JP2014160293A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive image forming apparatus configured to prevent transfer failure at a tip of a transfer material, while preventing leak black dot, transfer memory black dot or fog.SOLUTION: An image forming apparatus includes: a transfer bias power supply unit which applies a transfer bias to a transfer roller so as to transfer a toner image to a transfer material when the transfer material passes through a transfer nip part; a discharger arranged downstream of the transfer nip part when seen in a moving direction of the transfer material; an anti-static bias power supply unit which applies an anti-static bias opposite the transfer bias to the discharger; and a control unit which controls the transfer bias power supply unit and the discharger. The control unit causes the discharger to start applying the anti-static bias before the start of applying the transfer bias, and stabilizes output of the transfer bias during the time from when the tip of the transfer material reaches the transfer nip part to when a boundary between a non-image area and an image area of the transfer material passes through the transfer nip part.

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a fax machine.

Conventionally, the following is known as an image forming apparatus employing an electrophotographic system.
The image forming apparatus has a photosensitive drum as an image carrier, and a photosensitive layer made of OPC (Organic Photoconductor) or a-Si (amorphous silicon) is formed on the surface of the photosensitive drum. ing.

  The surface of the photosensitive drum is uniformly charged by a charging unit, and then partially exposed with a laser or LED according to image information, whereby an electrostatic latent image is formed. Next, the electrostatic latent image is developed into a toner image by a developing unit, and the toner image is transferred to a transfer material by a transfer unit. Thereafter, the transfer material is separated from the photosensitive drum by the separating unit, and the toner image on the transfer material is fixed to the transfer material by the fixing device.

  The transfer unit includes a transfer bias power supply unit, and the transfer bias power supply unit applies a transfer bias to the transfer roller when the transfer material passes through a transfer nip portion between the photosensitive drum and the transfer roller. When a transfer bias having a polarity opposite to that of the toner image is applied by the transfer bias power supply unit, the toner image on the photosensitive drum is transferred to a transfer material by a transfer electric field.

  The separating means includes, for example, a saw blade-shaped discharge member and a neutralizing bias power supply unit, and the discharge member is installed downstream of the transfer nip portion. The discharge member is applied with a discharge bias having a polarity opposite to the transfer bias by a discharge bias power supply unit.

  As an image forming apparatus of this type, in the image forming apparatus disclosed in Patent Document 1, the supply of the static elimination bias is started in synchronization with or before the transfer bias supply start. According to this image forming apparatus, it is considered that overshoot at the start of supplying the transfer bias is prevented, and the occurrence of memory on the image carrier is prevented or reduced.

Japanese Patent Laid-Open No. 11-007200

  In the image forming apparatus disclosed in Patent Document 1, the output of the transfer bias applied to the transfer roller rises before the leading edge of the transfer material reaches the transfer nip (see FIG. 2 of Patent Document 1). In this case, even if the overshoot of the transfer bias is suppressed, when an air gap occurs in the transfer nip portion near the tip of the transfer material due to the vibration of the transfer roller, the discharge voltage increases in the air gap portion, Abnormal discharge occurs on the photosensitive drum.

  In the case of a photosensitive drum (a-Si drum) in which the photosensitive layer is made of a-Si, a part of the photosensitive layer may be destroyed due to abnormal discharge due to an increase in discharge voltage at the air gap portion. Further, since the charge cannot be sufficiently held in the portion where the photosensitive layer is broken, a black spot (leak black spot) may occur in the transfer material.

  In the case where the photosensitive layer is a positively charged OPC photosensitive drum (OPC drum), a part of the photosensitive layer is charged with a polarity opposite to that of charging by the charging means due to abnormal discharge due to an increase in the discharge voltage at the air gap. As a result, a part of the photosensitive layer may not be charged to a sufficient potential by the charging means. Then, the positively charged toner is excessively developed on the portion of the photosensitive layer that is insufficiently charged, and a black spot (transfer memory black spot) on the transfer material or a fog on which the toner gets on the white paper portion may occur.

