Reproduction Appratus With Liquid Developer

Abstract

Apparatus for developing latent electrostatic images with a liquid developer by forming a wave to contact the image with sufficient turbulence, such that developer particles are suspended uniformly, but gentle enough so as not to destroy the electrical characteristics of the image thereby rendering a high quality print. The wave is formed by pumping liquid developer from a sump pit into a header pipe which is immersed below the surface of a development tray filled with liquid developer. Pressurized gas is introduced into the sump means to maintain uniform suspension of the particles in the developer liquid. In a preferred embodiment, an array of liquid development units of different colors are positioned at a development station to produce multicolor prints by selectively controlling the formation of different waves to contact latent images formed sequentially on a photoreceptor member advanced past the developing units. The liquid developed images are then transferred onto a support sheet in superposed relationship.

Description

This invention relates generally to electrostatic printing and more particularly to apparatus for developing latent electrostatic images with liquid developer.

In the art of electrostatic printing various methods are employed to develop electrostatic images. One such method is known as liquid development. In this method, electrostatic images residing on a surface are developed by applying to the surface a dispersion of finely divided pigment or toner in an insulating carrier liquid. During development, the toner is electrostatically extracted from the carrier liquid and held on the surface in an image pattern. Liquid development of electrostatic images is described in many publications and patents, such as U.S. Pat. No. 3,053,688 issued Sept. 11, 1962 to Harold C. Greig and U.S. Pat. No. 3,084,043 issued Apr. 2, 1963 to R. W. Gundlach.

Liquid development has a number of advantages over dry techniques. In most dry techniques, for example, the toner particles are electrostatically bound to the carrier. As a result, a minimum of force must be overcome before it is even possible for the toner particles to be attracted to the electrostatic image. This minimum force or threshold is not present in the case of liquid developers where the toner particles are freely suspended. Another obvious advantage of liquid developers arises by virtue of the fact that very minute toner particles can be readily suspended in the liquid medium while they are not so readily handled and distributed in dry form.

As a consequence of these and other advantages, it is possible to obtain higher image resolutions with liquid developers than can be accomplished by dry methods. In practice, however, liquid developers have not proved so ideal. In particular, it has been found that when a latent image-bearing surface is brought into contact with a volume of liquid developer, portions of the developer in immediate contact with the latent image areas tend to become depleted of toner particles, thereby depriving more weakly charged areas of sufficient development. If in an attempt to overcome this depletion, turbulence is introduced into the developer, there occurs, on the other hand, a tendency to mechanically and triboelectrically destroy the very delicate latent electrostatic image.

Accordingly it is a general object of the present invention to improve development of latent electrostatic images with liquid development compositions.

It is another object of the present invention to introduce sufficient agitation into liquid developer to maintain uniform suspension of toner particles during development of latent electrostatic images and yet not destroy the electrical properties of such images.

It is a further object of the present invention to provide multicolor electrostatic printing processes and apparatus more efficient than heretofore.

It is yet a further object of the present invention to produce a multicolor print of high quality or ordinary paper.

The foregoing objects and others are accomplished in accordance with the present invention by contacting a latent electrostatic image with a development wave generated with sufficient agitation to suspend developer particles uniformly but not destroy the electrical characteristics of the image rendering a high quality print. Due to the manner in which the wave is produced and controlled, this type of development is well adapted for the production of multicolor prints.

For a better understanding of the nature of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in conjunction with the accompanying drawings in which:

FIG. 1 is a partially cut away isometric view of development apparatus according to the present invention;

FIG. 2 is a side sectional view of development apparatus according to the invention;

FIG. 3 is a sectional view taken along lines 3--of FIG. 2; and

FIG. 4 is a partially schematic and partially side sectional view illustrating a multicolored printing system constructed in accordance with the principles of the present invention.

Referring now to FIGS 1-3 of the drawings, there is shown a tank or container 10 having sidewalls and a bottom wall and being of a size capable of holding a substantial quantity of liquid developer 12. Liquid developer 12 basically consists of bulk solvent carrier having a high electrical resistivity, such as 10.sup.15 ohm-cm., and pigment or coloring forming particles that are capable of being maintained as a dispersion in the carrier. Any suitable liquid developer may be used. Typical developer compositions are described in U.S. Pat. Nos. 2,890,174, 2,891,911, 2,899,335, 2,907,674, and 3,301,675.

Positioned in the lower or sump portion of the tank 10 are a plurality of conduits 15 which are connected to an air pump 17. Conduits 15 are formed with numerous small perforations 19 which are sufficiently minute such that the liquid developer 12 cannot normally overcome surface tension effects through them, but, as a result of pressure from air pump 17, air continuously seeps through perforations 19 and into the body of liquid developer contained within tank 10. This results in a bubbling action which provides continuous agitation within the liquid developer maintaining pigment or toner particles in a uniform dispersion throughout the body of the liquid developer. Such dispersion is highly desirable since if the pigment particles are effectively dispersed, proper electrical charge is maintained for high quality development.

