JP2013214042A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of forming a half-tone image having reduced stripe-like unevenness.SOLUTION: An image forming apparatus includes: an image carrier 11a on which an electrostatic latent image is formed by light exposure; an exposure device 12a that has a plurality of light sources 14a and a plurality of lenses 15a that are provided along a longitudinal direction of the image carrier, and irradiates the image carrier with light; and a developing device that is arranged separated from the image carrier by a constant interval, and is for developing the electrostatic latent image on the image carrier into a visible image with two-component developer by application of DC bias.

Description

  Embodiments described herein relate generally to an image forming apparatus.

  In recent years, small-sized electronic devices represented by LEDs have attracted attention as exposure apparatuses in electrophotographic apparatuses.

  Although it is LED, OLED, and small, the number of light emitting points is many orders of magnitude compared with a laser optical system or the like. In addition, since the SELFOC (registered trademark) lens array is used, the number of lenses is large, and variations in optical characteristics occur in the main scanning direction. Due to variations in the characteristics of the light emitting points and the characteristics of the SELFOC lens, the respective beam profiles are different. Therefore, when a halftone image is printed, density unevenness (vertical stripes, stripe unevenness) occurs on the stripes.

  In order to reduce density unevenness in a halftone image, a correction process called beam diameter correction is generally performed on the exposure apparatus side such as an LED. However, beam diameter correction can be counterproductive if conditions change. For this reason, for example, a technique has been proposed in which the dot diameter correction intensity is changed to perform stable dot diameter correction against environmental changes and the like.

  Even when such a method is adopted, dot diameter correction cannot be adjusted particularly when the distance between the LED and the photosensitive member is deviated from the focal position. Due to the characteristics of the SELFOC lens array, the distance between the LED and the photoconductor is extremely small, such as a focal length of about several tens of μm. Unfortunately, the dot diameter correction effect cannot be obtained because the characteristics of the respective light emitting points are disturbed.

Japanese Patent No. 3214124

  The problem to be solved by the present invention is to provide an image forming apparatus capable of forming a halftone image with reduced stripe unevenness.

  According to one embodiment, an image forming apparatus includes an image carrier on which an electrostatic latent image is formed by exposure, a plurality of light sources and a plurality of lenses provided along the longitudinal direction of the image carrier. And an exposure device for irradiating the image carrier with light and a fixed distance from the image carrier, and applying the DC bias to convert the electrostatic latent image on the image carrier into two components And a developing device for developing a visible image with a developer.

It is a figure showing the front side section of the image forming device concerning one embodiment. 3 is a perspective view showing a writing unit in the image forming apparatus. FIG. Schematic showing the structure of a writing part. Schematic showing the structure in the longitudinal direction of a LED printer head. FIG. 2 is a schematic diagram showing a photosensitive drum and an LED printer head. The enlarged view of a LED printer head. FIG. 3 is a schematic diagram illustrating a configuration in the vicinity of a developing device. FIG. 3 is an enlarged view of a developing device and a photosensitive drum. The conceptual diagram of beam diameter correction | amendment. FIG. 5 is a schematic diagram without washing the relationship between the exposure profile and reproduction dots, and the photoreceptor characteristics and development characteristics. The graph showing the relationship between exposure light quantity and surface potential. FIG. 6 is a graph showing the relationship between the distance between the photosensitive member and the developing roller and the isolated point dot diameter. FIG. 6 is a graph showing the relationship between the distance between the photosensitive member and the developing roller and the isolated point dot diameter.

  The embodiment will be specifically described below.

  Electrophotographic two-component development has milder development characteristics (gamma) than single-component development, and is stable against fluctuations in the environment. In particular, when developing with a DC electric field, if there is a certain distance between the developing roller and the photosensitive member, the development of minute points is promoted by the edge effect of the electric field. As a result, the reproducibility of isolated points and the like is improved, and minute dots are emphasized larger than the electrostatic latent image and developed on the photosensitive member.

  When a solid-state exposure apparatus such as an LED is used for exposure, it is a halftone image that causes streaks due to variations in beam diameter. The main cause is non-uniformity of minute points and fine lines. By combining the two-component development to which a DC electric field is applied as described above with a solid-state exposure apparatus such as an LED, even if a slight variation occurs in the beam diameter, stripe unevenness in a halftone image can be reduced. it can.

