Digital printing apparatus and process using curable dry toner

Abstract

A digital printing process for xerography printing with curable dry toner. The process includes: forming a latent image as a pattern of electric charge on a surface of an imaging member; transferring dry toner onto a development member; developing the latent image by transferring dry toner from the development member onto the imaging member in accordance with the pattern; transferring the dry toner from the imaging member to a first substrate; applying a second substrate on the transferred dry toner, fusing the transferred dry toner, and bonding the second substrate to the first substrate. The fusing is done before and/or during and/or after the applying of the second substrate. After application of the second substrate, the dry toner is irradiated with actinic radiation or particle beams to cure at least the fused transferred dry toner. The irradiating is done after and/or during the fusing.

Claims

1. A digital printing process for xerography printing with curable dry toner, wherein said process comprises: forming a latent image as a pattern of electric charge on a surface of an imaging member; transferring dry toner onto a development member; developing the latent image by transferring dry toner from the development member onto the imaging member in accordance with the pattern; transferring the dry toner from the imaging member to a first substrate; applying a second substrate on the transferred dry toner, fusing the transferred dry toner, and bonding the second substrate to the first substrate; wherein the fusing is done before and/or during and/or after the applying of the second substrate; after application of the second substrate, irradiating the dry toner with actinic radiation or particle beams to cure at least the fused transferred dry toner; wherein the irradiating is done after and/or during the fusing, such that the temperature of the dry toner during curing is higher than the glass transition temperature Tg thereof.

2. The process of claim 1, wherein the second substrate is provided with an adhesive layer on a face thereof facing the first substrate.

3. The process of claim 1, wherein the first substrate is provided with an adhesive layer on a face thereof where the dry toner is transferred.

4. The process of claim 1, further comprising applying a clear toner according to a clear toner pattern on the first substrate and/or on the second substrate, said clear toner pattern being such that at least the areas of the first substrate not covered with transferred dry toner, are covered by the clear toner.

5. The process of claim 1, wherein the bonding of the second substrate to the first substrate is caused by heat and/or pressure and/or actinic radiation and/or by particle beams.

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

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

8. The process of claim 1, wherein the second substrate and the dry toner are selected such that both the first and the second substrate adhere to the cured dry toner after the curing step.

9. The process of claim 1, wherein the irradiating is done in line with the fusing, wherein a distance measured on the first substrate between a fusing location and an irradiating location is less than 0.7 m; and wherein the first substrate moves from the fusing location to the irradiating location at a speed which is higher than 16 emfs.

10. The process of claim 1, wherein the fusing is done during the applying of the second substrate by applying a heated rotating member, comprising a fusing roller, against the second substrate such that the second substrate is pressed against the first substrate.

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

12. 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.

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

14. A digital printing apparatus for xerography printing with curable dry toner, 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 dry toner, and to develop said latent image by transferring said dry toner onto said imaging member in accordance with said pattern, wherein the image forming unit is further configured to transfer the dry toner from the imaging member to a first substrate; second substrate application unit configured to apply a second substrate on the transferred dry toner on the first substrate; a fusing means configured to fuse the transferred dry toner before and/or during and/or after the applying of the second substrate; and a curing means configured to irradiate the transferred dry toner with actinic radiation or particle beams to cure the transferred and fused dry toner; wherein the curing means are arranged downstream of the fusing means such that a distance measured on the first substrate between the fusing means and the curing means is less than 0.70 m; and wherein the curing means are arranged directly downstream of the fusing means such that the temperature of the transferred dry toner during curing is higher than the glass transition temperature Tg thereof.

15. The apparatus of claim 14, wherein the apparatus is configured to move the first substrate from the fusing means to the curing means at a speed which is higher than 16 emfs.

16. The apparatus of claim 14, wherein the curing means are arranged in a separate curing station downstream of the fusing means, and wherein the curing station is configured for heating the transferred and fused dry toner prior to curing.

