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The present invention is applicable to digital printing and substrate processing systems that require protection on contact with any other parts, especially, but not only, in the moments after printing. The protection may be required on any surface of the substrate. The substrate may be of any shape. In the case of flat substrates, the surface to be protected may be the upper and/or lower surface. The protection may be necessary to avoid contact of the substrate with the freshly printed area of other substrates printed in the same process, or to avoid contact with other elements of the production line. Therefore, the substrate could be, for example, a sanitary substrate that, from a certain moment of its process, should not come into direct contact with the transport elements of the production line. However, the present invention is particularly applicable in the field of digital printing where there is a subsequent stage, immediately after digital printing, of stacking the substrates.
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In digital printing systems with stacking, after applying the print to the substrate, curing and/or drying said print, the substrates are stacked on top of each other, with the new printed substrate, which may be identical to the one below, being stacked on top and so on. The printed area on the upper side of a lower substrate is brought into contact with the lower surface of the upper substrate. Any possible manner of contamination due to contact between substrates can occur from one side to the other. For example, there may be a migration of substances contained in the printed ink, even if it is completely fixed, towards the contact surface or simply a migration of substances, particles or dirt that may be on the non-printed surface towards the printed area.
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Contamination or migration will depend on the type of inks and materials used. In the case of printed substrates that are going to be used as packaging for food products, the migration that could occur from the printed side to the non-printed side of another substrate during stacking could subsequently cause the compounds that migrated to the inside of the packaging to later migrate to the food contained in said packaging when it is used. The range of materials whose components must not be transferred to food is very wide: plastic, glue, paper and cardboard, varnish, aluminum, color dyes, etc. Contamination can also occur, for example, by soiling due to contact between surfaces.
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In order to avoid such contamination, it is known to apply varnishes on top of the entire substrate or only on top of the printed area. For example, in the case of the manufacture of metal plates for sanitary containers, a decoration is first applied on the printed area by means of inkjet printing and then a finishing varnish is applied on top of the printed area. The finishing varnish contains solvents that have to evaporate before further handling of the plates. To ensure that the solvent evaporates, the substrates pass through an evaporation tunnel. This process requires times that may even be longer than one hour. In addition, it requires high temperatures, which means that all the materials applied must be resistant to high temperatures and also entails high energy costs. The lower surface is also varnished with a layer which is designed to prevent migrations and which is applied in liquid form. Said liquid layer must also be evaporated, which adds more time to the process. In short, the application of varnishes lengthens the production process. Moreover, the presence of a liquid with contaminating substances (the varnish) also poses a risk of contamination in itself between its application and its final solidification.
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It is an objective of the present invention to disclose means for providing a solution to the above-mentioned problem of contamination.
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More particularly, the present invention discloses a method for printing substrates by means of injecting ink, comprising the step of printing one side of a substrate in a printed area. The method further comprises a step of applying, preferably on the printed side, a protective layer on the printed area and/or on selected protective areas by means of bringing the substrate into contact with a transfer film carrying a protective layer. Preferably, the material transferred from the transfer film is solid.
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Preferably, the substrate is food packaging. For example, the substrate can be a film forming part of a bag-type food container, a flat or curved metal film forming part of a metal beverage can, etc.
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More preferably, the substrate has only one printed side.
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Preferably, the protective layer is applied immediately after the mentioned printing and, if necessary, subsequent curing of said print.
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In certain applications, the protective layer is preferably transparent. When the protective layer covers the printed area, it is particularly advantageous for the protective layer to be transparent, since it will allow the printed area to be seen. However, it would also be possible for the protective layer to be colored, particularly when said layer does not cover the entire printed area. If the protective layer does not coincide with the printed area, it may be transparent or colored.
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Preferably, the protective layer protrudes with respect to the substrate more than the printing material in the printed area. In certain applications, the protective film or layer surrounds the printed area.
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Preferably, the protective layer in the selected areas creates a distribution of raised areas that prevents contact with the printed area of another substrate placed on top of the substrate. In a particularly preferred embodiment, the protective layer is distributed, continuously or discontinuously, around the outer perimeter of the printed area. When the protective layer is located on the printed area, it may cover the printed area completely, or it may be distributed in sub-areas over the entire printed area, preferably in such manner that a distribution or network of protruding spots is created that prevent contact of another substrate with the printed area.
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An advantage associated with the distribution or network of protruding spots is that it may be easier, since the entire surface of the substrates is not in contact, to subsequently remove said substrates when unstacking, as no "vacuum" force, electrostatic charge force or any other type of force is generated between the surfaces of the substrates when trying to move them.
