JP3970592B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus represented by a digital copying machine, a laser printer, a laser facsimile, and the like. In particular, the present invention relates to a measure for avoiding image quality degradation at the time of low-speed image formation for an image forming apparatus in which the image forming speed is variable according to the image resolution requested by the user.
[0002]
[Prior art]
Conventionally, as an image forming operation in an electrophotographic image forming apparatus such as a laser printer, first, an electrostatic latent image is formed on a photosensitive drum based on image data transmitted from a personal computer (hereinafter simply referred to as a personal computer) or the like. An image is formed, toner is attached to the electrostatic latent image, and the image data is visualized on the photosensitive drum. Thereafter, the toner image is transferred from the photosensitive drum onto the surface of the recording paper conveyed through the paper conveyance path. Then, the recording paper is passed through a fixing roller, and the toner image is fixed on the recording paper by heating and pressurization with the fixing roller.
[0003]
In addition, as a mechanism for performing the transfer operation, a transfer roller is disposed facing the photosensitive drum, and a toner image on the photosensitive drum is recorded on the recording paper by applying a predetermined bias voltage to the transfer roller. It is configured to transfer to. Specifically, a transfer bias voltage necessary for transferring the toner image is applied to the transfer roller by a high-voltage power supply for transfer, and when the recording paper passes between the photosensitive drum and the transfer roller, A positive charge is applied to the recording paper, and the toner image on the photosensitive drum (the toner is negatively charged in advance) is transferred onto the recording paper. After this transfer, switching is performed so that the current (transfer bias) applied to the transfer roller is lowered (supply current OFF or a current lower than that at the time of transfer) by constant current control. This switching causes toner on the photosensitive drum to adhere to the transfer roller during non-transfer (when there is no recording paper between the photosensitive drum and the transfer roller), or due to a photosensitive memory described later. Thus, the occurrence of paper marks on the photosensitive drum is prevented. In general, the transfer bias is switched at an inter-sheet portion (area between sheets continuously conveyed with a predetermined interval) outside the leading and trailing edges of the recording sheet.
[0004]
Incidentally, in recent years, in this type of image forming apparatus, there is an increasing demand for higher speed and higher resolution. For example, in the above-described laser printer, the limits of the image forming speed and the image resolution are determined by the laser scanner. In other words, in a laser printer that rotates the polygon mirror and raster scans the laser beam, the limit of the rotational speed of the drive motor for rotating the polygon mirror at a high speed directly determines the upper limit of the image forming speed and image resolution of the printer. It will end up. For this reason, one means for obtaining a high image forming speed while maintaining a high resolution is to employ a high-speed drive motor.
[0005]
However, if such a high-speed drive motor is employed to achieve both high speed and high resolution, the cost of the entire apparatus will increase, resulting in lack of practicality.
[0006]
Therefore, as a technique for achieving higher resolution on demand without increasing the rotational speed of the polygon mirror, the image forming speed is made variable. That is, when high-resolution image formation is not required, the high-speed mode is set, and the image formation speed (process speed) of the printer is set high. On the other hand, in the high resolution mode that requires high resolution image formation, the image forming speed of the printer is reduced. Specifically, when a relatively low-resolution image formation operation with a resolution of 600 dpi is sufficient, a relatively high process speed that can achieve 12 ppm with A4 size paper is set, while a relatively high-resolution image formation with a resolution of 1200 dpi is formed. When operation is required, a relatively low process speed that can achieve the equivalent of 6 to 7 ppm with A4 size paper is set. As a result, high-speed image formation is possible in the case of an image having a resolution of 600 dpi such as characters, while high-resolution output of 1200 dpi is possible in the case of an image image such as a photograph. By making the image forming speed variable in this way, it is possible to increase the resolution of an image as necessary without causing an increase in the cost of the printer.
