Excess developing liquid removing device

Abstract

A device comprising a rotatable roller disposed for removing excess developing liquid from the developed surface of a photoreceptor in electrophotographic copying apparatus, along with a developing liquid removing means for removing the developing liquid from the roller. By adjusting the roller speed and the gap between the photoreceptor surface and the roller surface passing in opposite directions the thickness of the developing liquid on the photoreceptor surface may be regulated to produce better and drier copies.

Description

BACKGROUND OF THE INVENTION

This invention relates to excess developing liquid removing devices, and more particularly involves a device for removing excess developing liquid from photoreceptors in electrophotographic copying apparatus.

In one type of electrophotographic copying apparatus known in the art, a drum-shaped photoreceptor is electrically charged and then exposed to an optical image of an original while the photoreceptor is rotated to form thereon an electrostatic latent image which is developed by a developing liquid into a visible or toner image and printed on copy sheets by transfer printing after excess developing liquid is removed from the surface of the photoreceptor by an excess developing liquid removing device. By this process, duplicates of the original are produced upon fixing of the toner images transferred to the copy sheets.

Various types of excess developing liquid removing devices have hitherto been used. There is a squeeze roller type in which the roller is brought into pressing engagement with the surface of the photoreceptor for squeezing out excess developing liquid from the photoreceptor. An air knife type uses a stream of air blown onto the surface of the photoreceptor to remove excess developing liquid from its surface. In a corona discharge squeeze type, excess developing liquid on the surface of the photoreceptor is removed by means of corona discharge, whereas in a proximity squeeze type excess developing liquid on the surface of the photoreceptor is removed by a blade arranged in close proximity to the photoreceptor.

Some disadvantages are associated with these types of excess developing liquid removing devices. The squeeze roller type tends to damage the toner image on the photoreceptor because the roller is brought into pressing engagement with the surface of the photoreceptor. Difficulty is experienced in obtaining a roller having a smooth and planar peripheral surface, in maintaining good adherence of toner images on the photoreceptor, and in synchronizing the rotation of the roller with the photoreceptor. In the air knife type, the air tends to become polluted by vaporization of the developing liquid and, the image on the photoreceptor tends to become splashed with the developing liquid. Also, noise is caused by the air stream, and the toner on the photoreceptor is blown by the air to cause distortion of the image. It is also difficult to apply a uniform air stream to the photoreceptor. In the corona discharge squeeze system, corona discharge produces ozone which tends to cause deterioration of the photoreceptor and the roller. Moreover, this type is low in efficiency when production of duplicates is carried out at high speed. The blade or the like is spaced apart from the photoreceptor in the proximity squeeze type, so that it is not possible to remove excess developing liquid sufficiently to produce dry duplicates by this system.

SUMMARY OF THE INVENTION

The present invention provides an excess developing liquid removing device wherein a roller, disposed in close proximity to the periphery of a photoreceptor, is rotated such that the surface of the roller disposed near the surface of the photoreceptor moves in a direction substantially opposite to the direction of movement of the surface of the photoreceptor so as to remove excess developing liquid from the surface of the photoreceptor and adjust the amount of developing liquid thereon to any level as desired in accordance with the peripheral velocity of the roller and the dimension of the gap between the roller and the photoreceptor, and the developing liquid collected on the roller is removed therefrom by a developing liquid removing means. The excess developing liquid removing device of the character described obviates the aforementioned disadvantages of the prior art and is highly efficient in removing excess developing liquid from the surface of the photoreceptor. The device permits dry duplicates to be obtained of a quality equal to or higher than that of the duplicates obtained by using the air knife type remover.

