Digital printing apparatus and processing using liquid toner

Abstract

A digital printing process for xerography printing with liquid toner is disclosed, said liquid toner including a curable carrier liquid and imaging particles suspended in the carrier liquid. The process includes: forming a latent image as a pattern of electric charge on a surface of an imaging member; transferring the liquid toner onto a development member; developing the latent image by transferring liquid toner from the development member onto the imaging member in accordance with the pattern; transferring the liquid toner from the imaging member to a first substrate; applying a second substrate on the transferred liquid toner; after application of the second substrate, irradiating the liquid toner with actinic radiation or particle beams to cure the carrier liquid.

Claims

1. A digital printing process for xerography printing with liquid toner, said liquid toner comprising a curable carrier liquid and imaging particles suspended in the curable carrier liquid, wherein said process comprises: forming a latent image as a pattern of electric charge on a surface of an imaging member; transferring the liquid toner onto a development member; developing the latent image by transferring liquid toner from the development member onto the imaging member in accordance with the pattern; transferring the liquid toner from the imaging member to a first substrate; applying a second substrate on the transferred liquid toner; after application of the second substrate, irradiating the liquid toner with actinic radiation or particle beams to cure the curable carrier liquid.

2. The process of claim 1, wherein the first substrate and the curable carrier liquid are selected such that a layer of the curable carrier liquid remains present on the first substrate after the step of transferring the liquid toner from the imaging member to the first substrate.

3. The process of claim 1, wherein the curable carrier liquid is an electron beam-curable carrier liquid, and the irradiating step comprises irradiating the liquid toner with electron beams.

4. The process of claim 1, wherein the first and/or the second substrate are transparent.

5. The process of claim 1, wherein the second substrate and the curable carrier liquid are selected in such a way that both the first and the second substrate adhere to the cured carrier liquid after the irradiating step.

6. The process of claim 1, wherein the second or the first substrate is removed from the cured carrier liquid after the irradiating step.

7. The process of claim 6, wherein the second or first substrate that is being removed is chosen to be a substrate with a predetermined surface roughness or surface pattern on the side that is in contact with the transferred liquid toner, such that the removal of said substrate creates a complementary surface roughness or surface pattern at a top surface of the cured carrier liquid.

8. The process of claim 1, wherein the irradiating is done through the second substrate and/or through the first substrate.

9. The process of claim 1, wherein the first substrate and the second substrate are provided as a continuous web during printing; and wherein, during printing, the development member and the imaging member are continuously rotating members.

10. The process of claim 1, wherein the first substrate and the second substrate comprises any one of the following: plastic film, metallic film, thermal paper, paper, and combinations thereof.

11. The process of claim 1, wherein the first or second substrate is a plastic foil.

12. The process of claim 1, wherein the imaging particles are any one of the following: chargeable colour pigments, chargeable coated colour pigments, chargeable toner particles with colour pigments or dyes.

13. The process of claim 1, wherein the liquid toner is transferred from the imaging member to the first substrate either directly or via an intermediate member.

14. The process of claim 1, wherein the curable carrier liquid comprises a radiation curable dispersing agent.

15. The process of claim 1, further comprising applying additional curable liquid: on the first substrate after the step of transferring from the imaging member to the development member, and before the step of applying the second substrate; and/or on a side of the second substrate before the second substrate is applied on the first substrate and such that the side is brought in contact with the transferred liquid toner on the first substrate.

16. The process of claim 15, wherein the applying of additional curable liquid is done in accordance with a coating pattern which is substantially complementary to the pattern used in the developing step to control a thickness of the liquid layer on the first substrate to be substantially the same in all areas of the first substrate.

17. A digital printing apparatus for xerography printing with liquid toner, wherein the liquid toner used in said printing apparatus comprises a curable carrier liquid and imaging particles suspended in the curable carrier liquid, and wherein said apparatus comprises: an image forming unit comprising an imaging member adapted to sustain a pattern of electric charge forming a latent image on its surface, a development member arranged to receive liquid toner, and to develop said latent image by transferring said liquid toner onto said imaging member in accordance with said pattern, wherein the image forming unit is further configured to transfer the liquid toner from the imaging member to a first substrate; a second substrate application unit configured to apply a second substrate on the transferred liquid toner on the first substrate; a curing unit downstream of the second substrate application unit, said curing unit being configured to irradiate the transferred liquid toner with actinic radiation or particle beams to cure the curable carrier liquid.

18. The apparatus of claim 17, further comprising a first substrate feeding means configured to feed the first substrate as a continuous web during printing, wherein the second substrate application unit is configured to apply the second substrate as a continuous web during printing, and wherein the imaging member and the development member are configured to rotate during printing.

