Image forming apparatus

An image forming apparatus includes an image carrier, a transfer member and a cleaning member. The image forming apparatus: determines whether the electric resistance of a recording medium is low or high; applies a first transfer bias to the transfer member if the electric resistance is low; applies a second transfer bias to the transfer member if the electric resistance is high; applies a first cleaning bias to the cleaning member if the electric resistance is low; and applies a second cleaning bias to the cleaning member so that a potential difference between a portion of a surface of the image carrier against which the cleaning member is rubbed and the cleaning member becomes smaller than that in a case of applying the first cleaning bias if the electric resistance is high.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2012-218496 filed on Sep. 28, 2012, the contents of which are incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to an electrophotographical image forming apparatus.

As an electrophotographical image forming apparatus, there is known a printer which includes photosensitive elements for carrying developer images, and transfer members for transferring the developer images formed on the photosensitive elements onto a paper sheet.

For example, there has been proposed a color printer which includes four photosensitive drums that are provided corresponding to colors of black, yellow, magenta, and cyan, respectively, a conveyor belt that conveys paper sheets so that the paper sheets come into contact with all photosensitive drums, and four transfer rollers that are provided corresponding to the four photosensitive drums, respectively, so that the conveyor belt is interposed between the transfer rollers and corresponding photosensitive drums.

In this color printer, a resistance value between the most upstream photosensitive drum and a transfer roller facing the corresponding photosensitive drum is measured, and a target value for an electric current to flow between the photosensitive drum and the transfer roller (a transfer current target value) is set based on a resistance value before a paper sheet is conveyed and a resistance value when a paper sheet is being conveyed.

Then, a voltage to be applied between the photosensitive drum and the transfer roller is controlled such that the electric charge to flow between the photosensitive drum and the transfer roller approximates to the transfer current target value.

SUMMARY

However, in cases where the resistance value of a paper sheet is large, such as a case where the humidity of the surroundings is low, the absolute value of the voltage to be applied between the photosensitive drum and the transfer roller is increased in order to approximate the electric current to flow between the photosensitive drum and the transfer roller to the transfer current target value.

In this case, due to the electric current flowing between the photosensitive drum and the transfer roller, the charged state of developer may change between the photosensitive drum and the transfer roller. Specifically, positively charged developer may change to a rarely charged state (uncharged state) or a negatively charged state.

If the developer having changed in the charged state remains on the surface of the photosensitive drum, it becomes difficult to electrostatically clean the surface of the photosensitive drum.

Therefore, an object of an aspect of the present disclosure is to provide an image forming apparatus capable of reliably removing substances attached to an image carrier by a cleaning member.

The aspect of the present disclosure provides the following arrangements:

An image forming apparatus comprising:

an image carrier configured to carry a developer image;

a transfer member configured to face the image carrier and transfer the developer image from the image carrier onto a recording medium;

a cleaning member configured to rub against the image carrier, and clean the image carrier of substances after the developer image is transferred onto the recording medium;

a control device configured to:

determine whether the electric resistance of the recording medium is comparatively low or high;

apply a first transfer bias to the transfer member when it is determined that the electric resistance of the recording medium is comparatively low;

apply a second transfer bias having the same polarity as that of the first transfer bias and having an absolute value larger than that of the first transfer bias, to the transfer member when it is determined that the electric resistance of the recording medium is comparatively high;

apply a first cleaning bias to the cleaning member when it is determined that the electric resistance of the recording medium is comparatively low; and

apply a second cleaning bias to the cleaning member so that a potential difference between a portion of a surface of the image carrier against which the cleaning member rubs and the cleaning member becomes smaller than that in a case of applying the first cleaning bias to the cleaning member when it is determined that the electric resistance of the recording medium is comparatively high.

An image forming apparatus comprising:

an image carrier configured to carry a developer image;

a transfer member configured to face the image carrier and transfer the developer image from the image carrier onto a recording medium;

a cleaning member configured to be rubbed against the image carrier, and clean substances attached to the image carrier after the developer image is transferred onto the recording medium;

a humidity sensor configured to measure relative humidity;

a control device configured to:apply a first transfer bias to the transfer member when the relative humidity measured by the humidity sensor is higher than predetermined relative humidity;

apply a second transfer bias having the same polarity as that of the first transfer bias and having an absolute value larger than that of the first transfer bias, to the transfer member the relative humidity measured by the humidity sensor is equal to the predetermined relative humidity or less;

apply a first cleaning bias to the cleaning member the relative humidity measured by the humidity sensor is higher than the predetermined relative humidity; and

apply a second cleaning bias to the cleaning member so that a potential difference between a portion of a surface of the image carrier against which the cleaning member is rubbed and the cleaning member becomes smaller than that in a case of applying the first cleaning bias the relative humidity measured by the humidity sensor is equal to the predetermined relative humidity or less.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

1. Overall Configuration of Printer

As shown inFIG. 1, a printer1which is an example of an image forming apparatus is a tandem type direct color printer which is horizontally installed.