[0007]
[Problems to be solved by the invention]
However, in the image forming apparatus that can change the process speed according to the required resolution, there is a problem that the photoconductor memory is likely to be generated when the process speed is switched to the low speed side (high resolution side). This photosensitive memory is generated when the transfer current excessively flows to the photosensitive drum. In other words, the transfer current value is set so that a transfer current that normally does not cause a transfer defect flows to the photosensitive drum, but as described above, the transfer bias is switched outside the leading and trailing edges of the recording paper. Since this is performed in the inter-paper portion, when the leading and trailing end portions of the recording paper reach the transfer nip portion between the transfer roller and the photosensitive drum, the current easily flows to the photosensitive drum side, and is concentrated. Thus, a current flows toward the photosensitive drum. Moreover, since the process speed is low, the time required for the recording paper to pass through the transfer nip (transfer time) becomes longer than when the process speed is high. The amount flowing into the photosensitive drum side increases in proportion to the passage time, and as a result, the occurrence of the photoconductor memory becomes more remarkable.
[0008]
In this way, when a photoconductor memory is generated due to excessive current flowing through the photoconductive drum when the leading and trailing edges of the recording paper pass, image quality degradation (generally, due to the photoconductive memory during the next image formation) This is called edge missing memory. In particular, in a halftone image, black streak-like stains occur in the photoconductor memory generation portion, and the image quality is significantly deteriorated.
[0009]
The present invention has been made in view of the above point, and an object of the present invention is to suppress an excessive current from flowing through a photoconductor in an image forming apparatus having a variable process speed. Thus, it is possible to prevent the occurrence of image quality degradation caused by.
[0010]
[Means for Solving the Problems]
    -Summary of invention-
  In order to achieve the above object, the present invention provides a transfer unit in which the image forming speed becomes lower when the image forming speed is lower due to factors such as the need for high-resolution image formation. Transfer output (transfer roller bias current)The valueBy appropriately changing this, generation of the photoconductor memory is suppressed.
[0011]
    -Solution-
  Specifically, the toner on the surface of the photosensitive member when the recording medium passes through a transfer area formed between the photosensitive member, a toner image forming unit that forms a toner image on the surface of the photosensitive member, and the photosensitive member. A transfer means for transferring the image onto the recording medium,Between high speed mode and low speed modeIt is assumed that the image forming apparatus has a variable image forming speed. For this image forming device,Low-speed mode transfer, which is the transfer output of the transfer means when transferring the toner image in the low-speed mode, in contrast to the high-speed mode transfer output, which is the transfer output of the transfer means, when the toner image is transferred in the high-speed mode. The time output is set low, and in the high speed mode, in the low speed mode, the output in the high speed mode, which is the transfer output of the transfer means when there is no recording medium in the transfer area, The low speed mode non-transfer output, which is the transfer output of the transfer means when there is no recording medium in the transfer area, is set low.Output changing means is provided.The high speed mode transfer output is set higher than the high speed mode non-transfer output, and the low speed mode transfer output is set higher than the low speed mode non-transfer output.
[0012]
Due to this specific matter, even when the time for the transfer current (transfer output) to flow toward the photosensitive member becomes longer due to the low image forming speed, the transfer current of the transfer means at this time is set low. Therefore, the transfer current flowing through the photosensitive member is very small. In other words, it is possible to prevent an excessive current from flowing to the photosensitive member when the leading and trailing ends of the recording medium pass through the transfer area, and it is possible to prevent the generation of the photosensitive member memory and to avoid image quality deterioration. Is possible.
[0013]
  In the high speed mode, the transfer output is switched from the high speed mode non-transfer output to the high speed mode transfer output before the downstream end in the transport direction of the recording medium passes through the transfer area. After the upstream end passes through the transfer area, the transfer output is switched from the output during high speed mode transfer to the output during high speed mode non-transfer, while in the low speed mode, the downstream end in the recording medium transport direction passes through the transfer area. After this, and before the image forming area on the recording medium reaches the transfer area, the transfer output is switched from the low speed mode non-transfer output to the low speed mode transfer output, and the image forming area on the recording medium is transferred. After passing the area and before the upstream end in the transport direction of the recording medium passes the transfer area, the transfer output is changed from the low speed mode to the low speed mode. Constitute the output changing means so as to switch the transfer output.
[0014]
According to this specific matter, when the image forming speed is set low, the transfer output is switched on the inner side of the front and rear end edges of the recording medium. That is, when the image forming speed is set low, the transfer output is set high when the recording medium always exists in the transfer area. For this reason, it is possible to avoid the occurrence of a situation in which the transfer current easily flows to the photoconductor side outside the front and rear end edges. Therefore, even with this solution, it is possible to prevent an excessive current from flowing to the photosensitive member when the leading and trailing ends of the recording medium pass through the transfer area, and image quality deterioration is caused by preventing the generation of the photosensitive member memory. It can be avoided.
