Image forming apparatus

Abstract

 in an image forming apparatus, after charging, exposure, and developing operations are performed as a drum- shaped Image carrier (27) is rotated, an image formed on the surface of the image carrier (27) by the use of a developer is transferred to a copying paper (P) by a transfer device (35A), and the developer Image after the transfer is fixed onto the copying paper (P) by a fixing device (36). In this apparatus, the diameter of the image carrier (27) is limited to 80 mm at most,

Description

[0001] The present invention relates to an image forming apparatus with a photosensitive drum, more specifically to an image forming apparatus for reproducing an image of an original paper on a copying sheet through the medium of a photosensitive drum.

[0002] In a conventional image forming apparatus, a copying paper is peeled from an image carrier, such as a photosensitive drum, by the following methods. In a first method, an elongate sharp edge portion of a plate member is first abutted against the outer peripheral surface of the photosensitive drum at an acute angle. Then, the edge portion is inserted between the outer peripheral surface of the photosensitive drum and the copying paper clinging thereto as the photosensitive drum rotates. Thus, the copying paper is mechanically separated from the photosensitive drum. In a second method, the forward end of the copying paper is previously bent a little to form a bent strip. A plate member is disposed near the outer peripheral surface of the photosensitive drum so that an elongate sharp edge portion of the plate member is not in contact with the outer peripheral surface of the photosensitive drum. Then, the bent strip of the copying paper clinging to the outer peripheral surface of the rotating drum is caused to abut against the edge portion. Thus, the copying paper is mechanically separated from the photosensitive drum. In a third method, a fast stream of air is first blown against the forward end of the copying paper sticking to the outer peripheral surface of the photosensitive drum. Then, an air stream layer is passed between the outer peripheral surface of the photosensitive drum and the copying paper. Thus, the copying paper is separated from the photosensitive drum. In a fourth method, a suction drum is brought into rolling contact with the copying paper sticking to the outer peripheral surface of the photosensitive drum. Thus, the copying paper is separated from the photosensitive drum by the sucking force of the suction drum. A fifth method suitably combines the first to fourth methods.

[0003] According to the first and second methods, however, the copying paper cannot enjoy full-width transfer, and the effective area for transfer is limited. The third and fourth methods require large size and a large-capacity blower for the production of the fast air stream or the sucking force of the suction drum. According to the third and fourth methods, moreover, it is difficult to peel the copying paper securely from the photosensitive drum even with use of the large-capacity blower, since the force of adhesion between the outer peripheral surface of the photosensitive drum and the copying paper is great if the transferred image covers the whole surface of the copying paper.

[0004] The object of the present invention is to provide an image forming apparatus which is inexpensive, compact, and capable of securely peeling copying paper from an image carrier without the use of the conventional peeling means.

[0005] According to an aspect of the present invention, there is provided an image forming apparatus which reproduces an image of an object on a sheet, and which comprises an image carrier in the form of a drum with a given diameter and rotated in one direction, charging means for charging the surface of the image carrier, exposure means exposing the charged surface of the image carrier to the image of the object to be reproduced, thereby forming an electrostatic latent image corresponding to the image of the object on the surface of the image carrier, developing means for visualizing the electrostatic latent image by means of a developer, transfer means for electrostatically transferring the image visualized by the developer to the sheet, and peeling means for electrostatically attracting the sheet after transfer so that the sheet is separated from the surface of the image carrier, wherein the diameter of the image carrier is limited to 80 mm at most.

[0006] In order to attain the above object, the image forming apparatus is characterized in that the diameter of the photosensitive drum as an image carrier is reduced to increase the force to separate the paper from the outer peripheral surface of the photosensitive drum.

[0007] This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a front view schematically showing one embodiment of the image forming apparatus of the present invention;

Fig. 2 is a model diagram illustrating how to find the bending strength of paper;

Figs. 3 to 5 are diagrams showing the relationships between attraction angle 8 and the bending strength of paper obtained with use of photosensitive drums with diameters of 120 mm, 90 mm and 78 mm, respectively;

Fig. 6 is a diagram showing the relationship between the diameters of photosensitive drums and the bending strength of paper obtained with use of an attraction angle of 18°; and

Fig. 7 is a front view schematically showing the principal part of the image forming apparatus for illustrating the admission angle of paper.

[0008] There will now be described in detail one embodiment of the image forming apparatus of the present invention with reference to the accompanying drawings.

