HK1055154B - Image forming apparatus with an element for removing paper dust from photosensitive member - Google Patents

Description

Image forming apparatus with device for removing paper dust from photosensitive member

Technical Field

The present invention relates to xerographic imaging devices such as laser printers.

Background

As is well known, electrophotographic image forming apparatuses such as laser printers typically include a photosensitive drum, a charger, a laser scanner, a developing roller, and a transfer roller. After the photosensitive drum surface is uniformly charged by the charger, the laser scanner emits a laser beam to be irradiated onto the photosensitive drum surface, and forms an electrostatic latent image according to predetermined image data.

The developing powder mounted on the developing roller is conveyed to the electrostatic latent image formed on the surface of the photosensitive drum. The developer powder deposited on the surface of the photosensitive drum is transferred to the paper sheet passing between the photosensitive drum and the transfer roller.

Disclosure of Invention

When the paper sheet passes between the photosensitive drum and the transfer roller, paper dust is deposited on the surface of the photosensitive drum. If any residual paper dust is present on the photosensitive drum, the charger is prevented from uniformly charging the surface of the photosensitive drum, thereby degrading the printing quality. The image forming apparatus constructed according to the present invention can effectively remove paper dust deposited on the surface of the photosensitive drum.

According to the present invention, there is provided an image forming apparatus comprising: a photosensitive member; a charging device for uniformly charging the photosensitive member; an irradiating device for selectively irradiating the charged photosensitive memberIrradiating to form an electrostatic latent image; a developing device for supplying a developer to the photosensitive member based on the electrostatic latent image formed on the photosensitive member; a transfer device for transferring the developer supplied from the developing device and held on the photosensitive member onto a recording medium; a paper dust removing member for removing paper dust deposited on the photosensitive member; a power supply for applying a bias to the paper dust removing member; and a diode connected between the paper dust removing member and the power supply; wherein a bias potential applied to the paper dust removing member is set between an initial potential of the photosensitive member charged by the charging means and a potential of an unirradiated portion of the photosensitive member after the developer is transferred to the recording medium, and a volume resistance of the paper dust removing member is less than 10⁶Ohm cm.

According to the present invention, there is also provided a method of removing paper dust from a photosensitive member of an image forming apparatus, the method comprising the steps of: removing paper dust deposited on the photosensitive member by a paper dust removing member; applying a bias voltage to the paper dust removing member by a power supply, the bias voltage applied to the paper dust removing member being set between an initial potential of the photosensitive member charged by a charging device and a potential of an unirradiated portion of the photosensitive member after a developer is transferred onto a recording medium; preventing current from flowing from the photosensitive member to the power supply through a diode, and setting a volume resistance of the paper dust removing member to be less than 10⁶Ohm cm.

Drawings

A preferred embodiment of the present invention will be described in detail with reference to the following drawings, in which:

FIG. 1 is a side sectional view of the main part of a laser printer according to one embodiment of the present invention;

FIG. 2 is a side sectional view of an important part of the photocopying unit of the laser printer of FIG. 1; and

fig. 3 shows the amount of overlap of the conductive brush on the surface of the photosensitive drum.

Detailed Description

Fig. 1 is a side sectional view of a main part of a laser printer 1.

The paper feed tray 6 is detachably mounted on the bottom of the housing 2. The platen 7 is mounted on the sheet feed tray 6 to support and press upward the sheets 3 stacked in the sheet feed tray 6. The pickup roller 8 and the separation pad 9 are mounted above one end of the paper feed tray 6, and the registration rollers 12a, 12b are located downstream of the pickup roller 8 with respect to the sheet conveying direction.

The sheets 3 can be stacked on top of the platen 7. The end of the platen 7 remote from the feed roller 8 is pivotally supported so that the end of the platen 7 near the feed roller 8 is vertically movable. A spring (not shown) on the back of the pressure plate 7 pushes the pressure plate 7 upward. When the number of stacked sheets increases, the pressing plate 7 swings downward about its end away from the paper feed roller 8 against the urging force of the spring. The paper feed roller 8 and the paper feed pad 9 are both disposed oppositely. A spring 13 provided on the back of the paper feed pad 9 pushes the paper feed pad 9 toward the paper feed roller 8.

The uppermost sheet 3 of the stack on the platen 7 is pressed against the feed roller 8 by a spring mounted on the back of the platen 7, and when the feed roller 8 rotates, the uppermost sheet 3 is sandwiched between the feed roller 8 and the feed pad 9. Then, the printing paper 3 is fed from the uppermost side one by one.

After the paper dust removing roller 10 removes paper dust on the paper 3, the paper 3 is conveyed by the conveying roller 11 to the registration rollers 12a, 12 b. The registration rollers 12a, 12b are composed of two rollers, i.e., a drive roller 12a provided for the housing 2 and a driven roller 12b provided for a photocopying unit 17, which will be described later. The drive roller 12a and the driven roller 12b are in face-to-face contact with each other. When the sheet 3 is nipped between the drive roller 12a and the driven roller 12b, it is further conveyed downstream by the conveying roller 11.