Considering only the prevention of such leak black spots and transfer memory black spots, it is desirable to start applying the transfer bias after the transfer material reaches the transfer nip. However, the conventional transfer bias power supply unit has a problem that variations in transfer bias rise time are large.
Because of this problem, when starting to apply transfer bias after the transfer material reaches the transfer nip, the transfer bias may not rise in time for the determined image area, especially in high-speed image forming devices. There is. In such a case, a transfer bias is not sufficiently applied to the leading end side of the transfer material, and transfer failure occurs.

Therefore, apart from the use of a more expensive power supply, the configuration using the conventional inexpensive transfer bias power supply unit prevents the occurrence of a leak black spot or transfer memory while preventing the transfer omission on the front end side of the transfer material. It was difficult.
The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an image forming apparatus that can prevent the occurrence of leak black spots or transfer memory black spots while preventing transfer defects on the front side of the transfer material at low cost. It is to provide.

In order to achieve the above-described object, according to one aspect of the present invention, an image carrier having a photosensitive layer on which a toner image is formed and a transfer nip through which a transfer material in a conveyance process passes are formed together with the image carrier. A transfer roller, a transfer bias power supply unit that applies a transfer bias to the transfer roller so that the toner image is transferred to the transfer material when the transfer material passes through the transfer nip, and a transfer material conveyance direction. A discharge device disposed downstream of the transfer nip, a discharge bias power supply unit that applies a discharge bias having a polarity opposite to the transfer bias to the discharge device, a transfer bias power supply unit, and a discharge bias power supply unit are controlled. Prior to the start of application, the neutralization bias power supply unit starts application of the neutralization bias, and after the leading edge of the transfer material reaches the transfer nip portion, Image forming apparatus the image boundary area and the image area is a control unit for stabilizing the output of the transfer bias until passes through the transfer nip portion is provided.
Preferably, the control unit terminates the application of the static elimination bias immediately after the output of the transfer bias is stabilized.

Preferably, in this aspect, the transfer bias power supply unit is one of a transfer bias mode for outputting a transfer bias, a reverse bias mode for outputting a reverse bias having a reverse polarity to the transfer bias, and an off mode having an output of zero. The control unit can change the operation of the transfer bias power supply unit from the reverse bias mode to the off mode before the tip of the transfer material reaches the transfer nip portion, and the transfer nip portion transfers the transfer material to the transfer nip portion. The operation of the transfer bias power supply unit is changed from the off mode to the transfer mode so that the output of the transfer bias rises after the leading edge of the toner reaches.
In this embodiment, the photosensitive layer of the image carrier is preferably made of amorphous silicon or an organic photoconductor.

  The above-described objects can be achieved by still another aspect of the present invention. For example, an image carrier having a photosensitive layer on which a toner image is formed, a transfer roller that forms a transfer nip portion with the image carrier through which a transfer material in a conveyance process passes, and the transfer material passes through the transfer nip portion. A transfer bias power supply unit for applying a transfer bias to the transfer roller, and a downstream of the transfer nip portion in the transfer material conveyance direction, so that the toner image is transferred to the transfer material. A discharge unit, a neutralization bias power supply unit that applies a neutralization bias having a polarity opposite to that of the transfer bias to the discharger, and a control unit that controls the transfer bias power supply unit and the neutralization bias power supply unit. Before starting the application of the transfer bias, to start the application of the static elimination bias to the static elimination bias power supply unit, An image forming apparatus characterized by causing the transfer bias power supply unit to start applying the transfer bias so that the output of the transfer bias rises after the end of the transfer material reaches the transfer nip portion. Provided.

  In this image forming apparatus, by applying the static elimination bias before starting the application of the transfer bias, the rise time of the transfer bias is shortened, and variations in the rise time are suppressed. For this reason, even if the transfer speed of the transfer material is high, the transfer bias reliably rises to a predetermined level while the non-image area of the transfer material, for example, 3 to 4 mm from the leading edge passes through the transfer nip portion.

  The rise time is shortened and the variation is suppressed in this way due to the effect of the neutralization bias having the opposite polarity to the transfer bias discharged in the vicinity of the transfer roller, and the output load increases when the transfer bias is applied. This is because the output voltage increases.

