JP2011209380A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the following problem in a conventional image forming apparatus: image quality deteriorates due to a so called reverse transfer phenomenon in which a developer image transferred before to a medium to be transferred moves onto an image carrier 41 arranged on the downstream side.SOLUTION: The image forming apparatus 10 includes a medium-to-be transferred conveying means 30 for conveying the medium to be transferred and a plurality of image forming means 40 arranged along the conveying direction of the medium to be transferred and transferring the developer image to paper. In the image forming apparatus 10, the image forming means 40 has: the image carrier 41 for carrying the developer image; a transfer member 49 which faces the image carrier 41 and is arranged shifted by a constant distance to the downstream side in the conveying direction of the medium to be transferred from the image carrier 41; and a contact region where the image carrier 41 and the transfer member 49 are opposite to each other and the developer image is transferred to the paper. The image forming means 40 arranged downstream side in the conveying direction of the medium to be transferred has the image carrier 41 and the transfer member 49 which are arranged greatly shifted compared to that in the image forming means 40 arranged on the upstream side.

Description

  The present invention relates to an image forming apparatus that transfers a developer image formed on an image carrier to a transfer medium.

  2. Description of the Related Art Conventionally, an image forming apparatus such as a color electrophotographic printer includes an image carrier of a photoconductor (photosensitive drum or the like), a charging unit, an exposure unit, a developing unit, and the like as described in, for example, Patent Document 1 below. A technology is known in which four printing mechanisms are arranged side by side to form black, yellow, magenta, and cyan image forming means, and a developer image such as toner is sequentially transferred onto a transfer medium such as paper conveyed on a conveyance belt. ing.

JP 2004-258281 A

  However, the conventional image forming apparatus has a problem that the image quality deteriorates due to a so-called reverse transfer phenomenon in which the developer image transferred earlier is transferred onto the image carrier of the image forming means disposed on the downstream side. It was.

  The image forming apparatus according to the present invention includes a transfer medium transfer unit that transfers a transfer medium in the transfer medium transfer direction, and a transfer medium that transfers the developer image to the transfer medium. An image forming apparatus having a plurality of image forming means, wherein the image forming means has an image carrier that carries the developer image, and is opposed to the image carrier and to the image carrier. A transfer member arranged at a certain distance on the downstream side in the transport direction of the transfer medium, a contact area where the image carrier and the transfer member face each other, and transfer the developer image to the transfer medium And the image forming means disposed on the downstream side in the transport direction of the transfer medium includes the transfer member and the image carrier as compared with the image forming means disposed on the upstream side. The fixed distance is large.

The image forming apparatus of the present invention has the following effects (A) and (B).
(A) Deterioration in image quality due to reverse transfer phenomenon can be prevented.

  (B) In addition, the developer image in the image forming mechanism disposed on the downstream side in the transfer medium conveyance direction by suppressing a change in the developer charge amount of the developer image previously transferred to the transfer medium. Can be improved.

FIG. 1 is a schematic configuration diagram showing a transfer nip region of the image forming apparatus in Embodiment 1 of the present invention. FIG. 2 is a schematic configuration diagram illustrating the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a schematic configuration diagram showing a control circuit of the image forming apparatus of FIG. FIG. 4 is a flowchart showing the operation of the image forming apparatus in Embodiment 1 of the present invention. FIG. 5 is a diagram illustrating a set value of the transfer roller installation distance in FIG. 1. 6 is a diagram showing installation distance conditions (1) and (2) of the transfer roller of FIG. FIG. 7 is a diagram illustrating a toner charge amount transition of a black (K) toner image. FIG. 8 is a diagram showing density values and reverse transfer levels. FIG. 9 is a diagram illustrating the reverse transfer level of the black (K) toner image. FIG. 10 is a diagram illustrating the transfer property of the first toner image charge amount and the second toner image. FIG. 11 is a graph showing the transition of the toner charge amount of the black (K) toner image in Example 1 of the present invention. FIG. 12 is a diagram illustrating the reverse transfer level received by the black (K) toner image according to the first embodiment of the present invention. FIG. 13 is a schematic configuration diagram showing an image forming apparatus in Embodiment 2 of the present invention. FIG. 14 is a schematic configuration diagram showing a control circuit of the image forming apparatus of FIG. FIG. 15 is a flowchart showing the operation of the image forming apparatus according to the second embodiment of the present invention. FIG. 16 is a diagram showing conditions for calculating the correction value of the transfer voltage in FIG. FIG. 17 is a diagram illustrating the relationship between humidity and the surface resistivity of plain paper A. FIG. 18 is a diagram illustrating the relationship between the surface resistivity of the plain paper A and the reverse transfer level. FIG. 19 is a diagram showing the relationship between the surface resistivity of plain paper A and the good transfer voltage range. FIG. 20 is a diagram showing whether or not Example 2 is applied and the total reverse transfer level. FIG. 21 is a flowchart showing the operation of the image forming apparatus according to the third embodiment of the present invention. FIG. 22 is a diagram illustrating a voltage obtained by correcting the voltage applied to the developing roller in FIG. FIG. 23 is a schematic configuration diagram showing an image forming apparatus in Embodiment 4 of the present invention. FIG. 24 is a diagram showing the nip amount between the developing roller and the photosensitive drum in the printing mechanism of FIG. FIG. 25 is a diagram illustrating a relationship between the shift amount and the nip amount in the fourth embodiment of the present invention.

  Modes for carrying out the present invention will become apparent from the following description of the preferred embodiments when read in light of the accompanying drawings. However, the drawings are only for explanation and do not limit the scope of the present invention.

(Configuration of Example 1)
FIG. 2 is a schematic configuration diagram illustrating the image forming apparatus according to the first embodiment of the present invention.

  The image forming apparatus 10 is an apparatus capable of printing an image based on image data input from an external host computer on a sheet as a recording medium. The image forming apparatus 10 supplies a sheet, a guide 23, a sensor 26, 27, the conveyance belt 30, the driving roller 31, the driven roller 32, the suction roller 33, and four image forming units (for example, “printing mechanism”) 40 disposed on the conveyance belt 30 (= 40) -1, 40-2, 40-3, 40-4), a cleaning mechanism disposed under the conveyor belt 30, a sensor 60, a fixing mechanism 65, a discharge sensor 70, a guide 71, and a stacker 72. And have.