In the following description, when directions of the printer1are stated, the upper side and lower side of the printer refer to a state where the printer1is horizontally installed. That is, the upper side of the drawing sheet ofFIG. 1is referred to as the upper side of the printer1(one side in a vertical direction (a first direction)) and the lower side of the drawing sheet ofFIG. 1is referred to as the lower side of the printer1(the other side in the vertical direction). Further, the left side of the drawing sheet ofFIG. 1is referred to as the front side of the printer1(one side in a front-rear direction (a second direction)), and the right side of the drawing sheet ofFIG. 1is referred to as the rear side of the printer1(the other side in the front-rear direction). Furthermore, the left side and right side of the printer1refer to the state of the printer as seen from the front side. That is, a direction toward a viewer ofFIG. 1is referred to as the right side of the printer (one side in a left-right direction (a third direction)), and a direction away from the viewer ofFIG. 1is referred to as the left side of the printer (the other side in the left-right direction).

The printer1includes a main body casing2having substantially a box shape. At an upper end portion of the main body casing2, a top cover4for opening and closing a main body opening3is provided to be able to swing on a rear end portion of the front cover. The printer1includes a plurality of (four) process cartridges5, a plurality of (four) LED units6, a transfer unit7, and a fixing unit8which is an example of a fixing device.

All process cartridges5are removably installed in parallel at intervals inside the main body casing2. The plurality of (four) process cartridges5correspond to a plurality of (four) colors (black, yellow, magenta, and cyan), respectively.

Each process cartridge5includes a drum cartridge9, and a developing cartridge10that is removably mounted on the drum cartridge9.

The drum cartridge9includes a photosensitive drum11which is an example of an image carrier, a scorotron charger12, and a drum cleaning roller24which is an example of a cleaning member.

The photosensitive drum11is formed in a substantially cylindrical shape long in a left-right direction (longitudinal direction), and is rotatably provided at the rear end portion of the drum cartridge9.

The scorotron charger12is disposed to face the upper rear side of the photosensitive drum11.

The drum cleaning roller24is disposed below the scorotron charger12behind the photosensitive drum11. The drum cleaning roller24comes into contact with the photosensitive drum11from the rear side. The drum cleaning roller24is formed substantially in a columnar shape extending in the left-right direction.

The developing cartridge10includes a developing roller13, and a feeding roller14for feeding toner to the developing roller13.

The developing roller13is supported at a lower end portion of the developing cartridge10such that it is rotatable and is exposed to the rear side. The developing roller13comes into contact with the photosensitive drum11from the upper front side. The developing roller13is formed substantially in a columnar shape extending in the left-right direction.

The feeding roller14is rotatably supported at the upper front side of the developing roller13such that the feeding roller is in contact with the developing roller13. The feeding roller14is formed substantially in a columnar shape extending in the left-right direction.

The developing cartridge10includes a layer-thickness regulating blade15for regulating the thickness of toner fed on the developing roller13. Further, in a portion of the developing cartridge10on the developing roller13and the feeding roller14, toner is stored as an example of developer.

The plurality of LED units6is supported on the top cover4so as to face the tops of the photosensitive drums11of the plurality of process cartridges5, respectively.

The transfer unit7is disposed to face the lower sides of the plurality of process cartridges5. The transfer unit7includes a driving roller16, a driven roller17, a conveyor belt18, and a plurality of (four) transfer rollers19which is examples of a transfer member.

The driving roller16is rotatably supported at the rear end portion of the transfer unit7.

The driven roller17is rotatably supported at the front end portion of the transfer unit7.

The conveyor belt18is disposed around the driving roller16and the driven roller17such that the upper portion of the conveyor belt18be in contact with all photosensitive drums11. Driving of the driving roller16and following of the driving roller16cause the conveyor belt18to circulate such that the upper portion of the conveyor belt18moves from the front side toward the rear side.

The plurality of transfer rollers19is disposed below the plurality of photosensitive drums11such that they face the photosensitive drums11, respectively, with the upper portion of the conveyor belt18interposed therebetween.

The fixing unit8is disposed to face the rear side of the transfer unit7. The fixing unit8includes a heating roller20, and a pressing roller21that faces the heating roller20.

Then, the toner contained in the developing cartridges10is triboelectrically and positively charged between the feeding rollers14and the developing rollers13, and is carried, as thin layers having a constant thickness, on the development rollers13by the layer-thickness regulating blades15.

Meanwhile, the surfaces of the photosensitive drums11are uniformly and positively charged by the scorotron chargers12, and then are exposed based on predetermined image data by the LED units6. As a result, on the surfaces of the photosensitive drums11, electrostatic latent images based on the image data are formed. Then, the toner carried on the developing rollers13is fed to the electrostatic latent images formed on the surfaces of the photosensitive drums11, whereby toner images (developer images) are carried on the surfaces of the photosensitive drums11.

Paper sheets P which are examples of a recording medium are stored in a paper feeding tray22provided at the bottom of the main body casing2, and are conveyed to make a U-turn toward the upper rear side by various rollers, and are fed one by one into the gaps between the photosensitive drums11and the conveyor belt18at predetermined timings. Then, each paper sheet is conveyed from the front side toward the rear side through the gaps between all the photosensitive drums11and all the transfer rollers19by the conveyor belt18. At this time, if transfer biases (to be described below) are applied to the transfer rollers19, the toner images are transferred onto a paper sheet P.

