JP2007147776A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent current leakage without provision of a grounding circuit etc., by lowering a transfer bias voltage in a final transfer member, and to maintain stable image quality regardless of environmental temperature and humidity. <P>SOLUTION: The configuration to eliminate the need for the grounding circuit by lowering the transfer bias voltage of a transfer roller 74 itself (alleviating a potential difference with respect to a grounding level) is employed. In order to realize such configuration, a photoreceptor 5 is determined at -1,000V, first intermediate transfer drums 68, 70 are determined at -350V, a second intermediate transfer drum 72 is determined at 50V, and a transfer roller 74 at 400V. These numerical values are employed at the time of the high temperature and high humidity. Namely, the regulation of the polarity of the toner and the polarity of the transfer bias voltage to be applied to the intermediate transfer rollers 68, 70 to the same polarity is made possible by regulation of the shape (true sphere) of the toner and the surface roughness and hardness of the first intermediate transfer drums 68, 70. As a result, the transfer bias voltage to be applied to the transfer roller 74 is regulated to the voltage at which the current leakage does not occur. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  In the present invention, the surface of the image carrier is uniformly charged, and an electrostatic latent image is formed by irradiating the uniformly charged image carrier with a light beam according to image data. Development using toner charged to positive or negative polarity supplied from the toner, and transferring the toner image to a recording sheet by a final transfer member via an intermediate transfer member, and fixing the transferred toner image The present invention relates to an image forming apparatus including an image forming engine for forming an image.

  Conventionally, in an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine to which this type of electrophotographic method is applied, the photosensitive drum as an image bearing member is centered and the circumferential surface of the photosensitive drum is opposed. A charging unit, an optical scanning unit, a developing unit, a transfer unit, and the like are arranged as described above.

  That is, the surface of the photosensitive drum is uniformly charged by applying a predetermined voltage to the charging unit, an electrostatic latent image is formed by the light beam from the optical scanning unit, and toner is supplied to the developing unit for development. Then, for example, after the toner image is transferred to an intermediate transfer member or the like in the transfer portion, the image is transferred to the recording paper by the final transfer member.

  The recording sheet on which the image has been transferred is subjected to a fixing process in the fixing unit while being conveyed to the discharge port.

  In this way, in order to transfer the toner image on the photosensitive drum to the recording paper by the final transfer member via the intermediate transfer member, each member (photosensitive drum, intermediate transfer member, final transfer member) has a predetermined value. A transfer bias voltage is applied to transfer toner charged positively or negatively (usually negatively charged).

  Here, as the number of transfer processes increases, the transfer bias voltage tends to increase as it goes downstream, and the potential difference between the transfer bias voltage applied to the final transfer body and the ground (GND) is very large. It has become.

  When this potential is increased, a transfer failure occurs due to the transfer current escaping to GND through the water-containing recording paper or the plastic recording paper during the image forming process on the water-containing recording paper or the plastic recording paper. There is.

  As a technique related to the transfer bias voltage, Patent Document 1 discloses that a transfer bias voltage is not applied to at least one intermediate transfer member when forming a potential gradient in each part of the image carrier, the intermediate transfer member, and the final transfer member. Is disclosed.

In order to eliminate the leakage of the current, a ground circuit is provided via an element such as a diode or a varistor for a metal member or a fixing unit disposed around the final transfer member. As a technique of such a grounding circuit, Patent Document 2 discloses at least one of the upstream side and the downstream side of the transfer portion in the final transfer body in a recording paper transport system on which the toner image is transferred by the final transfer body. In other words, it is disclosed to be electrically grounded through an impedance element.
Japanese Patent Laid-Open No. 2004-20860 JP 2004-205559 A

  However, providing a grounding circuit as in the prior art increases the number of parts and lowers the assembly workability.

  In addition, the resistance value of each part increases with time, and the transfer bias voltage must be further increased accordingly, so that current leakage from the ground circuit occurs. Such a phenomenon becomes prominent in an environment of high temperature and high humidity or low temperature and low humidity.

  In addition, in this application, high temperature high humidity says 26 degreeC-30 degreeC, 65RH% -90RH%, and low temperature low humidity says 10 degreeC-15 degreeC, 10RH% -20RH%. It should be noted that each numerical value needs to be corrected within a certain range depending on the surrounding steady environment.

