EP0091074B1 - Indirect electrostatic printer - Google Patents

Description

The present invention relates to electrostatic printers and more particularly to those using an intermediate support on which a latent electrostatic image is formed and a final support, advantageously made of plain paper, onto which is transferred, after development, the previous latent electrostatic image.

In commonly used electrostatic printers, also known as direct electrostatic printers, the image is printed directly on a special final support, composed of a conductive substrate on which a dielectric layer a few micrometers thick is deposited. The image printing process is carried out in several successive operations carried out in separate stations assigned to these operations. It consists, essentially, in creating a latent electrostatic image on this special support, in a printing station, by means of a printing head with multiple electrodes of small dimensions, arranged in several possible arrangements, for example in one or several rows or in matrix form, by electrical discharge, in the air of the interval between electrodes and dielectric layer of the special support, then in developing this latent image, in an image development station, by inking obtained by depositing fine particles of magnetic powder or ink by means of a magnetic brush or other inking means, and finally to fix the ink thus deposited, in an image fixing station, by heat treatment for example by passing through an oven.

In certain electrostatic printers also known, of the type of those targeted by the present application and also called indirect electrostatic printers, the latent electrostatic image generated on an intermediate support then developed is transferred to a final support, generally made of paper, on which it is then fixed. The printing steps or operations in these indirect electrostatic printers are carried out by means similar to those used in the successive stations of direct electrostatic printers, that of ink transfer from the intermediate support to the final support being intercalated between image development and fixation. This ink transfer can advantageously be obtained in an image transfer station, by bringing this final plain paper support into intimate contact with the inked latent image of the intermediate support, with, if necessary, application of a suitable voltage on a so-called transfer electrode disposed on the back of the final support, promoting this transfer.

In the latter type of electrostatic printers, the intermediate support may consist of a conductive substrate on which a thin dielectric layer is deposited, but may also alternatively consist of a simple film of dielectric material.

Thus document US-A-3198 648 describes an electrostatic printing process, according to which one uses, for cost reasons, an intermediate support formed by a dielectric strip and a final support in plain paper. In the printer, the dielectric strip is mounted in an endless strip between two rollers. One of the electrically conductive rollers constitutes the counter electrode associated with the head of multiple electrodes which prints the latent image on the external surface of the dielectric strip supported by this roller, while an image development system, near this same roller ensures the inking of the image.

At the other roller supporting the dielectric strip, a pressure roller ensures the intimate contact of the final plain paper on the previously inked image, for the transfer of the ink particles on the paper to which they will be subsequently fixed by application. higher pressure forces than image transfer. Finally, the dielectric strip is subjected to cleaning and to erasing of the electrostatic charges after this image transfer operation for a new printing at the level of the first roller mentioned.

Document GB-A-1 064 980 also describes a mode of indirect electrostatic printing in which a dielectric strip is used as an intermediate image support and strip paper as a final image support. In the printer, the dielectric strip is delivered from a supply roll, passes successively through a latent image printing station, an image development station then a transfer station on the paper and, after cleaning, is rewound on a take-up roll. Along these stations, the dielectric strip is driven along a quasi-rectilinear path by a first roller, upstream of these stations, and by a second drive roller and a pressure counter-roller, downstream of these stations.

In the printing station, the points of each latent image line are successively generated on the dielectric strip, as it advances, by a pair of electrodes, one rotating having on its periphery the different characters. possible to print and the other fixed, arranged on either side of the dielectric strip. At the transfer station, each of the latent image lines, after development, is transferred to the strip paper, driven transversely to the dielectric strip, by two transfer electrodes of length at least equal to that of an image line, between which the dielectric and paper strips pass in contact and perpendicular to each other. To avoid spreading and slipping of image lines

when transferring to paper, perfect synchronization of training must be achieved between the bands.

The object of the present invention is to define a specific organization of the various means used in an indirect electrostatic printer with flexible intermediate support, such as a strip of dielectric material, allowing the reproduction of images printed on a final support, such as plain paper in sheets, and having a simple and compact structure.

It therefore relates to an indirect electrostatic printer with intermediate image support in the form of a flexible strip of dielectric material and with final image support in paper comprising a latent image printing station an image development station and an image transfer station on a final support through which said strip is successively driven along its path between a supply roller and a take-up roller and comprising, in addition to said image transfer station, a fixing station for image through which said final support is driven, characterized in that said image transfer station and said image fixing station are constituted simultaneously by an auxiliary roller for folding the trajectory of said strip and an associated pressure roller receiving between them said strip and said final support and in that said auxiliary roller and said associated pressure roller constitute, in addition with said strip, the means for dragging to a station for returning copies of said final support in the direction of movement of said strip, said final support being capable of being in the form of individual sheets.

The invention will be described below in more detail using an embodiment illustrated in the single figure of the accompanying drawing.

In this figure, we have schematically illustrated the organization of a compact electrostatic printer with image reproduction on plain paper.

This indirect electrostatic printing printer comprises an intermediate image support 1, formed of a flexible strip of dielectric material, of great length, coming from a supply roller 2 and received by a take-up roller 3. Between these two rollers 2 and 3, the strip 1 is mounted in an open loop by means of an auxiliary roller 4 ensuring a folding of its trajectory along which it is driven from the supplying roller to the receiving roller.

This flexible strip 1 is constituted by a dielectric film of small thickness and width at least equal to that of the image to be printed and to be restored.