Then, when the paper sheet P passes through the gap between the heating roller20and the pressing roller21, the paper sheet P is heated while being pressed. At this time, the toner image is thermally fixed on the paper sheet P.

Thereafter, the paper sheet P is conveyed to make a U-turn toward the upper front side, and is discharged to a paper discharge tray23provided at the top cover4.

2. Details of Photosensitive Drums, Transfer Rollers, and Drum Cleaning Rollers

As shown inFIG. 2, each photosensitive drum11includes a main drum body31, and a shaft32.

The main drum body31is made of a metal substantially in a cylindrical shape extending in the left-right direction. On the peripheral surface of the main drum body31, a photosensitive layer is formed. In both end portions of the main drum body31in the left-right direction, brush members (not shown) are relatively rotatably fit, respectively.

The shaft32is made of a metal substantially in a columnar shape extending along the central axis of the main drum body31. The shaft32is supported such that it passes through the radial centers of the brush members (not shown) and cannot rotate with respect to the brush members (not shown). The shaft32is electrically connected to the inner surface of the main drum body31through an electrically conductive member (not shown) made of a metal. Further, the shaft32is earthed (grounded) in the main body casing2.

Then, each photosensitive drums11is rotated, counterclockwise as seen in a right side view, for example, at a peripheral speed of, for example, 165 mm/s.

Each transfer roller19includes a transfer roller shaft33and a main transfer roller body34.

The transfer roller shaft33is made of a metal substantially in a columnar shape extending in the left-right direction.

The main transfer roller body34is made of an electrically conductive resin material and so on, substantially in a cylindrical shape extending in the left-right direction, such that the main transfer roller body covers the transfer roller shaft33so as to expose both end portions of the transfer roller shaft33in the left-right direction.

(3) Drum Cleaning Rollers

Each drum cleaning roller24includes a drum-cleaning-roller shaft35and a main drum-cleaning-roller body36.

The drum-cleaning-roller shaft35is made of a metal substantially in a columnar shape extending in the left-right direction.

The drum-cleaning-roller body36is made of foam of silicon resin or urethane resin, or the like having semiconductivity. The main drum-cleaning-roller body36is formed substantially in a cylindrical shape extending in the left-right direction, such that the main drum-cleaning-roller body covers the drum-cleaning-roller shaft35so as to expose both end portions of the drum-cleaning-roller shaft35in the left-right direction.

Then, each drum cleaning roller24is rotated, clockwise as seen in a right side view, at a peripheral speed of, for example, 219 mm/s faster than the peripheral speed of the photosensitive drums11. That is, the drum cleaning rollers20are rubbed against the photosensitive drums11.

3. Electrical Configuration of Printer

Inside the main body casing2, a power supply board41, a humidity sensor43, and a control device42are provided.

The power supply board41includes a power supply44, a transfer circuit45for supplying electric power to the transfer rollers19, and a drum cleaning circuit46for supplying electric power to the drum cleaning rollers24.

The power supply44is electrically connected to the transfer circuit45and the drum cleaning circuit46through wiring lines.

The transfer circuit45is electrically connected to the transfer roller shafts33of the transfer rollers19through wiring lines. The transfer circuit45adjusts a voltage from the power supply board41to predetermined transfer biases based on control of the control device42, and applies the predetermined transfer biases to the transfer roller shafts33.

The drum cleaning circuit46is electrically connected to the drum-cleaning-roller shafts35of the drum cleaning rollers24through wiring lines. The drum cleaning circuit46adjusts a voltage from the power supply44to predetermined drum cleaning biases based on control of the control device42, and applies the predetermined drum cleaning biases to the drum-cleaning-roller shafts35.

The humidity sensor43is a sensor for measuring the relative humidity of the inside of the main body casing2, and is electrically connected to the control device42through a signal line.

The control device42is configured by a CPU, a memory, and so on. The control device42is a component which is implemented in software by program processing of the CPU, and includes a transfer bias control unit47that is an example of a first control unit for controlling the transfer circuit45, and a drum-cleaning-bias control unit48that is an example of a second control unit for controlling the drum cleaning circuit46.

In the memory of the control device42, a transfer bias table, a drum cleaning bias table and a program are stored. In the transfer bias table, set values for transfer currents are stored corresponding to relative humidity levels. In the drum cleaning bias table, set values for the drum cleaning biases are stored corresponding to relative humidity levels.

4. Image Forming Operation

(1) Setting of Transfer Currents and Drum Cleaning Biases

When the above described image forming operation is performed, first, the transfer biases are set in the transfer bias control unit47, and at the same time, the drum cleaning biases are set in the drum-cleaning-bias control unit48.

The transfer bias control unit47sets the transfer currents corresponding to the relative humidity of the inside of the main body casing2measured by the humidity sensor43, with reference to the transfer bias table.

The drum-cleaning-bias control unit48sets the drum cleaning biases corresponding to the relative humidity of the inside of the main body casing2measured by the humidity sensor43, with reference to the drum cleaning bias table.