For this reason, the transfer bias output is not raised before the transfer material reaches the transfer nip portion, and when the region (image area) where the image of the transfer material is to be formed passes through the transfer nip portion, it is sufficient. It is possible to control so that an appropriate transfer bias is applied.
As a result, according to this image forming apparatus, the occurrence of leak black spots or transfer memory black spots is prevented, and transfer defects on the front end side of the transfer material are prevented. In addition, since this image forming apparatus can use a conventional transfer bias power supply unit, it is provided at a low cost without causing an increase in cost.

  Preferably, the transfer bias power supply unit is one of a transfer bias mode for outputting the transfer bias, a reverse bias mode for outputting a reverse bias having a reverse polarity to the transfer bias, and an off mode in which the output is zero. The control unit changes the operation of the transfer bias power supply unit from the reverse bias mode to the off mode before the leading edge of the transfer material reaches the transfer nip portion, and the transfer unit The operation of the transfer bias power supply unit is changed from the off mode to the transfer mode so that the transfer bias output rises after the leading edge of the transfer material reaches the nip portion.

According to this image forming apparatus, even when the operation of the transfer bias power supply unit is switched from the reverse bias mode to the transfer idle mode via the off mode, the transfer bias rise time is short and the variation thereof is small.
This is because the charge of the reverse bias charged to the transfer roller is difficult to come off when the transfer bias is turned off due to the effect of the neutralization bias, and therefore the transfer roller is charged with the opposite polarity to the transfer bias and in the vicinity of the transfer roller. This is because, due to the effect of the neutralizing bias having a reverse polarity to the discharged transfer bias, the output load further increases when the transfer bias is applied, and the output voltage of the transfer bias increases.
On the other hand, according to this image forming apparatus, by applying a reverse bias to the transfer roller in the reverse bias mode, undesired toner or the like adhering to the transfer roller is moved to the image carrier.
Therefore, according to this image forming apparatus, in addition to preventing the generation of black spots and transfer omission, backside contamination of the transfer material is prevented.

Preferably, the timing for ending application of the static elimination bias differs depending on the thickness of the transfer material.
According to this image forming apparatus, for example, when the transfer material is cardboard, the application of the static elimination bias is stopped when the transfer bias rises and stabilizes. Because the thick paper is strong, it is not necessary to apply a neutralization bias to separate the thick paper from the image carrier. By stopping the neutralization bias application, the toner particles are removed by the static elimination vise. Decrease in the adsorption power of is prevented. As a result, the toner particle fixing property is prevented from being lowered, and a good image is formed on cardboard.

On the other hand, according to this image forming apparatus, for example, when the transfer material is plain paper, the static elimination bias is continuously applied until the separation from the photosensitive drum is sufficiently performed. Since the stiffness of plain paper is weak, separation of the plain paper from the image carrier is promoted by removing the back surface of the plain paper by the charge removal bias.
Therefore, according to this image forming apparatus, in addition to preventing the occurrence of black spots and transfer omissions, it is possible to prevent the deterioration of the fixing property of thick paper or the separation failure of plain paper.

Preferably, the photosensitive layer of the image carrier is made of amorphous silicon.
This image forming apparatus is excellent in durability because the photosensitive layer of the image carrier is made of amorphous silicon and has high hardness.

Alternatively, the photosensitive layer of the image carrier is made of an organic photoconductor.
This image forming apparatus is inexpensive because an image carrier having a photosensitive layer made of an organic photoconductor is highly versatile.

  As described above, according to the present invention, it is possible to provide an image forming apparatus that can prevent the occurrence of leakage black spots, transfer memory black spots, or fogging, and also prevents transfer defects on the front side of the transfer material at low cost.