  In addition, the influence of the variation in beam diameter on development can be reduced by adjusting the light energy applied to the photosensitive drum as the image carrier to a certain range. In the present embodiment, it is preferable to set the exposure energy to a value that is not less than 1/2 and not more than twice the half exposure amount of the photosensitive drum. As a result, it is possible to further reduce streak unevenness that occurs in the halftone image.

  When the exposure energy is set within this range under normal conditions, the reproducibility of minute dots and fine lines deteriorates and the line drawing reproducibility deteriorates. In combination with the two-component development method using a DC electric field as described above, it has become possible to achieve good line image reproduction.

  FIG. 1 shows the configuration of the front side of an image forming apparatus according to an embodiment.

  The image forming apparatus 100 shown in the figure has a developer applied to the electrostatic latent images held by the first to fourth photosensitive drums 11a to 11d and the photosensitive drums 11a to 11d as image carriers that hold electrostatic latent images. The developing devices 17a to 17d that supply and form developer images, the transfer belt 19 that sequentially holds the developer images held by the photosensitive drums 11a to 11d, the cleaner 20 that removes the developer remaining on the transfer belt 19, and the transfer belt 19 A secondary transfer roller 27 for transferring the developer image held by the toner image to a sheet that is a transparent resin sheet such as plain paper or an OHP sheet, and fixing for fixing the developer image transferred to the sheet by the secondary transfer roller 27 to the sheet And an exposure device for forming latent images on the photosensitive drums 11a to 11d. The exposure apparatus will be described in detail later.

  Reference numeral 10 is a scanner unit for reading an original, and reference numeral 30 is a main power switch.

  The first to fourth developing devices 17a to 17d are arbitrary colors of Y (yellow, yellow), M (magenta), C (cyan), and Bk (black, black) used for obtaining a color image by subtractive color mixing. The latent image held by each of the photosensitive drums 11a to 11d is visualized in any one of Y, M, C, and Bk colors. The order of each color is determined in a predetermined order according to the image forming process and the characteristics of the developer.

  The transfer belt 19 holds the developer images of the respective colors formed by the first to fourth photoconductor drums 11a to 11d and the corresponding developing devices 17a to 17d in the order of forming the developer images, and is used for plain paper or OHP. It transfers to the sheet | seat which is a transparent resin sheet like a sheet | seat.

  The sheet feeding cassette 21 accommodates a sheet of an arbitrary size, and a pickup roller (not shown) takes out the sheet from the corresponding cassette according to an image forming operation. The sheet size corresponds to the magnification required for image formation and the size of the developer image to be formed.

  The registration roller 23 and the image quality maintenance control unit 25 transfer the selected sheet in contact with the secondary transfer roller 27 and the transfer belt 19 in accordance with the timing at which the secondary transfer roller 27 transfers the developer image from the transfer belt 19. Send to position.

  If necessary, a sheet can be supplied from the manual feed tray 32 to form a developer image on a desired sheet.

  The sheet on which the developer image has been transferred by the secondary transfer roller 27 is discharged to the discharge tray 31 after the developer image is fixed by the fixing device 29.

  FIG. 2 is a perspective view of the writing unit in the image forming apparatus. As shown in the figure, LED print heads 12a to 12d are arranged close to the first to fourth photoconductive drums 11a to 11d, respectively. The photoconductor drums 11a to 11d have a shape having a longitudinal direction, and the LED print heads 12a to 12d are arranged along the longitudinal direction. As shown in FIG. 3, LED print head abutment / separation levers 13a to 13d are provided in each pair including the photosensitive drums 11a to 11d and the LED print heads 12a to 12d.

  The writing unit irradiates the photosensitive drums 11a to 11d with LED light based on a digital image signal sent from a scanner, USB, network, or the like, thereby forming an electrostatic latent image on the photosensitive drum. . The image signals are applied to the photosensitive drums 11a to 11d by the LED print heads 12a to 12d, respectively.

  FIG. 4 shows a schematic configuration in the longitudinal direction of the LED print head 12a and a part of the beam profile. As shown in FIG. 4, the LED print head 12a has a shape extending in the longitudinal direction of the photosensitive drum 11a, and includes a plurality of light sources and a plurality of lenses. Since there are many light emitting points and lenses, the beam profile of the light emitted from the LED print head is not uniform. As shown in the figure, the beam profile varies, and the degree of variation varies.