[0016]
  Each of the above-described means changes the transfer output and changes the transfer output switching timing in accordance with the image forming speed. Each of the following means is a case where the image formation speed changes as the requested or set image resolution is changed, and the transfer output changes according to the image resolution.FurtherIt is what I do. This will be specifically described below.
[0017]
  First, a toner image on the surface of the photoconductor is recorded when the recording medium passes through a transfer area formed between the photoconductor, a toner image forming unit that forms a toner image on the surface of the photoconductor, and the photoconductor. A transfer means for transferring onto the medium,Between low resolution mode and high resolution modeAn image forming apparatus having a variable image resolution is assumed. For this image forming device,High-resolution mode transfer, which is the transfer output of the transfer means when transferring the toner image in the high-resolution mode, in contrast to the output of the transfer means, which is the transfer output of the transfer means when transferring the toner image in the low-resolution mode. In the low resolution mode, in the low resolution mode, in the high resolution mode, the output in the low resolution mode, which is the transfer output of the transfer means when there is no recording medium in the transfer area, Set the low-resolution mode non-transfer output, which is the transfer output of the transfer means when there is no recording medium in the transfer area.Output changing means is provided.The output at the time of low resolution mode transfer is set higher than the output at the time of low resolution mode non-transfer, and the output at the time of transfer of high resolution mode is set higher than the output at the time of non-transfer of high resolution mode.
[0018]
  In the low resolution mode, the transfer output is switched from the low resolution mode non-transfer output to the low resolution mode transfer output before the downstream end in the transport direction of the recording medium passes through the transfer area. After the upstream end passes through the transfer area, the transfer output is switched from the low resolution mode transfer output to the low resolution mode non-transfer output, while in the high resolution mode, the downstream end of the recording medium in the transport direction passes through the transfer area. After this, and before the image formation area on the recording medium reaches the transfer area, the transfer output is switched from the output at the time of non-transfer from the high resolution mode to the output at the time of transfer at the high resolution mode, and the image formation area on the recording medium is transferred. After passing through the area and before the upstream end in the transport direction of the recording medium passes through the transfer area, the transfer output is output during high resolution mode transfer. Constitute the output changing means so as to switch in the output high-resolution mode nontranscribed from.
[0020]
Further, as a specific configuration for controlling the transfer output of the transfer unit, the transfer output of the transfer unit at the time of toner image transfer and the transfer output of the transfer unit when no recording medium exists in the transfer area are determined. The output changing means is configured to set by current control. When the transfer output of the transfer unit is controlled by constant current control in this way, the transfer voltage is output while following the impedance of the recording medium. Therefore, compared with the case of controlling the transfer output of the transfer unit by constant voltage control. Transferability can be maintained well.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In this embodiment, a case where the present invention is applied to a laser printer which is an electrophotographic image forming apparatus will be described.
[0022]
-First embodiment-
FIG. 1 schematically shows the internal configuration of a laser printer M according to this embodiment. FIG. 2 shows a connection state between the laser printer M and an external device (personal computer) 20.
[0023]
As shown in these drawings, the laser printer M executes a printing operation when the control device 17 receives a print signal from the personal computer 20 via the communication cable 19 and the I / O port 16. Hereinafter, the configuration of the laser printer M will be described.
[0024]
As shown in FIG. 1, the laser printer M includes a laser scanning unit 3, an electrophotographic process unit A for forming an image on a recording sheet S as a recording medium, and the supply of the recording sheet S to the electrophotographic process unit A. A paper feeding / conveying unit B for carrying the paper and the recording paper S is provided. Hereinafter, each part will be described.
[0025]
<Description of Laser Scanning Unit 3>
The laser scanning unit 3 forms an electrostatic latent image on the photosensitive drum 1 by scanning and exposing the photosensitive drum 1 of the electrophotographic process section A with laser light based on image data received from the personal computer 20. It is.
