JP2000214706A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an image forming device small in size and, furthermore, to reduce the occurrences of image omissions as much as possible even in use of cardboard as paper. SOLUTION: An image carrier 1 and a fixing roller 12 are arranged so that the tangent line H of the fixing means at a part 13b of the recessed part 13a of an elastic pressure member 13 formed in pressure contact with the fixing member 12 which part 13b is the nearest to the image carrier 1 is situated on the side of a transfer roller 11 with respect to a nip point P where the image carrier 1 and a transfer means 11 are in pressure contact with each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a printer, a facsimile, a copying machine, and the like, and more particularly to an image forming apparatus for preventing an image transferred from an image carrier such as a photoreceptor or the like onto a sheet from being clogged. It relates to improvement of the device.

[0002]

2. Description of the Related Art Generally, in an image forming apparatus such as a printer, a facsimile, a copying machine or the like utilizing an electrophotographic system, a toner is visualized by attaching toner to an electrostatic latent image formed on a photosensitive member as an image carrier. Then, the visualized toner image is transferred to a sheet by a transfer unit.

On the other hand, when a toner image is transferred from a photoconductor to a paper by the above-described transfer means, an image (especially, a character) formed by fine lines is not transferred to the paper and remains on the photoconductor, and the image is formed in the image. A detachment phenomenon may occur.

FIG. 4 shows a state in which fine lines are formed on the photoconductor 1. Since the width S of the toner image 2 is formed to be narrow, the area that comes into contact with the sheet is reduced. Therefore, when the paper 3 is pressed against the toner image by the force F via the transfer unit, the pressure applied to the toner image 2 of the thin line increases. Since the central portion of the toner image 2 is formed with a small number of flat portions, the pressure is extremely large, so that the toner image 2 is not transferred to the sheet 3 while being pressed against the photosensitive member 1. The above process is considered to be a cause of image dropout.

FIG. 5 shows a state in which a thick line is formed on the photosensitive member 1. Since the width S of the toner image 4 is formed to be large, the area in contact with the sheet is increased. Therefore, even if the paper 3 is pressed against the toner image by the force F via the transfer unit, the force F is dispersed, and the thick line toner image 4 is less likely to drop out.

[0006] In order to reduce the occurrence of the above-described fine line image hollowing as much as possible, conventionally, it has been practiced to perform a slip transfer by providing a speed difference between the peripheral speed of the photosensitive member and the conveying speed of the conveyed paper. Have been done.

[0007] To achieve this, the following method is used, for example. As shown in FIG. 6, when the VP transport speed is given to the paper 3 by the peripheral speed VR of the pair of registration rollers 10 which are the paper transport rollers for transporting the paper (VP and V
R is substantially the same), and the peripheral speed VD of the photoconductor 1 is set to be several percent slower than the peripheral speed VR of the registration roller 10. Further, the peripheral speed V of a transfer roller 11 which is a transfer unit that contacts the peripheral surface of the photoconductor 1
B is faster than the peripheral speed VR of the registration roller 10 (VB> V
R) Set.

As described above, when the transfer process is performed while setting VB> VR, VP (VP and VR are substantially the same)> VD, the shearing to remove the toner from the toner image 2 shown in FIG. Power will be added. As a result, the occurrence of hollow images is reduced as much as possible.

In the slip transfer system, the registration roller 10 bears much of the sheet conveyance force. for that reason,
The paper that has passed through the registration rollers 10 tends to have a reduced conveying force, and cannot achieve a desired paper speed. In particular, in the case of cardboard such as a postcard made by a government, this is remarkable and a hollow portion is easily generated.

In order to avoid a decrease in the sheet speed, the nip force of the fixing roller 12 as the fixing means is increased or the peripheral speed VH is increased.
It is also conceivable to increase the conveying force to increase the conveying force.

However, both methods cause wrinkling of thin paper and deteriorate MQ (Motion Quality). Therefore, it is not preferable to increase the nip force of the fixing roller 12 or increase the peripheral speed VH as a countermeasure for hollowing out. .

For this reason, the peripheral speed VH of the fixing roller 12 is usually set slightly lower than the transport speed of the thin paper so that the thin paper has an appropriate slack.

[0013]

The fixing roller 12 as the above-mentioned fixing means is generally formed of a rigid body such as a metal containing a heat source such as a heat lamp inside, and the elastic pressing member 13 is a roller. When the elastic pressing member 13 is pressed against the fixing roller 12 formed of an elastic body such as rubber, the peripheral surface of the elastic pressing member 13 is recessed along the peripheral surface of the fixing roller 12 to form a concave portion 13a. .