(1-1) Case of Normal Humidity Environment

For example, in a case where the relative humidity of the inside of the main body casing2exceeds 30% and is less than 60%, the transfer bias control unit47and the drum-cleaning-bias control unit48determine that the electric resistance of a paper sheet P is comparatively low.

Then, the transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to black, to, for example, −14 μA.

The transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to yellow, to, for example, −11 μA.

Further, the transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to magenta, to, for example, −13 μA.

Furthermore, the transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to cyan, to, for example, −14 μA.

The drum-cleaning-bias control unit48sets the drum cleaning biases to be applied to all drum cleaning rollers24such that the potential differences from the surfaces of the photosensitive drums11become, for example, −300 V. That is, to the rearmost drum cleaning roller24, a drum cleaning bias (a first cleaning bias) for making a potential difference from the surface of a corresponding photosensitive drum11, for example, −300 V is applied.

(1-2) Case of Low Humidity Environment

For example, in a case where the relative humidity of the inside of the main body casing2is less than 30%, the transfer bias control unit47and the drum-cleaning-bias control unit48determine that the electric resistance of a paper sheet P is comparatively high.

Then, the transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to black, to, for example, −15 μA.

The transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to yellow, to, for example, −11 μA.

Further, the transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to magenta, to, for example, −14 μA.

Furthermore, the transfer bias control unit47sets a transfer current to flow between a photosensitive drum11and a transfer roller19corresponding to cyan, to, for example, −15 μA.

That is, in a case of determining that the electric resistance of a paper sheet P is comparatively high, the transfer bias control unit47sets the transfer currents to flow between the photosensitive drums11and the transfer rollers19, to transfer currents whose absolute values are larger than those in a case of determining that the electric resistance of a paper sheet P is comparatively low.

The drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby setting the potential of a drum cleaning roller24corresponding to the rearmost photosensitive drum11(the photosensitive drum11of cyan) to a float potential, such that the potential difference between the rearmost drum cleaning roller24and the surface of the rearmost photosensitive drum11becomes 0 V. That is, to the rearmost drum cleaning roller24, a drum cleaning bias (a second cleaning bias) equal to the potential of the surface of the rearmost photosensitive drum11is applied.

The drum-cleaning-bias control unit48sets the drum cleaning biases for drum cleaning rollers24corresponding to photosensitive drums other than the rearmost photosensitive drum11such that the potential differences from the surfaces of the corresponding photosensitive drums11become, for example, −300 V.

(2) Transferring and Cleaning Operations

(2-1) Transferring and Cleaning Operations in Normal Humidity Environment

When the above described image forming operation is performed, for example, if the relative humidity of the inside of the main body casing2exceeds 30% and is less than 60%, the transfer bias control unit47controls the transfer circuit45, thereby applying transfer biases (first transfer biases) to the plurality of transfer rollers19, respectively, such that the above described transfer currents constantly flow between the transfer rollers19and corresponding photosensitive drums11(constant current control).

The drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby applying the above described drum cleaning biases to the plurality of drum cleaning rollers24, respectively.

Then, when a paper sheet P passes through portions where the photosensitive drums11and the transfer rollers19face each other, toner images carried on the photosensitive drums11are transferred onto the paper sheet P.

At this time, on the peripheral surfaces of the photosensitive drums11, toner which is an example of attached substances having not been transferred onto the paper sheet P (hereinafter, referred to as post-transfer residual toner) may remain.

Thereafter, rotating of the photosensitive drums11causes the post-transfer residual toner remaining on the peripheral surfaces of the photosensitive drums11to face corresponding drum cleaning rollers24.

Then, the post-transfer residual toner is electrostatically held on the peripheral surfaces of the corresponding drum cleaning rollers24by the drum cleaning biases.

(2-2) Transferring and Cleaning Operations in Low Humidity Environment

When the above described image forming operation is performed, for example, if the relative humidity of the inside of the main body casing2is less than 30%, the transfer bias control unit47controls the transfer circuit45, thereby applying transfer biases (second transfer biases) to the plurality of transfer rollers19, respectively, such that the above described transfer currents constantly flow between the transfer rollers19and corresponding photosensitive drums11(constant current control).

The drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby setting the drum cleaning bias for the drum cleaning roller24corresponding to the rearmost photosensitive drum11to the float potential, and applying the above described drum cleaning biases to the other drum cleaning rollers24, respectively.

Further, the drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby applying the above described drum cleaning biases to the plurality of drum cleaning rollers24, respectively.

The drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby changing the drum cleaning bias to be applied to the rearmost drum cleaning roller24to the float potential before a paper sheet P is fed between the rearmost photosensitive drum11and the rearmost transfer roller19.

Then, when a paper sheet P passes through portions where the photosensitive drums11and the transfer rollers19face each other, toner images carried on the photosensitive drums11are transferred onto the paper sheet P.

Thereafter, rotating of the photosensitive drums11causes the post-transfer residual toner remaining on the peripheral surfaces of the photosensitive drums11to face corresponding drum cleaning rollers24.

Then, the post-transfer residual toner remaining on the surface of the rearmost photosensitive drum11is scraped off the surface of the rearmost photosensitive drum11by rubbing a corresponding drum cleaning roller24against the surface of the rearmost photosensitive drum11, and is held on the peripheral surface of the corresponding drum cleaning roller24.

