EP3599105B1 - Method for producing a security feature by means of laser beams - Google Patents

Description

The invention relates to a method for producing a security feature for a security element, security paper or a data carrier. In the method, a carrier is provided whose color impression can be modified by the action of laser radiation. By guiding a laser beam over the surface of the carrier, on the one hand continuous openings are made in the carrier and on the other hand the color impression of the carrier is modified in marking areas.

It is known to cut or perforate a substrate made of paper or film with a laser beam and to remove or discolor a coating applied to the substrate in some areas in order to thereby produce visually attractive effects arranged in register with one another. Examples of such methods can be found in the publications DE 43 34 848 C1 , WO 2009/003 587 A , WO 2010/072 329 A1 , WO 2011/154112 A1 , DE 10 2010 053 052 A1 and DE 10 2008 046 513 A1 .

In the methods mentioned, the laser impact often triggers not only the cutting effect of the laser beam, but also an additional effect, for example a discoloration of the substrate in the edge region of the cut hole. The securing effect is created by the exact and difficult to re-adjust fit of the cut hole with the colored edge of the hole.

The disadvantage, however, is that the production speed of such processes decreases with more complex designs. To remedy this, for example, in the WO 2009/003 587 A1 been proposed to perform the hole generation and the color effect generation with two laser modules connected in series. However, this method also reaches its limits when using conventional laser sources.

Another disadvantage of known methods is that the laser beam for cutting a paper substrate, for example, ideally has a focus that is as fine as possible. A fine focus means that little paper is burned and it can be cut quickly. However, because of the small focus size, the line width of the edge effect produced during cutting is also small and only narrow and visually less attractive edges can be produced. When using a larger focus diameter, wider and more visually noticeable color edges can be produced, but the cutting speed decreases. A compromise must therefore always be found between working speed and a good visual appearance.

From the EP 3 335 900 A1 a method for producing a security feature is known, in which a first laser beam introduces holes into a substrate, the color of which can be modified by laser radiation, and a second laser beam carries out a modification of the color of the substrate. In the method, the two laser beams are combined to form a single laser beam, with the combined laser beam being guided onto the surface of the substrate.

the WO 2014/106517 A1 relates to a method for producing a security element, in which a laser-sensitive recording layer, which has a laser-absorbing mixture component, is exposed in first and second sub-areas to laser radiation from the same laser source, but with different laser parameters, in order to produce different target appearances in the sub-areas.

Proceeding from this, the invention is based on the object of specifying a method of the type mentioned at the outset which avoids the disadvantages of the prior art and which in particular enables visually attractive security features to be produced at high production speeds.

This object is solved by the features of independent claim 1. Developments of the invention are the subject matter of the dependent claims.

According to the invention, in a generic method, the focal position and/or the focal size of the laser beam are rapidly changed when the laser beam is guided over the carrier surface in order to produce a variable width of the continuous openings and/or a variable width of the marking areas.

The term "rapid" describing the change in the focus size or focus position always refers as a comparison variable to the feed rate at which the cutting or marking laser beam is guided over the carrier. In general, a change in the focal size or focal position is referred to as rapid if it enables a visually recognizable change in the cutting width or the marking width during the advance of the laser beam.

With regard to the focus position, which indicates the position of the focus of the laser beam perpendicular to the processing plane, a rapid change according to the invention consists in a change in the focus position per laser feed distance of more than 1 mm/mm, in particular of more than 2 mm/mm and preferably of more than 5 mm/mm. This means that the focus position per laser feed can be shifted from 1 mm up or down by 1 mm (or 2 mm or 5 mm) perpendicular to the working plane.

Alternatively or additionally, with regard to the focus size, which indicates the diameter of the laser spot in the focus of the laser beam, there is a rapid change according to the invention in a change in the focus size per laser feed distance of more than 10%/mm, in particular of more than 50%/mm and preferably greater than 100%/mm. This means that the focus size can be increased or decreased by 10% (or 50% or 100%) per laser advance of 1mm.

The focus size of the laser beam indicates the size of the laser spot in the focus, which is not necessarily in the processing plane (the carrier surface). The spot size of the laser in the processing plane corresponds to the focus size when the focus is in the processing plane. With a focus position above or below the processing plane, the spot size of the laser in the processing plane is usually larger than the focus size, so that the focus size represents a minimum size of the laser spot that can be achieved by shifting the focus.

