Image forming apparatus

Abstract

An image forming apparatus has an image bearing member. A transfer member forms a nip with the image bearing member and transfers a toner image to a surface of a transfer sheet guided to the nip. A guide is immediately upstream of the nip in a sheet conveying direction. The guide is in sliding contact with one side of the transfer sheet and guides the transfer sheet to the nip. The guide includes a rigid member having a flat surface along the sheet conveying direction. A cutout is formed in the flat surface of the rigid member. The guide also has a flat elastic member placed on the flat surface of the rigid member to cover the cutout and to be held in sliding contact with the transfer sheet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an essential portion of an image forming apparatus according to one embodiment of the invention.

FIG. 2 is a side view of a guide member of the image forming apparatus.

FIG. 3 is a plan view of the guide member of the image forming apparatus.

FIG. 4 is a section showing an essential portion of the image forming apparatus.

FIG. 5 is a diagram showing an experimental example showing grounds for an elastic force.

FIG. 6 is a table showing a relationship between second members made of elastic flat members and elastic forces of transfer sheets.

FIG. 7 is a table showing judgment results on the adaptability of second members made of elastic flat members in an actual machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, an image forming apparatus according to one embodiment of the present invention is described with reference to the accompanying drawings.

FIG. 1 is a diagram showing an essential portion of the image forming apparatus according to one embodiment of the invention, FIG. 2 is a side view of a guide member of the image forming apparatus, FIG. 3 is a plan view of the guide member of the image forming apparatus, and FIG. 4 is a section showing an essential portion of the image forming apparatus.

As shown in FIG. 1, the image forming apparatus includes a photoconductive drum 1 as an image bearing member having a photoconductive layer on the outer surface thereof. A charger 2, an exposing device 3, a developing device 4, a transfer roller 5 as a transfer member, a sliding-contact member (roller) 6, a cleaning blade 7 and a charge remover 8 such as an LED are arranged around the photoconductive drum 1 in this order in a rotating direction of the photoconductive drum 1. Identified by S in FIG. 1 is a transfer sheet (recording sheet). A heating element 9 including a rod heater or surface heater is held in contact with the sliding-contact member 6.

The photoconductive drum 1 and transfer roller 5 face each other and are pressed in contact, thereby forming a nip portion 10. A guide member 11 for guiding the transfer sheet S to the nip portion 10 is disposed immediately upstream of the nip portion 10 in a conveying direction of the transfer sheet S.

The photoconductive drum 1 is for bearing an electrostatic latent image formed on the outer surface thereof. An amorphous silicon photoconductive drum may be, for example, used as the photoconductive drum 1. This photoconductive drum is constructed such that a carrier injection inhibition layer made of Si:H:B:O or the like, a carrier excitation/transport layer (photoconductive layer) made of Si:H or the like, and a surface protection layer made of SiC:H or the like are successively laminated on a conductive substrate.

The charger 2 is disposed above the photoconductive drum 1 for uniformly charging the photoconductive drum 1.

The exposing device 3 is for forming an electrostatic latent image on the photoconductive drum 1, for example, based on an image data of a document read by an unillustrated image data input unit or an image data outputted from a personal computer or the like.

The developing device 4 is for forming a toner image by supplying a developer to the outer surface of the photoconductive drum 1 formed with the electrostatic latent image. It should be noted that the developer used in this embodiment contains toner particles and magnetic powder and abrasive fine particles are fixed to the outer surfaces of the toner particles. Fine particles having high hardness such as metal oxides of alumina, zirconia and titania are suitable as the abrasive fine particles. In addition to the abrasive fine particles, fine powder of silica or the like may be added to regulate fluidity and charging property.

The transfer roller 5 is a roller member for transferring the toner image on the photoconductive drum 1 to the transfer sheet S. The transfer roller 5 applies a bias voltage to the transfer sheet S from the rear side of the transfer sheet S when the transfer sheet S passes the nip portion 10. By giving transfer charges to the transfer sheet S, the toner image on the photoconductive drum 1 is transferred to the transfer sheet S.

The sliding-contact member 6 is rotatably disposed to face the photoconductive drum 1 and comes into contact with the outer surface of the photoconductive drum 1 to abrade off ion products formed on the outer surface of the photoconductive drum 1 in an image forming process. Here, the sliding-contact member 6 is biased toward the photoconductive drum 1 by an unillustrated spring. The developer contains titanium oxide or the like as an abrasive agent as described above, and the outer surface of the photoconductive drum 1 is abraded by this titanium oxide when the sliding-contact member 6 comes into contact with the outer surface of the photoconductive drum 1.

The cleaning blade 7 is disposed above the sliding-contact member 6 for cleaning the residual toner particles on the outer surface of the photoconductive drum 1 by being held in sliding contact with the photoconductive drum 1. The cleaning blade 7 is made of urethane rubber or the like and is pressed in contact with the photoconductive drum 1. The residual toner particles scraped off by the cleaning blade 7 is collected by a toner collecting screw (not shown).