Before the sheet 3 comes into contact with the drive roller 12a, the drive roller 12a is not driven. After the sheet 3 comes into contact with the drive roller 12a and the direction is corrected by the drive roller 12a, the drive roller 12a rotates to convey the sheet 3 downstream.

A manual feed tray 14 for manually feeding the sheets 3 and a manual feed roller 15 for feeding the sheets 3 stacked on the manual feed tray 14 are provided at the front portion of the housing 2. The separation pad 25 is disposed opposite to the manual feeding roller 15. A spring 25a provided on the back of the separation pad 25 pushes the separation pad 25 toward the manual paper feeding roller 15. When the manual feed roller 15 rotates, the sheets 3 stacked on the manual feed plate 14 are nipped by the manual feed roller 15 and the separation pad 15 and fed one by one.

Also included in the housing 2 are a scanner unit 16, a photocopy unit 17, and a fixing unit 18.

The scanning unit 16 is provided at an upper portion of the housing 2 and has a laser emitting portion (not shown), a polygon mirror 19, lenses 20 and 21, and reflecting mirrors 22, 23, 24. The laser beam emitted from the laser emitting portion is modulated according to predetermined image data. The laser beam passes through or is reflected by the optical element in sequence, i.e. by the polygon mirror 19, the lens 20, the mirrors 22 and 23, the lens 21, the mirror 24 in the order indicated by the dashed line in fig. 1. The laser beam is then directed to the photosensitive drum 27 and scanned over the surface thereof at high speed, wherein the photosensitive drum 27 will be described later.

Fig. 2 is an enlarged sectional view of the photocopying unit 17. As shown in fig. 2, the photocopying unit 17 is disposed below the scanner unit 16, and includes a drum stand 26 detachably mounted on the housing 2, and a developing stand 28 detachably mounted on the drum stand 26.

The drum frame 26 includes therein a photosensitive drum 27, a corona charger 29, a transfer roller 30, and a conductive brush 51.

The developing frame 28 includes therein a developing roller 31, a blade 32, an ink supply roller 33, and a toner cartridge 34.

The toner cartridge 34 contains positively charged non-magnetic single component toner as a developer. The toner used in this example is a polymerized toner obtained by copolymerization, formed from a styrene-based monomer such as styrene, and an acryl-based monomer such as acrylic acid, alkyl (C1-C4) acrylate, alkyl (C1-C4) methyl acrylate, by a known polymerization method such as suspension polymerization. The particles of such polymerized toner are spherical and thus have excellent fluidity.

The polymerized toner is added with a colorant such as carbon black and wax. An external additive such as silicone oil is also added to the polymerized toner to improve fluidity. The particle size of the polymerized toner is about 6 to 10 microns.

A rotary shaft 35 is provided in a central portion of the toner cartridge 34, and the toner in the toner cartridge 34 is agitated by an agitator 36 supported on the rotary shaft 35 and flows out from a toner supply port 37 on the side of the toner cartridge 34. A toner inspection window 38 is provided on a side wall of the toner cartridge 34. The toner inspection window 38 is wiped clean by a cleaner 39 supported on the rotation shaft 35.

Ink supply roller 33 is rotatably disposed near toner supply port 37. The developing roller 31 is rotatably disposed opposite to the ink supply roller 33.

The ink supply roller 33 is formed by covering a conductive foam material on a metal roller shaft. The developing roller 31 is formed by covering a metal roller shaft with a conductive rubber material. More specifically, the developing roller 31 is covered with a conductive urethane or silicone rubber containing fine carbon particles, and the surface layer is covered with a fluorine-containing urethane or silicone rubber. The ink supply roller 33 and the developing roller 31 are in contact with each other, and therefore they are deformed to an appropriate degree by being pressed against each other. A predetermined developing bias is applied to the developing roller 31 with respect to the photosensitive drum 27.

The layer thickness regulating blade 32 is disposed near the developing roller 31 to regulate the thickness of the toner layer formed on the surface of the developing roller 31. The layer thickness regulating blade 32 includes a metal leaf spring and a pressing portion 40, wherein the pressing portion 40 is disposed at the tip of the leaf spring and is made of an insulating silicone rubber in a shape of a semicircle in cross section. The other end of the leaf spring is supported on the developing frame 28 so as to be close to the developing roller 31. The pressing portion 40 is pressed against the developing roller 31 by the elastic force of the leaf spring.

When ink supply roller 33 rotates, toner discharged from toner supply port 37 by agitator 36 is conveyed to developing roller 31. The toner is positively charged between the toner supply roller 33 and the developing roller 31 due to friction. After passing between the pressing portion 40 and the developing roller 31, the toner forms a thin layer having a predetermined thickness on the developing roller 31.

