JP4622344B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a printer or a copying machine employing an electrophotographic method, and more particularly to a transfer bias control technique for transferring an image from an image carrier such as a photosensitive drum to an intermediate transfer member.

Japanese Patent Laid-Open No. 2002-372876

  Conventionally, as an image forming apparatus such as a printer or a copying machine adopting this type of electrophotographic system, the surface of a photosensitive drum as an image carrier is charged to a predetermined potential by a charger, and then according to image information. Image exposure is performed to form an electrostatic latent image, and the electrostatic latent image is visualized as a toner image, and the toner image is transferred and fixed onto a transfer sheet via an intermediate transfer member. Is configured to form

  In such an image forming apparatus, when a toner image formed on a photosensitive drum is transferred onto an intermediate transfer member, a transfer bias voltage is applied to the intermediate transfer member, and the toner image formed on the photosensitive drum is applied. Is electrostatically transferred onto the intermediate transfer member. The resistance value of the intermediate transfer member and the voltage applied to the intermediate transfer member are determined mainly from the viewpoint of giving priority to the transfer performance of the toner image from the photosensitive drum. As a technique relating to the control of the transfer bias voltage and the like, for example, there is one disclosed in JP-A-2002-372876.

  Further, in the image forming apparatus, after the toner image is transferred from the photosensitive drum to the intermediate transfer member, a charging ghost appears as a latent image history corresponding to the toner image on the surface of the photosensitive drum. Therefore, the image forming apparatus is configured to erase the charged ghost by irradiating the surface of the photosensitive drum after the transfer process is completed with an erase lamp.

  Further, since the photosensitive drum is rotated while being in contact with the intermediate transfer member, the photosensitive drum is photosensitive for the purpose of suppressing charge injection to the photosensitive drum side by the voltage applied to the intermediate transfer member. It is configured to control charge injection resistance by selecting material characteristics of the body drum.

  However, the above prior art has the following problems. That is, in the case of the conventional image forming apparatus described above, the interaction with the charging ghost of the photosensitive drum can be prevented by using another member such as an erase lamp together or by selecting the material characteristics of the photosensitive drum. In many cases, this is handled by controlling the charge injection property, which increases costs as the number of parts increases, and narrows the overall system design latitude to cope with charging ghosts and device maintainability. Had the problem of becoming.

  More specifically, in a system in which a toner image formed on a photosensitive drum is transferred to an intermediate transfer member to form an image, the charging potential applied to the photosensitive drum is opposite to the charged potential by a transfer bias voltage applied to the intermediate transfer member. Polarity charge injection is likely to occur, and there has been a problem that the charging voltage of the photosensitive drum is lowered and the charger is deteriorated due to an excessive load of the charger. In particular, when a charger that applies only a DC bias voltage is used, the above-described problem becomes significant because the charging capability is relatively low.

  Further, the charging ghost is caused by the charging voltage being lowered because the previous latent image formation history cannot be recovered at the next charging due to insufficient charging ability of the charger. This charging ghost is usually prevented by eliminating the latent image history using an erase lamp or the like as described above, but in this case, as described above, the cost increases as the number of parts increases. The problem of inviting up occurs.

  Further, the charging ghost can be eliminated by setting the resistance value of the intermediate transfer member to be low and positively injecting charge from the intermediate transfer member to the photosensitive drum by the transfer electric field. Can do.

  However, in this case, the intermediate transfer member is configured to have a relatively low resistance in order to positively generate charge injection, but on the other hand, the presence or absence of a developed image in the transfer portion, and resistance change due to temperature. A new problem arises that it is relatively susceptible to influence.

  That is, when a developed image is present in the transfer portion, the developer is interposed between the photosensitive drum and the intermediate transfer member, so that charge injection is unlikely to occur, whereas no developer is interposed in the background portion. Therefore, charge injection is likely to occur. At this time, if the resistance value of the intermediate transfer member is too low, the background latent image ghost is developed by the excessive charge injection in the background portion without developing the charged potential of the background portion in the next cycle. Will occur. On the other hand, when the resistance value of the intermediate transfer member is too high, if a history removing means such as an erase lamp is not provided, charge injection from the intermediate transfer member is not performed, and a positive latent image ghost is generated.