For this reason, the sheet 3 once guided between the fixing roller 12 and the elastic pressing member 13 is tangential to the fixing roller 12 at the position 13b closest to the photoconductor 1 side of the concave portion 13a (tangent to the fixing means). ) Conveyed along H.

As shown in FIG. 6, when the tangent line H is located on the photosensitive member 1 side with respect to the nip point P where the photosensitive member 1 contacts the transfer roller 11, the photosensitive member 1 and the fixing roller 1
2 (that is, the distance from the nip point P to the position 13b closest to the photoconductor 1 side of the concave portion 13a from the nip point P) when the thick paper 3 such as an official postcard is conveyed. The thick paper 3 is positioned between the nip point P and the position 13b of the concave portion 13a closest to the photoconductor 1, as shown in FIG. At this time, since the thick paper 3 has high stiffness, that is, is hard to be deformed and has high rigidity, a force F ′ is applied to the photoreceptor 1 side by its resilient force so that its own posture follows the roller tangent H. .

In this way, the thick paper 3 is
When the force F 'is applied to the photoconductor 1 so as to conform to the above, the force F of the sheet 3 applied to the toner image 2 shown in FIG.

On the other hand, when thin paper is conveyed as the paper 3, it is similarly positioned between the nip point P and the position 13b of the concave portion 13a closest to the photosensitive member 1, but the thin paper has a weak stiffness. Therefore, it is appropriately bent by gravity. For this reason, the force F is not applied to the photoreceptor 1 and no hollowing occurs.

The above-described photosensitive member 1 and fixing roller 12
Is set to be shorter than the length of transportable paper (for example, government-made postcard) handled by the apparatus.

In view of the circumstances described above, the present invention is compact,
In addition, it is an object of the present invention to provide an image forming apparatus capable of minimizing the occurrence of image dropout even when thick paper is used as a sheet.

[0020]

In order to solve the above-mentioned problems, the present invention provides an image carrier for holding a toner image,
Transfer means for forming a nip point in contact with the image carrier and transferring the toner image to a sheet, fixing means for fixing the toner image formed on the sheet conveyed from the image carrier, and fixing means An image forming apparatus having at least an elastic pressing member that presses against the image bearing member and a sheet conveying path in which a distance between the image carrier and the fixing unit is set shorter than a length of a sheet that can be conveyed. The image bearing member is configured such that a tangent line of the fixing unit at a position closest to the image carrier side of the concave portion of the elastic pressing member formed by pressing the image bearing member is positioned on the transfer unit side with respect to the nip point. The body and the fixing means are arranged.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described in detail.

FIG. 1 shows an image forming apparatus 20 according to the present invention.
In the principal part enlarged conceptual diagram of FIG.

Also in this image forming apparatus 20, slip transfer is performed by providing a speed difference between the peripheral speed VD of the photoreceptor 1 and the transport speed VP of the transported paper, thereby making it possible to prevent image dropout. In order to prevent this, the peripheral speed of the registration roller 10 is set to VR, the transport speed of the sheet 3 is set to VP, and the peripheral speed of the photoconductor 1 is set to V
D, when the peripheral speed of the transfer roller 11 is VB and the peripheral speed of the fixing roller 12 is VH, VB> VR, VP (VP and VR
Are substantially the same)> VD, and VB> VR, VP (VP and VR
Are substantially the same)> VH.

Also in this image forming apparatus 20, the size of the apparatus is reduced, and the registration roller 10 is separated from the nip tangent line Q passing through the nip point P where the photosensitive member 1 and the transfer roller 11 serving as the transfer means abut. The sheet conveying path is curved at the transfer roller 11 side. Further, the distance between the photoconductor 1 and the fixing roller 12 as a fixing unit is also set shorter than the length of a transportable sheet (for example, a postcard made by government) handled by the apparatus. The fixing roller 12 of the embodiment is also formed of a rigid body such as a metal, and the elastic pressing member 13 is also formed of a roller and an elastic body such as a rubber.

For this reason, the elastic pressure member 1
When the pressing member 3 is pressed, the peripheral surface of the elastic pressing member 13 is recessed along the peripheral surface of the fixing roller 12, and a concave portion 13a is formed there. As a result, the sheet once guided between the fixing roller 12 and the elastic pressing member 13 is brought into contact with the tangent H of the fixing roller 12 (tangent to the fixing means) at a position 13b of the concave portion 13a closest to the photoconductor 1 side. It will be conveyed along.

In the image forming apparatus 20, the photosensitive member 1 as the image carrier and the fixing roller 12 are arranged such that the tangent H of the fixing means to the nip point P is located on the transfer roller 11 side. ing.