In addition, the roller used is electrically insulated and a bias voltage may be forcibly impressed on the roller to thereby minimize background staining in the duplicates produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an electrophotographic copying apparatus in which the present invention is incorporated;

FIG. 2(a) and FIG. 2(b) are a sectional front view and a sectional side view, respectively, of the excess developing liquid removing device illustrating one embodiment of the invention;

FIG. 3(a) and FIG. 3(b) are a sectional front view and a sectional side view, respectively, of the excess developing liquid removing device illustrating another embodiment of the invention;

FIG. 4 is a view in explanation of the operation of the illustrated embodiments;

FIG. 5 and FIG. 6 are diagrams showing the characteristics of the illustrated embodiments;

FIGS. 7(a)-(b) through FIGS. 9(a)-(b) are views in explanation of the influences exerted by the quality of the surfaces of the photoreceptor and the roller on the results achieved by the invention; and

FIGS. 10 through 13 are fragmentary schematic side views of other embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

A drum-shaped photoreceptor 11 of the type used with the present invention is shown in FIG. 1, driven by drive means (not shown) to rotate at a predetermined rate, and with an electrical discharge device 12, an exposing device 13, a developing device 14, an excess developing liquid removing device 15, a transfer-printing device 16 and a cleaning device 17, all arranged about the photoreceptor 11. In operation, the photoreceptor 11 first has its entire peripheral surface charged electrically by device 12 and the charged peripheral surface of the photoreceptor is then exposed to an optical image of an original by the exposing device 13 to form thereon an electrostatic latent image of the original. The electrostatic latent image is developed into a visible image with a developing liquid by the developing device 14 and excess developing liquid on the surface of the photoreceptor 11 is removed by the excess developing liquid removing device 15. The visible image is then printed on a copy sheet 18 by the transfer-printing device 16, and the photoreceptor 11 is cleaned by the cleaning device 17.

The transfer-printing device 16 may use a transfer-printing roller, or it may be an electrostatic transfer-printing device. A copy sheet 18 is fed between the photoreceptor 11 and transfer-printing device 16 by a copy sheet feed device 19 at a rate which is in synchronism with the rotation of photoreceptor 11, and the sheet is separated from the photoreceptor 11 by separating means (not shown) after the visible or toner image on the periphery of the photoreceptor 11 is printed on the copy sheet by the transfer-printing device 16. The separated copy sheet is ejected onto a suitable duplicate discharge tray.

The developing device 14 may comprise a pump 21 for delivering developing liquid to a supply means 22 from which the developing liquid is supplied to the photoreceptor 11. The developing liquid spilled by the supply means 22 is recovered by a vessel 23.

The excess developing liquid removing device 15 comprises a roller 24 and a blade means 25 disposed in the excess developing liquid collecting vessel 23. As shown in FIG. 2(a) and FIG. 2(b), the roller 24 has supported at opposite ends thereof insulated smaller length rollers 26 and 27 having built-in roller bearings which are supported for rotation relative to stub shafts 28 and 29 coaxially therewith. The stub shafts 28 and 29 are supported for rotation by bearings 30 and 31 respectively. The roller 24 is supported by well-known means and electrically insulated from the other parts.

The bearings 30 and 31 are supported by support plates 32 and 33 respectively for movement toward and away from the axis of the photoreceptor 11. A spring 34 is mounted between the support plate 32 and bearing 30 and a spring 35 is mounted between the support plate 33 and bearing 31, so that the bearing 30 and 31 are urged by the biasing forces of springs 34 and 35 to move upwardly toward the photoreceptor 11.

The smaller length rollers 26 and 27 each have a radius which is greater than the radius of roller 24 by a value in a range from 0.01 to 1 millimeter and are urged by the biasing forces of springs 34 and 35 into pressing engagement with opposite end portions of the periphery of photoreceptor 11. Thus the roller 24 is spaced apart from the periphery of photoreceptor 11 by a gap d in a range from 0.01 to 1 millimeter.