The statement that continuous openings are made in the carrier and the color impression of the carrier is modified in marking areas includes the possibility that only one through opening is made and the color impression of the carrier is modified only in one marking area. Instead of continuous openings, the laser radiation can also be used to produce recesses in the carrier material in some areas, which do not extend completely through the carrier, and which can also be combined with a colored marking area. Such recesses can be provided, for example, in the form of weakening lines.

In a preferred embodiment, the marking areas form marked edge areas of the continuous openings, at least in partial areas. The marked edge areas are preferably generated at the same time as the associated continuous openings by the same laser beam.

Provision is advantageously made for the laser beam to pass through an adjustment device before it is applied to the carrier surface, in which the beam diameter and/or the convergence or divergence of the laser beam are adjusted. The rapid change in the focus position and/or the focus size are preferably generated by setting variations in the setting device mentioned.

The setting variations in the setting device are made with particular advantage using one or more linear axes.

An advantageous development of the method provides that a first laser beam is provided to produce the continuous openings, a second laser beam is provided to produce the marking areas, that the first and second laser beams are combined with a beam combiner and the combined laser beam is guided over the carrier surface is used in order to introduce the mentioned openings into the carrier and to modify the color impression of the carrier in the mentioned marking areas.

Advantageously, both openings are made in the carrier and colored markings are produced at the same time, at least in part. It is of course also possible to produce only a color change in the carrier in partial areas without cutting the carrier.

When the combined laser beam is guided over the carrier surface, the focus position and/or the focus size of the first laser beam are advantageously changed quickly in order to produce a variable width of the continuous openings.

Alternatively or additionally, when the combined laser beam is guided over the carrier surface, the focus position and/or the focus size of the second laser beam are advantageously changed quickly in order to produce a variable width of the marking areas.

The carrier is advantageously provided with a laser-modifiable marking substance. In particular, the carrier is coated with a laser-modifiable marking substance and/or a laser-modifiable marking substance is introduced into the volume of the carrier.

In advantageous configurations, the carrier is formed from paper or comprises a paper layer. Supports made of plastic or with a plastic layer are also possible. The carrier can in particular be a composite substrate, for example with the layer sequence film/paper/film or paper/film/paper.

Further exemplary embodiments and advantages of the invention are explained below with reference to the figures, which are not shown to scale and proportions in order to improve clarity.

Fig. 1

eine schematische Darstellung einer Banknote mit einem Merkmalsbereich mit einem Laserloch mit variablem Randeffekt,

Fig. 2

eine Detailaufsicht auf den Merkmalsbereich von Fig. 1,

Fig. 3

schematisch eine Vorrichtung zur schnellen Veränderung der Fokusgröße und Fokuslage eines Laserstrahls,

Fig. 4

schematisch eine Laserbearbeitungsvorrichtung zur Erzeugung eines kombinierten Laserstrahls mit unabhängig einstellbarer Breite von Zentralbereich und Außenbereich, und

Fig. 5, 6

zwei Beispiele für Laserlöcher mit variablem Randeffekt und variabler Schnittbreite wie sie mit einer Vorrichtung nach Fig. 4 erzeugt werden können.

Show it:

1

a schematic representation of a bank note with a feature area with a laser hole with variable edge effect,

2

a detail view of the feature area of 1 ,

3

schematic of a device for rapidly changing the focus size and focus position of a laser beam,

4

a schematic of a laser processing device for generating a combined laser beam with an independently adjustable width of the central area and the outer area, and

Figures 5, 6

two examples of laser holes with a variable edge effect and variable cutting width as they are made with a device 4 can be generated.

The invention will now be explained using the example of a security feature of a bank note 10 . figure 1 shows a schematic representation of a banknote 10, which is in an in 2 Feature area 12, shown in more detail in the top view, is provided with a laser hole 14 with a variable edge effect.