1 is a diagram illustrating a schematic configuration of a copier according to an embodiment of the present invention. FIG. 2 is an enlarged view showing an image forming mechanism of the copying machine of FIG. 1. FIG. 2 is a block diagram showing a schematic electrical configuration of the copier of FIG. 1. FIG. 2 is a block diagram illustrating a schematic configuration of a transfer bias power supply unit of the copying machine of FIG. 1. FIG. 6 is a timing chart for explaining the operation when the copying machine of FIG. 1 prints on plain paper. The upper part of FIG. 5 shows the timing when the transfer material passes through the transfer nip portion, and the middle part of FIG. The time change of the output of the bias power supply unit is shown, and the lower part of FIG. 5 shows the time change of the output of the transfer bias power supply unit. FIG. 7 is a timing chart for explaining the operation when the copying machine of FIG. 1 prints on thick paper. The upper part of FIG. 6 shows the timing when the transfer material passes through the transfer nip part, and the middle part of FIG. The time change of the output of the power supply unit is shown, and the lower part of FIG. 6 shows the time change of the output of the transfer bias power supply unit. FIG. 8 is a timing chart for explaining an operation when a conventional copying machine prints on thick paper. The upper part of FIG. 7 shows the timing when the transfer material passes through the transfer nip portion, and the middle part of FIG. The time change of the output of the unit is shown, and the lower part of FIG. 7 shows the time change of the output of the transfer bias power supply unit. 7 is a chart showing temporal changes in output of a transfer bias power supply unit when operating at the timing of FIG. 6. It is a chart which shows the time change of the output of a transfer bias power supply unit when it operate | moves at the timing of FIG.

Hereinafter, a copying machine 10 will be described with reference to the drawings as an image forming apparatus according to an embodiment of the present invention.
An image reading mechanism 14 is provided above the main housing 12 of the copying machine 10. A cover 16 is disposed on the image reading mechanism 14 so as to be openable and closable. Although not shown, the image reading mechanism 14 has a contact glass plate, and optically reads an image of a document in contact with the contact glass plate.

Paper cassettes 18 and 20 are detachably inserted into the lower portion of the main housing 12 from the front. In the paper feed cassettes 18 and 20, paper (transfer material) on which an image read by the image reading mechanism 14 is printed is stored.
A movable manual feed tray 22 is attached to the side of the main housing 12, and printing can be performed on a sheet placed on the manual feed tray 22.

An image forming mechanism 24 is provided at the center of the main housing 12, and an image is printed on a sheet by the image forming mechanism 24.
The main housing 12 is provided between the image reading mechanism 14 and the image forming mechanism 24 and is integrally provided with paper discharge trays 26 and 28, and the printed paper is placed on the paper discharge trays 26 and 28. Discharged. A movable paper discharge tray 30 is attached to the side of the main housing 12 of the copying machine 10 so that printed paper can be discharged to the paper discharge tray 30.

Inside the main housing 12, there is provided a paper conveyance path 32 extending from the paper feed cassettes 18, 20 and the manual feed tray 22 to the paper discharge trays 26, 28, 30 through the image forming mechanism 24.
Specifically, the conveyance path 32 includes a paper feed path 34 extending from the paper feed cassettes 18 and 20 and the manual feed tray 22 to the image forming mechanism 24, and a paper discharge path extending from the image forming mechanism 24 to the paper discharge trays 26, 28, and 30. 36 and a re-conveyance path 38 that connects the paper discharge path 36 and the paper supply path 34 for duplex printing.

  A plurality of roller pairs are arranged on the conveyance path 32 and a path switching member (not shown) is arranged. The paper is conveyed through the conveyance path 32 by cooperation between the roller pairs and the path switching member. .

FIG. 2 shows an enlarged schematic configuration of the image forming mechanism 24.
The image forming mechanism 24 has a photosensitive drum 40 as an image carrier. The photosensitive drum 40 includes a drum body 41 and a photosensitive layer 42 made of a-Si (amorphous silicon) formed on the surface of the drum body 41. Therefore, hereinafter, the photosensitive drum 40 is also referred to as an a-Si drum 40. The photosensitive drum 40 is driven to rotate clockwise as viewed in FIG. 2 by a motor (not shown).

  The image forming mechanism 24 has a charging roller 44 as a means (charging device) for uniformly charging the photosensitive layer 42 to positive polarity, and the charging roller 44 is disposed in the vicinity of the photosensitive drum 40. .

  Further, the image forming mechanism 24 exposes the photosensitive drum 40 in accordance with the image read by the image reading mechanism 14 and forms a latent image on the photosensitive layer 42 (exposure device) as a laser. It has a scanning unit 46. The laser scanning unit 46 irradiates the photosensitive layer 42 of the photosensitive drum 40 with laser light at a position downstream of the charging roller 44 in the rotational direction of the photosensitive drum 40.