As shown in FIGS. 2 and 3, the guide member 11 is such that a first member 12, which is a rigid flat member, and a second member 13, which is an elastic flat member, are placed one over another.

The first member 12 is formed at the leading edge located at a downstream side in a sheet conveying direction with a cutout 12a having a specified width W along a sheet width direction normal to the sheet conveying direction. It is sufficient for the first member 12 to have a flat surface along the sheet conveying direction, and it is not always necessary to use a flat member.

The second member 13 is attached to the front surface of the first member 12 so as to cover the cutout 12a and come into sliding contact with one side (rear side) of the transfer sheet S.

As shown in FIG. 4, the leading end of the first member 12 is located closer to the transfer roller 5 than a tangent P at the nip portion 10. Contrary to this, the leading end of the second member 13 is located closer to the photoconductive drum 1 than the tangent P. Thus, the transfer sheet S is conveyed along such a path “p” as to enter the nip portion 10 from a side closer to the photoconductive drum 1 than the tangent P.

The first member 12 can be made of a material having a high shape retaining property such as a hard resin or metal. Further, in the case of considering the conveyance of a postal card as the transfer sheet S, the width of the cutout 12a in the sheet width direction may be set to be slightly wider (for example, 106 mm) than the width of the postal card normal to the conveying direction of the postal card.

The second member 13 is preferably made of a conductive material. This can prevent the friction charging of the guide member 11 resulting from the sliding contact of the transfer sheet being conveyed with the second member 13, thereby preventing the guide member 11 from being smeared by the scattered toner particles. Further, the second member 13 is preferably made of a material having an abrasion resistance and a low coefficient of friction (0.15 or lower). This can ensure a sufficient resistance against the sliding contact of the transfer sheet S and reduce a frictional force during the sliding contact of the transfer sheet S. The second member 13 preferably has a specified elastic force selected from a range from 0.002 gf/mm² (inclusive) to 0.02 gf/mm² (inclusive).

FIG. 5 is a diagram showing an experimental example showing grounds for the above elastic force, FIG. 6 is a table showing a relationship between second members 13 and elastic forces of transfer sheets S, and FIG. 7 is a table showing judgment results on the adaptability of the respective second members 13 in an actual machine.

In FIG. 5, an experiment was conducted using high-molecular polyethylene sheets as one embodiment of the second member 13. The second members 13 made of high-molecular polyethylene sheets are placed on a base 21 and pressed by a retainer 22, and leading end portions thereof are caused to overhang. Projecting amounts (free lengths) were 50 mm. After hanging amounts of the leading end portions due to their own weights were measured, amounts of displacement were measured in the case where a load of 1 Kg was applied to the leading end portions. Numerical values obtained by load (1 Kg)÷(amount of actual displacement×sample width) using numerical values obtained by subtracting the hanging amounts from the amounts of displacement as actual amounts of displacement are written as elastic force in FIG. 6.

Sheet thickness, free length, and transfer defects in FIG. 7 are respectively the thickness of the second member 13, the depth T (see FIG. 3) of the cutout 12a, and phenomenon in which the entire sheet surface becomes white due to excessive transfer charges. As shown in FIG. 7, good print results free from transfer defects and white spots could be obtained if the sheet thickness is 280 μm to 410 μm (elastic forces of 0.002 gf/mm² to 0.02 gf/mm²).

As described above, the image forming apparatus of this embodiment is constructed such that the cutout 12a having the width W larger than the width of postal cards and the depth T is formed in the middle part of the rigid first member 12 and is covered by the second member 13 having a specified elasticity. By this construction, a conveyance load can be reduced utilizing the elastic force of the second member 13 upon guiding postal cards as thick sheets to the nip portion 10, whereby the conveyance speed of the transfer sheet S can be maintained while being unlikely to cause the formation of abnormal images such as those with hollow defects.

Not only by forming the cutout 12a, but also by covering the cutout 12a with the second member 13, the formation of abnormal images (transfer defects and white spots) caused by abnormal discharge of a transfer voltage can be prevented on the transfer sheets S of normal sizes wider than the wide W of the cutout 12a and thick sheets narrower than the width W of the cutout 12a.

Although the second member 13 preferably has substantially the same size as the first member 12 as shown in solid line in FIG. 3, it may have a slightly larger size than the cutout 12a as shown in chain double-dashed line in FIG. 3.

The specific embodiment described above mainly embraces inventions having the following constructions.