  In consideration of the above-mentioned facts, the present invention can reduce the transfer bias voltage in the final transfer member, prevent current leakage without providing a ground circuit, and maintain stable image quality regardless of environmental temperature and humidity. An object of the present invention is to obtain an image forming apparatus capable of achieving the above.

  In the first invention, after the surface of the image carrier is uniformly charged and an electrostatic latent image is formed by irradiating the uniformly charged image carrier with a light beam according to image data, Development is performed using positively or negatively charged toner supplied from a developing unit, and the toner image is transferred to a recording sheet by a final transfer member via an intermediate transfer member, and the transferred toner image is fixed. An image forming apparatus including an image forming engine for forming an image by performing a predetermined transfer bias voltage so as to have a predetermined potential gradient between the image carrier, the intermediate transfer body, and the final transfer body. Is applied, and the potential gradient forming means applies a transfer bias voltage having the same polarity as that of the toner to at least one intermediate transfer member in contact with the image carrier. .

  According to the first invention, the polarity of the transfer bias voltage from the image carrier to the intermediate transfer member to which the toner image is transferred is the same as that of the toner. A level at which no current leakage occurs can be achieved.

  This eliminates the need for a ground circuit or the like via a diode or varistor for preventing current leakage, reduces the number of components, and improves the assembly workability.

  Further, according to the above, since the transfer bias voltage of the final transfer body is lowered, it is possible to suppress an increase in resistance with time and to prevent occurrence of transfer failure.

  In the first invention, the potential gradient forming unit sets a transfer bias voltage to the final transfer body as a reference transfer bias voltage according to environmental conditions, correlates with the set reference transfer bias voltage, and performs the intermediate transfer. A transfer bias voltage to the transfer body and the image carrier is set, and respective potential gradients are formed.

  For example, when the environmental conditions are temperature and humidity, transfer defects are likely to occur at high temperatures and high humidity and low temperatures and low humidity. Also, the appropriate transfer bias voltage differs between high temperature and high humidity and low temperature and low humidity.

  Therefore, based on the environmental conditions, first, the reference transfer bias voltage is set as the transfer bias voltage of the final transfer member, and the transfer bias to the intermediate transfer member and the image carrier is set in correlation with the reference transfer bias voltage. Thus, a predetermined potential gradient is formed.

  In the first aspect of the invention, the toner has a shape factor of 125 or less and the surface roughness of the intermediate transfer member under the condition that the polarity of the transfer bias voltage applied to the intermediate transfer member is the same as the polarity of the toner. RZ is 1.5 μm or less and the hardness is 50 AskerC or less.

  The transfer efficiency involves not only the potential gradient due to the application of the transfer bias voltage but also the shape of the toner, the surface roughness of the intermediate transfer member that receives the transfer of the toner image, and the hardness. Therefore, the transfer efficiency can be maintained by numerically defining these.

  In a second aspect of the invention, after the surface of the image carrier is uniformly charged and an electrostatic latent image is formed by irradiating the uniformly charged image carrier with a light beam according to image data, Development is performed using positively or negatively charged toner supplied from a developing unit, and the toner image is transferred to a recording sheet by a final transfer member via an intermediate transfer member, and the transferred toner image is fixed. An image forming apparatus including an image forming engine for forming an image by performing a predetermined transfer bias voltage so as to have a predetermined potential gradient between the image carrier, the intermediate transfer body, and the final transfer body. The potential gradient forming means sets a transfer bias voltage to the final transfer body as a reference transfer bias voltage, and from the final transfer body to which the transfer bias voltage is applied, in front Retroactively the processing steps of the image forming engine, the intermediate transfer body, is characterized by setting the respective transfer bias voltage to the image carrier.

  According to the second invention, the transfer bias voltage of the final transfer body with the highest transfer bias voltage is set in advance as the reference transfer bias voltage at which no current leakage can occur. Based on this reference transfer bias voltage, the transfer bias voltage to the intermediate transfer member and the image carrier is set from the final transfer member by going back through the processing steps of the image forming engine.

  Thereby, the transfer bias voltage to the final transfer member can be set to a level at which no current leakage occurs.

  This eliminates the need for a ground circuit or the like via a diode or varistor for preventing current leakage, reduces the number of components, and improves the assembly workability.