In communication with the liquid developer 12 in tank 10 is a conduit 21 which is connected to an impeller-type pump 23 which serves to draw the liquid developer from tank 10 into a header pipe 25 immersed in a developer tray 27 filled with developer liquid. A series of small spaced apertures or openings 31 extend along the full length of header pipe 25 which is centrally disposed and extends laterally across the width of the latent electrostatic image to be developed. Apertures 31 desirable are of a size ranging from about 0.010 to about 0.050 inches and have spacings ranging from about 0.05 to about 0.25 inches.

In accordance with the present invention as liquid developer 12 emerges from apertures 31 in header pipe 25, a wave is generated on the surface of developer tray 27. The wave formed contacts an electrostatic latent image bearing member 35 which has already been charged and selectively discharged to have a latent electrostatic image on the surface in contact with the crest of the wave. Image bearing member 35 has a layer of photoconductive material on its exposed surface and may, for example, be of the type disclosed in the above mentioned patents. Those regions of the image which are charged attract the pigment or color particles in liquid developer 12 while the uncharged regions remain unaffected by the developer. It should be noted that liquid developer flows over the top of the sides of developer tray 27 and falls back into the large supply of developer in tank 10 where it can be recirculated again in the manner described.

It will be appreciated that the development wave formed on the surface of tray 27 has sufficient agitation to achieve uniform dispersion of the toner particles in the liquid carrier and yet movement is sufficiently gentle so that flow conditions do not destroy the electrical properties of the image. Liquid is pumped out of header pipe 25 at an even rate regardless of the liquid level in tank 10 as long as the pump 23 is covered. Also the relative capacities of the pump 23 and header pipe 27 are such that a controlled smooth wave is formed on the surface of the tray 27. In order to ensure optimum performance drive motor 29 can be of the variable speed type to enable selective flow conditions for the development of the image. Alternatively a solenoid actuated valve may be used to regulate flow conditions. It should be noted that wave height depends upon such factors as the viscosity of the liquid, the pressure of the fluid and the spacing and size of apertures 31 in the header pipe. With respect to the latter, it should be understood that a slot could be substituted in place of the apertures.

After the electrostatic latent image, which is in motion through the developing wave, leaves the developing area, its excess or surplus liquid is wiped away in any suitable manner, such as, by an air knife or by squeezing rollers which may be made of suitable resilient material so that sufficient pressure can be applied to remove the excess liquid.

Because developer particles on transfer are being consumed without being restored to the developing system, that which is consumed is replenished from a suitable dispensing unit, not shown, containing developer particles that are dispensed in metered amounts to the liquid developer as is known by those skilled in the art. The dispenser is operative in response to a regulatory control set to effect the dispensing in proportion to the rate of consumption.

From the foregoing it is apparent that due to the gentle touching of the wave to the electrostatic latent image which in effect kisses the image, high quality development is accomplished without destroying the electrical properties of the image. It is also apparent that due to the continuous circulation of the developer fluid and of the air bubbles through the liquid, the liquid in the development wave is continually replenished and is always at maximum developing strength. Thus no adjustment to the speed of the electrostatic latent image is ever necessary to provide an effectively constant development time throughout the life of the developer fluid.

The compatibility of the liquid development apparatus of the invention as previously described in a multicolor printing system is quite advantageous. Production of multicolor prints in accordance with the invention may best be understood with reference to FIG. 4 which illustrates apparatus embodying such a system. An endless flexible plate 50 comprising a photoconductive layer on a flexible conductive backing is continuously advanced over suitable guide rolls one of which is driven by a drive motor M-1. As plate 50 advances a uniform electrostatic charge is first applied on its surface by a corona generating device 52 which is energized from a suitable potential source. On further advancement the charged plate is exposed to a light image of activating radiation from flash lamps 53 which is reflected from a document 54 supported on a glass platen 55 and which is optically aligned with an objective lens 56 which projects the image onto the charged plate resulting in an electrostatic latent image. At one point of the optical path, there is positioned a filter plate 58 which is shown as a disc incorporating a number of filters 59 each for a different color as cyan, yellow, and magenta. Filters 59 are interchanged by rotation of filter plate 58 using a drive motor M-2 for positioning the filter plate. Both the timing of flash lamps 54 and the actuation of motor M-2 are controlled by the signal from a lamp 60 and photocell 61 positioned along the path of plate 50 to detect one or more logic marks on the plate for ensuring registration of the images.

Subsequent to exposure the light image plate 50 is advanced to a developing station at which development of the electrostatic latent image occurs. Arranged at the developing station are a plurality of development units 65 which develop a latent electrostatic image by generating a wave as previously described. Each development unit has a different colored pigment to accomplish development for its respective color as, for example, cyan, yellow, magenta, and black. Development units 65 are activated from a control logic 70 which determines which color is to be used in accordance with a predetermined input which may be manual or automatic. It is to be understood that color development desirably has the same sequence as exposure filters 59. As already mentioned above, activation of a development unit is quite rapid and requires only actuation of the motor drive for pumping liquid developer to generate the development wave. Hence, the use of this kind of development for color printing is quite advantageous. Each development unit has an associated air knife 72 which subjects the developed image to air pressure thereby wiping away any surplus liquid.

