JP3605961B2 - Image forming device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that forms an image by transferring a toner image to a recording medium by a contact transfer method and fixing the toner image to the recording medium.
[0002]
[Prior art]
2. Description of the Related Art A conventional electrophotographic image forming apparatus can output high-quality images at high speed and is widely used in various apparatuses such as an electrophotographic copying machine and a printer. In such an image forming apparatus, a contact charging device is used as a device for charging the surface of an image carrier such as a photoconductor drum or a photoconductor belt, and a device for transferring a toner image formed on the image carrier to a sheet is used as a device. Contact transfer devices are used respectively.
[0003]
Among them, the contact charging device is provided with a charging member such as a charging bias roll that is pressed against the surface of the image carrier, and the surface of the image carrier is charged by applying a predetermined charging bias voltage to the charging member. Do. One of the contact transfer devices is also provided with a transfer member such as a transfer bias roll that is pressed against the surface of the image carrier, and is applied to the transfer member by applying a predetermined transfer bias voltage to be attracted to the surface of the image carrier. The transferred toner image is transferred to paper.
[0004]
It is known that the polarity of a bias voltage applied between a charging member and a transfer member is different in an image forming apparatus that performs a reversal developing method, which is a mainstream of today. In the reversal development method, a toner is used which is charged to the same polarity as the image carrier (generally, the image carrier is negatively charged, so the toner is also negative). Applies a negative charging bias voltage. Therefore, in the transfer member, a transfer bias voltage having a polarity opposite to that of the image carrier (generally, positive polarity) is applied to attract the negatively charged toner on the surface of the image carrier onto the sheet.
[0005]
However, when the above-described reversal development method is performed, there are the following problems. First, the contact charging device has a drawback that the charging area is extremely narrow, so that the time required for the surface of the image carrier to pass through the charging area is extremely short, and thus the charging ability is extremely low. The charging potential of the surface of the image carrier to which the reverse polarity is applied by the contact transfer device is reduced by the inflow of the reverse polarity transfer charge, and the surface of the image carrier is charged to the originally intended charging potential even after the recharging process by the contact charging device. The case that cannot be done occurs. In such a case, since the surface potential of the image carrier drops to near the developing bias voltage, a phenomenon (transfer memory fogging) in which the image is visualized in a fogging state by the developing device occurs.
[0006]
In particular, in a region outside the paper where the surface of the image carrier is in direct contact with the transfer member, it is likely to be charged with a polarity opposite to the charge potential due to the transfer bias voltage applied to the transfer member, and the above-described transfer memory fog is likely to occur. In particular, in an image forming apparatus capable of performing double-sided printing, moisture contained in the sheet evaporates due to the heat fixing step in the image forming step on the first side, and the resistance of the sheet significantly increases. For this reason, in the second surface transfer step, it is necessary to apply a higher transfer bias voltage value to the transfer member than at the time of transfer on the first surface. Also, although the transfer charge flows through the paper to the surface of the image carrier and does not occur until the transfer memory fog, the phenomenon that the halftone dots such as halftone dots are developed to be large, and the apparent halftone dot density becomes dark More likely to occur.
[0007]
Second, as a problem when a contact charging device having insufficient charging ability is used, there is a peel discharge development that occurs when a sheet is peeled from an image carrier. The paper is charged by the contact transfer device with a polarity opposite to that of the image carrier, and when the trailing edge of the paper is peeled off from the image carrier, the charged charges on the paper are discharged to the surface of the image carrier and the image carrier is charged. Since charging of the opposite polarity occurs on the body surface, there is a problem that the history of the peeling discharge portion is developed after one round of the image carrier, and black line-like stains occur (peeling memory) as in the case of transfer memory fog.
[0008]
As a method of solving the above-mentioned problem, a voltage for monitoring is applied to the transfer member at the start of the image forming process or at the start of the transfer process to detect the amount of current flowing out to the image bearing member. Obtain current characteristic information and select and control the transfer bias voltage so as not to exceed the charging capability of the contact charging device, or output the nip area between the transfer member and the image carrier by using a static elimination member located near the downstream of the transfer member. Immediately after application, a method is proposed in which a voltage having the same polarity as the charging polarity of the image carrier surface is applied to the back surface of the paper and the outer area of the image carrier surface to reduce the charge on the paper and reduce the reverse charging of the image carrier. Have been.
[0009]
Further, the amount of outflow current when a predetermined transfer bias voltage is applied to the transfer member is detected, and it is determined whether or not the detected outflow current amount has reached a desired value. A technique for feeding back the setting is described in JP-A-4-258980.
[0010]
[Problems to be solved by the invention]
However, in the above-described method, even when the transfer is performed with the same transfer bias voltage, when transferring the rear end area of the sheet, the leading end of the sheet comes into contact with a pre-grounded fixing roll and passes through the fixing roll. Transfer charges easily escape. That is, the grounding condition during transfer varies between a region on the rear end side and a front end side of the predetermined position on the sheet, and the transfer charge decreases in the region on the rear end side. There is a problem that the binding power is weakened.
[0011]
Further, after the trailing end of the sheet leaves the nip portion of the transfer roll, the sheet trailing end area bends downward, so that the distance between the sheet and the charge eliminating member is shortened, and the charge eliminating action on the sheet trailing end area is enhanced. As a result, the charging potential in the trailing edge region of the sheet is reduced, and the electrostatic binding force between the toner image and the sheet is further reduced.
[0012]
Furthermore, the electrostatic binding force between the toner image and the paper is further reduced by strengthening the paper static elimination action by the static elimination member in order to prevent peeling discharge generated when the trailing edge area of the paper is peeled from the image carrier. Would.
[0013]
As described above, when the electrostatic binding force between the toner image and the sheet is weakened in the rear end area of the sheet, the following problems may occur.
[0014]
(1) In the rear end area of the sheet, a phenomenon (fringe stain) occurs in which the toner on the sheet after the transfer scatters around the image after passing through the transfer nip.
[0015]
(2) A phenomenon in which a toner image is disturbed by a slight impact generated at a seam of a guide member conveyed to a fixing device, and a toner image is disturbed due to a local potential change due to a flow of charge of paper into the guide member (explosion) Occurs.
[0016]
(3) Fixing where the surface of the fixing roller is triboelectrically charged, and the toner on the paper is easily transferred electrostatically to the fixing roller, and the toner image fixed on the image surface of the paper at the front circumference is transferred in the cycle of the fixing roller. A ghost phenomenon (electrostatic offset) occurs.
[0017]
(4) Particularly in a high-temperature and high-humidity environment, the resistance of the sheet becomes extremely low. Therefore, when a charge-eliminating bias is applied to the charge-eliminating means when the sheet and the image carrier are discharged, the transfer bias passes through the sheet to the charge-eliminating device. As a result, a phenomenon (transfer failure at high temperature and high humidity) occurs in which a current necessary for transfer cannot be obtained, good transfer cannot be obtained, and an image is missing.
[0018]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and a stable image quality can be obtained by correcting and uniformizing a decrease in electrostatic binding force between a sheet and a toner image in a sheet rear end area. An object of the present invention is to provide an image forming apparatus capable of exhibiting performance.
[0019]
[Means for Solving the Problems]
In order to achieve the above object, an image forming apparatus according to claim 1, wherein a latent image forming means for forming an electrostatic latent image corresponding to an image to be formed on an image carrier, and And developing means for forming a toner image on the image carrier by adsorbing the toner on the image carrier, and nipping and transporting the recording medium between the image carrier and the developing means. A transfer unit that transfers the toner image onto the recording medium by applying a voltage, and a transfer unit that is disposed on the downstream side of the transfer unit in the conveying direction of the recording medium in a state where the transfer unit is grounded. Fixing means for fixing on the recording medium,Transferred position on the recording medium when the leading end of the recording medium comes into contact with the fixing unit.When the toner image is transferred to a region on the rear end side of the image forming apparatus, a voltage (transfer bias voltage) applied to the transfer unit is increased.Makes the charged potential of the recording medium uniformTransfer voltage control means.
