JP2006337500A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which forms a high-quality toner image by restraining the sudden increase of a transfer current when the trailing edge of paper passes through a transfer position and preventing the occurrence of a white stripe caused by the insufficiency of toner. <P>SOLUTION: When detecting that the trailing edge of the paper 3 reaches a sensor 80 (S4: Yes), a transfer set current I to be supplied to a transfer roller 30 is set to a transfer current I<SB>1</SB>=-14μA (S5). Counting a time T<SB>1</SB>taken until the trailing edge of the paper 3 reaches the transfer position (t=t<SB>1</SB>) after the trailing edge of the paper 3 is detected (t=t<SB>0</SB>) is started (S5). When deciding that counting the time T<SB>1</SB>is finished (S6: Yes), setting of the transfer set current I is switched from the transfer current I<SB>1</SB>=-14μA to a transfer current I<SB>2</SB>=-6μA (S7). Counting a time T<SB>2</SB>=90 ms taken until the paper 3 advances nearly by a little longer than 40 mm (t=t<SB>2</SB>) after the trailing edge of the paper 3 reaches the transfer position (t=t<SB>1</SB>) is started (S8). When deciding that counting the time T<SB>2</SB>is finished (S9: Yes), the setting of the transfer set current I is restored from the transfer current I<SB>2</SB>=-6μA to the transfer current I<SB>1</SB>=-14μA again (S10). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a laser printer.

  In an image forming apparatus such as a laser printer, usually, a photosensitive drum, and a charger, a scanner device, a developing roller, and a transfer roller are sequentially provided around the photosensitive drum according to the rotation direction of the photosensitive drum.

  As the photosensitive drum rotates, the surface of the photosensitive drum is first uniformly charged by a charger, and then exposed by high-speed scanning of a laser beam from a scanner device, and an electrostatic latent image based on image data is generated. It is formed. Next, the toner carried on the developing roller when it is opposed to the developing roller is supplied to the electrostatic latent image formed on the surface of the photosensitive drum and is selectively carried, whereby a toner image ( A visible image) is formed. Thereafter, the toner image carried on the surface of the photosensitive drum is opposed to the transfer roller, and a transfer bias is applied to the transfer roller while the paper passes between the photosensitive drum and the transfer roller. Is transcribed.

  In a transfer bias application power source having a transfer bias application circuit configured to apply such a transfer bias to the transfer roller, the transfer bias application circuit may, for example, have a resistance value (photosensitive drum) on the transfer roller side due to environmental changes. Ordinarily, constant current control is performed so that a constant transfer current can always flow even if the resistance value including the sheet and the same (hereinafter the same) changes.

  For example, a forward transfer bias application circuit configured such that a forward transfer bias is applied to the transfer roller by a transfer bias application power source lower than the surface potential of the photosensitive drum opposed to the transfer roller; and the surface potential of the photosensitive drum A reverse transfer bias application circuit configured to apply a higher reverse transfer bias to the transfer roller, and the forward transfer bias application circuit is configured to apply the forward transfer bias by constant current control. A forward transfer output voltage detection circuit for detecting the voltage of the forward transfer bias application circuit, and the forward transfer bias application circuit detected by the forward transfer output voltage detection circuit during constant current control by the forward transfer bias application circuit. Based on the output voltage and the resistance value on the reverse transfer bias application circuit side, the resistance value on the transfer roller side is detected by the CPU. See Patent Document 1).

  According to Japanese Patent Application Laid-Open No. 2004-133867, the CPU determines the transfer roller side based on the output voltage of the forward transfer bias application circuit detected by the forward transfer output voltage detection circuit and the resistance value on the reverse transfer bias application circuit side. Since the resistance value is detected and controlled, the resistance value of the transfer roller can be accurately detected, and the current value can be controlled to a desired value. Therefore, it is possible to calculate an appropriate constant current value with high accuracy. Further, if constant current control is performed using a CPU, the configuration of the transfer bias application power source can be simplified, leading to miniaturization of the image forming apparatus main body.

JP 2004-029601 A

  However, as shown in FIG. 9, when a high impedance sheet is used in Patent Document 1, a transfer output current during transfer that has been output with a specified transfer setting current until now is the rear end of the sheet is a photosensitive drum. Since the photosensitive drum and the transfer roller are in direct contact with each other when passing through the transfer position between the transfer roller and the transfer roller, the size rapidly increases. For example, if the transfer setting current supplied to the transfer roller is set to -14 μA, when the trailing edge of the sheet passes the transfer position, the output current actually output increases by about −10 μA |, and is about −25 μA. It was getting bigger. In Patent Document 1, the CPU that controls the transfer current can control the transfer current only at a constant period. In addition, a forward transfer bias application circuit is provided to supply a predetermined transfer forward bias to the transfer roller. Since it takes time to discharge the electric charge stored in the capacitive component, the followability is poor, and it takes time to return the increased current value to the specified value. As a result, the photosensitive portion in contact with the transfer roller is exposed. The amount of charge on the drum also suddenly increases, and there is a problem in that so-called white streaks occur where toner is insufficient only at the contact portion.

  Therefore, the present invention has been made in view of such problems, and can suppress a sudden increase in transfer current when the trailing edge of the paper passes the transfer position. An object of the present invention is to provide an image forming apparatus capable of preventing generation of stripes and forming a high-quality toner image.

  In order to achieve the above object, in the image forming apparatus according to claim 1, a photoconductor that carries a developer image, a transfer unit that transfers the developer image carried on the photoconductor onto a recording medium, A supply unit that supplies a transfer current to the transfer unit; and a first transfer current is supplied to the transfer unit by the supply unit, and the rear end of the recording medium passes or passes through a transfer position where the photosensitive member and the transfer unit face each other. Control means for switching the output value of the transfer current so that the second transfer current smaller than the first transfer current is supplied to the transfer means by the supply means immediately before or after the transfer.

  3. The image forming apparatus according to claim 2, wherein a recording medium detecting unit that detects a trailing end of the recording medium, and a timing that starts counting a predetermined time when the trailing end or the leading end of the recording medium is detected by the recording medium detecting unit. And a control means that, when timed by the timekeeping means, measures the output value of the transfer current so that the supply means supplies the second transfer current to the transfer means in place of the first transfer current. It is characterized by switching.