  In the lower part of the image forming apparatus 10, the paper feeding mechanism for supplying paper to the conveyance path 22 is provided. In this paper feeding mechanism, the paper storage cassette 20, the hopping roller 21 that feeds the paper stored in the paper storage cassette 20, the guide 23, and the paper skew (the state in which the paper is obliquely fed is referred to as skew). In this case, the pinch roller 25 that corrects the paper, the registration roller 24 that feeds the paper between the suction roller 33 and the conveyance belt 30, and the paper is pressed against the driven roller 32 to electrostatically adhere to the upper surface of the conveyance belt 30. And a suction roller 33 to be sucked.

  A sensor 26 is disposed on the upstream side of the registration roller 24 in the sheet conveyance direction, which is the recording medium conveyance direction, and a sensor 27 is disposed on the downstream side. The sensors 26 and 27 have a function of detecting the position of the paper.

  The conveyor belt 30 is made of a high-resistance semiconductive plastic film formed in a seamless endless shape, and an upper surface portion of the conveyor belt 30 is an image carrier (for example, “photosensitive drum”) of each printing mechanism 40. ) 41 and each transfer member (for example, “transfer roller”) 49. The conveyance belt 30 is pressed against and contacted with the photosensitive drum 41 by each transfer roller 49 to form a transfer nip region (transfer contact region). The conveyor belt 30 is stretched between a driving roller 31 and a driven roller 32, pulled by the driven roller 32 in the direction of the arrow f and applied with tension, and driven by the rotation of the driving roller 31 in the direction of the arrow e. The conveyance belt 30, the driving roller 31, and the driven roller 32 are one of medium conveyance units that convey paper.

  The cleaning mechanism is installed on the lower surface of the transport belt 30 and has a cleaning blade 34 and a waste toner tank 35. The driven roller 32 and the cleaning blade 34 are installed at positions facing each other so as to sandwich the lower surface portion of the transport belt 30. The cleaning blade 34 is made of a flexible rubber material or plastic material, and has a function of scraping off the developer (for example, toner) adhering and remaining on the surface of the transport belt 30 to the waste toner tank 35.

  Four independent printing mechanisms 40 (= 40-1, 40-2, 40-3, 40-4) are a plurality of developing means for developing a toner image as a developer image on a sheet, and black (K ), Yellow (Y), magenta (M), and cyan (C), and is arranged along a conveyance path 22 including a conveyance belt 30 that conveys a sheet. Each printing mechanism 40 includes a charging roller 42, a photosensitive drum 41 whose surface is uniformly charged by the charging roller 42, and a light emitting diode (hereinafter referred to as “an electrostatic latent image” formed on the charged photosensitive drum 41 by light. LED ”) head 48, developing roller 43, developing blade 45, and supply roller 44, which form a toner image on photosensitive drum 41 on which an electrostatic latent image is formed, and toner to this developing unit A toner cartridge 47 for supplying the toner, and a static elimination light source 46 for neutralizing the surface of the photosensitive drum 41.

  A conveyance belt 30 is movably disposed between the photosensitive drum 41 and the transfer roller 49. Further, each printing mechanism 40 is arranged so as to be separated from the conveyor belt 30 by a separation mechanism (not shown). In the first embodiment, the outer diameter of the photosensitive drum 41 is φ30, and the outer diameter of the transfer roller 49 is φ20. The transfer roller 49 is a transfer member that transfers a toner image to a sheet when a predetermined voltage is applied.

  On the downstream side of the conveying belt 30 on the driving roller 31 side, a sensor 60 for checking a sheet that has failed to be separated from the conveying belt 30 or detecting the rear end position of the sheet is installed. A fixing mechanism 65 is further installed on the side.

  The fixing mechanism 65 includes a heat roller 61, a pressure roller 62 that presses the heat roller 61, and a thermistor 63 that detects the temperature of the heat roller 61. The heat roller 61 is driven by a heat motor 87 (not shown), and the pressure roller 62 rotates with the heat roller 61. The heat roller 61 incorporates a heater 64 made of a halogen lamp that functions as a heat source. The fixing mechanism 65 has a function of fixing the toner image by heating and melting the toner on the paper. A thermistor 63 is disposed near the surface of the heat roller 61 to monitor the temperature of the heat roller 61. A discharge sensor 70 is installed on the downstream side of the heat roller 61 to monitor paper jam and paper winding around the heat roller 61 in the fixing mechanism 65. A guide 71 is provided on the downstream side of the discharge sensor 70 to convey the paper to the stacker 72 at the top of the housing of the image forming apparatus 10, and the printed paper is discharged to the stacker 72.

  FIG. 1 is a schematic configuration diagram showing a transfer nip region of the image forming apparatus in Embodiment 1 of the present invention.

  In the printing mechanism 40-1 arranged on the most upstream side in the paper conveyance direction, the transfer roller 49 (= 49-1) is downstream by a distance t1 with respect to the central axis of the photosensitive drum 41 (= 41-1). Installed. A fixed distance in which the transfer roller 49 (= 49-1 to 49-4) is shifted downstream from the photosensitive drum 41 (= 41-1 to 41-4) is hereinafter referred to as a “shift amount”. .

In the printing mechanism 40-2 disposed on the downstream side of the printing mechanism 40-1, the transfer roller 49-2 is shifted downstream by the shift amount t2 with respect to the position just below the central axis of the photosensitive drum 41-2. Has been. In the printing mechanism 40-3 disposed on the downstream side of the printing mechanism 40-2, the transfer roller 49-3 is shifted to the downstream side by a shift amount t3 with respect to the central axis of the photosensitive drum 41-3. Has been. In the printing mechanism 40-4 disposed on the downstream side of the printing mechanism 40-3, the transfer roller 49-4 is shifted downstream by a shift amount t4 with respect to the central axis of the photosensitive drum 41-4. Has been. At this time, the shift amounts t1 to t4 between the photosensitive drum 41 and the transfer roller 49 satisfy Expression (1).
t1 <t2 <t3 <t4 ≦ 2 [mm] (1)

  The reason why the upper limit value of each shift amount t1 to t4 of the transfer roller 49 with respect to the photosensitive drum 41 is 2 [mm] is that the medium transportability is deteriorated when it is 2 [mm] or more, and the color misregistration is reduced. This is because of the above.

