CN115666961A - Retroreflective relief structures in an embossing lacquer - Google Patents

Abstract

The invention relates to an embossing tool for producing a flat retroreflector having a retroreflective pyramidal prism relief structure formed in a curable embossing lacquer, in particular a UV lacquer, and which retroreflects electromagnetic radiation incident from the incident side. The embossing tool has a number n of mutually adjacent depressions and/or elevations on one side, wherein each of the n depressions and/or elevations is formed by three planes which are arranged at a specific angle (α') to one another. The invention further relates to a method for producing such an imprint template. According to the invention, the specific angle (α ') is smaller than 90 ° by a specific value X, i.e., α' =90 ° -X, so that the number n of mutually adjoining elevations and/or depressions which are formed into the embossing lacquer by the embossing tool, after hardening of the hardenable embossing lacquer, each consist of three planes which are arranged at an angle of 90 ° to one another. The elevations of the embossing tool correspond to the depressions in the embossing lacquer and the depressions in the embossing tool correspond to the elevations of the embossing lacquer.

Description

Retroreflective relief structures in an embossing lacquer

The invention relates to an embossing tool for producing a flat retroreflector having a retroreflective pyramidal prism relief structure formed in a curable embossing lacquer, in particular a UV lacquer, and which retroreflects electromagnetic radiation incident from the incident side. The embossing tool has a number n of mutually adjacent depressions and/or elevations on one side, wherein each of the n depressions and/or elevations is formed by three planes which are arranged at a specific angle (α') to one another. The invention further relates to a method for producing such an imprint template.

Retroreflective structures reflect incident electromagnetic radiation, such as light, in the direction of radiation incidence largely independently of the direction of incidence and the orientation of the reflector. This is known as retroreflection. In contrast, in a flat mirror, the back radiation depends on the mirror orientation, which is oriented only exceptionally perpendicular to the observer. Planar retroreflectors have a large number of individual retroreflectors arranged side by side. Retroreflective films are a common realization of planar retroreflectors. Retroreflective films are used in a wide variety of applications, in particular in traffic technology, but also in security technology, for example as security elements for the authentication of documents of value, such as banknotes, credit cards or identification cards, according to US 2014/0226212 A1. Most traffic signs and truck signs are also provided with retroreflective structures to improve nighttime visibility or to improve security (see US 56565656336).

Two variants of retro-reflectors are widely used: embedded high refractive microspheres which reflect on the bottom side, for example as described in US 4763985 and US 2009/0300953 A1. The optical principle is based here on the so-called luneberg lens (luneburg-linden), which reflects incident light rays in the direction of the illumination source. Another variant in the field to which the invention relates is based on relief structures consisting of flat surfaces, so-called corner-cube relief structures, which have the shape of cube corners. In this case, the incident light rays are successively redirected on three planes (so-called triple prisms) arranged at exactly 90 ° angles to one another and are reflected towards the illumination source. This geometry has the highest retroreflective efficiency at normal incidence of light; efficiency decreases with increasingly oblique incident light. The geometry of such a structure known from the prior art is shown in fig. 1. Fig. 1a shows a schematic side view of a relief structure of a planar retroreflector, in which a retro-reflective beam path is provided. The retroreflector is designed as a film composite comprising a carrier film 1 and a layer 2 made of an embossing lacquer, wherein the reflective structure 3 is molded into the layer 2. The reflecting structure 3 consists of planes which are arranged at an angle of 90 ° to each other, so that the incident radiation is deflected a number of times and finally retroreflected. Fig. 1b illustrates in a top view the path of the retroreflection as occurs in the triangular faces of a triangular prism, from the sequential arrangement of which a relief structure for the retroreflection of a planar retroreflector can be constructed. Fig. 1c and 1d show suitable cube-corner faces for relief structures in a top view or in a perspective view.

Retroreflective films for use in truck signage and road signs are typically manufactured in a hot pressing process. In this case, the structure of the impression tool is converted into the thermoplastic almost as it were.

It is known, for example from WO 2018/151959A1, WO 2018/151960A1 or WO 2018/151964A1, to produce an embossing mold with a pyramidal prism structure by means of a rotating engraving knife.

Cube-corner prism structures used for retroreflective films are more demanding in embossing due to their typical depth of 15 μm to 80 μm. In the case of embossing into a curable embossing lacquer, for example a UV lacquer which is cured after irradiation by electromagnetic radiation in the Ultraviolet (UV) wavelength range, a UV lacquer with very good flow properties is required in order to avoid the inclusion of bubbles and in order to more easily release the UV lacquer and the embossing mold. This UV lacquer, on the other hand, produces a relatively high volume shrinkage during its hardening, which is not isotropic, but rather protrudes in the direction perpendicular to the film surface more than in the horizontal direction, since the UV lacquer is fixed to the film in a form-fitting and friction-fitting manner. This "anisotropic" shrinkage results in flattening of the corner-cube structure and an increase in the corner-cube angle (depending on lacquer viscosity, chemical composition, substrate, temperature \8230;).