The post-transfer residual toner remaining on the peripheral surfaces of photosensitive drums11other than the rearmost photosensitive drum11is electrostatically held on the peripheral surfaces of corresponding drum cleaning rollers24by the drum cleaning biases.

(1) According to the printer1, in a case where it is expected that the relative humidity of the inside of the main body casing2is low and the electric resistance of a paper sheet P is comparatively high, the transfer bias control unit47increases the set values for the transfer currents.

Then, in the portions where the photosensitive drums11and the transfer rollers19face each other, the transfer currents excessively act on the toner, whereby the charged state of the toner becomes likely to vary.

Especially, in a tandem type direct color printer like the printer1, a transfer current to flow between a photosensitive drum11and a transfer roller19located on a more downstream side (more rear side) in the conveyance direction of a paper sheet P (a direction from the front side toward the rear side) is set to be larger than a transfer current to flow between a photosensitive drum11and a transfer roller19located on a more upstream side (more front side).

For this reason, in a portion where the rearmost photosensitive drum11and the rearmost transfer roller19face each other, the charged state of the toner becomes likely to vary.

Therefore, the drum-cleaning-bias control unit48sets a drum cleaning bias for a drum cleaning roller24corresponding to the rearmost photosensitive drum11to the float potential such that the potential difference between the surface of the rearmost photosensitive drum11and the rearmost drum cleaning roller24becomes substantially 0 V.

Therefore, even if the charged state of post-transfer residual toner attached to the rearmost photosensitive drum11varies, it is possible to physically scrape the post-transfer residual toner off the rearmost photosensitive drum11by rubbing the rearmost drum cleaning roller24against the rearmost photosensitive drum11.

As a result, it is possible to reliably remove the post-transfer residual toner attached to the photosensitive drums11by the drum cleaning rollers24.

(2) According to the printer1, the transfer bias control unit47controls the transfer biases such that the transfer currents become constant.

For this reason, it is possible to prevent the transfer currents from varying due to a change in the electric resistance of a paper sheet P.

As a result, it is possible to reliably transfer toner images on the surfaces of the photosensitive drums11onto a paper sheet P.

(3) According to the printer1, in a case where the relative humidity measured by the humidity sensor43is less than 30%, the drum-cleaning-bias control unit48determines that the electric resistance of a paper sheet P is comparatively high.

For this reason, it is possible to smoothly reduce the potential differences between the surfaces of the photosensitive drums11and the drum cleaning rollers24by predicting the electric resistance of a paper sheet P from the relative humidity measured by the humidity sensor43.

In a case where the relative humidity is 30% or less, and thus the charged state of toner is likely to vary, it is possible to effectively clean post-transfer residual toner off the photosensitive drums11.

(4) According to the printer1, the drum-cleaning-bias control unit48sets the drum cleaning bias for the rearmost drum cleaning roller24to the float potential before a paper sheet P is fed between the rearmost photosensitive drum11and the rearmost transfer roller19.

For this reason, it is possible to reliably set the drum cleaning bias for the rearmost drum cleaning roller24to the float potential before the rearmost drum cleaning roller24acts on post-transfer residual toner.

Therefore, it is possible to more reliably remove post-transfer residual toner attached to the surface of the rearmost photosensitive drum11.

(5) According to the printer1, the cleaning bias for the rearmost drum cleaning roller24is set to the float potential such that the potential difference between the surface of the rearmost photosensitive drum11and the rearmost drum cleaning roller24becomes 0 V.

For this reason, it is possible to reliably reduce the potential difference between the surface of the rearmost photosensitive drum11and the rearmost drum cleaning roller24by a simple configuration.

As a result, it is possible to more reliably remove post-transfer residual toner attached to the surface of the rearmost photosensitive drum11.

(6) According to the printer1, the drum-cleaning-bias control unit48controls only the drum cleaning bias to be applied to a drum cleaning roller24corresponding to the rearmost photosensitive drum11.

For this reason, it is possible to reliably remove post-transfer residual toner from the rearmost photosensitive drum11to which post-transfer residual toner whose charged state having changed is especially likely to attach.

5. Second Embodiment

A second embodiment of the printer1will be described with reference toFIG. 2. In the second embodiment, members identical to those of the above described first embodiment are denoted by the same reference symbols, and will not be described.

(1) Outline of Second Embodiment

In the above described first embodiment, for example, in a case where the relative humidity of the inside of the main body casing2is 30% or less, the transfer bias control unit47and the drum-cleaning-bias control unit48determine that the electric resistance of a paper sheet P is comparatively high.

In contrast, in the second embodiment, in a case where a print job has been input from an external personal computer51or the like to the printer1as shown by a virtual line inFIG. 2, if the length of a paper sheet P in the left-right direction designated in the print job is in a predetermined ratio or less with respect to the length of a maximum image forming area of a photosensitive drum in the left-right direction, the transfer bias control unit47and the drum-cleaning-bias control unit48determine that the electric resistance of the paper sheet P is comparatively high.