In FIG. 1, reference numeral d denotes a vertical distance from the nip point P to a tangent H of the fixing means. Next, the operation of the above-described image forming apparatus 20 will be described.

As shown in FIG. 1, the tangent H of the fixing means is
The photoconductor 1 and the fixing roller 12 are disposed so that the photoconductor 1 and the transfer roller 11 are located on the transfer roller 11 side with respect to a nip point P where the photoconductor 1 and the transfer roller 11 abut. At this time, when the thick paper 3 such as an official postcard longer than the distance of the paper conveyance path 15 formed between the photoreceptor 1 and the fixing roller 12 is conveyed, as shown in FIG. Paper 3 is the photoconductor 1
Between the nip point P and the position 13b of the concave portion 13a which is closest to the photoconductor 1 side, the thick paper 3 has Due to the strength, a force F ″ is applied to the transfer roller 11 so that the user's posture follows the tangent line H of the fixing unit.

As described above, when the force F ″ is applied to the transfer roller 11 so that the thick paper sheet 3 follows its tangential line H to the fixing means, the thick paper sheet 3 is applied to the toner image 2 shown in FIG. The force F is reduced by that amount, and the occurrence of image dropout is reduced as much as possible.

The vertical distance d (millimeter) from the nip point P to the tangent line H of the fixing means shown in FIG. 1 was changed, and the degree of occurrence of image voids on thick paper such as government postcards was examined by experiments. However, the experimental results shown in the graph of FIG. 3 were obtained.

In this graph, the vertical axis indicates the degree of occurrence of image dropout due to the thick paper 3. G0 indicates a level at which no image dropout has occurred, G1 indicates a level at which image dropout occurs but has no practical problem, and G4 indicates a level at which the center of most characters is lost and only outlines are transferred.
The horizontal axis represents the tangent H of the fixing unit from the nip point P shown in FIG.
Is the vertical distance d (millimeters).

As is clear from the graph of FIG. 3, it was found that the range where the degree of hollowing of the image was practically acceptable was d = about 5 mm or more.

[0033]

According to the image forming apparatus of the present invention, when thick stiff paper is conveyed to the fixing means, the pressure applied to the image carrier by the stiffness of the thick paper is reduced so that the image is hollowed out. Can be reduced as much as possible.
This is because the tangent line H of the fixing unit at the position closest to the image carrier side of the concave portion of the elastic pressing member formed by pressing the fixing unit is pressed to the transfer unit side with respect to the nip point at which the image carrier and the transfer unit press. This is because the image bearing member and the fixing unit are arranged so as to be positioned at the same position. As described above, a small-sized image forming apparatus with higher image quality can be provided at extremely low cost by a simple design change such as a change in component arrangement.

[Brief description of the drawings]

FIG. 1 is an enlarged conceptual view of a main part of an image forming apparatus according to the present invention.

FIG. 2 is an enlarged conceptual view of an essential part showing an operation of the image forming apparatus according to the present invention.

FIG. 3 is an experimental graph for verifying the degree of occurrence of image hollowing when a vertical distance d from a nip point P to a tangent H of a fixing unit is changed.

FIG. 4 is a diagram illustrating a principle of occurrence of a hollow image;

FIG. 5 is a diagram showing a principle of occurrence of a hollow image;

FIG. 6 is a conceptual diagram showing slip transfer.

FIG. 7 is an enlarged conceptual view of a main part of the image forming apparatus in a state where thick paper is conveyed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Image carrier (photoreceptor) 3 ... Paper 11 ... Transfer means (transfer roller) 12 ... Fixing means (fixing roller) 13 ... Elastic pressure member 13a ... Recess 13b ... Position closest to the image carrier among the recesses 15: paper transport path 20: image forming apparatus P: nip point H: tangent to fixing unit d: vertical distance from nip point to tangent to fixing unit

Claims (2)

[Claims] An image carrier for holding a toner image; a transfer unit for forming a nip point by contacting the image carrier to transfer the toner image to a sheet; A fixing unit for fixing the formed toner image, an elastic pressing member pressed against the fixing unit, and a distance between the image carrier and the fixing unit is set to be shorter than a length of a transportable sheet. An image forming apparatus having at least a sheet conveying path, wherein a tangent line of the fixing unit at a position closest to the image carrier side in a concave portion of the elastic pressing member formed by pressing against the fixing unit, An image forming apparatus, wherein the image carrier and the fixing unit are arranged so as to be located on the transfer unit side with respect to a point. 2. An image forming apparatus according to claim 1, wherein a vertical distance from said nip point to a tangent to said fixing means is about 5 mm or more.