FIELD OF THE INVENTION The present invention relates generally to imaging and more particularly to the use of tensioned substrates in image transfer.

BACKGROUND OF THE INVENTION

In electrostatic image transfer a toner or ink image defined on an image support surface is transferred electrostatically to a substrate. It may be fixed on that substrate, or alternatively transferred to a further substrate which is the intended image carrier.

In conventional electrostatic copiers, a developed image is transferred to loosely held paper by electrostatic and adhesion forces. The loosely held paper is brought into contact with the image as it is simultaneously charged to a high voltage. This technique, which is adequate for medium quality photocopying is unsuitable for more exacting processes, such as color proofing, for two principle reasons: loose mounting of the paper makes registra- tion of one image with another difficult, and mottle results from topographical variations in the loosely held paper surface. Such variations result in variation in the pressure of the paper on a wet image, which result in variations in dot size. The variations in dot size translate into low frequency grey-level variations, here referred to as "mottle".

The use of tensioned substrates in electrostatic image trans¬ fer is known in the literature. U.S. Patent 4,015,027 describes an electrophotographic toner transfer and fusing method which employs a tensioned transfer belt for transferring a toner image from a photoconductive drum or a photoconductive insulating sheet onto paper.

In the embodiment of U.S. Patent 4,015,027, the transfer belt is tensioned only along the direction of its travel. Such tensio¬ ning tends to produce surface irregularities in the belt along directions transverse to the direction of travel. As a result the uniformity of transfer is adversely affected, lowering the general level of image quality produced by the apparatus due to mottle and

TITUTE SHEET

rendering the transfer technique incompatible with registration requirements as for multi-color operations.

SUMMARY OF THE INVENTION It is an aim of the invention to provide apparatus and a method of use thereof, for use in image transfer, so as to overcome the limitations of the prior art.

In accordance with a preferred embodiment of the invention there is provided imaging apparatus comprising a flexible sub- strate, apparatus for tensioning the flexible substrate in at least two directions and apparatus for bringing the tensioned flexible substrate into image transfer engagement with an image bearing surface.

Further in accordance with a preferred embodiment of the invention, the imaging apparatus also comprises apparatus for subsequently bringing the tensioned flexible substrate bearing an image into image transfer engagement with an image receiving surface.

In accordance with one embodiment of the invention, the apparatus for bringing the tensioned flexible substrate into image transfer engagement produces motion of the flexible substrate and the image bearing surface in a travel direction and the at least two directions of tensioning include the travel direction and a direction transverse thereto in the plane along which the flexible substrate lies when in image transfer engagement.

In accordance with an embodiment of the invention, the image bearing surface is a gravure cylinder. In accordance with another embodiment of the invention, the image bearing surface is a photo¬ conductive surface. In accordance with an embodiment of the invention, the image comprises charged particles and apparatus is provided for genera¬ ting an electrical field in the region where image transfer engage¬ ment occurs to facilitate image transfer.

Further in accordance with a preferred embodiment of the present invention, the apparatus for tensioning is operative for supporting the flexible substrate in a generally planar orientation and contacts the flexible substrate only outside the regions of

SUBSTITUTE SHEET

image transfer engagement.

Additionally in accordance with a preferred embodiment of the invention, the apparatus for bringing the tensioned flexible sub¬ strate into image transfer engagement with an image bearing surface includes apparatus for providing synchronized motion of the image bearing surface and of the flexible substrate in image transfer engagement therewith to prevent relative sliding motion there¬ between.

In accordance with a preferred embodiment of the invention, the image bearing surface comprises a generally cylindrical surface and the apparatus for tensioning is operative for supporting the flexible substrate in a generally planar orientation, in generally tangential orientation relative to the generally cylindrical surface during image transfer engagement. In accordance with an embodiment of the invention, the flexi¬ ble surface is in generally curved surface contact with a portion of the cylindrical image bearing surface during image transfer engagement.