  Further, according to the above, by lowering the reference transfer bias voltage of the final transfer body, it is possible to suppress an increase in resistance with time and to prevent occurrence of transfer failure.

  Further, when setting the reference transfer bias voltage, it is preferable to take into consideration that no transfer failure occurs in the image carrier-intermediate transfer member or intermediate transfer member-final transfer member due to the transfer bias voltage set retroactively.

  In the second aspect of the invention, the reference transfer bias voltage is set based on conductivity of the recording paper and a conveyance system of the recording paper.

  The reference transfer bias voltage is set when a recording sheet having a high conductivity, such as a water-containing recording sheet or a plastic recording sheet, is used as the recording sheet, or based on the conductivity of a conveyance system that conveys the recording sheet.

  In the first invention and the second invention, the image forming engine includes a plurality of image carriers for different colors as the image carrier, and one of the plurality of image carriers as the intermediate transfer member. A first upstream intermediate transfer member that contacts the first portion, a first downstream intermediate transfer member that contacts a remaining part of the plurality of image carriers, the first upstream intermediate transfer member, and the first intermediate transfer member. A second intermediate transfer body in which the toner image is transferred from the first downstream intermediate transfer body after the toner image is transferred from the first upstream intermediate transfer body. And the final transfer member is opposed to the second intermediate transfer member.

  As a structure of the image forming engine, for example, when forming a full-color image, it is necessary to superimpose at least four CMYK images.

  Therefore, a first upstream intermediate transfer member to which a toner image is transferred from two of them (for example, an image carrier corresponding to C color and an image carrier corresponding to M color) and the remaining two colors ( For example, a first downstream intermediate transfer member to which a toner image is transferred from an image carrier corresponding to Y color and an image carrier corresponding to K color is provided. Further, a toner image for two colors is transferred from the first upstream intermediate transfer member, and then the remaining two-color toner images are transferred from the first downstream intermediate transfer member. And the second intermediate transfer member and the final transfer member are opposed to each other.

  In a state where all four colors are overlapped, a recording sheet is sandwiched and conveyed between the second intermediate transfer member and the final transfer member, whereby a full color image can be formed on the recording sheet.

  As the number of intermediate transfer members as described above increases, the transfer bias voltage tends to increase. According to the first or second invention, the increase in the transfer bias voltage is suppressed while maintaining the image quality. be able to.

  As described above, according to the present invention, the transfer bias voltage at the final transfer body can be lowered, current leakage can be prevented without providing a ground circuit, and stable image quality can be maintained regardless of the environmental temperature and humidity. It has an excellent effect of being able to.

[First Embodiment]
(Schematic configuration of image forming apparatus)
FIG. 1 shows an outline of an image forming apparatus 10 according to the first embodiment. The image forming apparatus 10 includes an engine unit 12, and a paper feeding unit 14 is provided below the engine unit 12.

  The paper feed unit 14 includes a paper tray 22 on which recording paper is stacked and a paper feed roll 24 that sends out the recording paper from the paper tray 22, and the recording paper sent out by the paper feed roll 24 is Then, the paper passes through the paper feed path 30 through the transport rolls 26 and 28, and is transported to the transfer roll 74.

  After the toner image is transferred to the recording paper by the transfer roll 74 and fixed by the fixing roll 32A of the fixing unit 32, the position of the switching claw 34 is selected so that the discharge roller 36 or the discharge roll 38 causes the toner image to be placed on the upper portion of the engine unit 12. It is discharged to the first discharge tray 16 or the second discharge tray 18 provided.

  Here, in the case of double-sided printing, after the printing of the front surface is completed in the above-described order, before the recording paper is completely discharged to the first discharge tray 16, the discharge roll 36 is reversed, and the recording paper Is supplied to the inversion path 40. Then, the paper is returned to the paper feed path 30 through the transport rolls 42, 44, 46, and 48, and the back side of the recording paper is printed. In the case of manual printing, by placing the recording paper on the manual feed tray 20, the recording paper is transported from the manual feed roll 49 to the paper feed path 30 via the transport roll 48 and printed.

  In the fixing unit 32, a fixing roll 32A is heated to a predetermined temperature by lighting a lamp (for example, a halogen lamp), and the toner image is applied to a recording sheet by heating and pressurization by the heated fixing roll 32A. The toner image is fixed.