19. A digital printing process for xerography printing with liquid toner, said liquid toner comprising an electron beam curable carrier liquid and imaging particles suspended in the electron beam curable carrier liquid, wherein said process comprises: forming a latent image as a pattern of electric charge on a surface of an imaging member; transferring the liquid toner onto a development member; developing the latent image by transferring liquid toner from the development member onto the imaging member in accordance with the pattern; transferring the liquid toner from the imaging member to a first substrate; applying a second substrate on the transferred liquid toner; after application of the second substrate, irradiating the liquid toner with electron beams to cure the electron beam curable carrier liquid, wherein the first and/or the second substrate are transparent.

20. The process of claim 19, wherein the first substrate and the electron beam curable carrier liquid are selected such that a layer of the electron beam curable carrier liquid remains present on the first substrate after the step of transferring the liquid toner from the imaging member to the first substrate.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of devices of the present invention. The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is block diagram of an exemplary embodiment of a digital printing apparatus;

(3) FIG. 2 is block diagram of another exemplary embodiment of a digital printing apparatus;

(4) FIGS. 3A and 3B illustrate schematically a cross section of a printed image before and after a foil S2 is applied, respectively; and

(5) FIG. 4 illustrates a block diagram of another exemplary embodiment of a digital printing apparatus.

DESCRIPTION OF EMBODIMENTS

(6) In electrophotographic processes operating with liquid toner (also called liquid toner dispersion), imaging particles (also called marking particles) are supplied as solid particles suspended in a carrier liquid. In embodiments of the invention a curable carrier liquid is used. The imaging particles may be chargeable colour pigments, chargeable coated colour pigments, chargeable toner particles with colour pigments, dyes. Toner particles comprise pigment grains, typically embedded in a small bead of resin. A dispersing agent, also called dispersant is added to the mix to avoid clustering of the imaging particles. Dispersants deflocculate the imaging particles and reduce the viscosity of the liquid toner dispersion.

(7) The curable carrier liquid in a liquid toner according to the present invention can be any suitable liquid having the desired conductivity and viscosity characteristics and capable of becoming cured to form a solid. The curable carrier liquid has typically a conductivity lower than 50 pS/cm, more preferably lower than 20 pS/cm and even more preferable lower than 5 pS/cm. Further, the liquid carrier must be capable of permitting the imaging particles of the liquid toner to migrate through the carrier liquid to develop electrostatic latent images.

(8) Typical carrier liquids suitable as the curable liquid vehicle include ethylenically unsaturated compounds, including monomers, dimers, or oligomers having one or more ethylenically unsaturated groups such as vinyl or allyl groups, and polymers having terminal or pendant ethylenic unsaturation. Examples of suitable curable liquids include, but are not limited to, acrylate and methacrylate monomers or polymers containing acrylic or methacrylic group(s). Also suitable are epoxy monomers or epoxy containing polymers having one or a plurality of epoxy functional groups. Further examples of suitable curable materials include vinyl ether monomers, oligomers, or polymers containing vinyl ether groups. Also suitable are internal or terminal alkenes and polyenes, fatty acid mono-, di-, tri- and polyesters based on unsaturated fatty acids and/or unsaturated alcohols and alkyl carbonates based on unsaturated alcohols. Depending on the functionality of the curable carrier liquid (number of double bounds) the bonding strength and flexibility of the print with cured carrier liquid can be adjusted.

(9) In an exemplary embodiment the curable carrier liquid is an isostearyl acrylate.

(10) Other examples of curable carrier liquids are Ebecryl and UceCoat from Allnex, Radia, Radiasolve, Radiasurf and Radiamuls from Oleon and Alpha Olefins from Ineos Oligomers.

(11) In electrophotographic processes liquid toner (also called liquid toner dispersion) is a dispersion of marking particles in a carrier liquid. In other words the toner particles are substantially undissolved in the carrier liquid. The marking particles may comprise coloured particles (also called ink particles or pigment) and a binder resin although non-pigmented resin systems also can be used containing e.g. a phosphor or taggant or UV active material. Typically, the diameter of the marking particles is about 0.5 to 4.0 m. The marking particles have a concentration of about 40-95% of binder resin. The binder resin is a polymer, preferably transparent, that embeds the ink particles. Preferably, a polyester resin is used as binder resin. Also other types of resin having a very low or no compatibility with the carrier liquid and dispersing agent can be used. Preferably, the resin has a high transparency and provides good colour developing properties. The coloured particles and other optional components of the marking particles such as wax, plasticizer or other additives may be embedded in the binder resin. The marking particles may be extrudates of the binder resin and the coloured particles.