Additionally in accordance with a preferred embodiment of the invention, the apparatus for tensioning comprises a frame having an expandable element associated therewith along its periphery, where¬ by expansion of the expandable element provides tensioning of a flexible substrate mounted on the frame in at least two directions. Further in accordance with an embodiment of the invention, the apparatus for tensioning comprises pressure rollers engaging the flexible substrate so as to apply pressure thereto during image transfer engagement.

In accordance with one embodiment of the invention, the image receiving surface comprises a curved surface, such as a cylinder. Alternatively the image receiving surface comprises a non-cylindri¬ cal surface, whose cross section varies smoothly in a direction transverse to the direction of travel between the flexible sub¬ strate and the image receiving surface.

Where the image receiving surface comprises a cylinder, it may be a circular cylinder or alternatively a non-circular cylin¬ der, such as an elliptical cylinder. When the cylinder has a non- circular cross section, the apparatus for subsequently bringing the

tensioned flexible substrate bearing an image into image transfer engagement with the image receiving surface preferably comprises spring loaded means for rotatably mounting the image receiving surface. Additionally in accordance with a preferred embodiment of the present invention there is provided a method for transfer of images from an image bearing surface -onto a flexible substrate comprising the steps of tensioning the flexible substrate in at least two directions and bringing the tensioned flexible substrate into image transfer engagement with an image bearing surface.

Further in accordance with a preferred embodiment of the invention, the method also comprises the step of subsequently bringing the tensioned flexible substrate bearing an image into image transfer engagement with an image receiving surface. In accordance with one embodiment of the invention, the step of bringing the tensioned flexible substrate into image transfer engagement includes the step of producing motion of the flexible substrate and the image bearing surface in a travel direction, and the step of tensioning in at least two directions of tensioning includes the step of tensioning the flexible substrate in the travel direction and a direction transverse thereto "in the plane along which the flexible substrate lies when in image transfer engagement.

In accordance with an embodiment of the invention wherein the image comprises charged particles, the method also comprises the step of generating an electrical field in the region where image transfer engagement occurs to facilitate image transfer.

Further in accordance with a preferred embodiment of the present invention, the step of tensioning is operative for suppor- ting the flexible substrate in a generally planar orientation and contacting the flexible substrate only outside the region of image transfer engagement.

Additionally in accordance with a preferred embodiment of the invention, the step of bringing the tensioned flexible substrate into image transfer engagement with an image bearing surface includes the step of providing synchronized motion of the image bearing surface and of the flexible substrate in image transfer

engagement therewith to prevent relative sliding motion there¬ between.

In accordance with a preferred embodiment of the invention wherein the image bearing surface comprises a generally cylindrical surface, the step of tensioning is operative for supporting the flexible substrate in a generally planar orientation, in generally tangential orientation relative to the generally cylindrical sur¬ face during image transfer engagement.

Additionally in accordance with a preferred embodiment of the invention, the step of tensioning comprises the steps of mounting the flexible substrate on a frame having an expandable element associated therewith along its periphery, and expanding the expan¬ dable element to provide tensioning of a flexible substrate mounted on the frame in at least two directions.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings, in which: Figs. 1A, IB and 1C each illustrate, in schematic form, image transfer apparatus constructed and operative in accordance with a preferred embodiment of the present invention in three respective versions;

Figs. 2A, 2B and 2C each illustrate, in schematic form, image transfer apparatus constructed and operative in accordance with another preferred embodiment of the present invention in three respective versions;

Fig. 3 is a partial perspective pictorial illustration of the apparatus shown in Fig. 1; Fig. 4 is a cross-sectional view taken along line IV-IV in Fig. 3;

Figs. 5A, 5B and 5C are schematic side view illustrations of the operation of synchronized drive apparatus employed in accor¬ dance with an embodiment of the present invention in respective starting, intermediate and end positions;

Fig. 6 is a pictorial illustration of part of the syn¬ chronized drive apparatus of Figs. A- C;

SUBSTITUTE sneer

Figs. 7A-7E are schematic side view illustrations of five variations of apparatus for printing on a curved substrate in accordance with preferred embodiments of the present invention; and

Fig. 8 is a sectional illustration of part of the apparatus of Fig. 7E, taken along the lines VIII-VIII in Fig. 7E.