The photosensitive drum 27 is rotatably disposed in the drum frame 26 and is in contact with the developing roller 31. The photosensitive drum 27 is formed by coating a positively charged polycarbonate photosensitive layer on a grounded cylindrical aluminum drum.

The charger 29 is disposed above the photosensitive drum 27 at a predetermined pitch. The charger 29 is a corona charger for uniformly charging the surface of the photosensitive drum 27 with a positive electric charge by generating corona discharge from a tungsten wire. The charger 29 is designed to bring the potential of the surface of the photosensitive drum 27 to about 900 volts.

The transfer roller 30 is disposed below the photosensitive drum 27, and is rotatably supported on the drum frame 26 so as to face the photosensitive drum 27. The transfer roller 30 is formed by covering a conductive rubber material on a metal roller shaft. A power source (not shown) is connected to the roller shaft, and a predetermined transfer bias is applied to the roller shaft when the toner on the photosensitive drum 27 is transferred to the sheet 3.

As shown in fig. 2 and 3, the conductive brush 51 includes a metal base 54 having a substantially L-shape, and a brush 55 inserted into one end of the base 54. The brush 55 is made of acrylic resin in which conductive particles such as carbon particles or conductive fillers are dispersed. The base 54 is fixed to a brush holder 56, and the brush holder 56 is integrated with the drum holder and extends toward the photosensitive drum 27. The tip of the brush 55 is in contact with the surface of the photosensitive drum 27. The position of the conductive brush 51 facing the photosensitive drum 27 with respect to the rotational direction of the photosensitive drum 27 is downstream of the transfer roller 30 and upstream of the charger 29. The brush 55 contacts the photosensitive drum 27 along the entire length of the photosensitive drum 27.

A dc power supply 53 is connected to the other end of the base 54, and a diode 52 is connected between the dc power supply 53 and the base 54 to prevent reverse current. The diode 52 is connected to allow current to flow from the dc power supply 53 to the conductive brush 51, but to prevent current from flowing from the conductive brush 51 to the dc power supply 53.

A dc power supply 53 and a diode 52 are mounted in the housing 2. The dc power supply 53 applies a bias of about 400 volts to the conductive brush 51.

As shown in fig. 1, the fixing unit 18 is disposed downstream of the printing unit 17, and includes a heating roller 41, a pressure roller 42 that presses against the heating roller 41, and a pair of conveying rollers 43 located downstream of the heating roller 41 and the pressure roller 42. The heating roller 41 is formed by covering an aluminum tube, in which a halogen lamp is mounted, with silicone rubber. The heat generated by the halogen lamp is transferred to the paper 3 through the aluminum tube. The pressure roller 42 is made of silicone rubber so that the paper 3 can be easily moved from the heating roller 41 and the pressure roller 42.

When the sheet 3 passes between the heating roller 41 and the pressure roller 42, the toner transferred onto the sheet 3 by the photocopying unit 17 is melted and fixed on the sheet 3 by the heat. After the fixing is completed, the sheet 3 is conveyed downstream by the conveying roller 43. The sheet discharge path 44 is located downstream of the conveying rollers 43, and serves to reverse the sheet conveying direction and guide the sheet 3 to a sheet discharge tray 46 provided on the top surface of the laser printer 1. A pair of sheet discharge rollers 45 is provided at the upper end of the sheet discharge path 44 for feeding the sheet 3 into the sheet discharge tray 46.

A reverse conveying unit 47 is provided in the laser printer 1 so that images can be formed on both sides of the sheet 3. The reverse conveying unit 47 includes a sheet discharge roller 45, a reverse conveying passage 48, a flapper 49, and a plurality of pairs of reverse conveying rollers 50.

The pair of sheet discharge rollers 45 can be switched between forward and reverse rotations. The sheet discharging roller 45 feeds the sheet 3 into the sheet discharging tray 46 when rotating forward, and reverses the sheet conveying direction when rotating backward.

The reverse conveyance path 48 is vertically provided for guiding the sheet 3 from the sheet discharge roller 45 to a reverse conveyance roller 50 disposed above the sheet feed tray 6. The upstream end of the reverse conveyance path 48 is adjacent to the sheet discharge roller 45, and the downstream end is adjacent to the reverse conveyance roller 50.

A flapper 49 is swingably provided at a branch point near the sheet discharge path 44 and the reverse conveyance path 48. The position of the flapper 49 can be switched between a first position shown in solid lines and a second position shown in dashed lines. Flapper 49 is repositioned by switching the energization of a solenoid (not shown).