  The photosensitive drum 11a and the LED print head 12a are arranged at a constant interval by being arranged via a gap spacer 16a as shown in FIG. The gap spacer 16a is a regular replacement part in consideration of gap deviation due to wear.

  As shown in the enlarged view of FIG. 6, the LED print head 12a has an LED (light emitting diode) 14a as a light source, and the photosensitive drum 11a is irradiated with LED light through a lens 15a. The focal depth of the lens 15a and the photosensitive drum 11a is about ± 15 μm.

  The electrostatic latent image formed on the surface of the photosensitive drum 11a by the irradiation of the LED light is developed by supplying a developer from the developing device.

  As shown in FIG. 7, the developing devices 17a to 17d are replenished with the developer from the developer replenishing devices 34a to 34d, respectively. In the present embodiment, a two-component developer containing toner particles and carrier particles is used. As a two-component developer that can be used, for example, a ferrite carrier having a diameter of 30 to 60 μm with a silicone or acrylic resin coating on the surface, a 4 to 10 μm polyester or acrylic resin and silica, etc. A toner obtained by mixing 3 to 20% by weight of a toner having a diameter of 4 to 12 [mu] m including an external additive consisting of the above is used. The toner may be either one produced by a pulverization method or one produced by a polymerization method. Also, the carrier may have a form in which the core is not ferrite and the magnetic material is dispersed in the resin.

  Each developer is stirred in the developing devices 17a to 17d, and the toner particles are charged with a negative polarity and the carrier particles are charged with a positive polarity by the friction. The charged toner particles are supplied to the surface of the photosensitive drums 11a to 11d by a magnet roller (not shown), and adhere to a portion where the potential of the photosensitive drum is low with respect to the developing bias applied to the magnet roller. In this embodiment, a DC electric field is applied as the developing bias.

  Through these steps, images are formed on the surfaces of the photosensitive drums 11a to 11d. The developer that has not been used for image formation is collected in a waste developer box 36.

  As shown in FIG. 8, the developing device 17a includes a developing drum 18a disposed so as to face the photosensitive drum 11a. By supplying developer to the surface of the photosensitive drum 11a by the developing roller 18a, the electrostatic latent image formed on the surface of the photosensitive drum 11a is developed (visualized).

  In the present embodiment, the developing roller 18a is disposed at a certain distance from the photosensitive drum 11a. The distance between the developing roller 18a and the photosensitive drum 11a is preferably 0.15 mm to 0.55 mm. “Distance between developing roller and photosensitive drum” is synonymous with “distance between developing means and image carrier”.

  As described above, the LED print head has variations in the beam diameter due to the large number of light emitting points and lenses. The dot diameter obtained after development also varies, and a halftone image with reduced streak unevenness cannot be obtained. In order to obtain a halftone image with reduced streak unevenness, the scattered beam diameter must be corrected and made uniform. Here, a conceptual diagram of beam diameter correction is shown in FIG.

  As shown in FIG. 9A, it is assumed that a beam profile of the type represented by profiles A and B exists. Since profile A is narrower than profile B, developer image A is smaller than developer image B. As shown in FIG. 9B, dot correction is performed by increasing the light amount of the profile A that is further reduced so that the dot diameters at the development threshold are the same.

  Usually, the LED print head is subjected to correction processing for each light emitting point by a current value or the like corresponding to the beam profile. For this reason, as long as the correction process is optimally performed and a uniform dot diameter is maintained, streak unevenness does not occur even in a halftone image.

  However, as described above, the depth of focus between the LED print head (lens) and the photoconductor is said to be about ± 15 μm, and the beam profile varies as the depth of focus varies. Considering the cost and durability of the apparatus, it is difficult to adjust the positions of the photosensitive member and the LED head so that the depth of focus can always be reliably maintained within a predetermined range.

  As can be seen from FIG. 9A, when the development threshold exists near the bottom of the beam profile, the variation of the dot diameter at the development threshold when the profile fluctuates increases. In the region where the exposure energy is higher, there are overlapping portions of the two beam profiles. Therefore, if the development threshold value can be set at a position with higher exposure energy (for example, the center portion of the profile), it is expected that the influence on the dot diameter will be reduced even if the beam profile varies.