DETAILED DESCRIPTION OF THE INVENTION Reference is now made to Figs. 1A-1C which illustrate, in schematic form, three variations of image transfer apparatus con- structed and operative in accordance with a preferred embodiment of the present invention. For simplicity and conciseness, the common features of all three variations will be described initially, followed by an explanation of the differences therebetween.

There is thus shown a metal drum 2 onto which is mounted an image bearing surface here embodied in a cylindrical gravure prin¬ ting plate 4 bearing an engraved image. The drum 2 typically com¬ prises end disks 6 which are mounted onto a shaft 8 by a key 10 so that the assembly is operative to rotate together with shaft 8. Shaft 8 is driven in any appropriate manner (not shown) in the direction of an arrow 9- Shaft 8, and the entire drum 2 and plate 4, are grounded, by a connection indicated by reference numeral 17. An image defining material, such as a liquid toner, which normally contains a dispersion of polymer-supported pigmented particles with added charge director in an insulating, nonpolar, nontoxic liquid, is circulated from any suitable source (not shown) through a pipe 16 into a development tray 18 from which it is drawn through a pipe 20 for recirculation.

A preferred image defining material is described in the examples in Published Applications GB 2l694l6A en GB 2176904A, the disclosures of which are incorporated herein by reference. Alter¬ natively, any other suitable image defining material, such as toner or printing ink, may be employed.

Development electrodes 22, which may be appropriately biased as known to the art, assist in toning the electrostatic latent image as it passes into contact with the developing liquid. Charged toner particles suspended in the developing liquid pass by elec- trophoresis to the gravure plate 4 and cover the entire surface. A

scraper 23 removes the material from the non-image areas.

Once the excess image defining material has been removed, the image is ready for transfer onto a flexible substrate, such as a sheet of paper, single or double coated chroma paper, a material suitable for silk screening, a material such as POLYMON, commer¬ cially available from Swiss Silk Bolting Cloth Manufacturing Co. Ltd. of Zurich, Switzerland under part number PES-1/1/SRC-, or a plastic sheet, and is rotationally conveyed towards a transfer station, referenced generally 40. With additional reference to Figs. 3 and 4, the flexible substrate 42 is maintained under tension in at least two directions and is conveyed generally tangentially along cylindrical plate 4, in a direction shown by an arrow 44, which is preferably one of the directions along which the substrate is tensioned. As will be explained below, it is an important feature of the present inven¬ tion that there is provided synchronized drive apparatus to syn¬ chronize relative motion between plate 4 and substrate 42. The structure and operation of one such synchronized drive apparatus is described below with reference to Figs. 5A-8 ^' . In electrostatic image transfer, using the preferred image defining material described in the examples in Published Applica¬ tions GB 2169416A and GB 2176904A, for example, at transfer statio 40, a corona discharge device 46 impresses upon the rear of sub¬ strate 42 a charge of polarity opposite to that of the toner par- tides forming the image. Constituent particles ^' of the image ar thus drawn towards substrate 42. As seen in the drawings, due t electrostatic forces the substrate 42 tends to be drawn to an conform to the curved configuration of the cylindrical plate 4.

A cleaning roller 56, formed of any appropriate syntheti resin, is driven in a direction opposite to that of the plate t scrub clean its surface. To assist in this action, a suitabl cleaning liquid may be fed through a pipe 58 to the surface o cleaning roller 56. A wiper blade 60 completes the cleaning of th plate. Alternatively, the cleaning roller 56 and the associate wiper blade 60 may be eliminated.

In the embodiment of Fig. 1A, the tensioned flexible sub strate 42 is arranged to define a relatively narrow area of contac

with the gravure plate 4 prior to operation of corona discharge device 46. Upon operation of the corona discharge device, the generated electric field causes the flexible substrate to be drawn to the gravure plate 4 and thus to define a broader area of con- tact, as shown in Fig. 1A. In the embodiment of Fig. IB, the arrangement is such that the substrate defines a much wider contact with the image bearing surface of gravure plate 4. In the embodi¬ ment of Fig. IC a pair of rollers 48 is also provided so as to enhance contact between substrate 42 and the image bearing surface of the gravure plate 4.