  By the way, on the right side of FIG. 1 of the image forming apparatus 10, four toner cartridges 64 filled with a developer (consisting of toner and magnetic carrier) for each color are arranged. The toner cartridge 64 is connected to developing units 60Y, 60M, 60K, and 60C, which will be described later, arranged in order from the top of FIG. 1 by a developer supply path 65, and the developer in the toner cartridge 64 is connected to the developing unit. Supplied to 60Y, 60M, 60K, and 60C.

  An exposure unit 62 is disposed on the left side of the toner cartridge 64 in FIG. 1, and the four laser beams L (Y), L (M), and L (K) corresponding to the image signal are provided from the exposure unit 62. , L (C) are the photosensitive drums 52Y, 52M, 52K, and 52C constituting the photosensitive unit 50 arranged on the left side of the exposure unit 62 in FIG. ) To form a latent image on the photosensitive drum 52.

  The photosensitive drum 52 is for yellow (52Y), magenta (52M), black (52K), and cyan (52C) from the top of FIG.

  The exposure unit 62 outputs laser light L (Y), L (M), L (K), and L (C) (hereinafter collectively referred to as laser light L) of each color Y, M, K, and C. And a modulation processing unit that modulates and scans the laser light L, an fθ lens that corrects the scanning speed on the exposure surface, and a cylindrical lens for surface tilt correction that has lens power in the scanning direction. And an optical system.

  In the exposure unit 62, the laser light L of each color emitted from the light source unit enters the modulation processing unit, is modulated according to the image information for each color, and is scanned by the polygon mirror 67 rotated by the polygon motor 63. (Main scanning). The laser beams L of the respective colors scanned by the polygon mirror 67 are reflected by the mirror group 69 in the arrangement direction of the photosensitive drums 52 corresponding to the respective colors and formed on the respective photosensitive drums 52.

  The photoconductor unit 50 is provided with a charging roll 56 and a refresh roll 54 corresponding to each photoconductor drum 52 (in FIG. 1, only those corresponding to the photoconductor unit 50Y are indicated by symbols). It is provided so as to rotate in contact with the body drum 52. In the charging roll 56, the photosensitive drum 52 is uniformly charged, and the toner flying from the magnet roll 80 provided in the developing device 58 is attached to the surface of the photosensitive drum 52. On the other hand, the refresh roll 54 discharges the photosensitive drum 52 to remove residual toner adhering to the surface of the photosensitive drum 52, thereby preventing ghosts and the like caused by the toner remaining on the surface of the photosensitive drum 52.

  Here, the developing device 58 is disposed on the lower right side of FIG. 1 of each photoconductor unit 50, and corresponds to each photoconductor drum 52 (52Y, 52M, 52K, 52C). 60M, 60K, and 60C are arranged in the vertical direction.

  On the other hand, an intermediate transfer unit 66 is disposed on the left side of the photoconductor unit 50 in FIG. 1, and three drum-shaped intermediate transfer drums 68, 70, and 72 are provided. The two first intermediate transfer drums 68 and 70 are arranged vertically in the vertical direction, and the upper first intermediate transfer drum 68 is arranged in the two photosensitive drums 52Y, 52Y, The lower first intermediate transfer drum 70 rotates in contact with the two photosensitive drums 52K and 52C disposed in the lower portion. Further, the second intermediate transfer drum 72 rotates in contact with both the first intermediate transfer drums 68 and 70, and the transfer roll 74 described above rotates in contact with the second intermediate transfer drum 72.

  Accordingly, the toner images are transferred from the photosensitive drums 52Y and 52M to the first intermediate transfer drum 68, and the toner images are transferred from the photosensitive drums 52K and 52C to the first intermediate transfer drum 70, respectively. The two-color toner images transferred to the first intermediate transfer drums 68 and 70 are transferred to the second intermediate transfer drum 72 to form four colors. The four-color toner images are transferred to the recording paper by the transfer roll 74. Will be.

  In the vicinity of these intermediate transfer drums 68, 70, and 72, a cleaning roll 76 and a cleaning brush 78 are disposed, respectively, to remove residual toner on the surface of the intermediate transfer drums 68, 70, and 72.