FIG. 3 is a schematic configuration diagram showing a control circuit of the image forming apparatus of FIG.
The control circuit of the image forming apparatus 10 is controlled by a host interface unit 81, a command / image processing unit 82 to which the host interface unit 81 is connected, an LED head interface unit 83, and the LED head interface unit 83. A plurality of LED heads 48 (= 48-1 to 48-4), a mechanism control unit 80, sensors 26, 27, 60, 70, a heater 64, a thermistor 63, a hopping motor 84, and the hopping motor 84 The hopping roller 21 driven by the registration motor 85, the registration motor 85, the registration roller 24 driven by the registration motor 85, the belt motor 86, the drive roller 31 driven by the belt motor 86, the heat motor 87, The heat driven by the heat motor 87 A troller 61, a drum motor 88, a plurality of printing mechanisms 40 (= 40-1 to 40-4) driven by the drum motor 88, a high pressure control unit 90, and charging controlled by the high pressure control unit 90. A voltage generator 91, a development voltage generator 92, a supply voltage generator 93, a transfer voltage generator 94, and a plurality of charging rollers 42 (= 42-1 to 42-4) to which a voltage is applied by the charging voltage generator 91. ), A plurality of developing rollers 43 (= 43-1 to 43-4) to which a voltage is applied by the developing voltage generator 92, and a plurality of supply rollers 44 (= 44) to which a voltage is applied by the supply voltage generator 93. -1 to 44-4) and a plurality of transfer rollers 49 (= 49-1 to 49-4) to which a voltage is applied by the transfer voltage generator 94.

  The host interface unit 81 is connected to a host computer (not shown) and a command / image processing unit 82. The host interface unit 81 includes a connector and a communication chip, and has a communication interface function of a physical hierarchy with the host computer.

  The command / image processing unit 82 is connected to the host interface unit 81 and the mechanism control unit 80. The command / image processing unit 82 interprets commands and image data from the host computer and develops the image data into a bitmap. The command / image processing unit 82 is a microprocessor, a random access memory (hereinafter referred to as “RAM”) and a not-shown image. It consists of special hardware for development and has a function of controlling the entire image forming apparatus 10.

  The LED head interface unit 83 is connected to the command / image processing unit 82, the host interface unit 81, and each LED head 48. The LED head interface unit 83 includes a semi-custom LSI (Large Scale Integration) and a RAM (not shown), and the image data developed by the command / image processing unit 82 into a bitmap is matched with the interface of the LED head 48 of each color. Has the function of processing.

  The mechanism control unit 80 includes a command / image processing unit 82, an LED head interface unit 83, sensors 26, 27, 60, 70, a heater 64, a thermistor 63, a hopping motor 84, a registration motor 85, A belt motor 86, a heat motor 87, a drum motor 88, and a high voltage controller 90 are connected.

  The mechanism control unit 80 has a function of controlling each part of the printing mechanism 40 and the driving mechanism of the image forming apparatus 10. The mechanism control unit 80 monitors input signals from the sensors 26, 27, 60, and 70 in accordance with commands from the command / image processing unit 82, while monitoring a hopping motor 84, a registration motor 85, a belt motor 86, and a heat motor 87. The drum motor 88 is driven, the input signal from the thermistor 63 is monitored, the heater 64 is controlled, and the high voltage controller 90 is further controlled.

  The driving mechanism drives a hopping motor 84, a registration motor 85, a belt motor 86, and a drum motor 88 for driving the printing mechanism 40, a heat motor 87 for driving the heat roller 61, and these motor groups. A driver circuit group. The heater 64 is formed of a halogen lamp and is disposed in the heat roller 61. A thermistor 63 is disposed near the surface of the heat roller 61 and has a function of detecting the temperature of the heat roller 61.

  The high voltage controller 90 includes a microprocessor or a custom LSI, and has a function of generating a charging voltage, a developing voltage, a supply voltage, and a transfer voltage supplied to each printing mechanism 40. The charging voltage generator 91 has a function of generating and stopping a charging voltage to the charging roller 42 of each printing mechanism 40. The development voltage generation unit 92 has a function of generating and stopping the development voltage to the development roller 43 of each printing mechanism 40. The supply voltage generation unit 93 has a function of generating and stopping supply voltage to the supply roller 44 of each printing mechanism 40. The transfer voltage generation unit 94 has a function of generating and stopping a transfer voltage to the transfer roller 49 and has a current detection circuit or a voltage detection circuit (not shown) so that the current flowing through the transfer roller 49 becomes a constant current. And a function of performing control so that the voltage applied to the transfer roller 49 becomes a constant voltage.

(Operation of Example 1)
FIG. 4 is a flowchart showing the operation of the image forming apparatus in Embodiment 1 of the present invention.

  When the process starts, the image forming apparatus 10 waits until image data is received from a host computer (not shown) in step S1. When the image data is received, in step S2, the command / image processing unit 82 instructs the mechanism control unit 80 to start warming up the fixing mechanism 65, and the mechanism control unit 80 drives the heat motor 87. The rotation of the heat roller 61 of the fixing mechanism 65 is started, and while maintaining the rotation, the temperature is controlled to the fixing possible temperature, and the conveyance of the sheet is awaited.

  In step S <b> 3, the mechanism control unit 80 refers to the output signal of the thermistor 63, controls on / off of the heater 64, and adjusts the temperature of the heat roller 61. When the temperature of the heat roller 61 reaches a temperature at which the toner image on the recording medium can be fixed, the process of step S4 is performed to start the printing operation.