Reference is now made to Figs. 2A-2C which illustrate, in schematic form, three variations of image transfer apparatus con¬ structed and operative in accordance with another preferred embodi¬ ment of the present invention. For simplicity and conciseness, the common features of all three variations will be described initial¬ ly, followed by a explanation of the differences therebetween.

There is thus shown a metal drum 2 onto which is mounted a cylindrical image bearing surface in the form of a photoconductive surface 4 bearing a latent image. The drum 2 typically comprises end disks 6 which are mounted onto a shaft 8 by a key 10 so that the assembly is operative to rotate together with shaft 8. Shaft 8 is driven in any appropriate manner (not shown) in the direction of an arrow 9« Shaft 8, and the entire drum 2 and plate 4, are grounded, by a connection indicated by reference numeral 17■>

The image to be reproduced is projected by an optical system 14 onto trie charged photoconductor. Since shaft 8 is grounded at 17 and disks 6 are conductive, the areas impinged upon by light dis¬ charge to ground and the remaining charged areas constitute a latent electrostatic image.

An image defining material such as a liquid toner, which normally contains a dispersion of polymer-supported pigment with added charge director in an insulating, nonpolar, nontoxic liquid, is circulated from any suitable source (not shown) through a pipe into a development tray 18 from which it is drawn through a pipe 20 for recirculation.

A preferred image defining material is described in the

examples in Published Applications GB 2169416A and GB 2176904A, the disclosure of which is incorporated herein by reference. Alterna¬ tively, any other suitable image defining material, such as powdered or liquid toner may be employed. Development electrodes 22, which may be appropriately biased as known to the art, assist in toning the electrostatic latent image as it passes into contact with the developing liquid. Charged toner particles suspended in the developing liquid pass by elec- trophoresis to the electrostatic latent image. If a selenium photo- conductor is used and is, for example, positively charged, in a method of positive development, the pigment particles are nega¬ tively charged. In a method of reverse development, however, the pigment particles are positively charged and, as known in the art, are attracted to those areas of the photoconductor that have been discharged as described above.

Since the amount of liquid on the photoconductor surface is normally too great for satisfactory subsequent transfer of the developed image, a roller 24, whose surface moves in a direction opposite to that of the photoconductor surface is spaced therefrom and is adapted to shear excess liquid from the developed image without disturbing it. An exemplary roller is shown in U.S. Patent No. 3,907.423, te contents of which are incorporated herein by reference.

Roller 24 is driven by any appropriate means, such as by a drive belt 26 which is driven by any appropriate known speed-con¬ trollable motor means (not shown) . The roller is kept clean by a wiper blade 28. Once any excess liquid has been removed by roller 24, the developed image is ready for transfer onto a chosen trans¬ fer substrate, and is rotationally conveyed towards a transfer station, referenced generally 40.

As noted above in connection with Figs. 1A-1C, a flexible substrate 42, such as a sheet of paper, single or double coated chroma paper, a material suitable for silk screening, or a material such als P0LYM0N, commercially available from Swiss Silk Bolting Cloth Manufacturing Co. Ltd. of Zurich, Switzerland under part number PES-1/l/SRC, is maintained under tension in at least two directions and is conveyed generally tangentially along cylindrical

plate 4, in a direction shown by an arrow 44, which is preferably one of the directions along which the substrate is tensioned.

As will be explained below, it is an important feature of the present invention that there is provided synchronized drive appara- tus to synchronize relative motion between plate 4 and substrate 42. The structure and operation of the synchronized drive apparatus is described below with reference to Figs. 5A-8.

In electrostatic image transfer, using the preferred image defining material described in the examples in Published Applica- tions GB 2169416A and 2176904A, for example, at transfer station 4θ, a corona discharge device 46 impresses upon the rear of sub¬ strate 42 a charge of polarity opposite to that of the toner par¬ ticles forming the image. Constituent particles of the image are thus drawn towards substrate 42. As noted above in connection with Figs. 1A-1C, the electrostatic forces tend to draw the substrate 42 into shape conforming engagement with the photoconductor 4.