[0021]
Also,Claim 2Image forming apparatus describedThe placement is a latent image forming means for forming an electrostatic latent image corresponding to the image to be formed on the image carrier, and by adsorbing toner on the image carrier corresponding to the electrostatic latent image, Developing means for forming a toner image on the image carrier, and a recording medium sandwiched and conveyed between the image carrier, and a voltage is applied in the sandwiched conveyance state, so that the toner image is transferred to the recording medium. Transfer means for transferring the toner image on the recording medium to the transfer means, and a fixing means for fixing the toner image on the recording medium onto the recording medium; When the toner image is transferred from the rear end of the medium to a region on the rear end side of a position more distant to the front end side than the distance between the transfer position by the transfer unit and the charge removing unit, the voltage is applied to the transfer unit. Increase the voltage (transfer bias voltage) And transfer voltage controlling means for equalizing the charge potential of the recording medium and,The transfer unit is disposed in the vicinity of the downstream side in the transport direction of the recording medium and below the transport path of the recording medium, and a voltage having a polarity opposite to that of the transfer bias voltage is applied to remove the charge of the recording medium. Performing static elimination means, voltage applying means for applying a predetermined voltage to the static elimination means,It is characterized by having.
[0022]
Also,Claim 3In the described image forming apparatus,Claim 15. The image forming apparatus according to claim 1, further comprising: a discharging unit that is disposed near a downstream side of the transfer unit in a conveying direction of the recording medium with respect to the transfer unit and that applies a voltage having a polarity opposite to that of the transfer bias voltage to discharge the recording medium. And the recording mediumRear endImmediately before peeling off from the image carrier, further comprising: a static elimination voltage control means for increasing a voltage (static elimination bias voltage) applied to the static elimination means, wherein the transfer voltage control means increases the static elimination bias voltage. The transfer bias voltage is increased accordingly.
[0023]
Also,Claim 4In the described image forming apparatus,Claim 15. The image forming apparatus according to claim 1, further comprising: a discharging unit that is disposed near a downstream side of the transfer unit in a conveying direction of the recording medium with respect to the transfer unit and that applies a voltage having a polarity opposite to that of the transfer bias voltage to discharge the recording medium. And a neutralization voltage application unit capable of applying two or more different voltages having a polarity opposite to that of the transfer bias voltage to the neutralization unit, wherein the transfer voltage control unit includes a voltage applied to the neutralization unit. It is characterized in that the transfer bias voltage is increased to a different upper limit value according to (static charge bias voltage).
[0024]
Also,Claim 5In the above-described image forming apparatus, claim 1 is provided.Or Claim 2Wherein the transfer voltage control means increases the transfer bias voltage based on at least one of an increase rate and an upper limit of the transfer bias voltage set according to environmental conditions. I do.
[0025]
Also,Claim 6In the image forming apparatus described above, claims 1 toClaim 5In the image forming apparatus according to any one of the above, the recording medium having the toner image fixed on one surface thereof by the fixing unit is inverted by transferring the recording medium such that the other surface faces the image carrier. The image forming apparatus further includes a double-sided printing conveying unit that conveys the toner image to the transfer position, and the transfer voltage control unit increases the transfer bias voltage only when the toner image is transferred to the other surface.
[0026]
Also,Claim 7In the image forming apparatus described above, claims 1 toClaim 6In the image forming apparatus according to any one of the above, the transfer voltage control means increases the transfer bias voltage at a voltage increase rate within a predetermined upper limit value with respect to the conveyance distance of the recording medium.
[0027]
In the image forming apparatus according to the first aspect, first, an electrostatic latent image corresponding to an image to be formed is formed on an image carrier by a latent image forming unit, and toner is formed by a developing unit in accordance with the electrostatic latent image. A toner image is formed on the image carrier by causing the toner image to be adsorbed on the image carrier. On the other hand, the image forming apparatus is provided with a transfer unit, and the transfer unit sandwiches and conveys the recording medium between the image forming apparatus and the image carrier. When a voltage is applied to the transfer unit in the nipping and conveying state, the toner image formed on the image carrier is transferred to a recording medium.
[0028]
Further, in the image forming apparatus, the fixing unit is disposed in a state of being grounded downstream of the transfer unit in the transport direction of the recording medium. That is, the recording medium on which the toner image has been transferred by the transfer unit is transported in a predetermined transport direction, and reaches the fixing unit. The fixing unit fixes the unfixed toner image on the recording medium onto the recording medium.
[0029]
By the way, since the fixing means is grounded, when the recording medium comes into contact with the fixing means, transfer charges on the recording medium (charges adsorbing the recording medium and the toner image) escape through the fixing means. In the image forming apparatus according to the first aspect of the present invention, the transfer bias voltage is increased by the transfer voltage control unit when the transfer is performed to a region on the rear end side of the predetermined position on the recording medium. . As a result, even if the transfer charge on the recording medium easily escapes, it is possible to prevent a decrease in the attraction force between the recording medium and the toner image.
[0030]
That is, by performing the transfer process by increasing the transfer bias voltage, the charge that binds the toner over the entire surface of the recording medium is made uniform, so that the attraction force between the toner image and the recording medium is uniformly stabilized. Can be done. As a result, the above-described fringe contamination, explosion, electrostatic offset, and transfer failure at high temperature and high humidity can be prevented, and stable image quality performance can be exhibited.
[0031]
The predetermined position at which the transfer bias voltage starts increasingIsThe transfer position can be set at the recording medium when the leading end of the recording medium comes into contact with the fixing unit. With this setting, it is possible to prevent a decrease in the attraction force between the recording medium and the toner image from the beginning when the transfer charge on the recording medium becomes easy to escape via the fixing unit, and to prevent the attraction force over the entire surface of the recording medium. Can be uniformly stabilized.
[0032]
next,Claim 2In the image forming apparatus described above, the charge removing means for removing the charge of the recording medium is disposed near the downstream of the transfer means in the transport direction of the recording medium and below the transport path of the recording medium. By applying a voltage having a polarity opposite to that of the transfer bias voltage to the charge removing means by the voltage applying means, the charge removal of the recording medium and the relaxation of the peeling discharge when the recording medium described below is peeled from the image carrier are performed. .
[0033]
In such an image forming apparatus, when the rearmost end of the recording medium is separated from the transfer means, the vicinity of the rearmost end is a free end, and the vicinity of the rearmost end is bent downward by its own weight. At this time, the static elimination means is located at a distance between the transfer position of the transfer means and the static elimination means and at a position away from the rearmost end of the recording medium toward the front end (in FIG. 13, a distance L between the transfer position J1 and the static elimination means 41, (A position indicated by an arrow A away from the rearmost end of the recording medium 35 toward the front end side, here, position A). Since the position A of the recording medium and the static eliminator approach, the static elimination by the static eliminator is performed. The action (the action of applying a voltage having a polarity opposite to the transfer bias voltage) works stronger than before.
[0034]
For this reason, in the region on the rear end side from the position A, the transfer charge may be smaller than in the region on the front end side from the position A, and the transferability may be reduced.Claim 2In the image forming apparatus described above, the position at which the increase of the transfer bias voltage is started is set at least on the front end side of the position A. Therefore, since the transfer bias voltage applied to the transfer means at the time of transfer is increased at least in the region on the rear end side of the position A in the recording medium, even if the static elimination function works strongly as described above, the transfer charge Of the recording medium and the toner image can be prevented from lowering.
[0035]
next,Claim 3In the image forming apparatus described above, a static elimination unit for neutralizing the recording medium is disposed near the transfer unit downstream of the transfer unit in the conveying direction. The recording medium is neutralized by applying a voltage having a polarity opposite to that of the transfer bias voltage to the neutralizing unit. Furthermore, the recording mediumRear endBy increasing the discharging bias voltage immediately before the recording medium is separated from the image carrier, the separation discharge generated when the recording medium is separated from the image carrier is reduced.
[0036]
Increasing the discharging bias voltage as described above may decrease the transfer charge and decrease the attraction force between the recording medium and the toner image.Claim 3In the image forming apparatus described above, the transfer voltage control unit increases the transfer bias voltage in accordance with the increase in the charge removal bias voltage. As a result, even if the neutralization bias voltage increases, the transfer bias voltage also increases in accordance with the increase, so that it is possible to prevent a decrease in the attraction force between the recording medium and the toner image.