  In addition, when the resistance value of the intermediate transfer member decreases as the temperature rises, the current flowing into the transfer portion increases, so that the negative latent image ghost deteriorates as described above.

  Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and its object is to increase the number of parts such as erase lamps and to select the material characteristics of the photosensitive drum. An object of the present invention is to provide an image forming apparatus that can cope with charging ghosts and charge injection properties to an image carrier without depending on the cost, and can reduce the cost of the apparatus and widen the latitude of the design of the entire system. .

In order to achieve the above object, the invention described in claim 1 is characterized in that a toner image formed on an image carrier charged by contact-type charging means to which only a DC voltage is applied is applied to the image carrier. The image is formed by transferring the image onto the transfer sheet once after being transferred onto the intermediate transfer body to which the transfer bias having a polarity opposite to the charged polarity of the image carrier is applied, and the previous time. In an image forming apparatus that does not include a history removing unit that removes the history of image formation of
The intermediate transfer member disposed so as to be in contact with the image carrier has a releasable outermost surface layer,
Before the surface potential of the image portion of the image carrier shifts to the next image forming cycle, the image is subjected to charge injection from the intermediate transfer body during charging of the toner image and charging by the charging means. The voltage applied to the intermediate transfer member is set so that the surface potential of the non-image portion of the carrier is 95% to 105%, and the number of images formed in one image forming operation of the transfer sheet is set. Accordingly, the image forming apparatus is characterized in that the voltage applied to the intermediate transfer member is gradually lowered toward a preset value.

  According to the second aspect of the present invention, the charge injection from the intermediate transfer body during transfer of the toner image until the surface potential of the image portion of the image carrier shifts to the next image forming cycle, and 2. The image according to claim 1, wherein the voltage applied to the intermediate transfer member is set to be equal to the surface potential of the non-image portion of the image carrier after being charged by the charging unit. Forming device.

Furthermore, the invention described in claim 3 is arranged so that the toner image formed on the image carrier charged by the contact-type charging means to which only a DC voltage is applied is in contact with the image carrier. The image is transferred onto an intermediate transfer member to which a transfer bias having a polarity opposite to the charging polarity of the image carrier is applied, and then transferred onto a transfer sheet to form an image, and the previous image formation history In an image forming apparatus that does not include a history removing means for removing
The intermediate transfer member disposed so as to be in contact with the image carrier has a releasable outermost surface layer,
In a state where a transfer bias capable of obtaining a predetermined transfer efficiency is applied to the intermediate transfer member, the surface potential of the image portion of the image carrier is shifted to the next image forming cycle before transferring the intermediate image. After receiving the charge injection from the transfer body and charging by the charging means, the resistance value of the intermediate transfer body is set so that the surface potential of the non-image part of the image carrier is 95% to 105%. And setting the voltage applied to the intermediate transfer body gradually toward a preset value in accordance with the number of images formed in one image forming operation of the transfer paper. The image forming apparatus.

  The invention according to claim 4 is characterized in that the intermediate transfer member is formed by coating a conductive rubber on a core material made of a conductive material. The image forming apparatus according to any one of the above.

  Furthermore, the invention described in claim 5 is characterized in that the intermediate transfer member is configured in a belt shape having at least the surface thereof made of conductive rubber. This is an image forming apparatus.

  According to the present invention, it is possible to cope with the charge ghost and the charge injection property to the image carrier without depending on the increase in the number of parts such as the erase lamp and the selection of the material characteristics of the photosensitive drum. Therefore, it is possible to provide an image forming apparatus capable of reducing the cost and widening the design latitude of the entire system.

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

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

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

  As shown in FIG. 3, the photosensitive drums 11, 12, 13, and 14 are arranged in parallel to each other at a constant interval so as to have a common tangential plane M.sub.2. Further, the first primary intermediate transfer drum 51 and the second primary intermediate transfer drum 52 are surfaces whose rotational axes are parallel to the photosensitive drums 11, 12, 13, and 14 and have predetermined symmetry planes as boundaries. They are arranged in a symmetrical relationship. Further, the secondary intermediate transfer drum 53 is arranged so that the rotation axis thereof is parallel to the photosensitive drums 11, 12, 13, and 14.