  The four photosensitive drums 52Y, 52M, 52K, and 52C, the two first intermediate transfer drums 68 and 70, the one second intermediate transfer drum 72, and the transfer roll 74 have predetermined transfer bias voltages, respectively. Is applied.

  The toner images developed on the photosensitive drums 52Y, 52M, 52K, and 52C are sequentially transferred by the potential gradients of the respective transfer bias voltages, and finally, the second intermediate transfer drum 72, the transfer roll 74, and the like. The image is transferred to a recording sheet that is nipped and conveyed.

(Schematic configuration of the control system of the entire image forming apparatus)
FIG. 2 is a block diagram of a control system for image formation in the engine unit 12.

  The main power management unit 200 is connected to a commercial power source (not shown), generates LVPS (low voltage power source) and HVPS (high voltage power source), and supplies power to each unit through a power supply line.

  A user interface 204 is connected to the main controller 202, and an instruction relating to image formation or the like is given by a user's operation, and information about the image formation or the like is notified to the user.

  The main controller 202 is connected to a network line with an external host computer (not shown) so that image data is input.

  When the image data is input, the main controller 202 analyzes, for example, the print instruction information included in the image data and the image data, and converts them into a format suitable for the engine unit 12 (for example, bitmap data). The image data is sent to the image formation processing control unit 206 constituting a part of the MCU.

  In the image formation processing control unit 206, based on the input image data, together with the image formation processing control unit 206, the optical scanning system control unit 208, the drive system control unit 210, the charger control unit 212, and the development that constitute the MCU, respectively. Each of the apparatus control unit 214 and the fixing unit control unit 216 is synchronously controlled to execute image formation.

  A state management unit 218 is connected to the image formation processing control unit 206, and determines the operating state of the engine unit 12 (for example, during the processing mode, during the sleep mode, during startup from the sleep mode, during processing, etc.). It is like that. The operating state determined by the state management unit 218 is sent to the main controller 202.

  The main controller 202 is connected with a temperature sensor 221 and a humidity sensor 222 for detecting the environment. The temperature sensor 221 and the humidity sensor 222 detect the environmental temperature and humidity in the engine unit 12.

  Further, the main controller 202 is connected with a density sensor 224 that is essential for performing output image density correction and toner density correction.

  In the following, as shown in FIG. 3, environmental regions depending on environmental temperature and humidity are as follows: high temperature is 26 ° C. to 30 ° C. or higher, low temperature is 10 ° C. to 15 ° C. or lower, and high humidity is 65% RH to 90% RH. As described above, the regions of high temperature and high humidity and low temperature and low humidity are set with reference to the low humidity of 10% RH to 20% RH or less.

  Here, transfer bias voltages are applied to the above-described photosensitive drum 52, first intermediate transfer drums 68 and 70, second intermediate transfer drum 72, and transfer roll 74 in a correlated manner.

  The photosensitive drum 52 determines a transfer bias voltage to the photosensitive drum 52 so that the electrostatic latent image can be reliably transferred (toner development) (to increase the transfer rate). Based on this, the downstream transfer bias voltage is determined.

  For example, conventionally, as shown in FIG. 4B, the photosensitive drum 52 is −400V, the first intermediate transfer drums 68 and 70 are 250V, the second intermediate transfer drum 72 is 650V, and the transfer roll 74 is 1000V. It is determined.

  Here, since the transfer bias voltage of the transfer roll 74 is 1000 V, which is a very high potential difference with respect to the ground level, the recording material is water-containing paper, the transfer roll 74 or the recording medium, particularly in a high temperature and high humidity environment. If there are metal members arranged around the paper conveyance system, current may leak through these metal members.

  In order to avoid this, conventionally, a ground circuit having elements such as diodes and varistors interposed therein has been provided.

  On the other hand, in the first embodiment, the transfer bias voltage of the transfer roll 74 itself is lowered (the potential difference is reduced with respect to the ground level), thereby eliminating the need for the ground circuit as described above.

  As a result, the photosensitive drum 52 is determined to be −1000 V, the first intermediate transfer drums 68 and 70 are determined to be −350 V, the second intermediate transfer drum 72 is determined to be 50 V, and the transfer roll 74 is determined to be 400 V (see FIG. 4A). ). That is, the first embodiment of the present invention is characterized in that the first intermediate transfer members 68 and 70 that receive toner from the photosensitive drum 52 have the same polarity (both negative) as the toner.