[0037]
next,Claim 4In the image forming apparatus described above, a static elimination unit for neutralizing the recording medium is disposed near the transfer unit downstream of the transfer unit in the conveying direction. The recording medium is neutralized by applying a voltage having a polarity opposite to that of the transfer bias voltage to the neutralizing unit. Moreover, the applied voltage (static bias voltage) to the static elimination means can be switched to two or more different voltages by the static elimination voltage applying means.
[0038]
BookClaim 4In the image forming apparatus described above, the transfer voltage control unit increases the transfer bias voltage to a different upper limit value according to the charge eliminating bias voltage even if the charge eliminating bias voltage is switched to another voltage value. Therefore, for example, even if the neutralization bias voltage is switched to a higher voltage value, the transfer bias voltage is increased to a higher upper limit value corresponding to the higher voltage value, so that the attraction force between the recording medium and the toner image is reduced. It can be prevented from lowering.
[0039]
As is clear from the transfer bias voltage-transfer current characteristics for each environmental condition such as temperature and humidity shown in FIG. 5, an optimum transfer current (2.0 μA to 4.0 μA in the example of FIG. 5) is obtained in the transfer unit. Transfer bias voltages for the high temperature and high humidity environment (H / H environment in FIG. 5), the normal temperature and humidity environment (N / N environment in FIG. 5), and the low temperature and low humidity environment (L / L environment in FIG. 5) Different. Therefore, it is desirable that the control for increasing the transfer bias voltage is also performed according to the environmental conditions. In particular,Claim 5As in the invention described above, it is desirable that the transfer voltage control means increases the transfer bias voltage based on at least one of an increase rate and an upper limit value of the transfer bias voltage set according to environmental conditions.
[0040]
next,Claim 6In the image forming apparatus described above, the recording medium on which the toner image is fixed on one surface (hereinafter, referred to as a first surface) by a double-sided printing conveying means is transferred to the other surface (hereinafter, referred to as a second surface). Can be conveyed to the transfer position after being inverted so as to face the image carrier. As a result, the toner image can be transferred to the second surface of the recording medium, and thereafter, the toner image can be fixed by the fixing unit to form an image on the second surface of the recording medium. That is, double-sided printing can be performed.
[0041]
As described above, when the image formation on the first side of the recording medium is completed in the double-sided printing, the moisture content of the recording medium is lower than before the start of the processing on the first side, and the resistance value of the recording medium is reduced. Is higher. Therefore, the transfer bias voltage at the time of transfer to the second surface needs to be set higher than the transfer bias voltage at the time of transfer to the first surface.
[0042]
In other words, a higher transfer bias voltage is required during the transfer to the second surface than during the transfer to the first surface.But,I mentioned earlierWhen transferring to the first side,Leakage of transfer charge from the fixing means, excessive static elimination due to the proximity of the recording medium to the static elimination means, and the like, can lead to a reduction in the attraction force between the recording medium and the toner image.
[0043]
Therefore, when performing double-sided printing, of course, it is most preferable to perform control to increase the transfer bias voltage not only at the time of transfer to the second surface, but also at the time of transfer to the first surface.Claim 6As in the invention described above, only when the toner image is transferred to the other surface (second surface), the transfer bias voltage may be controlled by the transfer voltage control means so as to increase the image quality of the formed image. Very effective to keep good.
[0044]
By the way, the transfer voltage control meansClaim 7As in the invention described above, it is desirable to increase the transfer bias voltage at a voltage increase rate within a predetermined upper limit value with respect to the conveyance distance of the recording medium. As an example, in a normal temperature and humidity environment of a temperature of 20 ° C. and a humidity of about 50% RH, a transfer current obtained from an experimental result when the transfer bias voltage is +1200 volts (hereinafter, unit volt is abbreviated to V) From the characteristic graph (FIG. 8), it is desirable to increase the transfer bias voltage at a transfer bias voltage increase rate of 5% or less per 1 mm in the conveyance direction of the recording medium.
[0045]
By regulating the rate of increase of the transfer bias voltage in this way, the transfer bias voltage rises sharply and a part of the transfer current flows into the surface of the image carrier, and before and after the control for increasing the transfer bias voltage is started. It is possible to prevent a difference in surface potential of the image carrier from occurring at the position (2) and a change in image density at the time of the next image formation.
[0046]
BEST MODE FOR CARRYING OUT THE INVENTION
[First Embodiment]
Hereinafter, the first embodiment according to the present invention will be described in detail.
[0047]
[Overall Configuration of Image Forming Apparatus]
FIG. 1 shows a schematic configuration of a laser beam printer 11 as an example of the image forming apparatus of the present invention. As shown in FIG. 1, the laser beam printer 11 is provided with a laser scanning device 13 for irradiating a laser beam to a photosensitive drum 19 as an image carrier described later. Is provided with a semiconductor laser (not shown). The semiconductor laser is modulated based on print data sent from an information processing device (not shown) directly connected to the laser beam printer 11 or an information processing device connected via a network such as a LAN (local area network). And output the laser beam. The outputted laser beam is deflected by the polygon mirror 15 rotating in a predetermined direction, and enters the fθ lens 16. The laser beam emitted from the fθ lens 16 is sequentially reflected by the first mirror 17 and the second mirror 18 and irradiates an exposure position 22 on a surface of a photosensitive drum 19 (hereinafter, referred to as a drum surface) described later. Is done. The optical path of the laser beam is indicated by a dotted line 21.
[0048]
The cylindrical photosensitive drum 19 is rotated by a main motor (not shown) at a constant angular velocity in a predetermined rotation direction indicated by an arrow P, and the laser beam is deflected by the rotating polygon mirror 15. Scanning is repeatedly performed in 19 axial directions (main scanning direction). The laser beam scans the drum surface at a constant speed by the action of the fθ lens 16. Slightly upstream of the exposure position 22 along the drum rotation direction indicated by the arrow P, a charging roll 23 is in contact with the drum surface so as to uniformly charge the drum surface. The photoconductor drum 19 of the present embodiment uses an organic photoconductor, and has a characteristic that should be negatively charged in order to form an electrostatic latent image. A negative voltage with a bias voltage superimposed is applied.
[0049]
At the exposure position 22, an electrostatic latent image corresponding to the print data is formed on the drum surface uniformly charged negatively. The electrostatic latent image is developed by a cartridge type developing device 25 disposed downstream of the exposure position 22 along the drum rotation direction indicated by the arrow P.
[0050]
In the developing device 25, a developing roll 26 and a toner supply mechanism 27 are arranged. The toner supply mechanism 27 is a mechanism for sequentially supplying the toner in the developing position 25 to the developing roll 26. The developing roll 26 magnetically raises the toner to form an electrostatic latent image on the drum surface. A roll for forming a toner image corresponding to the electrostatic latent image by approaching or contacting the area. A negative developing bias voltage in which a negative DC bias voltage is superimposed on an AC voltage is applied to the developing device 25.
[0051]
In the present embodiment, an electrostatic latent image is formed on the negatively charged drum surface, and the potential of the formation area of the electrostatic latent image is close to 0 volt. On the other hand, since a negative developing bias voltage is applied to the developing device 25, the potential difference between the negatively charged toner of the developing device 25 and the drum surface is small in a region where the electrostatic latent image is not formed, whereas It becomes large in the area where the electrostatic latent image is formed. For this reason, the toner is attracted only to the formation area of the electrostatic latent image.
[0052]
The toner image formed on the drum surface by the developing device 25 moves to a position where the transfer bias roll 29 is in rolling contact with the drum surface by the rotation of the photosensitive drum 19. Here, the transfer of the toner image to the paper 35 sent via the registration roll 31 is performed. In FIG. 1, the conveyance path of the paper 35 is indicated by a dotted line 32.
[0053]
The laser beam printer 11 includes a power supply unit 102 for supplying power to each device in the laser beam printer 11 and a CPU 101A as shown in FIG. And a control device 101 for controlling the control.
[0054]
Next, the paper transport path of the laser beam printer 11 will be described. At the bottom of the apparatus main body 12, a cassette tray 33 is detachably arranged. Paper 35 cut into a predetermined size is stacked on the cassette tray 33. The stacked papers 35 are sent out one by one to the right in FIG. 1 by intermittent rotation of the half-moon rolls 36. In addition to the half-moon roll 36, another roll such as a retard roll or a mechanism for sucking and feeding the paper 35 may be used.