17. The apparatus of claim 14, 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.

18. A digital printing apparatus for xerography printing with curable dry toner, 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 dry toner, and to develop said latent image by transferring said dry toner onto said imaging member in accordance with said pattern, wherein the image forming unit is further configured to transfer the dry toner from the imaging member to a first substrate; second substrate application unit configured to apply a second substrate on the transferred dry toner on the first substrate; a fusing means configured to fuse the transferred dry toner before and/or during and/or after the applying of the second substrate; and a curing means configured to irradiate the transferred dry toner with actinic radiation or particle beams to cure the transferred and fused dry toner; wherein the second substrate application unit is configured to apply a second substrate which is provided with an adhesive layer facing the first substrate; wherein the fusing means is integrated with the second substrate application unit and comprises a heated rotating member to apply the second substrate against the first substrate; the apparatus optionally further comprising a winding means configured for winding the resulting first substrate with the cured dry toner and applied second substrate; wherein optionally the curing means is an electron beam curing means or a UV source; wherein optionally the first and/or the second substrate are transparent; wherein optionally the image forming unit is further configured to transfer clear toner on the first substrate according to a pattern which is complementary to the pattern associated with the latent image; wherein the apparatus optionally further comprises an additional image forming unit configured to transfer clear toner on the first and/or second substrate, upstream of the second substrate application unit.

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-3C illustrate schematically three cross sections of three possible combined substrate structures S′ obtained with embodiments of the digital printing method; 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 dry toner, the dry toner comprises radiation curable resin material and a coloring agent such as a pigment. The radiation curable resin material is composed of one or more radiation curable resins. The radiation curable resin material may be a UV-light curable resin material, or another radiation curable resin material, e.g. an electron-beam curable resin material. The radiation curable resin material may be a mixture of one or more radiation curable resins. Further, the radiation curable resin material may be mixed with a non-radiation curable resin material comprising one or more non-radiation curable resins. In that case, the weight percent of radiation curable resin material with respect to the total amount of resin material (i.e. the sum of the radiation curable resin material and non-radiation curable resin material) is preferably higher than 85 weight %, more preferably higher than 90 weight %, and most preferably more than 95 weight %. Useful UV curable resins are resins based on (meth)acryloyl containing polyester. The term polyester includes all polymers with a backbone structure based on a polycondensation of an alcohol, preferably one or more polyols having 2 to 5 hydroxyl groups, and a carboxylic acid-containing compound. Examples of such UV curable resins are unsaturated polyesters based on terephthalic and/or isophthalic acid as the carboxylic acid-containing component, and on neopentylglycol and/or trimethylolpropane as the polyol component and whereon afterwards an epoxy-acrylate such as glycidyl (meth)acrylate may be attached. Another UV curable resin is a polyester-urethane acrylate polymer which may be obtained by the reaction of a hydroxyl-containing polyester, a polyisocyanate and a hydroxy-acrylate. Another useful curable resin material is composed of a mixture of an unsaturated polyester resin in which maleic acid or fumaric acid is incorporated and a polyurethane containing a vinyl ether. If a non-radiation curable resin material is included, the weight percentage of the non-radiation curable resin material is preferably less than 5 weight % of the total resin amount. The non-radiation curable resin material may contain one or more of the following resins: poly condensation polymers (e.g. polyesters, polyamides, co(polyester/polyamides), etc), epoxy resins, addition polymers. The radiation curable resin is preferably BPA free.

(7) For example, the radiation curable resin material may be a resin material comprising a blend of a (meth)acrylated polyester resin and a meth(acrylated) polyurethane resin. Preferably, the milli-equivalent amount of double bounds per gram of said radiation curable resin is more than 0.5 meq/g, more preferably more than 0.7 meq/g.

(8) In addition to the radiation curable resin material, the prepared toner particles may comprise any one or more of the following: a photo-initiator, a wax, a thermal initiator, a flowability improving agent, a charging agent, a filler, etc.

(9) The use of curable toner has the advantage that the internal cohesive strength of the toner layer after curing is higher compared to a non curable toner. Without being limit to any theory the hypothesis is that due to the crosslinking of the toner layer the cohesive strength of a toner layer can be significantly increased. The internal cohesive strength of a toner layer can be seen as the resistance to a split in the toner layer itself (and not the resistance to detach from the first or second substrate).

(10) When starting with a resin with a higher molecular weight or a resin that is (slightly) crosslinked the internal cohesive strength can be improved but the viscosity of the toner will increase and thus the fusing of the toner will become more critical resulting in limitations of substrates that can be printed on. Therefore the use of a low viscous curable toner results in a broad fusing window on many different substrates and a high cohesive strength after curing. The use of BPA as a monomer constituent also helps in this internal cohesion aspect, but the presence of this monomer or chemical alikes is not tolerated anymore in food packaging applications. Embodiments of the invention can solve this problem by curing a BPA free curable dry toner in accordance with embodiments of the invention.