After development the developed image is then transferred onto a support sheet 80 turning in synchronization with moving plate 50 by a corona generating device 81. Support sheet 80 is supplied from a paper feeder 82 to a rotating drum 84 to which it is secured by any suitable securing device. The image transferred to support sheet 80 is fixed by any suitable fixing device. 86. After image transfer, liquid developer remaining on plate 50 is removed by a suitable cleaning squeegee 88.

It will be appreciated that projection, developing and fixing of a second stage step and succeeding steps are similarly carried out. By repeating the above described procedure for the required number of stages, each comprising projection, development and fixing and then lifting support sheet 80 off form drum 84, a reproduction print corresponding to a color print of original 54 is obtained. After release of the sheet 80 from drum 84, it is moved by a transport belt 90 into output tray 92.

By suitably selecting the filters and developing toners in the liquid developer material the production of multicolor prints is possible according to a two-color method, a three-color method, or a multicolor method using more colors. Also by a suitable control system triggered by one or more logic marks on plate 50 as is well known in the art, the programming of exposure at the exposure station may be controlled in order to maintain the distance between repetitive images at a minimum. As a result of this mode of operation, it is possible to form images having a width corresponding to the plate width of any length up to the length of the web at the exposure station while maintaining the amount of unexposed web between and around the images to a minimum. Additionally it is possible to form only a two-color print even though the image bearing surface is moved through all of the development units. This may be conveniently accomplished as, by example, disabling the development units not in use by deenergizing the motor drive for pumping liquid developer thereby drooping the wave so that no development can occur. Alternatively a solenoid actuated valve may be used to activate or deactivate a wave development unit.

It will be appreciated that the multicolor system of the invention affords color printing without the necessity of a large number of printing plates as in the case of conventional multicolor printing processes. Since coloration is determined by the colors of the filters and the colors of the toners in the liquid developer, multicolor is possible from any kind of original pattern irrespective of its being a negative or positive or its being transparent or opaque. Another advantage is that printing is possible on ordinary paper for ordinary black and white printing. Due to the fact that the apparatus of the invention can be operated automatically after placing the original pattern in the correct position as in the case of ordinary monochromatic electrostatic printing systems, the apparatus can be easily operated. Also since the construction is simple, the apparatus can be manufactured at low cost. It can be readily appreciated that the present invention affords a multicolor printing system which can be effectively applied to a wide range of uses.

While the invention has been described with reference to the structures disclosed herein, it is not to be confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements of the following claims.

Claims

What is claimed is:

1. Liquid developer apparatus comprising

sump means for holding a supply of liquid developer,

tray means disposed above said sump means for containing a supply of liquid developer,

nozzle means including manifold having openings disposed below the surface of developer contained in said tray means, said openings being directed substantially at right angles toward the surface to control the flow of developer in the path of latent electrostatic images to be developed,

conduit means defining a flow path for said developer from said sump means to said manifold, and

pump means for impelling liquid developer through said conduit means and manifold openings to create a wave on the surface of said tray means with sufficient agitation to develop latent electrostatic images brought into contact with the wave but without destroying the electrical properties of the images.

2. Apparatus according to claim 1 including means to introduce a pressurized gas into said sump means for providing continuous agitation of the liquid developer particles throughout the body of the liquid.

3. Apparatus according to claim 1 wherein said sump means has wall members extending beyond the boundaries of said tray means to catch falling developer.

4. Multicolor printing apparatus comprising

a photoreceptor member,

means for advancing said photoreceptor past a plurality of processing stations along a predetermined path,

charging means at a first station for applying a uniform charge to said photoreceptor member,

exposure means at a second station for causing monocolor light of different colors to form sequential latent electrostatic images on said photoreceptor member,

liquid development means at a third station comprising a plurality of development units, each development unit being of a different color, and operative to contact the photoreceptor member in the form of a wave with sufficient agitation to develop the image without destroying the electrical properties thereof,

logic means for controlling said development units by generating different wave forms at predetermined intervals to develop each image with its respective color in proper sequence, and

transfer means at a fourth station for transferring the liquid developed images to a support sheet in superposed relationship.

5. Multicolor printing apparatus comprising

a photoreceptor member,

means for advancing said photoreceptor past a plurality of processing stations along a predetermined path,

charging means at a first station for applying a uniform charge to said photoreceptor member,

exposure means at a second station for causing monocolor light of different colors to form sequential latent electrostatic images on said photoreceptor member,

liquid development means at a third station comprising a plurality of development units, each development unit being of a different color, said development units each including a tray containing liquid developer with a submerged manifold having openings directed substantially toward the surface to control the flow of liquid developer through the manifold openings to contact the photoreceptor member in the form of a wave with sufficient agitation to develop the image without destroying the electrical properties thereof,

logic means for controlling said development units by generating different wave forms at predetermined intervals to develop each image with its respective color in proper sequence, and

transfer means at a fourth station for transferring the liquid developed images to a support sheet in superposed relationship.