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

  Around each of the photosensitive drums 11, 12, 13, and 14, an image forming process for each color is executed by a well-known electrophotographic method. First, as the photoconductor drums 11, 12, 13, and 14, for example, photoconductor drums using an OPC photoconductor having a diameter of 20 mm are used. These photoconductor drums 11, 12, 13, and 14 are, for example, It is rotationally driven at a rotational speed of 95 mm / sec. As shown in FIG. 3, the surfaces of the photosensitive drums 11, 12, 13, and 14 are applied with a DC voltage of about -800 V to charging rolls 21, 22, 23, and 24 as contact-type charging means. For example, it is charged to about -300V. The contact type charging means includes a roll type, a film type, a brush type, etc., but any type may be used. In this embodiment, a charging roll generally used in an electrophotographic apparatus in recent years is employed. Further, in order to charge the surfaces of the photosensitive drums 11, 12, 13, and 14, in this embodiment, a charging method in which only DC is applied is adopted.

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

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

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

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

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

  The toner supplied onto the developing roll 401 is in the form of a magnetic brush composed of a carrier and toner by the magnetic force of the magnet roll, and this magnetic brush comes into contact with the photosensitive drums 11, 12, 13, and 14. ing. A developing bias voltage of AC + DC is applied to the developing roll 401 to develop the toner on the developing roll 401 into an electrostatic latent image formed on the photosensitive drums 11, 12, 13, and 14. It is formed. In this embodiment, the development bias voltage is about 4 kHz for AC, 1.5 kVpp, and about −230 V for DC.

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

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

The first and second primary intermediate transfer drums 51 and 52 used in this embodiment have an outer diameter of 42 mm, for example, and a resistance value of about 10 ⁸ Ω. As shown in FIG. 4, the first and second primary intermediate transfer drums 51 and 52 are cylindrical rotating bodies having a single layer or a plurality of layers whose surfaces are flexible or elastic. In this case, a low resistance elastic rubber layer 56 (R = 10 ² to 10 ³ Ω) represented by conductive silicone rubber or the like is formed on a metal pipe 55 as a metal core made of Fe, Al or the like with a thickness of 0.1. It is provided at ~ 10mm. Further, the outermost surfaces of the first and second intermediate transfer drums 51 and 52 are typically made of a high release layer 57 (R = 10 ⁵⁾ having a thickness of 3 to 100 μm of fluororubber in which fluororesin fine particles are dispersed. ˜10 ⁹ Ω) and bonded with a silane coupling agent-based adhesive (primer) 58. What is important here is the resistance value and the releasability of the surface, and the resistance value of the high release layer is about R = 10 ⁵ to 10 ⁹ Ω. It is not limited.

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

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

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

  Next, the final toner image from the single color image to the quadruple color image formed on the secondary intermediate transfer drum 53 is tertiary transferred onto the transfer paper P passing through the paper transport path by the final transfer roll 60. Is done. As shown in FIG. 2, the transfer paper P passes through a registration roller 90 from a paper feed cassette 91 through a paper feeding process, and is fed into a nip portion between the secondary intermediate transfer drum 53 and the transfer roll 60. After this final transfer step, the final toner image formed on the transfer paper 101 is fixed by heat and pressure by the fixing device 70, and is discharged onto a discharge tray T provided at the top of the full-color printer 01. A series of image forming processes is completed.

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

  Further, when images are formed on both sides of the transfer paper P, as shown in FIG. 2, the transfer paper P on which the image is formed on one side is not directly discharged onto the discharge tray T, but is turned upside down. In this state, the transfer paper P is conveyed again to the transfer section via the double-sided paper conveyance path 07, and an image is formed on the back surface of the transfer paper P.

  In FIG. 2, reference numeral 08 denotes a paper feed roll for transporting the transfer paper P supplied from a manual feed tray (not shown), 09 denotes a control circuit, and 10 denotes a power supply circuit.

  By the way, in this embodiment, the toner image formed on the image carrier charged by the contact-type charging means to which only a DC voltage is applied is disposed so as to contact the image carrier, and In an image forming apparatus that forms an image by temporarily transferring onto an intermediate transfer member to which a transfer bias having a polarity opposite to the charging polarity of the image carrier is applied, and then transferring the image onto a transfer sheet, the image portion of the image carrier Until the surface potential of the toner image is transferred to the next image forming cycle, after being charged by the intermediate transfer member and charged by the charging means at the time of transfer of the toner image, the non-image portion of the image bearing member The voltage applied to the intermediate transfer member is set so as to have a value of 95% to 105% of the surface potential.