  Usually, transfer is performed with a predetermined potential gradient because the polarity is opposite to that of the toner.

In the first embodiment contrary to this, the reason why the transfer is performed efficiently even if the polarity of the toner and the first intermediate transfer bodies 68 and 70 are the same is the toner shown in the following “reason”. It is presumed that it depends on the material of the first intermediate transfer members 68 and 70.
of.

(reason)
The transfer bias voltage cannot be simply lowered for the transfer roller 74, and the transfer bias voltage to the photosensitive drum 52, the first intermediate transfer drums 68 and 70, and the second intermediate transfer drum 72 having the correlation needs to be changed. In order to make this possible, the transfer rate is improved by considering the shape of the toner, the surface roughness of the first intermediate transfer drums 68 and 70 (and / or the second intermediate transfer drum 72), and the hardness.

  That is, the transfer rate depends not only on the transfer bias voltage but also on the shape factor of the toner, the surface roughness of the first intermediate transfer drums 68 and 70, and the hardness.

  Table 1 shows the primary transfer rate with respect to the toner shape factor, and the presence / absence of occurrence of a residual image (transfer residual ghost) (indicated by ◯ when there is no, Δ when it is small, and x when it is large).

  For the toner shape factor, the maximum length (ML) and projected area (A) of 100 or more toners are measured by taking an optical microscope of the toner dispersed on the slide glass into a Luzek image analyzer via a video camera. , (25π × ML (2 / A)) average value.

  References relating to the toner shape factor include JP 2001-125315 A, JP 2002-189313 A, JP 2002-328535 A, and the like.

  Table 2 shows the primary transfer rate with respect to the surface roughness of the first intermediate transfer drums 68 and 70 (the same applies to the second intermediate transfer drum 72), and the presence or absence of afterimages (transfer residual ghosts). △, and a large number is indicated by ×).

  Table 3 shows the primary transfer rate with respect to the roll hardness of the first intermediate transfer drums 68 and 70 (the same applies to the second intermediate transfer drum 72) and the presence or absence of occurrence of afterimages (transfer residual ghosts). (Triangle | delta, the case where there are many is described by x) is shown.

In Tables 1 to 3, it can be seen that the range in which no afterimage (transfer residual ghost) occurs is a toner shape of 125 or less, a surface roughness of 1.5 R _Z (μm) or less, and a roll hardness of 50 (Asker C) or less. .

  In Tables 1 to 3, if the afterimage evaluation is set to “◯”, the transfer bias voltage of each part can be set as shown in Table 4 below in a high-temperature and high-humidity environment ( (See FIG. 4A). In Table 4, the transfer bias voltage of each conventional part is also shown for comparison.

  Based on the above “reason”, in the first embodiment, the transfer bias voltage at each part in the transfer process is set to the numerical values shown in Table 4 (and FIG. 4A) at high temperature and high humidity. At this time, by making the polarity of the toner and the polarity of the transfer bias voltage applied to the first intermediate transfer rolls 68 and 70 the same, the transfer bias voltage applied to the transfer roll 74 is a voltage that does not cause current leakage. It becomes possible.

  The operation of the first embodiment will be described below.

(Flow of image forming process)
Around each photosensitive drum 52, an image forming (printing) process for each color by a known electrophotographic method is performed as follows.

  First, each photosensitive drum 52 is rotationally driven at a predetermined rotational speed.

  As shown in FIG. 1, the surface of the photosensitive drum 52 is uniformly charged to a predetermined level by applying a DC voltage of a predetermined charging level to the charging roll 56. In the first embodiment, only a DC voltage is applied to the charging roll 56, but an AC component may be superimposed on the DC component.

  Next, the surface of each photosensitive drum 52 having a uniform surface potential is irradiated with laser light L corresponding to each color by the exposure unit 62, and an electrostatic latent image corresponding to image information for each color is formed. The As a result, the surface potential of the exposed portion of the photosensitive drum 52 by the laser light L is neutralized to a predetermined level.

  The electrostatic latent image formed on the surface of each photoconductive drum 52 is developed by each corresponding developing device 58 and visualized on each photoconductive drum 52 as a toner image of each color.