A cleaning roller 6, formed of any appropriate synthetic resin, is driven in a direction opposite to that of the plate to scrub clean its surface. To assist in this action, a suitable cleaning liquid may be fed through a pipe 58 to the surface of cleaning roller 56. A wiper blade 60 completes the cleaning of the plate.

Any residual charge left on the photoconductive drum is extinguished by flooding the photoconductor surface with light from a lamp 62.

In the embodiment of Fig. 2A, the tensioned flexible sub¬ strate 42 is arranged to define a relatively narrow area of contact with the photoconductor 4 prior to operation of corona discharge device 46. Upon operation of the corona discharge device, the generated electric field causes the flexible substrate to be drawn to the gravure plate 4 and thus to define a broader area of con¬ tact, as shown in Fig. 2A. In the embodiment of Fig. 2B, the arrangement is such that the substrate defines a much wider con¬ tact, here termed curved area contact, with the image bearing surface of photoconductor 4. In the embodiment of Fig. 2C a pair of rollers 48 is also provided so as to enhance contact between sub¬ strate 42 and the image bearing surface of the photoconductor 4.

Alternatively the photoconductor 4 can be replaced by an electrostatic master such as a master commercially available from

DuPont under the tradename EPIC. In such a case, the apparatus of

Figs. 2A-2C is employed with the exception of lens 14 and lamp 62, which are eliminated.

Referring particularly to Figs. 3 and 4, transfer substrate 42 is mounted under tension on a channel-section frame 64, the open side of the channel facing outwards. Peripheral portions of sub¬ strate 42 are wrapped around frame 64, which is made preferably from a material having magnetic properties, and are secured to inward facing portions thereof as by magnetic strips 66.

A bottom surface 8l of frame 64 preferably slopes downwards towards an edge 82 which defines a support plane for substrate 42. It is to be appreciated that any other suitable method of securing substrate 42 to frame 64 may be employed in an alternative embodiment of the invention. In the illustrated embodiment of the invention, strips, referenced 68, of a high friction medium such as emery cloth are used to further secure substrate 42 to frame 64.

Housed within frame 64, preferably along the entire peripher thereof, is tensioning apparatus which, in a preferred embodimen of the invention, comprises an expansible tube 70, expansion o which applies to substrate 42 an outward, tensioning force i multiple directions as indicated by arrows 71.

When the flexible substrate is tensioned in at least tw directions, as by the apparatus shown in Figs. 3 and 4 and a described in conjunction therewith, surface variations in th substrate surface are substantially reduced and enhanced transfe uniformity is realized, producing reduced mottle. This is due t the substantially uniform pressure at which the tensioned substrat is brought into contact with the image.

Reference is now made to Figs. 5A, B and 5C, which illustra te image transfer apparatus for transferring an image onto a curve image receiving surface in three different operating orientations The apparatus comprises a fixed support surface 80. Disposed i predetermined locations with respect to the support surface are a image bearing surface 82, typically in the form of a cylindrica drum of the type described hereinabove in connection with any o

Figs. 1A-2C, and an image receiving surface 84, such as a cylindri¬ cal object, for example, a beverage can.

