JP2008173866A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus with improved reproducibility of fine lines, and an image forming method. <P>SOLUTION: By using line heads 101K-101Y in which light emitting parts are arranged in a line in the axial direction (main scanning direction) of an image carrier, optical writing is performed on image carrying bodies 41K-41Y moving in the direction (sub-scanning direction) orthogonal to axis. Respective light emitting parts are controlled by a line head controlling circuit 73 so that they are turned on continuously for a period where the image carriers move by one pixel in the sub-scanning direction. When respective pixels are written to respective light emitting parts, on every time, a corrected value on the amount of light is transferred. The corrected value on the amount of light is modified so that photo-energies for writing are different when a fine line in the main scanning direction is drawn on the image carrying body and when the fine line in the sub-scanning direction is drawn thereon. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method with improved reproducibility of fine lines.

  As an image forming apparatus using a line head, one using an LED as a light source or one using an organic EL element is known. In such a line head, the latent image may be deformed by the movement of the photosensitive member (image carrier). This point will be described with reference to FIGS. 6 (a) and 6 (b). Since the LED line head has a large light power of the light emitting unit, the lighting time is shortened to perform time-division driving. Particularly in recent LEDs, the light power is sufficiently large, so that an exposure time of 1 / several to several tens of minutes is sufficient for the time required for the photosensitive member to move by one pixel in the sub-scanning direction. It is not necessary to consider the deformation of the latent image due to the movement of the photosensitive member during the operation.

  FIG. 6B shows the surface potential on the photoconductor when the ratio of the LED lighting time is set to 1/10 of the one-pixel moving time with respect to the exposure conditions shown in the present embodiment as described later. The distribution (latent image distribution) is shown. Since the distribution of optical power is a perfect circle, it can be seen that the distribution of the surface potential is almost a perfect circle. The X direction is the axial direction (main scanning direction) of the photoconductor, and the Y direction is the direction (sub scanning direction) orthogonal to the axial direction.

  In the organic EL line head, since the light power of the light emitting unit is smaller than that of the LED, it is necessary to take a long lighting time. In particular, when almost all the photosensitive element is lit during the period in which the photosensitive element moves by one pixel in the sub-scanning direction, the light is lit while moving in the sub-scanning direction by the length of one pixel. As can be seen from the surface potential distribution, the latent image extends (deforms) in the sub-scanning direction. In this example, the exposure is performed while moving 42.3 μm with respect to the imaging spot size of 60 μm.

  Patent Document 1 discloses a method of controlling the width of a thin line by controlling the pulse width of a laser in writing on a photosensitive member using a light beam such as a laser. Patent Document 2 discloses a method of controlling a latent image of a fine line by changing the duty of a vertical line (sub-scanning direction) to other than 100%.

JP 2001-158127 A Japanese Patent Laid-Open No. 10-193683

  As described above, when exposure is performed using a light source having a relatively low light amount such as an organic EL, particularly when a thin line is exposed by lighting for a time corresponding to one pixel, a one-pixel width extending in the vertical direction (sub-scanning direction). The line width is different between the thin line and the horizontal (one pixel width thin line extending in the main scanning direction. Especially when a drawing is printed by CAD or the like, for example, a thin line having one pixel width is often used as a dimension line or the like. However, if the width or density of the fine lines in the vertical direction and the horizontal direction are different, the image quality deteriorates and becomes very unsightly.

  The method described in Patent Document 1 or Patent Document 2 adjusts the width of the thin line by the pulse width, but has a problem that it cannot be applied when the gradation is controlled by the pulse width. Further, both Patent Documents 1 and 2 are technologies based on the premise of exposure by laser scanning, and there is a problem that it is difficult to directly apply to a line head.

  The present invention has been made in view of such various problems of the prior art, and an object of the present invention is to improve the reproducibility of fine lines when a line head is used as an exposure light source, and An object is to provide an image forming method.