However, corner cube prism structures whose angles between the respective planes are no longer 90 but differ from 90 by more than +/-0.5 almost completely lose their retroreflective properties.

The object of the present invention is therefore to develop an embossing tool for corner cube structures of the type according to the invention in such a way that the disadvantages of the prior art are eliminated. The object of the invention is therefore to provide retroreflective films having corner cube structures in embossing lacquer, in particular UV lacquer.

This object is achieved by the features of the independent claims. The invention is characterized in that it is provided with a plurality of design elements.

According to the invention, the specific angle (α ') is smaller than 90 ° by a specific value X, i.e., α' =90 ° -X, so that a number n of mutually adjacent elevations and/or depressions, which are formed (or embossed) into the embossing lacquer by the embossing tool, after hardening of the hardenable embossing lacquer, each consist of three planes which are arranged at an angle of 90 ° to one another. The elevations of the embossing tool correspond to the depressions in the embossing lacquer and the depressions in the embossing tool correspond to the elevations of the embossing lacquer.

The invention therefore provides an embossing tool which maintains or compensates for the shrinkage of the lacquer in such a way that the embossing structures are produced sharply. Thus, the imprint mold itself does not exhibit retroreflective properties. A further particular advantage of the invention results from the fact that the embossing tool according to the invention can be bent into a cylinder or a cylinder section and that adjoining edges of the cylinder or adjoining edges of adjoining cylinder sections can be welded to one another by means of an electromagnetic beam, for example a laser beam, to form the embossing cylinder. Since the laser beam reaches the structure of the embossing tool which is not retroreflective, the laser beam is in principle not reflected back to the laser. In contrast, with embossing tools having retroreflective structures, the laser beam is in principle reflected back to the laser and can damage or destroy it here.

According to the invention, a curable impression lacquer is an impression lacquer which is applied to a substrate as a carrier in a plastically deformable state and in which an impression structure is formed by an impression tool. Such an embossing die is, for example, an embossing punch or an embossing cylinder. The embossing lacquer is subsequently hardened, for example by UV radiation or thermal radiation, so that the embossing lacquer is no longer plastically deformable and the embossed structure can no longer be changed.

As radiation-hardening embossing lacquer, for example, free-radical or cationically hardening lacquers are used, which can be used on a large number of substrates, such as PET, PVC, PE, PP, PMMA or PC. In order to stably fix the UV paint on the substrate, an intermediate layer for increasing adhesion may be used.

The imprint template according to the invention has a number n of mutually adjoining recesses. The geometry and dimensions of the n number of mutually adjacent projections formed by this embossing tool in the curable embossing lacquer conform to the corresponding recesses of the embossing tool before the embossing lacquer has cured. After the hardening of the embossing lacquer, the geometry and dimensions of the elevations in the embossing lacquer no longer correspond to the corresponding recesses of the embossing mold. Due to the shrinkage during hardening, the projections collapse, so that the angle between the three planes respectively arranged on each other increases. According to the invention, the projections are compressed by a specific value X due to shrinkage during hardening, so that the angle between the three planes respectively arranged one above the other is 90 DEG and the projections exhibit retroreflective properties.

The specific angle (α') is smaller than 90 ° by the value of X, and during the hardening process the embossing lacquer collapses by the value of X, which depends on the hardenable embossing lacquer used, the substrate to which the embossing lacquer is applied and the hardening conditions, and which differs for different hardenable embossing lacquers. If the value X is not known for the particular curable impression lacquer associated with the substrate to be used, the value X must be determined by preliminary experiments. Here, the relief is formed in the embossing varnish by means of a test embossing tool having recesses with a first angle α', and the angle α of the flank of the relief produced is measured after the embossing varnish has hardened. The difference between the measured angle α and 90 ° yields a correction value, which must be corrected in the course of the following procedure for the angle α'. This process is repeated as necessary until the angle alpha is equal to 90 deg..

