US3654896A

US3654896A - Apparatus for developing electrostatic images

Info

Publication number: US3654896A
Application number: US573294A
Authority: US
Inventors: Gerhard Marx; Hans Heist
Original assignee: Kalle GmbH and Co KG
Current assignee: Kalle GmbH and Co KG
Priority date: 1965-08-21
Filing date: 1966-08-18
Publication date: 1972-04-11
Anticipated expiration: 1989-04-11
Also published as: ES330402A1; GB1144890A; FR1490096A; CH447815A; BE685765A; NL6611306A; SE318783B; AT267327B; DE1497166B2; NL155667B; DE1497166A1

Abstract

Apparatus for developing latent electrostatic images with liquid developer including means for passing a carrier past a developing station and a nozzle means for applying the developer at the station. The nozzles are placed below the carrier path and produce an upwardly directed laminar flow curtain which impinges the carrier at an angle between 10* and 45* to the vertical and in the direction of carrier motion. Means are provided for deflecting the curtain in the absence of the carrier and returning the liquid to a storage container.

Description

United States Patent Marx etal.

[151 3,654,896 [451 Apr. 11, 1972 [54] APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES [72] Inventors: Gerhard Marx, Wiesbaden-Kastel; Hans Heist, Wiesbaden-Schierstein, both of Germany Kalle Aktiengesellschaft, Wiesbaden- Biebrich, Germany [22] Filed: Aug. 18,1966

[21] Appl.No.: 573,294

[73] Assignee:

[30] Foreign Application Priority Data Aug. 21, 1965 Germany ..I( 56946 [56] References Cited UNITED STATES PATENTS 2,419,132 4/1947 Friedman ..1 18/325 X 2,487,440 11/1949 Heer ..1 18/301 3,141,793 7/1964 Oliphant ..1 18/325 X 3,144,211 8/1964 Goldman ..239/523 X 3,192,846 7/1965 Wright ....1 18/637 UX 3,202,526 8/1965 Ostensen ....1 18/637 UX 3,237,871 3/1966 Vicard et a1. ..239/523 X 3,334,613 8/1967 Young ....118/637 UX 3,405,627 10/1968 Day et a1 ..118/325 X FOREIGN PATENTS OR APPLICATIONS 255,578 7/1926 Great Britain ..118/325 Primary ExaminerJohn P. McIntosh Attorney-James E. Bryan [57] ABSTRACT Apparatus for developing latent electrostatic images with liquid developer including means for passing a carrier past a developing station and a nozzle means for applying the developer at the station. The nozzles are placed below the carrier path and produce an upwardly directed laminar flow curtain which impinges the carrier at an angle between 10 and 45 to the vertical and in the direction of carrier motion. Means are provided for deflecting the curtain in the absence of the carrier and returning the liquid to a storage container.

8 Claims, 6 Drawing Figures PATENTEUAPR l 1 I972 SHEET 1 [IF 3 PATENTEDAPR H I972 3, 654, 896

sum 2 0r 3 2 H I H ////Hbbbsw .MA 5 b m w w 250 v H zsb INVENTOR GERHARD MARX HANS HEIST BY O 44 M ATTO EY PATENTEDAFR 1 1 I972 SHEET 3 [1F 3 FIG. 6

INVENTORS GERHARD MARX HANS HEIST (9M Z M A ORNEY APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES This invention relates to an apparatus for rendering latent electrostatic images visible by bringing the electrophotographic material carrying such an image into contact with a so-called dispersion developer.

Dispersion developers as used in electrophotography are fluid developers and comprise a dispersion of solid toner particles in a fluid medium having a high specific resistance and a low dielectric constant. When such a developer is brought into contact with a surface bearing an electrostatic image, the toner particles are attracted to the charge pattern on the surface while the dispersion medium flows away. A visible print is thus obtained. Depending on the sign of the charge of the latent electrostatic image to be developed and the sign of the charge of the dispersed particles, either positive or negative copies of the image to be reproduced are obtained. If the image charge and the charge of the dispersed particles are the same, a negative print is obtained, whereas with opposite charges a positive copy is produced. The latter case is the normal one, and the known processes and apparatuses for developing latent electrostatic images with dispersion developers are adapted to this process. In contrast, the present invention relates to an apparatus preferably for reversal development of latent electrostatic images with a dispersion developer.

