INTRODUCTION

De present patent application relates to a method for applying a pattern on a panel.

Screen printing, also called serigraphy, is a printing technique that is used for a plurality of graphical purposes, such as decoration of textile, posters, art printing and textile printing. This technique is also used in industry for the production of printed circuit boards and solar cells, for example.

The principle of screen printing is rather simple. A piece of fine silk, polyester or steel mesh, for example, is stretched over a frame. Next, a light- sensitive layer is typically applied at the screen printing frame and is illuminated by a (positive) film. During development, dark spots present on the positive film are washed away, such that permeable spots occur at the screen printing frame through which ink is pressed. This ink (or solder paste) is applied at the frame and is spread using a squeegee, as a result of which the model of the template is printed at the object to be printed (the image carrier). This technique can be repeated with different colours and shapes that are printed next to each other or on top of each other.

Screen printing ink is opaque; this provides that it is possible to print on dark image carriers. This contrary to flat-bed printing, relief printing and intaglio printing, wherein always transparent inks are used.

With a correct printing ink or paste, screen printing can be used to print on nearly each material, for example also round bottles of glass. At present this technique is used on a large industrial scale and screen printing also has become an integral part of the world of arts.

Ink is a liquid colour substance that is used for writing or printing. In general, ink is black or blue however, different coloured inks are also available.

Strictly speaking, ink is a solution or colloidal solution of toners or dyes. In this way ink differs from paint which consist of a spread containing pigments. There are several types of ink, such as printing ink, of which offset ink is a special type. These inks, in fact, are paints because same comprise pigments instead of toners. A squeegee is a type of distribution combe for pushing the ink through the mesh of the screen printing frame. In screen printing, with this instrument, through the still open parts in the mesh (image), ink is physically applied on the substrate to be printed.

Several documents relate to screen printing.

WO 2009/106912 (A1 ) discloses a roller for rotary screen printing, in particular for decorating ceramic products.

WO 2007/124551 (A1 ) discloses, for example, a method for UV- serigraphy for preparing FRP boards, using non-saturated polyester resin inks on a FRP laminate and curing thereof in two steps.

EP 1710095 (A1 ) discloses, for example, a method for producing a strong metal foil on a carrier plate and for precisely positioning parts using a laser.

Several documents disclose devices suitable for screen printing, specific materials used in screen printing and so on.

The documents mentioned above relate to general and specific developments in the technique of screen printing.

A typical disadvantage of the prior art is that patterns can not be applied with high precision. This is an inherent disadvantage of the technology used and/or devices and/or materials.

The prior art neither provides materials or methods which can be used at elevated temperatures.

Further, the prior art does not provide a combination of techniques and/or materials that comply to high optical standards.

Accordingly, there is still a need for an improved screen printing method that solves one or more of the mentioned problems and disadvantages, without endangering the favourable features.

SUMMARY OF THE INVENTION

The present invention relates to a method for applying a pattern on a panel, preferably a flat panel, which panel has suitable optical properties and, at its circumference, on at least one flat side thereof, the panel has bevelled edges, comprising the steps of :

providing the panel,

applying a pattern on the panel using a suitable, adhesive and curable ink,

wherein, before applying the ink, the panel is preferably washed and dried, and

curing the ink.

The present invention is particularly suitable for producing transparent panels having a pattern by which an image, called panoramagram, positioned beneath the panel, can be viewed in three-dimensional, 3D, form. That is, a viewer standing at a certain distance form the panel gets a three-dimensional or spatial visual impression of the in fact two-dimensional, 2D, image. For this, it is required that the 2D image is produced in a suitable manner. In practise, this technology is also called autostereoscopy.

In the present description and claims, the term "panel" is used to indicate a three-dimensional object having a certain length, width and thickness, wherein the thickness is significantly smaller than the width and/or length, and wherein the thickness is sufficient to provide to the panel an intrinsic strength against bending. The panel further may have any shape. In the light of the present invention, the panel preferably has a constant thickness.

The panel has suitable optical properties to achieve the best effect in terms of providing a sharp and clear 3D impression. After elaborated and extensive tests, the inventor has established that the total absorbed energy is an important parameter in this respect. This absorption is, in an example, as small as possible. This can be achieved, for example, by keeping the number of absorption spots (measured per cm ³ ) in the panel as small as possible. Absorption spots are, for example, metal parts and metal ions, such as Zn or Fe. In an example, the concentration thereof is less than 100 ppm, preferably less than 10 ppm such as less than 5 ppm. These tests have also shown that, in an example, the direct energy transmission through the panel is as high as possible, which is also the case for the transmission of light. To achieve an as much as possible homogeneous image for a 3D visualisation, it has been determined that the RD65 colour yield, in an example, is as high as possible. Further, in an example, it has been determined that the shadow coefficient is as high as possible, to achieve an optimal contrast.