  According to a third aspect of the present invention, the set value of the second transfer current is changed depending on the type of material of the recording medium.

  According to a fourth aspect of the present invention, the image forming apparatus further includes an input unit capable of inputting to change the set value of the second transfer current.

  The image forming apparatus according to claim 5, wherein an output current detection unit that detects an output value of a transfer current supplied to the transfer unit by the supply unit, and an output value of a transfer voltage that is supplied to the transfer unit by the supply unit are detected. And an output voltage detection means for controlling the output current of the transfer current before the recording medium detected by the output current detection means and the output voltage detection means is sandwiched between the photosensitive member and the transfer means. From the output value of the transfer voltage, the output value of the transfer current when the recording medium detected by the output current detection means and the output voltage detection means is sandwiched between the photosensitive member and the transfer means, and the output value of the transfer voltage, (2) A setting value of a transfer current is calculated and changed.

  According to the image forming apparatus of the first aspect, the second transfer current smaller than the first transfer current is supplied to the transfer unit by the supply unit when the rear end of the recording medium passes the transfer position, or immediately before or after the recording medium passes. The output value of the transfer current is switched so that the transfer current can be prevented from rapidly increasing when the trailing edge of the recording medium passes the transfer position. Therefore, the charge amount of the photosensitive member at the contact portion with the transfer means of the photosensitive member is rapidly increased, and white streaks due to the lack of toner only at the contact portion can be prevented. Thereby, a high quality image can be formed.

  According to the image forming apparatus of the second aspect, the first transfer current supplied by the supplying unit can be switched to the second transfer current at an accurate timing by the recording medium detecting unit and the time measuring unit.

  According to the image forming apparatus of the third aspect, every time a recording medium of a different material is used, the set value of the second transfer current is set to a value that causes a shortage of toner only in the contact portion of the photosensitive member with the transfer means. It is possible to change to a setting value most suitable for preventing Thereby, even if the material of the recording medium is changed, a high-quality image can be maintained.

  According to the image forming apparatus of the fourth aspect, the set value of the second transfer current is set to the most suitable set value for preventing the occurrence of white streaks due to the lack of toner only at the contact portion of the photosensitive member with the transfer means. It can be changed as appropriate. Thereby, even if the material of the recording medium is changed, a high-quality image can be maintained.

  According to the image forming apparatus of the fifth aspect, since the control unit calculates and changes the set value of the second transfer current, it is most suitable for preventing the occurrence of white stripes without requiring the user to change the set value. It can be changed to the set value. Thereby, even if the material of the recording medium is changed, a high-quality image can be maintained.

[First Embodiment]
(overall structure)
First, the overall configuration of a laser printer as an image forming apparatus of the present invention will be described with reference to FIGS. FIG. 1 is a side sectional view of an essential part showing an embodiment of a laser printer 1. FIG. 2 is a block diagram of the main configuration of the transfer bias application circuit. The laser printer 1 includes, in a main casing 2, a paper feeding unit 4 for feeding paper 3 as a recording medium, an image forming unit 5 for forming an image on the fed paper 3, and the like. ing.

  In the following, in the laser printer 1, the side on which the front cover 2a is provided is referred to as “front side”, and the opposite side is referred to as “rear side”.

<Configuration of paper feed unit>
The paper feed unit 4 is detachably mounted on the bottom of the main casing 2, a paper pressing plate 7 provided in the paper feed tray 6, and above the front end of the paper feed tray 6. A paper feed roller 8 and a separation pad 9 provided in the paper, a paper dust removing rollers 10 and 11 provided downstream of the paper feed roller 8 in the conveying direction of the paper 3, and a paper 3 for the paper dust removing rollers 10 and 11. And a registration roller 12 provided on the downstream side in the conveying direction.

  The sheet pressing plate 7 can stack the sheets 3 in a stacked manner, and is supported at the rear end so as to be swingable, so that the front end can be moved in the vertical direction. Moreover, it is urged | biased by the spring which is not shown in figure from the back side. For this reason, the sheet pressing plate 7 is swung downward against the urging force of the spring with the rear end portion as a fulcrum with respect to the sheet feeding roller 8 as the amount of stacked sheets 3 increases. The paper feed roller 8 and the separation pad 9 are disposed so as to face each other, and the separation pad 9 is pressed toward the paper feed roller 8 by a spring 13 provided on the back side of the separation pad 9.

  The fed paper 3 is conveyed to the registration roller 12 after the paper dust is removed by the paper dust removing rollers 10 and 11. The registration roller 12 includes a pair of rollers, and conveys the paper 3 to the transfer position after registration. Note that the transfer position is a position at which the toner image on the photosensitive drum 27 serving as the photosensitive member is transferred to the sheet 3, and is a contact position between the photosensitive drum 27 and the transfer roller 30 in this embodiment.

  The feeder unit 4 further includes a multi-purpose tray 14, a multi-purpose side feed roller 15 and a multi-purpose side separation pad 25 for feeding the paper 3 stacked on the multi-purpose tray 14. ing. The multi-purpose side feed roller 15 and the multi-purpose side separation pad 25 are arranged to face each other, and the multi-purpose side separation pad 25 is supplied to the multi-purpose side by a spring 25 a provided on the back side of the multi-purpose side separation pad 25. It is pressed toward the paper roller 15.

  In addition, a sheet detection sensor 80 as a recording medium detection unit is provided on a conveyance path downstream of the registration roller 12 in the sheet conveyance direction and upstream of the photosensitive drum 27 in the sheet conveyance direction. When the paper detection sensor 80 is pressed by the paper 3 that has been passed and conveyed, it is detected that the leading edge of the paper 3 has reached the paper detection sensor 80. While the paper 3 is passing through the paper detection sensor 80, the paper detection sensor 80 continues to be pressed by the paper 3, and if the pressure is released, the trailing edge of the paper 3 has reached the paper detection sensor 80. Is detected.

<Configuration of image forming unit>
The image forming unit 5 includes a scanner unit 16, a process cartridge 17, a fixing unit 18, and the like. Hereinafter, each of these components will be described in detail.