(12) In some embodiments, the marking particles may be curable marking particles, preferably marking particles curable by a particle beam such as an electron beam.

(13) The carrier liquid may further contain variable amounts of charge control agent (CCA), wax, plasticizers, and other additives, although they also can be incorporated into the imaging particle itself. The carrier liquid may be volatile or non-volatile. Typically, the toner liquid may have a solid concentration between 5% and 60 wt %. The high-shear viscosity, as measured at a shear rate of 3000 s1 at 25 C. with a cone plate geometry of C60/1 and a gap of 52 m, is preferably in the range of 5-500 mPa.Math.s.

(14) FIG. 1 illustrates schematically an exemplary embodiment of a digital printing apparatus using liquid toner. The apparatus comprises a first image forming unit 100a for applying liquid toner Ta having a first colour, e.g. black, onto a first substrate S1, a second image forming unit 100b for applying liquid toner Tb having a second colour, e.g. cyan, onto the first substrate S1, a third image forming unit 100c for applying liquid toner Tc having a third colour, e.g. magenta, onto the first substrate S1, and a fourth image forming unit 100d for applying liquid toner Td having a fourth colour, e.g. yellow, onto the first substrate S1.

(15) The first image forming unit 100a comprises a toner reservoir 110a, a feed member 120a, a first development member 130a, a first imaging member 140a, and an optional intermediate member 150a. The first imaging member 140a is adapted to sustain a first pattern of electric charge forming a first latent image on its surface. The first development member 130a is arranged to receive first liquid toner Ta from the feed member 120a, and to develop said first latent image by transferring a portion of said first liquid toner Ta onto first imaging member 140a in accordance with said first pattern. Similarly, the second image forming unit 100b comprises a toner reservoir 110b, a feed member 120b, a second development member 130b, a second imaging member 140b, and an optional intermediate member 150b. The second imaging member 140b is adapted to sustain a second pattern of electric charge forming a second latent image on its surface. The second development member 130b is arranged to receive second liquid toner Tb from the feed member 120b, and to develop said second latent image by transferring a portion of said second liquid toner Tb onto second imaging member 140b in accordance with said second pattern. The third and fourth imaging member 100c, 100d may be implemented in a similar manner.

(16) The first substrate S1 is supported on a substrate support assembly which comprises in the illustrated embodiment first, second, third and fourth support members 200a, 200b, 200c, 200d for supporting the first substrate S1 during the subsequent transfer of first, second, third and fourth liquid toner Ta, Tb, Tc, Td from the first, second, third and fourth image forming unit 100a, 100b, 100c, 100d, respectively, whilst the first substrate S1 moves in a movement direction M from the first image forming unit 100a to the fourth image forming unit 100d.

(17) In the development stage, imaging particles travel from a development member 130a supplied with a thin, film-like layer of liquid toner Ta, onto the imaging member 140a that carries the first latent image. In a subsequent step, the developed first latent image is transferred from the imaging member 140a onto the intermediate member 150a. In the final transfer step, the developed image is transferred from the intermediate roller 150a onto the first substrate S1, which is supported by the support roller 200a that may be kept at a suitable potential. Similar development stages apply for the second, third and fourth image forming units 100b, 100c, 100d.

(18) Throughout the application, the various stages of the image forming units 100a, 100b, 100c, 100d and of the support assembly 200a, 200b, 200c, 200d have been described as members. These members may be rotating rollers, but the skilled person will appreciate that the same principles may be applied with other members, e.g. comprising a suitably designed rotating belt with a roll and/or a belt tracking shoe.

(19) This process results in a thin layer of liquid toner being applied to the first substrate S1, see FIG. 3A. When four colours are printed the thickness of the liquid toner layer on the first substrate is typically between 0.5 and 15 micron. For example, in areas where an image with four colours is printed the thickness may be approximately 11 to 13 micron, in areas where no image is present the thickness may be approximately 0.5 to 2 micron, and in areas with an image with one colour the thickness may be approximately 3 to 5 micron.

(20) The digital printing apparatus further comprises a second substrate application unit 300 configured to apply a second substrate S2, e.g. a foil, on the transferred liquid toner on the first substrate S1, and a curing unit 400 configured to irradiate the transferred liquid toner through said second substrate S2 and/or through the first substrate S1 with actinic radiation or particle beams to cure the carrier liquid on and/or in the first substrate S1. When the first substrate S1 does not absorb the carrier liquid, the carrier liquid on the first substrate S1 is cured. When the first substrate S1 partially absorbs the carrier liquid, the carrier liquid on and in the first substrate S1 is cured. Also, when the carrier liquid is partially absorbed in the second substrate S2, the carrier liquid in the second substrate S2 is cured. In other words, preferably the resulting substrate S (i.e. the first substrate S1 with the printed image beneath second substrate S2) does not contain any uncured carrier liquid.