[0055]
The paper 35 sent out from the cassette tray 33 is transported to the registration roll 31 via transport rolls 38 and 39 in order, and once stopped there, in synchronization with the rotation of the photosensitive drum 19, leftward in FIG. Transported to A predetermined positive DC bias voltage is applied to the transfer bias roll 29 only when the paper 35 passes between the photosensitive drum 19 and the transfer bias roll 29. As a result, a toner image composed of negatively charged toner particles on the photosensitive drum 19 is electrostatically attracted to the transfer bias roll 29, and the toner image is transferred to the paper 35.
[0056]
The paper 35 on which the toner image has been transferred is discharged from behind (from the side opposite to the side where the toner image has been transferred) by a discharging needle 41 arranged adjacent to the exit side of the transfer bias roll 29. The discharged paper 35 is separated from the drum surface, advances leftward in FIG. 1 along the transport path 32, and advances to a position where the heat roll 42 and the pressure roll 43 constituting the fixing device 40 are in rolling contact with each other. The heat roll 42 is maintained at a constant high temperature, and the pressure roll 43 presses the paper 35 against the heat roll 42, so that efficient heat transfer is performed in a nip area between the two, and the toner image is transferred onto the paper 35. Is established. The paper 35 after the fixing passes through the transport roll 47, and is transported upward along the transport path 32 with the switching piece 44 facing upward and left in FIG. Then, the sheet is discharged to a discharge tray 48 provided at an upper portion of the apparatus main body 12 through a transport roll 45 and a discharge roll 46 in order. This is the transport path when printing is performed on only one side of the paper 35.
[0057]
When printing is performed on both sides of the paper 35, the paper 35 on which the toner image has been transferred and fixed on the first surface is once transported upward through the transport path 32 from the switching piece 44. Then, the conveyance is temporarily stopped at a position where the rear end of the paper 35 is removed from the switching piece 44 via the conveyance roll 45. In this state, the switching piece 44 switches to the position shown by the outline 44A, and the transport roll 45 rotates in the reverse direction to transport the paper 35 downward this time. As a result, the paper 35 travels along the transport path 51 shown by the dashed line and sequentially passes through the transport rolls 52, 53, and 54 to reach the transport roll 38 used for transfer to the first surface. At this time, the first surface is facing upward.
[0058]
The paper 35 is transported again along the transport path 32 and passes between the photosensitive drum 19 and the transfer bias roll 29. At this time, the toner image is transferred to the second surface of the paper 35. Thereafter, the paper is conveyed in the same manner as when printing is performed only on one side as described above, and the paper 35 on which the second fixing is performed is discharged to the discharge tray 48.
[0059]
On the other hand, the toner remaining on the surface of the photosensitive drum 19 even after the transfer of the toner image by the transfer bias roll 29 is scraped off from the drum surface by a cleaning device 56 disposed between the transfer bias roll 29 and the charging roll 23. To be removed. The photosensitive drum 19 whose surface has been cleaned in this way is uniformly charged again by the charging roll 23, and is ready for the next image forming process.
[0060]
[Configuration of main part]
Next, the transfer bias roller 29, the static elimination needle 41, the conveyance guides 71 and 72, and the fixing device 40, which are main components of the present invention, will be described in detail with reference to FIG.
[0061]
As shown in FIG. 2, a static elimination needle 41 as a static elimination means of the present invention is provided downstream of the transfer bias roll 29 in the sheet transport direction. After a positive charge is applied to the paper 35 by the transfer bias roll 29, the paper 35 is limited by the charge eliminating needle 41 so that the positive charge is reduced. This is because the charging potential by the transfer bias roll 29 greatly differs depending on the type of the paper 35. That is, when a high-resistance resin film for OHP or a thick paper having high resistance is used as the paper 35, the surface thereof may be charged to +5 kV or more. When the paper 35 comes into contact with or approaches a conductive member such as the fixing device case 81 during such high charging, the charged charges leak and the unfixed toner image on the paper 35 is disturbed. The paper 35 rises from the first conveyance guide 71 or the second conveyance guide 72 disposed on the sheet and is jammed without the tip of the paper 35 being conveyed to the transfer portion (fixing nip J2) between the heat roll 42 and the pressure roll 43. May cause it. On the other hand, if the charging potential of the paper 35 is low, the attraction force between the paper 35 and the unfixed toner is reduced, so that the unfixed toner is electrostatically attracted to the surface of the heat roll 42 at the fixing nip J2 and is transferred. As a result, an electrostatic offset phenomenon that causes unfixed toner to be fixed on the surface of the paper 35 occurs. Therefore, a configuration is adopted in which a constant charge-elimination bias voltage is applied to the charge-eliminating needle 41 to keep the charged potential at about +2 kV regardless of the type of the paper 35.
[0062]
On the other hand, the heat roll 42 is grounded via a diode 85. Therefore, when the paper 35 is conveyed to the fixing nip J2 and the leading end of the paper 35 comes into contact with the heat roll 42, the charged charges of the paper 35 tend to leak. That is, the grounding condition of the paper 35 during the transfer is changed from the time when the leading end of the paper 35 comes into contact with the heat roll 42, and the state is changed to a state in which the charged electric charge easily leaks.
[0063]
Therefore, in the first embodiment, an example is shown in which the control device 101 (see FIG. 1) controls the transfer bias voltage to increase from the time when the leading end of the conveyed paper 35 comes into contact with the heat roll 42.
[0064]
However, if the transfer bias voltage is suddenly changed during the transfer process, an abrupt electric field change is caused in a transfer area where the transfer is not affected, and if the toner is transferred from the photosensitive belt drum 19 to the paper 35 once, the toner is again transferred to the photosensitive drum 19. This causes a problem that retransfer occurs, or that the transfer efficiency of the image toner on the photosensitive drum 19 changes abruptly, resulting in a difference in image density. Therefore, in the first embodiment, control is performed so that the rate of increase of the transfer bias voltage is within 5% per 1 mm of sheet feed.
[0065]
As shown in FIG. 2, in order to detect that the leading end of the paper 35 has been conveyed to the fixing nip J2, a transmission optical sensor 105 including a pair of light-emitting elements and a light-receiving element is attached to the fixing nip J2. The transmission type optical sensor 105 is provided near the upstream side in the sheet conveyance direction, and a detection signal is transmitted to the control device 101 (see FIG. 1). Similarly, a transmissive optical system for detecting that the leading end of the paper 35 has approached the transfer nip J1 is located near the upstream of the contact portion (transfer nip J1) between the photosensitive drum 19 and the transfer bias roll 29. A sensor 103 is provided, and a transmission optical sensor 104 for detecting that the rear end of the paper 35 has passed the transfer nip J1 is provided near the downstream of the transfer nip J1.
[0066]
[Configuration of Transfer Bias Roll and Transfer Bias Voltage]
In the present embodiment, the roll portion of the transfer bias roll 29 that comes into contact with the photosensitive drum 19 is a rubber material such as urethane, EPDM (polyisoprene blend elastomer), CR (chloroprene rubber), silicone rubber, or a foamed sponge made of these materials. It is composed of The hardness of the roll portion is around 40 degrees under a 500 gram (hereinafter abbreviated as g) load of ASKER C as JIS standard. This hardness is effective not only for stabilizing the transporting performance of the paper 35 but also for preventing the hollowing of characters at the time of transfer of the toner image and preventing the photosensitive drum 19 from being scraped. The volume resistance value of the transfer bias roll 29 is 10⁷-10¹⁰Set to about.
[0067]
As a method of measuring the volume resistance value of the transfer bias roll, as shown in FIG. 3, the transfer bias roll 29 to be measured is placed on a sheet metal 61 as a conductor, As described above, a force of 500 g load is applied downward. In this state, a DC voltage of 1000 V is applied to one end of the rotating shaft 62. Then, the current output to the sheet metal 61 at this time is measured by the ammeter 64, and the volume resistance value of the transfer bias roll 29 is calculated from the voltage and the measured current.
[0068]
In order to obtain good transferability regardless of whether the paper 35 has high resistance or low resistance, the resistance value of the transfer bias roll 29 must be 10⁷-10¹⁰It is said that a resistance of the order of magnitude is preferred. When the resistance value of the transfer bias roll 29 is within the above range, the voltage applied to the transfer bias roll 29 (hereinafter, referred to as a transfer bias voltage) is optimally in a range of about 400 V to 5 kV. The high-resistance paper is, for example, a high-resistance resin film for an OHP (overhead projector), paper left in a low-temperature and low-humidity environment, or after transfer of a first-side toner image when performing double-sided printing. Paper or the like that has undergone thermal fixing and has a low water content and a high resistance is applicable.