[0026]
<Description of electrophotographic process part A>
The electrophotographic process part A includes the photosensitive drum 1 as a photosensitive member. The photosensitive drum 1 is rotated in a direction indicated by an arrow R1 in FIG. 1 by driving means (not shown), and an electrostatic latent image is formed on the outer peripheral surface by the laser scanning unit 3.
[0027]
Around the photosensitive drum 1, a charging roller 2, a developing device 4, a transfer unit 5, a cleaning device 6, and a static eliminator (not shown) are arranged.
[0028]
The charging roller 2 charges the surface of the photosensitive drum 1 before the electrostatic latent image is formed to a predetermined potential. The developing device 4 develops the electrostatic latent image formed on the surface of the photosensitive drum 1 into a visible image with a developer (toner or the like). The transfer unit 5 transfers the toner image formed on the surface of the photosensitive drum 1 onto the recording paper S. Further, the transfer unit 5 includes a transfer roller 51 as a transfer unit that comes into contact with the outer peripheral surface of the photosensitive drum 1 with a predetermined pressing force, and serves as a transfer area between the transfer roller 51 and the photosensitive drum 1. A transfer nip portion N is configured. The cleaning device 6 removes the developer remaining on the surface of the photosensitive drum 1 after the toner image is transferred. The static eliminator removes residual charges on the surface of the photosensitive drum 1.
[0029]
<Description of Paper Feed Unit B>
The paper feeding / conveying unit B includes a paper feeding cassette 7, a paper feeding tray 8, a discharge unit 15 formed on the top of the laser printer M, and a conveyance from the discharge side of the paper feeding cassette 7 and the paper feeding tray 8 to the discharge unit 15. A path C is provided. Further, the paper feeding / conveying unit B takes out the recording paper S, S,... Stored in the paper feeding cassette 7 or the paper feeding tray 8 one by one, and the recording paper S is extracted from the photosensitive drum 1 of the electrophotographic process section A. And the transfer unit 5 are supplied at a predetermined timing. A registration roller 11 for supplying the recording sheet S to the electrophotographic process unit A at a predetermined timing is disposed in the vicinity of the electrophotographic process unit A in the transport path C. Further, pickup rollers 9 and 10 are provided on the discharge side of the paper feed cassette 7 and the paper feed tray 8 so that one recording paper S can be taken out.
[0030]
A fixing unit 14 is disposed on the discharge side of the transfer unit 5. The fixing device 14 includes a pair of fixing rollers. With these fixing rollers, the toner image on the recording paper S that has passed through the transfer unit 5 is heated and fixed and discharged to the discharge unit 15.
[0031]
<Image forming operation>
With the configuration described above, when an image is formed on the recording paper S, first, a print signal and an image signal from the personal computer 20 are transmitted through the communication cable 19 and via the I / O port 16 of the image forming apparatus M. It is transmitted to the control device 17. The control device 17 controls the high-voltage power supply 18 and also controls the laser scanning unit 3, the electrophotographic process unit A, and the paper feeding / conveying unit B. Specifically, the surface of the photosensitive drum 1 is charged to a predetermined potential by the charging roller 2, and an electrostatic latent image is formed on the surface of the photosensitive drum 1 by the laser scanning unit 3. Thereafter, the developing device 4 develops a visible image with a developer on the surface of the photosensitive drum 1, and the recording sheet S fed from the paper feeding / conveying unit B and waiting on the registration roller 11 is placed on the photosensitive drum 1. The toner image is guided and supplied to the upper and lower guides 12 and 13 so as to match the timing of the toner image, and the toner image is transferred to the recording paper S by the transfer unit 5. In this transfer operation, the toner image on the photosensitive drum 1 is electrostatically transferred onto the recording material S when the high voltage power supply 18 applies a transfer bias to the transfer roller 51. Specifically, a transfer bias voltage (plus charge) necessary for transferring the toner image is applied to the transfer roller 51 by the high-voltage power supply 18, and the transfer is performed when the recording paper S passes through the transfer nip N. A positive charge is applied from the roller 51 to the recording paper S, and the toner image on the photosensitive drum 1 (the toner is negatively charged in advance) is transferred onto the recording paper S.
[0032]
Thereafter, the recording sheet S is heated by the fixing device 14 provided on the conveyance path C, and after the toner image is fixed, the recording sheet S is discharged to the paper discharge unit 15.