(2) Setting of Transfer Currents and Drum Cleaning Biases

In the second embodiment, in a case where the length of a paper sheet P in the left-right direction is, for example, in 80% or less, preferably, 60% or less, with reference to the length of the maximum image forming area of a photosensitive drum, the transfer bias control unit47and the drum-cleaning-bias control unit48determine that the electric resistance of the paper sheet P is comparatively high.

Then, likely in the above described first embodiment, the transfer bias control unit47sets a transfer current to flow between each of the plurality of photosensitive drums11and a corresponding transfer roller19.

The drum-cleaning-bias control unit48sets the drum cleaning biases to be applied to the plurality of drum cleaning rollers24, respectively.

(3) Effects of Second Embodiment

Even in the second embodiment, it is possible to achieve the same effects as those of the above described first embodiment.

Specifically, in the second embodiment, in a case where the size of a paper sheet P (the length in the left-right direction) is in the above described ratio or less with respect to the length of the maximum image forming area of a photosensitive drum, the transfer bias control unit47determines that the electric resistance of the paper sheet P is comparatively high and sets the transfer currents to flow between the photosensitive drums11and the transfer rollers19to be large likely in the first embodiment, such that sufficient transfer currents flow in the paper sheet P having a comparatively small size.

Even in this case, likely in the above described first embodiment, the drum-cleaning-bias control unit48sets a drum cleaning bias for a drum cleaning roller24corresponding to the rearmost photosensitive drum11to the float potential such that the potential difference between the surface of the rearmost photosensitive drum11and the rearmost drum cleaning roller24becomes substantially 0 V.

For this reason, it is possible to reliably remove the post-transfer residual toner attached to the photosensitive drums11by the drum cleaning rollers24.

A third embodiment of the printer1will be described with reference toFIG. 3. In the third embodiment, members identical to those of the above described first embodiment are denoted by the same reference symbols, and will not be described.

(1) Outline of Third Embodiment

In the above described first embodiment, the relative humidity of the inside of the main body casing2is detected by the humidity sensor43, and the electric resistance of a paper sheet P is determined based on the detected relative humidity.

In contrast, in the third embodiment, the electric resistance between the transfer rollers19and the photosensitive drums11is measured, and the electric resistance of a paper sheet P is determined based on the measured electric resistance.

(2) Configuration of Third Embodiment

In the third embodiment, as shown inFIG. 3, ammeters61and voltmeters62that are examples of a detecting means are provided.

The ammeters61are interposed between the transfer circuit45and the transfer roller shafts33. The ammeters61measure the current values of currents flowing from the transfer circuit45to the transfer roller shafts33. The ammeters61are electrically connected to the control device42through signal lines (shown by a broken line inFIG. 3). The ammeters61transmit the measured current values to the control device42.

The voltmeters62are electrically connected to the transfer roller shafts33and the shafts32. The voltmeters62measure voltages applied between the transfer roller shafts33and the shafts32. The voltmeters62are electrically connected to the control device42through signal lines (shown by a broken line inFIG. 3). The voltmeters62transmit the measured voltages to the control device42.

Then, the control device42computes the electric resistance between the transfer rollers19and the photosensitive drums11based on the current values measured by the ammeters61and the voltages measured by the voltmeters62.

(3) Setting of Transfer Currents and Drum Cleaning Biases

In the third embodiment, for example, in a case where the electric resistance between the transfer rollers19and the photosensitive drums11is equal to 1.0×1012Ω or more, the transfer bias control unit47determines that the electric resistance of a paper sheet P is comparatively high.

Then, likely in the above described first embodiment, the transfer bias control unit47sets a transfer current to flow between each of the plurality of photosensitive drums11and a corresponding transfer roller19.

The transfer bias control unit47controls the transfer circuit45, thereby applying transfer biases (second transfer biases) to the plurality of transfer rollers19, respectively, such that the above described transfer currents constantly flow between the transfer rollers19and corresponding photosensitive drums11(constant current control). The applied transfer biases are measured by the voltmeters62.

Then, in a case where the transfer biases measured by the voltmeters62are equal to the second transfer biases or more, the drum-cleaning-bias control unit48determines that the electric resistance of a paper sheet P is comparatively high.

Then, likely in the above described first embodiment, the drum-cleaning-bias control unit48sets the drum cleaning biases to be applied to the plurality of drum cleaning rollers24, respectively.

Thereafter, the drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby changing the drum cleaning bias to be applied to the rearmost drum cleaning roller24to the float potential after the paper sheet P is fed between the rearmost photosensitive drum11and the rearmost transfer roller19until the post-transfer residual toner faces the rearmost drum cleaning roller24.

(4) Effects of Third Embodiment

(4-1) According to the printer1of the third embodiment, in a case where the transfer biases measured by the voltmeters62are equal to the second transfer biases or more, the drum-cleaning-bias control unit48determines that the electric resistance of a paper sheet P is comparatively high.

For this reason, it is possible to more effectively reduce the potential difference between the surface of the rearmost photosensitive drum11and the rearmost drum cleaning roller24by measuring the transfer biases.

(4-2) According to the printer1of the third embodiment, the drum-cleaning-bias control unit48changes the drum cleaning bias to be applied to the rearmost drum cleaning roller24to the float potential after a paper sheet P is fed between the rearmost photosensitive drum11and the rearmost transfer roller19until post-transfer residual toner faces the rearmost drum cleaning roller24.