In the case of older, known methods for developing latent electrostatic images with dispersion developers, the electrophotographic material generally is brought into contact with the developer by immersing it or drawing it through a container holding developer fluid.

It is also known to develop the electrophotographic material by bringing it into contact with a thin film of fluid developer on the surface of a rotating roller. Apparatuses, in which this process is used, have been developed from known older devices such as those used in the fields of diazotype and silver photography. Since impressions easily result when developing by roller application, i.e. the toner particles still oriented by the previous developing process in the form of a print produce a weak imprint on the newly introduced electrophotographic material, it already has been suggested that the developer fluid should be sprayed onto the roller with a nozzle in order to clear the roller and avoid such impressions.

The known devices are suitable for positive development and some give outstanding positive copies. However, they can not be used to produce reverse copies of a satisfactory quality. In the case of reversal development, the uncharged portions of the image-bearing surface must be made visible, i.e. the dispersed toner agent must be deposited on them. Naturally this can be achieved only by means of the field generated by the charged portions. This field must exert an influence as far as possible into the uncharged portions so that sufficient deposition of the toner can take place. The field distribution during the developing process obviously is also influenced by the components of the developing apparatus. However, it is scarcely possible to make theoretical statements as to how the developing apparatus should be constructed to achieve optimum reversal development because of the complexity of the matter. However, it is a fact that the known developing devices do not offer optimum conditions for reversal development. Very blurred and low contrast copies are always obtained from developing apparatus with roller application in which the surface layer of the copying material comes directly into contact with the material of the roller. If the copying layer has a backing of paper, which is generally the case, in known devices in which the rear of the copying material also comes in contact with the fluid developer, copies are always obtained which are smeared on the back. If the backing paper has a high resistance, it also can happen that a mirror image negative copy is formed on the back since the field of the layer projects through the paper and since, at the back, the charge of the layer is of the same sign as the charge of the dispersed particles.

To solve the above problem it already has been proposed that apparatus should be used in which the copying material should come in contact only with the film of dispersion developer on a roller without being pressed. The backing is supported freely from the rear during development. Using this device reverse copies of a fairly good quality can be obtained which are also clean on the back. However, if copies of a very high quality indeed are to be produced without any background at all, the copy just developed has to undergo additional rinsing with pure dispersion medium in this apparatus.

There is, therefore, a need for a process and apparatus for dispersion development of latent electrostatic images which is specially suitable for reversal development and gives satisfactory copies without any background. The achievement of this involves constructing the developing apparatus in such a way that the rear of the copying material does not come into contact with the dispersion developer and the front of the copying material does not come into direct contact with a fixed application device during the developing process.

There is a similar requirement in wet development of latent electrostatic images. It already has been suggested in this case that development should be carried out with a slot nozzle from which the developer liquid issues and over which the copying material is drawn with the layer side facing downwards. However, in the case of wet development it is essential that the developer should not be applied in excess. In the above apparatus, fluid is therefore supplied only from the slot in a quantity such that a bead of fluid is formed from which the electrostatic forces draw the required amount of fluid.

The above-mentioned apparatus can, in principle, be used for developing with a dispersion developer as well but the copies obtained in this way are blurred and also are not free from background. The copies are also as a rule striped since the coated side of the copying material tends to come into contact with the metal edges of the slot nozzle.

The present invention provides an apparatus for developing latent electrostatic images by means of a dispersion developer, wherein material to be developed is passed, with the imagebearing surface downwards, over an upwardly directed spray of developer fluid, and, after development, the surplus developer fluid is removed, a jet of developer spray being directed, in the form of a curtain of fluid at least as wide as the material to be developed and of substantially uniform height over the relevant area. The jet has such an angle to the vertical that, when unimpeded, the spray curtain assumes, under the influence of gravity, a parabolic trajectory, and the spray pressure is so controlled that the trajectory is maintained substantially constant and of such a height that the material under development can be located in the spray curtain at a distance of between 10 and mm. below the highest point of the trajectory, the material being thus located during the development.

