DE102023005457A1 - Method for producing an array of microlenses made of a plastic or resin on a sheet-shaped substrate - Google Patents

Abstract

The invention relates to a method for producing an array of microlenses consisting of a plastic or resin on a sheet-shaped substrate, wherein in a sheet-fed printing machine a layer of liquid or plasticized plastic or resin is applied to at least one specimen of the substrate using a screen printing process, then the array of microlenses to be produced is formed from the layer consisting of the plastic or resin using an embossing process by means of a molding tool, after which the layer formed into microlenses on the respective specimen of the substrate is cured by irradiation with UV radiation, wherein a molding tool is used which, for forming each of the microlenses to be produced in the array with a lens width of maximum 100 µm, has shaping cavities with a pair of opposing concave or convex surfaces in the transport direction of the substrate to be transported through the sheet-fed printing machine and/or transversely to the transport direction of the substrate to be transported through the sheet-fed printing machine, wherein the distance between the respective pair of opposing concave or convex surfaces is at least 10% of the respective lens width.

Description

Through the DE 11 2010 000 957 T5 A lens array for imaging a plurality of image elements in an object plane is known, the lens array comprising a plurality of microlenses formed in or on one side of a transparent or translucent material, the image elements being arranged on the opposite side of this material.

Through the DE 11 2017 003 672 T5 An embossing tool for use with an embossing roll is known, the embossing tool comprising: a tool body having a tool surface; and an array of depressions formed in the tool surface to form a desired embossing surface profile, at least two of the depressions being interconnected by a connecting passage to enable fluid communication therebetween during embossing.

Through the DE 11 2009 002 538 T5 A registration method for an embossed security feature on a security document is known, in which a substrate is provided; at a first printing station, an embossable printing ink is applied to a region of the substrate; at an embossing station, at least part of the region of the embossable printing ink is embossed by means of embossing means to form the embossed security feature; wherein the method further comprises, at the first printing station, printing a registration area of embossable printing ink on the substrate; at least part of the registration area is embossed to form a recognizable registration key; the position of the registration key is recognized; the position of the registration key is compared with at least one registration mark provided at least in part by a printing means, and in response to the comparison of the position of the embossed registration key with that of the at least one registration mark, the position of the printing means is adjusted relative to the embossing means.

Through the US 2015 077 856 A1 An optical mold having a spacer cavity portion, a lens cavity portion, and a flow stop control portion to allow the flow of optical lens material between the spacer cavity portion and the lens cavity portion, as well as an optical lens assembly formed therefrom, is known.

Such microlenses can now be manufactured from a plastic or resin in an industrial manufacturing process, using, for example, an embossing process. For this purpose, the plastic or resin is first molten. The viscous melt is then applied to a substrate, after which the microlenses are formed from the viscous melt, using, for example, an embossing process using a mold. The thus formed microlenses are then cured by irradiation with UV radiation, i.e., the plastic or resin is polymerized.

However, unlike metals, for example, plastics or resins have very poor castability. The reason for this unfavorable property lies in the length of the molecules contained in the plastic or resin, which are predominantly long-chain. These long molecular chains mean that the melt of a plastic or resin is usually too viscous to ideally fill a mold, which would be necessary to form microlenses with the desired shape and/or contour. Instead, it has been observed that when the microlenses are embossed, a protrusion and/or projection consisting of the plastic or resin often forms at their respective edges. The mass of plastic or resin forming a protrusion and/or projection usually represents a shortage in the mold of the mold used to form the microlenses, so that the mold in question is only partially filled with the plastic or resin. In practice, it has been determined that this shortage can even amount to between 30% and 50% of the filling volume of the mold. As a result, the microlenses are significantly malformed during embossing. This malformed microlens formation impairs the desired optical effect of these microlenses in an intolerable manner, especially in an arrangement of several microlenses, each with very small dimensions in the micrometer range. This malformed formation leads, for example, to an optically distorted and/or incomplete reproduction of an object viewed through the microlenses, in particular a printed image.

The invention is based on the object of providing a method for producing an array of microlenses consisting of a plastic or a resin on a sheet-shaped substrate, with which these microlenses are formed in an embossing process largely without projections and/or shoulders.

The problem is solved by a method having the features of claim 1. The dependent claims show advantageous embodiments and/or further developments of the solution found.

The advantages achievable with the invention consist in particular in the fact that microlenses made of a plastic or a resin are formed in an embossing process in such a way that an optically impairing, in particular distorting and/or incomplete reproduction of an object to be viewed through them is avoided as a result of projections and/or lugs formed on these microlenses during the embossing process.