For this reason, it is possible to more reliably remove post-transfer residual toner attached to the surface of the rearmost photosensitive drum11.

(4-3) Even in the third embodiment, it is possible to achieve the same effects as those of the above described first embodiment.

A fourth embodiment of the printer1will be described with reference toFIG. 3. In the fourth embodiment, members identical to those of the above described third embodiment are denoted by the same reference symbols, and will not be described.

(1) Outline of Fourth Embodiment

In the above described third embodiment, in the transfer bias table, set values for the transfer currents are stored corresponding to relative humidity levels. Then, the transfer bias control unit47controls the transfer circuit45, thereby applying transfer biases to the plurality of transfer rollers19, respectively, such that the transfer currents set based on the transfer bias table flow (constant current control).

In contrast, in the fourth embodiment, in a transfer bias table, set values for the transfer biases are stored corresponding to relative humidity levels. Then, the transfer bias control unit47controls the transfer circuit45, thereby applying transfer biases set based on the transfer bias table to the plurality of transfer rollers19, respectively (constant voltage control).

(2) Setting of Transfer Currents and Drum Cleaning Biases

(2-1) Case of Normal Humidity Environment

In the fourth embodiment, if the electric resistance between the transfer rollers19and the photosensitive drums11is, for example, 1.0×1011Ω or less, the transfer bias control unit47determines that the electric resistance of a paper sheet P is comparatively low.

Then, the transfer bias control unit47sets a transfer bias (the first transfer bias) to be applied to a transfer roller19corresponding to black to, for example, −3500 V.

The transfer bias control unit47sets a transfer bias (the first transfer bias) to be applied to a transfer roller19corresponding to yellow to, for example, −3500 V.

Further, the transfer bias control unit47sets a transfer bias (the first transfer bias) to be applied to a transfer roller19corresponding to magenta to, for example, −3500 V.

Furthermore, the transfer bias control unit47sets a transfer bias (the first transfer bias) to be applied to a transfer roller19corresponding to cyan to, for example, −3500 V.

Like in the above described first embodiment, the drum-cleaning-bias control unit48sets the drum cleaning biases to be applied to the plurality of drum cleaning rollers24, respectively.

Then, the transfer bias control unit47controls the transfer circuit45, thereby applying the above described transfer biases to the plurality of transfer rollers19, respectively (constant voltage control). The applied transfer biases are measured by the voltmeters62.

Then, since the transfer biases measured by the voltmeters62are the first transfer biases, the drum-cleaning-bias control unit48determines that the electric resistance of a paper sheet P is comparatively low.

Then, likely in the above described first embodiment, the drum-cleaning-bias control unit48sets the drum cleaning biases to be applied to the plurality of drum cleaning rollers24, respectively.

Then, the drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby applying the above described drum cleaning biases to the plurality of drum cleaning rollers24, respectively.

(2-2) Case of Low Humidity Environment

If the electric resistance between the transfer rollers19and the photosensitive drums11is, for example, 1.0×1012Ω or less, the transfer bias control unit47determines that the electric resistance of a paper sheet P is comparatively high.

Then, the transfer bias control unit47sets a transfer bias (the second transfer bias) to be applied to a transfer roller19corresponding to black to, for example, −5000 V.

The transfer bias control unit47sets a transfer bias (the second transfer bias) to be applied to a transfer roller19corresponding to yellow to, for example, −4500 V.

Further, the transfer bias control unit47sets a transfer bias (the second transfer bias) to be applied to a transfer roller19corresponding to magenta to, for example, −5000 V.

Furthermore, the transfer bias control unit47sets a transfer bias (the second transfer bias) to be applied to a transfer roller19corresponding to cyan to, for example, −5000 V.

That is, in a case of determining that the electric resistance of a paper sheet P is comparatively high, the transfer bias control unit47sets the transfer biases to be applied to the transfer rollers19to transfer biases having absolute values larger than those in a case of determining that the electric resistance of a paper sheet P is comparatively low.

Further, the transfer bias control unit47controls the transfer circuit45, thereby applying the above described transfer biases to the plurality of transfer rollers19, respectively (constant voltage control). The applied transfer biases are measured by the voltmeters62.

Then, in a case where the transfer biases measured by the voltmeters62are equal to the second transfer biases or more, the drum-cleaning-bias control unit48determines that the electric resistance of a paper sheet P is comparatively high.

Then, likely in the above described first embodiment, the drum-cleaning-bias control unit48sets the drum cleaning biases to be applied to the plurality of drum cleaning rollers24, respectively.

Thereafter, the drum-cleaning-bias control unit48controls the drum cleaning circuit46, thereby changing the drum cleaning bias to be applied to the rearmost drum cleaning roller24to the float potential after the paper sheet P is fed between the rearmost photosensitive drum11and the rearmost transfer roller19until the post-transfer residual toner faces the rearmost drum cleaning roller24.

(3) Effects of Fourth Embodiment

According to the fourth embodiment, the transfer bias control unit47performs control such that the transfer biases are constant. Further, in a case where the transfer biases measured by the voltmeters62have been switched to the second transfer biases, the drum-cleaning-bias control unit48determines that the electric resistance of a paper sheet P is comparatively high.

