Image forming apparatus having image bearing members

An image forming apparatus includes: a first image forming unit including a first image bearing member and a first rotating member configured to rub a surface of the first image bearing member; a second image forming unit including a second image bearing member and a second rotating member configured to rub a surface of the second image bearing member; and a transfer unit configured to convey a sheet between the first image bearing member and the second image bearing member and transfer a developer on the first image bearing member and the second image bearing member, onto the sheet. The first image forming unit is disposed upstream of the second image forming unit in a sheet conveying direction. A force of the second rotating member rubbing the surface of the second image bearing member is less than a force of the first rotating member rubbing the surface of the first image bearing member.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2013-146324, which was filed on Jul. 12, 2013, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to an image forming apparatus including a rotating member for rubbing a surface of an image bearing member.

2. Description of the Related Art

There is known a direct tandem image forming apparatus including a plurality of photoconductive drums. This image forming apparatus includes cleaning members capable of contacting the respective photoconductive drums. Each of the cleaning members rubs a surface of a corresponding rotating one of the photoconductive drums to remove foreign matters, e.g., paper dust from the photoconductive drum.

SUMMARY

Incidentally, foreign matters, e.g., paper dust are easily attached to a photoconductive drum located on the most upstream side in a sheet conveying direction at a position near an opening for loading of a sheet. Thus, a force of the cleaning member rubbing the photoconductive drum is preferably made large. However, if forces of all the cleaning members rubbing the respective photoconductive drums are made large, unnecessary loads are applied to the respective photoconductive drums other than the most upstream photoconductive drum. This may inhibit smooth rotation of the photoconductive drums, leading to a printing failure such as banding.

This invention has been developed to provide an image forming apparatus enabling better removal of foreign matters from a surface of an upstream photoconductive drum in a sheet conveying direction and smooth rotation of a downstream photoconductive drum.

The present invention provides an image forming apparatus including: a first image forming unit including a first image bearing member and a first rotating member configured to rub a surface of the first image bearing member; a second image forming unit including a second image bearing member and a second rotating member configured to rub a surface of the second image bearing member; and a transfer unit configured to convey a recording sheet between the first image bearing member and the second image bearing member and transfer a developer on the first image bearing member and the second image bearing member, onto the recording sheet. The first image forming unit is disposed upstream of the second image forming unit in a direction in which the recording sheet is conveyed. A force of the second rotating member rubbing the surface of the second image bearing member is less than a force of the first rotating member rubbing the surface of the first image bearing member.

DETAILED DESCRIPTION OF THE EMBODIMENT

Hereinafter, there will be described one embodiment of the present invention by reference to the drawings. In the following description, an overall structure of a color printer1as one example of an image forming apparatus will be explained first, and features of the present invention will be thereafter explained.

In the following description, directions are defined with respect to a user using this color printer1. That is, a left side, a right side, a back side, and a front side inFIG. 1are respectively defined as a front side, a rear side, a left side, and a right side. Also, an up and down direction inFIG. 1are defined as an up and down direction.

<Overall Construction of Color Printer>

As illustrated inFIG. 1, the color printer1includes a body housing10, an upper cover11, a sheet-supply portion20for supplying a sheet S as one example of a recording sheet, an image forming portion30for forming an image on the supplied sheet S, and a sheet-output portion90for discharging the sheet S on which the image is formed.

The upper cover11is provided on an upper portion of the body housing10so as to pivot about a pivot shaft12located at a rear portion of the body housing10such that a front portion of the upper cover11moves upward and downward with respect to the body housing10. This movement of the upper cover11opens and closes an opening10A formed in an upper face of the body housing10.

The sheet-supply portion20includes a sheet-supply tray21, provided in a lower portion of the body housing10, for storing sheets S, and a sheet-supply mechanism22for supplying the sheets S from the sheet-supply tray21to the image forming portion30. The sheets P in the sheet-supply tray21are separated one by one by the sheet-supply mechanism22and supplied to the image forming portion30.