[0033]
On the other hand, the developer remaining on the surface of the photosensitive drum 1 is removed by the cleaning device 6 and the residual charge on the surface of the photosensitive drum 1 is removed by the static eliminator. Thereby, one cycle of the image forming operation (printing operation) on the recording paper S is completed. By repeating this cycle, it is possible to continuously form images on a plurality of recording sheets S, S,. Incidentally, a collecting means such as a transport pipe (not shown) is provided between the cleaning device 6 and the developing tank of the developing device 4, and the toner collected by the cleaning device 6 is supplied to the developing device 4 to be reused. It can be used.
[0034]
The laser printer M according to this embodiment has a variable image forming speed (process speed) as one of its features. Specifically, the image forming speed can be changed in two stages. When high-resolution image formation is not required, the high-speed mode is selected. When high-resolution image formation is required, the low-speed mode is selected. It has become. For example, the high-speed mode is a mode in which a relatively low-resolution image forming operation with a resolution of 600 dpi is sufficient, and a sheet conveyance speed of 122 mm / sec and a relatively high process speed that can achieve 27 ppm equivalent with A4 size paper. Set to On the other hand, the low-speed mode is a case where a relatively high-resolution image forming operation with a resolution of 1200 dpi is required, and the paper conveyance speed is 61 mm / sec, and a relatively low process speed that can achieve 14 ppm equivalent with A4 size paper. Set to As a result, a high-speed output is possible when the image is a resolution of 600 dpi such as characters, while a high-resolution output of 1200 dpi is possible when the image is an image such as a photograph. In other words, by making the image forming speed variable in this way, a high-speed polygon motor is not required, and higher image resolution can be achieved as needed without causing an increase in printer cost. It can be done. The process speed is switched by operating a switch on the operation panel 21 of the laser printer M. The process speed may be switched by operating the personal computer 20. Further, the process speed may be automatically switched by recognizing the image information.
[0035]
<Description of output changing means>
Next, the characteristic part which concerns on this invention is demonstrated. The control device 17 is provided with output changing means 17A for controlling a transfer bias applied from the high-voltage power supply 18 to the transfer roller 51. The output changing unit 17A is configured to transfer the transfer bias of the transfer roller 51 at the time of toner image transfer and the recording sheets S, S,... The transfer bias of the transfer roller 51 when the area between them is located at the transfer nip portion N is changed according to the image forming speed.
[0036]
Specifically, as shown in FIG. 3 (the transfer bias in the high speed mode is indicated by a thick line, and the transfer bias in the low speed mode is indicated by a thin line), the transfer nip portion N in the high speed mode is shown. In the case where the transfer bias of the transfer roller 51 is 12 μA and the transfer bias at the time of toner image transfer is 18 μA when there is no recording S paper, the recording paper is transferred to the transfer nip N when the low speed mode is switched. Both transfer biases are lowered so that the transfer bias of the transfer roller 51 when S is not present is 8 μA, and the transfer bias when transferring the toner image is 12 μA. The transfer bias in each mode is changed by such constant current control. In this embodiment, the timing for switching to increase the transfer bias when the downstream end (the leading end in the drawing) of the recording sheet S passes through the transfer nip N is the same as in the conventional case. It is set to a timing before the downstream end in the transport direction of S passes through the transfer nip portion N. In addition, when the upstream end (the rear end in the drawing) of the recording paper S passes through the transfer nip portion N, the timing for switching so as to lower the transfer bias is the same as in the prior art. The timing is set after the upstream end in the direction passes through the transfer nip portion N.
[0037]
As described above, in this embodiment, when the mode is switched to the low speed mode, the transfer bias of the transfer roller 51 when the recording sheet S is not present in the transfer nip portion N and the transfer bias at the time of toner image transfer are both high. It is set lower than each transfer bias in the speed mode. For this reason, even if the time during which the transfer current (transfer output) flows toward the photosensitive drum 1 becomes longer due to the low image forming speed, the transfer current of the transfer roller 51 at this time is set low. Therefore, the transfer current flowing through the photosensitive drum 1 is very small. Conventionally, a transfer current indicated as a transfer bias in the high speed mode in the drawing is applied regardless of the image forming speed. Therefore, a high current (18 μA) flows through the photosensitive drum 1 while the region T in the drawing passes through the transfer nip portion N. The time for this region T to pass through the transfer nip N is a short time in which no photoconductor memory is generated in the high speed mode, but in the low speed mode, there is a possibility that the photoconductor memory is generated. High and relatively long time. For this reason, there has been a concern about the occurrence of the photoconductor memory in the low speed mode. According to this embodiment, the transfer bias switching operation causes an excessive current to flow to the photosensitive drum 1 when the leading and trailing ends of the recording sheet S pass through the transfer nip portion N even in the low speed mode. Can be suppressed. As a result, it is possible to prevent the photoconductor memory from being generated and to avoid image quality deterioration.