When flapper 1 is in the first position, sheet 3 guided along sheet discharge path 44 is fed by sheet discharge roller 45 to sheet discharge tray 46. When the flapper is at the second position, the sheet 3 is conveyed to the reverse conveyance path 48 by the reverse-rotation sheet discharge roller 45.

Pairs of reverse conveyance rollers 50 are arranged above the paper feed tray 6 in the horizontal direction. A pair of reverse conveyance rollers 50 on the most upstream side is located near the lower end of the reverse conveyance path 48. The pair of reverse conveyance rollers 50 on the most downstream side is located below the registration rollers 12a, 12 b.

The operation of the reverse conveying unit 47 when images are formed on both sides of the sheet 3 will be described below.

The sheet 3 with the printed image on one side thereof is conveyed by the conveying roller 43 along the sheet discharge path 44 to the sheet discharge roller 45. At this time, the flapper 49 is in the first position. The sheet discharge roller 45 nips the sheet 3 and rotates forward, temporarily conveying the sheet 3 to the sheet discharge tray 46. When the sheet 3 is almost completely fed into the sheet output tray 46, the sheet output roller 45 stops rotating forward, and the trailing end of the sheet 3 is still gripped by the sheet output roller 45.

In this state, the flapper 49 is switched to the second position, and then the discharge roller 45 is rotated backward. The sheet 3 is conveyed in the reverse direction along the reverse conveyance path 48. After the entire sheet 3 is conveyed into the reverse conveyance path 48, the flapper 49 is switched to the first position.

Then, the sheet 3 is conveyed to the reverse conveyance roller 50, and is further conveyed upward by the reverse conveyance roller 50 to the registration rollers 12a, 12 b. The sheet 3 is then conveyed to the photocopying unit 17 with the printed side facing downward. As a result, images are printed on both sides of the sheet 3.

The imaging operation will now be described. The charger 29 uniformly positively charges the surface of the photosensitive drum 27. The surface potential of the photosensitive drum 27 is about 900 volts. When the surface of the photosensitive drum 27 is irradiated with the laser beam emitted from the scanner unit 16, the electric charge of the laser beam irradiated portion is eliminated, and the surface potential of the portion becomes about 200 volts. In this way, the surface of the photosensitive drum 27 is divided into a high potential portion (non-irradiated portion) and a low potential portion (irradiated portion), thereby forming an electrostatic latent image.

The surface potential of the non-irradiated portions is about 900 volts, while the surface potential of the irradiated portions is about 200 volts.

When the positively charged toner on the developing roller 31 is directed to the photosensitive drum 27, the toner is conveyed to the low potential portion of the photosensitive drum 27, which has been irradiated. As a result, the electrostatic latent image formed on the photosensitive drum 27 is visualized.

The developing roller 31 recovers the toner remaining on the surface of the photosensitive drum 27. The residual toner is toner that has been conveyed onto the photosensitive drum 27 but has not been transferred from the photosensitive drum 27 onto the paper 3. The residual toner adheres to the developing roller 31 due to the potential difference between the photosensitive drum 27 and the developing roller 31, and is recovered into the developing frame 28. By this means, there is no need for a scraper for scraping off residual toner from the photosensitive drum 27, and no need for a reservoir for scraping off toner. Therefore, the structure of the laser printer can be simplified to be more compact, and the production cost can be reduced.

When the sheet 3 passes between the photosensitive drum 27 and the transfer roller 30, the toner forming the visible image on the photosensitive drum 27 is transferred to the sheet 3 by the coulomb force generated due to the potential difference between the sheet 3 and the surface of the photosensitive drum 27. The surface potential of the non-irradiated portion of the photosensitive drum 27 is reduced from about 900 volts to about 300 volts due to the transfer bias applied to the transfer roller 30.

When the toner is transferred onto the sheet 3, paper dust contained in the sheet 3 adheres to the surface of the photosensitive drum 27. If the next charging process is carried out with paper dust deposited on the surface of the photosensitive drum 27, the surface of the photosensitive drum 27 may not be uniformly charged, resulting in a reduction in print quality.

In the laser printer 1 of the present embodiment, the surface of the photosensitive drum 27 faces the brush 55. Thus, the paper dust deposited on the photosensitive drum 27 is collected by the brush 55 by a physical method. Further, when a bias of about 400 volts is applied to the brush 55, the paper dust is also electrically collected by the brush 55.

After the toner is transferred to the sheet 3, the surface potential of the non-irradiated portion of the photosensitive drum 27 is about 300 volts, which is different from the bias voltage applied to the brush 55 of about 400 volts by about 100 volts. Due to this potential difference, the paper dust 55 can be effectively collected by the brush 55.

Since the transfer bias is applied to the transfer roller 30 during the transfer of the toner onto the sheet 3, the surface potential of the non-irradiated portion of the photosensitive drum 27 becomes 300 volts. When the application of the transfer bias is stopped upon completion of toner transfer, the brush 55 may come into contact with the non-irradiated portion on the surface of the photosensitive drum 27, in which the surface of the photosensitive drum 27 is held at the initial potential of 900 volts by the charger 29.