A sprocket wheel 36 is mounted on stub shaft 29, and a chain 38 is trained over sprocket 36 and another sprocket 37 which is connected to a drive shaft of a variable speed motor 39. Thus, the roller 24 may be rotated at any peripheral velocity as desired as shown in FIG. 2(b) by the variable speed motor 39 through the sprocket wheel 37, chain 38, and sprocket wheel 36. The small length rollers 26 and 27 engage and are rotated by the photoreceptor 11 in the opposite direction that is clockwise in FIG. 2(b). The movement of the surface of the roller 24 in a direction opposite to the adjacent surface of the photoreceptor 11 removes excess developing liquid from the photoreceptor 11 and adjusts the amount of developing liquid on the photoreceptor in accordance with the rate of rotation of the roller 24 and the dimension of the gap d between the photoreceptor 11 and roller 24. By varying the number of revolutions of motor 39 through the use of a speed control 39', it is thus possible to control the amount of developing liquid on the periphery of photoreceptor 11. The blade 25 is maintained in pressing engagement with the roller 24 for removing developing liquid from the periphery of the roller 24 so that the roller 24 is cleaned and the removed developing liquid is collected in the vessel 23.

The excess developing liquid removing device may also be constructed as shown in FIG. 3(a) and FIG. 3(b). This embodiment comprises a roller 40 similarly disposed in the excess developing liquid collecting vessel 23 and juxtaposed to the photoreceptor 11. The roller 40 is supported by stub shafts 41 and 42 which in turn are rotatably supported by levers 45 and 46 through insulated bearings 43 and 44 respectively. The levers 45 and 46 are each pivotally supported substantially at their central portions by a shaft 47, and smaller length rollers 48 and 49 are mounted through eccentric screws 50 and 51 on one end portion of each of the levers 44 and 45, or on the side of roller 40. Thus, by turning adjusting the eccentric screws, it is possible to readily and positively adjust the gap between the periphery of photoreceptor 11 and the periphery of roller 40. Also, this eliminates the need to work on the roller 40 by precision machining to impart to it a desired diameter, and the roller 40 can be juxtaposed to photoreceptor 11 along the entire length thereof.

A spring 52 is mounted between an immovable support plate member and the other end of lever 45 and a spring 53 is mounted between an immovable support plate member and the other end of lever 46, so that the levers 45 and 46 are urged by the biasing forces of springs 52 and 53 to move counterclockwise about shaft 47 to bring the smaller length rollers 48 and 49 into pressing engagement with the photoreceptor 11. The roller 40 is spaced apart from the photoreceptor 11 by a gap in a range from 0.01 to 1 millimeter, and the gap can be varied by turning and adjusting the eccentric screws 50 and 51 to adjust the eccentricity of smaller length rollers 48 and 49.

An insulated sprocket wheel 54 is mounted on stub shaft 42, and a chain 56 is trained over sprocket 54 and another sprocket 55 connected to a drive shaft of a variable speed motor 57. Thus, the roller 40 is driven by the variable speed motor 57 through the sprocket wheel 55, chain 56 and sprocket wheel 54 to rotate at any peripheral velocity as desired through the use of a speed control 57' in the same direction of rotation of the photoreceptor 11 (both counterclockwise in FIG. 3(b) so that excess developing liquid on the photoreceptor 11 can be removed and the amount of developing liquid thereon can be adjusted to any level as desired in accordance with the peripheral velocity of roller 40 and the dimension of the gap between the roller 40 and photoreceptor 11. The amount of developing liquid on the photoreceptor 11 can be adjusted by adjusting the eccentric screws 50 and 51 and variable speed motor 57.

A blade means 58 is maintained in pressing contact with the roller 40 to remove excess developing liquid from the roller 40 and clean the same. The removed excess developing liquid is returned to the vessel 23.

Operation of the roller of the excess developing liquid removing device 15 will now be described. The developing liquid on the photoreceptor 11 after application to the electrostatic latent image thereon has a thickness of several hundred microns due to surface tension. By virtue of the rotation a flow of developing liquid of a speed U oriented in the direction of rotation of photoreceptor 11 takes place on the peripheral surface of photoreceptor 11 as shown in FIG. 4. Likewise, a flow of developing liquid of a speed V takes place on the roller 24 (40) in its direction of rotation. Thus, a flow of developing liquid of a speed U - V = W takes place between the photoreceptor 11 and roller 24 (40), so that the developing liquid is torn apart in two streams.