When this apparatus is used, the backs of the copies obtained are entirely free from any soiling since the developer is applied from below. Also, no stripes or blurring can be seen in the developed print since the copying layer does not come into contact with a fixed application device. However, applying the fluid from below without any contact, as stated above, is not a sufiicient condition for obtaining satisfactory reverse copies. It is of fundamental importance for the quality of the reverse copies to be obtained with the process in accordance with the invention that the force of impingement of the developer fluid during the developing process should remain within certain limits. As difiiculties arise in measuring the pressure of impingement upon the copying material, the pressure range is most easily defined by the difference in height between the highest point of the undisturbed fluid trajectory and the path of the copying material in the zone of application. If the difference in height is below the given range, the pressure is too small. The print then will be only partially developed and edge effects easily occur. If the difference in height is larger, the pressure is too great and the toner will be lightly deposited on the background where there is no image, so that the quality of the copies will be reduced by background. If the electric field from the print is strong, the pressure should be maintained towards the lower limit of the given range. Conversely, a pressure towards the upper limit of the range should be chosen if the electric field is weak. Since it is primarily the pressure of the fluid which determines the quality, the height of the fluid curtain from the path of the copying material to the place of origin of the fluid curtain is of less importance. For minimizing the overall height of the apparatus, it should not be too large. In experiments, good copies always were obtained with fluid trajectories having an overall height of 20-150 mm. provided the given conditions with regard to the pressure were maintained.

The inclination at which the fluid curtain is incident on the copying material also affects the quality of the copies obtained with the process of the invention. Basically, any angle of incidence is possible since more or less usable copies are always obtained. However, very oblique angles are eliminated since the range of the trajectory would be too great. The quality of the copies also is impaired if the fluid curtain rises vertically or approximately vertically so that it is incident perpendicularly or approximately perpendicularly on the copying material, or if it is directed in the opposite direction to the direction of motion of the copying material. The best results are obtained if the inclination of the rising curtain of fluid at its base is l45 with respect to the vertical to the path of the copying material in the direction of motion of the copying material. Particularly good results are obtained if the angle is l-30. This range of inclination is presumably particularly favorable because, on the one hand, the motion of the paper is not substantially disturbed by the spray and, on the other hand, the spray is quite compact when it strikes the coated side of the copying material. It also is advantageous probably that the flow of the spray has a tangential component in the direction of motion of the copying material. For this reason the developer fluid is substantially displaced in this direction.

Particularly advantageous results are obtained if laminar flow is set up in the curtains of fluid, as long as they are freely formed, so that up to the highest point the fluid forms a continuous film which divides into individual drops only as it descends.

The quantity of developer fluid available to the latent electrostatic image to be developed must be such that sufficient blackening of the print is achieved. This is generally the case when about 5-50 ml. of dispersion are available per dcm. of surface of the print carrier. Entirely satisfactory copies, however, can be obtained with smaller or larger quantities since the speed of developing depends not only upon the available quantity of developer but also upon the nature and composition of the dispersion.

As a rule, for a speed of progression of the copying material of 5-20 m. per min. sufficient development is achieved after passing only one curtain of fluid. If sufficient development is not achieved at a higher speed of progression with one contact zone, then the copying material must be brought into contact with one or more further curtains of fluid. However, it has been established that sufiicient development generally is always achieved with at most three curtains of fluid, provided that the speed of progression is not greater than about 50 m./min. Higher speeds of the copier are theoretically possible but difficulties then arise in removing the excess dispersion developer.