  In this embodiment, the intermediate transfer member is formed by coating a conductive rubber on a core made of a conductive material.

  The intermediate transfer member is not limited to this, and at least the surface of the intermediate transfer member may be configured as a belt made of conductive rubber.

  That is, in this embodiment, as shown in FIGS. 2 and 3, the surfaces of the photoconductive drums 11, 12, 13, and 14 are placed on charging rolls 21, 22, 23, and 24 as contact-type charging means. By applying only a DC voltage of −800 V, for example, after charging to about −300 V, image exposure is performed to form an electrostatic latent image, and the electrostatic latent image is developed into developing devices 41, 42, 43, The toner image formed on the photosensitive drums 11, 12, 13, 14 is primarily transferred onto the first or second primary intermediate transfer drums 51, 52. Has been.

  At that time, for example, a transfer bias voltage of about +380 V is applied to the first and second primary intermediate transfer drums 51 and 52. Since the first and second primary intermediate transfer drums 51 and 52 are in contact with the surface of the photosensitive drums 11 and 12 or the photosensitive drums 13 and 14, the photosensitive drums 11 and 12 or the photosensitive drum 13 are in contact with each other. , 14, when the toner image is primarily transferred onto the first or second primary intermediate transfer drums 51, 52, the surface of the photosensitive drums 11, 12 or the photosensitive drums 13, 14 is injected by charge injection. It has the effect of discharging or charging the positive polarity opposite to the original charged polarity.

  As a result, the surface potentials of the photosensitive drums 11 and 12 or the photosensitive drums 13 and 14 are, as shown in FIG. 1, a non-image portion (background portion) charged to −300 V and -60 V by image exposure. The image portion whose potential has been attenuated by about a negative potential is affected by the transfer electric field of the first and second primary intermediate transfer drums 51 and 52 to which the transfer bias voltage of about +380 V is applied by the transfer bias power source 54. Decreases and the potential becomes uniform.

  Thereafter, the surfaces of the photosensitive drums 11 and 12 and the photosensitive drums 13 and 14 are set to about −300 V by charging rolls 21, 22, 23, and 24 as contact-type charging means in preparation for the next image forming cycle. Uniformly charged.

  However, the surface potential of the photosensitive drums 11 and 12 or the photosensitive drums 13 and 14 depends on whether the surface of the photosensitive drum was an image portion or a non-image portion at the time of the previous image formation. The charging history at the time was different, and in the case of the image portion, the potential was about -60 V, whereas in the case of the non-image portion, the potential was about -300 V, about 5 times. There is a difference.

  Therefore, even in the case where a charging process for uniformly charging to about −300 V is performed by the charging rolls 21, 22, 23, and 24 as contact-type charging means in preparation for the next image forming cycle, the above-described charging history As a result, the target potential is not uniformly charged to about -300V. Note that the influence of the charging history or the like does not affect the plurality of image forming cycles, and it is sufficient to consider the image forming cycles before and after taking into account the natural potential decay of the photosensitive drum.

  Incidentally, as shown in FIG. 1, the present inventors uniformly charge the surfaces of the photosensitive drums 11, 12, 13, and 14 by charging rolls 21, 22, 23, and 24 as contact-type charging means. In the printer, the first and second primary intermediate transfer drums 51 and 52 which are in contact with the surfaces of the photosensitive drums 11, 12, 13 and 14 are also used as charging means, like the charging rolls 21, 22, 23 and 24. When the transfer bias voltage applied to the primary intermediate transfer drums 51 and 52 is changed within a range where good transfer efficiency can be obtained based on the idea that it must function, these primary intermediate transfer drums 51 and 52 An experiment was conducted in a laboratory environment to confirm what conditions the applied voltage of, 52 could meet to prevent the generation of charging ghosts.