In this case, a reverse current from the brush 55 to the dc power supply 53 is generated due to a potential difference between the surface potential of the photosensitive drum 27 and the bias voltage applied to the brush 55. Then, the paper dust collected by the brush 55 is released onto the photosensitive drum 27 due to coulomb force.

At this time, the transfer of the toner onto the sheet 3 is not performed, and no new paper dust adheres to the photosensitive drum 27. Therefore, the brush 55 is not required to collect the newly deposited paper dust from the photosensitive drum 27, but only the brush 55 is required to hold the paper dust that has been collected. As long as the paper dust that has been collected by the brush 55 is prevented from returning to the photosensitive drum 27, the brush 55 can satisfactorily collect the paper dust without reducing the ability to collect the paper dust.

In the laser printer 1 of the present embodiment, since the diode 52 is provided between the conductive brush 51 and the dc power supply 53, no current is generated from the brush 55 to the dc power supply 53. Therefore, the potential of the brush 55 is equal to the surface potential (900 volts) of the photosensitive drum 27. There is no potential difference between the brush 55 and the photosensitive drum 27, and therefore no coulomb force acts on the paper dust that has been collected by the brush 55. Therefore, the paper dust remains on the brush 55 without returning to the photosensitive drum 27.

The diode 52 provided between the conductive brush 51 and the direct current power supply 53 can prevent the current from flowing from the photosensitive drum 27 to the direct current power supply 53 even when the surface potential of the photosensitive drum 27 becomes higher than the bias applied to the conductive brush 51 due to on/off of the transfer bias and the transfer current variation of the transfer roller 30. Therefore, the paper dust collected by the brush 55 remains on the brush 55 without returning to the photosensitive drum 27.

The bias voltage applied to the brush 51 is set at 400 volts, between the surface potential of the non-irradiated portion of the photosensitive drum 27 after toner transfer (about 300 volts) and the initial potential of the photosensitive drum 27 charged by the scorotron charger 29 (about 900 volts). Therefore, the discharge between the conductive brush 51 and the photosensitive drum 27 can be reliably prevented, and paper dust can be satisfactorily removed.

The volume resistance of the brush 55 in the conductive brush 51 is less than 10⁶Ohm-cm, preferably 10²-10⁴Ohm cm. When the volume resistance of the brush 55 is less than 10⁶Ohm cm, the potential difference generated between the brush 55 and the photosensitive drum 27 is sufficient for the brush 55 to be able to collect the charged paper dust. In this state, paper dust can be collected more efficiently by applying a bias voltage to the brush 55 with the dc power supply 53.

On the other hand, when the volume resistance of the brush 55 is equal to or greater than 10⁶At ohm cm, the electric field strength generated between the brush 55 and the photosensitive drum 27 is not strong enough to collect the charged paper dust, so that the brush 55 has a reduced ability to remove the paper dust.

The brush 55 is made of acrylic resin dispersed with conductive particles such as carbon particles or conductive fillers, and is moderately hard.

On the other hand, if the brush is made of resin coated with metal, it is too hard to rub against the surface of the photosensitive drum 27, thereby promoting the generation of paper dust or toner film on the photosensitive drum 27. If the brush used is too soft, the ability to remove paper dust is reduced.

By using the brush 55 in the present embodiment, generation of a thin film can be suppressed, and a sufficient paper dust removing ability can be provided.

In the present embodiment, the length of the brush 55 is preferably 6 mm or more, and the overlapping amount of the brush 55 and the photosensitive drum 27 is preferably 1 mm or more, and more preferably 1 to 4 mm. When the overlapping amount of the brush 55 is more than 1 mm, the tip of the brush 55 is slightly bent when it is in contact with the surface of the photosensitive drum 27. Thus, the brush 55 can provide a sufficient paper dust removing ability while suppressing the generation of a thin film on the photosensitive drum 27.

When the length of the brush 55 is less than 6 mm and the overlapping amount of the brush is less than 1 mm, the tip of the brush 55 may rub against the surface of the photosensitive drum 27, thereby possibly generating a thin film on the photosensitive drum 27. Conversely, when the overlapping amount of the brush 55 is more than 4 mm, the brush 55 may be too bent to reduce the paper dust removing ability.

As shown in fig. 3, the overlapping amount of the brush 55 is defined as a length X, which is obtained by subtracting a distance Y between the base 54 and the surface of the photosensitive drum 27 from a length L of the brush 55. The portion corresponding to the overlapping amount X is curved along the surface of the photosensitive drum 27 to the downstream side in the rotational direction of the photosensitive drum 27. Therefore, the middle of the brush 55 is in contact with the photosensitive drum 27, not the tip of the brush 55.