(21) It is noted that the first substrate S1 and/or the second substrate S2 may be transparent. For example, the first substrate S1 may be a non-transparent substrate and the second substrate S2 may be a transparent film. In this example the irradiating is preferably done through the second substrate S2. However, it is also possible to print on a thin transparent first substrate S1 and to irradiate through the first substrate S1. In such an embodiment the second substrate S2 may be non-transparent. In yet another embodiment the first substrate S1 may be a transparent substrate, and the second substrate S2 may be a non-transparent substrate, i.e. non-transparent for visible light but transparent for the radiation of electron beams used.

(22) The second substrate application unit 300 is arranged downstream of the image forming units 100a, 100b, 100c, 100d. FIG. 3A shows the first substrate S1 on which a layer of carrier liquid CL containing imaging particles P has been applied, and FIG. 3B shows the first substrate S1 downstream of the second substrate application unit 300, after a second substrate S2 has been applied over the layer of carrier liquid CL. Applying a second substrate S2 may cause a slight smoothening of the upper surface of the layer of carrier liquid CL.

(23) The curing unit 400 is preferably an electron beam (EB) curing unit. EB penetration depends amongst others upon the mass density and thickness of the material. EB curing has the advantage that electrons are substantially color blind and that penetration is not affected by pigments and opaque substrates. This has also the advantage that EB curing can take place through a non-transparent substrate, for example a metallic foil. An EB curing unit typically comprises electrically operated filaments and grids contained within a vacuum chamber. The electrons are accelerated through a window/foil structure to reach the area to be cured at atmospheric pressure. In an embodiment of the invention low-voltage EB equipment operating from about 70 to 125 kV may be used for most applications. EB penetration may be controlled by varying the accelerating potential (voltage) of the EB curing unit. If the first substrate S1 does not absorb the carrier liquid, preferably, the control is such that the penetration in the first substrate S1 itself is limited. However, the effect of the electron beams on the first substrate S1 may in certain embodiments be beneficial. E.g. cross-linking may enhance the properties of some polyethylene based substrates. Also, EB-induced ionization of the substrate surface may result in enhanced adhesion. Electron beams can also potentially be used for simultaneous curing of the carrier liquid and surface sterilization of the substrates S1, S2. Such embodiments may be useful for food packaging materials.

(24) Embodiments of the invention have the advantage that the fusing step which is typically applied downstream of the image forming units 100a, 100b, 100c, 100d may be omitted. In that way it is avoided that the substrate is heated up to a high temperature since the curing by the curing unit 400 can be performed at lower temperatures. However, it is still possible to perform a fusing step downstream of the image forming units 100a, 100b, 100c, 100d, before or after the second substrate application unit 300, e.g. to enhance the mixing of imaging particles of different colors.

(25) Particular embodiments of the invention relate to the field of digital printing apparatus and processes for so-called continuous webs, i.e. printing systems where a continuous roll of substrate is run through the printer, in particular to print large numbers of copies of the same image(s), or alternatively, series of images, or even large sets of individually varying images. The digital printing apparatus comprises to that end a first substrate feeding means 500 configured to feed the first substrate S1 as a continuous web during printing. Further the second substrate application unit 300 may be configured to apply the second substrate S2 as a continuous web during printing. The resulting substrate S with the printed image beneath second substrate S2 may then be rolled on a roll 600.

(26) FIG. 2 illustrates another exemplary embodiment of a digital printing apparatus of the invention in which components similar to the components of the embodiment of FIG. 1 have been indicated with the same reference numerals. The digital printing apparatus comprises a first substrate feeding means 500 for feeding a first substrate S1, an image forming unit 100 for forming a printed image by transferring liquid toner on the first substrate S1, a second substrate application unit 300 configured to apply a second substrate S2, e.g. a foil, on the transferred liquid toner on the first substrate S1, and a curing unit 400 configured to irradiate the transferred liquid toner through said second substrate S2 and/or through the first substrate S1 with actinic radiation or particle beams to cure the carrier liquid on and/or in the substrate S1. The digital printing apparatus of FIG. 2 further comprises a second substrate removal means 620 downstream of the curing unit 400, and a first substrate winding means 610 downstream of the curing unit 400. The second substrate removal means 620 is configured to remove the second substrate S2 after curing. The first substrate winding means 610 is configured for winding the first substrate S1 with the cured liquid toner after removal of the second substrate S2 by the second substrate removal means 620. The second substrate removal means 620 comprises a second substrate reel spool 625 for winding the removed second substrate S2. In such an embodiment the first substrate S1 and the second substrate S2 may be opaque. If the irradiating is performed through the second substrate S2, the second substrate S2 may be opaque as long as it allows the used actinic radiation or particle beams to pass through.