The image forming portion30includes four LED units40, four process units50, a transfer unit70, and a fixing unit80.

Each of the LED units40is pivotably supported by the upper cover11via a holder14and disposed on an upper side of a corresponding one of photoconductive drums52in a state in which the upper cover11is closed. This LED unit40illuminates or exposes a surface of the electrically charged photoconductive drum52by blinking, based on image data, of a light emitting portion, i.e., an LED, provided at a distal end of the LED unit40.

The process units50are arranged in parallel in the front and rear direction between the upper cover11and the sheet-supply tray21so as to be mountable on and removable from the body housing10substantially in the up and down direction through the opening10A of the body housing10which is exposed when the upper cover11is open.

The process units50are constituted by process units50K,50Y,50M,50C respectively containing black toner, yellow toner, magenta toner, and cyan toner and arranged in this order from an upstream side in a sheet conveying direction (in which the sheet S is conveyed) that is directed from a front side to a rear side. In other words, the process unit50K for black toner as one example of a first image forming unit is disposed upstream of the process unit50C for cyan toner as one example of a second image forming unit in the sheet conveying direction, and the process unit50Y for yellow toner as one example of a third image forming unit and the process unit50M for magenta toner as one example of a fourth image forming unit are arranged between the process unit50K and the process unit50C, the process unit50Y being disposed upstream of the process unit50M. The process unit50K for black toner is disposed near an opening which is formed in the image forming portion30for loading of the sheet S.

Each of the process units50includes a drum unit51and a developing unit61which is removably mounted on the drum unit51.

The drum unit51includes a drum frame59, the photoconductive drum52as one example of an image bearing member provided on the drum frame59, a charging unit53, and a cleaning roller100as one example of a rotating member. The cleaning roller100will be explained later. It is noted that rotational speeds of the photoconductive drums52of the respective process units50are set to be the same.

The developing unit61includes a developing roller62, a supply roller63, a layer-thickness limiting blade64, and a toner container65for containing toner as one example of a developer which is positively charged.

Each of the developing rollers62is provided corresponding to one of the photoconductive drums52and bears toner on its surface. This developing roller62supplies toner onto the photoconductive drum52when the developing roller62contacts the photoconductive drum52in a state in which a positive developing bias is applied to the developing roller62.

The transfer unit70is provided between the sheet-supply tray21and the process units50and includes a drive roller71, a driven roller72, an endless conveyor belt73looped over the drive roller71and the driven roller72, and the four transfer rollers74. An outer surface of the conveyor belt73is held in contact with the photoconductive drums52, and the conveyor belt73conveys the sheet S between the process unit50K and the process unit50C. The transfer rollers74are arranged inside the conveyor belt73so as to be opposite the photoconductive drums52, with the conveyor belt73interposed between the transfer rollers74and the photoconductive drums52.

The fixing unit80is provided at a rear of the process units50and the transfer unit70and includes a heated roller81and a pressure roller82disposed so as to opposite the heated roller81to press the heated roller81.

In the image forming portion30, the surface of the photoconductive drum52is electrically charged uniformly by the charging unit53, and then illuminated and exposed by the LED units40, so that an electrostatic latent image based on image data is formed on the photoconductive drum52.

The toner in the toner container65is supplied to the developing roller62via the supply roller63and then to a position between the developing roller62and the layer-thickness limiting blade64and borne on the developing roller62as a thin layer having a constant thickness. In this process, the toner is frictionally charged positively between the developing roller62and the supply roller63and between the developing roller62and the layer-thickness limiting blade64.

The toner borne on the developing roller62is supplied to an exposed region of the photoconductive drum52, which forms an visible image from the electrostatic latent image, that is, a toner image is formed on the photoconductive drum52. The sheet S supplied from the sheet-supply portion20is thereafter conveyed through an area between the photoconductive drums52and the conveyor belt73, whereby the toner images formed on the respective photoconductive drums52are transferred to the sheet S. The sheet S on which the toner images are transferred is conveyed through a position between the heated roller81and the pressure roller82, whereby the toner images are fixed to the sheet S by heat.