Further, the image forming mechanism 24 has a developing unit 48 as a means (developing device) for developing the electrostatic latent image to form a toner image. The developing roller 50 constituting the developing unit 48 is located in the vicinity of the photosensitive drum 40 and is located downstream of the irradiation position of the laser beam. The developing roller 50 supplies positively charged toner particles corresponding to the electrostatic latent image to the surface of the photosensitive layer 42, whereby the toner is reversely developed on the photosensitive layer 42.
The developing unit 48 is supplied with a developer containing toner particles from the toner tank 52.

Furthermore, the image forming mechanism 24 includes a transfer roller 54 as a means (transfer device) for transferring the toner image on the photosensitive layer 42 to a sheet (transfer material). The transfer roller includes a metal shaft portion 55 and a roller portion 56 provided integrally and coaxially with the shaft portion 55.
The roller unit 56 is, for example, an electronic conductive type foamed sponge roller, and includes a base polymer made of EPDM (ethylene propylene diene rubber) and carbon particles dispersed in the base polymer and imparting conductivity to the roller unit 56. .

The transfer roller 54 is in contact with the photosensitive drum 40 while being urged by the urging means, and cooperates with the photosensitive drum 40 to form a transfer nip portion through which a sheet in the conveyance process passes. Yes. The transfer nip portion is positioned downstream of the developing roller 50 in the rotational direction of the photosensitive drum 40. A transfer bias power supply unit 57 is connected to the transfer roller 54.
The paper speed (conveyance speed) when passing through the transfer nip is, for example, in the range of 40 mm / sec to 500 mm / sec, and preferably in the range of 80 mm / sec to 460 mm / sec.

  Further, the image forming mechanism 24 includes a cleaning unit 58 as a means (cleaning device) for cleaning the photosensitive layer 42 of the photosensitive drum 40 after the toner image is transferred.

  The cleaning unit 58 includes a cleaning roller 60 and a cleaning blade 62 disposed in the vicinity of the photosensitive drum 40. The cleaning roller 60 and the cleaning blade 62 are located between the transfer nip portion and the charging roller 44 when viewed in the rotation direction of the photosensitive drum 40. The cleaning roller 60 is in sliding contact with the surface to polish the surface of the photosensitive layer 42, and the cleaning blade 62 scrapes off toner particles and the like from the surface of the photosensitive layer 42.

  The transfer nip portion constitutes a part of the transport path 32, and a registration roller pair 64 that also constitutes a part of the transport path 32 is provided upstream of the transfer nip portion of the transport path 32. Although not shown, a motor for rotationally driving the registration roller pair 64 is mechanically connected to the registration roller pair 64, and the registration roller pair 64 feeds the sheet to the transfer nip portion at a predetermined timing in a correct posture. It is.

  On the other hand, a fixing roller pair 66 that also constitutes a part of the transport path 32 is provided downstream of the transfer nip portion of the transport path 32. The fixing roller pair 66 is a means for heating the paper on which the toner image is transferred and fixing the toner image on the paper.

A corona discharger 68 is disposed between the transfer nip portion and the fixing roller pair 66. The corona discharger 68 constitutes a static elimination means (separation means) for neutralizing the charging on the back surface of the paper.
The corona discharger 68 is connected to a static elimination bias power supply unit 70.

FIG. 3 is a block diagram showing an electrical configuration of the copying machine 10.
The copying machine 10 includes a control unit 72 that executes control of the entire copying machine 10. The control unit 72 includes, for example, an MPU (microprocessor), a memory, a storage device, and the like.

Connected to the control unit 72 are a main switch 74 for a user or the like to turn on / off the main power and an operation panel 76 for inputting a command to the copier 10.
The control unit 72 is also connected to the image reading mechanism 14 and the image forming mechanism 24. The control unit 72 operates the image reading mechanism 14 and the image forming mechanism 24 based on a command input by a user or the like. Let That is, the operation of the charging roller 44, the laser scanning unit 46, the developing unit 48, the transfer bias power supply unit 57, and the static elimination bias power supply unit 70 is controlled by the control unit 72.