  In step S4, the mechanism control unit 80 starts driving the belt motor 86 to drive the driving roller 31, and in step S5, starts the control of the drum motor 88 to drive each printing mechanism 40, and in step S6. The high-voltage control unit 90 is instructed to start supplying a high-voltage bias. Since the mechanism control unit 80 processes steps S4 to S6 without waiting, the operations of steps S4 to S6 start at the same timing. The high voltage control unit 90 controls the high voltage bias to be supplied to the charging voltage generation unit 91, the development voltage generation unit 92, and the supply voltage generation unit 93.

  Here, a toner image forming operation in each printing mechanism 40 will be described. Here, the black (K) printing mechanism 40-1 will be described as a representative. The yellow (Y), magenta (M), and cyan (C) printing mechanisms 40-2, 40-3, and 40-4, except for the toner color difference, are the black (K) printing mechanisms 40. Same as -1.

  The high voltage controller 90 controls the charging voltage generator 91, whereby a charging voltage of −1100 V is supplied to the charging roller 42. The charging roller 42 comes into contact with the photosensitive drum 41 and charges its surface to −600V.

  The high voltage controller 90 controls the developing voltage generator 92 and the supply voltage generator 93 to supply a developing voltage of −200 V to the developing roller 43 and a supply voltage of −250 V to the supplying roller 44. . Due to the difference between the voltages applied to the developing roller 43 and the supply roller 44, an electric field in the direction from the developing roller 43 toward the supply roller 44 is formed in the vicinity of the nip region between the development roller 43 and the supply roller 44. The toner cartridge 47 of the printing mechanism 40-1 contains black (K) toner. The toner supplied from the toner cartridge 47 is strongly rubbed by the developing roller 43 and the supply roller 44 and frictionally charged to a negative polarity. The toner that is triboelectrically charged to the negative polarity adheres on the developing roller 43 by the Coulomb force received from the electric field directed from the developing roller 43 toward the supply roller 44. The adhered toner is carried to the contact portion between the developing roller 43 and the developing blade 45 as the developing roller 43 rotates, and is made uniform by the developing blade 45 to form a toner layer. Further, as the developing roller 43 rotates, the toner layer is carried to the nip region with the photosensitive drum 41.

  In step S <b> 7, the mechanism control unit 80 drives the hopping motor 84 to rotate the hopping roller 21 and sends only one sheet of the sheet storage cassette 20 to the guide 23. In step S8, the output of the sensor 26 is monitored, and when it is detected that the leading edge of the sheet has reached between the registration roller 24 and the pinch roller 25, the hopping motor 84 is stopped in step S9.

  In step S <b> 10, the mechanism control unit 80 rotates the registration roller 24 by driving the registration motor 85, and sends the sheet to the nip region between the driven roller 32 and the suction roller 33. Further, the driving roller 31 is rotated by driving the belt motor 86, and the sheet electrostatically attracted to the upper surface of the conveying belt 30 is sent to the nip region between the photosensitive drum 41 and the conveying belt 30.

  Next, in each printing mechanism 40, an operation in which the LED head 48 writes an electrostatic latent image on the photosensitive drum 41 and an operation in which the electrostatic latent image is developed with toner will be described. The command / image processor 82 waits until the paper reaches the nip area formed by the photosensitive drum 41 and the conveyor belt 30 after detecting the leading edge of the paper by the sensor 27 in step S11. Image data is transmitted to the head interface unit 83.

  The LED head interface unit 83 blinks the LED of the LED head 48 corresponding to the transmitted image data, exposes the photosensitive drum 41 charged to −600 V, and writes the electrostatic latent image discharged to −50 V. The electrostatic latent image written on the surface of the photosensitive drum 41 reaches the nip region with the developing roller 43 as the photosensitive drum 41 rotates.

  Since the exposed portion on the photosensitive drum 41 is discharged to −50 V by exposure, an electric field in the direction from the photosensitive drum 41 toward the developing roller 43 is formed. On the other hand, since the non-exposed portion on the photosensitive drum 41 is not discharged at −600 V, an electric field opposite to the exposed portion is formed in the direction from the developing roller 43 toward the photosensitive drum 41. Accordingly, the negatively charged toner layer on the developing roller 43 selectively adheres to the exposed portion on the photosensitive drum 41, and the electrostatic latent image is developed as a toner image.

  In step S13, the mechanism controller 80 causes the transfer voltage generator 94 of the high voltage controller 90 to transfer the transfer rollers 49-1 to 49- for black (K), yellow (Y), magenta (M), and cyan (C). 4 is supplied with each transfer voltage. At this time, the sheet is transported by the transport belt 30 and the leading edge sequentially reaches the transfer nip regions of black (K), yellow (Y), magenta (M), and cyan (C). In the first embodiment, a transfer voltage of 2600 V is supplied to the black (K) transfer roller 49-1, a transfer voltage of 2700 V is supplied to the yellow (Y) transfer roller 49-2, and magenta (M) transfer is performed. A transfer voltage of 2800 V is supplied to the roller 49-3, and a transfer voltage of 2900 V is supplied to the cyan (C) transfer roller 49-4. These transfer voltages are stored in a storage unit (not shown) of the mechanism control unit 80.

  In the transfer nip region, an electric field is formed in the direction from the conveyance belt 30 toward the photosensitive drum 41, so that the toner developed on the photosensitive drum 41 is transferred to a sheet on the conveyance belt 30.

  The sheet on which the transfer process has been completed is continuously conveyed by the conveyance belt 30 and sent to the fixing mechanism 65. In step S <b> 14, when the sensor 60 detects the paper, the mechanism control unit 80 continues to rotate the heat roller 61 of the fixing mechanism 65 by driving the heat motor 87. The sheet is nipped and conveyed by the heat roller 61 that has reached the fixable temperature in step S3 and the pressure roller 62 that is in pressure contact with the heat roller 61, thereby fixing the toner image.