The sheet-output portion90includes a sheet output passage91for guiding the sheet S conveyed from the fixing unit80, and a plurality of conveying rollers92for conveying the sheet S. The sheet S on which the toner image is fixed by heat, i.e., the sheet P on which the image is formed is conveyed by the conveying rollers92through the sheet output passage91, discharged to the outside of the body housing10, and placed onto a sheet-output tray13.

There will be next explained the cleaning rollers100.

As illustrated inFIG. 2, the cleaning rollers100are rotatably provided for the respective photoconductive drums52. Each of the cleaning rollers100scrubs the surface of the corresponding photoconductive drum52to remove foreign matters (such as paper dust and toner) from the photoconductive drum52. In use, the cleaning roller100and the photoconductive drum52are rotated in the same direction, but at an area of the cleaning roller100which contacts the photoconductive drum52, the traveling direction of the cleaning roller100is reverse to the traveling direction of the photoconductive drum52.

In the following description, the word “first” is affixed to the members corresponding to black as needed, the word “third” to the members corresponding to yellow, the word “fourth” to the members corresponding to magenta, and the word “second” to the members corresponding to cyan. Furthermore, each of the reference numerals for the components relating to the colors of toner such as the photoconductive drum52and the cleaning roller100may contain a corresponding one of the signs “K”, “Y”, “M”, and “C” respectively representing black, yellow, magenta, and cyan.

Each of the cleaning rollers100is constituted by a roller shaft110having a circular cylindrical shape and a roller portion120formed of foam rubber and covering the roller shaft110. In other words, the surface of the cleaning roller100which contacts the photoconductive drum52is formed of foam rubber. In the present embodiment, the roller shafts110of the respective cleaning rollers100have the same construction, and the photoconductive drums52also have the same construction. Also, the cleaning rollers100have the same outside diameter, and the roller shafts110and shafts of the respective photoconductive drum52are respectively spaced from each other at the same distance.

Specifically, the first roller portion120K is formed of ethylene propylene rubber, and the second roller portion120C is formed of silicon rubber. Because of these constructions, the hardness of the second cleaning roller100C is less than that of the first cleaning roller100K. Accordingly, in a case where the cleaning rollers100are in contact with the respective photoconductive drums52so as to be compressed by the same amount, the resilience of the second roller portion120C is less than that of the first roller portion120K. That is, a pressure at which the second cleaning roller100C is in contact with the second photoconductive drum52C is less than a pressure at which the first cleaning roller100K is in contact with the first photoconductive drum52K.

The third roller portion120Y is formed of ethylene propylene rubber like the first roller portion120K, and the fourth roller portion120M is formed of silicon rubber like the second roller portion120C. Accordingly, a pressure at which the third cleaning roller100Y is in contact with the third photoconductive drum52Y is equal to the pressure at which the first cleaning roller100K is in contact with the first photoconductive drum52K, and a pressure at which the fourth cleaning roller100M is in contact with the fourth photoconductive drum52M is equal to the pressure at which the second cleaning roller100C is in contact with the second photoconductive drum52C.

In the present embodiment, the rotational speed of the second cleaning roller100C is set to be lower than the rotational speed of the first cleaning roller100K. Accordingly, the difference in rotational speed between the second cleaning roller100C and the second photoconductive drum52C is smaller than the difference in rotational speed between the first cleaning roller100K and the first photoconductive drum52K.

The rotational speed of the third cleaning roller100Y is set to be equal to that of the first cleaning roller100K, and the rotational speed of the fourth cleaning roller100M is set to be equal to that of the second cleaning roller100C. Accordingly, a difference in rotational speed between the third cleaning roller100Y and the third photoconductive drum52Y is equal to the difference in rotational speed between the first cleaning roller100K and the first photoconductive drum52K, and a difference in rotational speed between the fourth cleaning roller100M and the fourth photoconductive drum52M is equal to the difference in rotational speed between the second cleaning roller100C and the second photoconductive drum52C.