In an example, the panel is a flat panel, such as used in bus stops or the like. Alternatively, in an example, the variations in the thickness of the panel over the surface is as small as possible, such as less than a variation of 1/100 compared to the total thickness.

After time consuming experiments, it has been established that the risk of cracks in the panel during heating and cooling is drastically reduced by providing circumferential bevelled edges at the panel. This is particular relevant because the panel, in the present method, is relatively quickly heated up and is quickly cooled down. This is required to apply the pattern with a required precision. In an example, the bevelled edges are polished, because polished edges are even better capable to absorb tensions in the panel.

In the present description and claims the term bevelled edges also comprises rounded edges or stepwise formed edges and the like.

The ink applied is an ink with a sufficient adhesive power to the panel. In an example, the ink does not or hardly not transmits any light. In an example, the ink cures good and quickly, for example at an increased temperature. In an example, the ink receives a hardness that is comparable to quartz (7 at the ohs scale). Such an ink is sufficient durable for use in, for example, a bus stop. The ink and panel can be selected having properties such that a suitable adhesion is achieved.

The ink is cured. Curing can take place by an increase of temperature, by chemical activation of a hardener present in the ink, such as by adding an activator, by using light, such as UV-radiation, by evaporation of solvents present in the ink, by use of an electrical field, by baking, or combinations thereof, for example.

Accordingly, the present invention provides a method with which a pattern can be applied with a very high precision. After applying a pattern having a particular width on a panel, during the present process, the width does hardly vary, that is less than 1/1000 of its width, often even less than 1/10000 of its width. As an example, a line or strip having a width of 1 mm will show a width across the total length of the line or strip that differs less than 1 micrometer (μιη) from this width of 1 mm, that is the width will be between 0.999 mm and 1.001 mm. It can be stated that the width of the line is essentially constant. This is of particular importance for achieving an optimum 3D image.

The present invention further provides a panel, in particular a panel having optical properties, having a pattern applied thereon in accordance with the method disclosed above. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of applying a pattern on a panel, and a panel comprising such a pattern, in particular an optical panel.

In an example, the panel is made of glass, quartz or safety glass in particular. Glass and quartz comprise to a certain extend the optical properties relevant for this panel.

Preferably, the glass or quartz safety glass satisfies the EN-14179 and/or de EN-12600 Standard for tempered glass or an equivalent material. Such a panel is, in particular, suitable for use in environments where breakage of the panel may occur, for example due to vandalism.

In an example, at one flat side and preferably at both flat sides (upper side and lower side), the panel has bevelled edges which form an angle with the normal at the surface of the panel between 30 and 60 degrees, preferably between 40 and 50 degrees, such as 45 degrees. From experimenting it has been determined that an angle between 30 and 60 degrees provide the best results in terms of minimizing the risk of breakage and/or damaging of the panel. An angle of 45 degrees provides the best results.

In an example, the ink is an enamel ink, preferably a black enamel ink. After curing, such an ink is resistant to scratches, which is desired for achieving durability and optical properties. In an example, the ink receives a hardness that is comparable to quartz (6 to 8 at the ohs scale). Such an ink does transmit little or no light, which is particularly desired. Such an ink is available from companies such as Pemco, Ferro, en Johnson atthey, for example.

Further, the ink may comprise an accelerator, a dilution agent, an anti-foam agent, an anti-flux agent, an equalizer, a curing agent, an adhesive agent, a thickener, a delustring agent, a weakener, an anti-static agent or a combination thereof.

In an example the ink has a viscosity of 1000-5000 mPa.sec, preferably of 2000-4000 mPa.sec. It is important to set the viscosity of the ink correctly. A too low viscosity will cause to much fading of the pattern, while a too high viscosity does not provide well-bounded lines, having a constant thickness and width over the entire length of the line, for example. After time consuming experiments, the inventors have been successful in establishing a suitable range for the viscosity. Parameters that further determine a suitable viscosity are the type of powder particles in the ink, the type of screen that is used, the type of dilution agent, the thickness of the end layer, etcetera.

In an example, for applying the ink, a screen having more than 75 wires per cm, preferably more than 100 wires per cm, such as 120 wires per cm is used, preferably a screen having plastic or metal wires and preferably a metal frame, such an aluminum frame. Such a screen has a high quality value, such as T120, by which lines can be accurately positioned adjacent to each other. Among others, with such a screen the above mentioned advantages can be achieved, such as non-f!owing of the tines during application and curing. The screen itself, in an example, exerts sufficient pressure to apply ink using a squeegee.

In an example, the ink is applied using a squeegee, which squeegee exerts a pressure of more than 0.1 kPa, preferably between 0, 1 en 100kPa, such as 2 kPa, at the ink. The pressure of the squeegee is higher than the pressure of a screen that is used. The pressure is typically adjustable.