  In step S16, the process waits until the discharge sensor 70 detects the sheet, and then waits in step S17 until the discharge sensor 70 detects no sheet. Then, the fixing process in the sheet fixing mechanism 65 is completed, and the sheet is guided. Guided by 71 and discharged to the stacker 72, the operation of FIG.

  Here, features of the first embodiment will be described. In the transfer step, in order to obtain a color image, the toner image must be transferred onto the toner image previously transferred onto the paper by the printing mechanism 40 having different color toners. A part of the toner contained in the toner image is considered to be charged to the reverse polarity due to the influence of the transfer current in the transfer nip region, but the toner image that has been transferred onto the paper and charged to the reverse polarity first is The image quality deteriorates due to the “reverse transfer phenomenon” that moves from the paper to the photosensitive drum 41 when it comes into contact with the photosensitive drum 41 of the printing mechanism 40 having toner of different colors.

  The reverse transfer phenomenon can be reduced by shifting the position of the transfer roller 49 disposed at the position facing the photosensitive drum 41 from the position immediately below the photosensitive drum 41 to the downstream side in the paper transport direction by a certain distance. This is because the gap discharge amount generated at the rear side of the transfer nip region is increased, the toner charge amount of the transferred toner image is increased, and the toner charged to the reverse polarity can be decreased.

FIG. 5 is a diagram illustrating a set value of the transfer roller installation distance in FIG. 1.
The shift amount t1 is a distance at which the transfer roller 49 is shifted and installed on the downstream side with respect to the photosensitive drum 41 of the black printing mechanism 40-1 installed on the most upstream side. Hereinafter, similarly, the shift amount t2 is a distance where the transfer roller 49 is shifted and installed on the downstream side with respect to the photosensitive drum 41 of the yellow printing mechanism 40-2. The shift amount t3 is a distance at which the transfer roller 49 is shifted and installed on the downstream side with respect to the photosensitive drum 41 of the magenta printing mechanism 40-3. The shift amount t4 is a distance at which the transfer roller 49 is shifted and installed on the downstream side with respect to the photosensitive drum 41 of the cyan printing mechanism 40-4.

6 is a diagram showing installation distance conditions (1) and (2) of the transfer roller of FIG.
Condition (1) is that the transfer roller 49 is shifted with respect to the photosensitive drum 41 from the black printing mechanism 40-1 installed on the most upstream side to the cyan printing mechanism 40-4 installed on the most downstream side. It is shown that it is installed.

  Condition (2) is that the transfer roller 49 is downstream of the photosensitive drum 41 from the black printing mechanism 40-1 installed at the most upstream side to the cyan printing mechanism 40-4 installed at the most downstream side. It is shown that it is shifted by 1.5 mm.

FIG. 7 is a diagram illustrating a toner charge amount transition of a black (K) toner image.
As can be seen from FIG. 7, when the shift amount t of each transfer roller 49 is 1.5 mm, the toner charge amount is greatly increased compared to the case where the shift amount t is 0.0 mm. This is because by shifting the position of the transfer roller 49 to the downstream side in the paper conveyance direction, the gap discharge amount generated at the rear side of the transfer nip region is increased, and the toner charge amount of the transferred toner image is increased.

FIG. 8 is a diagram showing density values and reverse transfer levels.
The reverse transfer level shown in FIG. 8 is a quantitative indication of how much image quality degradation has occurred due to the reverse transfer phenomenon, with the original density value being 0. The reverse transfer phenomenon is a phenomenon in which the toner charged to the reverse polarity included in the toner image previously transferred to the paper moves onto the photosensitive drum 41 disposed on the downstream side, and thus the image quality is deteriorated. Say.

  The concentration values shown in FIG. 8 are values measured with an X-rite 528 spectral densitometer (X-rite). Originally, printing is performed with a print pattern in which the density value is 1.50. However, as the reverse transfer phenomenon occurs, the density value decreases, and when the density value becomes 1.40, deterioration in image quality is visually observed. Can be recognized. This was defined as level 10, and the reverse transcription level was defined for each density value of 0.01. In FIG. 8, the level with the least degradation in image quality is level 0 and the level with the greatest degradation in image quality is level 10.

FIG. 9 is a diagram illustrating the reverse transfer level of the black (K) toner image.
As can be seen from FIG. 9, the shift amounts t1 to t4 of the respective transfer rollers 49-1 to 49-4 in the condition (2) are compared with the case where the shift amounts t1 to t4 in the condition (1) are all 0.0 mm. Are 1.5 mm, the reverse transfer levels in yellow (Y), magenta (M), and cyan (C) are low, and deterioration in image quality can be prevented. The total reverse transfer level in condition (1) is 5 + 4 + 3 = 12, while the total reverse transfer level in condition (2) is 3 + 2 + 1 = 6. The total value of the reverse transfer level is lower in the condition (2) than in the condition (1), and therefore the image quality is less deteriorated.

  Further, in order to obtain a color image in the image forming apparatus 10 according to the first exemplary embodiment, the second, third, and fourth color toner images must be transferred onto the first toner image that has been previously transferred. However, by shifting the transfer rollers 49-1 to 49-4 to suppress the reverse transfer phenomenon, the toner charge amount of the previously transferred toner image increases. The increase in the toner charge amount of the toner image can suppress the reverse transfer phenomenon, but on the other hand, the transferability of the toner images of the second, third and fourth colors which are transferred onto the toner image transferred in advance. The problem of worsening occurs. This is because the transfer voltage at which the transfer of the toner image is good differs depending on the toner charge amount of the previously transferred toner image.

  FIG. 10 is a diagram illustrating the transfer property of the first toner image charge amount and the second toner image.

  The horizontal axis of FIG. 10 indicates the toner charge amount [μC / g], the vertical axis indicates the transfer voltage [V], and the rectangle indicates the range of transfer voltage that can be satisfactorily transferred at each toner charge amount. The horizontal broken line indicates the range of transfer voltage that can be transferred satisfactorily when there is no previously transferred toner image (when the toner charge amount is 0 [μC / g]).

  The pattern of the image to be printed includes a pattern in which the second toner image is transferred to a place where the first toner image transferred earlier and a place where the first toner image transferred earlier is not present. There is a pattern to which a toner image of 2 is transferred.