The thickness of the rising film at the height of the plane of motion of the copying material, measured in the undisturbed curtain of fluid, is preferably between 0.5 and 2 mm. As the copying material with the coating on the underside passes by, a contact zone between the fluid and the film surface is formed which is in this case 1-3 cm. wide; development takes place in this zone. Unexpectedly, with the process of the invention it is not necessary either for there to be a metal body which influences the field of the electrostatic print or for voltage to be applied for reverse development. Devices for carrying out the process therefore can be constructed in a relatively simple manner.

Nozzles, baffle plates or suitably modified combinations of both can be used, for example, to produce the curtains of fluid.

The copying material can be transported e.g. by means of rollers or suction components such as those commonly used in printing for moving individual sheets.

Excess developer can be removed e.g. by means of squeezing rollers or an air nozzle.

It has been found that, from the point of view of the quality of the copies, it is immaterial whether a copy sheet is transported over the fluid curtain by means of transport rollers or by means of a suction system, provided that the means of conveyance is adapted to the particular circumstances. On the other hand it has been found that it does make a difierence which means of transport is combined with which means for removing excess developer. Definite combinations have proved to be advantageous and these are described below.

The invention will be further illustrated by reference to the accompanying drawings, in which:

FIGS. 1 to 4 show cross-sections through three representative and preferred forms of apparatus according to the invention FIG. 5 shows an individual component in a perspective view, and

FIG. 6 shows a modification of the embodiment FIG. 1.

FIGS. 1 to 3 show forms of apparatus for carrying out the process in accordance with the invention which generally comprise elements for transporting the electrophotographic copying material in a horizontal or inclined plane (E-E), one or more nozzles 1 (or la, lb) for producing and upwardly directed spray curtain of developer fluid extending over the width of the path of the copying material, the nozzles being positioned somewhat below the path of the copying material in such a way that the curtain or curtains of fluid 2 (or 2a, 2b) when undisturbed have the form of trajectory parabolas, a continuously operating fluid pump (not shown) for feeding the dispersion developer to each nozzle at a rate such that the difference in height h between the apex of the trajectory parabola and the path of the copying material is 10-70 mm., preferably 20-40 mm., and elements mounted in the path of the copying material for removing excess developer fluid.

FIGS. 1 and 2 illustrate an embodiment of the apparatus with a wide slot nozzle; FIG. 1 shows the apparatus running empty, and FIG. 2 shows it with copying material passing therethrough. FIG. 3 shows a form of the apparatus with two nozzles comprising perforated tubes.

FIG. 4 shows a further apparatus for developing latent electrostatic prints, preferably for reverse development, by means of the process of the invention, which generally comprises elements for transporting the electrophotographic copying material in a plane (E E), a nozzle 12 with downwardly inclined outlets 13 extending over the whole or part of the width of the path of the copying material, a preferably parabolic deflector with unequal arms positioned in such a way that the dispersion developer coming from the nozzle is incident tangentially or approximately tangentially on the saucer shaped lower part of the deflector and that the curtain of fluid leaving the upper edge thereof has the form of a parabolic trajectory in free flight, a continuously operating fluid pump with a feed rate such that the difference in height h between the apex of the trajectory and the plane of motion of the copying material, is 10-70 mm., but preferably 20-40 mm., and elements positioned in the path of the print carrier to remove excess developer fluid.

In the devices shown in FIGS. 1, 2, and 3, a pair of feed rollers 3 is used to transport the electrophotographic copying material in the plane E E, in the direction from E to E, into a guide channel 4. To remove the excess dispersion developer in these devices there is a pair of squeeze rollers 5, the pressure of which can be regulated. The lower roller coming into contact with the freshly developed print has a smooth, metallic surface.

On the other hand, the device shown in FIG. 4 uses a suction system 22 to move the copying material, which in this case may be only a single sheet. The sheet is held under the,

necessary tension during development by a second fixed suction device 21. The excess developer is removed by means of an air nozzle 23 mounted beyond the curtain of fluid and directed upwardly at an inclination in the opposite direction to the direction of the motion of the copying material.