[0038]
  −Reference example−
  next,Reference exampleWill be described. In the first embodiment, the transfer bias of the transfer roller 51 when the toner image is transferred and the transfer bias of the transfer roller 51 when the recording sheet S is not present in the transfer nip N are changed according to the image forming speed. Thus, the output changing means 17A is configured.This exampleInstead, the transfer bias switching timing when the downstream end in the transport direction and the upstream end in the transport direction of the recording paper S pass through the transfer nip portion N is output so as to be changed according to the image forming speed, respectively. The changing means 17A is configured.
[0039]
Specifically, as shown in FIG. 4 (the transfer bias in the high speed mode is indicated by a thick line, and the transfer bias in the low speed mode is indicated by a thin line), the transfer nip N is placed in the transfer nip N in any mode. The transfer bias of the transfer roller 51 when there is no recording S paper is 12 μA, and the transfer bias at the time of toner image transfer is 18 μA.
[0040]
In the high speed mode, similarly to the case of the first embodiment, switching is performed so that the transfer bias is increased before the downstream end in the transport direction of the recording paper S passes the transfer nip N, and After the upstream end in the conveyance direction of the recording paper S passes through the transfer nip N, the transfer bias is switched to be low.
[0041]
On the other hand, in the low speed mode, the downstream end in the conveyance direction of the recording paper S passes through the transfer nip portion N, and no image formation is performed on the non-image area (on the recording paper S). In the non-image area, the transfer bias is switched to be low before the upstream end in the transport direction of the recording paper S passes through the transfer nip N. .
[0042]
  in this way,This exampleWhen the mode is switched to the low speed mode, the transfer bias is switched high after the downstream end in the transport direction of the recording paper S passes through the transfer nip portion N, and the upstream end in the transport direction of the recording paper S is in the transfer nip portion N. The switching timing is changed so that the transfer bias is switched to a low level before passing through. For this reason, when the image forming speed is set to be low, the transfer bias is set to be high when the recording sheet S is always present in the transfer nip portion N. That is, when the leading and trailing edges of the recording paper S pass through the transfer nip N, the state where the recording paper S does not exist in the transfer nip N and the state where the transfer bias is high continues for a relatively long time. It can be avoided. In other words, in the low speed mode, it is possible to avoid a situation where the recording paper S is not present in the transfer nip portion N and the transfer bias is high. For this reason, it is possible to suppress the excessive current from flowing to the photosensitive drum 1, and as a result, it is possible to prevent the generation of the photosensitive memory and to avoid the deterioration of the image quality.
[0043]
  -No.2Embodiment
  Next, the first of the present invention2Embodiments will be described. This embodiment is the same as the first embodiment described above.Reference exampleAre combined. That is, when the mode is switched to the low speed mode, the transfer bias of the transfer roller 51 when the recording sheet S does not exist in the transfer nip portion N and the transfer bias at the time of transferring the toner image are both transferred in the high speed mode. Set lower than the bias. In this low speed mode, the transfer bias is switched to a high level after the downstream end in the transport direction of the recording paper S passes through the transfer nip N, and the upstream end in the transport direction of the recording paper S passes through the transfer nip N. Change the switching timing so that the transfer bias is switched low before.
[0044]
Specifically, as shown in FIG. 5 (the transfer bias in the high speed mode is indicated by a bold line and the transfer bias in the low speed mode is indicated by a thin line), when the transfer speed is switched to the low speed mode, the transfer nip Both the transfer biases are lowered so that the transfer bias of the transfer roller 51 when the recording paper S does not exist in the portion N is 8 μA, and the transfer bias when transferring the toner image is 12 μA. In addition, after the downstream end in the conveyance direction of the recording paper S has passed through the transfer nip portion N, switching is performed so as to increase the transfer bias in the non-image area, and the upstream end in the conveyance direction of the recording paper S Is switched so as to lower the transfer bias in the non-image area before passing through the transfer nip portion N.