An image forming apparatus according to one aspect of the present invention comprises an image bearing member for bearing a toner image; a transfer member arranged to face the image bearing member to form a nip portion together with the image bearing member, and adapted to transfer the toner image to a surface of a transfer sheet guided to the nip portion; a guide member arranged immediately upstream of the nip portion in a sheet conveying direction so as to guide the transfer sheet to the nip portion and to be held in sliding contact with one side of the transfer sheet, wherein the guide member includes a first member made of a rigid member, having a flat surface along the sheet conveying direction and having a cutout with a specified width along a sheet width direction normal to the sheet conveying direction formed in the flat surface at a leading end portion located at a downstream side in the sheet conveying direction, and a second member made of an elastic flat member and placed on the flat surface of the first member so as to cover the cutout and to be held in sliding contact with the one side of the transfer sheet.

According to this construction, when a thick transfer sheet of a size corresponding to that of the cutout is guided to the nip portion, the rigid first member is not present by the presence of the cutout and the transfer sheet can be conveyed to the nip portion while being held in sliding contact only with the elastic second member. Therefore, hollow defects and a reduction in conveying speed can be prevented for thick postal cards and the like.

In the above construction, if the first member is also a flat member, the guide member can have a simple structure. In this case, the first member is preferably made of a hard metal or a material having a high shape retaining property such as a metal.

In this construction, when a postal card is conveyed as the transfer sheet, the width of the cutout in the sheet width direction is preferably slightly wider than the width of the postal card normal to a conveying direction of the postal card. According to this construction, a conveyance load can be normally reduced utilizing the elastic force of the second member when a postal card as a thick sheet is guided to the nip portion.

In this construction, it is preferable that the leading end of the first member is located closer to the transfer member than a tangent at the nip portion and that of the second member is located closer to the image bearing member than the tangent. Thus, the transfer sheet is conveyed along such a path as to enter the nip portion from a side closer to the image bearing member than the tangent at the nip portion. Therefore, in consideration of the elastic deformation of the second member, the transfer sheet can be better brought to the nip portion.

In this construction, the second member is preferably made of a conductive material.

According to this construction, the guide member can be prevented from being smeared by the scattered toner particles by preventing the friction charging of the guide member caused by the sliding contact of the transfer sheet being conveyed with the second member.

The second member preferably has an elastic force selected from a range from 0.002 gf/mm² (inclusive) to 0.02 gf/mm² (inclusive). This enables good print results free from transfer defects and white spots to be obtained.

Further, the second member is preferably made of material having an abrasion resistance and/or a low coefficient of friction. This can ensure resistance against the sliding contact of the transfer sheet and reduce a frictional force during the sliding contact of the transfer sheet.

A high-molecular polyethylene sheet can be suitably used as the second member.

This application is based on patent application No. 2006-235057 filed in Japan, the contents of which are hereby incorporated by references.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to embraced by the claims.

Claims

1. An image forming apparatus, comprising: an image bearing member for bearing a toner image;a transfer member arranged to face the image bearing member to form a nip portion together with the image bearing member, and adapted to transfer the toner image to a surface of a transfer sheet guided to the nip portion;a guide member arranged immediately upstream of the nip portion in a sheet conveying direction so as to guide the transfer sheet to the nip portion and to be held in sliding contact with one side of the transfer sheet,wherein the guide member includes: a first member made of a rigid member, having a flat surface along the sheet conveying direction and having a cutout with a specified width along a sheet width direction normal to the sheet conveying direction formed in the flat surface at a leading end portion located at a downstream side in the sheet conveying direction, anda second member made of an elastic flat member and placed on the flat surface of the first member so as to cover the cutout and to be held in sliding contact with the one side of the transfer sheet.

2. An image forming apparatus according to claim 1, wherein the first member is a flat member.

3. An image forming apparatus according to claim 2, wherein the first member is made of a material having a high shape retaining property such as a hard resin or metal.

4. An image forming apparatus according to claim 1, wherein, when a postal card is conveyed as the transfer sheet, the width of the cutout in the sheet width direction is slightly wider than the width of the postal card normal to a conveying direction of the postal card.

5. An image forming apparatus according to claim 1, wherein the leading end of the first member is located closer to the transfer member than a tangent at the nip portion and that of the second member is located closer to the image bearing member than the tangent.

6. An image forming apparatus according to claim 1, wherein the second member is made of a conductive material.

7. An image forming apparatus according to claim 1, wherein the second member has an elastic force selected from a range from 0.002 gf/mm2 (inclusive) to 0.02 gf/mm2 (inclusive).

8. An image forming apparatus according to claim 1, wherein the second member is made of an abrasion resistant material.

9. An image forming apparatus according to claim 1, wherein the second member is made of a material having a low coefficient of friction.

10. An image forming apparatus according to claim 1, wherein the second member is made of a material having an abrasion resistance and a low coefficient of friction.

11. An image forming apparatus according to claim 10, wherein the second member is a high-molecular polyethylene sheet.