To make the rotational speeds of the cleaning rollers100different from one another, gear ratios of gear trains for driving the respective cleaning rollers100are made different from one another, for example.

There will be next explained operations and effects of the color printer1including the cleaning rollers100having the constructions described above.

The cleaning rollers100are rotated in the same direction as the photoconductive drums52while scrubbing the surfaces of the respective photoconductive drums52. Here, the cleaning roller100and the surface of the photoconductive drum52scrub each other at their respective contact portions, so that foreign matters (such as paper dust and toner) on the surface of the photoconductive drum52are removed by the cleaning roller100.

Incidentally, foreign matters, e.g., paper dust are easily attached in particular to the first photoconductive drum52K located near the opening for loading of the sheet S on the most upstream side in the sheet conveying direction among the photoconductive drums52. Thus, a force of the first cleaning roller100K rubbing the first photoconductive drum52K is preferably made larger. In this case, if a force of the second cleaning roller100C rubbing the second photoconductive drum52C is also made large, an unnecessary load is applied to the second photoconductive drum52C which is located on the most downstream side among the photoconductive drums52and to which foreign matters, e.g., paper dust are less attached when compared with the first photoconductive drum52K. This unnecessary load inhibits smooth rotation of the second photoconductive drum52C, leading to a printing failure such as banding.

In the present embodiment, however, the difference in rotational speed between the second cleaning roller100C and the second photoconductive drum52C is less than the difference in rotational speed between the first cleaning roller100K and the first photoconductive drum52K. Thus, a frictional force by which the second cleaning roller100C scrubs the second photoconductive drum52C is smaller than a frictional force by which the first cleaning roller100K scrubs the first photoconductive drum52K. That is, since the force of the second cleaning roller100C rubbing the surface of the second photoconductive drum52C is smaller than the force of the first cleaning roller100K rubbing the surface of the first photoconductive drum52K, a load applied from the second cleaning roller100C to the second photoconductive drum52C is smaller than a load applied from the first cleaning roller100K to the first photoconductive drum52K. Accordingly, foreign matters can be reliably removed from the surface of the first photoconductive drum52K, and the second photoconductive drum52C can be rotated smoothly.

It is noted that the above-described difference in rotational speed can be set as needed. While the cleaning roller100and the photoconductive drum52are rotated in the same direction in the present embodiment, in a case where the difference in rotational speed between the second cleaning roller100C and the second photoconductive drum52C is less than the difference in rotational speed between the first cleaning roller100K and the first photoconductive drum52K, at least one of the cleaning rollers100may be rotated in a direction reverse to the rotational direction of the photoconductive drum52(that is, the traveling direction of the cleaning rollers100coincides with the traveling direction of the photoconductive drum52). Also, the rotational speed of each cleaning roller100may be set as needed as long as the above-described conditions are satisfied. For example, in a case where the rotational direction of the cleaning roller100and that of the photoconductive drum52are reverse to each other, the rotational speed of each cleaning roller100may be made greater than the rotational speed of the photoconductive drum52.

The pressure at which the second cleaning roller100C is in contact with the second photoconductive drum52C is less than the pressure at which the first cleaning roller100K is in contact with the first photoconductive drum52K. Accordingly, the force of the second cleaning roller100C rubbing the surface of the second photoconductive drum52C is smaller than the force of the first cleaning roller100K rubbing the surface of the first photoconductive drum52K, enabling more smooth rotation of the second photoconductive drum52C.

Incidentally, foreign matters, e.g., paper dust are attached to the third photoconductive drum52Y located next to the most upstream first photoconductive drum52K in the sheet conveying direction at the second highest frequency. Thus, a force of the third cleaning roller100Y rubbing the third photoconductive drum52Y is preferably set to be relatively large. In the present embodiment, the contact pressure and the rotational speed of the third cleaning roller100Y are equal to those of the first cleaning roller100K, so that the magnitude of the force of the third cleaning roller100Y rubbing the third photoconductive drum52Y is equal to that of the force of the first cleaning roller100K rubbing the first photoconductive drum52K. Accordingly, the third photoconductive drum52Y can be rubbed by a force identical to the force by which the first photoconductive drum52K is rubbed.