  The image forming apparatus of the present invention that achieves the above object uses a line head in which light emitting portions are arranged in a line in the axial direction (main scanning direction) of an image carrier, and the direction (sub-scanning) is perpendicular to the axial direction. When performing optical writing on the image carrier that moves in the direction), each of the light emitting units is controlled to be continuously lit for a period in which the image carrier moves by one pixel in the sub-scanning direction, and A light amount correction value is transferred each time each pixel is written to each light emitting unit, and when writing a fine line in the main scanning direction and writing a thin line in the sub-scanning direction on the image carrier The light quantity correction value is corrected so that the writing light energy is different.

  The image forming apparatus of the present invention is characterized in that the light amount correction value of the fine line in the main scanning direction is controlled to be smaller than the light amount correction value of the thin line in the sub-scanning direction.

  In the image forming apparatus of the present invention, the line width of the thin line is controlled by controlling the pulse width of the driving pulse of each light emitting unit.

  The image forming apparatus of the present invention is characterized in that means for determining whether the direction of the thin line written on the image carrier is the main scanning direction or the sub-scanning direction is provided.

  The image forming apparatus of the present invention is characterized in that each light emitting unit uses an organic EL element.

  The image forming apparatus according to the present invention includes an image forming station in which image forming units including a charging unit, the line head described above, a developing unit, and a transfer unit are arranged around an image carrier. At least two or more are provided, and a color image is formed by a tandem method by passing a transfer medium through each station.

The image forming method of the present invention moves in a direction (sub-scanning direction) orthogonal to the axial direction using a line head in which light emitting portions are arranged in a line in the axial direction (main scanning direction) of the image carrier. When performing optical writing on the image carrier,
The light emitting units are controlled to be continuously lit during a period in which the image carrier moves by one pixel in the sub-scanning direction, a light amount correction value for each light emitting unit is formed, and each pixel When writing a thin line in the main scanning direction on the image carrier, and writing light energy in the sub scanning direction are different on the image carrier. And correcting the light amount correction value as described above.

  The image forming method of the present invention is characterized in that the image formation is performed by simultaneously forming a plurality of color images using a plurality of the line heads.

  In the embodiment of the present invention, when a line head is used as the exposure light source, the line widths of the vertical and horizontal lines can be made equal even when the exposure time is long with respect to the photosensitive member moving time for one pixel. This makes it possible to improve the reproducibility of fine lines. Moreover, since the organic EL element is used as the light emitting part, it is not necessary to reduce the diameter of the light emitting part, and the light power of the light emitting part can be increased. For this reason, it is possible to use an organic EL material having a low luminous efficiency.

  An embodiment of the present invention will be described. First, “resolution” according to an embodiment of the present invention will be described. In the embodiment of the present invention, “resolution” is defined as follows. In a line head using an arrayed light source, such as an LED head, the resolution in the main scanning direction is the arrangement of the light emitting units when the direction perpendicular to the moving direction of the photosensitive member (image carrier) is the main scanning direction. It is determined by the pitch and cannot be varied.

  In the embodiment of the present invention, the “resolution” of the line head is defined as the reciprocal of the pitch of the light emitting units in the main scanning direction. More precisely, “resolution” is the reciprocal of the pitch in the main scanning direction of the image of the light emitting portion projected onto the surface of the photoreceptor. However, in the sub-scanning direction, the resolution can be freely changed by controlling the frequency of turning on / off the light source. Since the resolution in the sub-scanning direction can be freely set in this way, the unit of image processing in the sub-scanning direction is defined as “pixel” in the sub-scanning direction, and the reciprocal of the pixel pitch is defined as the resolution in the sub-scanning direction.

  Next, control of the organic EL head in the embodiment of the present invention will be described. When a light-emitting element having a relatively low optical power, such as an organic EL element, is used as a light source, it is necessary to give sufficient exposure energy to the photoreceptor. FIG. 7 is a timing chart showing driving pulses of the organic EL element. In FIG. 7, t is a time, and ts is a scanning cycle (a time required for the photosensitive member to move by one pixel in the sub-scanning direction). That is, ts corresponds to the time from when the pulse is turned on at time 0 to when the pulse is turned off at time Ta. As shown in FIG. 7, each light emitting portion of the organic EL element is turned on by almost all time drive pulses in the scanning period ts.