The brush 55 should have a fiber density in excess of 7.75, more preferably in excess of 10.85, and most preferably in excess of 15.5 kilo-fibers per square centimeter.

The unit of measurement of density, thousand fibers per square centimeter, is used to represent the number of fibers per square centimeter, and 7.75 thousand fibers per square centimeter represents 7750 fibers implanted per square centimeter.

When the density of the brush 55 is equal to or less than 7.75 thousand pieces/square centimeter, the paper dust can pass through the brush 55. When the density of the brush 55 is more than 7.75 thousand roots per square centimeter, the paper dust can be satisfactorily collected. The ability to remove paper dust can be further improved.

The brush 55 preferably has a fiber thickness of about 330 dtex/48 or less. The unit of measure of fiber coarseness "dtex/48 is used herein. "dtex" means the thickness of one gram of filament when elongated to 10,000 meters. 330 dtex/48 means that the total thickness of 48 fibers is 330 times the thickness of one gram of filament when drawn to 10,000 meters.

When the fiber thickness of the brush 55 is more than 330 dtex/48, the brush 55 becomes hard and rubs against the photosensitive drum 27, thereby possibly causing a film to be generated on the photosensitive drum 27. In contrast, when the fiber thickness of the brush 55 is about 330 dtex/48 or less, the brush 55 is less likely to cause generation of a thin film, and can satisfactorily remove paper dust.

The conductive brush 51 may be replaced with a roller with a brush 55 in contact with the surface of the photosensitive drum 27 to remove paper dust.

Experimental examples

The advantages of the above-described brush 55 will now be described more specifically with reference to experimental examples in which various types of brushes are used. The laser printer used in the experiment was constructed the same as that of the printer 1.

Experimental example 1: resistance of brush

Three brushes with different volume resistances were used to evaluate their ability to remove paper dust. These brushes are identical except for the resistance. The following brushes were used:

I. volume resistance of 10²Ohm-cm brushes;

volume resistance of 10⁴Ohm-cm brushes;

volume resistance of 10⁶Ohm cm brush.

Brushes I and II have substantially the same paper dust removal capacity. The brush III has a lower ability to remove paper dust.

Experimental example 2: length and overlap of brush

Three brushes of different lengths and overlapping amounts were used to evaluate the extent to which they caused film formation. These brushes are identical except for length and overlap. The following brushes were used:

I. a brush having a 5.5 mm length and a 0.5 mm overlap;

a brush with a 6.5 mm length and a 1.5 mm overlap;

brush with 7.5 mm length with 2.5 mm overlap.

The tip of the brush I rubs against the photosensitive drum and causes much film to be generated. Brush II resulted in less film formation and achieved satisfactory results. Brushes II and III differed little in the degree of film formation that resulted, but brush III earlier resulted in film formation.

Experimental example 3: fiber density of brush

Three brushes with different fiber densities were used to evaluate their ability to remove paper dust. These brushes were identical except for the fiber density. The following brushes were used:

I. a brush with a fiber density of 7.75 thousand fibers/cm;

II, a brush with a fiber density of 10.85 thousand roots per square centimeter;

brush with fiber density of 15.5 thousand roots per square centimeter.

The brush I cannot sufficiently remove paper dust. The brush II can satisfactorily remove paper dust. The brush III almost completely removed the paper dust.

Experimental example 4: fiber coarseness of brush

Two brushes with different fiber thickness were used to evaluate the extent to which they caused film formation. These brushes were identical except for fiber coarseness. The following brushes were used:

I. brushes with 330 dtex/48 fiber thickness;

brush with fiber thickness of 440 dtex/48.

The brush I did not cause the generation of a thin film. Brush II resulted in the formation of some film because the fibers of this brush were too stiff.

As described above, it is desirable to have a volume resistance of 10²-10⁴The ohm-cm brush 55 can effectively remove paper dust from the photosensitive drum 27. When printing a sheet with a lot of paper dust, the paper dust adhering to the photosensitive drum 27 easily gathers in the vicinity of the developing roller 31 and to the ink supply roller 33 through the developing roller 31. This causes poor charging of the toner on the developing roller 31 after printing on the paper multiple times.

This means that toner that is poorly charged is used to form an image on paper and remains on the photoreceptor roll after printing. The toner that is poorly charged is captured by the brush 55, resulting in a poor image and poor paper dust removal ability of the brush 55. This disadvantage occurs when the current through the brush 55 is large, i.e. the volume resistance of the brush is small. Has a volume resistance of 10⁷-10⁹The ohm-cm brush 55 can obtain a good image for a long time because the toner which is poorly charged is not captured much by the brush 55. Only a bad image is initially present in a short time, but the ability to remove paper dust in a short time is not yet strong.