FIG. 4 is a block diagram showing a more detailed configuration of the transfer bias power supply unit 57 together with the control unit 72 and the transfer roller 54.
The transfer bias power supply unit 57 includes a control unit 78, and the control unit 78 operates the power supply unit 80 based on a command from the control unit 72 and applies a predetermined voltage to the transfer roller 54 at a predetermined timing. . In the present embodiment, the control unit 78 operates the power supply unit 80 by appropriately switching between the three modes of the transfer bias mode, the reverse bias mode, and the off mode.

In the transfer bias mode, a voltage (transfer bias) having a polarity opposite to the polarity of the toner image (toner particles) is applied to the transfer roller 54.
Here, the transfer bias power supply unit 57 is subjected to constant current control.

  On the other hand, in the reverse bias mode, a voltage having the same polarity as the toner image is applied to the transfer roller 54. In the off mode, the output of the power supply unit 80 is set to zero, and the current supplied to the transfer roller 54 is set to zero.

  In addition, in this embodiment, a static elimination bias (separation bias) is applied from the static elimination bias power supply unit 70 to the corona discharger 68 at a predetermined timing. The neutralizing bias has a polarity opposite to that of the transfer bias, and thus has the same polarity as that of the toner image. In the present embodiment, the corona discharger 68 is applied with a positive charge eliminating bias having a polarity opposite to the transfer bias.

Hereinafter, the operation timing of the transfer bias power supply unit 57 and the neutralization bias power supply unit 70 when plain paper is used as the transfer material will be described with reference to FIG.
The upper part of FIG. 5 shows the presence or absence of the transfer material in the transfer nip part, that is, the timing when the transfer material passes through the transfer nip part, and the middle part of FIG. 5 shows the time change of the output of the static elimination bias power supply unit 70. The lower part shows the time change of the output of the transfer bias power supply unit 57.

  As shown in FIG. 5, the transfer bias power supply unit 57 is operated in the reverse bias mode sufficiently before the transfer material reaches the transfer nip portion. The operation of the transfer bias power supply unit 57 is switched from the reverse bias mode to the off mode immediately before the transfer material reaches the transfer nip portion, and the transfer bias output rises immediately after the transfer material reaches the transfer nip portion. The mode is switched from the off mode to the transfer bias mode.

  On the other hand, the neutralization bias power supply unit 70 starts the neutralization bias output before the operation of the transfer bias power supply unit 57 is switched from the reverse bias mode to the off mode, and the transfer material is sufficiently separated from the photosensitive drum. When it passes to the position where it stops, the neutralization bias output stops.

Next, the operation timing of the transfer bias power supply unit 57 and the neutralization bias power supply unit 70 when using thick paper as the transfer material will be described with reference to FIG.
The upper stage, middle stage, and lower stage of FIG. 6 are the timing of the transfer material passing through the transfer nip portion, the time change of the output of the neutralization bias power supply unit 70, and the output of the transfer bias power supply unit 57, as in FIG. Each time change is shown.
As apparent from FIG. 6, the thick paper is different from the plain paper in that the application of the neutralizing bias is stopped immediately after the transfer bias is stabilized.

Furthermore, the operation timing of the transfer bias power supply unit 57 and the neutralization bias power supply unit 70 when using thick paper as the transfer material in the conventional image forming apparatus will be described with reference to FIG.
The upper, middle, and lower stages in FIG. 7 are the same as those in FIGS. 5 and 6, the timing at which the transfer material passes through the transfer nip portion, the temporal change in the output of the static elimination bias power supply unit 70, and the transfer bias power supply unit 57. The time change of the output of each is shown.
As can be seen from FIG. 7, when printing on thick paper by a conventional image forming apparatus, the charge removal bias is not applied.

  Here, the chart shown in FIG. 8 shows the transfer when the transfer bias power supply unit 57 and the neutralization bias power supply unit 70 are operated with the pattern shown in FIG. 6 when printing on thick paper using the copying machine 10. The time change of the output voltage of the bias power supply unit 57 is shown.