[0045]
  Thus, in the present embodiment, the first described above.Embodiments and reference examplesThus, it is possible to reliably prevent an excessive current from flowing to the photosensitive drum 1 when the leading and trailing edges of the recording paper S pass through the transfer nip portion N. As a result, it is possible to prevent the photoconductor memory from being generated, and it is possible to reliably avoid image quality deterioration.
[0046]
-Other embodiments-
In each of the above-described embodiments, the case where the present invention is applied to a laser printer has been described. The present invention is not limited to this, and can be applied to various image forming apparatuses such as a digital copying machine, a laser facsimile, and a complex machine of these.
[0047]
In each of the above-described embodiments, the laser printer in which the image forming speed can be changed in two stages has been described. However, the laser printer can be applied to an image forming apparatus in which the image forming speed can be changed in three stages or more. In this case, it is preferable that the transfer bias and the transfer bias switching timing can be switched to three or more stages.
[0048]
Further, in each of the above embodiments, the high speed mode is set when high resolution image formation is not required, and the low speed mode is set when high resolution image formation is required. That is, the high speed mode and the low speed mode are switched according to the required resolution. The present invention is not limited to this, and the high speed mode and the low speed mode may be switched by factors other than the resolution. For example, when the present invention is applied to a color printer, regardless of the resolution, the high-speed mode may be set when outputting a monochrome image, and the low-speed mode may be set when outputting a color image.
[0049]
Further, the current value of each transfer bias is not limited to that described above. However, when each transfer bias is set lower than that in the high speed mode when the mode is switched to the low speed mode (in the case of the first embodiment), the photoconductor memory stores the transfer bias at the time of transferring the toner image. It is necessary to set the value so that the toner image can be satisfactorily transferred without being generated.
[0050]
【The invention's effect】
As described above, according to the present invention, when the image forming speed is low for an image forming apparatus in which the image forming speed is variable depending on the required image resolution or the like, compared to when the image forming speed is high until then. Thus, the transfer output of the transfer means is reduced. For this reason, even in a situation where the transfer current (transfer output) flows toward the photoconductor becomes long, the transfer current of the transfer means at this time is set low, and the transfer current flowing through the photoconductor is As a result, the generation of the photoconductor memory can be prevented.
[0051]
In addition, when the image forming speed is low, the transfer output is switched to high after the downstream end in the conveyance direction of the recording medium passes through the transfer area, and the transfer is performed before the upstream end in the conveyance direction of the recording medium passes through the transfer area. The switching timing is changed to switch the output low. For this reason, when the image forming speed is set low, the transfer output is set high when the recording medium always exists in the transfer area, and when the leading and trailing edges of the recording medium pass through the transfer area. It is possible to suppress an excessive current from flowing through the photoconductor, and this can also prevent the photoconductor memory from being generated.
[0052]
Further, when the transfer output of the transfer unit is controlled by the constant current control in each of the above operations, the transfer voltage is output while following the impedance of the recording medium. Therefore, the transfer output of the transfer unit is controlled by the constant voltage control. Compared to the case of controlling, it becomes possible to maintain the transferability better, and the practicality of the image forming apparatus can be improved.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an outline of an internal configuration of a laser printer according to an embodiment.
FIG. 2 is a diagram illustrating a connection state between a laser printer and a personal computer.
FIG. 3 is a diagram for explaining a control operation of a transfer bias in each of a high speed mode and a low speed mode in the first embodiment.
[Fig. 4]Reference exampleFIG. 6 is a diagram for explaining a transfer bias control operation in each of a high speed mode and a low speed mode in FIG.
FIG. 52FIG. 6 is a diagram for describing a transfer bias control operation in a high speed mode and a low speed mode in the embodiment.
