EP1333976B1 - Method for the production of screen cavities in a rotogravure form and base body applicable in said method - Google Patents

Description

The invention relates to a method according to the preamble of patent claim 1 and a base body of a gravure printing form according to the preamble of patent claim 8.

State of the art

From DE-A 2 218 393 a non-generic method for the production of gravure forms with electron beams is known. In the production of gravure printing forms with electron beams, difficulties were encountered in the production of screen wells with a depth below 10 microns. DE-A 2 218 393 has therefore proposed coating the surfaces to be engraved with electron beams with a separating layer of silver or copper sulfide having a layer thickness of only 0.1 μm. A second layer of copper with a layer thickness of 15 μm was then applied to the separating layer. Separation layer and second layer served only to reduce ablation depth. After engraving with electron beams, where Wells were produced with a depth of up to 20 microns, remained on the subsequent removal of the separation and copper layer only wells with a depth of 5 microns; ie below 10 microns, as desired.

The process described in DE-A 2 218 393 represented a complicated process due to the production of two layers with different materials and working in a vacuum.

From DE-A 30 35 714 a further method for the production of pressure cups for a gravure printing form is known. Here, the still "raw" gravure mold was coated with an etchant-resistant paint. The varnish was then removed with an electronic engraving device at the locations where later sieve cups should be present. The electronic engraving device used was a stylus, a laser beam or an electron beam. After the targeted removal of the paint, an etching process was carried out to produce the sieve cups. The manufacturing process described here was complicated and time consuming.

A method analogous thereto is described in DE-A 2 344 233.

1. 1. Sehr starke Reflexion der Kupferschicht;
2. 2. Hohe Schmelz- bzw. Verdampfungstemperatur von Kupfer;
3. 3. Hohe Schmelz- bzw. Verdampfungswärme von Kupfer;
4. 4. Gute Wärmeleitfähigkeit von Kupfer und damit starke Wärmeabgabe an die Umgebung der Rastemäpfchen.

In EP-B 0 473 973 it is now proposed to carry out a direct engraving of the detent cups for the intaglio printing plate by means of laser radiation, it being pointed out here that laser processing of intaglio printing plates with an outer layer of copper was associated with the following difficulties:

1. 1. Very strong reflection of the copper layer;
2. 2. High melting or evaporation temperature of copper;
3. 3. high melting or heat of vaporization of copper;
4. 4. Good thermal conductivity of copper and thus strong heat transfer to the environment of Rastemäpfchen.

The laser engraving of copper layers also resulted in a projecting ejection crater edge on the well. This edge then had to be removed in a laborious way.

For this reason, EP-B 0 473 973 proposes to produce the wells no longer in copper, but in zinc.

Although the process described in EP-B 0 473 973 can be used, it is disadvantageous in the intaglio printing forms produced therewith that the entire gravure printing technique now once aligned with copper as a material in which the sieve cups are located.

Object of the invention

The object of the invention is to provide a method and to provide a base body in which or on the Rastemäpfchen a gravure mold directly by means of laser radiation preferably in copper, but also in other materials without ejection crater edge, i. burr-free Rastemäpfchen can be produced.

Solution of the task

The object is achieved in that a preferably only a single ablation support layer is applied to the base body via the upper layer regions provided for the information embossing, through which sieve cups are laser ablated by material ablation (vaporization and / or ejection of molten material) into the jacket regions are introduced and then this support layer is removed, whereupon burr-free Rastemäpfchen be obtained. The laser radiation is a temporally modulated in their intensity course radiation. Usually you will use a pulsed radiation, but this is not mandatory. Laser spikes, Q-switch, mode-locking etc. are also possible. Upon removal of the backing layer, there is no change in the snap-in cups in the topsheet regions. The quality of the Rastmäpfchen so produced without burr is so good that without aftertreatment, a hard layer, especially a chromium layer, can be applied. The chromium layer in such intaglio printing plates will preferably be applied with a layer thickness between 4 μm and 30 μm, in particular between 8 μm and 10 μm.

In particular, the burr-free well cups, preferably in copper, can be achieved by selecting the support layer to allow good energy launch for the laser radiation with good ablation of material to the underlying material with minimized directional radiation backscatter. Minimized radiation backscatter is important to prevent radiation from entering the laser cavity. This would namely amplified there and could cause damage to the optical components. A good energy coupling of the laser radiation is important, because then only a small amount of radiation remains, the even for a return reflection yet could be considered. On the other hand, a good energy input causes a strong heating of the material of the support layer. Once the support layer has become liquid, there is virtually no need to worry about radiation absorption.

If one now selects this material of the support layer still such that at its essential material content of the melting point is low, the high radiation absorption occurs quickly. However, the melting point should definitely be lower than that of the underlying upper layer material, in which then the Rastemäpfchen lie. If the Rastemäpfchen are in copper, the melting point should be below 1083 ° C. Only at the melting point would the metals be 961 ° C silver, 660 ° C aluminum, 1063 ° C gold (which, however, would immediately be cost-off), gallium and germanium 937 ° C, indium 927 ° C, lead at 327 ° C, tin at 232 ° C, zinc at 419 ° C, etc. Reasonably usable, however, are only materials whose vapors are not harmful to health, otherwise would have to be made large expenditure for a vapor extraction. By a substantial proportion of material of the layer material is meant a percentage which causes the above-mentioned property. Depending on the material, a substantial proportion of material is likely to be between 80% and almost 100%.