If the rate of rotation V2 of roller 23 (40) is increased, the speed V will be increased and the thickness t.sub.2 of the developing liquid removed from the photoreceptor 11 will be increased. The removal value of V2 becomes saturated when the rate of rotation of the roller 24 (40) exceeds a certain level depending upon the coefficient of viscosity, surface tension and specific gravity of the developing liquid, the diameter and degree of surface smoothness of the photoreceptor and roller, and other conditions. Experiments have been carried out using ISOPAR G (trade name) with a viscosity of 1.31 cp and a specific gravity of 0.76 as the developing liquid, and rotating the roller in the same direction as the photoreceptor and then stopping its rotation and rotating it in a direction opposite to the direction of rotation of the photoreceptor. It was ascertained that when the peripheral velocity of the roller exceeds 1,200 mm/sec, the the value of t.sub.2 is constant even if the dimension of the gap T is varied.

When the peripheral velocity of roller 24 (40) was varied by adjusting the motor 39 (57), the amount of developing liquid transferred from the photoreceptor 11 to the copy sheet 18 was as shown in FIG. 5 with the sheet 18 being of a size A4 Japanese Industrial Standards, 297 .times. 210 mm.sup.2 and of high quality paper with the gap T being 0.1 millimeter. When the gap T was varied in dimension, the amount of transferred developing liquid was as shown in FIG. 6. In FIG. 5 and FIG. 6, the dotted line l is the limit below which the copy sheets ejected onto the duplicate-receiving tray are not wet to the touch. That is, when the amount of developing liquid adhering to the copy sheets is below the limit l, they are dry to the touch. It was found that when the roller was rotated in the same direction as the photoreceptor or the rotation of the roller was stopped, the amount of developing liquid adhering to the photoreceptor was increased by over eightfold above the level of minimum amount of developing liquid adhering to the photoreceptor when the roller was rotated in a direction opposite to the direction of rotation of the photoreceptor. It was thus ascertained that by rotating the roller at a peripheral velocity of over 800 mm/sec in a direction opposite to the direction of rotation of the photoreceptor with the gap T being in a range from 0.05 to 0.1 millimeter, excess developing liquid was removed without damaging the toner image on the photoreceptor and dried duplicates of high quality could be produced by using the roller 24 (40) alone.

It is important that the thickness of developing liquid adhering to the photoreceptor immediately after the electrostatic latent image has been developed be greater than the gap T. For example, when the developing liquid is small in amount and its thickness is reduced in part below the level of T, or when the roller is disposed such that the liquid film is torn apart in part as the roller increases its peripheral velocity in rotating in a direction opposite to the direction of rotation of the photoreceptor, the thickness t.sub.2 is reduced and the thickness t.sub.1 is increased. As a result, the amount of developing liquid adhering to the photoreceptor will be greater than is necessary. Thus, it was ascertained that the roller should be disposed near the developing position and immediately below the photoreceptor. Also, it was ascertained that distortion of the developed image was less likely to occur if the roller was disposed near the developing position, because the force of the photoreceptor to attract the toner electrically was reduced in going from the developing device in the direction of rotation of the photoreceptor. It was also ascertained that the smaller the diameter of the roller, the greater was the amount of developing liquid removed.

The distribution of the speeds U, V and W of the flows of developing fluid can be varied by varying the rate of revolution of the photoreceptor and the roller. That is, if the rate of movement of the peripheral surface of the photoreceptor 11 is reduced, then the distribution of speed U changes from that shown in FIG. 7(a) to that shown in FIG. 7(b). If the rate of rotation of the roller 24 (40) is increased, then the distribution of speed V is changed from that in FIG. 8(a) to that in FIG. 8(b). If the combination of the photoreceptor shown in FIG. 7(a) and the roller shown in FIG. 8(a) is changed into the combination of the photoreceptor shown in FIG. 7(b) and the roller shown in FIG. 8(b), then the distribution of speed W is changed from that shown in FIG. 9(a) to that shown in FIG. 9(b). This indicates that the efficiency of removing developing liquid is increased and the thickness of developing liquid on the photoreceptor is reduced.