[0009] Fig. 1 is a longitudinal sectional front view schematically showing an image forming apparatus, e.g., an electrostatic copying apparatus, according to one embodiment of the present invention. In Fig. 1, numeral 20 designates an original table fixed on the top of a copying apparatus housing 21. Inside the housing 21, an exposure device 22 underlies the original table 20. The exposure device 22 includes a lamp 23 and a first mirror 24 moving together along the horizontal direction of Fig. 1 so that the surface of an original paper set on the original table 20 can be scanned and illuminated. Also, the exposure device 22 is provided with a second mirror 26 which reflects a reflected light from the first mirror 24 to project it on a lens 25 located ahead of the second mirror 26. The second mirror 26 is moved so that the length of an optical path from the first mirror 24 to the lens 25 is fixed. The exposure device 22 is further provided with fixed mirrors 28 for projecting the light transmitted through the lens 25 on an image carrier, such as a photosensitive drum 27, which is rotatably set at the substantially central portion of the interior of the copying apparatus housing 21. The photosensitive drum 27 has a drum body and a photosensitive layer covering the whole outer peripheral surface of the body. An image of the original paper illuminated by the lamp 23 is formed on the surface of the photosensitive drum 27 by the exposure device 22.

[0010] A charger 29 is disposed near the photosensitive drum 27. By the illumination at the image formation, an electrostatic latent image is formed on the surface of the photosensitive drum 27 charged by the charger 29. The electrostatic latent image is visualized when a developer is applied thereto by a developing device 30. Disposed at one side portion of the copying apparatus housing 21 is a paper feed device 33 which includes a cassette 31 storing a pile of copying papers _P and a delivery roller 32. A copying paper P delivered from the paper feed device 33 runs against a pair of resist rollers 34 to be oriented thereby, and is then carried onto the surface of the photosensitive drum 27 on which the visible image is formed. A transfer device 35A and a peeling charger 35B are arranged side by side right under the photosensitive drum 27 at a space therefrom. The visible image is transferred by the transfer device 35A to the paper P as the paper P is passed over the transfer device 35A. Then, the paper P is passed over the peeling charger 35B to be separated electrostatically thereby from the photosensitive drum 27. Thereafter, the image is fixed onto the paper P by a fixing device 36. Subsequently, the paper P with the image thereon is carried out into a tray 37 at the other side portion of the housing 21. A cleaning device 38 is disposed near the photosensitive drum 27. After the transfer, the surface of the photosensitive drum 27 is cleaned by the cleaning device 38. In Fig. 1, a two-dots and dash line represents a path for the paper P extending from the one side portion of the housing 21 to the other.

[0011] The peeling charger 35B is used for the following reason. Electrostatic charges emitted toward the photosensitive drum 27 by the transfer device 35A neutralize those electrostatic charges on the outer circumferential surface of the photosensitive drum 27 which absorb the developer forming the visible image on the outer peripheral surface of the photosensitive drum 27, and transfer the developer onto the paper P. Meanwhile, the electrostatic charges from the transfer device 35A are directly applied to that portion of the outer peripheral surface of the photosensitive drum 27 which bears no developer thereon, i.e., a no-image portion. The electrostatic attraction of the electrostatic charges constitutes one of the causes for the sticking of the paper P to the outer peripheral surface of the photosensitive drum 27. Thus, in order to separate the paper P from the photosensitive drum 27, it is necessary to use the peeling charger 35B which neutralizes the electrostatic charges on the no-image portion.

[0012] It has already been found that the neutralization of the electrostatic charges by the peeling charger 35B is not enough for the complete separation of the paper P from the photosensitive drum 27.

[0013] According to the present invention, therefore, the diameter of the photosensitive drum 27 is limited to 80 mm at most for the perfect separation of the paper P from the drum 27. This is done for the following reason.

[0014] As seen from Fig. 2, flexure y of the paper P is given by

and distributed load w (kg/mm) is given by

where E is the Young's modulus of the paper P, I is geometrical moment of inertia acting on the cross section of the paper P, and R is the radius of the photosensitive drum 27. By four times differentiating the flexure y of equation (I) with respect to x and substituting for d⁴y dx⁴ in equation (II), we obtain bending strength compared with the length of the paper P as follows:

[0015] Let us suppose, for example, that the paper P used is of the size A4. Then, if the bending strength of the paper P varying with the diameter d of the photosensitive drum 27 is calculated on the basis of equation (III), the results shown in Figs. 3 to 5 may be obtained. Here the A4-size paper P is supposed to have Young's modulus of 9.75 x 10² kg/mm, density of 0.73 x 10-⁶ kg/mm³, width of 210 mm, and thickness varying from 50 µ to 150 µ. Figs. 3 to 5 show the relationships between attraction angle (θ of Fig. 2) and bending strength obtained with the thickness as a parameter. Experiments were conducted to see how the paper P is separated from photosensitive drums with various diameters. The results are as follows.

Example 1

[0016] A paper P with a thickness of 90 p (basic weighing 65 g/m2) was able to be separated from a photosensitive drum with diameter d of 120 mm, while papers with thicknesses of 57 µ (basic weighing 47 g/m2) or less were not.

Example 2

[0017] A paper P of 57-p thickness (basic weighing 47 g/m²) was able to be separated from a photosensitive drum with diameter d of 90 mm, while a paper of 50-p thickness (basic weighing 42 g/m²) was not.

Example 3

[0018] All of papers P with thicknesses of 50 p to 150 µ (basic weighing 42 g/m2 to 129 g/m²) were able to be separated from a photosensitive drum with diameter d of 78 mm.