  The transport speed of the thick paper is 300 mm / sec, and the off mode period is 10 msec. Accordingly, during the off mode, the cardboard is transported by about 3 mm. Then, about 8.3 msec after the operation of the transfer bias power supply unit 57 was switched from the off mode to the transfer bias mode, the application of the static elimination bias was stopped. Therefore, the cardboard was transported by about 2.5 mm after the operation of the transfer bias power supply unit 57 was switched from the off mode to the transfer bias mode until the application of the neutralizing bias was stopped.

  On the other hand, the chart shown in FIG. 9 shows the time change of the output voltage of the transfer bias power supply unit 57 when the transfer bias power supply unit 57 and the neutralization bias power supply unit 70 are operated in the pattern shown in FIG. Yes.

  As shown in FIGS. 8 and 9, according to the copying machine 10 of the above-described embodiment, the rising time of the transfer bias can be obtained by applying the static elimination bias before starting to apply the transfer bias. And the variation in the rise time is suppressed. Specifically, when the application of the transfer bias was started without applying the static elimination bias, the transfer bias rising time was 32.8 msec, but the application of the transfer bias was started after the static elimination bias was applied. In this case, the rise time of the transfer bias was 24.8 msec.

  For this reason, even if the transfer speed of the transfer material is high, the transfer bias reliably rises to a predetermined level when the non-image area of the transfer material 3 to 4 mm from the leading edge passes through the transfer nip portion.

In this way, the rise time is shortened and the variation is suppressed due to the effect of the neutralizing bias having the opposite polarity to the transfer bias discharged in the vicinity of the transfer roller, and the output load increases when the transfer bias is applied. This is because the output voltage increases. In addition, when the transfer roller is reverse biased, the transfer roller is opposite in polarity to the transfer bias because the charge of the reverse bias charged to the transfer roller is difficult to escape when the transfer bias is turned off due to the neutralization bias. This is because the output load is further increased when the transfer bias is applied, and the output voltage of the transfer bias rises due to the effect of the neutralizing bias that is charged to the opposite side of the transfer bias and discharged in the vicinity of the transfer roller. is there.
Therefore, the transfer bias is not applied to the transfer roller so that the transfer bias output rises before the transfer material reaches the transfer nip portion, and there is an area (image area) where an image of the transfer material is to be formed. When passing through the transfer nip portion, a sufficient transfer bias is applied.

  As a result, according to the copying machine 10, even when the photosensitive drum 40 having the photosensitive layer 42 made of a-Si is used, the occurrence of leak black spots is prevented and the transfer on the leading end side of the transfer material is prevented. Omission is prevented. Further, since the copying machine 10 can use a conventional transfer bias power supply unit, it is provided at a low cost without causing an increase in cost.

  According to the copying machine 10 of the embodiment described above, even when the operation of the transfer bias power supply unit 57 is switched from the reverse bias mode to the transfer idle mode through the off mode, the transfer bias rise time is short and The variation is small.

On the other hand, according to the copying machine 10, undesired toner particles adhering to the transfer roller 54 are moved to the photosensitive drum 40 by applying a reverse bias to the transfer roller 54 in the reverse bias mode. .
Therefore, according to this copying machine 10, in addition to preventing the occurrence of leaked black spots and transfer omission, backside contamination of the transfer material is prevented.

  Further, according to the copying machine 10 of the above-described embodiment, for example, when the transfer material is thick paper, the application of the static elimination bias is stopped when the transfer bias rises and becomes stable. Since the stiffness of the thick paper is strong, it is not necessary to apply a static elimination bias in order to separate the thick paper from the photosensitive drum 40. By stopping the application of the static elimination bias, the thick paper due to the static elimination bias removing the transfer material. A reduction in the adsorbing power of the toner particles on the toner is prevented. As a result, the toner fixing property is prevented from being lowered, and a good image is formed on the cardboard.

On the other hand, according to the copying machine 10, for example, when the transfer material is plain paper, the static elimination bias is continuously applied until the separation from the photosensitive drum is sufficiently performed. Since the stiffness of the plain paper is weak, separation of the plain paper from the photosensitive drum 40 is promoted by eliminating the back surface of the plain paper with the static elimination bias.
Therefore, according to the copying machine 10, in addition to preventing the occurrence of black spots and missing transfer, it is possible to prevent a decrease in the fixing property of thick paper or a separation failure of plain paper.