Next, the distribution of the latent image when the photosensitive member is exposed by a line head using an organic EL element will be described with reference to the characteristic diagrams of FIGS. Here, the latent image distribution when an organic photoconductor (OPC) with half exposure energy of 0.8 μJ / cm ² and initial charge potential of −600 V is exposed with an average exposure energy of 0.3 μJ / cm ² during one pixel period exposure is shown. Shall be shown. Further, it is assumed that the imaging spot size is 1 / e ² in diameter and 60 μm in both the main scanning direction and the sub-scanning direction. The resolution in the sub-scanning direction is the same as that in the main scanning direction. That is, the photosensitive member moves while being lit in the sub-scanning direction by the same distance as the pitch of the main scanning spot. Furthermore, the spread (blurring) of the charge when the charge moves through the film of the photoconductor is not considered.

  FIG. 2 is a three-dimensional graph showing a latent image distribution of lines in the main scanning direction (X direction), that is, horizontal lines. In the main scanning direction of FIG. 2, the exposure position is determined by the position of the light emitting portion, so that unevenness is seen at the peak of the waveform diagram. FIG. 3 shows a latent image distribution of lines in the sub-scanning direction (Y direction), that is, vertical lines. In the sub-scanning direction, one light emitting unit is continuously lit and exposed, so that the waveform diagram does not show the irregularity as in the main scanning direction.

  FIG. 4 is a characteristic diagram in which the above-described two-dimensional latent image distribution is an average latent image distribution having a cross section perpendicular to the line direction. As can be seen from FIG. 4, the line (horizontal line) in the main scanning direction is wider than the distribution of the line (vertical line) in the sub-scanning direction, and the peak height is slightly lower. This is because the line in the main scanning direction is exposed while moving by one pixel in the direction perpendicular to the line.

  As shown in FIG. 4, since the distribution of the latent image is widened in the horizontal line, the line width is widened. Therefore, in the embodiment of the present invention, when a horizontal line is printed, control is performed to reduce the amount of light only by the light emitting unit corresponding to the corresponding pixel. Specifically, control is performed to reduce the light amount correction value of each light emitting unit at a constant ratio. If the light amount correction value for each pixel is corrected at this time, strictly speaking, the uniformity of the image density cannot be maintained. However, such a phenomenon is remarkable in the halftone image, but the fine line that is the subject of the present invention is used. Since there is little influence on the image, there is no problem.

  FIG. 1 is a block diagram of a control unit for performing the control of the present invention. Image data to be printed and data such as a print command sequence defining graphics and characters are sent to the printer control unit 71 from the external host computer 70 or the like. The print command sequence is interpreted by the printer controller 72 and developed into two-dimensional image data together with image data sent from the outside. For this image data, conversion of position and size, in the case of color, conversion to the developer color of the printer (in many cases cyan, magenta, yellow), levels such as halftone dots and lines, etc. A tone screen process is performed.

  The gradation data converted into the gradation data of each pixel of the line head by the printer controller 72 is sent to the line head control circuit 73. The line head control circuit 73 outputs image data to the line heads 101K, 101C, 101M, and 101Y for each color, and exposure processing is performed on each of the photoconductors 41K, 41C, 41M, and 41Y. Predetermined data is transmitted and received between the printer controller 72 and the engine controller 74.

  In the embodiment of the present invention, since the light amount correction value for the horizontal line is corrected, when the pixel data corresponding to the horizontal line is sent from the printer controller 72, information (flag or the like) indicating that the horizontal line is applied is displayed. The data is transferred to the line head control circuit 73 simultaneously with the adjustment data. The line head control circuit 73 multiplies the original correction value by a certain coefficient based on the information to form a light amount correction value and sends it to the line heads 101K to 101Y. As described above, in order to realize the present embodiment, the circuit for transferring the gradation value and the light amount correction value to the line head every time data for drawing each pixel is transferred to the line head control circuit 73. The configuration is provided in the line head control circuit 73.