The material of the backing layer is intended to cause a material removal in the underlying material carrying the printing information. That is, it should be done by the introduced with the laser radiation local heat energy as quickly as possible reproduced melting of the underlying material. As tests have shown, this reproducible melting is only given if the layer thickness of the Abtragungsunterstützungsschicht is everywhere the same thickness. If this is the case, precisely the cell volume to be generated can be specified via the irradiated maximum pulse intensity and the pulse shape. The cell volume can be determined most simply experimentally. Good results have been found in copper as an information-bearing layer and zinc as Abtragungsunterstützungsschicht at their layer thickness between 1 .mu.m to 15 .mu.m, preferably between 5 .mu.m and 10 .mu.m with a Schichtdicketoleranz of less than 10 ^-3 , preferably of better than 5 × 10 ^-5 , A zinc layer with such accuracy is best applied by electroplating.

It was also found by experiments that the material of the ablation support layer should have the highest possible vapor pressure. The "background material" ejected from the information-carrying layer by the laser pulse, which liquid still falls on the support layer, causes it to melt and evaporate and is then thrown away by the steam with a further loss of heat. The vapor pressure of the "base material" should be at least five times smaller than that of the material lying thereon. If left in the above example, zinc has a vapor pressure about 100 times higher than copper.

The material of the ablation support layer should be well removable, especially chemically, without attacking the information-bearing cladding regions.

The wavelength of the laser radiation used is to be adapted to the absorption of the material of the ablation support layer. Also, the wavelength has to be adapted to the dimensions of the sieve cups to be produced in accordance with the optical imaging laws. For Rastemäpfchen with a diameter greater than 10 microns, a CO ₂ laser (wavelength 10.6 microns) can be used. For small diameters, it is preferable to use a Nd: YAG laser (1.06 μm). A pulse shaping as well as an optical setup for the beam guidance of the laser will preferably be carried out as described in EP 00 810 552.0. If an Nd: YAG laser is used, zinc has also proved to be a material of the ablation support layer in this case as well.

The ablation support layer not only initiates a material removal in the underlying material, it also causes a turn-on delay for the drilling operation in the underlying layer. The laser pulse has thus already risen to a higher intensity value compared to its initial pulse value, which results in a drilling intensity increase. This results in a good, i. a hemispherical sieve cup shape.

Further advantages of the invention and the variants are also apparent from the text below.

Hereinafter, an embodiment is given, which, however, according to the above embodiments in a wide range and material is variable.

From the following detailed description and the totality of the claims, there are further advantageous embodiments and feature combinations the invention.

Brief description of the drawings

Fig. 1

einen Querschnitt durch den efindungsgemässen Grundkörper in vergrösserter Darstellung mit einem Rastemäpfchen erzeugenden, gepulsten Laserstrahl,

Fig. 2

einen zu Figur 1 analogen Querschnitt, wobei hier die Abtragungsunterstützungsschicht entfernt ist und

Fig. 3

einen zu den Figuren 1 und 2 analogen Querschnitt, wobei hier eine Hartschicht aufgebracht ist

The drawings used to explain the embodiments show:

Fig. 1

a cross section through the eferungsgemässen base body in an enlarged view with a Rastemäpfchen generating, pulsed laser beam,

Fig. 2

a similar to Figure 1 cross-section, in which case the Abtragungsunterstützungsschicht is removed and

Fig. 3

a cross section analogous to Figures 1 and 2, wherein here a hard layer is applied

Basically, the same parts are provided with the same reference numerals in the figures.

Ways to carry out the invention

Metallic rotary gravure printing plates are usually constructed of several functional elements. As the main body 1 is usually a steel cylinder. On the steel cylinder, a copper layer 3 is applied with a thickness of several Millimetem The copper layer 3 is the information-carrying intaglio printing plate. The information consists of an arrangement of a plurality of Rastemäpfchen 5, which absorb the color required for the print. To increase the resistance, a chromium coating 7 having a typical thickness of about 10 μm is applied as the uppermost layer

The printing information is now introduced directly into the copper layer 3 in the upper layer region 8 with a beam 9 of a pulsed Nd: YAG laser by material removal. In order to achieve this direct material removal with burr-free Rastemäpfchen 5 , the copper surface 11 is galvanic with a zinc layer 13 as ablation support layer a small thickness tolerance (less than 5 · 10 ^-5 ) provided. Burr freedom is a prerequisite for perfect quality in the printing process.

The laser pulse 9 for generating in each case a detent cup 5 pierces the zinc layer 13 while melting . Solid zinc has an absorption for the radiation 9 of the Nd: YAG laser of approximately 50%. Furthermore, solid zinc shows virtually no directional reflection. Does the zinc go because of its relatively low melting point and its low thermal conductivity in relation to copper in the liquid state, there is a nearly 100 percent radiation absorption. There is a strong local heating of the zinc, which further passes in the absorbent state to the underlying copper, whereupon this also goes into the liquid state. Copper is now of almost 100 percent reflection for the radiation 9 of the Nd: YAG laser (the reflection but does not come to effect, since the copper is still covered by zinc) in the solid state, now liquid in an approximately 100- percent absorption.