  FIG. 10 schematically shows the relationship between the light amount setting (exposure profile) and the reproduction dots, and the photoreceptor characteristics and the development characteristics. Photoreceptor characteristics are represented by changes in the photoreceptor surface potential with respect to changes in exposure energy, and development characteristics are represented by changes in development contrast potential with respect to changes in image ID. The size of the filled area in the beam profile corresponds to the size of the solid portion.

  In FIG. 10A, the development threshold is 17%, and is set to a predetermined light amount here. FIG. 10B shows the result obtained under the same conditions except that the light amount setting is lowered, and shows that the development threshold is 30%. From this result, it can be seen that the development threshold shown on the beam profile is moved to the central portion of the beam profile by reducing the light amount setting.

  With reference to FIG. 11, the relationship between the amount of light and the surface potential will be described.

In the figure, E _{hf on} the horizontal axis represents a half light amount, and E2 and E3 represent three times and three times the half light amount, respectively. FIG. 10 shows the relationship between the exposure light amount and the surface potential for three different adhesion suppression potentials Vc represented by curves Vc1, Vc2 and Vc3.

As shown in I, in the light quantity region less than the half light quantity (E _hf ), even if the adhesion suppression potential Vc changes from Vc1 to Vc3 and the charging potential changes, the ratio of the change in the photoreceptor potential with respect to the light quantity change. Will not change greatly. That is, the slopes of the three curves Vc1, Vc2 and Vc3 do not change significantly.

  In the region of about twice the amount of half light quantity (E2) shown in II, when the adhesion suppression potential Vc varies from Vc1 to Vc3 and the charging potential changes, the rate of change in the photoreceptor potential is large. That is, the three curves have a large change in inclination.

  As shown in III, in the region exceeding 3 times the half light quantity (E3), even if the adhesion suppression potential Vc changes from Vc1 to Vc3 and the charging potential changes, the change in the photoreceptor potential with respect to the light quantity fluctuations. The ratio does not change greatly. In other words, since the potential is completely reduced, the three curves have little change in the inclination.

  Based on these results, it is understood that the light intensity energy of the exposure apparatus is preferably less than twice the half exposure amount. In consideration of the stability and reproducibility of the potential of the photoconductor, the light amount energy of the exposure apparatus is preferably ½ or more of the half exposure amount.

Next, dots were formed by combining LED exposure and two-component development, and the size of the obtained dot diameter was examined. As the exposure apparatus, 600 dpi manufactured by Oki Digital Imaging Corporation was used. The exposure energy of the LED was 4 nJ / nm ² and the LED was subjected to dot correction processing.

  The two-component developer used is composed of a carrier made of Powdertec Co., Ltd., coated with a silicone-based ferrite, and a polyester-based toner.

  First, only a DC bias (−300 v) was applied, and the distance between the photosensitive drum and the developing roller was changed between 0.05 mm and 0.7 mm to form dots on the photosensitive member. The focus position of the LED was shifted by about −15 μm, and the dot diameter correction was performed slightly out of order. Such a deviation of the focal position always occurs due to component tolerances and the like, and even in the initial state, such a fluctuation is a condition that can sufficiently occur. Development was performed with the above-described two-component developer, and the experimental results of reproducing minute points were examined.

Specifically, the reproducibility of 1 dot (ideal diameter 42.3 μm) at 600 dpi was examined. Half-value exposure value of the photosensitive drum is 1.5 nJ / nm ^2, the exposure energy is 4 nJ / nm ^2. Of the isolated points formed on the photoconductor, 100 diameters are photographed with a CCD camera and then measured with ImagePro. The maximum and minimum values are summarized in Table 1 below.

Table 1 below also shows the streaks in the halftone image. The halftone image was evaluated by forming a gradation image of 32 gradations and determining mainly the image quality at the streak level. If no streak irregularity was confirmed by visual inspection, “◯” was indicated. If streak irregularity was confirmed, “x” was indicated. The maximum dot diameter and the minimum dot diameter are also shown in the graph of FIG.

In addition, an experiment was performed under the same conditions as described above except that an AC bias (pp5 kv 1 kHz) was superimposed, and the reproducibility of minute points was examined. The results for maximum dot diameter, minimum dot diameter, and halftone streaks are summarized in Table 2 below. The maximum dot diameter and the minimum dot diameter are also shown in the graph of FIG.

  From the comparison between Table 1 and Table 2, it can be seen that when only the DC bias is applied, the dot diameter is reproduced larger than when the AC bias is superimposed. In addition, in this case, the difference between the maximum dot diameter and the minimum dot diameter is also small, indicating that the dot diameter is very stable.