  Since it is necessary to add information for each pixel as described above in order to determine whether each pixel is a horizontal line, the amount of data transferred from the printer controller 72 to the line head control circuit 73 increases. . In order to avoid such an increase in the amount of data, it is only necessary to provide a determination circuit for determining whether the corresponding pixel is a pixel included in the horizontal line on the line head control circuit 73 side. Specifically, for pixels in a square area of, for example, 5 × 5 squares centering on the relevant pixel, pixels that are continuous from the relevant pixel in the horizontal direction (main scanning direction) are turned on, and other pixels are turned off. If it is, it is judged as a horizontal line. Many such image recognition techniques have already been announced, but it is desirable that the method be as simple as possible so as not to increase the circuit scale.

  The degree to which the amount of light is corrected depends on the characteristics of the electrophotographic process to be used, and thus cannot be decided unconditionally. For example, in FIG. 4, if development is performed from about −200 V to 0 V, ideally, the width of the potential distribution line with −200 V as the slice level may be equalized between the vertical line and the horizontal line. However, in practice, if the exposure energy is decreased to make the widths the same, the peak potential also decreases (the absolute value increases), and therefore it is not always necessary to match the widths at a certain slice level.

  FIG. 5 is a characteristic diagram showing a result of controlling the light amount correction value so that the optical power of the horizontal line becomes 65% of the vertical line. In FIG. 5, the width of the horizontal line latent image at a surface potential of −200 V is still slightly larger than the vertical line, but the line width actually developed is sufficiently narrow. Thus, in the embodiment of the present invention, the light amount correction value is controlled so that the writing light energy of the vertical line and the horizontal line are different.

As already described, since the lighting time of one pixel is sufficiently long in the organic EL element, the line width of the thin line can be controlled by pulse width control when gradation control is not performed on the thin line. is there. In this case, for example, in FIG. 7, the on-time of the pulse is set to 0 to Tb shorter than the scanning cycle ts.
In the case of controlling to increase the pulse width, in the case of a horizontal line, that is, a thin line extending in the main scanning direction, the pulse width can be increased beyond the period of one pixel. In this case, the on-time of the pulse is set to 0 to Tc longer than the scanning cycle ts.

  In the above case, the line width of the horizontal line is increased. However, when the exposure energy is different or the development process is different, the relationship between the line widths of the vertical line and the horizontal line may be reversed. Even in such a case, control can be performed in the same manner as described above.

  When an organic EL element is used as the light emitting element, it is not necessary to reduce the diameter of the light emitting part in the embodiment of the present invention, so that the optical power of the light emitting part can be increased. For this reason, it is possible to use an organic EL material having a low luminous efficiency. In the embodiment of the present invention, since the exposure pixels are arranged at a higher density than in a normal line head, the number of pixels increases dramatically. Although it is possible to apply the present invention to a line head using an LED as a light source which has been conventionally used, an LED array chip provided with a large number of LEDs is mounted on a substrate with high positional accuracy, and the number of pixels is higher than usual. Therefore, the number of bondings connecting the chip and the substrate increases, which makes manufacturing difficult.

  On the other hand, when an organic EL element is used as a light source, a large number of pixels can be formed on a glass substrate at a high density and with high accuracy, which is optimal as an embodiment of the present invention. Further, in the embodiment of the present invention, the circuit configuration is simplified, and it becomes easy to make a drive circuit with a thin film transistor on the same glass substrate as the light emitting portion. Various types of thin film transistors such as amorphous silicon, low-temperature polysilicon, high-temperature polysilicon, and organic transistors can be used.

  In the embodiment of the present invention, a tandem color printer that exposes four photoconductors with four line heads, simultaneously forms four color images, and transfers them onto one endless intermediate transfer belt (intermediate transfer medium). The target is a line head used in (image forming apparatus). FIG. 8 is a vertical side view showing an example of a tandem image forming apparatus using an organic EL element as a light emitting element. In this image forming apparatus, line heads 101K, 101C, 101M, and 101Y using four organic EL element arrays having the same configuration are combined with four corresponding photosensitive drums (image carriers) 41K having the same configuration. , 41C, 41M, and 41Y, respectively, and is configured as a tandem image forming apparatus.