• 1 vorteilhafte Konturen eines zur Ausbildung von Mikrolinsen verwendeten Formwerkzeugs;
• 2 eine schematische Darstellung einer Druckmaschine zur Durchführung eines Verfahrens zur Herstellung von Mikrolinsen.

An embodiment is shown in the drawings. They show:

• 1 advantageous contours of a mold used to form microlenses;
• 2 a schematic representation of a printing machine for carrying out a process for producing microlenses.

In optics, the term "lens" refers to a component that is transparent to light and has at least one light-refracting surface arranged in the path of the light beam. The term light here refers to the part of electromagnetic radiation that is visible to the human eye. In the electromagnetic spectrum, the range of light covers wavelengths from around 380 nm (violet) to 780 nm (red). The following is based on convex lenses, i.e. lenses that focus incoming light, in particular plano-convex lenses. Common designs are rotationally symmetrical, spherical or aspherical lenses, with the respective axis of symmetry of the lens in question and its optical axis being congruent. The optical axis is therefore a straight line that usually runs through the center of curvature of a convex lens surface. With a flat lens surface, the optical axis is perpendicular to it. The curvature of a light-refracting lens, e.g. B. convex surface is specified by its radius of curvature, where the radius of curvature has its origin on the optical axis. A flat lens surface is defined by an infinitely large radius of curvature. In a spherically shaped lens, the refractive surface is a section of a sphere, e.g. in the shape of a spherical cap. An aspherically shaped lens has at least one refractive surface that deviates from the spherical or flat shape. The shape of rotationally symmetric aspherical surfaces is usually specified as a conic section (circle, ellipse, parabola, hyperbola) plus a correction polynomial for higher-order deformations.

A lens has two surfaces intersected by the light beam path, so-called enveloping surfaces. With regard to light bundling, it is defined that light enters a plano-convex converging lens at its convex curved enveloping surface and exits this lens at its flat enveloping surface. The enveloping surfaces are interfaces between different media, in each of which the light propagates. One of these media is formed by the material of the lens in question. At least one other medium is the space, usually filled with air, in which the lens in question is located. Since at least two of the media arranged in the light beam path have at least their respective optical material properties different from one another, the light is refracted at the interface between these adjacent media. Thus, light refraction occurs at at least one of the enveloping surfaces of the respective lens, in particular at its curved enveloping surface. The optical material property related to the refraction of light is expressed by the respective refractive index of the medium in question. The refractive index is a dimensionless physical quantity that indicates the factor by which the wavelength and phase velocity of light are smaller in the medium in question than in a vacuum. Of two media with different refractive indices that form a common interface, the medium with the higher refractive index is said to be the optically denser. The Abbe number, also called Abbe's number, is a dimensionless quantity used to characterize the optical dispersive properties of a lens and indicates how much its refractive index changes with the wavelength of light. The ability of a lens to create an optical image of an object viewed through it depends on the refractive index of the material of the respective lens and on the shape of its enveloping surfaces that form interfaces between different media.

The principal plane of a lens is a plane arranged orthogonally to the axis of symmetry of the lens in question. In the case of a thin lens, where its greatest dimension along the axis of symmetry, i.e. the thickness of the lens, is considered to be very small compared to the radius of curvature of its convex enveloping surface, because the radius of curvature of the convex enveloping surface is, for example, at least five times greater than this thickness, only a single principal plane can be used with sufficient accuracy to consider the properties of the lens in question. In a plano-convex lens, this principal plane coincides with the flat lens surface. The focal length of a lens is the distance between the principal plane of the lens in question and its focus (focal point), where The focus of a lens is the intersection point of parallel light rays bundled by the lens and entering the lens. The parallel light rays entering the lens do not necessarily enter parallel to its optical axis, but rather at any angle of incidence, particularly an acute angle, with respect to the principal plane of the lens in question. A plane arranged at the focus orthogonal to the optical axis is called a focal plane.

The enveloping surface of the lens in question, which serves to admit light, has two opposite edge points that are axially symmetrical to its optical axis and delimit this enveloping surface, for example, located in the principal plane of this lens. The distance between these two edge points determines the width of the lens in question, i.e. its lens width. The aperture or opening width of a lens refers to its free opening or diameter through which light rays can be received unhindered and corresponds at most to the lens width. The point that lies at the intersection of the optical axis with the enveloping surface of the lens in question, which serves to admit light, is called the vertex. The vertex is located on the enveloping surface, which serves to admit light, furthest from the focus of the lens.