The banknote paper of the banknote 10 represents a carrier 20, the color impression of which in the feature area 12 can be modified by the action of laser radiation or has already been modified during production by laser exposure. The laser modifiability of the banknote paper can be produced, for example, by a coating of the paper that is locally removed or changed in color by the laser impact. The laser modifiability can also be produced, for example, by laser-modifiable substances introduced into the volume of the paper, such as effect pigments.

The laser hole 14 shows a complex design that is difficult to imitate, with a central opening 22 passing through the carrier 20, a narrow colored border 24 running around the opening 22, and with a plurality of colored pentagons 26 that register precisely on protuberances of the central opening 22 are put on. The color impression of the pentagons 26 is the same as that of the colored edge 24, for example a red color impression against a white background. As a special feature, the pentagons 26 are formed with different line widths, from a narrow, fine line 26-1 at the top of the opening 22 over several successive increasing line widths 26-2, 26-3, 26-4 up to the bold lines 26-5 of the pentagons lying at the bottom of the opening 22.

In order to be able to produce such a complex laser hole 14 at high production speed, the laser beam is guided over the paper with a rapidly variable focus size and/or focus position when the laser is applied to the banknote paper 20, as explained in more detail below. As already explained in more detail above, the term "fast" describing the change in the focus size or focus position always refers to the feed rate at which the cutting or marking laser beam is guided over the carrier.

figure 3 FIG. 1 schematically illustrates the structure of a laser processing device 30 with a focus size and focus position of a laser beam that can be changed quickly. The essentially parallel laser beam 32 of a laser source 34, for example a CO ₂ laser, is first expanded with a beam expander 36, and the expanded laser beam 38 is thereby generated. The expanded laser beam 38 can, as left in 3 shown, can be a parallel laser beam, but it can also be set slightly convergent or slightly divergent by the beam expander 36 (beams 38', 38" further to the right in 3 ). The expanded laser beam 38 is deflected and focused by a scanning device 40 and the deflected laser beam 42 is pivoted in the processing plane via a mirror arrangement of the scanning device, as shown in FIG 3 indicated schematically by the arrows 46. In 3 The focal length f of the focusing device, the focus F of the laser beam, the focus size F _G and the focus position F _L of the deflected laser beam 42 are also shown schematically. The carrier to be processed is typically arranged in the focal plane 44 of the focusing device at a distance of a focal length f from the scanning device 40 .

If the expanded laser beam 38 is a parallel laser beam, as left in 3 shown, the focus F generated by the scanning device 40 lies in the focal plane 44 of the focusing device and generates a laser spot of focus size F _G at the processing location.

If the expanded laser beam 38' is set to be slightly convergent, as shown diagrammatically on the right below in 3 shown, the focus F of the deflected laser beam 42' generated by the scanning device 40 is slightly above the focal plane 44, so that the spot size of the laser in the focal plane is slightly larger than the focal size F _G . If the widened laser beam 38" is set slightly divergent, as schematically shown in the following on the right 3 shown, the focus F of the deflected laser beam 42" generated by the scanning device 40 is slightly below the focal plane 44, so that the spot size of the laser in the focal plane is also slightly larger than the focal size F _G here.

Although the focus size F _G remains almost the same with these changes in the focus position F _L , a different spot size in the focal plane 44 and thus at the processing location is achieved by shifting the focus position F _L . The laser beam 38, 38', 38" can be set parallel, convergent or divergent by small changes in a lens in the beam expander 36. Since the required variations are very small, they can be made very quickly in relation to the feed rate of the laser beam. In addition, in particular a linear axis in the beam expander 36 can be used.

In addition to the focus position F _L , the focus size F _G can also be changed quickly, namely in particular by the extent of the beam expansion in Beam expander 36. Since the numerical aperture of a focusing lens increases with its illumination and the minimum achievable focus size FG is inversely proportional to the numerical aperture, a smaller focus diameter _FG can be generated with a more widely expanded laser beam 38 _‴ , as shown schematically on the far right in 3 illustrated. The widening of the laser beam 38, 38‴ can also be adjusted as desired by small changes in the beam expander 36 and can therefore be carried out very quickly.