  As a result, the present inventors have confirmed that the charge from the primary intermediate transfer drums 51, 52 during the transfer of the toner image before the surface potential of the photosensitive drums 11, 12, 13, 14 shifts to the next image forming cycle. After being injected and charged by the charging rolls 21, 22, 23, 24, the surface potential of the non-image area of the photosensitive drums 11, 12, 13, 14 is 95% to 105%. It has been found that if the voltage applied to the primary intermediate transfer drums 51 and 52 is set, the generation of the charging ghost can be prevented or suppressed to a level that does not cause a problem.

  That is, the inventors use a printer manufactured by Fuji Xerox Co., Ltd. having the same configuration as the printer shown in FIGS. 2 and 3 and a test bench modified based on the product name “DPC1616”. The surface potential (non-image portion) of the photosensitive drums 11, 12, 13, and 14 is set to -300V, the surface potential of the image portion is set to -60V, and the density of the image portion is set to 100%. When an image was formed by changing the applied voltage V1 to 52 from −380V at the center of setting to the minus direction and the plus direction every 50V, an experiment was conducted to visually confirm the state of occurrence of the charging ghost.

  The generation state of the charging ghost was evaluated by grading. Grade 0 is a state where no charging ghost is generated, Grade 1.0 is a state where charging ghost is generated, but is a level that does not cause a problem in image quality, and Grade 2.0 is a state where charging ghost is generated. In the case of grade 3.0 or higher, a charged ghost is clearly generated, indicating a state in which the image quality is at a problem level. Some cases were evaluated every 0.5. As a result, grade 1.5 indicates a state in which a charging ghost has occurred and the image quality is problematic.

  FIG. 5 is a chart and graph showing the results of the experiment.

  As apparent from FIG. 5, the surface potential Vai of the image portions of the photosensitive drums 11, 12, 13, and 14 is ± 5% of the surface potential (non-image portion) of the photosensitive drums 11, 12, 13, and 14. = If the voltage is within the range of about ± 15 V, the generation grade of the charging ghost is within the allowable range of 1.0 or less, and the generation of the charging ghost can be performed without depending on other means such as an erase lamp. It was found that it can be reduced to a problem-free level.

  Further, by setting the applied voltage V1 to the primary intermediate transfer drums 51 and 52 to be higher by + 100V than the nominal set value, the surface potentials of the image areas and the non-image areas of the photosensitive drums 11, 12, 13, and 14 are set. Was found to be equal.

  It should be noted that even if the surface potential of the image areas of the photoconductive drums 11, 12, 13, 14 is set to other values, the same tendency is shown, and the non-image areas of the photoconductive drums 11, 12, 13, 14 are shown. It was confirmed that the surface potential should be set to 95% to 105%.

  As described above, according to the above-described embodiment, the charging ghost and the charge injection property to the image carrier can be handled without depending on the increase in the number of parts such as the erase lamp and the selection of the material characteristics of the photosensitive drum. Therefore, it is possible to reduce the cost of the apparatus and widen the latitude of the design of the entire system.

  In this configuration, since the charger needs to perform charging by supplementing the charge injection received from the intermediate transfer member, the load tends to be excessive. Therefore, it is desirable to set the transfer voltage as low as possible. However, in order to obtain the original transfer property, a certain level of transfer voltage is required.

  The resistance value of the intermediate transfer member decreases as the temperature rises due to the printing operation of the image forming apparatus. If the applied voltage is constant, an excessive current flows and the load on the charger increases.

  For this reason, it has been found that the temperature rise of the intermediate transfer member depends not on the running mode but on the number of printed sheets. Therefore, the correction width is determined from the accumulated number of sheets, and the voltage applied to the intermediate transfer member when the temperature rise is saturated. It is desirable that the voltage be gradually lowered by about 150V with respect to the initial set value. Further, a correction according to the time of the voltage applied to the intermediate transfer member may be used together. For example, in the printer described above, every time 16 sheets are printed, the voltage is lowered by about 5 V, and when printing is stopped, so-called resting, control is performed to gradually return to the original applied voltage using a timer.