  The thick paper can be defined by the thickness of the paper. For example, it can be defined as a paper having a mass per unit area (weight per unit area) of 120 g / mm 2 or more. Alternatively, the cardboard can be defined as a paper having a stiffness that is separable from the photosensitive drum 40 as an image carrier without applying a static elimination bias.

  Furthermore, the copying machine 10 of the above-described embodiment is excellent in durability because the photosensitive layer 42 of the photosensitive drum 40 is made of amorphous silicon and has high hardness.

The present invention is not limited to the above-described embodiment, and various modifications can be made.
For example, even when printing on plain paper using the copying machine 10, if the plain paper can be separated from the photosensitive drum 40, the application of the neutralizing bias may be stopped immediately after the transfer bias is stabilized. That is, it is sufficient that the static elimination bias is applied until at least the transfer bias is stabilized.

In the above-described embodiment, the photosensitive layer 42 is formed of a-Si, but may be formed of an organic photoconductor (OPC). The photosensitive drum 40 in which the photosensitive layer 42 is made of OPC is highly versatile and inexpensive.
Even in the copying machine 10 using the photosensitive drum 40 having the photosensitive layer 42 made of OPC, the transfer bias is predetermined when the non-image area of the transfer material 3 to 4 mm from the leading edge passes through the transfer nip portion. Get up to the level without fail. For this reason, generation of black spots on the transfer memory is prevented and transfer omission on the leading end side of the transfer material is prevented.

In the above-described embodiment, the copying machine 10 has been described as the image forming apparatus. However, the image forming apparatus may be a printer or a facsimile.
The present invention has been described above with specific embodiments. However, the present invention is not limited to these descriptions, and a configuration obtained by appropriately combining configurations described as different embodiments is also a technique of the present invention. A configuration obtained by appropriately combining a conventionally known technique with the configuration obtained in this manner is also included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Copy machine 14 Image reading mechanism 24 Image forming mechanism 32 Conveyance path 40 Photosensitive drum 42 Photosensitive layer 54 Transfer roller 57 Transfer bias power supply unit 68 Corona discharger (discharger)
70 Static elimination bias power supply unit 72 Control unit

Claims (5)

An image carrier having a photosensitive layer on which a toner image is formed;
A transfer roller that, together with the image carrier, forms a transfer nip through which the transfer material in the conveyance process passes;
A transfer bias power supply unit that applies a transfer bias to the transfer roller so that the toner image is transferred to the transfer material when the transfer material passes through the transfer nip portion;
A discharger disposed downstream of the transfer nip portion as viewed in the conveying direction of the transfer material;
A neutralizing bias power supply unit that applies a neutralizing bias having a polarity opposite to that of the transfer bias to the discharger;
The transfer bias power supply unit and the discharge bias power supply unit are controlled to start application of the charge removal bias to the charge removal bias power supply unit before the start of application of the transfer bias, and the leading edge of the transfer material is the transfer material An image forming apparatus comprising: a control unit that stabilizes an output of the transfer bias during a period from reaching the nip portion until a boundary between the non-image region and the image region of the transfer material passes through the transfer nip portion. The image forming apparatus according to claim 1.
The image forming apparatus, wherein the control unit terminates the application of the neutralizing bias immediately after the output of the transfer bias is stabilized. The image forming apparatus according to claim 1, wherein
The transfer bias power supply unit includes:
It is possible to operate in one of a transfer bias mode for outputting the transfer bias, a reverse bias mode for outputting a reverse bias having a reverse polarity to the transfer bias, and an off mode in which the output is zero.
The control unit is
Before the tip of the transfer material reaches the transfer nip portion, the operation of the transfer bias power supply unit is changed from the reverse bias mode to the off mode,
The image forming is characterized in that the operation of the transfer bias power supply unit is changed from the off mode to the transfer mode so that the output of the transfer bias rises after the leading edge of the transfer material reaches the transfer nip portion. apparatus. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus, wherein the photosensitive layer of the image carrier is made of amorphous silicon. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus, wherein the photosensitive layer of the image carrier is made of an organic photoconductor.

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