[Explanation of symbols]
1 Photosensitive drum (photoconductor)
4. Developing device (toner image forming means)
17A Output changing means
51 Transfer roller (transfer means)
N Transfer nip (transfer area)
S Recording paper (recording medium)

Claims (5)

When the recording medium passes through a transfer area formed between the photosensitive member, a toner image forming unit that forms a toner image on the surface of the photosensitive member, and the photosensitive member, the toner image on the surface of the photosensitive member is recorded on the recording medium. in the image forming apparatus image forming speed is varied between with and a transfer unit for transferring a high speed mode and the low speed mode,
Low-speed mode transfer, which is the transfer output of the transfer means when transferring the toner image in the low-speed mode, in contrast to the high-speed mode transfer output, which is the transfer output of the transfer means, when the toner image is transferred in the high-speed mode. While setting the hour output low,
In the high speed mode, there is no recording medium in the transfer area in the low speed mode as compared to the high speed mode non-transfer output which is the transfer output of the transfer means when there is no recording medium in the transfer area. Output changing means for setting the low-speed mode non-transfer output low, which is the transfer output of the transfer means at the time ,
The image forming method is characterized in that the high speed mode transfer output is set higher than the high speed mode non-transfer output, and the low speed mode transfer output is set higher than the low speed mode non-transfer output. apparatus. The image forming apparatus according to claim 1.
In the high speed mode, the output changing means switches the transfer output from the high speed mode non-transfer output to the high speed mode transfer output before the downstream end in the transport direction of the recording medium passes through the transfer area. While the upstream end of the transport direction passes through the transfer area, the transfer output is switched from the output during high speed mode transfer to the output during high speed mode non-transfer,
In the low speed mode, the transfer output after the downstream end in the conveyance direction of the recording medium passes through the transfer area and before the image forming area on the recording medium reaches the transfer area is changed from the output in the low speed mode non-transfer mode. In addition to switching to low-speed mode transfer output, the transfer output is set to low-speed mode after the image forming area on the recording medium has passed the transfer area and before the upstream end in the transport direction of the recording medium has passed the transfer area. An image forming apparatus configured to switch from an output during transfer to an output during low-speed mode non-transfer. When the recording medium passes through a transfer area formed between the photosensitive member, a toner image forming unit that forms a toner image on the surface of the photosensitive member, and the photosensitive member, the toner image on the surface of the photosensitive member is recorded on the recording medium. And an image forming apparatus in which the image resolution is variable between the low resolution mode and the high resolution mode .
High-resolution mode transfer, which is the transfer output of the transfer means at the time of toner image transfer in the high-resolution mode, in contrast to the low-resolution mode transfer output, which is the transfer output of the transfer means, at the time of toner image transfer in the low-resolution mode. While setting the hour output low,
In the low resolution mode, there is no recording medium in the transfer area in the high resolution mode as compared to the low resolution mode non-transfer output which is the transfer output of the transfer means when there is no recording medium in the transfer area. Output changing means for setting the output in the high-resolution mode non-transfer, which is the transfer output of the transfer means at a low time ,
The image formation characterized in that the output at the time of low resolution mode transfer is set higher than the output at the time of non-transfer of the low resolution mode, and the output at the time of transfer of the high resolution mode is set higher than the output at the time of non-transfer of the high resolution mode. apparatus. The image forming apparatus according to claim 3.
In the low resolution mode, the output changing means switches the transfer output from the low resolution mode non-transfer output to the low resolution mode transfer output before the downstream end in the transport direction of the recording medium passes through the transfer area. While the upstream end in the transport direction passes through the transfer area, the transfer output is switched from the low resolution mode transfer output to the low resolution mode non-transfer output,
In the high resolution mode, the transfer output from the high resolution mode non-transfer output is after the downstream end in the transport direction of the recording medium has passed through the transfer area and before the image forming area on the recording medium reaches the transfer area. In addition to switching to high resolution mode transfer output, the transfer output is set to high resolution mode after the image forming area on the recording medium has passed the transfer area and before the upstream end in the transport direction of the recording medium has passed the transfer area. From transfer output to high resolution mode non-transfer output An image forming apparatus characterized by being configured to switch. The image forming apparatus according to any one of claims 1 to 4 ,
The output changing unit is configured to set the transfer output of the transfer unit at the time of toner image transfer and the transfer output of the transfer unit when there is no recording medium in the transfer area by constant current control. Image forming apparatus.

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