[0069]
The transfer of the charge from the transfer bias roll 29 to the paper 35 when transferring the toner image formed on the photosensitive drum 19 to the paper 35 is performed in a region where the transfer bias roll 29 is nipped. In most cases, the paper 35 mostly moves in a region slightly separated on both sides near the nip. That is, the charge is mainly moved by the discharge due to the dielectric breakdown of air according to the Paschen's rule.
[0070]
[Setting of transfer bias voltage according to environment]
As described above, the optimum ranges of the volume resistance and the transfer bias voltage of the transfer bias roll 29 exist because the volume resistance of the transfer bias roll changes as shown in FIG. 4 depending on the use environment of the image forming apparatus. That's because It should be noted that the L / L environment abbreviated in FIG. 4 is a low-temperature and low-humidity environment, a temperature and humidity environment of about 10 ° C./15% RH, and the N / N environment is a normal environment of about 20 ° C./50% RH. The H / H environment is a high-temperature and high-humidity environment, which is a temperature / humidity environment of about 28 ° C./85% RH. The names of N environment and H / H environment are used.
[0071]
Since the resistance of the paper 35 also changes due to the change in the water content due to the environmental change described above, the conductive material contained in the transfer bias roll is adjusted so that the volume resistance of the transfer bias roll 29 also changes in the same manner as the paper 35. I have. By doing so, the relative resistance difference between the transfer bias roll 29 and the paper 35 in each environment is kept substantially constant, and the above-mentioned overcurrent outflow of the transfer bias current to the area outside the paper is suppressed.
FIG. 5 shows the results of confirming several types of applied transfer bias voltages, transfer currents flowing out at that time, and the presence or absence of image defects on paper when printing was performed using the same image forming apparatus under each environment. Is shown. As is apparent from FIG. 5, since the volume resistance of the transfer bias roll 29 and the paper 35 changes depending on the environment, the transfer current flowing out for each environment differs even if the same transfer bias voltage value is applied. . Further, it can be seen that the transfer performance depends on the transfer charge amount, that is, the transfer current, and there is an optimum current range. As described above, a transfer memory occurs when an excessive current flows into the photoconductor, and a transfer failure occurs when the current is insufficient. However, the amount of electric current flowing per unit area of the sheet varies depending on the sheet feeding speed of the image forming apparatus. Therefore, the illustrated optimum transfer area differs depending on the sheet feeding speed. FIG. 5 shows the result when the paper feeding speed of the image forming apparatus main body is set to 60 mm / sec.
[0072]
FIG. 6 shows a transfer current flowing out of the transfer bias roll 29 and an image defect on a sheet when a transfer bias voltage of +1200 V is applied to a sheet of various sheet basis weights in a normal environment (N / N) to form an image. The result of confirming the presence or absence of is shown. As is apparent from FIG. 6, it is understood that stable transfer performance cannot be obtained for all paper basis weights with the same transfer bias voltage. In the present invention, a transfer bias roll in which conductive particles such as carbon are mixed with urethane rubber and the volume resistance in an N / N environment is adjusted to about 1 × 10E8 cmΩ is used, and the paper feeding speed is 60 mm / sec.
[0073]
As is clear from FIGS. 5 and 6, in order to obtain good transfer performance in any environment, an appropriate amount of transfer charge, that is, a transfer current, exists depending on the paper feeding speed. For this reason, a constant current circuit for compensating a constant current amount during transfer has been used as a conventional transfer bias application power supply. However, as a drawback when the constant current circuit is used, the sheet resistance decreases in a high-temperature and high-humidity (H / H) environment, and the transfer current flows out through contact with a sheet conveyance guide or a fixing device after transfer or through the ground of the nip member. As a result, a discharge due to dielectric breakdown does not occur in a minute gap in the transfer nip region, and transfer failure occurs. Also, when a high-resistance recording medium such as OHP paper is used, the transfer current becomes difficult to flow, so that the transfer bias voltage rises significantly, resulting in extremely large damage to the photosensitive drum photosensitive layer in the area outside the paper. I had to give it. The transfer bias power supply of the image forming apparatus using the transfer bias roll has a transfer current monitor circuit in order to solve such a problem at the time of using the constant current circuit, and a necessary transfer charge amount is determined based on information from the monitor circuit. A current compensation type constant voltage circuit that determines a supplyable voltage value and applies a constant transfer bias voltage during transfer is being used. With this configuration, it is possible to prevent the occurrence of transfer failure due to the decrease in the transfer application voltage in the H / H environment described above and to prevent the application of a transfer bias voltage higher than necessary during OHP paper printing.
[0074]
[Configuration of transfer bias voltage application circuit and charge removal bias voltage application circuit]
FIG. 7 shows a configuration diagram of the transfer bias voltage application circuit 180 and the charge removal bias voltage application circuit 160. As shown in FIG. 7, the transfer bias voltage application circuit 180 is provided with a drive current control circuit 184 and an output voltage control circuit 186, and the operation of these circuits is controlled by a transfer voltage output on / off circuit 182. The operation of the transfer voltage output on / off circuit 182 is controlled by a control signal from the CPU 101A of the control device 101. The transfer bias voltage is output from the transfer voltage output terminal 190 after being transformed by the transformer 188 in the transfer bias voltage application circuit 180.
[0075]
Further, the transfer bias voltage application circuit 180 is provided with an output current detection circuit 192 for detecting a transfer current output from the transfer bias roll 29 to the photosensitive drum 19. The information of the transfer current detected by the output current detection circuit 192 is notified to the CPU 101A, and an appropriate transfer bias voltage is set by the CPU 101A and fed back to the control of the drive current control circuit 184 and the output voltage control circuit 186.
[0076]
On the other hand, the charge eliminating bias voltage applying circuit 160 is also provided with a drive current control circuit 164 and an output voltage control circuit 166, and the operation of these is controlled by the charge eliminating voltage output on / off circuit 162. The operation of the static elimination voltage output on / off circuit 162 is controlled by a control signal from the CPU 101A of the control device 101. The static elimination bias voltage is output from the static elimination voltage output terminal 170 after being transformed by the transformer 168 in the static elimination bias voltage application circuit 160.
[0077]
[Operation of First Embodiment]
Hereinafter, the operation of the first embodiment will be described. In the following, after the paper 35 is separated from the photosensitive drum 19, the discharge needle 41 and the paper 35 come close to each other, and after the paper 35 due to the strengthening of the discharging operation by the discharge needle 41 to prevent peel discharge. The charging potential at the trailing edge of the paper due to a change in the grounding condition of the paper during transfer due to the leading edge of the paper 35 coming into contact with the fixing device 40 on the assumption that the decrease in the charging potential at the edge is very small. An example of preventing the drop will be described.
[0078]
When the control device 101 receives a print processing request from an external information processing device, the CPU 101A starts execution of the control routine in FIG. In step 202 of FIG. 9, the subroutine of the transfer bias voltage setting process (see FIG. 10) is executed as follows. In step 252 of FIG. 10, a drive-on signal of the main drive for driving the photosensitive drum 19 is sent to rotate the photosensitive drum 19. In the next step 254, a high voltage application ON signal is sent from the power supply 102 to the charging roller 23, and the drum surface is charged with the charging potential V._H(−400 V).
[0079]
Then, after the uniformly charged surface of the photosensitive drum 19 reaches the transfer nip J1, a monitor voltage of +1000 V is applied to the transfer bias roll 29 in step 256, and in the next step 258, the current value flowing from the transfer bias roll 29 ( Hereinafter, the outflow current value I (t) will be detected.
[0080]
In the next step 260, it is determined whether or not the outflow current value I (t) is 4 μA (hereinafter abbreviated as μA) or more. If the outflow current value I (t) is 4 μA or more, step 264 is performed. The program proceeds to step 266, where it is determined that the environment is the H / H environment. In the next step 266, the transfer bias voltage for the H / H environment + 400 V is set as the transfer bias voltage.