The mode of operation of the apparatus of the invention will now be explained in detail with reference to the drawings. With reference to FIGS. 1 and 2: the copying material, which generally consists of a backing sheet, normally paper, with a photoconductive film surface, is charged and illuminated imagewise. It is then placed on the plate 6 with the coated side downwards, then taken by the rollers 3 and fed through the guide channel 4 into the curtain of fluid 2. A small part of the dispersion developer suspended above the plane of motion of the copying material falls on the rear side of the material. The greatest part flows off sideways. A small remaining part is spread over the leading part of the back of the copying material by the squeezing rollers 5. However, this quantity is so small that it does not impair the quality of the final print. To ensure that the copying material enters the gap between the squeezing rollers even when the material is not very stiff, a deflector 8 is mounted in front of the pair of rollers 5. This ensures that the sheet is always fed between the pair of rollers 5. After passing through the pair of squeezing rollers 5, the developed print is already fairly dry and resistant to smudging, and it passes to the delivery table 7 from which the print can be removed.

After the copy sheet has passed through the apparatus, the fluid sprayed upwardly immediately reverts to a parabolic trajectory. In order that the fluid does not reach the upper squeezing roller, the trajectory is bounded by a deflector 9. The developer flows sideways out of the groove 9a formed by the lower part of the deflector plate 9, into the container 10, and from there to a storage container (not shown) from which it is returned to the nozzle by a pump, not shown.

To ensure that the pressure of the jet of developer fluid is not altered by deflection of the copying material, particularly at the leading edges of individual sheets, the pair of rollers 3 or, if this is not possible for reasons of space, the guide channel 4, should be as close as possible to the spray curtain. In this way the sheet is held with only a short margin protruding into the spray curtain so that its stiffness counteracts deflection due to the spray pressure.

As already mentioned above, the spray nozzles can be of various forms. Wide slot nozzles, such as those shown in FIGS. 1 and 2 are especially advantageous. These generally consist of two separate metal jaws pressing on one another, of which the upper internal surfaces forming the opposite boundaries of the passage from which the fluid emerges are of the highest possible quality. The passage inside the nozzle should be of such a length and breadth that a substantially laminar flow is set up therein. The spray emerging from the wide slot nozzle then forms a uniform curtain of fluid. The gap width of the slot nozzle should be between 0.l and 2.0 mm., preferably between 0.2 and 1 mm.

Nozzles that can be more easily produced than wide slot nozzles comprise a tube with one or more straight rows of perforations. Small, individual sprays thus emerge from each hole. The individual sprays merge to form a more or less continuous curtain of fluid only at a certain distance from their exit positions. Even in this case the pressure is not uniform throughout; it falls slightly between the centers of the individual jets. If only one row of holes is used, this drop in the pressure can be detected by longitudinal stripes on the copy.

The stripes correspond to the distance between the holes. To eliminate this undesired effect, when nozzles with rows of holes are used, there must be at least two rows of holes, the holes of one row being displaced with respect to the adjacent row or rows. Copies of a high quality then can be obtained with such nozzles. The diameter of the holes in the tubes can be about 0.2 to 2 mm. Smaller holes are somewhat less desirable since the pressure head then is correspondingly large.

Larger holes are also undesirable since too large a volume of fluid is emitted.

An embodiment of the apparatus in accordance with the invention with two perforated nozzles 1a and lb is shown in FIG. 3. The perforated nozzles produce jets 2a and 2b. To achieve a uniform spray over the width of the nozzle and therefore a horizontal summit line, care must be taken that the pressure is unifonn at the outlet points for the fluid. This can be achieved either by dividing the fluid feed into parallel passages which connect with the nozzle at points spaced uniformly over its width, or by arranging a bafile in the nozzle at a small distance in front of the outlet holes; preferably both measures are combined.