  As shown in FIG. 10, when the toner charge amount of the first toner image transferred earlier increases, the transfer voltage range in which the second toner image transferred onto the first toner image can be satisfactorily transferred. Shifts to the high voltage side.

  In addition, when the toner charge amount of the first toner image transferred first becomes -40 [μC / g] or more, the second toner image in the case where there is no first toner image transferred first is displayed. There is no overlap between the transfer voltage range in which transfer can be satisfactorily and the transfer voltage range in which the second toner image can be satisfactorily transferred when there is the first toner image transferred earlier. In this case, no matter how the transfer voltage is selected, either the case where there is the first toner image transferred first or the case where there is no first toner image transferred first is good. Transfer cannot be performed, resulting in transfer failure.

  Referring to FIG. 7, when the shift amounts t1 to t4 of the transfer rollers 49-1 to 49-4 are all 1.5 mm, the toner charge amount of the black (K) toner image after magenta transfer is −40 [ μC / g] or more. In this case, when there is a black (K) toner image previously transferred in cyan (C), there is no transfer voltage range that can be satisfactorily transferred, and there is no black (K) toner image transferred earlier. Since the range of transfer voltages that can be satisfactorily transferred does not overlap, transfer defects occur regardless of the transfer voltage selected.

In the first embodiment, in view of the above, as shown in FIG. 5, the shift amounts t1 to t4 of the transfer rollers 49-1 to 49-4 of the printing mechanisms 40-1 to 40-4 are set to the downstream side. By gradually increasing the size of the installed printing mechanism 40, it is possible to suppress the reverse transfer phenomenon and to secure the transferability of the toner image transferred on the paper.
FIG. 11 is a graph showing the transition of the toner charge amount of the black (K) toner image in Example 1 of the present invention.

  FIG. 11 shows that the toner charge amount of the black (K) toner image after passing through the magenta (M) transfer nip region can be suppressed to −40 [μC / g] or less according to the first embodiment. .

  FIG. 12 is a diagram illustrating the reverse transfer level received by the black (K) toner image according to the first embodiment of the present invention.

  FIG. 12 shows that the total value of the respective reverse transfer levels in yellow (Y), magenta (M), and cyan (C) can be reduced to 10 or less where no deterioration in image quality is observed according to the first embodiment. At this time, when transferring the cyan (C), there is a transfer voltage range in which transfer can be performed satisfactorily when there is a black (K) toner image transferred first, and a black (K) toner image transferred first. If not, the transfer voltage ranges that can be transferred satisfactorily overlap. Therefore, transfer can be performed satisfactorily by applying a transfer voltage in the overlapping range to the transfer roller 49.

(Effect of Example 1)
The image forming apparatus 10 according to the first exemplary embodiment has the following effects (A) and (B).

  (A) Deterioration in image quality due to reverse transfer phenomenon can be prevented.

  (B) At the same time, it is possible to improve the transferability of the toner image in the printing mechanism 40 arranged on the downstream side in the paper conveyance direction by suppressing an increase in the toner charge amount of the toner image previously transferred to the paper. Can do.

(Configuration of Example 2)
FIG. 13 is a schematic configuration diagram illustrating an image forming apparatus according to the second embodiment of the present invention. Elements common to those in FIG. 2 illustrating the first embodiment are denoted by common reference numerals.

  The image forming apparatus 10A according to the second embodiment has a humidity detecting unit 89 in addition to the same configuration as the image forming apparatus 10 shown in FIG.

  FIG. 14 is a schematic configuration diagram illustrating a control circuit of the image forming apparatus in FIG. 13. Elements common to those in FIG. 3 illustrating the first embodiment are denoted by common reference numerals.

  The control circuit of the image forming apparatus 10A of the second embodiment has the same configuration as the control circuit of the image forming apparatus 10 of FIG. 3 showing the first embodiment, and further includes a humidity detection unit 89, a transfer voltage correction unit 95, and the like. have. The humidity detection unit 89 and the transfer voltage correction unit 95 are connected to the mechanism control unit 80. The humidity detector 89 is an operating environment acquisition unit that acquires humidity, which is one of the environmental information of the image forming apparatus 10A. The transfer voltage correction unit 95 corrects the transfer voltage based on the humidity, which is the environmental information. Voltage correction means.

(Operation of Example 2)
FIG. 15 is a flowchart showing the operation of the image forming apparatus according to the second embodiment of the present invention. Elements common to those in FIG. 4 showing the first embodiment are denoted by common reference numerals.

  When the processing starts, the image forming apparatus 10A waits until image data is received from a host computer (not shown) in step S1 as in the first embodiment. If the image data is received, unlike the first embodiment, the humidity is detected by the humidity detection unit 89 in step S20, and the transfer voltage correction unit 95 corrects the transfer voltage based on this humidity in step S21. Calculate the value.

Henceforth, the process of step S2-S12 is the same as that of Example 1. FIG.
In step S13A, the image forming apparatus 10A supplies the transfer roller 49 with a transfer voltage in consideration of the correction value calculated in step S21. Henceforth, the process of step S14-S17 is the same as that of Example 1. FIG.

FIG. 16 is a diagram showing conditions for calculating the correction value of the transfer voltage in FIG.
A transfer voltage correction value is determined in step S21 based on the humidity measured in step S20 and the transfer voltage correction condition shown in FIG. When the humidity is less than 30 [% RH], the correction value is 0 V, when the humidity is 31 to 60 [% RH], the correction value is −200 V, and when the humidity is 61 [% RH] or more, the correction value is − It shows that it is 400V. The transfer voltage correction value is determined by the high voltage control unit 90 from the transfer voltage generation unit 94 to transfer the black (K), yellow (Y), magenta (M), and cyan (C) transfer rollers 49-1 to 49-4. When supplying the voltage, the transfer voltage correction value determined in step S21 is added to the preset transfer voltage. In the second embodiment, the transfer voltage to the black (K) printing mechanism 40-1 is 2600 V, the transfer voltage to the yellow (Y) printing mechanism 40-2 is 2700 V, and the magenta (M) printing mechanism 40-3. The transfer voltage to 2800 V and the transfer voltage to cyan (C) printing mechanism 40-4 are set in advance to 2900 V, and the transfer voltage correction value shown in FIG. 16 is added to these transfer voltages.