For commercially available hardenable embossing lacquers, the specific value X typically has a value of less than or equal to 3.5 °, preferably 0.3 ° to 3.5 °, whereby the specific angle (α') is greater than or equal to 86.5 °, preferably 86.5 ° to 89.7 °. Embossing lacquers known from the prior art are, for example, the mixed polymers for micro-optical systems, model Ormocer, from the company "micro resist technology GmbH", which can be cured by UV light. For example (7 months as of 2015), impression paints

Or "InkOrmo" has a relative volume shrinkage (english volume shrinkage) of 5% to 7%,

from 4% to 6%, "Ormocore" and "Ormoclad" from 2% to 5%,

is 3% to 5% "

10' is<2% and "

30' is<<2％。

Thus, X is the value of the shrinkage of the embossing lacquer after hardening, typically 0.3 ° to 3.5 °. For example, structures in a specific UV lacquer show an opening angle of 92.5 ° due to shrinkage of the lacquer after UV imprinting by an imprint mold having recesses with an opening angle α' of 90 °, i.e. X =2.5 °. In order to reserve this shrinkage or this angular difference X in the next mold, the structures in the imprint mold are manufactured with a corner cube angle of 90 ° -2.5 ° =87.5 °.

The design of the embossing tool depends on the structure of the retroreflective film and in particular on whether the viewer observes the cube-corner structure through the UV lacquer or through a layer of complementarily flattened UV lacquer. In the first case, the mold will have recesses in the form of cube corners, the opening angle of which changes according to the invention with respect to the intended paint shrinkage. In the second case, the embossing mold will have protrusions of modified cube-corner design, which leave corresponding recesses in the UV lacquer.

The invention also relates to a method for producing an imprint template according to the invention. The embossing tool is composed of a substrate, wherein recesses are introduced into one side of the substrate by means of a chisel, wherein the chisel has a corner at its tip of 70.52 ° -X. Such a chisel is made of, for example, diamond, solid carbide, HSS-Co, siC, tiC, tungsten or TiCN.

In the prior art, the corresponding impression molds are produced by diamond cutting. A diamond engraving knife with a 70.52 ° angle at the tip is used here, which scribes a grid of parallel grooves in the substrate of the imprint mold. Next, two further sets of meshes are drawn, which are twisted by 60 ° or 120 ° with respect to the first mesh. Therefore, when a 70.52 ° diamond carver is used, a corner cube having an opening angle α' of 90 ° may remain in the imprint mold.

The substrate of the imprint mold is preferably composed of brass (an alloy of copper and zinc), copper, nickel phosphorus, nickel vanadium, zinc white copper (an alloy of nickel, copper and zinc), or cobalt. Softer carrier substrates are also often used, on the surface of which hard coatings are deposited, such as Cr, crN, ti, tiN, carbides, W, DLC (diamond-like carbon).

In order to produce an impression tool according to the invention with a sharper opening angle of 90 ° to X, a correspondingly sharper diamond chisel is used, which has a corner of 70.52 ° to X at the tip. For example, for paint shrinkage with a value of X =2.5 °, a diamond graver with an opening angle of 70.52 ° -2.5 ° =68.02 ° needs to be used.

It is clear that the features mentioned above and still to be explained below can be used not only in the combination shown but also in other combinations without leaving the scope of the invention, provided this is included in the scope of protection of the claims.

The advantages of the invention are elucidated on the basis of the following examples and supplementary figures. These examples represent preferred embodiments, to which, however, the invention is in no way limited. Moreover, the drawings in the figures are highly schematic and do not reflect reality for a better understanding. In particular, the proportions shown in the figures do not correspond to relationships that exist in reality and are only used to improve the intuitiveness. In addition, for better understanding, the embodiments described in the following embodiments are simplified as basic core information. In a practical implementation much more complex patterns or images can be applied.

In the drawings schematically and in detail:

fig. 1 shows a planar retroreflector known from the prior art, whereby fig. 1a shows the retroreflector in a side view, fig. 1b shows the retroreflection paths occurring on the triangular faces of a triangular prism in a top view, and fig. 1c and 1d show suitable cube-corner faces for the relief structure in a top view or a perspective view,

figure 2 shows a first embodiment of a retro-reflector manufactured by means of an imprint template according to the invention in a side view,

figure 3 shows a first embodiment of an imprint template according to the invention in a side view,

fig. 4 shows the curable impression lacquer embossed by the impression tool in fig. 3 and before the hardening of the impression lacquer in fig. 4a, after the hardening of the impression lacquer in fig. 4b,

figure 5 shows a second embodiment of a retro-reflector manufactured by means of an imprint template according to the invention in a side view,

figure 6 shows a second embodiment of an imprint template according to the present invention in side view,

fig. 7 shows the curable impression lacquer embossed by the impression tool in fig. 6 and in this case before the impression lacquer is cured in fig. 7a and after the impression lacquer is cured in fig. 7 b.