Even if a brush having a high volume resistance is used, the volume resistance is reduced according to the ambient humidity, and a small current can flow from the brush 55 to the photosensitive drum 27 or reversely. Therefore, the electronic element, i.e., the diode, connected between the brush 55 and the power supply can effectively prevent the current from flowing from the brush 55 to the power supply 53 even in the case where the volume resistance of the brush 55 is high. The power supply 53 and other electrically connected components are thus protected from unexpected overcurrents.

In general, when paper dust is electrically collected by applying a bias voltage to a conductive brush, if the potential difference between the bias voltage applied to the conductive brush and the surface potential of the photosensitive drum is too large, a discharge phenomenon may occur between the conductive brush and the photosensitive drum. Therefore, the bias voltage applied to the conductive brush should be set so as not to differ too much from the surface potential of the photosensitive drum.

The surface potential variation of the photosensitive drum depends largely on the change of the transfer current in the transfer roller and the on/off state of the transfer bias. When the bias voltage applied to the conductive brush is not too different from the surface potential of the photosensitive drum, the high-low relationship between the voltage applied to the conductive brush and the surface potential of the photosensitive drum may be reversed. In this case, paper dust that has been collected by the conductive brush will be released to the surface of the photosensitive drum.

To solve this problem, the surface potential of the photosensitive drum should be kept stable at any time, and the relationship between the surface potential of the photosensitive drum and the bias voltage applied to the conductive brush should be kept constant. For this purpose, a discharge tube may be provided between the downstream of the transfer roller and the upstream of the conductive brush with respect to the rotational direction of the photosensitive drum.

The discharge tube is advantageous in that the potential difference between the surface potential of the photosensitive drum and the bias voltage applied to the conductive brush can be kept stable, and thus the conductive brush can stably collect the paper dust. However, such discharge tubes have recently been eliminated in order to simplify the structure and reduce the cost.

According to the invention, a diode is arranged between the conductive brush and the power supply for the conductive brush. Therefore, paper dust deposited on the photosensitive drum can be electrically collected in a stable manner without providing a discharge tube.

The diode is used to prevent current from flowing from the photosensitive drum to the power supply.

The diode prevents a current from flowing from the photosensitive drum to the conductive brush even when the surface potential of the photosensitive drum varies due to a change in transfer current and an on/off state of a transfer bias, and even when a high-low relationship between a bias applied to the conductive brush and the surface potential of the photosensitive drum is reversed. No potential difference is generated between the photosensitive drum and the conductive brush. Thus, the paper dust remains on the conductive brush.

Since the paper dust collected on the conductive brush is not discharged to the photosensitive drum, the potential difference between the bias voltage applied to the conductive brush from the power supply and the surface potential of the photosensitive drum can be made small. In the present embodiment, the bias voltage applied to the conductive brush is set at 400 volts, and thus differs by 100 volts from the surface potential of the photosensitive drum set at 300 volts. With such a potential difference, no electric discharge is generated between the conductive brush and the photosensitive drum, and therefore the ability of the conductive brush to remove paper dust can be improved.

The bias voltage applied to the conductive brush is set between the initial potential of the photosensitive drum charged by the charger and the potential of the non-irradiated portion of the photosensitive drum after the visible image is transferred onto the sheet.

However, when the transfer bias is canceled, the surface potential of the photosensitive drum may become about 900 volts, and thus is greatly different from the bias (400 volts) applied to the conductive brush. In this case, the diode disposed between the conductive brush and the power supply may prevent current from flowing from the conductive brush to the power supply. Therefore, no potential difference is generated between the conductive brush and the photosensitive drum.

When the transfer bias is canceled, there is no paper between the photosensitive drum and the transfer roller, and therefore no paper dust adheres to the photosensitive drum. In this case, if a potential difference is generated between the conductive brush and the photosensitive drum, paper dust collected on the conductive brush may be released onto the photosensitive drum. However, in the present embodiment, no potential difference is generated between the conductive brush and the photosensitive drum, and thus the paper dust collected on the conductive brush remains there.

Therefore, the paper dust removing ability can be maintained without controlling the surface potential of the photosensitive drum with a discharge tube. This structure without the discharge tube in the present embodiment is advantageous for reducing the production cost of the image forming apparatus.

By making the brush itself conductive, such as dispersing conductive particles such as carbon particles or conductive fillers in the brush 51, paper dust deposited on the photosensitive drum can be collected physically and electrically. Therefore, the ability of the brush to remove paper dust can be improved.

By setting the volume resistance of the conductive brush to less than 10⁶Ohm cm, a sufficiently large potential difference is obtained so that the brush can collect paper dust electrically.

The conductive brush is preferably made of acrylic resin dispersed with conductive particles or fillers.