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Another advantage of the distribution or network of protective spots or zones, particularly when the material of the protective layer is resistant and/or has a higher coefficient of friction than the ink in the printing area, is that said areas make it difficult for one substrate to slide over another while the stack of substrates is being formed or while substrates are being removed from the stack. Ultimately, the present invention will not only protect against contamination but also against the lack of adequate friction between substrates when stacking or unstacking. This problem of lack of friction is caused due to the low coefficient of friction that certain inks can exhibit, once cured.
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The distribution of the protective areas, according to the present invention, may present some patterns that are more intense than others, i.e. a higher density of spots and/or extension of the same, depending on the weight of the pile that each specific substrate has to support.
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In certain applications, particularly when the protective layer does not interfere with the printed area, preferably the combined thickness of the protective layer and the adhesive that adheres it to the substrate is greater than the thickness of the printing material deposited on the printed area.
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Preferably, the adhesive is an adhesive cured by means of ultraviolet radiation, and the curing is effected by means of ultraviolet radiation.
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The protective layer can be applied on the same side as the printed area and/or on an opposite side, whether printed or not. In either case, the protective layer will be located on the printed area or area to be printed, or outside of it, preferably surrounding it. When the protective layer is located on one such opposite, non-printed side, it is understood that the protective layer is located "in the printed area" when it coincides with the projection of the printed area on mentioned opposite side, "outside the printed area" and "preferably surrounding the printed area" when it is located outside said projection and surrounding it, respectively.
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The printed area can be printed before, during or after the application of the protective layer. The protective layer can be continuous or discontinuous.
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In a particularly preferred embodiment, in addition to a protective layer on the printed side, a further protective layer is applied on the non-printed side by means of contact with a transfer film carrying a protective layer, on the entire mentioned side or on selected protective areas of this side.
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The process to which the present invention relates may advantageously comprise a subsequent step of stacking printed substrates, one directly on top of the other.
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The present invention can be advantageously applied to metallic substrates, particularly in processes in which the substrate subsequently undergoes a welding process. In these cases, the welding process will preferably take place in an area without a protective layer.
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The present invention utilizes the application by means of a system of transfers of a protective layer which is intended to protect against migrations from the printing material to other substrates at later stages of the substrate treatment. It also protects against possible contamination of the medium, or from the medium to the substrates, etc.
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The present invention arranges a laminated material on a carrier film or foil and transfers it from the carrier foil to the substrate. This method ("transfer method") is known from other processes and for different purposes. Document
MX2022006644A explains the method, applying it to obtain effects on part of the printed area. Basically, it involves selecting, by means of applying an adhesive on the substrate or on the carrier film of the material to be applied, the areas on which it is desired to transfer the film onto the substrate. The transfer occurs when the film comes into contact with the glue applied on the substrate or, if the glue was applied on the film, when the glue comes into contact with the substrate.
MX2022006644A discusses the different ways of applying this method (hot or cold glues, different ways of applying the glue, e.g. roller, inkjet, flexography, etc.).
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The application of films by means of transfers in accordance with the present invention also has additional advantages. If adhesives curable by means of ultraviolet (UV) radiation are used in the invention, the final curing of the adhesive improves its efficiency when a film is bonded on top, because curing takes place in the absence of oxygen, due to the isolation from the environment caused by the protective layer.
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On the other hand, the application of films, as opposed to the application of a liquid, which then has to be converted to a solid, is a much more efficient process (for example, a large energy cost involved in evaporating the solvent is avoided). In addition, laminated application avoids the appearance of defects in the protective layer itself, typical of layers applied in liquid form. The present invention, by replacing the varnish with the application of a solid layer, also reduces the possibilities of migration during the production process itself, which are associated with the presence of liquids during the process.
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The transfer film can preferably comprise the mentioned protective layer, a release layer and a carrier layer. In use, the protective layer remains adhered to the adhesive while the release layer, together with the carrier layer, is separated from the protective layer. More preferably, the protective layer is a single layer made of a single material. Even more preferably, the transfer film comprises only a protective layer, a release layer and a carrier layer.
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The protective layer can preferably have a higher resistance and/or coefficient of friction than the inks in the printed area.
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As mentioned above, the application of the protective layer by contact transfer greatly improves efficiency compared to protective processes using varnish. In addition, the protective layer(s) can easily be applied in a selective manner such that the protective layer only covers selected areas. This makes it possible, for example, to leave parts of the substrate that will be affected by subsequent processes uncovered. For example, a part of a metallic substrate, which will later form a cylindrical container, can be left without a protective layer in the area which will later be welded to form the mentioned cylindrical container.