  As shown in FIG. 8, this image forming apparatus is provided with a driving roller 51, a driven roller 52, and a tension roller 53. The tension roller 53 applies tension to the image forming apparatus and stretches it in the direction indicated by the arrow (counterclockwise direction). ) Is circulated to the intermediate transfer belt (intermediate transfer medium) 50. Photosensitive members 41K, 41C, 41M, and 41Y having photosensitive layers are arranged on the outer peripheral surface as four image carriers arranged at predetermined intervals with respect to the intermediate transfer belt 50.

  K, C, M, and Y added after the reference sign mean black, cyan, magenta, and yellow, respectively, and indicate that the photoconductors are black, cyan, magenta, and yellow, respectively. The same applies to other members. The photoreceptors 41K, 41C, 41M, and 41Y are rotationally driven in the direction indicated by the arrow (clockwise) in synchronization with the driving of the intermediate transfer belt 50. Around each photoconductor 41 (K, C, M, Y), charging means (corona charger) 42 (K) for uniformly charging the outer peripheral surface of the photoconductor 41 (K, C, M, Y), respectively. , C, M, Y) and the outer peripheral surface uniformly charged by the charging means 42 (K, C, M, Y) are synchronized with the rotation of the photoconductor 41 (K, C, M, Y). A line head 101 (K, C, M, Y) as described above of the present invention for sequentially scanning the lines is provided. The line head 101 (K, C, M, Y) performs optical writing on the photoconductors (image carriers) 41K, 41C, 41M, 41Y.

  Further, a developing device 44 (K, C, and M) that applies a toner as a developer to the electrostatic latent image formed by the line head 101 (K, C, M, and Y) to form a visible image (toner image). M, Y) and a primary transfer roller 45 (K, Y) as transfer means for sequentially transferring the toner image developed by the developing device 44 (K, C, M, Y) to the intermediate transfer belt 50 as a primary transfer target. C, M, Y) and a cleaning device 46 (K, C, M, Y) as a cleaning means for removing the toner remaining on the surface of the photoreceptor 41 (K, C, M, Y) after being transferred. ).

  Here, each line head 101 (K, C, M, Y) is installed such that the direction of the organic EL element array is along the bus line of the photosensitive drum 41 (K, C, M, Y). Then, the emission energy peak wavelength of each organic EL element array exposure head 101 (K, C, M, Y) and the sensitivity peak wavelength of the photoconductor 41 (K, C, M, Y) are set so as to substantially match. Has been.

  The developing device 44 (K, C, M, Y) uses, for example, a non-magnetic one-component toner as a developer, and the one-component developer is conveyed to the developing roller by a supply roller, for example, and adhered to the developing roller surface. The film thickness of the developed developer is regulated by a regulating blade, and the developing roller is brought into contact with or increased in thickness by the photosensitive body 41 (K, C, M, Y), whereby the photosensitive body 41 (K, C, M, Y). The toner is developed as a toner image by attaching a developer according to the potential level.

  The black, cyan, magenta, and yellow toner images formed by the four-color single-color toner image forming station are intermediated by the primary transfer bias applied to the primary transfer roller 45 (K, C, M, Y). The toner image, which is sequentially primary transferred onto the transfer belt 50 and sequentially superposed on the intermediate transfer belt 50 to become a full color, is secondarily transferred to a recording medium P such as paper by a secondary transfer roller 66, and serves as a fixing unit. The toner is fixed on the recording medium P by passing through the fixing roller pair 61, and is discharged onto a paper discharge tray 68 formed in the upper part of the apparatus by a paper discharge roller pair 62.

  In FIG. 8, 63 is a paper feed cassette in which a large number of recording media P are stacked and held, 64 is a pickup roller for feeding the recording media P one by one from the paper feed cassette 63, and 67 is a secondary transfer roller. 66, a pair of gate rollers for defining the supply timing of the recording medium P to the secondary transfer portion 66; a secondary transfer roller 66 as a secondary transfer means for forming a secondary transfer portion with the intermediate transfer belt 50; Is a cleaning blade as a cleaning means for removing the toner remaining on the surface of the intermediate transfer belt 50 after the secondary transfer.