  As the distance between the photosensitive drum and the developing roller increases, the dot diameter tends to increase. This is because an edge effect is generated in the electric field because the gap is large, and the dots are developed with emphasis more than actual. In this case, the size of the beam diameter is not reproduced faithfully to the dot diameter. However, on the other hand, streak unevenness can be reduced by stabilizing the diameter of minute dots that tend to be unstable due to variations in beam diameter.

  When only the DC bias is applied, as shown in Table 1, if the distance between the photosensitive drum and the developing roller is within a range of 0.15 to 0.55 mm, no halftone streak is confirmed. .

  On the other hand, when the AC bias is superimposed, the difference between the maximum dot diameter and the minimum dot diameter reaches 30 mm at the maximum. Moreover, halftone streaks occur at any distance regardless of the distance between the photosensitive drum and the developing roller.

  From the above results, the streak level on the image is excellent because only the DC bias at which dots are stably reproduced is applied, and the distance between the photosensitive drum and the developing roller is set to 0.15 to .0. It was confirmed that it was a case where it was set within a range of 55 mm.

  Next, exposure was performed by changing the light energy, and streaks in the obtained halftone image were examined.

Using a photoconductor drum having a half-exposure amount of 1.5 nJ / mm ² , the amount of light was varied to determine streaks in the halftone image. As described with reference to FIG. 10, when the light amount setting is lowered, the development threshold value moves. Therefore, the beam profile variation should be further reduced.

The larger the focal position shift, the greater the variation in beam profile. Therefore, the gap between the photosensitive drum and the LED was set to 30 μm and 50 μmn, a halftone image was formed by the same method as described above, and the generated stripe unevenness was evaluated. The results are summarized in Table 3 below. In Table 3, “Δ” indicates that some streak unevenness occurred.

As shown in Table 3, if the light quantity is within the range of 0.75 to 3 nJ / mm ^2, no halftone streak is confirmed regardless of the gap between the photosensitive drum and the LED. Since the half-exposure amount of the photosensitive drum is 1.5 nJ / mm ² , the range of the light amount corresponds to 1/2 to 2 times the half-exposure amount.

  It has been confirmed that the unevenness of the stripes can be further reduced by setting the light energy to ½ or more and half or less of the half exposure amount of the photosensitive drum. By setting and using these conditions, even if the parts that determine the gap are scraped due to parts accuracy, environment, continuous use, etc., the occurrence of streaks in the halftone image is reduced. It is predicted that

  According to at least one embodiment described above, the image carrier has a plurality of light sources and a plurality of lenses provided along the longitudinal direction of the image carrier, and the image carrier is irradiated with light. For exposing the electrostatic latent image on the image carrier to a visible image with a two-component developer by applying a direct current bias. By providing the developing device, it is possible to form a halftone image with reduced stripe unevenness.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus; 10 ... Scanner unit 11a-11d ... Photosensitive drum; 12a-12d ... LED print head 13a-13d ... LED print head contact | separation lever 14a ... LED (light emitting diode); 15a ... Lens 16a ... Gap Spacers; 17a to 17d ... developing device;
18 ... developing roller; 19 ... transfer belt; 20 ... cleaner 23 ... registration roller; 25 ... image quality maintenance control unit; 27 ... secondary transfer roller 29 ... transfer device; 30 ... main power switch; 31 ... discharge tray 32 ... manual feed Tray; 34 ... Developer supply device; 36 ... Waste developer box.

Claims (4)

An image carrier on which an electrostatic latent image is formed by exposure; and
An exposure apparatus having a plurality of light sources and a plurality of lenses provided along a longitudinal direction of the image carrier, and irradiating the image carrier with light;
A developing device disposed at a certain distance from the image carrier and developing the electrostatic latent image on the image carrier into a visible image with a two-component developer by applying a DC bias; An image forming apparatus comprising:   The image forming apparatus according to claim 1, wherein a distance between the developing device and the image carrier is 0.15 mm or more and 0.55 mm or less.   3. The image according to claim 1, wherein the image bearing member is a photoconductor, and the light intensity energy of the exposure device is ½ or less and less than twice the half-exposure amount of the photoconductor. Forming equipment.   The image forming apparatus according to claim 1, wherein the exposure device is an LED.

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