(27) Preferably the first substrate S1 comprises any one of the following: plastic film, metallic film, thermal paper, paper, and combinations thereof. The first substrate S1 may also have a multilayer structure. The invention is especially advantageous with first substrates S1 in which the carrier liquid is not absorbed (or only to a limited extent) such that the carrier liquid remains present on top of the first substrate S1. Examples of such first substrates are plastic or metallic films. Suitable plastics are e.g. polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyester, polycarbonates, polyvinyl acetate, polyolefins and particularly polyethylenes (PE), like polyethylene of high density (HDPE), polyethylene of middle density (MDPE), linear polyethylene-middle density (LMDPE), polyethylene low-density (LDPE), linear low density polyethylene (LLDPE), and (biaxially oriented) polypropylene (PP). Examples of metallic films are foils comprising any one or more of the following: iron, steel, copper, aluminium and its alloys. Preferably, a metallic film comprises a polymer foil on which a metal coating is applied.

(28) In an exemplary embodiment where the second substrate S2 is not removed, the first substrate S1 may be non-transparent and the second substrate S2 may be transparent. However it is also possible to apply a (removable) non-transparent foil as the second substrate S2. If the second substrate S2 has to be transparent, preferably a polymer foil is chosen. Examples of suitable plastic foils are: PE foils, PP foils, polyester foils, etc. In such an embodiment the irradiating is preferably done through the second substrate S2, and the second substrate S2 is then preferably thin, e.g. between 15 and 50 micron. Alternatively the printing may be performed on a thin first substrate S1 and the irradiating may be performed through the first substrate S1.

(29) FIG. 4 illustrates another exemplary embodiment of a digital printing apparatus of the invention in which components similar to the components of the embodiment of FIGS. 1 and 2 have been indicated with the same reference numerals. The digital printing apparatus comprises a first substrate feeding means 500 for feeding a first substrate S1, an image forming unit 100 for forming a printed image by transferring liquid toner on the first substrate S1, a coating station 700 configured for applying additional curable liquid, a second substrate application unit 300 configured to apply a second substrate S2 on the transferred liquid toner on the first substrate S1, and a curing unit 400 configured to irradiate the transferred liquid toner through said second substrate S2 and/or through the first substrate S1 with actinic radiation or particle beams to cure the carrier liquid on and/or in the first substrate S1, and a substrate winding means 600 downstream of the curing unit 400, for winding the resulting substrate S. When the first substrate S1 does not absorb the carrier liquid, the carrier liquid on the first substrate S1 is cured. When the first substrate S1 partially absorbs the carrier liquid, the carrier liquid on and in the first substrate S1 is cured. Also, when the carrier liquid is partially absorbed in the second substrate S2, the carrier liquid in the second substrate S2 is cured. In other words, preferably the resulting substrate S does not contain any uncured carrier liquid. In an exemplary embodiment the apparatus further comprises a coating station for applying curable liquid on the first substrate between the image forming unit and the second substrate application unit. The coating station 700 can be configured to add the curable liquid image-wise, e.g. according to a pattern that is complementary to the pattern used by the image forming unit, or non-image-wise, e.g. as an even layer. The coating station 700 may comprise for example an anilox roller and/or one or more inkjet heads. The inkjet heads may be configured to apply curable liquid according to a controllable pattern, e.g. according to a pattern that is complementary to the pattern used by the image forming unit. To that end there may be provided a controller (not shown) configured to receive image data about the image to be printed by the image forming unit 100, and to control the inkjet heads based on the received image data.

(30) The skilled person understands that also in the embodiment of FIG. 2, it may be envisaged to add a coating station 700 between the image forming unit 100 and the second substrate application unit 300. In yet another embodiment (not shown) the coating station 700 may be used to apply a curable liquid on the bottom side of the second substrate S2, whereupon the second substrate S2 may be applied with its coated bottom side against the printed image on the first substrate S1.

(31) While the invention has been described hereinabove with reference to specific embodiments and examples, this is done to illustrate and not to limit the invention. The skilled person will appreciate that other ways of implementing the inventive concept described herein are within the scope of the invention, as defined by the accompanying claims.