[0081]
On the other hand, if the outflow current value I (t) is less than 4 μA in step 260, the process proceeds to step 262, and it is determined whether the outflow current value I (t) is 2 μA or less. If the outflow current value I (t) is 2 μA or less, the process proceeds to step 268, where it is determined that the environment is the L / L environment, and in the next step 270, the transfer bias voltage for the L / L environment + 2400V is set as the transfer bias voltage. Set.
[0082]
On the other hand, if a negative determination is made in step 262, it can be determined that the outflow current value I (t) is greater than 2 μA and less than 4 μA, so the process proceeds to step 272, where it is determined that the environment is an N / N environment. Then, in the next step 274, the transfer bias voltage for the N / N environment + 1200V is set as the transfer bias voltage.
[0083]
As described above, the environment is determined based on the magnitude of the outflow current value I (t), an appropriate transfer bias voltage is set according to the determined environment, and the process returns to the main routine of FIG.
[0084]
On the other hand, after the above-described environment monitoring process is completed, after the uniformly charged surface of the photosensitive drum 19 reaches the laser writing position 22, a drive signal of a semiconductor laser (not shown) based on image information is sent from the CPU 101 to the photosensitive drum. A latent image is formed on the drum 19. Next, the latent image portion on the photosensitive drum 19 is developed by the developing roll 26 into a visible image with toner. The recording paper is transported along the recording paper transport path 32 in synchronization with the transport of the toner image to the rolling contact portion between the transfer bias roller 29 and the photosensitive drum 19.
[0085]
In step 204 following FIG. 9, it is determined whether or not the leading edge of the sheet has reached just before the transfer nip J1 based on the detection signal from the transmission optical sensor 103. When the leading end of the paper 35 reaches just before the transfer nip J1, the process proceeds to step 206, and the application of the transfer bias voltage is started.
[0086]
In the next step 212, it is determined whether or not the leading end of the paper 35 has reached the fixing nip J2 based on the detection signal from the transmission optical sensor 105. When the leading end of the paper 35 reaches the fixing nip J2, the process proceeds to step 214, and the voltage rise control is started. Specifically, the transfer bias voltage value is increased at an increasing rate of +60 V per 1 mm of the paper transport distance from +1200 V applied to the paper area up to that time. The increase rate of +60 V / mm is set as 5% with respect to +1200 V. Since the paper feed speed is 60 mm / sec, the transfer bias voltage increases at an increase rate of +3.6 kV / sec.
[0087]
In the next step 216, it is monitored whether or not the outflow current value I (t) has reached the upper limit value (4.0 μA in FIG. 5) of the optimum transfer current value. When the upper limit value is reached, the process proceeds to step 218, where the voltage rise control is stopped, and the application of the transfer bias voltage (upper limit transfer bias voltage) when the outflow current value I (t) is the upper limit value of the optimum transfer current value is maintained. Switch to Then, in the next step 220, the application of the upper limit transfer bias voltage is continued until it is determined from the detection signal from the transmission type optical sensor 104 that the rear end of the paper 35 has passed the transfer nip J1.
[0088]
According to the above-described first embodiment, even when the state in which the charge of the paper 35 easily leaks from the time when the leading end of the paper 35 contacts the heat roll 42, the transfer bias voltage is increased from that time. Since the control is performed, the charging potential of the paper 35 can be uniformly stabilized from the leading edge to the trailing edge of the paper. As a result, the above-described fringe contamination, explosion, electrostatic offset, and transfer failure at high temperature and high humidity can be prevented, and stable image quality performance can be exhibited.
[0089]
[Second embodiment]
Hereinafter, a second embodiment according to the present invention will be described. This second embodiment isClaims 2, 3, and 4In accordance with the invention described in the above, after the paper 35 is separated from the photosensitive drum 19, the static elimination needle 41 and the paper 35 come close to each other, and the static elimination action by the static elimination needle 41 is enhanced to prevent the separation discharge. An example is shown in which the transfer bias voltage is controlled to increase at the rear end of the paper 35 in order to cope with the decrease in the charged potential at the rear end of the paper 35. Also, an example is shown in which, in double-sided printing, the transfer bias voltage is increased to a different upper limit value corresponding to a different static elimination bias voltage between the first surface and the second surface.
[0090]
In the laser beam printer 11 according to the second embodiment, as shown in FIG. 13, a transmission optical sensor 106 is installed at a distance L from the transfer nip J1 on the upstream side in the paper transport direction. The control device 101 detects that the rear end of the paper 35 has passed the position on the upstream side by the distance L from the transfer nip J1 by using the transmission type optical sensor 106, and thereby the transfer nip J1 and the charge elimination from the rear end of the paper 35 are performed. It is detected that a position (arrow A) separated by a distance L from the needle 41 has reached the transfer nip J1, and control is performed so that the transfer bias voltage is increased from the detection point.
[0091]
Further, the control device 101 detects that the rear end of the paper 35 has peeled off from the photosensitive drum 19 by the transmission type optical sensor 104, and at the time of the detection, removes the bias voltage applied to the neutralization needle 41 to prevent the peeling discharge. Switch to a higher voltage value. In the second embodiment, in order to prevent the charging potential from being lowered only near the rear end of the paper 35 by switching the charge elimination bias voltage value to a higher voltage value, the transfer of the charging potential is performed near the rear end of the paper 35. The voltage increase rate or the upper limit value related to the transfer bias voltage increase control is set so that the transfer bias voltage that can compensate for the decrease in the transfer bias voltage is applied. That is, the transfer bias voltage may be switched to a higher voltage value along with the switching of the charge removal bias voltage value, but problems such as a transfer current flowing into the photosensitive drum 19 due to a sudden change in the transfer bias voltage are avoided. ing.
[0092]
Hereinafter, the operation of the second embodiment will be described with reference to FIGS. In the control routines shown in FIGS. 11 and 12, the same processes as those in the above-described control routines shown in FIGS.
[0093]
When the control device 101 receives a print processing request from an external information processing device, the CPU 101 starts execution of the control routine in FIG. In step 203 in FIG. 11, a subroutine for setting various bias voltages (see FIG. 12) is executed as follows. In steps 252A to 258A in FIG. 12, the photosensitive drum 19 is rotated to charge the drum surface to the charged potential V, as in steps 252 to 258 in FIG. 10 in the first embodiment._HIn the state of being uniformly charged to (−400 V), a monitor voltage of +1000 V is applied to the transfer bias roll 29, and the outflow current value I (t) from the transfer bias roll 29 is detected.
[0094]
According to the determination of the outflow current value I (t) in the next steps 260A and 262A, when the outflow current value I (t) is 4 μA or more, when the outflow current value I (t) is 2 μA or less, and when the outflow current value I (t) is 2 μA or less. When (t) is larger than 2 μA and smaller than 4 μA, it is classified into a total of three types.
[0095]
Here, when the outflow current value I (t) is larger than 2 μA and smaller than 4 μA (when a negative determination is made in steps 260A and 262A), the process proceeds to step 272A, where it is determined that the environment is an N / N environment, and the next In step 273, for example, the transfer bias voltages Vt1 and Vt2 for the N / N environment are set as follows.
[0096]
For example, in an N / N environment, the transfer current value obtained by applying the +1000 V monitor voltage to the transfer bias roll 29 is 2.5 μA from the graph of FIG. 5, and the CPU 101 determines that the applied voltage value (Vm = + 1000 V) The outflow current value (Im = 2.5 μA) is substituted into a pre-programmed equation to determine a bias voltage value to be applied during the transfer process. In the present invention, in order to obtain a transfer current of about 3.0 μA during transfer from FIG. 5, the operating environment of the laser beam printer 11 is changed according to the result obtained from the proportional expression (V = 1000 × 3 / 2.5 = 1200 V). It is determined that the environment is normal temperature and normal humidity, and transfer application voltage values Vt1 = + 1200V and Vt2 = + 1800V to be applied to the transfer bias roll 29 on the first surface and the second surface thereof during the transfer process are determined. Here, the transfer bias voltage value at the time of the second surface transfer is set to 1.5 times the first surface transfer bias voltage value in consideration of the fact that the paper 35 has increased the resistance through the first surface fixing step.