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The present invention also discloses the use in a production process of the application of a protective layer by contact with a transfer film carrying a protective layer for the protection of a substrate printed by means of inkjet against contamination of or from the substrate. Preferably, the production process comprises a method according to the present invention. More preferably, the process is carried out within a forming and/or filling process of food packaging, the substrate being part of the food packaging. In this case, the substrate may be metallic or non-metallic. The substrate may be, for example, a film forming part of a bag-type food container, a flat or curved metal film forming part of a metal beverage can, etc.
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For a better understanding, drawings of an embodiment of the subject matter of the present invention are attached as an explanatory, but non-limiting, example.
- Figure 1 shows a diagram of a first example of the printing system carrying out a method according to the present invention.
- Figure 2 shows a diagram of a second example of the printing system carrying out a second embodiment of the method according to the present invention.
- Figure 3 shows, schematically, a third printing system carrying out a third embodiment of the method according to the present invention.
- Figure 4 shows, schematically, a fourth printing system carrying out a fourth embodiment of the method according to the present invention.
- Figure 5 shows an elevation view, schematically showing a printed substrate obtained with a method according to the present invention, in which the protective material surrounds the printed area.
- Figure 6 shows an elevation view, schematically showing another printed substrate obtained with a method according to the present invention, in which the protective material covers the printed area.
- Figure 7 shows an elevation view, schematically showing yet another substrate obtained with a method according to the present invention, in which the protective material is located on the side opposite to the side on which the printed area is located.
- Figure 8 shows an elevation view showing, schematically, two substrates as shown in figure 7, stacked on top of each other.
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In the embodiment of figure 1, the already printed substrates 1 enter an adhesive applicator 10 in which the adhesive 2 is applied to the substrate 1 in selected areas by means of inkjet. Subsequently, it is partially gelled or dried in a dryer 20. Then, the transfer foil 34 is brought into contact with the substrate 1 in the transfer station 30. The transfer foil 34 has a solid layer of protective material on a laminar base. When the transfer foil 34 comes into contact with the substrate 1, in the selected areas, protective material adheres to the substrate, forming a protective layer 3. The transfer station 30 includes a feed reel 31, a receiving reel 33 and an applicator roller 32. The applicator roller 32 has the task of bringing the transfer foil 34 into contact with the substrate 1, and thereby bringing the protective layer of the transfer foil 34 into contact with the adhesive previously applied on the substrate 1, under conditions that cause the transfer of the protective layer from the transfer foil 34 to the substrate 1 in the areas where the adhesive is located. Curing or drying then takes place in a dryer 40 to finish fixing the adhesive located below the protective layer already applied.
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Figure 2 shows a variation of the system and method in figure 1. The same or equivalent elements have been identified with identical numerals, and will therefore not be explained in depth. In the case of figure 2, the adhesive is applied to the transfer foil 34. Subsequently, the transfer foil 34, with adhesive 2 in the selected areas, comes into contact with the already printed substrate 1, such that both the selected areas with adhesive 2 and the protective layers 3 are transferred from the transfer foil 34 to the substrate 1, and the result may be equivalent to that of figure 1.
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Figure 3 shows a system that applies a protective layer on both sides of the substrate. Elements that are the same or equivalent to those in the above figures have been identified with identical numerals, and will therefore not be explained in depth.
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The system shown in figure 3 starts with a primer area 70 and continues with a subsequent dryer/curer 71 of the primer, an inkjet printer 50, with its subsequent dryer/curer 51, and a conveyor station 60 that transports the substrates 1 along the different stations of the system. The system also comprises, for the printed side, an adhesive applicator 10, located behind the printer 50 and its corresponding dryer 20. The system also comprises a transfer station 30 similar to the one shown in figure 1, which applies a protective layer on the selected areas. For clarity, the protective layer and the selected areas are not shown in the figure. The system comprises a further transfer station 30' of similar type to the one in figure 2 for applying a protective layer on selected areas on the rear side of the substrate 1. Likewise for clarity, the selected areas and the protective layer are not shown in the figure. The transfer station 30' for the rear side also comprises an adhesive applicator 10' that applies adhesive to a transfer foil 34', a dryer 20' of the applied adhesive, an applicator roller 32' of the transfer foil 34' and a subsequent curer 40'. The transfer station also has a receiving reel 33' for collecting the already used transfer foil 34'.
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The system in figure 4 is similar to that in figure 3, with the particularity it includes a station for applying a white layer 80 prior to printing. The white layer is also applied by means of a transfer foil 84 that is brought into contact with the substrate 1 by means of an applicator roller 82 that is fed from a reel 81. Likewise, the station has a dryer 85. The rest of the elements are the same or equivalent to those in figure 3 and have been identified with identical numerals, therefore they will not be explained in depth. In this case, the white layer is applied as a base on which the printing is subsequently carried out. The white layer (not marked in the figure) therefore does not have the effect of protecting against subsequent contamination. The application of a white layer is useful, for example, in the printing of metallic substrates, providing a base on which to print and which subsequently allows the colors to be seen correctly.
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The selected areas in which the protective layer is subsequently applied may completely cover the printed area, or they may surround the printed area without covering it. In this case, it may be desirable for the thickness of the protective layer to be greater than the thickness of the printing material in the printed area. It is also possible that the selected areas are spots or areas distributed along one of the sides of the substrate. In this case, it is also desirable that the thickness of the adhesive and protective layer together is greater than the thickness of the printing material in the printed area. In this way, the selected areas with a protective layer form a series of raised spots that protect the printed area from contact. If the selected areas and the corresponding protective layer completely or partially coincide with the printed area, the material used for the protective layer is preferably transparent. This is not necessary for selected areas with a protective layer located outside the printed area. The protective layer applied on the opposite side, when said side is not printed, can cover the entire substrate, or a selected area or areas, preferably in conjunction with selected areas on the other side of the substrate. The application of a protective layer on both sides makes it possible to isolate the substrate, printed by inkjet, on its upper part and its lower part by means of corresponding films above or very close to the area printed by means of inkjet, thus avoiding contamination or migrations to or from the substrate which could lead to further contamination on these substrates or on other substrates at later stages of the production process. It is also possible to leave areas of the substrate without a protective layer, especially if the requirements of the subsequent production process so recommend. For example, in the case of the substrate being a metal plate, e.g. a plate for a beverage can, the part of the plate that will later have to be welded to form the cylindrical body of the may be left without printing and/or without a protective layer.
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Figure 5 shows a substrate 1 obtained following a method according to the present invention, in which the protective areas 5 surrounding the printed area 4 protrude with respect to the substrate 1 by a greater height than the printed area 4 from the substrate. The protective areas 5 comprise an adhesive layer 2, and a protective layer 3 which has been applied by means of transfer. The protective layer 3 could have been applied by any of the transfer processes shown in the figures above. The fact that the protective areas 5, as well as the adhesive layers 2 and protective layer 3 protrude with respect to the substrate 1 more than the ink of the printed area 4, allows protection against contamination by avoiding direct contact of the material of the printed area with other substrates, e.g. other substrates like the ones shown that are stacked on top of each other.
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Figure 6 shows another substrate 1 obtained following a method according to the present invention in which the protective area 5 covers the printed area 4. On the substrate 1 already printed, the printed area 4 is covered by an adhesive layer 2, and a protective layer 3. The protective layer 3 acts as a protective material for the substrate. Said protective layer 3 can be applied by means of the transfer process described above in the previous embodiments of the systems. Once the protective layer 3 has been applied, the respective curing of the adhesive 4 can be carried out in order to properly fix the adhesive. In this case, both the adhesive and the protective material must be transparent in order to allow the print under the protective area to be seen. The protective layer in this embodiment avoids any contamination by contact with the printed area.
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Figure 7 shows another substrate 1 obtainable following a method according to the present invention. Elements that are the same or equivalent to those shown in previous figures have been identified with identical numerals and will not be explained in detail. In this case, the protective areas 5 are located on the opposite side to the printed area 4. Furthermore, as can be seen, the arrangement of the protective area 5 surrounds the printed area 4 or, more precisely, the projection of the printed area 4 on the opposite side on which the protective areas are located. Although no protective areas have been depicted on the printed side, they could also be added in this embodiment. For example, the protections in figure 7 could be combined with those in figure 5 or 6.
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Figure 8 shows two substrates 1 as in figure 7 stacked on top of each other. It can be seen how the lower substrate 1 makes contact with the protective areas 5 of the upper substrate 1, avoiding a contact of the printed area 4 of the lower substrate 1 with the upper substrate 1.
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Although the invention has been presented and described with reference to embodiments thereof, it will be understood that these are not limiting of the invention, therefore multiple variable structural or other details could become evident to those skilled in the art after interpreting the subject matter disclosed in the present description, claims and drawings. Thus, all variants and equivalents will be included within the scope of the present invention if they can be considered as falling within the broader scope of the following claims.