  FIG. 9 is an enlarged perspective view schematically showing the line head 101 using the organic EL element array. In FIG. 9, the organic EL element array 81 is held in a long housing 80. The positioning pins 89 provided at both ends of the long housing 80 are fitted into the opposing positioning holes of the case, and fixing screws are screwed into the screw holes of the case through the screw insertion holes 88 provided at both ends of the long housing 80. By fixing, each line head 101 is fixed at a predetermined position.

  The line head 101 has a light emitting element (organic EL element) 83 of an organic EL element array 81 placed on a glass substrate 82 and is driven by a drive circuit 85 formed on the same glass substrate 82. A gradient index rod lens array (SLA) 65 constitutes an imaging optical system, and has a gradient index rod lens 84 arranged in front of the light emitting element 83. As the rod lens array 85, the “selfoc lens array” (abbreviated as SLA, trade name of Nippon Sheet Glass Co., Ltd.) as described above is frequently used.

  The light beam emitted from the organic EL element array 81 is formed on the surface to be scanned by the SLA 65 as an equal-size erect image. Thus, since the organic EL elements 83 are arranged on the glass substrate 82, the image carrier can be irradiated without impairing the light amount of the light emitting elements. Further, since the organic EL element can be controlled statically, the control system of the line head can be simplified.

  Although the image forming apparatus and the image forming method of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made.

It is a block diagram which shows embodiment of this invention. It is a characteristic view which shows embodiment of this invention. It is a characteristic view which shows embodiment of this invention. It is a characteristic view which shows embodiment of this invention. It is a characteristic view which shows embodiment of this invention. It is explanatory drawing which shows embodiment of this invention. It is a timing chart which shows embodiment of this invention. 1 is a longitudinal side view of an image forming apparatus showing an embodiment of the present invention. It is a perspective view of the line head concerning the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 41 ... Photosensitive body (image carrier), 42 ... Charging means, 44 ... Developing means, 45 ... Primary transfer member, 50 ... Intermediate transfer belt, 65 ... Gradient index rod lens array (SLA), 71 ... Printer control , 72 ... printer controller, 73 ... line head control circuit, 81 ... organic EL element array, 82 ... substrate, 83 ... light emitting element for image formation, 84 ... gradient index rod lens, 101 ... line head

Claims (8)

Optical writing is performed on an image carrier that moves in a direction (sub-scanning direction) perpendicular to the axial direction using a line head in which light emitting portions are arranged in a line in the axial direction (main scanning direction) of the image carrier. When performing, each of the light emitting units is controlled to be continuously lit for a period in which the image carrier moves by one pixel in the sub-scanning direction, and when writing each pixel to each of the light emitting units It is configured to transfer the light amount correction value every time, and the light amount correction value is corrected so that the writing light energy is different when writing the fine line in the main scanning direction on the image carrier and when writing the thin line in the sub scanning direction. An image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the light amount correction value of the fine line in the main scanning direction is controlled to be smaller than the light amount correction value of the thin line in the sub-scanning direction. 3. The image forming apparatus according to claim 1, wherein the line width of the thin line is controlled by controlling a pulse width of a driving pulse of each light emitting unit. 4. 4. The image formation according to claim 1, further comprising means for determining whether a direction of a thin line written on the image carrier is the main scanning direction or the sub-scanning direction. apparatus. The image forming apparatus according to claim 1, wherein each of the light emitting units uses an organic EL element. At least two image forming stations in which image forming units including a charging unit, a line head according to any one of claims 1 to 5, a developing unit, and a transfer unit are arranged around an image carrier. An image forming apparatus provided as described above, wherein a color image is formed by a tandem method when a transfer medium passes through each station. Optical writing is performed on an image carrier that moves in a direction (sub-scanning direction) perpendicular to the axial direction using a line head in which light emitting portions are arranged in a line in the axial direction (main scanning direction) of the image carrier. When doing
The light emitting units are controlled to be continuously lit during a period in which the image carrier moves by one pixel in the sub-scanning direction, a light amount correction value for each light emitting unit is formed, and each pixel When writing a thin line in the main scanning direction on the image carrier, and writing light energy in the sub scanning direction are different on the image carrier. And correcting the light quantity correction value as described above. The image forming method according to claim 7, wherein the image formation is performed by simultaneously forming a plurality of color images using a plurality of the line heads.

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