The copper material ejection or that of the zinc 15 lie on the zinc layer 13 and can easily be removed by chemically detaching them in a subsequent cleaning process. The exposed engraving (Rastemäpfchen 5) in copper 3 is burr-free and can be easily chromed.

With the thinly applied zinc layer 13 is now an economic, direct, burr-free laser engraving in copper 3 has become possible. In particular, zinc prevents the melting from adhering, reduces the initial reflection for the laser radiation 9 and therefore allows an efficient drilling process in copper 3.

Of course, the method just described is not limited to zinc 13 as a copper coating. As stated above, a number of other materials are possible. Also, the ablation support layer to be applied to copper 3 does not necessarily have to be a metal layer. Non-metals are also suitable, provided they have the required properties regarding absorption, directional reflection and melting point.

Instead of just a single ablation support layer 13 , multiple layers may be superimposed. However, it has proven the only zinc layer 13 for cost reasons and because of the ease of handling.

The main body 1 of a gravure form does not necessarily have to be cylindrical; it can also be semi-cylindrical, flat or otherwise shaped.

Claims (10)

1. A method for the production of a rotogravure form carrying screen cavities (5) as print information, the form preferably comprising a rotation-symmetrical base body (1), by means of time-modulated, particularly pulsed laser radiation (9), characterised in that an erosion support layer (13) is deposited on the base body (1) over its upper-layer regions (8) provided for information engraving, through which the screen cavities (5) are produced in the upper-layer regions (8) with the laser radiation (9) by means of material ablation, and this erosion support layer (13) is subsequently removed, after which burr-free screen cavities (5) are obtained.
2. The method according to claim 1, characterised in that only a single erosion support layer (13) is deposited, and after its removal a hard layer (7), in particular a chromium layer (7), preferably with a layer thickness between 4 µm and 30 µm, in particular between 8 µm and 10 µm, is deposited and the layer regions (8) provided for the information engraving are preferably made of copper.
3. The method according to claim 1 or 2, characterised in that the support layer (13) is selected in such a way that it enables good energy coupling for the laser radiation (9) with good material erosion initiation for the material (3, 8) lying thereunder at the same time as minimised directed radiation backscattering.
4. The method according to any one of claims 1 to 3, characterised in that the support layer (13) is deposited with a thickness that is constant up to a tolerance, in order that the screen cavity depth can be produced with a preselectable, reproducible form factor by an adjustable energy, or more precisely a time-modulated intensity course of the laser radiation (9), and that the support layer (13) is deposited with a thickness between 1 µm and 15 µm, in particular between 5 µm and 10 µm, especially galvanically, with particular regard to a layer thickness tolerance of less than 10^-3, preferably less than 5.10^-5.
5. The method according to any one of claims 1 to 4, characterised in that a material with a high vapour pressure, preferably with at least one that is higher by a factor five than copper, is selected for the support layer (13), and in particular the support layer (13) can be readily removed, in particular chemically, without attack of the information-carrying layer regions (8).
6. The method according to any one of claims 1 to 5, characterised in that a substantial material proportion of the support layer (13) is selected in such a way that it has a low melting point, preferably below that of copper, in particular below 500°C, and in particular is a metal, in particular zinc.
7. The method according to any one of claims 1 to 6, characterised in that a laser radiation (9) with a preferred wavelength between 0.8 µm and 11 µm, preferably the radiation of a CO₂ laser, especially in the case of screen cavities in the micrometer range the radiation of an Nd:YAG laser, is used.
8. A base body (1) of a rotogravure form, in the upper-layer region or regions (8) whereof screen cavities (5) are produced as print information by means of time-modulated, particularly pulsed laser radiation (9), with a method according to claims 1 to 7, characterised in that each upper-layer region (8) is covered with a single, removable layer acting as an erosion support layer (13), through which the screen cavities (5) can be produced with the laser radiation (9), a substantial material proportion of the support layer (13) is metal, and the support layer (13), compared with the surface region or regions, enables better energy coupling for the laser radiation (9) with good material erosion initiation for the material (3) lying thereunder at the same time as a minimised directed radiation backscattering.
9. The base body (1) according to claim 8, characterised in that the layer regions (8) provided for the information engraving are made of copper and the support layer (13) has a thickness between 1 µm and 15 µm, in particular between 5 µm and 10 µm, with a layer thickness tolerance of less than 10^-3, in particular less than 5.10^-5, whereby the support layer (13) is a material with a high vapour pressure, preferably with at least one that is higher by a factor five than copper, and in particular the support layer (13) can be readily removed, in particular chemically, without attack of the information-carrying layer regions (8).
10. The base body (1) according to claim 8 or 9, characterised in that a substantial material proportion of the support layer (13) has a low melting point, preferably below that of copper, in particular below 500°C, and in particular is zinc.

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