All types of pumps can be used to feed the dispersion developer provided that they produce only small variations in the pressure. Piston pumps therefore can be used only in conjunction with a buffer volume. All types of rotary pump are directly applicable; gear pumps have proved successful. Care must be taken when selecting the pump that any seals are made of material resistant to the dispersion medium of the developer since this is generally a hydrocarbon or chlorinated hydrocarbon. The feed rate of the pump to be chosen depends upon the speed of progression of the copying material, the width of the apparatus and the pressure head in the system. As a rule, a feed of 0.8 l./min. is required per cm. of width of the nozzle. This means that in apparatus for developing DIN A 4 prints with an operating width of 22 cm., the pump must have a feed capacity of about 20 l./min.

When selecting the dimensions of the pump, or more exactly when selecting the drive mechanism for the pump, it should be remembered that it is frequently necessary to incorporate a filter in the flow path to trap sludge and any impurities in the dispersion developer and that such a filter constitutes a flow resistance. Generally, if a filter is required, an effort should be made to incorporate it in the pressure-free return.

Basically, the width of the apparatus can be adapted to the circumstances. However, for very large widths it is advantageous to use perforated nozzles since large slot nozzles are expensive.

Furthermore, it should be borne in mind that the input pair of rollers 3 and the pair of squeezing rollers 5 should have the same circumferential speed so that the copying material is not compressed or stressed between the pairs of rollers. The lower roller of the pair of squeezing rollers preferably has a highly polished metal surface. Rollers of brass, chromed brass or high grade steel are particularly suitable. The upper roller must be soft and elastic to ensure uniform squeezing of the excess developer from the print carrier. A solvent-resistant rubber with a shore hardness of 30 35 has proved especially successful. The pressure of the upper roller on the lower can be regulated so that it can be adapted to the copying material as well as the developer. It also has proved suitable to make the upper roller removable so that when the apparatus is not used for long periods there is no deformation of the elastic roller due to plastic flow.

While the copying material runs through, the lower roller of the pair of rollers 5 is kept moistened by the squeezed out dispersion medium flowing back. With some kinds of toners this dampening is not sufficient to reliably ensure that the image of one print is not imprinted on a subsequent one. In these cases, it has proved successful to flush the pair of rollers 5, preferably the lower roller, with developer when the apparatus is running empty. This can be achieved by an appropriate shape of the deflector plate 9, by tilting the entire apparatus in the direction of motion of the copy material, or simply by positioning the elements of the apparatus in such a way that the plane through the lines of contact of the pairs of

rollers

3 and 5 is inclined at 45 to the horizontal in the direction of motion of the print carrier. In this case, part of the developer flowing from the deflector plate 9 continuously flows onto the lower roller of the pair of rollers 5.

The mode of operation of the apparatus shown in FIGS. 4 and 5 is as follows:

The copy sheet bearing the latent electrostatic image is passed into the gap formed by the supporting table 19 and the

suction devices

21 and 22, with the coated surface downwards. In doing this the front of the sheet trips the microswitch 20; this actuates the apparatus. A continuously operating air pump, not shown, with its suction side connected to the

suction devices

21 and 22 is actuated and the reduced pressure is imparted to the suction devices by means of a suitable valve. About half a second after the start of the suction, the suction device 22 is moved away from the initial position depicted towards the curtain of fluid, the air nozzle 23 and the delivery table 24. It carries with it the leading edge of the sheet to be developed. To keep the sheet stiff as it passes through the application zone, so that it is not deflected by the spray pressure of the fluid, the sheet is drawn against the fixed suction device 21. The suction power of the latter must be slightly less than that of the moving suction device 22 as otherwise the copy sheet will be detached from the latter. A short time after the end of the copy sheet has passed the switch 20 so that the latter has returned to its idle position, the forward motion of the suction device 22 ceases, the subatrnospheric pressure is terminated and the suction device 22 is returned to its initial position. The developed copy sheet then lies on the delivery table 24.