In an example, the thickness of the ink applied is set with a squeegee between a thickness of 10-200 μιτι, preferably between 20-100 μνη, rather between 50-90 m, such as 70-80 Mm. This thickness is measured during wetness. Typically, such thickness will be as small as possible to save material. However, the layer has to be sufficient thick to transmit (essentially) no light. Further, a dried ink is sufficiently resistant to scratches.

In an example, the panel is washed and dried before applying the ink. Washing can take place, for example, using water, for example having a low oxygen amount. Drying is typically performed in the air, whether or not at an increased temperature, at lower pressure, by removing air or a combination thereof. Accordingly, a clean surface is provided, to which the ink adheres better. This is advantageous for the durability and hardness of the ink.

In an example, the ink pattern comprises parallel lines or strips in a first direction, which lines or strips have one and the same constant width. Typically, the first direction is a vertical direction, seen when the panel is in use. By applying the present invention, lines or strips having a very good constant thickness are achieved. Even after applying the present method, using possible optional steps, the variation in width is less than 1/1000 of the mean width, even less than 1/10000.

In an example, the number of lines or strips is between 0.2 and 250 per cm, preferably between 0.5 and 100 per cm, rather between 1 and 50 per cm, more preferably between 2 and 10 per cm, such as between 4 and 8 per cm. The number of lines or strips per centimetre also depends of the number of picture lines per centimetre of the image to be positioned under the panel, that is these two numbers are in relation to each other. It has been found that the number of lines or strips per centimetre should not be too high, because too less light can pass the panel. With a too small number of lines, the 3D effect disappears. In an example, the number of lines is 2 per centimetre, 4 per centimetre, of 8 per centimetre. With this, the amount of fight is optimal and so the 3D effect provided.

In an example, there is an opening between the lines or strips of 1/10 and 1/2 of the width of the lines, preferably between 1/5 and 1/3 of the width. With a correct relation between the width of the lines or strips, the number of lines or strips per centimetre and the width of the gaps between the lines or strips, respectively, with a suitable panoramagram desired 3D effects can be achieved. This is extensively disclosed in patent application NL 2004797, "Method and device for producing a panoramagram for providing an autostereoscopic image" , from the present inventor, filed by applicant. For further details reference is made to this application, which application is herein incorporated by reference.

In an example, the width and thickness of the lines or strips and the width of the gaps are optically checked, preferably with a laser and a camera. A laser transmits a beam of light which is completely or partially blocked by a line or strip. With an optical system, such as a camera, the width of a line can be easily determined. Likewise the thickness of a line or strip can be determined. Accordingly, the quality of the method can be easily checked and, if necessary, adapted. A more or less continuous inspection of lines and gaps can be provided by moving one (or more) laser (or lasers) transversely to the lines at the panel. In this way the relative position of lines and gaps there between can be determined. It is also possible to reject a product that not complies to quality specifications.

In an example, the curing takes place at a temperature of higher than 500 °C, preferably higher than 600 °C, preferably during a time period of more than 15 minutes, such as more than 20 minutes, after which the panel is preferably quickly cooled down, preferably quicker than 40 °C/min, more preferred quicker than 75 °C/min, such as quicker than 100 °C/min. A too low temperature results therein, that the ink may somewhat flow, such that the above mentioned quality requirements can not be met. A too high temperature causes too much stress in the panel, with an increased risk of breakage as a result. A temperature of 500 °C-700 °C is preferred. Cooling down is typically till room temperature. Quickly cooling down provides, as an advantage, an optimal quality and hardness of the glass and ink. Besides this, the pattern as applied will essentially result after curing, for example in terms of accuracy.

In an example, during curing, the panel moves back and forth with a speed of between 0.1 and 5 metres/min. preferably between 0.2 and 3 metres/min, more preferably between 0.2 and 2 metres/min, such as 1 metre/min. With this, the tension in the panel will as much as possible be reduced. Also the flow in the panel is reduced by this, which could be caused in that part of the panel gets to hot, for example up to a temperature near flow point.

In an example, the dimensions of the panel are more than 80 x 100 cm ² , preferably more than 100 x 125 cm ² , preferably more than 110 x 150 cm ² such as more than 120 x 180 cm ² , and with a thickness of preferably 1-10 mm, preferably 3-7 mm, such as 5 mm. Accordingly, panels of a plurality of dimensions can be produced, which display a 2D image of a comparable size as a 3D image. The panels provided are substantially larger than can be provided with the prior art.