FIG. 17 is a diagram illustrating the relationship between humidity and the surface resistivity of plain paper A.
The horizontal axis of FIG. 17 indicates humidity [% RH], the vertical axis indicates the surface resistivity [Ω / □] of plain paper A, and the rhombic plot indicates the relationship between humidity and surface resistivity.

  In Example 2, surface resistivity having a correlation with the reverse transfer phenomenon will be described. FIG. 17 shows that the surface resistivity of the paper (plain paper A), which is a recording medium, decreases with moisture absorption as the humidity increases.

FIG. 18 is a diagram illustrating the relationship between the surface resistivity of the plain paper A and the reverse transfer level.
The horizontal axis of FIG. 18 shows the surface resistivity [Ω / □] of plain paper A, the vertical axis shows the reverse transfer level, and the rhombus plot shows the relationship between the surface resistivity [Ω / □] and the reverse transfer level. Is shown.

  FIG. 18 shows that the reverse transfer level deteriorates as the surface resistivity of the recording medium (plain paper A) decreases. As described above, it is considered that the toner on the paper is affected by the transfer current in the transfer nip portion and is charged to the reverse polarity, which is a factor of the reverse transfer phenomenon. Therefore, it is considered that the lower the surface resistivity of the paper, the more easily the toner on the paper is affected by the transfer current in the transfer nip portion and is charged to the reverse polarity, thereby deteriorating the reverse transfer level.

FIG. 19 is a diagram showing the relationship between the surface resistivity of plain paper A and the good transfer voltage range.
The transfer voltage at which the transfer is good varies depending on the resistance value of the recording medium (plain paper A). As can be seen from FIG. 19, the transfer voltage at which the transfer is good spreads to the low voltage side as the surface resistivity of the recording medium (plain paper A) decreases.

  That is, the lower the surface resistivity of the paper, the worse the reverse transfer level, but at the same time, the transfer voltage at which the transfer becomes good spreads to the low voltage side. Therefore, by lowering the transfer voltage, it is possible to reduce the influence of the transfer current in the transfer nip portion and to suppress the generation of toner charged to the reverse polarity, thereby suppressing the reverse transfer phenomenon. Furthermore, since the amount of gap discharge generated at the rear side of the transfer nip portion is reduced by lowering the transfer voltage, it is possible to suppress an increase in the toner charge amount of the toner image transferred to the paper first. The transferability of the toner image in the printing mechanism 40 disposed on the downstream side in the direction can be improved.

  In the second embodiment, in view of the above, the humidity of the environment in which the image forming apparatus 10A is placed is detected, and the relationship between the humidity and the resistance variation of the medium, which are experimentally obtained in advance, is determined. In addition, by performing the correction so as to obtain the optimum transfer voltage, the reverse transfer phenomenon can be suppressed and the transferability of the toner image to be transferred in an overlapping manner can be ensured.

FIG. 20 is a diagram showing whether or not Example 2 is applied and the total reverse transfer level.
FIG. 20 shows that application of Example 2 can prevent deterioration in image quality even under high humidity conditions where the surface resistivity of the recording medium is low.

(Effect of Example 2)
The image forming apparatus 10A according to the second embodiment has the following effects (C) and (D).

  (C) Humidity detection means including a humidity detection unit 89 is provided, and the optimum transfer voltage is determined from the relationship between the humidity and the resistance value of the paper that is the recording medium, thereby preventing image quality deterioration due to the reverse transfer phenomenon. .

  (D) At the same time, it is possible to suppress an increase in the toner charge amount of the toner image previously transferred to the paper, and to improve the transferability of the toner image in the printing mechanism 40 arranged on the downstream side in the paper transport direction. .

(Configuration of Example 3)
The configuration of the image forming apparatus 10 according to the third exemplary embodiment of the present invention is the same as the configuration of the image forming apparatus 10 illustrated in FIG.

(Operation of Example 3)
FIG. 21 is a flowchart showing the operation of the image forming apparatus according to the third embodiment of the present invention. Elements common to those in FIG. 4 showing the first embodiment are denoted by common reference numerals.

  When the processing is started, the image forming apparatus 10 of the third embodiment performs the processing of steps S1 to S5 as in the first embodiment. In step S6A, unlike the first embodiment, the developing rollers 43-1 to 43- are processed. The applied voltage of 4 is corrected. Specifically, the photosensitive drum 41-1 to 41-4 is less sensitive to the photosensitive drum than the black (K) developing roller 43-1 on the upstream side in the paper conveying direction with a small shift amount of the transfer rollers 49-1 to 49-4. Application of yellow (Y), magenta (M), and cyan (C) developing rollers 43-2, 43-3, and 43-4 on the downstream side where the shift amount of the transfer rollers 49-1 to 49-4 with respect to 41 is large. Increase the absolute value of the voltage.

  Henceforth, the process of step S7-S17 is the same as that of Example 1. FIG.

FIG. 22 is a diagram illustrating a voltage obtained by correcting the voltage applied to the developing roller in FIG.
The yellow (Y) developing roller 43-2 is larger in absolute value of the applied voltage by d [V] than the black (K) developing roller 43-1, and thereafter the downstream developing roller 43 is similarly d [ V], the absolute value of the applied voltage increases, so that the shortage of toner generated in the downstream printing mechanism 40 can be counteracted.

(Effect of Example 3)
According to the image forming apparatus 10 of the third embodiment, in addition to the effects of the first embodiment, there is the following effect (E).

  (E) Insufficient amount of toner generated in the downstream printing mechanism 40 can be canceled by the voltage applied to the developing roller 43.

(Configuration of Example 4)
FIG. 23 is a schematic configuration diagram illustrating an image forming apparatus according to the fourth embodiment of the present invention. Elements common to those in FIG. 2 illustrating the first embodiment are denoted by common reference numerals.