(11) In the present invention, the term “radiation curable” refers to curable by actinic radiation or by a particle beam. The term “actinic radiation” is understood to cover any kind of radiation that can induce a cross-linking reaction in the toner particles after coalescence. In the invention, suitable actinic radiation includes IR-radiation, visible light, UV-light and γ-radiation. Suitable particle beams include electron beams.

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

(13) The first image forming unit 100a comprises a mixing device 130a, a first development member 140a, a first imaging member (also called photoconductor member) 150a, and a transfer corona 160a. The first imaging member 150a is adapted to sustain a first pattern of electric charge forming a first latent image on its surface. The first development member 140a is arranged to receive mixed first dry toner from the mixing device 130a, and to develop said first latent image by transferring a portion of said first dry toner Ta onto first imaging member 150a in accordance with said first pattern. Similarly, the second image forming unit 100b comprises a second development member and a second imaging member. The second imaging member is adapted to sustain a second pattern of electric charge forming a second latent image on its surface. The second development member is arranged to receive second dry toner, and to develop said second latent image by transferring a portion of said second dry toner onto second imaging member in accordance with said second pattern. The third and fourth imaging member 100c, 100d may be implemented in a similar manner.

(14) The first substrate S1 is supported on a substrate support assembly (not shown) for supporting the first substrate S1 during the subsequent transfer of first, second, third and fourth dry toner 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. In the development stage, dry toner particles travel from the development member 140a onto the imaging member 150a that carries the first latent image. In the transfer step, the developed image is transferred from the imaging member 150a onto the first substrate S1 e.g. using transfer coronas 160a. Similar development stages apply for the second, third and fourth image forming units 100b, 100c, 100d.

(15) Throughout the application, the some stages of the image forming units 100a, 100b, 100c, 100d 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.

(16) 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 dry toner on the first substrate S1, a fusing means 350 configured to fuse the transferred dry toner, and a curing means 400 configured to irradiate the transferred dry toner through said second substrate S2 and/or through the first substrate S1 with actinic radiation or particle beams to cure the transferred dry toner. In the embodiment the fusing means 350 are integrated in the second substrate application unit 300, e.g. by using a pair of rolls comprising a fuser roll 350a and an optionally heated backing roll 350b. It is noted that further heating members may be provided downstream and/or upstream of the fusing means 350. For example, roll 330 and/or 340 may be heated to heat the backside of the first substrate S1 between the image forming unit 100d and the second substrate application unit 300. The fusing means 350 perform a fusing step downstream of the image forming units 100a, 100b, 100c, 100d, to heat the dry toner particles to a temperature above Tg which is advantageous for the performing of a good curing by the curing means 400. Further the fusing step may enhance the mixing of imaging particles of different colors.

(17) The fusing means 350 may also function to cause a bonding of the second substrate S2 to the first substrate S1. When the fusing means are not integrated in the second substrate application unit 300 (see also FIG. 4 and the discussion below), the second substrate application unit 300 may still comprise a pair of rolls 350a, 350b to bond the second substrate S2 to the first substrate S1, e.g. by pressure and/or heat.

(18) The second substrate S2 may be provided with an adhesive layer (see also FIG. 3A) facing the first substrate S1 in order to improve the bonding of the second substrate S2 to the first substrate S1. Alternatively or in addition first substrate S1 may be provided with an adhesive layer (see also FIG. 3B) facing the second substrate S2 in order to improve the bonding of the second substrate S2 to the first substrate S1.

(19) 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.

(20) The second substrate application unit 300 is arranged downstream of the image forming units 100a, 100b, 100c, 100d. FIG. 3A shows schematically the second substrate S2 comprising a base layer B and an adhesive layer A applied on a first substrate S1 on which dry toner particles P has been applied. Applying the second substrate S2 may cause a slight smoothening of the upper surface of the dry toner particles P. In the example of FIG. 3B both the first and second substrate S1, S2 comprise a base layer B1, B2 and an adhesive layer A1, A2.