While the embodiment of the present invention has been described above, it is to be understood that the invention is not limited to the details of the illustrated embodiment, but may be embodied with various changes and modifications, which may occur to those skilled in the art, without departing from the spirit and scope of the invention.

In the above-described embodiment, the difference in rotational speed between the second cleaning roller100C and the second photoconductive drum52C is less than the difference in rotational speed between the first cleaning roller100K and the first photoconductive drum52K. However, these differences in rotational speed may be equal to each other as long as the force of the second cleaning roller100C rubbing the second photoconductive drum52C is smaller than the force of the first cleaning roller100K rubbing the first photoconductive drum52K.

While the material of the first roller portion120K and that of the second roller portion120C are different from each other in the above-described embodiment, these roller portions may be formed of the same material as long as the force of the second cleaning roller100C rubbing the second photoconductive drum52C is smaller than the force of the first cleaning roller100K rubbing the first photoconductive drum52K.

FIG. 3illustrates one example of a configuration in which the cleaning rollers are rotated at the same speed, and the roller portions are formed of the same material. InFIG. 3, compression springs230having different urging forces are provided.

In this configuration, each of cleaning rollers200is constituted by a roller shaft210and a roller portion220.

The compression springs230respectively urge components such as bearings of the cleaning rollers200to the photoconductive drums52, and urging forces of the respective compression springs230are partly different from one another.

Specifically, the first compression spring230K as one example of a first urging member has the largest urging force among the compression springs230, with the third compression spring230Y and the fourth compression spring230M following in that order. The second compression spring230C as one example of a second urging member has the same construction as the fourth compression spring230M.

In this configuration, the first cleaning roller200K has the highest contact pressure among the cleaning rollers200, with the third cleaning roller200Y and the fourth cleaning roller200M following in that order. Also, the contact pressure of the second cleaning roller200C is equal to that of the fourth cleaning roller200M. It is noted that each of the third cleaning roller200Y and the fourth cleaning roller200M can be considered to as a second rotating member. The compression spring230is used as the urging member, but the present invention is not limited to this configuration, and the compression spring230may be a torsion spring, for example.

FIG. 4illustrates another example of the configuration in which the cleaning rollers are rotated at the same speed, and the roller portions are formed of the same material. InFIG. 4, roller portions320have different thicknesses.

In this configuration, each of cleaning rollers300is constituted by a roller shaft310and the roller portion320, and the roller shafts310have the same construction. Also, distances between axes of the photoconductive drums52and axes of the respective roller shafts310are also coincide with each other.

The first roller portion320K and the third roller portion320Y have the largest thickness (the same thickness) among the roller portions320, with the fourth roller portion320M and the second roller portion320C following in that order. In this configuration, the first roller portion320K and the third roller portion320Y have the largest amount of compression among the roller portions320when each pressed by the corresponding roller shaft310and the photoconductive drum52, with the fourth roller portion320M and the second roller portion320C following in that order.

In this configuration, the first cleaning roller300K and the third cleaning roller300Y have the largest contact pressure among the cleaning rollers300, with the fourth cleaning roller300M and the second cleaning roller300C following in that order.

As another example different from those illustrated inFIGS. 3 and 4,FIG. 5illustrates a configuration in which roller portions420formed of foam rubber have different thicknesses.

In this configuration, each of cleaning rollers400is constituted by a roller shaft410and the roller portion420. The roller shaft410K has the largest diameter among the roller shafts410, with the roller shafts410Y,410M,410C following in that order. A roller portion420K has the smallest thickness among the roller portions420, with the roller portions420Y,420M,420C following in that order. The cleaning rollers400have the same outside diameter.