For this reason, in a configuration in which transfer biases are applied to the transfer rollers19by constant voltage control, it is possible to reliably remove post-transfer residual toner attached to the photosensitive drums11by the drum cleaning rollers24.

It is possible to more effectively reduce the potential difference between the surface of the rearmost photosensitive drum11and the rearmost drum cleaning roller24by measuring the transfer biases.

Even in the fourth embodiment, it is possible to achieve the same effects as those of the above described third embodiment.

A fifth embodiment of the printer1will be described with reference toFIG. 4. In the fifth embodiment, members identical to those of the above described first embodiment are denoted by the same reference symbols, and will not be described.

(1) Outline of Fifth Embodiment

In the above described first embodiment, the printer1has been configured as a color printer.

In contrast, in the fifth embodiment, as shown inFIG. 4, a printer71is configured as a monochrome printer.

(2) Configuration of Fifth Embodiment

The printer71of the fifth embodiment does not have process cartridges5of yellow, magenta, and cyan but has a process cartridge5of black.

The printer71of the fifth embodiment has a transfer roller72as an example of a transfer member, instead of the transfer unit7.

The transfer roller72is rotatably supported at the rear end portion of the process cartridge5below a photosensitive drum11. The transfer roller72comes into contact with the photosensitive drum11from the lower side.

The printer71of the fifth embodiment includes a reverse path73as an example of a conveying means for conveying a paper sheet P having a toner image fixed thereon, from a fixing unit8to the gap between the photosensitive drum11and the transfer roller72as shown by a virtual line inFIG. 4.

The reverse path73is provided to extend the rear side of the fixing unit8in a vertical direction and extend the upper side of a paper feeding tray22and the lower sides of the fixing unit8and the process cartridge5in the front-rear direction. The front end portion of the reverse path73is on the substantially central portion of the process cartridge5in the front-rear direction. The rear end portion of the reverse path73is disposed to face the lower rear sides of paper discharging rollers74for discharging paper sheets P.

A paper sheet P having a toner image fixed thereon is introduced into the reverse path73by rotating of the paper discharging rollers74such that a surface (a first surface) having the toner image fixed thereon is directed downward.

Thereafter, the paper sheet P passes through the reverse path73from the rear side to the front side, and is fed to the gap between the photosensitive drum11and the transfer roller72such that a surface (a second surface) opposite to the surface (the first surface) having the toner image fixed thereon is directed upward.

In the printer71of the fifth embodiment, a main body opening3is formed at the front end portion of a main body casing2, and at the front end portion of the main body casing2, a front cover75for opening and closing the main body opening3is provided to be able to swing on a lower end portion of the front cover.

In the printer71of the fifth embodiment, the photosensitive drum11is exposed by a scanner unit76. The scanner unit76is disposed to face the upper side of the process cartridge5. The scanner unit76emits a laser beam toward the photosensitive drum11as shown by a solid line, based on image data.

(3) Setting of Transfer Current and Drum Cleaning Bias

In the fifth embodiment, when a print job has been input from an external personal computer51or the like to the printer71, in a case where duplex printing has been designated in the print job (that is, a case of consecutively printing both surfaces (a first surface and a second surface) of a paper sheet P), the transfer bias control unit47and the drum-cleaning-bias control unit48determine that the electric resistance of a paper sheet P is comparatively high.

Then, likely in the above described first embodiment, the transfer bias control unit47sets a transfer current to flow between the photosensitive drum11and the transfer roller72.

The drum-cleaning-bias control unit48sets a drum cleaning bias to be applied to the drum cleaning roller24.

(4) Effects of Fifth Embodiment

In the printer71of the fifth embodiment, when a paper sheet P passes through a fixing unit8, the paper sheet P is heated to be dried, and the electric resistance of the paper sheet P increases. Even in this case, it is possible to reliably remove post-transfer residual toner attached to the photosensitive drum11by the drum cleaning roller24.

The above described embodiments can be combined with each other.

For example, as shown inFIG. 2, the first embodiment and the second embodiment can be combined.

In this case, as described above, the control device42sets the transfer currents and the drum cleaning biases based on designation of a paper size in a print job and detection of the relative humidity by the humidity sensor43.

As shown inFIG. 3, the second embodiment and the third embodiment can be combined.

In this case, as described above, the control device42sets the transfer currents and the drum cleaning biases based on designation of a paper size in a print job and detection of the relative humidity by the humidity sensor43.

In the fifth embodiment, like in the above described first embodiment, it is possible to set the transfer current and the drum cleaning bias based on detection of the relative humidity by the humidity sensor43.

In the fifth embodiment, it is possible to apply at least one of the above described second embodiment, third embodiment, and fourth embodiment.

In the above embodiment, the control device42including the CPU and the memory carries out the cleaning bias control. However, the invention is not limited to this. For example, the control device42may be configured by a device including a plurality of CPU and memories for carrying out the cleaning bias control, configured only by a hardware circuit such as ASIC (Application Specific Integrated Circuit), or configured by a combination of a CPU and a hardware circuit such as ASIC.