<Configuration of scanner unit>
The scanner unit 16 is provided at an upper portion in the main body frame 2 and includes a laser light emitting unit (not shown), a polygon mirror 19 that is rotationally driven, lenses 20 and 21, and reflecting mirrors 22, 23, and 24. The laser beam based on the image data emitted from the laser emitting unit passes or reflects in the order of the polygon mirror 19, the lens 20, the reflecting mirrors 22 and 23, the lens 21, and the reflecting mirror 24, as indicated by the chain line, and processes. The surface of the photosensitive drum 27 of the cartridge 17 is irradiated with high-speed scanning.

<Configuration of process cartridge>
The process cartridge 17 is provided below the scanner unit 16. The process cartridge 17 includes a drum cartridge 26 that is detachably attached to the main body frame 2, and a developing cartridge 28 that is accommodated in the drum cartridge 26. As shown in FIG. 1, a front cover 2a that can be opened and closed with the lower end side as a central axis is provided on the front surface of the main body frame 2, and the process cartridge 17 opens the front cover 2a to open the main body frame 2. It is accommodated in a removable manner.

<Drum cartridge configuration>
The drum cartridge 26 includes a photosensitive drum 27 as a photosensitive member, a scorotron charger 29, a transfer roller 30 as a transfer unit, and a cleaning brush 64.

<Configuration of photosensitive drum>
The photosensitive drum 27 is disposed behind the developing roller 31 so as to face the developing roller 31 and is supported by the drum cartridge 26 so as to be rotatable in an arrow direction (clockwise). The photosensitive drum 27 includes a cylindrical drum body, and a metal drum shaft 27a that supports the drum body and is provided at the axis of the drum body. The drum body is made of an aluminum tube, and a positively chargeable photosensitive layer made of polycarbonate or the like is formed on the surface thereof. The drum shaft 27a is grounded as shown in FIG.

<Configuration of scorotron charger>
As shown in FIG. 1, the scorotron charger 29 is disposed above the photosensitive drum 27 so as to face the photosensitive drum 27 at a predetermined interval and is supported by the drum cartridge 26. The scorotron charger 29 is a positively-charged scorotron charger that generates corona discharge from a charging wire 29 a such as tungsten. The scorotron charger 29 includes a grid 29 b between the charging wire 29 a and the photosensitive drum 27. Is uniformly charged to a positive polarity. A predetermined charging bias voltage is applied to the charging wire 29a.

<Configuration of transfer roller>
The transfer roller 30 is disposed below the photosensitive drum 27 so as to face the photosensitive drum 27 and is supported by the drum cartridge 26 so as to be rotatable in the direction of the arrow (counterclockwise). In the transfer roller 30, a metal roller shaft 30a is covered with a roller made of a conductive rubber material.

  A bias application circuit 60 mounted on a high voltage power supply circuit board 52 is connected to the roller shaft 30a of the transfer roller 30, and the toner image carried on the developing roller 31 at the transfer position is transferred to the paper 3. During the transfer operation, a transfer setting current I as a transfer current is applied from the bias application circuit 60.

<Configuration of cleaning brush>
The cleaning brush 64 is provided so as to face the drum body of the photosensitive drum 27. The cleaning brush 64 is made of a conductive member, and is applied with a predetermined cleaning bias voltage and serves to remove paper dust adhering to the photosensitive drum 27.

<Configuration of developer cartridge>
The developing cartridge 28 is detachably accommodated in the drum cartridge 26 and includes a developing roller 31 as a developer carrier, a layer thickness regulating blade 32, a supply roller 33, and a toner hopper 34.

  The toner hopper 34 is filled with positively chargeable non-magnetic one-component toner as a developer. Examples of the toner include polymerizable monomers such as styrene monomers such as styrene, and acrylic monomers such as acrylic acid, alkyl (C1 to C4) acrylate, and alkyl (C1 to C4) methacrylate. Polymerized toners obtained by copolymerization by a known polymerization method such as suspension polymerization are used. Such a polymerized toner has a substantially spherical shape, has extremely good fluidity, and can achieve high-quality image formation.

  Such a toner is blended with a colorant such as carbon black, wax, and the like, and an additive such as silica is added to improve fluidity. The particle diameter is about 6 to 10 μm.

  Then, the toner in the toner hopper 34 is agitated by an agitator 36 supported by a rotating shaft 35 provided at the center of the toner hopper 34, and is discharged from a toner supply port 37 opened at the rear side portion of the toner hopper 34. The agitator 36 is rotationally driven in the direction of the arrow (clockwise) by the input of power from a motor (not shown). Note that windows 38 for detecting the remaining amount of toner are provided on both side walls of the toner hopper 34 (both side walls in the depth direction in FIG. 1), and are cleaned by a wiper 39 supported by the rotating shaft 35.

  A supply roller 33 is rotatably provided behind the toner supply port 37, and a developing roller 31 is rotatably provided facing the supply roller 33. The supply roller 33 and the developing roller 31 are in contact with each other in a state where each of them is compressed to some extent.

  The supply roller 33 has a metal roller shaft covered with a roller made of a conductive foam material. The supply roller 33 is rotationally driven in the direction of the arrow (counterclockwise) by the input of power from a motor (not shown).

  In the developing roller 31, a roller made of a conductive rubber material is coated on a metal roller shaft 31a. More specifically, the roller of the developing roller 31 is coated with a coating layer of urethane rubber or silicone rubber containing fluorine on the surface of a roller body made of conductive urethane rubber or silicone rubber containing carbon fine particles. Has been. A predetermined developing bias voltage is applied to the developing roller 31 during development. The developing roller 31 is rotationally driven in the direction of the arrow (counterclockwise) by the input of power from a motor (not shown).

  A layer thickness regulating blade 32 is provided in the vicinity of the developing roller 31. The layer thickness regulating blade 32 includes a pressing portion 40 having a semicircular cross section made of insulating silicone rubber at the tip of a blade body made of a metal leaf spring material. The layer thickness regulating blade 32 is supported by the developing cartridge 28 near the developing roller 31, and the pressing portion 40 is pressed onto the developing roller 31 by the elastic force of the blade body.