[0019] The commonly used paper P weighs 42 g/m² to 129 g/m², and has a thickness of 50 p to 150 µ. Therefore, as seen from Figs. 3 to 5 and Fig. 6 showing on the basis thereof the relationships between the diameter d of the photosensitive drum 27 and the bending strength of the paper P obtained with use of attraction angle 6 of 180°, as well as from Examples 1 to 3, papers P with bending strength of 0.2 g/mm or more can satisfactorily be separated from the photosensitive drum 27. In Fig. 6, it should be noted that a line in the case of a paper with a thickness of 150 is not shown because that its bending strength is more than 1.0 g/mm. Accordingly, good peeling can always be achieved if the diameter d of the photosensitive drum 27 is approximately 80 mm or less.

[0020] In the present invention, proper locations of the transfer device 35A and the separating charger 35B, as well as proper admission position of the paper P approaching the photosensitive drum 27, are essential to the satisfactory peeling. Thus, as shown in Fig. 7, a wire A of the transfer device 35A is located plumb under the center O of the photosensitive drum 27. A wire B of the peeling charger 35B is located in the position corresponding to the intersection of a straight line & passing through the wire A at right angles to a segment OA (a segment connecting the center 0 of the photosensitive drum 27 and the wire position A in the transfer device 35A) and the extension of a segment OD which extends at a clockwise central angle of 18° to the segment OA. The admission position of the paper _P is located on the intersection C of the circumference of the photosensitive drum 27 and a segment symmetrical with the segment OD with respect to the segment OA. By this location, the paper P can be satisfactorily peeled from the photosensitive drum 27.

Example 4

[0021] A developer with negative charging characteristic was used, and voltages of +6.0 kV DC and +5.2 kV AC were applied to the transfer device 35A and the peeling charger 35B, respectively. The positions of the transfer device 35A and the peeling charger 35B and the admission position or angle of the paper P were so set that 4AOD = 4AOC = 18°, and admission angle a (an angle to a horizontal line at which the paper P is admitted) is 12°, as shown in Fig. 7. Thus, a tangential angle formed between the admission passage of the paper P and a tangent which touches the circumference of the photosensitive drum 27 at the admission position C is 30°. A hundred thousand copies were made with use of the feeding speed of paper P of 130 mm/sec. The diameter of the photosensitive drum 27 used is 78 mm. In this case, no copying paper suffered defective peeling.

[0022] Although an illustrative embodiment of the present invention has been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to that embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.

[0023] For example, the tangential angle B of the paper P to the photosensitive drum 27 has been described as being 30°. However, this angle β is not limited to 30°, and may be any other angle less than an angle φ or 90° which is formed between the segment OC of Fig. 7 and the tangent at the point C where the forward end of the paper P first touches the surface of the photosensitive drum 27.

[0024] According to the present invention, as described in detail herein, the following effects may be obtained. First, the copying paper can satisfactorily be peeled from the photosensitive drum without using any mechanical means or a large-capacity blower. Accordingly, the image forming apparatus of the invention can be made more compact and the manufacturing cost can be reduced. Secondly, it is possible to satisfactorily peel the copying paper as thin as 50 p or less which conventionally could only be peeled by the use of mechanical separating means. Moreover, there may be provided a highly reliable image forming apparatus in which at least a hundred thousand copying papers can be peeled after transfer.

Claims

1. An image forming apparatus which reproduces an image of an object on a sheet, said image forming apparatus comprising:

an image carrier in the form of a drum with a given diameter and rotated in one direction;

charging means for charging the surface of the image carrier;

exposure means exposing the charged surface of the image carrier to the image of the object to be reproduced, thereby forming an electrostatic latent image corresponding to the image of the object on the surface of the image carrier;

developing means for visualizing the electrostatic latent image by means of a developer;

transfer means for electrostatically transferring the image visualized by the developer to the sheet; and

peeling means for electrostatically attracting the sheet after transfer so that the sheet is separated from the surface of the image carrier,

wherein the diameter of said image carrier is limited to 80 mm at most.

2. The image forming apparatus according to claim 1, wherein said image carrier includes a photosensitive drum.

3. The image forming apparatus according to claim 2, wherein said photosensitive drum includes a cylindrical drum body, and a photosensitive layer laid on the whole outer peripheral surface of the drum body.

4. The image forming apparatus according to claim 1, wherein said transfer means is located plumb under the center of the image carrier at a given space from the surface of the image carrier.

5. The image forming apparatus according to claim 4, wherein said separating means is adjacent to the transfer means along a sheet conveying direction.

6. The image forming apparatus according to claim 4, which further comprises conveyor means for carrying the sheet between the image carrier and the transfer means, said conveyor means carrying the sheet so that the angle formed between the sheet and a tangent which touches the circumferential surface of the image carrier at an admission point thereon for the sheet is less than 90°.

Drawing