  Further, in the above-described embodiment, before the surface potential of the image portion of the image carrier shifts to the next image forming cycle, the charge is injected from the intermediate transfer member at the time of transferring the toner image, and the charging means is used. Although the case where the voltage applied to the intermediate transfer member is set so as to have a value of 95% to 105% of the surface potential of the non-image portion of the image carrier after being charged has been described, the present invention is not limited thereto. In the state where a transfer bias capable of obtaining a predetermined transfer efficiency is applied to the intermediate transfer member, the surface potential of the image portion of the image carrier is not changed until the next image forming cycle. After receiving charge injection from the intermediate transfer member and charging by the charging means at the time of image transfer, the intermediate value is set to 95% to 105% of the surface potential of the non-image portion of the image carrier. Set the resistance value of the transfer body Good.

The resistance value of the intermediate transfer member is set to about 10 ⁸ Ω, for example. The resistance value of the intermediate transfer member can be set, for example, by changing the center value of the film thickness of the top layer. By changing the film thickness of the top layer of the intermediate transfer member, the charge property to be injected to the photosensitive drum accompanying the transfer operation is changed. Although there is no significant change in the resistance value as an intermediate transfer member, the apparent resistance as a system obtained from the current flowing between the intermediate transfer member and the photosensitive drum changes, and the resistance on the system is controlled by the film thickness Is possible.

FIG. 1 is an explanatory diagram showing a control principle in a tandem type full-color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a configuration diagram showing a tandem type full-color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 3 is a block diagram showing an image forming unit of a tandem type full color printer as an image forming apparatus according to Embodiment 1 of the present invention. FIG. 4 is a cross-sectional configuration diagram showing the primary intermediate transfer drum. FIG. 5 is a chart and graph showing experimental results.

Explanation of symbols

  11, 12, 13, 14: photosensitive drum (image carrier), 21, 22, 23, 24: charging roll (charging means), 51, 52: first and second primary intermediate transfer drums (intermediate transfer body) ), 54: Transfer bias power supply.

Claims (5)

A toner image formed on an image carrier charged by a contact-type charging means to which only a DC voltage is applied is disposed so as to contact the image carrier and is opposite to the charging polarity of the image carrier. An image forming apparatus that forms an image by transferring it onto an intermediate transfer member to which a polar transfer bias is applied and then transferring it onto a transfer sheet, and does not include a history removing unit that removes the previous image forming history. In
The intermediate transfer member disposed so as to be in contact with the image carrier has a releasable outermost surface layer,
Before the surface potential of the image portion of the image carrier shifts to the next image forming cycle, the image is subjected to charge injection from the intermediate transfer body during charging of the toner image and charging by the charging means. The voltage applied to the intermediate transfer member is set so that the surface potential of the non-image portion of the carrier is 95% to 105%, and the number of images formed in one image forming operation of the transfer sheet is set. Accordingly, the image forming apparatus is configured to gradually decrease the voltage applied to the intermediate transfer member toward a preset value. Before the surface potential of the image portion of the image carrier shifts to the next image forming cycle, the image is subjected to charge injection from the intermediate transfer body during charging of the toner image and charging by the charging means. The image forming apparatus according to claim 1, wherein a voltage applied to the intermediate transfer member is set to be equal to a surface potential of a non-image portion of the carrier. A toner image formed on an image carrier charged by a contact-type charging means to which only a DC voltage is applied is disposed so as to contact the image carrier and is opposite to the charging polarity of the image carrier. An image forming apparatus that forms an image by transferring it onto an intermediate transfer member to which a polar transfer bias is applied and then transferring it onto a transfer sheet, and does not include a history removing unit that removes the previous image forming history. In
The intermediate transfer member disposed so as to be in contact with the image carrier has a releasable outermost surface layer,
In a state where a transfer bias capable of obtaining a predetermined transfer efficiency is applied to the intermediate transfer member, the surface potential of the image portion of the image carrier is shifted to the next image forming cycle before transferring the intermediate image. After receiving the charge injection from the transfer body and charging by the charging means, the resistance value of the intermediate transfer body is set so that the surface potential of the non-image part of the image carrier is 95% to 105%. And setting the voltage applied to the intermediate transfer body gradually toward a preset value in accordance with the number of images formed in one image forming operation of the transfer paper. Image forming apparatus. 4. The image forming apparatus according to claim 1, wherein the intermediate transfer member is configured by covering a core made of a conductive material with a conductive rubber. 5. The image forming apparatus according to claim 1, wherein at least a surface of the intermediate transfer member is configured in a belt shape made of conductive rubber.

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