Needless to say, the width of the widened area of the transparent toner is not limited to the above, and a suitable value is determined according to the apparatus configuration, such as transfer efficiency and degree of scattering.
The present invention is not limited to an image forming apparatus, and may be the above-described method for setting a transparent toner image area. The method may be a program executed by a computer. Further, the program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like.

<Modification>
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above embodiment, the color toner image area existing in one page is specified based on the gradation values of all pixels for the same page for each color Y to K. However, the present invention is not limited to this. . The color toner image area can be specified by using, for example, a known character area determination method, a photograph, or a graphic area determination method with reference to Y to K color image data developed in a bitmap in a memory. it can.

  As the character area determination, for example, a method may be used in which a line or column is detected from a character sequence instead of a line segment constituting a character, and an area including a block including the detected line or column is specified as a character area candidate. it can. In this case, a color toner image area is formed by one block area having a line or column shape including a line segment of characters and a non-image area between the line segments. Therefore, the overlapping region 96 in the transparent toner image region (extended region 97) becomes a color toner image region represented by the one block, and the widened region 95 corresponds to the contour pixel of the color toner image region of the block. Thus, the region has a width corresponding to two pixels. In particular, the color toner particles constituting the character line segment located near the block outline in the block are prevented from being scattered outside the block region by the transparent toner particles existing in the widened region for the block. Can do. If there are a plurality of blocks, a widened portion is set for each block.

  (2) In the above-described embodiment, the width (width in the two-dimensional direction) of the widened area 95 is set to a predetermined amount (corresponding to two pixels) and the width is uniform (fixed). For each color toner image forming region (corresponding to an overlapping region) to be multiplex-transferred, the width of the widened region 95 can be changed to a larger value as the number of color toner layers in the forming region increases. .

  The increase in the number of color toner layers to be laminated means that the amount of color toner particles increases as compared with the case where there are few toner layers at the same coordinate position, and the height of the color toner layer from the surface of the intermediate transfer belt 21 is also high. Become. As a result, there is a risk that the amount of color toner particles that are about to scatter during secondary transfer may increase, or the scattering may spread over a wider range.To suppress the movement of such color toner particles, This is because it may be effective to increase the width (predetermined amount) of the widened region by the transparent toner.

Specifically, the width of the widened area is, for example, 2 for the color toner layers to be laminated (corresponding to the above embodiment), 4 for the 3 layers (lamination of Y, M, C, etc.). It is conceivable that the value can be changed to a different value, such as the equivalent of 6 pixels in the 4 layers (Y to K color stack).
The number of layers is determined by referring to the coordinate position of the pixel belonging to the image area for each color on the nth page, and for one pixel located at a certain coordinate, for example, the pixel is a Y-color image area and C If it belongs to the image area of the color and does not belong to the image area of the M color and the K color, the number of layers can be obtained such as two layers.

  When each pixel in one overlapping area (color toner image forming area) includes pixels with different numbers of layers (number of color colors), specifically, one pixel existing in one overlapping area In the case where the number of stacked pixels is 2, and the number of stacked other pixels is 3, for example, a method may be used in which the maximum value is regarded as the number of stacked color toners for the overlapping region or an average is taken. . For each overlap region, the width of the widened region corresponding to the number of color toner layers stacked in that region is changed to a larger value as the number of layers increases.

  Further, in the above, the transparent toner image forming area is set to an extended area including the overlapping area formed by widening the transparent toner image forming area by a predetermined amount with reference to the overlapping area overlapping the color toner image forming area. However, it is not limited to this. A transparent toner image is formed in an extended area obtained by extending an overlapping area overlapping with the formation area of the color toner image to be transferred so that the color toner image is covered with the transparent toner image on the paper after the secondary transfer. Any configuration can be used.

This configuration includes, for example, a configuration in which the transparent image forming region is the entire surface of one page as in the fifth embodiment of FIG. In this configuration, the entire non-image area other than the color toner image formation area in one page corresponds to an area expanded from the overlapping area. In this configuration, the transparent image covers the entire page. Image data for forming a transparent image is generated.
(3) In the above description, the widened region is set outward from the outline (outer edge) of the rectangular overlapping region. In the case of a shape such as a shape, a widened region is set for the inner edge (inner contour) of the overlapping region. In this case, among the non-image areas surrounded by the overlap area, an area that has entered the non-image area by a predetermined amount (for example, equivalent to two pixels) from the inner edge of the overlap area (overlapping when viewed from the overlap area) The area outside the inner edge of the area becomes the widened area, and the outer outline or the inner outline of the overlap area is separated from the overlap area by a predetermined amount from the outline of the overlap area. Each area becomes a widened area.