  Next, the toner images of the respective colors formed on the respective photoconductive drums 52 are electrostatically primary-transferred onto the corresponding primary intermediate transfer drums 68 and 70. Here, the Y and M toner images formed on the photosensitive drums 52Y and 52M are on the primary intermediate transfer drum 68, and the K and C toner images formed on the photosensitive drums 52K and 52C are the primary intermediate. Each image is transferred onto the transfer drum 70.

  Thereafter, the toner image formed on the primary intermediate transfer drums 68 and 70 is electrostatically secondary-transferred onto the secondary intermediate transfer drum 72. As a result, on the secondary intermediate transfer drum 72, toner images from a single color image to a quadruple color image of each color of Y, M, K, and C are formed.

  Finally, the toner image formed on the secondary intermediate transfer drum 72 is tertiary-transferred onto the recording paper passing through the recording paper conveyance path by the transfer roll 74. After the tertiary transfer of the recording paper, the toner image formed on the recording paper is heat-fixed by the fixing unit 32, and the image forming process ends.

  In the image forming apparatus 10, the transfer rate varies depending on the environment (high temperature and high humidity) during image formation. In order to stabilize the transfer rate, it is necessary to increase the transfer bias voltage of the transfer roll 74. Conventionally, a transfer bias voltage of −400 V is applied to the photosensitive drum 52, and thereafter, as it goes downstream. By giving the correlation, the transfer application voltage of the transfer roll 74 reached 1000V.

  However, in this case, if the recording paper is water-containing paper or if a metal member is present around the recording paper transport system, current may leak, and thus a grounding circuit for preventing leakage is necessary.

  On the other hand, in the first embodiment, the transfer bias of each part is considered in consideration of the shape of the toner, the surface roughness of the first intermediate transfer drums 68 and 70 (and / or the second intermediate transfer drum 72), and the hardness. The voltage was set. In this case, the polarity of the transfer bias voltage applied to the first intermediate transfer drums 68 and 70 is the same as the polarity of the toner (negative polarity in the first embodiment).

  At this time, the transfer bias voltage of the transfer roll 74 is set to a minimum voltage that does not cause the current leakage as described above and does not deteriorate the transfer rate, and hereinafter, each part is traced back to the image forming process. The transfer bias voltages were set while maintaining correlation with each other (see Table 4 and FIG. 4A).

  This eliminates the need for a grounding circuit for maintaining the transfer rate and preventing current leakage, maintaining stable image quality, and reducing the number of assembly steps by reducing the number of components.

  Although the toner and the first intermediate transfer bodies 68 and 70 have the same polarity, it is difficult to transfer the toner from the photosensitive drum 52 to the first intermediate transfer bodies 68 and 70 with the conventional transfer bias voltage. However, it can be estimated as one factor that the voltage is higher than the ground level (0 V).

  As other factors, it is presumed that the toner is close to a true sphere and is related to the surface roughness and hardness of the first intermediate transfer members 68 and 70.

[Second Embodiment]
The second embodiment of the present invention will be described below.

  Note that the second embodiment has the same configuration as the image forming apparatus 10 applied in the first embodiment, and is characterized by setting a transfer bias voltage specialized for low temperature and low humidity as an environmental condition.

  That is, in the first embodiment, the surface roughness of the first intermediate transfer drums 68 and 70 (and / or the second intermediate transfer drum 72), particularly with the transfer rate at high temperature and high humidity and current leakage as problems. The transfer bias voltage of the transfer roll 74 is set to the lowest under the conditions of the transfer rate based on the hardness and the shape of the toner (the degree of occurrence of the transfer residual ghost). The transfer rate is improved by further increasing the voltage (about 3400V to 4800V). In this case, there is no concern about current leakage, but due to resistance that changes over time (accumulation of processing amount), a white spot occurs when the transfer bias voltage to the transfer roll 74 is low (image quality defect A). If it is high, toner scattering occurs (image quality B).

  The change in resistance with respect to the processing amount tends to increase as the processing amount increases as shown in FIG.

  Conventionally, as shown by the chain line in FIG. 5, since the resistance is greatly increased, the voltage region where the image quality defect A and the image quality defect B do not occur is very narrow and the control is complicated.