Increase in the thickness of the roller portion420decreases elastic coefficient. Accordingly, in a case where the cleaning rollers400are respectively held in contact with the photoconductive drums52by the same amount of compression, the first roller portion420K presses the corresponding photoconductive drum52back by the largest force among the cleaning rollers400, with the third roller portion420Y, the fourth roller portion420M, and the second roller portion420C following in that order. Also, the first roller shaft410K has the largest diameter and its outer circumferential surface is spaced apart from the corresponding photoconductive drum52at the smallest distance among the roller shafts410, with the third roller shaft410Y, the fourth roller shaft410M, and the second roller shaft410C following in that order. Thus, the force of the first cleaning roller400K rubbing the corresponding photoconductive drum52is affected by the corresponding roller shaft410by the largest amount among the cleaning rollers400, with the third cleaning roller400Y, the fourth cleaning roller400M, and the second cleaning roller400C following in that order. Accordingly, the first cleaning roller400K is set to have the largest contact pressure among the cleaning rollers400, with the third cleaning roller400Y, the fourth cleaning roller400M, and the second cleaning roller400C following in that order.

The surface of the second cleaning roller100C is formed of the foam rubber in the above-described embodiment, but the present invention is not limited to this configuration. For example, the second cleaning roller100C may be constituted by a brush roller as illustrated inFIG. 6. It is noted that cleaning rollers500are set to be rotated at the same speed.

In this configuration, the first cleaning roller500K is constituted by a roller shaft510and a roller portion520formed of foam rubber.

Each of the second cleaning rollers500Y,500M,500C is a brush roller constituted by the roller shaft510and a brush layer541with a plurality of fibers540held on an outer circumferential surface of the roller shaft510. In this configuration, each of the second cleaning rollers500Y,500M,500C can remove foreign matters, e.g., paper dust from the surface of the second photoconductive drum52C such that the fibers540stroke the surface of the second photoconductive drum52C.

Here, each of the second cleaning rollers500Y,500M,500C constituted by the brush rollers is smaller than the first cleaning roller500K formed of the foam rubber in force for rubbing the corresponding photoconductive drum52, and consequently a smaller load is applied to the photoconductive drum52by each of the second cleaning rollers500Y,500M,500C, whereby the second photoconductive drum52C can be rotated smoothly. Also, the brush roller is less expansive than the roller formed of the foam rubber, resulting in reduced cost.

Only the first cleaning roller500K has the foam rubber inFIG. 6, but the present invention is not limited to this configuration. For example, as illustrated inFIG. 7, all cleaning rollers600K-600C may be constituted by brush rollers. It is noted that the cleaning rollers600are set to be rotated at the same speed.

In this configuration, each of the first cleaning rollers600K,600Y,600M is constituted by a roller shaft610and a brush layer651with a plurality of fibers650held on an outer circumferential surface of the roller shaft610, and the second cleaning roller600C is constituted by a roller shaft610and a brush layer641with a plurality of fibers640held on an outer circumferential surface of the roller shaft610. Each fiber650of the first cleaning rollers600K,600Y,600M is longer than each fiber640of the second cleaning roller600C, so that the area of contact of each fiber640with the second cleaning roller600C is smaller than the area of contact of each fiber650with the corresponding one of the first cleaning rollers600K,600Y,600M. Accordingly, the force of the second cleaning roller600C rubbing the corresponding photoconductive drum52is smaller than the force of each of the first cleaning rollers600K,600Y,600M rubbing the corresponding photoconductive drum52.

In the embodiment, the thickness of the fiber may be changed to make the force of each cleaning roller rubbing the corresponding photoconductive drum52different from each other.

In the above-described embodiment, the photoconductive drum52is used as one example of the image bearing member, but the present invention is not limited to this configuration. For example, a photoconductor belt may be used as the image bearing member.

While the present invention is applied to the color printer1in the above-described embodiment, the present invention may be applied to other image forming apparatuses such as a copying machine and a multifunction peripheral.

While the sheet S such as a thick paper sheet, a postcard, and a thin paper sheet is used as the recording sheet in the above-described embodiment, other types of sheets such as an OHP sheet may be used as the recording sheet.