A rotationally symmetrical, spherical, or aspherical lens focuses incident light into a cone, where the diameter of the base of this cone corresponds to the maximum lens width, and the height perpendicular to the base of this cone corresponds to the focal length of the lens in question. The numerical aperture describes the ability of a lens to focus light. It determines the minimum size of the light spot that can be created at its focus and is thus an important resolution-limiting parameter.

A plurality of lenses, each of which is rotationally symmetrical, spherical or aspherical, for example, and which are arranged in a preferably uniform grid consisting of either square or hexagonal grid cells, in particular each without gaps and without overlap, form a lens group, which is also referred to as a lens array.

A microlens is a miniaturized form of a conventional lens. The term "microlens" refers to a lens with a maximum lens width of 100 µm and preferably in the range between 20 µm and 65 µm. Microlenses have a focal length of less than 100 µm, for example, and preferably a maximum of 95 µm. Optically imaging structures consisting of microlenses are also referred to as micro-optical structures.

An optically imaging structure formed from microlenses is combined with a preferably flat printed image, particularly in value printing for the production of security elements or banknotes, for example, by applying this optically imaging structure, for example, to a substrate containing the printed image. This makes it possible to create various effects for a viewer viewing the printed image through the optically imaging structure. For example, an arrangement consisting of at least one printed image and at least one optically imaging structure can create so-called alternating images or wobbly images (flips) and/or spatial, i.e. three-dimensional effects and/or morphing effects and/or zoom effects and/or animations. These effects are perceptible to a viewer without optical aids when they view the printed image alternately through the microlenses from different viewing angles. The perception presented to the viewer through different viewing angles is also called a lenticular image.

The usually flat print image is formed on the preferably two-dimensional substrate, e.g. in an industrial production process, preferably with a printing press. The substrate is, for example, a printing material web or in the form of a printing sheet. The print image is, for example, applied to the substrate in a dot-shaped or line-shaped grid. The print image therefore consists, for example, of several, in particular a large number of pixels and/or lines. A pixel size or a line thickness is in a range of less than 100 µm, preferably less than 50 µm, in particular less than 20 µm, e.g. in the range of approximately 5 µm to 10 µm. In the following, it is assumed that the pixel size of pixels and/or the line thickness of lines, which are each involved in the formation of a print image used together with an optically imaging structure, e.g. B. are each at most as large, preferably smaller, in particular less than half as large as the respective lens width of the lenses involved in the construction of the respective optically imaging structure.

In optics, resolution refers to the ability to distinguish fine structures, i.e., the minimum distance that, for example, two pixels or two lines may or must be from each other in order to be perceived as separate pixels or lines. The resolution of the naked human eye varies from person to person. Adults with normal vision can usually still distinguish at a distance of 25 cm. Structures at a distance of 150 µm can be distinguished. This corresponds to a visual angle of approximately 2 angular minutes, which is referred to as angular resolving power. At low contrasts, the visual acuity of the human eye decreases noticeably, with visual acuity being the inverse of resolving power. The lens width of a microlens is therefore typically smaller than the resolving power of the naked eye of a normal-sighted adult.

The substrate is, for example, a fibrous printing material, in particular paper, or a film, preferably a polymer film. The substrate can be opaque or transparent. The substrate can be single-layered or multi-layered, in particular multi-layered in sections. Different layers of a multi-layer substrate can be made of different materials, e.g. one layer of paper and another layer of a polymer film. The substrate or at least one respective layer of this substrate has a material thickness, i.e. thickness, of e.g. less than 100 µm, preferably less than 50 µm, in particular approximately 25 µm. A print image formed on the substrate by means of applied printing ink has a layer thickness, e.g. less than 10 µm, preferably less than 5 µm, in particular in the range from 1 µm to 2 µm. The substrate can be printed on one or both sides.

In a preferred embodiment of the invention, the arrangement of the printed image and the optically imaging structure is a component of a security element or a document, in particular a security document. These documents include, for example, banknotes, credit cards, checks, securities, share certificates, passports, ID cards, driver's licenses, title deeds, travel documents such as airline or train tickets, admission tickets, student documents, and other official or governmental documents such as birth, death, or marriage certificates. This list is merely exemplary and by no means exhaustive. However, banknotes are preferred.

Microlenses can now be manufactured in an industrial production process. Microlenses made from plastic or resin can be produced using an embossing process. Embossing is a primary forming process and, according to DIN 8580, belongs to a group of manufacturing processes in which a workpiece is manufactured from a formless material. In this case, at least one microlens with a specific shape is formed from a liquid or plasticized mass of plastic or resin using a mold. Irradiation with ultraviolet (UV) radiation then creates a form-giving cohesion among the molecular chains contained in the plastic or resin. UV radiation covers a wavelength range from 100 nm (nanometers) to 400 nm and is the most energetic part of optical radiation. UV radiation is invisible to humans and cannot be perceived by other sensory organs. For the present application, UV radiation with a wavelength of approximately 315 nm has proven advantageous.