Coming back to the presentation of 2 the outline of the through opening 22 of the laser hole 14 is cut with a highly focused laser beam of high energy density. The laser source 34 is controlled in such a way that the energy density of the laser beam 42 in the central area of the laser spot at the processing location exceeds the threshold energy Ecut required for cutting the paper carrier 20 . In a narrow edge area of the laser spot, the energy required for cutting is no longer exceeded, but the energy density is above the threshold energy E _color <Ecut required for a color change in the feature area 12 . Outside of this narrow edge area, the carrier is neither discolored nor cut. In this way, when the opening 22 is cut, a circumferential colored edge 24 is produced at the same time, which is perfectly matched to the outline of the opening 22 because it was produced during the cutting process.

To produce the pentagons 26, the energy of the laser beam is also reduced in the central area to a value between the threshold energy E _Color for a color change and the threshold energy E cut required for cutting. At the same time, with one or more of the related 3 Measures described the spot size of the laser beam 42 varies at the processing location, so that the threshold energy E _Color in different large areas is exceeded, and thus differently wide colored borders 26-1 to 26-5 of the pentagons 26 are generated. Since these changes occur very quickly, the production speed of laser writing is practically not reduced by changing the mark widths.

The carrier can also be cut and the color changed with a combined laser beam, which is formed from a superimposition of two laser beams that are initially separate and set independently of one another in terms of convergence/divergence and beam width. This allows the width of the cut openings and the width of the markings adjacent to the opening to be adjusted independently.

figure 4 1 shows a schematic of a laser processing device 50 with a cutting laser 52 and a marking laser 54, the laser beams of which are provided for cutting or marking the material to be processed. The laser beams of the lasers 52, 54 each pass through an associated adjusting device 56, 58, in which the respective laser beam is expanded and its convergence properties are adjusted, as is fundamentally the case with FIG 3 described. The beam profile 72 of the adjusted cutting laser beam 62 and the beam profile 74 of the adjusted marking laser beam 64 are in 4 shown schematically as insets.

The cutting laser beam 62 and the marking laser beam 64 are combined with the aid of a beam combiner 60, which can be formed by a beam splitter working in reverse, to form the desired processing laser beam 66, the beam profile of which is 76 in 4 is also shown schematically as an inset. The beam profile 76 of the processing laser beam 66 has a central area Z of high energy density, which is used to cut the substrate, and an outer region A of lower energy density, which is used to produce a color change in the substrate.

The width of the central area Z and the width of the outer area A can be adjusted independently of one another with the aid of the adjustment devices 56, 58. When the carrier is subjected to the laser beam 66, the cutting width and the marking width can therefore be changed independently of one another.

As already in connection with 3 explained, the corresponding changes to the laser beams 62, 64 and thus also to the combined laser beam 66 can be made very quickly, so that the production speed of the laser writing is practically not reduced by changing the cutting width and/or the marking width.

If the cutting width or the marking width is not to be varied for an application, the corresponding adjustment device 56 or 58 can of course also be dispensed with.

Figures 5 and 6 FIG 4 can be generated.

First shows figure 5 a carrier 20 which can be modified by the action of laser radiation and which is provided with a plurality of through openings 80 of constant width. The openings 80 and the space between The opening 80 are provided with a color marking 82 of variable width.

To create this design, the combined laser beam 66 was the 4 guided from left to right over the carrier 20 and the cutting laser beam 62 is temporarily switched on or off with a constant focus size and focus position. At the same time, the focus size or focus position of the marking laser beam 64 and thus also the size of the outer area A in the combined laser beam 66 was changed continuously. The change takes place practically instantaneously, so that large changes in the marking width of the colored marking 82 can be achieved despite a high operating speed of the laser beam 66 on the carrier 20 .

figure 6 FIG. 12 shows a further design in which a carrier 20 which can be modified by the action of laser radiation is provided with an elongated cut opening 84 of variable width. Surrounding the elongated cut opening 84 is a paint marking 86 of varying width in an undulating manner.

To create this design, the combined laser beam 66 was the 4 guided from left to right over the carrier 20 and the focus size or focus position changed quickly for both the cutting laser beam 62 and for the marking laser beam 64 . Accordingly, both the size of the central area Z and the size of the outer area A in the combined laser beam 66 change quickly and independently of one another, so that the cutting width of the opening 84 and the marking width of the colored marking 86 can vary independently of one another.