The time lag between the end of the sheet passing the switch 20 and the final switching-off of the apparatus is determined according to the length of the printing path and the speed with which the sheet moves. The times can be set to any value by a time lag relay. The switch 20 actuates the air nozzle 23 at the same time as the suction devices. The air nozzle is suitably connected to the pressure side of the air pump, not shown. A strongly convergent air stream with a high velocity leaves the air nozzle and impinges on the excess developer fluid adhering to the copy sheet moving above the nozzle, causing such fluid to fall off in drops. The fluid drips into the funnel 18 which also acts as a cover for the container 17. In the container 17 is the pump 16 for feeding the dispersion developer. The pump is a liquid seal pump and preferably has an outlet aperture which is exactly as wide as the nozzle 12. The nozzle 12 in this apparatus always can be a perforated nozzle since the individual jets from the rows or holes of the nozzle are united in a continuous film by the deflector 14 without any differences in pressure in the transverse direction as it rises. This arrangement is thus very advantageous since perforated nozzles are simple and inexpensive to manufacture and particularly because there is no difficulty in producing them in large widths. In addition, with this arrangement it is not necessary for the nozzle to be exactly the same width as the curtain of fluid produced by it. It is, in fact, possible and even advantageous to make the nozzle narrower and to expand the curtain of fluid by means of a suitable shaped deflector, e.g. as shown in FIG. 5. This can be done advantageously with the ribs 25a or the generally triangular projections 25b on the inside of the deflector and radiating outwardly in a star shape from the nozzle. The ribs should not extend completely to the front edge of the deflector since they would affect the freely rising curtain of fluid. The ribs can be formed by pressing from sheet metal or separate metal strips about 3 mm. high also can be used. Advantageously, these are somewhat wedge-shaped in the direction of motion of the fluid. The inner surface of the deflector should be very smooth so that unnecessary friction of the moving fluid is avoided. Deflectors of highly polished metal have proved particularly successful.

FIG. 6 shows an apparatus similar to that of FIG. 1 in which the path of the image carrier is inclined downwardly in the direction of movement of the image carrier. The operation of this embodiment is as described above in connection with FIG. 1.

When the process and apparatus of the invention are used for reverse development of latent electrostatic prints, very good prints with a high contrast are obtained which can be used almost immediately after leaving the apparatus. Unexpectedly, areas at a distance of 1-2 cm. from the edge of the charged regions are uniformly develo ed as well. This means that even with heavy print no edge e ect is observed. In addition, the edges of the letters are very sharp and are not blurred into the empty space.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. An apparatus for developing latent electrostatic images with a dispersion developer which comprises means for moving an image carrier along a predetermined path, spray means extending across the path for producing only an upwardly directed freely-formed laminar flow spray curtain adapted to impinge on the image carrier at an angle between 10 and 45 to the vertical and in the direction of motion of the image car rier at a distance between 10 and mm. below the highest point of the uninterrupted spray curtain, pump means for maintaining a supply of developer liquid to the spray means, and means for removing excess developer from the developed image carrier.

2. An apparatus according to claim 1 in which the spray means includes a nozzle having a continuous slot with a width between 0.1 and 2.0 mm.

3. An apparatus according to claim 1 in which the spray means includes a tube having a series of perforations therein each of a diameter of 0.2 to 2.0 mm.

4. An apparatus according to claim 1 in which the path of the image carrier is inclined downwardly in the direction of movement of the image carrier.

5. An apparatus according to claim 1 in which the spray means includes deflector plate means of curved configuration whereby a spray incident thereon is deflected towards the path.

6. An apparatus according to claim 5 in which the deflector plate means has a plurality of outwardly radiating projections thereon.

7. An apparatus according to claim 1 in which the means for moving the image carrier includes a pair of feed rolls and a pair of squeeze rolls.

8. An apparatus according to claim 1 in which the means for moving the image carrier includes fixed and movable suction means.

Claims

1. An apparatus for developing latent electrostatic images with a dispersion developer which comprises means for moving an image carrier along a predetermined path, spray means extending across the path for producing only an upwardly directed freely-formed laminar flow spray curtain adapted to impinge on the image carrier at an angle between 10* and 45* to the vertical and in the direction of motion of the image carrier at a distance between 10 and 70 mm. below the highest point of the uninterrupted spray curtain, pump means for maintaining a supply of developer liquid to the spray means, and means for removing excess developer from the developed image carrier.