The distribution of the speeds U, V and W of the flows of developing liquid may also vary depending on the smoothness of the surfaces of photoreceptor and roller. Attention should be paid to the operation of the developing liquid removing blade 25 (59). If the pressure applied by the blade is too low or too much liquid flows through the blade means, the clinging liquid will increase the thickness t.sub.1, resulting in an increase in the amount of liquid on the photoreceptor.

The roller 24 (40) of the aforementioned excess developing liquid removing device 15 may be electrically insulated from other parts of the apparatus so that it may electrically float and have a self-bias effect. More specifically, image regions of the photoreceptor 11 shown in FIG. 10 have an electric potential ranging from 900 to 950 volts and non-image regions have an electric potential of about 150 volts. If an electric potential of about 300 volts is induced in the roller 24 (40), the electric potential of the roller will be lower than the image regions of the photoreceptor and higher than the non-image regions thereof. As a result, toner will flow to the image regions and the toner on the non-image regions will be attracted to the image regions. Thus, duplicates of high quality with a clear-cut image of high contrast having no background staining can be produced. The roller 24 (40) may be made of an electrically conducting material. However, a coat of rubber having a volume resistivity of over 10.sup.8 .OMEGA. cm may be applied to the roller, and any other material may be used for the roller so long as it has a self-bias effect.

Since the roller operates in the presence of a large amount of developing liquid immediately after the electrostatic latent image on the photoreceptor is developed, the developing step may be considered to be prolonged. In the case where the roller is electrically conducting, all the toner will be transferred to the photoreceptor by electrophoresis if the roller is used while being grounded but without being insulated from other parts. In such event, the toner will adhere to non-image regions of the photoreceptor having an electric potential of -200 volts, thereby causing background staining in the duplicates produced. Therefore, the roller should be insulated from other parts.

Also, when the gap between the photoreceptor and roller is gradually reduced, the surfaces of the photoreceptor and roller may be partially brought into contact with each other and the roller may become grounded. This may result in the aforementioned disadvantage. The surface of the electrically conducting roller should be treated with alumilite, resin coating or rubber coating to insulate the same.

A bias voltage EV may be forcibly impressed from a DC power source 59 to the roller 24 (40) of the excess developing liquid removing device as shown in FIG. 11 so that the roller may have a bias effect. The system is more effective than the aforementioned self-bias system in producing duplicates of uniform contrast in all the cases, because the results achieved by the self-bias system tend to show variations depending on the size of the original. The impressed bias voltage is set such that it is lower than the electrical potential of the image regions of the photoreceptor and higher than the electrical potential of the non-image regions. The impressed bias system can achieve better results than the self-bias system in reducing background staining of the duplicates produced and achieving higher contrast of the image. The roller 24 (40) may be made of an electrically conducting material, but it may be made of an insulating material and a bias potential may be imparted to it through corona discharge by means of a charging device.

The drum-shaped photoreceptor of the electrophotographic copying apparatus in which the present invention is incorporated may be replaced by a belt-shaped photoreceptor 60 as shown in FIG. 12 which may be trained over a number of rollers 61 and driven by drive means. If the roller 24 (40) is juxtaposed to one of the rollers 61, the gap between the roller 24 (40) and photoreceptor 60 will be constant.

In one type of electrophotographic copying apparatus known in the art, a photoreceptor in sheet form is electrically charged, exposed to an optical image of an original and developed so that the photoreceptor serves as a copy sheet for the duplicate. The present invention can have application in this type of apparatus also. More specifically, excess developing liquid on a photoreceptor sheet 62 shown in FIG. 13 can be removed therefrom by means of the roller 24 (40) and blade means 25 (58) after the electrostatic latent image on the photoreceptor sheet 62 is developed. The photoreceptor sheet 62 which is conveyed by a belt 65, moved by rollers 63 and 64 and other rollers, is attracted to the belt as air is drawn by suction through openings 68 formed in the belt 65 and openings 67 formed in a suction box 66 evacuated by means of a pump or the like. The photoreceptor sheet 62 while passing the suction box 66 is stable. Excess developing liquid on the photoreceptor sheet can be removed by the excess developing liquid removing device according to the invention while the sheet is stable on the suction box.