  The image forming apparatus 10B according to the fourth embodiment of the present invention has a plurality of printing mechanisms 40B (= 40B-1, 40B-2, 40B-3, 40B-4) different from the image forming apparatus 10 shown in FIG. ) Is the same as that of the image forming apparatus 10 of the first embodiment.

  FIG. 24 is a diagram showing the nip amount between the developing roller and the photosensitive drum in the printing mechanism of FIG.

  Each printing mechanism 40 </ b> B includes a photosensitive drum 41, a charging roller 42, a developing roller 43, a supply roller 44, and a transfer roller 49, similarly to each printing mechanism 40 of the first embodiment. A conveyor belt 30 is movably disposed between the photosensitive drum 41 and the transfer roller 49 as in the first embodiment. The photosensitive drum 41 and the charging roller 42, the photosensitive drum 41 and the developing roller 43, and the developing roller 43 and the supply roller 44 are in contact with each other. Here, N is defined as the nip amount (the amount that each roller pushes in) between the developing roller 43 and the photosensitive drum 41.

  FIG. 25 is a diagram illustrating a relationship between the shift amount and the nip amount in the fourth embodiment of the present invention.

  Unlike the printing mechanisms 40 of the first embodiment, each printing mechanism 40B increases the nip amount N between the developing roller 43 and the photosensitive drum 41 as the shift amount t between the photosensitive drum 41 and the transfer roller 49 increases. As shown, the developing roller 43 and the photosensitive drum 41 are arranged.

(Operation of Example 4)
The shortage of the amount of toner transferred from the photosensitive drum 41 to the paper caused by shifting the transfer roller 49 in the paper transport direction with respect to the photosensitive drum 41 is canceled by an increase in the nip amount N between the developing roller 43 and the photosensitive drum 41. Therefore, even the downstream printing mechanism 40 does not run out of toner.

(Effect of Example 4)
According to the image forming apparatus 10B of the fourth embodiment, in addition to the effects of the first embodiment, there is the following effect (F).

  (F) The shortage of toner amount generated in the downstream printing mechanism 40B can be canceled by increasing the nip amount N between the developing roller 43 and the photosensitive drum 41.

(Modification)
The present invention is not limited to the above-described embodiments, and various usage forms and modifications are possible. For example, the following forms (a) to (d) are used as the usage form and the modified examples.

  (A) In Examples 1 to 4, the latent image forming means is the LED head 48, but a laser light source or the like may be used as the latent image forming means.

  (B) In Embodiments 1 to 4, the printing mechanisms 40-1 to 40-4 are arranged in the order of black (K), yellow (Y), magenta (M), and cyan (C) from the upstream side of the sheet conveyance path. Explained the case. However, the arrangement order when there are a plurality of printing mechanisms 40 related to multicolor toner may be upstream of cyan, for example, and is not limited to the first to fourth embodiments.

  (C) In the first to fourth embodiments, four printing mechanisms 40 are described. However, two or more printing mechanisms 40 may be used.

  (D) In the image forming apparatus 10 according to the first to fourth embodiments, the toner is transferred using the sheet conveyed by the conveying belt 30 as the transfer medium. However, after the transfer belt 30 is used as a transfer medium and the toner is directly transferred to the transfer belt 30 by the image forming means of black, yellow, magenta, and cyan, the black, yellow, magenta, and cyan transferred to the transfer belt 30 are transferred. Even in an image forming apparatus that simultaneously transfers toner, the present invention is applied to a portion where the toner is directly transferred to the conveying belt 30 by the image forming means of black, yellow, magenta, and cyan. It is possible to obtain.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 40 Printing mechanism 41 Photosensitive drum 42 Charging roller 49 Transfer roller 64 Heater 82 Command / image processing part 80 Mechanism control part 89 Humidity detection part 90 High voltage control part 94 Transfer voltage generation part 95 Transfer voltage correction part

Claims (8)

A transfer medium transfer means for transferring the transfer medium in the transfer medium transfer direction;
An image forming apparatus including a plurality of image forming units disposed along the transfer medium conveyance direction and transferring a developer image to the transfer medium;
The image forming unit includes:
An image carrier that carries the developer image;
A transfer member disposed opposite to the image carrier and shifted by a certain distance on the downstream side in the transfer medium conveyance direction with respect to the image carrier;
The image carrier and the transfer member are opposed to each other, and a contact area for transferring the developer image to the transfer medium;
The fixed distance between the transfer member and the image carrier is larger in the image forming unit disposed on the downstream side in the conveyance direction of the transfer medium than the image forming unit disposed on the upstream side. An image forming apparatus.   The image forming apparatus according to claim 1, wherein the fixed distances in the plurality of image forming units are all 2.0 mm or less.   The image forming apparatus according to claim 1, wherein the transfer member transfers the developer image onto the transfer medium when a predetermined voltage is applied. Storage means for previously storing a voltage correction value for correcting the predetermined voltage applied to the transfer member for each operating environment;
An operating environment acquisition unit for acquiring environmental information related to the operating environment;
4. The image forming apparatus according to claim 3, further comprising a voltage correction unit that corrects the predetermined voltage based on the environmental information and the voltage correction value.   The image forming apparatus according to claim 4, wherein the environment information acquired by the operating environment acquisition unit is humidity.   The image forming apparatus according to claim 5, wherein the voltage correction value is smaller as humidity is higher. The image forming means further includes a developing roller that develops the image carrier when a developing voltage is applied,
The development voltage of the developing roller is higher in the image forming unit disposed on the downstream side in the conveyance direction of the transfer medium than in the image forming unit disposed on the upstream side. Item 2. The image forming apparatus according to Item 1. The image forming unit further includes a developing roller that develops the image carrier by being pushed into and in contact with the image carrier,
The image forming unit disposed on the downstream side in the conveyance direction of the transfer medium has a larger amount of the developing roller pushed into the image carrier than the image forming unit disposed on the upstream side. The image forming apparatus according to claim 1.

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