The panel, in an example, has a light transmission (τ _ν ) in the visible light of more than 89% such as more than 90%, such as more than 91 %, and has preferably a total energy absorption (a _e ) of less than 10%, preferably less than 5%, such as less than 3%, such as 2%, and has preferably a direct energy transmission (τ _ν ) of more than 85%, preferably more than 87%, such as more than 88%, such as 89% or 90%, and has preferably a Shading Coefficient (SC) higher than 0,97, preferably higher than 1 ,00, such as higher than 1 ,02, such as 1 ,03, and has preferably a colour reproduction - RD65 <R _d ) of more than 99, such as 100 (all according to EN-410, at a thickness of 6 mm). After extensive experiments, it has been found that the best results in terms of image forming, contrast, brightness, distortion, 3D reproduction, are achieved if panels are used with the above mentioned features.

In an example, the panel is provided with a pattern for forming a so called parallax barrier for use with a panoramagram for a 3D reproduction of an image. That is, the image and panel are produced according to the principles of the parallax barrier autostereoscopy.

The invention will now be further elucidated by the examples, which are in now way limiting to the scope of protection. The scope of protection is defined by the claims.

SHORT DESCRIPTION TO THE FIGURES

Figure 1a, b, show, schematically, in exploded view, an example of the present method.

Figure 2 shows, schematically, a section along the line ll-ll in figure

1a of a fragment of a panel produced in accordance with the example of figure 1 a, b.

DETAILED DESCRIPTION OF THE FIGURES

Figure 1 a, b show, schematically, an example of the present method.

Figure 1a shows a screen printing method. By a squeegee, ink is applied across a pattern image (120) in a screen printing frame, and through a screen (110). Accordingly, a pattern image (120) equivalent to the pattern is provided on a panel (100).

Figure 1b shows a fragment (encircled) of a pattern at the panel (100). This schematic image, in an example, relates to a pattern (170) comprised of parallel lines or strips (180) which have a certain width (231 ), and gap or opening (190) having a certain width (233) between these lines or strips (180), which lines or strips (180) (and gaps 190) have a certain density (235) which is expressed in the number of lines or strips per cm (or number of lines or strips per inch). Although not explicitly shown, the pattern (170) extend essentially across the entire surface of the panel (100).

EXAMPLES

For comparison, next some typical values of glass are provided.

Table 1 shows specific properties of several types of glass, at different thicknesses. Planibel and Matelux Clearvision are trademarks of AGC Europe. Planibel (thickness) atelux Clearvision (thickness)

4mm 5mm 6mm 4mm 5mm 6mm

Light transmission 90 89 89 91 91 91

Direct energy 84 83 81 90 90 89 transmission

Total energy 8 10 12 3 3 2 absorption

Colour yield RD65 99 99 99 100 100 100

Shading coefficient 1.00 0.98 0.97 1.03 1.03 1.03

Table 1. Properties of glass

In an example, a flat panel (100) of 120 x 180 cm ² of type Matelux Clearvision is provided. The thickness of the panel is 5 mm. First, the edges at both flat sides (140, 150) of the panel are bevelled (200, 210) and polished, such as schematically shown in figure 2, which is a section view of a fragment of the panel (100) along the line ll-ll in figure 1a. If necessary, the panel is sand blasted, provided with holes and/or grooves and further processed dependent on a final application.

To prevent impurities at the further processing, the panel is moved at a transport belt through a washing and drying tunnel, wherein the panel is washed with water comprising a surface-active agent, and thereafter dried in the air at 60 °C during 2 minutes. Next, by screen printing, an enamel layer (black, provided by the company Pemco) is applied at one side of the panel. The enamel layer of ink is balanced, which means that there is substantially no shrinkage or expansion during the further processing. The ink used has a viscosity of 2000 mPa.sec. The screen (1 10) used has an aluminum frame and plastic wires (120 per centimetre). The pressure at the squeegee is about 0.2 Pa, the processing time of the panel is about 30 seconds. With this, 8 lines per centimetre are produced, with a gap between the lines having a width which is 1/5 of the width of the lines. The thickness of the lines is about 80 pm. If necessary, the lines are somewhat narrowed, for example with a few micrometers towards each other, such that the image strips coincide with the density of the 2D image.

Next, in an oven, the panel is heated to 650 °C during 20 minutes by which the enamel ink is baked into the glass surface. The panel moves with a speed of 0.5 metre/minute back and forth in the oven. If necessary, a special transport belt provides that the curing is achieved without further distortion of the glass surface. Using a very strong blower installation, the hot glass is cooled down with 130 °C/minute till room temperature. During this, tensions occur between the inner kernel and the outer layers of the glass, which tensions are removed, amongst others, by the bevelled edges. As a result, amongst others, a mechanically very strong glass panel is provided. The quality of the lines are monitored by a laser and a camera. The hardness of the ink is also tested by a scratch test.

The invention is, of course, not limited to the above described example. From the previous description, different embodiments are readily possible for a person skilled in the art, which embodiments are regarded to be within the scope of the present claims.