  The toner discharged from the toner supply port 37 is supplied to the developing roller 31 by the rotation of the supplying roller 33. At this time, the toner is positively frictionally charged between the supplying roller 33 and the developing roller 31. Further, the toner supplied onto the developing roller 31 enters between the pressing portion 40 of the layer thickness regulating blade 32 and the developing roller 31 as the developing roller 31 rotates, and is developed as a thin layer having a constant thickness. It is carried on a roller 31.

<Configuration of fixing unit>
As shown in FIG. 1, the fixing unit 18 is provided on the rear downstream side of the process cartridge 17, the heating roller 41, the pressing roller 42 that presses the heating roller 41, and the downstream side of the heating roller 41 and the pressing roller 42. A pair of conveying rollers 43 provided in the apparatus is provided. The heating roller 41 is made of metal and includes a halogen lamp for heating, and is driven to rotate in the arrow direction (clockwise) by the input of power from a motor (not shown). Further, the pressing roller 42 is rotated in the direction of the arrow (counterclockwise) following the heating roller 41 in a state where the heating roller 41 is pressed. In the fixing unit 18, the toner transferred onto the paper 3 in the process cartridge 17 is thermally fixed while the paper 3 passes between the heating roller 41 and the pressing roller 42. 43 is conveyed to the paper discharge path 44.

  The laser printer 1 is provided with a reverse conveyance unit 47 in order to form images on both sides of the paper 3. The reverse conveyance unit 47 includes a paper discharge roller 45, a reverse conveyance path 48, a flapper 49, and a plurality of reverse conveyance rollers 50.

(Configuration of bias application circuit)
As described above, the bias application circuit 60 is for applying the transfer setting current I as a transfer current to the transfer roller 30 during the transfer operation.

  The bias application circuit 60 includes a CPU 61 as control means and a transfer output circuit 62 as supply means. The transfer output circuit 62 is connected to a connection line 90 connected to the roller shaft 30 a of the transfer roller 30. The bias application circuit 60 includes an output current detection circuit 83 as output current detection means for outputting a detection signal S2 corresponding to the value of the current flowing through the connection line 90. The transfer output circuit 62 is subjected to constant current control by PWM (Pulse Width Modulation) control of the CPU 61. Further, the CPU 61 is connected with a paper detection sensor 80 as a recording medium detection means and a timer 100 as a time measuring means, and the paper detection sensor 80 detects that the trailing edge of the paper 3 has reached the sensor 80. Then, the CPU 61 causes the timer 100 to count for a predetermined time. The paper detection sensor 80 is disposed in the paper transport path on the upstream side of the transfer roller 30.

<Configuration of transfer output circuit>
The transfer output circuit 62 will be described. The transfer output circuit 62 includes a transfer PWM signal smoothing circuit 70, a transfer transformer drive circuit 71, a transfer boosting / smoothing rectifier circuit 72, and an output voltage detection circuit 73 as output voltage detection means.

  Of these, the transfer PWM signal smoothing circuit 70 receives the PWM signal S 1 output from the PWM port 61 a of the CPU 61, smoothes it to remove an AC component, and gives it to the transfer transformer drive circuit 71. The transfer transformer drive circuit 71 is configured to cause an oscillation current to flow through the primary winding 75b of the transfer boosting / smoothing rectifier circuit 72 based on the received PWM signal S1.

  The transfer boosting / smoothing rectifier circuit 72 includes a transformer 75, a diode 76, a smoothing capacitor 77, and the like. The transformer 75 includes a secondary winding 75a, a primary winding 75b, and an auxiliary winding 75c. One end of the secondary winding 75 a is connected to the roller shaft 30 a of the transfer roller 30 via a connection line 90 via a diode 76. On the other hand, the other end of the secondary winding 75 a is grounded via the output current detection circuit 83. A smoothing capacitor 77 and a discharge resistor 78 are connected between one end and the other end of the secondary winding 75a, respectively.

  With such a configuration, the oscillation current of the primary speed winding 75 b is boosted and rectified in the transfer boosting / smoothing rectifier circuit 72 and applied to the roller shaft 30 a of the transfer roller 30 connected to the output terminal A of the bias applying circuit 60. Applied as transfer setting current I.

  The output voltage detection circuit 73 is connected to the auxiliary winding 75c of the transformer 75 of the transfer boosting / smoothing rectification circuit 72, and generates a transfer output voltage Vt generated between the auxiliary windings 75c during the transfer operation by the transfer output circuit 62. The detection signal S3 is detected and input to the A / D port 61b of the CPU 61.

  As described above, during the transfer operation, the CPU 61 applies the PWM signal S1 to the transfer output circuit 62 to drive it, and this current value is set to a constant current set value based on the detection signal S2 corresponding to the transfer output current It flowing in the connection line 90. The constant current control is executed to output the PWM signal S1 with the duty ratio appropriately changed so as to be I to the transfer PWM signal smoothing circuit 70. When software control is performed using the CPU 61 as described above, the followability for constant current control is reduced as compared with the case where hardware control is performed without using the CPU 61, but an appropriate constant current value with high accuracy is supplied. be able to. Further, the configuration of the bias application circuit 60 can be simplified, leading to a reduction in the size of the laser printer 1 main body.

  In the present embodiment, the detection signal S2 from the output current detection circuit 83 is fed back by the A / D port 61d during the transfer operation.

(Example of image forming operation)
In the laser printer 1 of the present embodiment as described above, the operation during printing will be described with reference to FIG.

  The uppermost sheet 3 on the sheet pressing plate 7 is pressed toward the sheet feeding roller 8 by a spring (not shown) from the back side of the sheet pressing plate 7, and the sheet feeding roller 8 and the separation pad 9 are rotated by the rotation of the sheet feeding roller 8. Are fed one by one. The fed paper 3 is conveyed to the registration roller 12 after the paper dust is removed by the paper dust removing rollers 10 and 11.

  When the sheets 3 are stacked on the multi-purpose tray 14, one sheet is sandwiched between the multi-purpose side feed roller 15 and the multi-purpose side separation pad 25 by the rotation of the multi-purpose side feed roller 15. Paper is fed every time. The fed paper 3 passes through the paper dust removing rollers 10 and 11 and is conveyed to the registration roller 12.