Along this open-loop trajectory, between the supply and take-up rollers and the auxiliary roll, the strip 1 passes successively through an electrostatic printing station 5, arranged between the supply roll and the auxiliary roll, in a development station for image 6 arranged in the immediate vicinity of the auxiliary roller and in a common transfer and fixing station 7 supplied with individual sheets of plain paper 8 from an adjacent dispenser 9.

The printing station 5 is as such of conventional type and comprises a printing head 10 composed of a plurality of individual electrodes arranged in one or more lines or in matrix form facing the outside surface of the strip 1 in open loop, and an associated counter-electrode constituted by a conductive roller 11 in contact with the internal surface of the strip 1. The latent electrostatic image is therefore printed on the external surface of the strip, as a function of control signals successively applied to the individual electrodes of the head by a suitable control circuit not shown.

Taking into account the almost smooth surface condition of the strip 1, the print head 10 further comprises spacing means between the electrodes and the external surface of the strip, in order to provide the necessary air film ion discharge performed for printing the latent image.

The development station 6 is as such known. It is here constituted by a magnetic brush 12 using a single-component or two-component powder taken from a reservoir 13, for inking the latent image previously generated. For reasons of compactness, this station is advantageously disposed in the immediate vicinity of the auxiliary roller 4 and is disposed substantially in the extension of the open loop of the strip 1.

The station for fixing and transferring the image of the strip 1 to the final plain paper support is constituted by the auxiliary roller 4 and a pressure roller 14 receiving an external pressure P coming to apply it against the strip 1 still supported by the roller 4. The distributor 9 of individual sheets of paper, with a guide outlet oriented tangentially to the rollers 4 and 14, feeds this image transfer and fixing station in synchronism with the generation of latent images in the printing station 5, as symbolized by the actuation control link C indicated between these two stations. This dispenser q is also as such known and is therefore not described; only its connection C with the printing station, specific to this application, then makes it possible to suitably trigger the output of sheets 8, successively, which are directly snapped up between the rollers 4 and 14.

The ink deposited on the latent image then passing between these rollers 4 and 14 is transferred and simultaneously fixed on the sheet of paper 8 then pressed between the strip and driven with it.

In this transfer and fixing station 7, the rollers 4 and 14 can be rollers kept cold metal between which are only strongly pressed the dielectric strip 1 and the sheet of paper 8. Depending on the type of image inking powder used in the development station 6, it is possible to create a potential difference between rollers 4 and 14, as symbolized by the voltage V applied to the roller 14, in order to facilitate image transfer.

Alternatively, the metal rollers 4 and 14 can be heated by means of lamps or filaments not shown. Under these conditions, the fixing of the image on the paper is improved and requires only a lower pressure. Simultaneously, the image transfer can also be improved by applying a potential difference between the rollers 14 and 4.

The sheets of paper 8, drawn along with the strip 1 between the rollers 4 and 14 and then on the strip which will be wound on its receiving roller 3, are thus conveyed to a station for returning printed copies. 15 disposed near the take-up roller 3. This restitution station 15 comprises a roll 16 for guiding the strip against which the internal face of the strip is applied, coming at least slightly to break 3a end of the trajectory in open loop of the strip 1, upstream of the receiving roller, and a separating member 17, mounted opposite the roller 16, at the end of the broken path, coming to receive the successive sheets of paper and guiding them to a return receptacle 18 where they are available.

The belt 1 is driven by the auxiliary roller 4 and the axis of the receiving roller 3, the supply roller 2 being mounted with slight friction on its axis so that the band 1 is properly tensioned when it passes through the station 5. The auxiliary roller 4 and 3rd pressure roller 14 mounted free in rotation on its axis receiving the pressure P constitute a means of gripping the individual sheets 8 delivered by the distributor and, associated with the strip 1, their means of drive to receptacle 18. Advantageously although not illustrated, the rollers 4 and 14 will each be covered with a material making it possible to adapt the coefficients of friction between the rollers, the paper and the dielectric strip and also to compensate for its flexibility roller machining faults.

In this printer, the long dielectric strip in a roll, running along an open loop path to a take-up roll, makes it possible to define a printer structure of great simplicity and great compactness due essentially to the fact of the use a common station for transferring and fixing a latent image on sheets of paper driven simultaneously with the strip, produced by the sole mounting of the pressure roller 14 associated with the auxiliary roller 4 for folding the trajectory of the strip.

Claims (3)

1. An indirect electrostatic printer with an intermediate image support in the form of a flexible dielectric tape (1) and with a final image support (8) made of paper, comprising a latent image printing station (5), an image developing station (6) and an image transfer station (7) on a final support (8) through which stations said tape (1) is successively driven along its path between a supply roller (2) and a receiving roller (3), and comprising, in addition to said image transfer station, an image fixing station, through which stations said final support (8) is driven, characterized in that said image transfer station and said image fixing station are simultaneously constituted by an auxiliary deflection roller (4) for deflecting said tape (1), and an associated pressure roller (14), these two rollers receiving between them said tape and said final support and in that said auxiliary roller (4) and said associated pressure roller (14) constitute together with said tape the driving means up to a copy delivery station (15) of said final support in the movement direction of said tape, said final support being constituted by individual sheets.

2. A printer according to claim 1, characterized in that said auxiliary (4) and pressure rollers (14) are metal rollers each covered by a material matching the friction coefficients between rollers, sheets of the final support and the dielectric tape and compensating their fabrication faults.

3. A printer according to one of claims 1 and 2, characterized in that said copy delivery station (15) comprises a tape guide roller (16) against which the inner surface of the tape is applied, deflecting the end of path of said tape, and that said delivery station comprises a separator element (17) mounted opposite the tape at the level of the deflected end of path, this element receiving the printed copies which are successively conveyed.

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