Reference List

10

bank note

12

feature area

14

laser hole

20

carrier

22

continuous opening

24

colored border

26

pentagons

26-1 to 26-5

lines of different widths

30

laser processing device

32

laser beam

34

laser source

36

beam expander

38, 38′, 38ʺ, 38‴

expanded laser beam

40

scanning device

42, 42′, 42ʺ, 42‴

deflected laser beam

44

focal plane

50

laser processing device

52

cutting laser

54

marking laser

56.58

adjustment devices

60

beam combiner

62

cutting laser beam

64

marking laser beam

66

Processing laser beam

72, 74, 76

beam profiles

f

focus

FG

focus size

FL

focal position

A

outdoor area

Z

central area

Claims (12)

1. A method for manufacturing a security feature for a security element, a security paper or a data carrier (10), in which

   - a carrier (20) is provided whose color impression is modifiable by the action of laser radiation, and

   - by guiding a laser beam (42) across the surface of the carrier (20), on one hand, through openings (22) are introduced into the carrier, and on the other hand, the color impression of the carrier (20) is modified in marking regions (24, 26-1, 26-2,....26-5),

   - when guiding the laser beam across the carrier surface, the focus position (F_L) and/or the spot size (F_G) of the laser beam (42) are quickly changed to produce a variable width of the through openings (22; 84) and/or a variable width of the marking regions (82, 86; 26-1, 26-2, ...26-5),
   characterized in that

   - when guiding the laser beam (42) across the carrier (20), the focus position (F_L) of the laser beam (42) per laser travel path is changed by more than 1 mm/ mm, and/or the spot size (F_G) of the laser beam (42) per laser travel path is changed by more than 10%/mm.
2. The method according to claim 1, characterized in that, due to the marking regions (82, 86; 26-1, 26-2, ...26-5), marked edge regions of the through openings (22; 84) are formed at least in sub-regions.
3. The method according to claim 2, characterized in that the marked edge regions are produced simultaneously with the associated through openings (22; 84) by the same laser beam.
4. The method according to at least one of claims 1 to 3, characterized in that, when guiding the laser beam (42) across the carrier, the focus position (F_L) of the laser beam (42) is quickly changed, namely in that the focus position per laser travel path is changed by more than 2 mm/mm and preferably by more than 5 mm/ mm.
5. The method according to at least one of claims 1 to 4, characterized in that, when guiding the laser beam (42) across the carrier, the spot size (F_G) of the laser beam (42) is quickly changed, namely in that the spot size per laser travel path is changed by more than 50%/mm and preferably by more than 100%/mm.
6. The method according to at least one of claims 1 to 5, characterized in that, prior to the impingement of the carrier surface, the laser beam passes through an adjustment device (36; 56, 58) in which the beam diameter and/or the convergence or divergence of the laser beam are adjusted, the quick change of the focus position and/or the spot size preferably being produced by adjustment variations in the adjustment device (36; 56, 58).
7. The method according to claim 6, characterized in that the adjustment variations in the adjustment device (36; 56, 58) are carried out with the aid of one or more linear axes.
8. The method according to at least one of claims 1 to 7, characterized in that a first laser beam (62) is provided for producing the through openings, a second laser beam (64) is provided for producing the marking regions, in that the first and second laser beam are combined with a beam combiner (60) and the combined laser beam (66) is guided across the carrier surface to introduce the said openings into the carrier and to modify the color impression of the carrier in the said marking regions.
9. The method according to claim 8, characterized in that, when guiding the combined laser beam (66) across the carrier surface, the focus position and/or the spot size of the first laser beam are quickly changed to produce a variable width of the through openings.
10. The method according to claim 8 or 9, characterized in that, when guiding the combined laser beam (66) across the carrier surface, the focus position and/or the spot size of the second laser beam are quickly changed to produce a variable width of the marking regions.
11. The method according to at least one of claims 1 to 10, characterized in that the carrier (20) is furnished with a laser-modifiable marking substance, especially in that the carrier (20) is coated with a laser-modifiable marking substance and/or in that a laser-modifiable marking substance is introduced into the volume of the carrier (20).
12. The method according to at least one of claims 1 to 11, characterized in that the carrier (20) is formed from paper or comprises a paper ply.

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