  Further, when the agitator 36 is rotated, the toner in the toner hopper 34 is stirred and discharged from the toner supply port 37 toward the supply roller 33. The toner released toward the supply roller 33 is supplied onto the developing roller 31 by the rotation of the supply roller 33, and at this time, the toner is positively frictionally charged between the supply roller 33 and the developing roller 31. The toner supplied onto the developing roller 31 enters between the pressing portion 40 of the layer thickness regulating blade 32 and the developing roller 31 as the developing roller 31 rotates, and becomes a thin layer having a certain thickness. It is carried on the developing roller 31.

  On the other hand, the surface of the photosensitive drum 27 is uniformly positively charged by a scorotron charger 29 and then exposed by high-speed scanning of a laser beam from the scanner unit 16 to form an electrostatic latent image based on image data. .

  Next, the electrostatic latent image formed on the surface of the photosensitive drum 27 when the toner carried on the developing roller 31 and positively charged comes into contact with the photosensitive drum 27 by rotation of the developing roller 31. That is, the surface of the photosensitive drum 27 that is uniformly positively charged is supplied to an exposed portion that is exposed to a laser beam and the potential is lowered, and is selectively carried to be visualized. Thus, a toner image is formed by reversal development.

  Thereafter, the photosensitive drum 27 and the transfer roller 30 are rotationally driven so as to sandwich and convey the sheet 3 therebetween, and the sheet 3 is conveyed between the photosensitive drum 27 and the transfer roller 30. A toner image carried on the surface of the photosensitive drum 27 is transferred onto the paper 3. Note that the paper dust adhering to the surface of the photosensitive drum 27 by contact with the paper 3 after the transfer is removed when the surface of the photosensitive drum 27 faces the cleaning brush 64 as the photosensitive drum 27 rotates.

  When the sheet 3 to which the toner image is transferred is conveyed to the fixing unit 18, the sheet 3 passes between the heating roller 41 and the pressing roller 42 through the toner transferred onto the sheet 3 in the fixing unit 18. Then, the sheet 3 is conveyed to the sheet discharge path 44 by the conveying roller 43. The sheet 3 conveyed to the sheet discharge path 44 is conveyed to the sheet discharge roller 45 and is discharged onto the sheet discharge tray 46 by the sheet discharge roller 45.

  When forming images on both sides of the paper 3, the flapper 49 is switched when the vicinity of the rear end of the paper 3 is sandwiched between the paper discharge rollers 45, and the paper discharge rollers 45 are rotated in reverse to reverse the paper 3. It is transported to the transport path 48 and then transported toward the reverse transport section 47. The sheet 3 conveyed to the reversal conveyance unit 47 is sent again to the registration roller 12 by a plurality of reversal conveyance rollers 50, and a new image is formed on the surface where the image is not formed by the image forming unit 5. .

(Transfer setting current control in image forming operation)
Next, control of the transfer setting current I applied to the transfer roller 30 in the image forming operation of the laser printer 1 will be described with reference to FIGS. 3, 4A, and 4B. FIG. 3 is a flowchart showing the control contents of the CPU 61 for suppressing a rapid change in the transfer output current It at the rear end of the sheet 3. 4A and 4B, from when the paper detection sensor 80 detects the rear edge of the paper 3 to after the rear edge of the paper 3 passes the transfer position between the photosensitive drum 27 and the transfer roller 30. 6 is a graph showing changes in transfer setting current I and transfer output current It.

First, the paper 3 is fed by the paper feed roller 8 and conveyed to the registration roller 12 (S1). The sheet 3 registered by the registration roller 12 is conveyed toward the transfer position where the photosensitive drum 27 and the transfer roller 30 are in contact with each other by the rotation of the registration roller 12. Then, it is determined whether or not it is detected by the paper detection sensor 80 provided in the conveyance path between the registration roller 12 and the transfer position that the leading edge of the paper 3 has reached the sensor 80 (S2). If it is determined by the paper detection sensor 80 that the leading edge of the paper 3 has reached the sensor 80 (S2: Yes), the time when the leading edge of the paper 3 is detected by the paper detection sensor 80 is defined as t ₀ ′. Supply of a transfer current of −14 μA to the transfer roller 30 is started by the bias application circuit 60 (S3). This transfer current is supplied while the paper 3 passes through the transfer roller 30. If it is not determined by the paper detection sensor 80 that the leading edge of the paper 3 has reached the sensor 80 (S2: No), the process of S2 is performed until it is detected that the leading edge of the paper 3 has reached the sensor 80. Is repeated. After the current supply to the transfer roller 30 is started, it is determined whether or not it is detected that the rear end of the sheet 3 has reached the sensor 80 (S4). If it is not detected by the paper detection sensor 80 that the trailing edge of the paper 3 has reached the sensor 80 (S4: No), the process of S4 is performed until it is detected that the trailing edge of the paper 3 has reached the sensor 80. Repeated. During this time, a transfer current of −14 μA is supplied to the transfer roller 30. When the paper detection sensor 80 detects that the trailing edge of the paper 3 has reached the sensor 80 (S4: Yes), a detection signal is transmitted to the CPU 61. The CPU 61 that has received the detection signal sets the transfer setting current I supplied to the transfer roller 30 by the transfer output circuit 62 to the transfer current I ₁ as the first transfer current (S5). In the present embodiment, the value of the transfer current I ₁ is set to −14 μA which is the same as that during the transfer. At the same time as the transfer current I ₁ is set, the timer 100 starts counting for a predetermined time with t ₀ when the trailing edge of the sheet 3 is detected by the sheet detection sensor 80 as shown in FIG. (S5). The predetermined time corresponds to a time T _{1 from} when the trailing edge of the sheet 3 is detected by the sheet detection sensor 80 (t = t ₀ ) until reaching the transfer position (t = t ₁ ). Next, it is determined whether or not the timer 100 has finished counting the time T ₁ (S6). When the timer 100 determines that the time T ₁ has been counted (S6: Yes), the CPU 61 determines that the trailing edge of the sheet 3 has reached the transfer position, and sets the transfer setting current I to the transfer current. Switching from I ₁ to a transfer current I ₂ as a second transfer current (S7). At this time, in this embodiment, the transfer setting current I is decreased from −14 μA to −−8 μA | and switched to −6 μA. That is, when the rear end of the sheet 3 reaches the transfer position, a transfer setting current I of I ₂ = −6 μA is supplied to the transfer roller 30. On the other hand, if it is not determined by the timer 100 that the counting of the time T ₁ has ended (S6: No), the process of S6 is repeated until the timer 100 determines that the counting of the time T ₁ has ended. During this time, −14 μA is continuously supplied as the transfer current I ₁ . When the setting of the transfer set current I is switched from the transfer current I ₁ to the transfer current I ₂ in the process of S7 (t = t ₁ ), the timer 100 starts counting for a predetermined time (S8). In the present embodiment, the predetermined time is a time T ₂ from when the rear end of the sheet 3 reaches the transfer position (t = t ₁ ) until the sheet 3 moves forward by about 40 mm (t = t ₂ ). Correspondingly, the time T ₂ is set to T ₂ = 90 ms. Then, it is determined by the timer 100 whether or not the time T ₂ has been counted (S9). When the timer 100 determines that the time T ₂ has been counted (S9: Yes), the CPU 61 determines that 90 ms has elapsed since the trailing edge of the sheet 3 reached the transfer position, and the transfer setting current I The setting is returned from the transfer current I ₂ to the transfer current I ₁ again (S10). That is, the transfer setting current I is returned from I ₂ = −6 μA to I ₁ = −14 μA. On the other hand, if it is determined that the count of the time by the timer 100 T ₂ has been completed (S9: No), the processing of S9 until the count of the timer 100 time T ₂ is determined to have ended are repeated. During this period, −6 μA is supplied as the transfer current I ₂ . When the transfer setting current I is reset from I ₂ = −6 μA to I ₁ = −14 μA and a predetermined time has elapsed (S10), it is determined whether there is a sheet 3 on which an image is to be formed next (S11). ). Next, if it is determined that there is no sheet 3 on which an image is to be formed (S11: Yes), the supply of current to the transfer roller 30 by the bias application circuit 60 is terminated (S12). When the current supply to the transfer roller 30 is completed (S12), the image forming process is completed. On the other hand, if it is determined that there is a sheet 3 on which an image is formed (S11: No), the process returns to the process from S1, and the control of the transfer set current I is executed again. At this time, since the transfer current is already ON, the process of turning ON the transfer current in S3 is not executed.