An advantageous method for producing an array of microlenses made of a plastic or resin provides for producing these microlenses on a sheet-shaped substrate in an industrial manufacturing process using a high-speed printing press, i.e., in particular, a sheet-fed printing press capable of printing several thousand copies of the sheet-shaped substrate per hour, in several successive process steps. Such a printing press is capable of transporting, for example, at least 10,000 sheet-shaped substrates, in particular 12,000 to 18,000 such substrates per hour, and optionally printing them, and of forming an array of microlenses made of a plastic or resin on each of these substrates. With regard to the production of an array of microlenses consisting of a plastic or resin, it is provided that, in a first process step, a layer of liquid or plasticized plastic or of liquid or plasticized resin with a layer thickness preferably in the range between 18 µm and 25 µm is applied to these specimens of the sheet-shaped substrate using a screen printing process. In a second process step, the array of microlenses to be produced is then formed in the printing press from the layer consisting of the plastic or resin using an embossing process by means of a mold embossed onto this layer. In a third process step, the layer formed on the respective specimen of the sheet-shaped substrate into the array of microlenses to be produced is cured by irradiation with UV radiation, this UV radiation having a wavelength of, for example, 315 nm. The curing of the plastic or resin occurs through polymerization of this plastic or resin stimulated by the high-energy radiation. In a preferred embodiment, both the substrate and the plastic or resin to be applied thereto are each transparent.

To avoid that a small amount of dust forms on the edges of the microlenses 01 when embossing them. made of the plastic or resin, a molding tool 07 is used in the printing machine 11, which is designed in particular as a sheet-fed printing machine, which molding tool 07 has, for forming each of the microlenses 01 to be produced in the arrangement with a lens width L of a maximum of 100 µm, shaping cavities with a pair of concave or convex surfaces 02 which are opposite one another at a distance a in the transport direction T of the specimens of the sheet-shaped substrate 03 to be transported through the printing machine 11 and/or transversely to the transport direction T of the specimens of the sheet-shaped substrate 03 to be transported through the printing machine 11, wherein the distance a between the respective pair of opposing concave or convex surfaces 02 is at least 10% of the respective lens width L. As a result, the cavities of the molding tool 07 which form the microlenses 01 take on a shape such as a rectangle or square instead of a rectangle or square. B. with an oval or double-T-shaped or x-shaped contour. Examples of advantageous contours of the shaping cavities of the mold 07 used to form the microlenses 01 are shown in the 1 The contours shown are each a sectional view of an individual shaping cavity of this molding tool 07 used to form the microlenses 01 on or parallel to the preferably printed surface of the respective substrate 03.

A further or additional measure to prevent a plastic or resin overhang and/or projection forming at the respective edge of the microlenses 01 during embossing consists in forming a trough or depression on the surface of the respective substrate 03 at the respective position of the microlens 01 to be formed, before the plastic or resin layer is applied there, comprising between 30% and 50% of the volume of the microlens 01 to be formed at the respective position, in which the plastic or resin is held during embossing. Such a trough or depression can be created, for example, by rolling a nub roller 12 on the substrate 03. A further or alternative embodiment can provide flow barriers formed on the surface of the respective substrate 03 for the plastic or resin to be applied, wherein these flow barriers preferably extend transversely to the transport direction T of the copies of the sheet-shaped substrate 03 to be transported through the printing press 11. A flow barrier, i.e. an obstacle to the spread of the flowing plastic or resin, can rise in the form of a wall from the surface of the substrate 03 in question or can be introduced into the surface of the substrate 03 in question in the form of a depression, e.g. a groove or notch.

In the 2 It is shown schematically and in simplified form that when carrying out the above-mentioned method for producing an arrangement of microlenses 01 consisting of a plastic or a resin, the substrates 03 in question are successively removed individually from a stack and are successively printed, preferably initially in at least one printing unit 04 of the printing press 11, wherein, for example, an offset printing process and/or an inkjet printing process and/or a gravure printing process and/or an intaglio printing process is/are used. A layer of liquid or plasticized plastic or of liquid or plasticized resin with a layer thickness preferably in a range between 18 µm and 25 µm is then applied to the substrate 03 printed in this way during its further transport in a screen printing unit 06 of the printing press 11, i.e. in a printing unit which carries out a screen printing process. The plastic or the resin is preferably transparent. The previously described embossing process is then carried out as the substrate 03 is transported through the printing press 11, the coated substrate 03 in question being guided, for example, through a nip between two interacting rollers 07; 08, the roller 07 rolling on the coated surface of the substrate 03 having a shell surface forming the molding tool with corresponding cavities, i.e. shaping cavities, each for receiving the plastic or resin and thus for embossing the microlenses 01. Furthermore, a radiation source 09 directed at the substrate 03 in question for irradiating the embossed microlenses 01 with UV radiation is then preferably arranged in the printing press 11 in the transport direction T of the substrate 03.