Although the brush can be made conductive by coating metal on the surface, the metal-coated brush becomes too hard to strongly rub against the surface of the photosensitive drum. The strong friction will promote the generation of paper dust or toner film on the photosensitive drum. Conversely, if the brush is soft, its ability to remove paper dust will be reduced.

For this purpose, as shown in the embodiments, it is preferable to use a brush made of acrylic resin dispersed with conductive particles or fillers. With this structure, the brush can be made moderately hard, thereby providing sufficient paper dust removal capability while suppressing the generation of a thin film.

The brush is configured such that the length thereof is equal to or greater than 6 mm and the overlapping amount with the photosensitive drum is equal to or greater than 1 mm.

When the length of the brush is less than 6 mm and the overlapping amount of the brush is less than 1 mm, the tip of the brush rubs against the surface of the photosensitive drum, and thus it is likely to cause generation of a thin film on the photosensitive drum.

In contrast, as shown in the embodiment, when the length of the brush is equal to or greater than 6 mm and the overlapping amount of the brush is equal to or greater than 1 mm, the tip of the brush is slightly bent when contacting the surface of the photosensitive drum. Therefore, the brush can provide a sufficient paper dust removing ability while suppressing the generation of a thin film on the photosensitive drum.

The brush preferably has a fiber density in excess of 7.75 thousand fibers per square centimeter.

When the fiber density of the brush is equal to or less than 7.75 thousand pieces/cm, the paper dust is likely to pass through the brush. When the brush has a fiber density of more than 7.75 thousand roots per square centimeter, the paper dust can be satisfactorily collected. Therefore, the ability of the brush to collect paper dust can be improved.

The brush preferably has a fiber thickness of about 330 dtex/48 or less. When the fiber thickness of the brush is more than 330 dtex/48, the brush becomes too hard and may cause generation of a thin film on the photosensitive drum.

Obviously, a brush satisfying the above requirements can provide an extremely high ability to remove paper dust deposited on the surface of the photosensitive drum.

Claims (13)

1. An image forming apparatus comprising:

a photosensitive member;

a charging device for uniformly charging the photosensitive member;

an irradiation device for selectively irradiating the charged photosensitive member to form an electrostatic latent image;

a developing device for supplying a developer to the photosensitive member based on the electrostatic latent image formed on the photosensitive member;

a transfer device for transferring the developer supplied from the developing device and held on the photosensitive member onto a recording medium;

a paper dust removing member for removing paper dust deposited on the photosensitive member;

a power supply for applying a bias to the paper dust removing member; and

a diode connected between the paper dust removing member and the power supply;

wherein a bias potential applied to the paper dust removing member is set between an initial potential of the photosensitive member charged by the charging means and a potential of an unirradiated portion of the photosensitive member after the developer is transferred to the recording medium, and a volume resistance of the paper dust removing member is less than 10⁶Ohm cm.

2. The image forming apparatus according to claim 1, wherein said paper dust removing member is a conductive brush.

3. The image forming apparatus as claimed in claim 1, wherein the conductive brush is made of acrylic resin dispersed with conductive particles or fillers.

4. The image forming apparatus according to claim 2, wherein a length of the conductive brush is equal to or greater than 6 mm, and an overlapping amount of the conductive brush on the photosensitive member is equal to or greater than 1 mm.

5. The imaging apparatus of claim 2, wherein the density of the conductive brushes is greater than 7.75 thousand roots per square centimeter.

6. The imaging apparatus of claim 2, wherein the conductive brush has a fiber thickness of about 330 decitex/48 or less.

7. A method of removing paper dust from a photosensitive member of an image forming apparatus, the method comprising the steps of:

removing paper dust deposited on the photosensitive member by a paper dust removing member;

applying a bias voltage to the paper dust removing member by a power supply, the bias voltage applied to the paper dust removing member being set between an initial potential of the photosensitive member charged by a charging device and a potential of an unirradiated portion of the photosensitive member after a developer is transferred onto a recording medium;

current is prevented from flowing from the photosensitive member to the power supply by a diode,

further comprising setting the volume resistance of the paper dust removing member to less than 10⁶Ohm cm.

8. The method according to claim 7, wherein the step of removing the paper dust from the photosensitive member is performed by a conductive brush.

9. The method according to claim 8, wherein said step of removing paper dust from the photosensitive member is performed by a conductive brush made of acrylic resin dispersed with conductive particles or fillers.

10. The method of claim 8, wherein the step of removing the paper dust is performed by a conductive brush having a length of more than 6 mm and an overlapping amount of the conductive brush on the photosensitive member of more than 1 mm.

11. The method of claim 8, wherein the step of removing paper dust is performed by a conductive brush having a density greater than 7.75 thousand roots per square centimeter.

12. The method of claim 8, wherein the step of removing paper dust is performed by a conductive brush having a fiber thickness of about 330 dtex/48 or less.

13. The method of claim 7, wherein the bias voltage is set to 400 volts.