[0097]
In FIG. 12, in the next step 275, based on the result obtained in the step 273, a period from when the paper 35 reaches the position of the static elimination needle 41 at the time of the first surface transfer until the rear end of the paper passes through the transfer nip J1. The bias voltage Vd1 = −100 V applied to the static elimination needle 41 used for static elimination of the paper 35, and the static elimination needle 41 for removing the history of the area outside the transfer material of the photosensitive drum 19 immediately after the rear end of the paper passes the transfer nip J1. Bias voltage Vd2 = −3000 V applied to the second surface transfer step, and the bias voltage Vd3 = −1000 V for the second surface paper neutralization applied to the neutralization needle 41 for the same reason in the second surface transfer step, the transfer material outside the second surface transfer The bias voltage Vd4 for region charge elimination is set to -3000V.
[0098]
The examples of setting the various bias voltages for the N / N environment have been described above. However, according to the determinations at steps 260A and 262A, the outflow current value I (t) is 4 μA or more, or the outflow current value I (t) is 2 μA. In the following cases as well, the transfer application voltage values Vt1 and Vt2 are set in steps 265 and 269 in the same manner as described above, and in steps 267 and 271 the bias voltages Vd1 and Vd3 for removing electricity and the bias voltage values Vd2 and Vd4 for removing history are set. Set.
[0099]
As described above, the environment is determined based on the magnitude of the outflow current value I (t), an appropriate transfer bias voltage is set according to the determined environment, and the process returns to the main routine of FIG.
[0100]
On the other hand, after the above-described environment monitoring process is completed, after the uniformly charged surface of the photosensitive drum 19 reaches the laser writing position 22, a drive signal of a semiconductor laser (not shown) based on image information is sent from the CPU 101 to the photosensitive drum. A latent image is formed on the drum 19. Next, the latent image portion on the photosensitive drum 19 is developed by the developing roll 26 into a visible image with toner. The recording paper is transported along the recording paper transport path 32 in synchronization with the transport of the toner image to the rolling contact portion between the transfer bias roller 29 and the photosensitive drum 19.
[0101]
In the main routine of FIG. 11, the application of the transfer bias voltage is started when the leading edge of the sheet reaches just before the transfer nip J1 (step 206A), and in the next step 208, it is determined whether or not the leading edge of the sheet has reached the neutralization position. The determination is made based on the detection signal from the transmission optical sensor 104. When the leading end of the sheet reaches the neutralization position, application of the neutralization bias voltage value Vd1 to the neutralization needle 41 is started in step 210.
[0102]
In the next step 213, it is determined whether or not a position separated from the rear end by the distance L between the transfer nip J1 and the charge removal needle 41 has reached the transfer nip J1 based on the detection signal from the transmission optical sensor 106. When the position distant from the rear end by the distance L reaches the transfer nip J1, the process proceeds to step 214A, and the voltage rise control is started.
[0103]
For example, in the image forming apparatus 11 of the present embodiment, since the distance L is 20 mm, the transfer bias voltage Vt1 (+1200 V) is increased at the timing when the position on the leading end side of 20 mm from the rear end of the paper 35 enters the transfer nip J1. At a rate of 4% per mm feed, ie, +48 V / mm. However, the upper limit is 1.3 times (+1560 V) the initial transfer bias voltage Vt1.
[0104]
Then, in the next step 216A, it is monitored whether or not the outflow current value I (t) has reached the upper limit of the optimum transfer current value (4.0 μA in FIG. 5). When the current value reaches the upper limit value, the voltage rise control is stopped, and switching is performed so as to maintain the application of the transfer bias voltage (upper limit transfer bias voltage) when the outflow current value I (t) is the upper limit value of the optimum transfer current value. (Step 218A) Thereafter, control is performed so that the voltage is applied until the rearmost end of the paper 35 passes through the nip of the transfer bias roll 29 (until an affirmative determination is made in Step 220A) while maintaining the upper limit transfer bias voltage value.
[0105]
When the last end of the paper 35 has passed through the nip of the transfer bias roll 29, the process proceeds to step 222, where the bias voltage Vd1 for static elimination is switched to the bias voltage Vd2 for history elimination (-3000V), and the photosensitive member 19 passing therethrough Removes positive charge history on the surface.
[0106]
The transfer to the first surface of the sheet 35 is completed as described above. Thereafter, the recording paper carrying the toner image is separated from the photosensitive drum 19 and is conveyed and fixed to a portion of the fixing device 40. On the other hand, the residual toner that has not been transferred onto the surface of the photosensitive drum 19 is removed by the cleaning device, and the surface of the photosensitive drum 19 is ready for the next image forming step.
[0107]
After the image formation on the first surface is completed, the process proceeds to step 226, where the transfer bias voltage is switched from Vt1 to Vt2, the bias voltage for static elimination is changed from Vd1 to Vd3, and the bias voltage for hysteresis removal is changed from Vd2 to Vd4. . On the other hand, the paper 35 is again transported along the transport path 32 through the recording paper transport path 51 to enter the image forming step of the second surface by the paper transport means, and is transported along the transport path 32 by the transmission optical sensor as described above. The end position is detected, and the process proceeds to the second surface image transfer process. In FIG. 11, the process returns to step 204A, and the processes of steps 204A to 222 are sequentially performed on the second surface of the paper 35.
[0108]
Also at this time, the predetermined transfer bias voltage Vt2 for the second surface = + 1800V, the bias voltage Vd3 = -1000V for removing the paper, and the bias voltage value Vd4 = + for removing the positive charge history in the area outside the paper of the photosensitive drum 19 are also determined. 3000 V is applied at the same timing as in the first-side transfer step, so that the toner image is transferred to the second-side sheet 35, the rear-end transfer bias voltage of the sheet is increased, the sheet 35 is discharged, and the surface of the photosensitive drum 19 is discharged. History removal. In this case, the increase in the transfer bias voltage at the rear end of the sheet is controlled at +72 V / mm at an increase rate of 4%, similarly to the first surface, because the previous transfer bias voltage Vt2 is +1800 V. However, the upper limit is 1.3 times (+2340 V) the initial transfer bias voltage Vt2.
[0109]
The rate of increase / upper limit voltage of the transfer bias voltage differs depending on the temperature and humidity environment and the conditions such as the time of transfer of the first and second surfaces, and the increase amount is 2% in the H / H environment where a difference in sheet charging is unlikely to occur. On the other hand, in the case of printing on the second side in an L / L environment where the paper is easily charged highly, the rate of increase per mm is 5% and the upper limit voltage is 1.4 times. Up to
[0110]
Such control under each condition is performed by the CPU 101 in accordance with a monitor value obtained in the monitor step of applying a monitor voltage to the transfer bias roll 29 and detecting a current in the above-described transfer bias roll 29 using a pre-programmed calculation formula. Vt1, Vt2, Vd1, Vd2, Vd3, Vd4, and the applied bias voltage values such as the rate of increase of the trailing edge of the paper are determined, and image forming is performed by applying the above-mentioned timing programmed in the CPU 101. It is.
[0111]
Therefore, the applied bias voltage values of Vt1, Vt2, Vd1, Vd2, Vd3, and Vd4 vary depending on the operating environment of the laser beam printer 11, and are constantly changed to the optimum values. The transfer effective current value including the sheet 35 obtained by detecting the transfer bias voltage value and the transfer current value during the transfer process to the sheet 35 is monitored when the sheet 35 is in the transfer nip J1, and the CPU 101 calculates again. By doing so, it is also possible to change the discharge bias voltage value, the rear end increasing rate, and the upper limit transfer bias voltage of the paper 35 that has reached the discharge needle 41 from the transfer nip J1 for each type of paper 35.
[0112]
When the transfer to both sides of the paper 35 is completed, an affirmative determination is made in step 224 in FIG. 11, and the process ends.
[0113]
In the second embodiment described above, the transfer bias voltage is gradually increased in accordance with the sheet transport distance from the point in time when the position separated from the rear end by the distance L between the transfer nip J1 and the charge eliminating needle 41 reaches the transfer nip J1. Therefore, a decrease in the charging potential near the rear end of the paper 35 and a decrease in the attraction force between the paper 35 and the toner image due to the strong static elimination action of the static elimination needle 41 near the rear end of the paper 35 are prevented. be able to. In other words, the charging potential of the paper 35 can be made uniform from the leading edge to the trailing edge of the paper 35, and image disturbance and electrostatic offset due to leakage at the time of conveyance can be eliminated, so that the quality of the formed image can be kept good.