  On the other hand, in the second embodiment, the surface roughness, hardness, and toner shape of the first intermediate transfer drums 68 and 70 (and / or the second intermediate transfer drum 72) are the same as those in the first embodiment. With this setting, as shown in FIG. 5, even if the resistance changes (rises) with time, the increase in resistance can be suppressed as compared with the conventional case. The transfer bias voltage to the transfer roll 74 in which neither of the above occurs is in a relatively wide range, and the control of the transfer bias voltage is simplified (see Table 5).

1 is a side view showing an image forming apparatus according to a first embodiment. It is a control block diagram of the engine part which concerns on 1st Embodiment. It is an environment area distribution characteristic view for specifying an environment area according to the first embodiment. (A) is a schematic diagram showing transfer bias voltages set for each part of the transfer process according to the first embodiment, and (B) is a schematic showing transfer bias voltages set for each part of the transfer process according to the conventional example. FIG. It is a print volume-resistance characteristic figure concerning a 2nd embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Engine part 52 Photosensitive drum 56 Charging roll 58 Developing apparatus 60 Developing device 62 Exposure unit 74 Transfer roll 202 Main controller 204 User interface 206 Image formation process control part 208 Optical scanning system control part 210 Drive system control part 212 Charger control unit 214 Developing device control unit 216 Fixing unit control unit 221 Temperature sensor 222 Humidity sensor 224 Density sensor

Claims (6)

The surface of the image carrier is uniformly charged, and an electrostatic latent image is formed by irradiating the uniformly charged image carrier with a light beam according to image data, and then supplied from a developing device. Development is performed using toner charged to positive or negative polarity, and the toner image is transferred to a recording sheet by a final transfer member via an intermediate transfer member, and the transferred toner image is fixed to form an image. An image forming apparatus provided with an image forming engine
A potential gradient forming unit that applies a predetermined transfer bias voltage to each of the image carrier, the intermediate transfer member, and the final transfer member so as to have a predetermined potential gradient;
The image forming apparatus, wherein the potential gradient forming unit applies a transfer bias voltage having the same polarity as the toner to at least one intermediate transfer member in contact with the image carrier.   The potential gradient forming unit sets a transfer bias voltage to the final transfer body as a reference transfer bias voltage according to environmental conditions, and correlates with the set reference transfer bias voltage to the intermediate transfer body and the image carrier. The image forming apparatus according to claim 1, wherein the transfer bias voltages are set to form respective potential gradients.   Assuming that the polarity of the transfer bias voltage applied to the intermediate transfer member is the same as the polarity of the toner, the shape factor of the toner is 125 or less, the surface roughness of the intermediate transfer member is RZ 1.5 μm or less, and the hardness is 50 AskerC. The image forming apparatus according to claim 1, wherein: The surface of the image carrier is uniformly charged, and an electrostatic latent image is formed by irradiating the uniformly charged image carrier with a light beam according to image data, and then supplied from a developing device. Development is performed using toner charged to positive or negative polarity, and the toner image is transferred to a recording sheet by a final transfer member via an intermediate transfer member, and the transferred toner image is fixed to form an image. An image forming apparatus provided with an image forming engine
A potential gradient forming unit that applies a predetermined transfer bias voltage to each of the image carrier, the intermediate transfer member, and the final transfer member so as to have a predetermined potential gradient;
The potential gradient forming means sets a transfer bias voltage to the final transfer member as a reference transfer bias voltage;
Image forming characterized in that the transfer bias voltage to the intermediate transfer member and the image bearing member is set from the final transfer member to which the transfer bias voltage is applied by going back through the processing steps of the image forming engine. apparatus.   The image forming apparatus according to claim 4, wherein the reference transfer bias voltage is set based on conductivity of the recording sheet and a conveyance system of the recording sheet. The image forming engine in the image forming apparatus according to any one of claims 1 to 5,
A plurality of image carriers for different colors as the image carrier,
As the intermediate transfer member, a first upstream intermediate transfer member that contacts a part of the plurality of image carriers, and a first downstream intermediate transfer member that contacts a remaining part of the plurality of image carriers. The first upstream intermediate transfer body and the first downstream intermediate transfer body, and after the toner image is transferred from the first upstream intermediate transfer body, the first downstream intermediate transfer body A second intermediate transfer body to which a toner image is transferred from the body,
An image forming apparatus, wherein the final transfer member is opposed to the second intermediate transfer member.

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