(21) The curing means 400 may be an electron beam (EB) curing means. 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. An EB curing means 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 means. 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 dry toner and surface sterilization of the substrates S1, S2. Such embodiments may be useful for food packaging materials.

(22) The curing means 400 may also function to cause or enhance a bonding of the second substrate S2 to the first substrate S1. To that end the adhesive layer of the first and/or second substrate S1, S2 may be a curable adhesive layer.

(23) Other curing means 400 are UV curing systems based on LED and/or (doped) mercury bulb. It is advisable that the absorption spectrum of the used photo-initiator match with the spectrum of the irradiated UV light in order to obtain an as good curing as possible.

(24) 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.

(25) 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 dry toner on the first substrate S1, a second image forming unit 200 to apply curable clear toner to the non image parts of the image, optionally a second image forming unit 200′ to apply an additional amount of curable clear toner to have the possibility to adjust the desired amount of clear toner, a second substrate application unit 300 configured to apply a second substrate S2, e.g. a foil, on the transferred dry toner on the first substrate S1, and a curing means 400 configured to irradiate the transferred dry toner through said second substrate S2 and/or through the first substrate S1 with actinic radiation or particle beams to cure the dry toner. A fusing means 350 may be integrated in the second substrate application unit 300, see reference numeral 350. In addition or alternatively there may be provided a fusing means between the image forming unit 200′ and the second substrate application unit 300, and/or a fusing means 350″ between the second substrate application unit 300 and the curing means 400. The fusing means 350, 350″ may function to fuse the dry toner and/or to cause the bonding of the first substrate S1 to the second substrate S2. Such an example is illustrated in FIG. 3C.

(26) 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. Examples of 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.

(27) 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 with an adhesive layer 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.

(28) 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; a first station 1000 comprising an image forming unit 100 for forming a printed image by transferring dry toner on the first substrate S1 and a second substrate application and fusing unit 300, 350 configured to apply a second substrate S2 on the transferred dry toner on the first substrate S1 and to fuse the transferred dry toner (before, during and/or after the application of the second substrate); and a curing station 1400 comprising a curing means configured to irradiate the transferred dry toner through said second substrate S2 and/or through the first substrate S1 with actinic radiation or particle beams to cure the dry toner, and optionally also a heating means to heat the resulting substrate S′ before and/or during curing; and a substrate winding means 600 downstream of the curing means 400, for winding the cured resulting substrate S′. The fusing means may be integrated in the second substrate application unit 300, as explained in connection with FIG. 1. In addition or alternatively there may be provided a fusing means between the image forming unit 100 and the second substrate application unit 300, and/or a fusing means between the second substrate application unit 300 and the curing station 1400. The fusing means 350 may function to fuse the dry toner and/or to cause the bonding of the first substrate S1 to the second substrate S2. Also, the curing station 1400 may function to cure the dry toner and/or to cause or enhance the bonding of the second substrate S2 to the first substrate S1.

(29) As illustrated in FIGS. 3A and 3B, the first and/or the second substrate S1, S2 may comprise a base layer and an adhesive layer. In the example of FIG. 3A, the second substrate S2 comprises an adhesive layer A and a base layer B, while the first substrate S1 may be any desirable substrate that can be bonded through the adhesive layer A of the second substrate S2. The adhesive layer A is preferably a layer which is dry when stored, but which can bond to the first substrate S1 e.g. when heated and/or pressed and/or cured, e.g. when passing through the second substrate application unit 300, and/or through the fusing means 350, 350′, 350″ and/or through the curing means 400. In the example of FIG. 3B, both the first and the second substrate S1, S2 comprises an adhesive layer A1, A2 and a base layer B1, B2. In that way the adhesive layers A1, A2 may be thinner. The adhesive layer A1 is a layer which is dry when transferring the dry toner particles thereon, but which can bond to the adhesive layer A2 e.g. when heated and/or pressed and/or cured, e.g. when passing through the second substrate application unit 300, and/or through the fusing means 350, 350′, 350″ and/or through the curing means 400. In yet another example (not illustrated) only the first substrate S1 may be provided with an adhesive layer. In the example of FIG. 3C clear toner C is applied on the non-image parts of the first substrate, whereupon fusing is performed and a second substrate S2 is applied thereon followed by curing.

(30) 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.