Fig. 2 shows schematically a first exemplary embodiment of a retroreflector produced by an imprint template according to the invention in a side view from above, i.e. from the top side of the imprint lacquer 2 corresponding to fig. 1 a. The embossing lacquer 2 is shown as a dashed line before it hardens and as a solid line after it hardens. Before hardening, the sides 3 'of the embossed structure have an angle α' with each other in the "valleys" of the embossed structure. During the hardening process, the height of the embossed structure shrinks, so that its sides 3 after hardening have an angle of α =90 ° with each other in their "valleys".

Fig. 3 shows schematically in a side view a first embodiment of an imprint template 4, in one side of which two recesses 5 are provided. The recess 5 has side faces which are oriented at an angle α' to one another. Fig. 4 shows a curable impression lacquer 2 which is embossed by such an impression tool 4. Before the embossing lacquer 2 hardens, the geometry and dimensions of the elevations 3' correspond, according to fig. 4a, to the recesses 5 of the embossing mold 4. After the embossing lacquer 2 has hardened, according to fig. 4b, the height of the embossed structure is shrunk so that its side faces 3 after hardening have an angle of α =90 ° with each other in their "valleys".

Fig. 5 shows a second exemplary embodiment of a retroreflector produced by an imprint template according to the invention, schematically in a side view from below, i.e. from the opposite side of the imprint template 2, viewed through the imprint template 2. In this case, the substrate 6 and the embossing lacquer 2 must be transparent, so that they allow the incident light to pass through with little attenuation and little scattering. In this case, the incident light is not reflected directly on the reflective upper side of the embossing lacquer, as in fig. 1a or fig. 2 or fig. 4b, but only after passing through the transparent substrate 6 and the transparent embossing lacquer 2. Although the reflective surface is the same in both cases, the opposite side of this surface will reflect light.

According to fig. 2a, the embossing lacquer 2 is shown by a dashed line before it is hardened and by a solid line after it is hardened. Before hardening, the sides 3 'of the embossed structure have an angle α' with each other at their tips. During the hardening process, the height of the embossed structure shrinks, so that the side faces 3 after hardening thereof have an angle of α =90 ° to one another at the tip thereof.

Fig. 6 shows schematically in a side view a second embodiment of an imprint template 4, on one side of which three protrusions 8 are applied. The projections 8 have flanks which are oriented at an angle α' to one another. Fig. 7 shows a curable embossing lacquer 2 embossed by such an embossing tool. Before the hardening of the embossing lacquer 2, the geometry and dimensions of the recesses 3' correspond to the elevations 8 of the embossing mold 7 according to fig. 7 a. After the embossing lacquer 2 has hardened, the height of the embossed structure is contracted according to fig. 7b, so that its side faces 3 after hardening have an angle of α =90 ° with each other at their tips.

The views in fig. 2 to 7 are not drawn to scale. For better visibility, the depth of the depressions 5 or the height of the elevations 8 of the respective embossing tool 4 or 7 and the height of the elevations before the hardening of the embossing lacquer 2 are shown, in particular, strongly exaggerated. Thus, the angle α' is of course also shown to be sharper than in reality. In contrast, the angles α are each shown almost proportionally as 90 ° angles.

Claims (4)

1. Embossing tool for producing a flat retroreflector having a retroreflective pyramidal prismatic relief structure formed in a curable embossing lacquer, in particular a UV lacquer, and retroreflecting electromagnetic radiation incident from an incident side, wherein the embossing tool has a number n of mutually adjoining depressions and/or elevations on one side, wherein each of the n depressions and/or elevations is formed by three planes arranged at a specific angle (α ') to one another, characterized in that the specific angle (α') is smaller by a specific value X than 90 °, so that the n depressions and/or elevations formed into the curable embossing lacquer by the embossing tool after curing of the embossing lacquer each consist of three planes arranged at an angle of 90 ° to one another.

2. An embossing mold as claimed in claim 1, characterized in that the value of X depends on the curable embossing lacquer used, the curing conditions and the substrate to which the embossing lacquer is applied, and that the value of X is different for different curable embossing lacquers.

3. The imprint template according to claim 2, wherein the specific value X has a value of less than or equal to 3.5 °, preferably of 0.3 ° to 3.5 °, whereby the specific angle (α') is greater than or equal to 86.5 °, preferably of 86.5 ° to 89.7 °.

4. Method for producing an imprint mold according to one of the preceding claims, characterized in that the imprint mold consists of a substrate, wherein recesses and/or protrusions are provided into one side of the substrate by means of a carving tool, wherein the carving tool has a corner at its tip of 70.52 ° -X.

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