  As described above, when the transfer set current I as shown in FIG. 4A is controlled, as shown in FIG. 9, conventionally, when the rear end of the sheet 3 passes through the transfer position, it becomes larger by about −10 μA |. The transfer output current It can be suppressed to a change of about ± 6 μA as shown in FIG. Therefore, the charge amount of the photosensitive drum 27 at the contact portion between the photosensitive drum 27 and the transfer roller 30 is rapidly increased, and the occurrence of white stripes due to the lack of toner at only the contact portion can be prevented. Thereby, a high quality image can be formed.

Further, the sheet detection sensor 80 and the timer 100 can switch the transfer setting current I supplied from the transfer output circuit 62 from the transfer current I ₁ to the transfer current I ₂ at an accurate timing.

[Modification]
The present invention has been described above based on the above embodiment, but the present invention is not limited to the above embodiment, and various improvements and modifications can be made without departing from the technical idea thereof.

  Although a high-quality image is obtained by controlling the transfer current value by the CPU 61, the same result can be obtained by controlling the transfer voltage value by the CPU 61.

The fact that the trailing edge of the sheet 3 has reached the transfer position has been determined by counting by the timer 100 for a time T ₁ after the trailing edge of the sheet 3 is detected by the sheet detection sensor 80. A method may be used in which it is determined that the trailing edge of the sheet 3 has reached the transfer position by counting for a predetermined time by the timer 100 after the leading edge of the sheet 3 is detected in 80.

Depending on the type and thickness of the paper 3, there is a difference in the transfer current change at the rear end of the paper 3 because there is a paper 3 with a high impedance and a paper 3 with a low impedance. For example, the paper 3 made of an insulating material such as OHP paper has a high impedance, and the thicker the paper 3 is, the higher the impedance is. Therefore, in the first embodiment, the transfer setting current I at the rear end of the sheet 3 is set to I ₂ = −6 μA, but this current value may be appropriately changed depending on the type of material of the sheet 3. As a method for changing the transfer setting current I, the change value of the transfer current I ₂ is stored in advance in a ROM (not shown) in the CPU 61 in accordance with the type and thickness of the paper 3, and the user can select the laser printer 1 or the like. While looking at the display of an information terminal device (not shown) connected to the laser printer 1 via a network, the type of the paper 3 can be determined using the input keys 200 as input means as schematically shown in FIG. There is a method in which the change value of the transfer setting current I corresponding to the input information is read from the ROM of the CPU 61 and set by inputting the thickness. Further, since the impedance value of the sheet 3 varies depending on the surrounding environment, the change value of the transfer setting current I ₂ may be directly input by the input key 200 and set. In this case, the setting of the current value of the transfer current I ₂ can be changed. In this way, the transfer setting current I most suitable for the type and thickness of the paper and the surrounding environment can be obtained, and a higher quality image can be formed.

The impedance of the transfer roller 30 before the paper 3 is passed through the transfer roller 30 and the impedance of the transfer roller 30 and the paper 3 when the paper is passed are calculated. Based on the calculation result, the rear end of the paper 3 is transferred. The set current I ₂ may be changed. As shown in FIG. 2, these impedances are the transfer output current It measured by the output current detection circuit 83 as the output current detection means and the transfer output measured by the output voltage detection circuit 73 as the output voltage detection means. It can be calculated by using the voltage Vt. As a result, the CPU 61 changes the set value of the transfer set current I ₂ as appropriate, so that the user can save time and effort.