[0114]
Accordingly, there is no need to provide a member for cleaning, a circuit for preventing electrostatic offset, and the like in the fixing device 40, and it is not necessary to add a power supply. Therefore, the configuration of the device is simplified, the size of the device is reduced, and the cost is reduced. be able to.
[0115]
Further, since the charged potential of the paper 35 is uniformly stabilized, the paper 35 after the transfer / discharge is conveyed to the fixing device 40 in a stable posture adsorbed by the conveyance guides 71 and 72, thereby preventing the occurrence of a jam. can do.
[0116]
In the above-described second embodiment, an example has been described in which the control for increasing the transfer bias voltage is performed both during transfer to the first surface and during transfer to the second surface in double-sided printing. The control for increasing the transfer bias voltage may be performed only at the time. That is, since the sheet has a higher resistance value and a higher transfer bias voltage is required during transfer to the second surface than during transfer to the first surface, excessive static elimination due to the close proximity of the paper to the static elimination needle. The action or the like leads to a decrease in the attraction force between the sheet and the toner image. It is very effective to perform control to increase the transfer bias voltage during such transfer to the second surface.
[0117]
Further, the transfer bias voltage is increased from a point in time when a position distant from the rear end by a distance L between the transfer nip J1 and the charge removal needle 41 reaches the transfer nip J1, but the transfer bias voltage is increased from a point earlier than the point in time. The increase may be started.
[0118]
Further, the transfer bias voltage value and the transfer current value during the transfer process to the paper 35 are detected, the resistance value of the paper 35 is calculated by the CPU 101, and the type of the paper 35 is determined. The voltage value, the rate of increase of the transfer bias voltage, and the upper limit value may be changed.
[0119]
Further, the control of the transfer bias voltage and the neutralization bias voltage is changed based on the voltage / current information of the transfer bias roll 29. For example, the paper transport roll is formed of the same conductive member as the transfer bias roll 29, and the paper transport is performed during the paper transport. By applying a voltage to the roll and monitoring the output current, the use environment and the condition of the used paper may be determined, and the control may be similarly performed according to the condition.
[0120]
Further, in order to enable application of a transfer bias voltage suitable for the paper basis weight shown in FIG. 6, the transfer current is monitored when the leading end of the paper enters the transfer nip J1, and information on the paper basis weight is obtained. The transfer bias voltage may be adjusted according to the paper basis weight.
[0121]
【The invention's effect】
As described above, according to the present invention, since the charging that binds the toner over the entire surface of the recording medium is made uniform, the attraction force between the toner image and the recording medium can be uniformly stabilized. As a result, the above-described fringe contamination, explosion, electrostatic offset, and transfer failure at high temperature and high humidity can be prevented, and stable image quality performance can be exhibited.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an image forming apparatus according to the present invention.
FIG. 2 is a configuration diagram of a main part of the image forming apparatus according to the first embodiment.
FIG. 3 is a diagram illustrating a configuration related to measurement of a volume resistance value of a transfer bias roll.
FIG. 4 is a graph showing a change in a volume resistance value of a transfer bias roll according to an environment.
FIG. 5 is a graph showing a transfer current characteristic for each environment of a transfer bias roll.
FIG. 6 is a graph showing a transfer current characteristic of a transfer bias roll according to a sheet amount;
FIG. 7 is a circuit configuration diagram of a transfer voltage application circuit and a static elimination needle bias voltage application circuit.
FIG. 8 is a graph showing characteristics of a transfer bias voltage increase rate-increase transfer current in a transfer bias roll.
FIG. 9 is a flowchart showing a main routine in the first embodiment.
FIG. 10 is a flowchart illustrating a subroutine of a setting process of a transfer bias voltage.
FIG. 11 is a flowchart showing a main routine in a second embodiment.
FIG. 12 is a flowchart showing a subroutine of setting processing of various bias voltages.
FIG. 13 is a configuration diagram of a main part of an image forming apparatus according to a second embodiment.
[Explanation of symbols]
11 Laser beam printer (image forming device)
13 Semiconductor laser scanning device
19 Photoconductor drum (image carrier)
23 Charging roll
25 Developing device (developing means)
29 Transfer bias roll
35 paper (recording medium)
40 Fixing device (fixing means)
41 static elimination needle (static elimination means)
101 control device
J1 transfer nip

Claims (7)

Latent image forming means for forming an electrostatic latent image corresponding to the image to be formed on the image carrier,
Developing means for forming a toner image on the image carrier by adsorbing toner on the image carrier corresponding to the electrostatic latent image;
Transfer means for nipping and transporting the recording medium between the image carrier and applying a voltage in the nipping and transporting state, thereby transferring the toner image to the recording medium;
Fixing means for fixing the toner image on the recording medium onto the recording medium, arranged in a state of being grounded on the downstream side in the conveying direction of the recording medium with respect to the transfer means,
When the toner image is transferred to a region on the rear end side of the transfer target position on the recording medium when the leading end of the recording medium comes into contact with the fixing unit, a voltage (transfer) applied to the transfer unit Transfer voltage control means for making the charged potential of the recording medium uniform by increasing the bias voltage);
An image forming apparatus having: Latent image forming means for forming an electrostatic latent image corresponding to the image to be formed on the image carrier,
Developing means for forming a toner image on the image carrier by adsorbing toner on the image carrier corresponding to the electrostatic latent image;
Transfer means for nipping and transporting the recording medium between the image carrier and applying a voltage in the nipping and transporting state, thereby transferring the toner image to the recording medium;
Fixing means for fixing the toner image on the recording medium onto the recording medium, arranged in a state of being grounded on the downstream side in the conveying direction of the recording medium with respect to the transfer means,
When transferring the toner image to a region at a rear end side of a position at least a distance between a transfer position by the transfer unit and the charge removing unit and a leading end side from a rearmost end of the recording medium, the transfer unit Transfer voltage control means for equalizing the charging potential of the recording medium by increasing the voltage (transfer bias voltage) applied to
The transfer unit is disposed in the vicinity of the downstream side in the transport direction of the recording medium and below the transport path of the recording medium, and a voltage having a polarity opposite to that of the transfer bias voltage is applied to remove the charge of the recording medium. Means for performing static elimination,
Voltage applying means for applying a predetermined voltage to the static elimination means,
An image forming apparatus having: Discharge means disposed in the vicinity of the transfer direction downstream of the recording medium with respect to the transfer means, the discharge bias means to remove the charge of the recording medium by applying a voltage having a polarity opposite to the transfer bias voltage,
Immediately before the trailing end of the recording medium separates from the image carrier, a charge elimination voltage control unit that increases a voltage (discharge bias voltage) applied to the charge elimination unit;
Further having
2. The image forming apparatus according to claim 1, wherein the transfer voltage control unit increases the transfer bias voltage according to an increase in a static elimination bias voltage. 3. Discharge means disposed in the vicinity of the transfer direction downstream of the recording medium with respect to the transfer means, the discharge bias means to remove the charge of the recording medium by applying a voltage having a polarity opposite to the transfer bias voltage,
A static elimination voltage application unit capable of applying two or more different voltages having polarities opposite to the transfer bias voltage to the static elimination unit;
Further having
2. The image forming apparatus according to claim 1, wherein the transfer voltage control unit increases the transfer bias voltage to a different upper limit value according to a voltage (static bias voltage) applied to the charge eliminating unit. 3. The transfer voltage control unit, according to claim 1 or claim 2, characterized in that increasing the transfer bias voltage based on at least one of the conditions of the growth rate of the transfer bias voltage set in accordance with the environmental conditions and the upper limit value the image forming apparatus according to. A double-sided printing transport unit that reverses the recording medium having the toner image fixed on one surface thereof by the fixing unit so that the other surface faces the image carrier and transports the recording medium to a transfer position by the transfer unit. Have more,
The transfer voltage control means, the only case of transferring the toner image on the other surface, the image forming apparatus according to any one of claims 1 to 5, characterized in that increasing the transfer bias voltage . The transfer voltage control means according to any one of claims 1 to 6, characterized in that increasing the transfer bias voltage in the voltage increase rate within a predetermined upper limit value for the conveying distance of the recording medium Image forming device.

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