List of reference symbols

01

microlens

02

Area

03

Substrat

04

printing unit

05

-

06

Screen printing unit

07

roller; forming tool

08

roller

09

Radiation source

10

-

11

printing press; sheet-fed printing press

12

dimpled roller

a

Distance

L

Lens width

T

Transport direction

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 11 2010 000 957 T5 [0001]
• DE 11 2017 003 672 T5 [0002]
• DE 11 2009 002 538 T5 [0003]
• US 2015 077 856 A1 [0004]

Claims (9)

A method for producing an array of microlenses (01) made of a plastic or resin on a sheet-shaped substrate (03), wherein, in a sheet-fed printing press (11), a) a layer of liquid or plasticized plastic or resin is applied to at least one specimen of the sheet-shaped substrate (03) fed to it using a screen printing process, b) the array of microlenses (01) to be produced is formed from the layer made of the plastic or resin using an embossing process by means of a molding tool (07) embossed onto this layer, c) the layer formed on the respective specimen of the sheet-shaped substrate (03) into the array of microlenses (01) to be produced is cured by irradiation with UV radiation, wherein a molding tool (07) is used which has shaping cavities with a lens width (L) of a maximum of 100 µm in the transport direction for forming each of the microlenses (01) to be produced in the array (T) of the substrate (03) to be transported through the sheet-fed printing press (11) and/or having a pair of concave or convex surfaces (02) each disposed transversely to the transport direction (T) of the substrate (03) to be transported through the sheet-fed printing press (11), said pair being spaced apart from one another at a distance (a), wherein the distance (a) between the respective pair of opposing concave or convex surfaces (02) is at least 10% of the respective lens width (L), wherein at least one flow barrier for the plastic or resin to be applied is formed on the surface of the respective substrate (03), said flow barrier extending transversely to its transport direction (T) through the sheet-fed printing press (11). Procedure according to Claim 1 , characterized in that a transparent plastic or a transparent resin is used and/or that an opaque or transparent substrate (03) is used. Procedure according to Claim 1 or 2 , characterized in that the plastic or resin is applied to the substrate (03) with a layer thickness in a range between 18 µm and 25 µm. Procedure according to Claim 1 or 2 or 3 , characterized in that the substrate in question (03) is printed before being coated with the plastic or the resin in at least one printing unit (04) of the sheet-fed printing machine (11) using an offset printing process or an inkjet printing process or a gravure printing process or an intaglio printing process. Procedure according to Claim 4 , characterized in that the screen printing process carrying out the coating with the plastic or with the resin is carried out with a screen printing unit (06) arranged in the sheet-fed printing machine (11) following the printing of the substrate (03) in the offset printing process or the inkjet printing process or the gravure printing process or the intaglio printing process. Procedure according to Claim 1 or 2 or 3 or 4 or 5 , characterized in that the irradiation of the layer formed on the respective specimen of the substrate (03) into the arrangement of microlenses (01) to be produced with UV radiation is carried out by a radiation source (09) arranged in the sheet-fed printing machine (11) and directed onto the respective substrate (03). Procedure according to Claim 1 or 2 or 3 or 4 or 5 or 6 , characterized in that one of two cooperating rollers (07; 08) is used as the forming tool (07), the coated substrate (03) being guided through a roller gap formed by these rollers (07; 08), the roller (07) rolling on the coated surface of the substrate (03) having a jacket surface forming the forming tool (07) with the shaping cavities. Procedure according to Claim 1 or 2 or 3 or 4 or 5 or 6 or 7 , characterized in that on the surface of the substrate in question (03) at a respective position of each microlens (01) to be formed, before the layer consisting of the plastic or the resin is applied there, a depression or trough having between 30% and 50% of the volume of the microlens (01) to be formed at the respective position. Procedure according to Claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 , characterized in that several substrates (03) are transported one after the other through the sheet-fed printing machine (11) at a transport speed of 12,000 to 18,000 substrates (03) per hour and are each provided with an arrangement of microlenses (01).

Images