  Also, the set value of the transfer set current I may be changed when the leading edge of the paper 3 reaches the transfer position. This is because, when the high impedance paper 3 is used, the constant current value output at the specified transfer output current It until the front end of the paper 3 reaches the transfer position, the front end of the paper 3 passes through the transfer position. Even in this case, the state in which the sheet 3 is interposed between the photosensitive drum 27 and the transfer roller 30 from the state where the photosensitive drum 27 and the transfer roller 30 are in direct contact with each other as shown in FIG. As such, it is to become rapidly smaller. Then, it takes time to return to the set value of the transfer set current I, and a shortage of transfer current may occur near the front end of the image forming area where the image of the paper 3 is formed, resulting in a transfer residual ghost.

  Therefore, the transfer setting current I may be controlled at the leading edge and the trailing edge of the sheet 3 as follows. FIG. 5 is a flowchart showing the control contents of the CPU 61 for suppressing a rapid change in the transfer output current It at the leading edge and the trailing edge of the sheet 3. 6A and 6B show a period from when the paper detection sensor 80 detects the leading edge of the paper 3 to after the rear edge of the paper 3 passes through the transfer position between the photosensitive drum 27 and the transfer roller 30. FIG. 6 is a graph showing changes in transfer setting current I and transfer output current It.

First, the paper 3 is fed by the paper feed roller 8 and conveyed to the registration roller 12 (S100). The sheet 3 registered by the registration roller 12 is conveyed toward the transfer position by the rotation of the registration roller 12. Then, it is determined whether or not the paper detection sensor 80 detects that the leading edge of the paper 3 has reached the sensor 80 (S101). If it is determined by the paper detection sensor 80 that the leading edge of the paper 3 has reached the sensor 80 (S101: Yes), supply of a transfer current of −14 μA to the transfer roller 30 is started by the bias application circuit 60 (S101: Yes). S102). This transfer current is supplied while the paper 3 passes through the transfer roller 30. If it is not determined by the paper detection sensor 80 that the leading edge of the paper 3 has reached the sensor 80 (S101: No), the process of S101 is performed until it is detected that the leading edge of the paper 3 has reached the sensor 80. Is repeated. During this time, a transfer current of −14 μA is supplied to the transfer roller 30. After a predetermined time has elapsed from the start of current supply to the transfer roller 30, the CPU 61 uses the transfer set current I supplied to the transfer roller 30 by the transfer output circuit 62 as the first transfer current I ₁ ′ that is the same as during the transfer. = -14 μA is set (S103). At the same time as the transfer current I ₁ ′ is set, as shown in FIG. 6A, the time when the leading edge of the paper 3 is detected by the paper detection sensor 80 is t ₀ ′, and the timer 100 starts counting for a predetermined time. (S103). The predetermined time corresponds to a time T ₁ ′ from when the leading edge of the paper 3 is detected by the paper detection sensor 80 (t = t ₀ ′) until reaching the transfer position (t = t ₁ ′). Next, it is determined by the timer 100 whether or not the time T ₁ ′ has been counted (S104). When the timer 100 determines that the time T ₁ ′ has been counted (S104: Yes), the CPU 61 determines that the leading edge of the paper 3 has reached the transfer position, and sets the transfer set current I to the transfer current. Switching from I ₁ to transfer current I ₃ (S105). At this time, the transfer setting current I is increased from −14 μA to | −8 μA | and switched to −22 μA. That is, when the leading edge of the sheet 3 reaches the transfer position, a transfer setting current I of I ₃ = −22 μA is supplied to the transfer roller 30. On the other hand, 'if it is not determined that the count has been completed (S104: No), the time T ₁ by the timer 100' time T ₁ by the timer 100 processes S104 until the count of is determined to have ended are repeated. During this time, −14 μA is continuously supplied as the transfer current I ₁ . In the process of S105, when the setting of the transfer setting current I supplied to the transfer roller 30 by the transfer output circuit 62 is switched from the transfer current I ₁ to the transfer current I ₃ (t = t ₁ ′), the timer 100 simultaneously The predetermined time count is started (S106). This predetermined time corresponds to a time T ₂ ′ from the time when the leading edge of the sheet 3 reaches the transfer position (t = t ₁ ′) until the sheet 3 advances slightly more than 40 mm (t = t ₂ ′). T ₂ ′ is set to T ₂ ′ = 90 ms. Then, it is determined by the timer 100 whether or not the time T ₂ ′ has been counted (S107). When the timer 100 determines that the time T ₂ ′ has been counted (S107: Yes), the CPU 61 determines that 90 ms has elapsed since the leading edge of the sheet 3 reached the transfer position, and the transfer setting current I The setting is returned from the transfer current I ₃ to the transfer current I ₁ again (S108). That is, the transfer setting current I is returned from I ₃ = −22 μA to I ₁ = −14 μA. Here, since the maximum value that the margin between the leading edge of the paper 3 and the leading edge of the image forming area of the paper 3 can be 40 mm, the transfer current I ₃ is from the non-image forming area where the image of the paper 3 is not formed. The image is supplied across the image forming area where the image is formed. On the other hand, 'if it is not determined that the count has been completed (S107: No), the timer 100 times T _2' time T ₂ by the timer 100 processes S107 until the count of is determined to have ended are repeated. During this period, −22 μA is continuously supplied as the transfer current I ₃ . When the transfer setting current I is reset from I ₃ = −22 μA to I ₁ = −14 μA and a predetermined time has elapsed (S108), it is determined whether or not it is detected that the trailing edge of the sheet 3 has reached the sensor 80. (S109). Henceforth, the process of S109-S117 is the same as the process of S4-S12 of the flowchart shown in FIG. If it is determined that there is a sheet 3 on which an image is to be formed next (S116: No), the process returns to the process from S100, and the control of the transfer set current I is executed again. At this time, since the transfer current is already ON, the process of turning ON the transfer current in S102 is not executed. While the transfer current I is ON, the transfer roller 30 is supplied with a transfer current I _{1 of} −14 μA.