(4) In the above embodiment, an example using toner having a negative charge polarity has been described, but the present invention is not limited to this. For example, a toner having a positive charging polarity may be used. In this case, the primary and secondary transfer bias voltages are negative voltages.
(5) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a tandem color digital printer has been described. However, the present invention is not limited to this. Among N (a plurality of three or more) image carriers, (N-1) different colored toner images are formed on each of (N-1) image carriers, and one image carrier A colorless transparent toner image is formed, and the transparent toner image and the (N-1) colored toner image are sequentially transferred onto the intermediate transfer member in this order, and transferred onto the intermediate transfer member. Any image forming apparatus having a configuration in which the toner images are secondarily transferred to a recording sheet at the secondary transfer position can be applied to, for example, a copying machine, a FAX, an MFP (Multiple Function Peripheral), and the like.

  In the above description, the photosensitive drum 1 is used as the image carrier. However, the image carrier is not limited to this, and a belt-shaped member may be used, for example. In the above description, the intermediate transfer belt 21 is used as the intermediate transfer member. However, the intermediate transfer member is not limited to this, and a drum-shaped member may be used. In the above description, an example of four colors Y, M, C, and K has been described as a colored color image. However, the color image is not limited to the four-color configuration, and a color image is obtained by multiple transfer of toner images of different colors. It can be applied to the configuration that forms.

  Further, the secondary transfer roller 35 is used as an example of the secondary transfer unit, and the secondary transfer bias voltage in which the alternating current is superimposed on the direct current is applied to the secondary transfer roller 35 from the secondary transfer bias voltage output unit 36. The secondary transfer means is not limited to the roller shape, and other shapes may be used as long as they can perform secondary transfer. Further, the developer accommodated in each developing device may be, for example, a one-component system or a two-component system.

  The contents of the above embodiment and the above modification may be combined.

  The present invention can be widely applied to image forming apparatuses that form color toner images and transparent toner images of different colors.

1Y, 1M, 1C, 1K, 1T Photosensitive drum 5Y, 5M, 5C, 5K, 5T Primary transfer roller 21 Intermediate transfer belt 35 Secondary transfer roller 36 Secondary transfer bias voltage output unit 56 Color toner image formation region detection unit 57 Transparent toner image formation region setting unit 92 Color toner image formation region 94 Contour pixel 95 Widening region 96 Overlapping region 97 Extended region 100 Printer 361 AC power supply unit 362 DC power supply unit

Claims (5)

A color toner image and a transparent toner image of different colors are formed on each of the plurality of image carriers, and the formed transparent toner image and the color toner images of different colors are transferred onto the intermediate transfer member in this order. And an image forming apparatus for secondary transfer of the toner image transferred onto the intermediate transfer member onto the recording sheet by a secondary transfer unit,
Voltage output means for providing the secondary transfer means with a voltage in which alternating current is superimposed on direct current as a secondary transfer bias;
The transparent toner image is expanded to an extended area that overlaps the formation area of the color toner image to be transferred so that the color toner image is covered with the transparent toner image on the recording sheet after the secondary transfer. An image forming apparatus that forms the image. Setting means for setting the extension region to a region including the overlap region formed by widening the overlap region by a predetermined amount;
The image forming apparatus according to claim 1, wherein a transparent toner image is formed in the set extended area. The setting means includes
First setting means for setting an area in which at least one color toner image is formed in one page as a color toner image area based on image data for forming the different color toner images in units of pages; ,
In the one page, an area showing the same coordinates as the set color toner image area is defined as the overlapping area of the transparent toner image, and is separated from the overlapping area by the predetermined amount from the outline of the overlapping area. A second setting means for setting an area as an enlarged area, and setting an area composed of the overlapping area and the enlarged area as the extended area;
The image forming apparatus according to claim 2, further comprising: The first setting means includes
For each different color, an image area in which a color toner image is formed is specified in the one page, and an area obtained by logically adding the image areas for the specified colors is set as the color toner image area. The image forming apparatus according to claim 3. The predetermined amount is
5. The variable value according to any one of claims 2 to 4, wherein the color toner layer is changed to a larger value as the number of color toner layers stacked in the formation region of the color toner image transferred onto the intermediate transfer member is increased. Image forming apparatus.

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