Claims

What is claimed is:

1. In an electrophotographic copying apparatus including a moving photoreceptor, electrostatic charging means, exposure means and transfer means, the combination comprising:

means for applying liquid developer onto the surface of the moving photoreceptor to convert an electrostatic latent image into a visible image;

a rotating roller means disposed adjacent to said liquid developer applying means for removing excess liquid developer from the surface of the moving photoreceptor to form a uniform film of liquid developer thereon;

means for supporting said rotating roller means to provide a gap between the periphery of said rotating roller means and the surface of the moving photoreceptor for thereby preventing said rotating roller means from damaging said visible image formed on the surface of the moving photoreceptor, said supporting means comprising a shaft on which said rotating roller means is rotatably supported, a pair of electrically insulated rollers rotatably supported on said shaft at opposite ends of said rotating roller means, said pair of electrically insulated rollers each having a diameter larger than that of said rotating roller means for thereby providing said gap, and biasing means for urging said pair of electrically insulated rollers into pressing contact with the surface of the moving photoreceptor;

driving means for rotating said rotating roller means such that the periphery of said rotating roller means moves at the gap in a direction substantially opposite to the direction of movement of the surface of the moving photoreceptor; and

means disposed in contact with the periphery of said rotating roller means for removing liquid developer therefrom.

2. The combination according to claim 1, in which said driving means comprises a variable speed motor

3. The combination according to claim 2, further comprising means for controlling said variable speed motor to vary the number of revolutions of said speed motor.

4. The combination according to claim 1, further comprising means for applying a bias voltage to said rotating roller means, the bias voltage being selected to be lower in magnitude than the electrical potential of image regions of the moving photoreceptor and higher than the electrical potential of non-image regions.

5. The combination according to claim 1, peripheral which the photoreceptor comprises a rotating drum.

6. The combination according to claim 1, in which the photoreceptor comprises a belt moved by rollers.

7. In an electrophotograhic copying apparatus including a moving photoreceptor, electrostatic charging means, exposure means and transfer means, the combination comprising:

means for applying liquid developer onto the surface of the moving photoreceptor to convert an electrostatic latent image into a visible image;

a rotating roller means disposed adjacent to said liquid developer applying means for removing excess liquid developer from the surface of the moving photoreceptor to form a uniform film of liquid developer thereon;

means for supporting said rotating roller means to provide a gap between the periphery of said rotating roller means and the surface of the moving photoreceptor for thereby preventing said rotating roller means from damaging said visible image formed on the surface of the moving photoreceptor, said supporting means comprising a pair of levers pivotally supported on fixed shafts, a pair of rollers rotatably supported by said levers respectively and held in contact engagement with the surface of the moving photoreceptor, a rotatable shaft on which said rotating roller means is rotatably disposed, said rotatable shaft being rotatably supported by said levers at an intermediate position between said pair of rollers and said fixed shafts to provide said gap, means cooperating with said pair of rollers for varying the gap between the moving photoreceptor and said rotating roller means, and biasing means urging said pair of levers to maintain said pair of rollers in contact with said moving photoreceptor;

driving means for rotating said rotating roller means such that the periphery of said rotating roller means moves at the gap in a direction substantially opposite to the direction of movement of the surface of the moving photoreceptor; and

means disposed in contact with the periphery of said rotating roller means for removing liquid developer therefrom.

8. The combination according to claim 7 in which said driving means comprises a variable speed motor.

9. The combination according to claim 7, further comprising means for applying a bias voltage to said rotating roller means, the bias voltage being selected to be lower in magnitude than the electrical potential of image regions of the moving photoreceptor and higher than the electrical potential of non-image regions.

10. The combination according to claim 7, in which the photoreceptor comprises a belt moved by rollers.