  As described above, when the transfer set current I as shown in FIG. 6A is controlled, as shown in FIG. 9, conventionally, when the rear end of the sheet 3 passes through the transfer position, it becomes larger by about −10 μA |. The transfer output current It can be suppressed to a change of about ± 6 μA as shown in FIG. Further, the transfer output current It when the leading edge of the paper 3 passes the transfer position can be suppressed to a change of about ± 6 μA. Therefore, the charge amount of the photosensitive drum 27 at the contact portion between the photosensitive drum 27 and the transfer roller 30 is rapidly increased, and the occurrence of white stripes due to the lack of toner at only the contact portion can be prevented. Further, the charge amount of the photosensitive drum 27 at the contact portion between the photosensitive drum 27 and the transfer roller 30 is abruptly reduced, and the transfer current is insufficient near the front end of the image forming area where the image on the paper 3 is formed. It is possible to prevent the occurrence of a transfer residual ghost due to being carried on the substrate. Thereby, a high quality image can be formed.

Further, the sheet detection sensor 80 and the timer 100 cause the transfer setting current I supplied from the transfer output circuit 62 to be transferred from the transfer current I ₁ to the transfer current I ₃ at an accurate timing, and from the transfer current I ₁ to the transfer current I. Can switch to ₂ .

Further, as another modified example, in the embodiment as a modified example, FIG. 4 (a) and FIG. 6 (a), the time T _2, T ₂ 'transferring the transfer setting current I between current I ₂ and was switched to the transfer current I _3, as shown in FIG. 7 (a) and FIG. 8 (a), subdivide switching of transfer set current I between time T _2, T ₂ ', the transfer current I ₂ Alternatively, not only the transfer current I ₃ but also the set value different between the times T ₂ and T ₂ ′ may be switched to the transfer current I ₂ or the transfer current I ₃ step by step. In this way, as shown in FIGS. 7B and 8B, the fluctuation of the transfer output current It between the times T ₂ and T ₂ ′ becomes small, and can be suppressed to a change of about ± 4 μA. it can.

1 is a side sectional view of an essential part showing an embodiment of a laser printer 1. It is a block diagram of the principal part structure of a transfer bias application circuit. 6 is a flowchart showing the control content of a CPU 61 for suppressing a rapid change in transfer output current It at the trailing edge of the paper 3; (a) shows the transfer set current I from when the paper detection sensor 80 detects the rear edge of the paper 3 until after the rear edge of the paper 3 passes the transfer position between the photosensitive drum 27 and the transfer roller 30. A graph showing a change, (b), from when the paper detection sensor 80 detects the rear edge of the paper 3 to after the rear edge of the paper 3 passes the transfer position between the photosensitive drum 27 and the transfer roller 30. It is a graph which shows the change of transfer output current It. 6 is a flowchart showing the control contents of a CPU 61 for suppressing a rapid change in transfer output current It at the leading edge and trailing edge of the sheet 3; (a) shows a change in the transfer setting current I from when the paper detection sensor 80 detects the leading edge of the paper 3 to after the trailing edge of the paper 3 passes through the transfer position between the photosensitive drum 27 and the transfer roller 30. (B) shows a transfer output from when the paper detection sensor 80 detects the leading edge of the paper 3 to after the rear edge of the paper 3 passes through the transfer position between the photosensitive drum 27 and the transfer roller 30. It is a graph which shows the change of electric current It. (a) is a graph showing changes in the transfer set current I at the time of performing a plurality of times to switch the transfer set current I between time T ₂ in FIG. 4 (a), the (b), the FIGS. 7 (a) It is a graph which shows the change of the transcription | transfer output current It when doing it. FIG. 6A is a graph showing changes in the transfer setting current I when the transfer setting current I is switched a plurality of times between times T ₂ ′ and T ₂ in FIG. 6A, and FIG. It is a graph which shows the change of the transfer output current It when doing as (a). Conventional transfer setting current I and transfer output current from when the paper detection sensor 80 detects the leading edge of the paper 3 until after the trailing edge of the paper 3 passes through the transfer position between the photosensitive drum 27 and the transfer roller 30. It is a graph which shows the change with It.

Explanation of symbols

1 Laser printer (image forming device)
3 paper (recording medium)
4 Paper Feeding Section 5 Image Forming Section 12 Registration Roller 27 Photosensitive Drum (Photoconductor)
30 Transfer roller (transfer means)
52 High Voltage Power Circuit Board 60 Bias Application Circuit 61 CPU (Control Unit)
62 Transfer output circuit (supply means)
80 Paper detection circuit (recording medium detection means)
100 timer (time measuring means)

Claims (5)

A photoreceptor carrying a developer image;
Transfer means for transferring a developer image carried on the photoreceptor onto a recording medium;
Supply means for supplying a transfer current to the transfer means;
A first transfer current is supplied to the transfer unit by the supply unit, and the rear end of the recording medium passes through a transfer position where the photoconductor and the transfer unit face each other, or immediately before or after the pass. An image forming apparatus comprising: a control unit that switches a set value of the transfer current so that a second transfer current smaller than the first transfer current is supplied to the transfer unit by a supply unit. Recording medium detecting means for detecting a rear end of the recording medium;
When the recording medium detecting means detects the trailing edge or the leading edge of the recording medium, the recording medium detecting means further comprises time measuring means for starting a predetermined time counting,
The control means sets the transfer current so that the second transfer current is supplied to the transfer means instead of the first transfer current by the supply means when the predetermined time is counted by the time measuring means. The image forming apparatus according to claim 1, wherein the image forming apparatus is switched. The image forming apparatus according to claim 1, wherein the set value of the second transfer current is changed according to a type of material of the recording medium. 4. The image forming apparatus according to claim 1, further comprising an input unit capable of inputting to change a setting value of the second transfer current. 5. Output current detection means for detecting an output value of the transfer current supplied to the transfer means by the supply means;
Output voltage detection means for detecting an output value of the transfer voltage supplied to the transfer means by the supply means,
The control means includes an output value of the transfer current and the transfer voltage before the recording medium detected by the output current detection means and the output voltage detection means is sandwiched between the photosensitive member and the transfer means. An output value, an output value of the transfer current and an output value of the transfer voltage when the recording medium detected by the output current detection unit and the output voltage detection unit is sandwiched between the photosensitive member and the transfer unit. 4. The image forming apparatus according to claim 1, wherein a set value of the second transfer current is calculated and changed from

Images