A tensioned flexible transfer substrate 86, typically a sheet of paper, single or double coated chroma paper or a material suitable for silk screening, such as POLYMON, commercially available from Swiss Silk Bolting Cloth Manufacturing Co. Ltd. of Zurich, Switzerland under part number PES-1/l/SRC, supported on a frame 88, such as the frame described in Figs. 3 and 4, is mounted for relatively unimpeded sliding motion along support surfacte 80, in a direction indicated by an arrow 90. The arrangement and linear motion of substrate 86 and the rotation of image bearing surface 82 and image receiving surface 84 are such that flexible substrate 86 sequentially engages image bearing surface 82 in image transfer engagement therewith and then engages image receiving surface 84 in image transfer engagement therewith. Corotrons $2 and 94 are posi¬ tioned in operative engagement with the flexible substrate 86 at the regions of engagement with the image bearing surface 82 and the image receiving surface 84 respectively for providing an appro¬ priate charge to the substrate 86 for enhancing transfer. In accordance with a preferred embodiment of the present invention, linear driving of the flexible substrate 86 is provided by a driving mechanism which will now be described with additional reference to Fig. 6. Initial acceleration of the flexible substrate 86 and frame 88 in the direction indicated by arrow 90 is provided by extension of a solenoid driven piston 96. Further driving of the flexible substrate 86 is provided by frictional engagement of the substrate 86 between a friction roller 98 and the cylindrical edge surface of image bearing surface 82. This frictional engagement ensures that the linear travel of the substrate 86 precisely cor- responds to the linear travel of the cylindrical image bearing surface 82, produced by rotation thereof in the direction by arrow 100. The linear surface rotation of image receiving surface 84 is precisely synchronized with that of the image bearing surface 82 and thus with the linear travel of the substrate 86, by means of suitable gearing (not shown) .

In order that engagement of friction roller 98 not interfere with sliding motion of the frame 88 and the substrate 86 when the

substrate is not in image transfer engagement with either of sur¬ faces 82 and 84, the friction roller 98 is spring mounted in asso¬ ciation with a cam follower roller 102 by means of a linkage 108 and a spring 109♦ It may be appreciated that at the onset of motion in the direction of arrow 90 and at the end of such motion, illustrated respectively in Figs. A and 0, roller 102 engages respective cams 104 and 106 which are mounted on frame 88, thus causing friction roller 98 to disengage from substrate 86. Intermediate the two extremes illustrated in Figs. 5A en 50, the roller 102 does not engage the cams and thus the friction roller 98 is caused to engage substrate 86 causing drum 2 to drive it in synchronized travel..with the surface travel of surfaces 82 and 84. Reference is now made to Figs. 7A-7E, which illustrate sche¬ matically a number of variations of the image transfer system described hereinabove in connection with Figs. 5A-5C Common refe¬ rence numerals have been used throughout to indicate similar struc¬ tures. Fig. 7A illustrates the system of Figs. 5A-5C generally wherein only line contact is provided between the substrate 86 and the image bearing surface 82 and between the same said substrate 86 and the image receiving surface 84 in the absence of the operation of the corotron, and relatively narrow area contact is produced by action of the corotron. Fig. 7B illustrates the same system wherein much broader area contact is provided with both the image bearing surface 82 and the image receiving surface 84 by suitable relative orientation of the surfaces relative to the substrate 86.

Fig. 70 illustrates an embodiment wherein a web of sheet material 110 is fed over a roller 112, to define a curved image receiving surface. Fig. 7-0 illustrates an image receiving cylinder 114 which has a non circular cross section and is supported by a spring loaded support 116. It is appreciated that the synchro¬ nization of the rotation drive of cylinder 114 must take into account the cross sectional configuration of the cylinder. Figs. 7E and 8 illustrate an embodiment wherein a non-cylin¬ drical image receiving surface 117 is provided and wherein the image receiving surface is curved in a direction perpendicular to

the direction of travel. Here a suitably curved corotron 4 is employed as shown in Fig. 8.

It will be appreciated that in most applications, particular¬ ly those described above in connection with Figs. ^~ JA- ^~ JE, the flexible substrate will be wetted with a transfer liquid such as Isopar H, prior to entering transfer engagement with an image bearing surface.

The apparatus and techniques of the present invention may be used for a wide variety of applications including printing on smooth surfaces such as plastic surfaces, printing on curved sur¬ faces such as metal cans and producing silk screen masters.

With the provision of a tensioned flexible substrate fixedly mounted on a rigid frame, the achievement of suitable alignment for precise registration of multi-pass color electrophotography, color proofing and other processes becomes a straightforward engineering task. Required color changes for electrophotography, for example, can be provided by sequentially draining and replacing toner in container 18 for successive colors as is well know in the art or, for both electrophotography and gravure, by the provision of mul- tiple transfer drums 2 as is also well known in the art.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. The scope of the invention is, rather, limited solely by the claims which follow: