ES2792990T3 - A process for the manufacture of a continuous pressure tape for ceramic slabs that have a structured surface - Google Patents

Abstract

A process for the manufacture of a continuous pressure tape (1) for the creation of ceramic tiles having a structured surface, comprising the following steps: - advancing the continuous pressure tape (1) by steps of a submultiple of a length of the continuous pressure tape (1); - in each advance stage, subjecting a strip of continuous pressure tape (1) having a width equal to the stage for driving at least one piezoelectrically controlled ink jet printing head (4), piloted by a graphic image, to obtain on the continuous pressure belt (1) a deposit of ejected material that reproduces the graphic image, the ink jet print head (4) can be moved transversely to the continuous pressure belt (1) and it is suitable for expelling an ink jet belonging to a class of inks that can be cured in a controlled way by ultraviolet radiation; - subjecting the strip of material emitted by the ink jet recording head (4) to the driving of an ultraviolet lamp (42) to harden the ink; - repeat the printing and hardening operations on the same strip at least once, where a virgin continuous pressure tape (1) is arranged stretched between two rollers (2), and after tensioning it a length of the pressure tape is detected continuous (1) to acquire an exact extension of the continuous pressure tape (1) tensioned), and to allow an electronic control system of the inkjet print heads (4) to automatically adapt the dimensions of the graphic image to be printed at the effective length of the continuous pressure tape (1).

Description

DESCRIPTION

A process for the manufacture of a continuous pressure tape for ceramic tiles that have a structured surface

TECHNICAL FIELD

[0001] The present invention relates to the manufacture of ceramic slabs, preferably with a small thickness, such as tiles for flooring and wall cladding, which have a side intended to be visible that comprises random reliefs, commonly defined as a structured surface or have a structured effect.

BACKGROUND

[0002] The "structured" effect gives the ceramic product a particularly pleasing aesthetic value, especially in the case of imitation of natural materials such as wood and stone.

[0003] Slabs or tiles with the structured side in sight are currently constructed by pressing powders in a common die for ceramic tiles, in which the punch intended to form the structured side includes a negative of the design that will appear in the surface of the slab or tile.

[0004] In particular, the punch is covered by an elastomer layer that carries, in negative, the design of the structured side.

[0005] The elastomer layer is constructed in the following manner.

[0006] A rigid die matrix bearing the positive design (ie, completely similar to that which will be reproduced on the ceramic tile or slab) is made by numerically controlled precision mechanical work from three-dimensional data.

[0007] Generally, the die is made of aluminum or other suitable material for rapid machining by chip removal or laser ablation.

[0008] When the die is finished it is used to vulcanize the elastomer layer (typically polyurethane with 90 Shore A hardness) applied to the active side of the punch. It is commonly defined as "vulcanizing matrix").

[0009] The vulcanization process requires heating to a suitable temperature (approximately 150 °) and a contemporaneous pressing action.

[0010] At the end of vulcanization, a layer of elastomer is present in the punch, generally a few millimeters thick and with the negative profile of the structure. Therefore, the punch is ready to be mounted as an "exposed" punch in a conventional die for pressing ceramic powders, allowing the production of tiles that have a surface completely similar to the surface of the initial metal matrix.

[0011] Apart from being complicated and expensive, this method is not suitable for the realization of continuous structured surface press conveyors suitable for continuous belt compaction lines.

[0012] In particular, the difficulty lies in making the upper pressing tape, intended to form the structured side of the slab, on which a relief structure is to be created, always diverse and continuous, that is to say, without interruptions.

[0013] Furthermore, it should be possible the rapid replacement of the relief structure.

[0014] The only prior art capable of producing high-strength structured, continuous steel tapes is chemical etching, in which the web is coated with an emulsion bearing the negative pattern and then dipped in aggressive baths that etch the metal where it is not protected.

[0015] This procedure is very complex, laborious and has a negative environmental impact.

[0016] Furthermore, the etching does not allow accurate differentiation of the incisions, with obvious aesthetic limits.

[0017] Laser ablation also has limitations and drawbacks, as the tape has to be mounted on a rotating drum with a horizontal axis that, due to current proportions, would be excessively large (more than 2 meters in diameter); and it implies a working time (tens of hours) incompatible with an economical and industrially exploitable production.

[0018] A method is also known to create three-dimensional structures on a substrate, by means of jet printers using inks that cure on the action of UV rays. However, this process, described in WO 02/053338, is not usable to make structured pressure tapes that have the requirements to create ceramic slabs continuously, since the substrate of the prior art is a tape of length indefinite, not looped, and the inkjet area is curved.

[0019] Furthermore, the known method is not suitable for creating a three-dimensional surface suitable for pressing ceramic tiles in the usual dies, since the nature of the structures created with the known method does not possess the necessary mechanical and wear resistance characteristics to withstand the large number of pressing cycles and the high pressures achieved in common ceramic dies.

[0020] EP0228570 A2 shows a process for manufacturing a continuous pressure tape using an image-controlled applicator.

SUMMARY OF THE INVENTION

[0021] An object of the invention is a process for the manufacture of a closed loop pressure tape for the continuous pressing of ceramic tiles having a structured surface.

[0022] The closed belt must present the characteristics and adequate requirements for the compaction action for a high pressure continuous compaction plant, as illustrated, for example, in the application PCT / IB2012 / 001977 belonging to the present applicant.

[0023] In particular, the tape should have a high tensile strength in order to withstand the stresses of accumulated prestressed and crushing action of the powders; a high elastic limit, in order to resist the alternating bending stresses due to the continuous winding and unwinding around the tension rollers; a high degree of dimensional stability and absence of joints, which would lead to visually perceptible surface defects, or defects identifiable by touch, to the ceramic slabs produced.

[0024] The tape must also present a structured surface, that is to say, comprising the relief pattern without interruptions, without seams, and resistant to high pressure compaction and abrasion due to dust.

[0025] The structured surface must adhere closely to the base tape, and be achievable with limited costs and very quickly.

[0026] All the objectives of the invention are achieved by means of a process having the characteristics indicated in the independent claim; the dependent claims relate to further advantageous features of the invention.

[0027] In particular, in an example not covered by the claims, the relief structure is made on the die intended to form the structured surface of the punch of a die. In the invention, the relief structure is made directly on the continuous press belt by additive synthesis processes, that is, by the ink jet printing process with photohardening plastic materials, that is, plastic materials that comprise a fraction heat hardening.

[0028] In a case in which the ink to be sprayed is 100% of the photohardening type, that is, composed of acrylate or epoxy monomers or oligomers, with the addition of photoinitiating substances, crosslinking occurs alone by the effect of UV radiation from a suitable lamp.

[0029] Alternatively, in order to give the layer exhibited on the tape special strength characteristics, the aforementioned ink may be of the two-stage type, with mixed UV and heat crosslinking.

[0030] Inks of this type contain a fraction, ranging from 10% to 90%, of monomers and oligomers selected from the following families: pure acrylics, polyester acrylates, polyurethane acrylates, epoxy acrylates, vinyls, epoxies.

[0031] These monomers and oligomers do not crosslink by UV effect, but they require for their radical polymerization a heat treatment at a temperature between 150 ° C and 200 ° C for a time of at least 15 minutes.

[0032] Therefore, the ultraviolet light functions as a temporary block that, by acting on the fraction sensitive to ultraviolet light, prevents the design from deteriorating; the final heat treatment definitely fixes the entire mass. In general, the final heat treatment of the structure permanently fixes the entire mass. In general, the structure can be made with a succession of applied layers of different materials (photohardening and bi-stage).

[0033] In the case of a punch, a steel die is located in a special drawing means that the stepped advance under a beam in which a mobile head can slide in a direction transverse to an axis of the die, in both directions.

[0034] The moving head carries a series of piezoelectrically controlled type inkjet heads, which can print at each stage a punch portion over a full width corresponding to the stage.

[0035] In each passage of the head, the die advances in a step equal to a full print width of the head p, that is, a submultiple ps thereof.

[0036] Each head is piloted in a known manner by means of a suitable image, to obtain a deposit of ejected material that reproduces the image on the matrix.

[0037] The die is used to vulcanize, on the visible side of the punch of the die, a layer of rubber that is capable of resisting wear caused by dust during printing operations.

[0038] Alternatively, the deposition of ejected material to create the structured surface can be done directly on the visible side of the punch.

[0039] In the case of a continuous pressure tape, a smooth pressure tape made of a non-absorbent material, generally metal or made of composite materials comprising glass, carbon or Kevlar fibers, is introduced on a pair of rollers and tense.

[0040] A beam is placed above the upper horizontal portion of the belt, on which a beam of a tailstock can slide in a direction transverse to the axis of the belt, in both directions.

[0041] The belt rotates in stages using motorized drawing means; the moving head carries a series of piezoelectrically controlled inkjet heads and can print at each stage on a portion of tape over a full width, the portion of which is equal to one stage.

[0042] In each passage of the head, the tape advances in a stage equal to a full print width of the head p, that is, a submultiple ps thereof.

[0043] As in the previous case, each head is piloted in a known way by means of a suitable image, in order to obtain, on the tape, a deposit of ejected material that reproduces the image.

[0044] The information that is sent to the heads is obtained when processing graphic images totally similar to normal digital images, with the difference that the data related to color is superfluous.

[0045] For this reason, gray scale images are used, with a color depth of at least 8 bits (corresponding to 256 shades of gray); the electronic control system will interpret the data correctly, assigning more decay volume where the image has a darker tone.

[0046] Suitable "tearing" algorithms, of known type, will be used to optimize the variability of the tones by means of the spatial distribution of the ink drops in such a way that the desired effect is obtained, thickening them where the required tone is darker and diluting them where it is clearest.

[0047] The ink projected from the head can be a UV crosslinking ink that allows perfect adhesion to the tape and, once irradiated with ultraviolet light, instantly crosslinks and hardens, or a mixture of UV crosslinking ink and crosslinking ink thermal.

[0048] For crosslinking using UV lamps which generally mount directly to the head, flanked on the printheads, to instantly solidify as soon as the drops are deposited in each passage.

[0049] To carry out heat crosslinking, heating means are used that act on the fully formed three-dimensional structure.

[0050] Head- projected materials in the class of UV-controlled curing inks are generally mixtures of acrylate or epoxy monomers and oligomers, with the addition of substances photoinitiators. Under normal conditions, these are easily expellable liquids (viscosity of approximately 10 mPas) when impacted by UV light of suitable wavelength, the electronic bonding configuration is modified, thereby activating the crosslinking process and starting a hardening. fast, until solidification. The liquid completely solidifies and there is no presence of volatile solvents (and therefore contaminants).

[0051] UV light is usually applied immediately after the projection of the drops, which fall on the tape as a liquid and solidify by the passage of the UV lamp, which is solidly restricted in the transverse movement to the injection heads of ink. Once solidified, the drops of material are covered in the following passages, until the desired thickness is obtained.

[0052] The UV light from the single passage can be modulated to a lower energy level to effect only the initiation of solidification (pinching) and thereby allow better adhesion of subsequent layers. At the end of the application, a higher intensity UV treatment is applied.

[0053] As an alternative to the use of UV inks, two-component plastic materials (for example, polyurethanes) can be used that project in sequence (with different heads) superimposed drops of the two components (for example, polyol and isocyanate, in the case of polyurethane ) that solidify during mixing, directly on the surface of tape 1. These materials harden under the action of heat.

[0054] Alternatively, in order to give the layer on the tape special characteristics of resistance, the above - mentioned ink may be the two - stage type, with mixed crosslinking UV and heat.

[0055] In this case, apart from the UV lamp, a heating module is obviously present, which completes the curing of the thermal curing resins.

[0056] While the UV lamp works together with the ink jet heads, the heating module works at the end of printing.

[0057] Although the tape is made of a material having a high modulus of elasticity, when mounted in the printing machine and properly tensioned, however, there will be an elongation, in general difficult to assess.

[0058] This could cause problems when the relief impression at the end of a passage has to be joined with great precision (approximately 0.05-0.1 mm). To avoid this problem, the method of the invention comprises the exact detection of the position of the tape by means of an encoder.

[0059] Once the virgin tape has been loaded and tension has been made, the tape is set in rotation. The head prints some reference markers on the outer tape of the web that will not participate in crushing the powder, and an optical sensing system determines the position of the markers, while also acquiring the exact longitudinal extent of the tension tape .

[0060] At this point the electronic control system of the printing heads, according to the invention, automatically adapts the graphic image to be printed, lengthening or restricting it proportionally by a small correction entity (the stretch function), not perceptible to the naked eye. . In this way, the printing of the tape will be perfectly adapted to the actual dimension, without any overlapping effects or unprinted stripes.

[0061] In a possible operative configuration, the head houses four piezoelectric inkjet heads of mod. 1001 GS6 (Xaar GB manufacturer), with a print width of d = 70mm, and arranged in such a way that it involves exactly the same print web.

[0062] Under these conditions, printing is performed at a resolution of 360 dpi, that is, the drops are arranged according to a square grid with a stage of 70 pm; the droplets have a maximum volume of 42 µl (1 picoliter = 10 ^-12 liters) and a printing pass having a diameter of approximately 80 µm. The thickness of each pass can be estimated at approximately 8-10 m, depending on the physical and rheological parameters of the liquid (density, viscosity, surface tension, temperature, etc.) and the surface of the strip. In general, therefore, it can be stated that in each printing pass the pattern grows approximately 10 pm in the Z direction perpendicular to the surface of the ribbon.

[0063] By increasing the number of heads working on the same printing strip, that is to say, reducing the stage p ^s (thereby increasing the number n of sub-stages, so that p = n times p ^s ), it is possible to easily deposit many overlapping droplets on each pixel in the image.

[0064] For example, when driving four heads with the same image, and when applying a reduction of the stage by a factor n = 4, there will be 16 drops for each physical pixel on the tape; therefore, at the end of the passage around the system printing, the ribbon will exhibit embossments (according to the design chart entered in the control system) which at the highest points will be up to 160 pm thick.

[0065] If this maximum "depth" value of the design graphic is not sufficient, the print can be repeated (obviously synchronized with the previous print) a certain number of times, until the desired height is reached.

[0066] In the case of a printing repeated 4 times (maximum theoretical height 4 x 4 x 4 x 10 pm = 640 pm), zones where heights greater than 500 pm have been reached have been obtained experimentally.

[0067] If the structure of the tape surface is to be changed, in the invention this can be done on the same machine that creates the structure, in which prior to the ink distributor head there is a removal system present that you can delete the structure created in it.

[0068] In another variant of the method, the station that creates the structure on the belt is not a separate machine with respect to the belt compactor, but is an integrated accessory of the compactor, located in a convenient inactive zone of the belt (for example, the top). Obviously, during this pressing stage the station is not working, while when a new structure is created, pressing is not active. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The advantages and constructive and functional characteristics of the invention will emerge from the detailed description that follows, with reference to the figures in the attached drawing tables, in which:

Figure 1 is a plan view of the machine for creating the structured surface of a die.

Figure 2 is the machine of Figure 1 shown in a side view.

Figure 3 is section III-III of Figure 1.

Figure 4 is a plan view of the machine for creating the structured surface of a pressure tape. Figure 5 illustrates the machine of Figure 4 in a side view.

Figure 6 shows sections VI-VI of Figure 4.

Figure 7 illustrates the machine for creating the structured surface integrated with the tile forming machine, in a side view.

Figure 8 shows sections VIII-VIII of Figure 7.

Figure 9 is section IX-IX of Figure 7.

DETAILED DESCRIPTION

[0070] Figures 1 to 3 illustrate a die 100 positioned on a conveyor belt 101 advancing in stages equal to p by a fraction ps of p.

[0071] The tape is intended to house at least two dies 100, which are perfectly aligned and equidistant from one side of the tape.

[0072] A description of the means for positioning the dies is omitted, as it is obvious to a person skilled in the art.

[0073] The beam 103 on which the print head 104 runs is positioned in a direction transverse to the direction of movement of the tape 101.

[0074] The mechanical means for moving the head 104 are not illustrated, as they are known to those skilled in the art.

[0075] The print head 104 carries the ink jet heads 141 that will decorate a strip of the matrix equal to the po ps stage.

[0076] An ultraviolet lamp 142 is located posterior to the heads 141, in the direction of movement, which causes the material just deposited on the belt to harden.

[0077] A sensor 144, located on board the printhead 104, is capable of following the edge of the dies during advancement to command suitable corrections of the print media in order to recover any transverse die errors.

[0078] Full width heating means 107, for example infrared heating means, are positioned posterior to the beam and transversely to the advance of the tape.

[0079] Position sensing sensors, eg photocell devices 105, signal the position of slab 100 to initiate the print cycle.

[0080] Figures 4 to 8 detect the belt 1 placed under tension between two rollers 2 of which one is step-driven by pop entities ^.

[0081] The belt 1 is made of a material selected from the following: steel, stainless steel, composite material with carbon and / or kevlar and / or glass fibers, and exhibits a thickness of 0.5 mm to 2 mm.

[0082] The beam 3 on which the print head runs is transverse to the direction of movement of the ribbon and covers the ribbon.

[0083] The mechanical means for moving the head 4 are not illustrated, as they are known to a person skilled in the art.

[0084] The print head 4 carries the ink jet heads 41 capable of decorating a strip of the ribbon 1 of a pop stage ^.

[0085] A UV lamp is located posterior to the heads 41, in the direction of movement, and this UV lamp causes a hardening of the material just deposited on the tape.

[0086] In the particular embodiment illustrated in Figure 7, the upper tape 1 can be mounted, in unused or virgin form, directly on the tile-forming machine, and covers the lower tape 10 that carries the layer 5 of powder.

[0087] Both in the embodiments of Figure 5 and in Figure 7, an abrasive device 6 is located in a position anterior to the beam 3, to remove the relief structure from the upper surface of the belt 1.

[0088] The abrasive device 6 comprises a rotating brush 61 with a horizontal axis, drawn at high speed by a motor group (not illustrated) which is pressed against the belt 1 by means of pressure (not illustrated). The dragging effect of the brush 61 causes the detachment of portions of the relief structure, which are distanced from the working area by suction devices 62 (see Figure 5).

[0089] Finally, near one end of the beam 3, a sensor 31 that reads a series of equidistant reference 310 stands, which are preprinted on the head 4 at an edge of the tape not interested in printing the objective of controlling the exact development of the same after tensioning to adapt the dimension of the printed image.

[0090] Furthermore, on board the recording head 4 is positioned a sensor 4, which detects even minimal transverse deviations of the ribbon to adapt the position of the image in real time.

Application example 1

[0091] A virgin continuous pressure tape is mounted, with a width of 2000 mm and a total extension of 7500 mm, wound in a loop, stretched on two rollers having horizontal axis, parallel to each other, approximately 2500 mm distant from each other, with a diameter of 750 mm. The printing system automatically tightens the ribbon, applying a force of approximately 100 kN, equal to approximately 25 N / mm.

[0092] The printing system is provided with 4 heads 1001 GS6 (Xaar GB manufacturer), arranged in a battery, capable of dispensing 42 picoliters per drop, at the optimum resolution of 360 dpi (pitch between pixels = 70 pm). The printing width is 70mm.

[0093] The belt is advanced with a longitudinal stage of% having a head width of (p ^s = 17.5 mm); in this way, four different nozzles pass through the same point on the web, increasing the density of the printed dots by a factor of 4.

[0094] By piloting the 4 heads with the same graphic image, an additional multiplication of the dispensed quantity is obtained by a factor of 4.

[0095] The printing process was repeated 3 times, obtaining a maximum growth factor of 4 x 4 x 3 = 48 drops per pixel on the tape, equal to a theoretical height of about 480 pm. The actual maximum height measured at the end of the print (using a three-dimensional profilometer) is 400 pm.

[0096] The ink used is of the UV curing type, belonging to the Unijet Thunder 1000 series produced by Unico Digital (BE).

[0097] Before printing the true length of the tensioned tape was evaluated, printing on the outer edge of the same a series of markers, read by an optical sensor. The image to be printed (initial length 7500 mm) was linearly deformed to adapt to the detected length, to perfectly cover the outer surface of the tape.

[0098] The result of the application of the method of the invention is a pressure conveyor suitable for continuous compaction lines, which has on an external surface of this a relief that is variable between 0 and 2 mm, preferably between 0 and 1 mm , made by applying inkjet of liquid materials polymerized by UV light, that is, polymerizable by mixing two or more liquid components.

[0099] The "structuring" treatment of the web driven by the present method allows the following objectives to be achieved: excellent spatial definition of the reliefs (up to 360 dpi and higher, depending on the inkjet heads used); possibility of having an endless relief over the entire extension of the band; exact control of the height of the relief (resolution greater than 10 pm); possibility of carrying out a high thickness transport (theoretically unlimited, practically around 1 mm); short completion times (approximately 30 min / m2); an economical solution and great flexibility of use.

[0100] The invention is not limited to the embodiments described, and any variant and improvement can be carried out without leaving the scope of the following claims, for example, the pressure tape, once provided with the structured surface, can be sectioned into portions, each of which is suitable for application to the active side of the punch of a normal ceramic powder die.

Claims (9)

1. A process for the manufacture of a continuous pressure tape (1) for the creation of ceramic slabs having a structured surface, comprising the following steps: - advancing the continuous pressure tape (1) by steps of a submultiple of a length of the continuous pressure tape (1); - in each advance stage, subjecting a strip of continuous pressure tape (1) having a width equal to the stage for driving at least one piezoelectrically controlled ink jet printing head (4), piloted by a graphic image, to obtain on the continuous pressure belt (1) a deposit of ejected material that reproduces the graphic image, the ink jet print head (4) can be moved transversely to the continuous pressure belt (1) and it is suitable for expelling an ink jet belonging to a class of inks that can be cured in a controlled way by ultraviolet radiation; - subjecting the strip of material emitted by the ink jet recording head (4) to actuation of an ultraviolet lamp (42) to harden the ink; - repeat the printing and hardening operations on the same strip at least once, where a virgin continuous pressure tape (1) is arranged stretched between two rollers (2), and after tensioning it a length of the pressure tape is detected continuous (1) to acquire an exact extension of the continuous pressure tape (1) tensioned), and to allow an electronic control system of the inkjet print heads (4) to automatically adapt the dimensions of the graphic image to be printed at the effective length of the continuous pressure tape (1). 2. The method of claim 1, characterized in that the continuous pressure tape (1) is made of a material selected from the following: metal or a composite material with glass, carbon or kevlar fibers. 3. The method of claim 1, characterized in that the ink is a mixture of acrylate or epoxy monomers and oligomers, with an addition of photoinitiating substances. The method of claim 1, characterized in that the UV energy of a single passage, apart from a last passage, is modulated to an energy level capable of performing only an initiation of curing to allow better adhesion of the ink layers applied in the following passages. 5. The method of claim 1, characterized in that the ejected ink contains a fraction between 10% and 90% of monomers and oligomers selected from the following families: pure acrylics, polyester acrylates, polyurethane acrylates, epoxy acrylates , vinyls, epoxies, which start solidification due to the action of UV rays and heat. 6. The method of claim 5, characterized in that it comprises an additional activity of subjecting the continuous pressure tape (1), after the application of all the layers, to a heat treatment at a temperature between 150 ° C and 200 ° C for a time of at least 15 minutes. The method of claim 1, characterized in that the information that is sent to the ink jet print heads (4) is obtained by processing gray scale images, with a color depth of at least 8 bits, which corresponds to 256 shades of gray. The method of claim 1, characterized in that the elongation of the continuous pressure tape (1) and the exact development of the same are detected by optical control of a series of reference markers fixed on the continuous pressure tape (1 ). The method of claim 1, characterized in that before the action of the ink jet recording head (4), the continuous pressure belt (1) is subjected to the action of abrasive means (6) that clean the surface of a previously printed image.