FR2666759A1 - Device for ablating, through the transparency of a plastic film, using a laser, and applications to spraying and to marking - Google Patents

Abstract

The invention relates to the methods making it possible to ablate, according to a pattern, through a plastic film, a layer, for example a metal layer, using a laser and, moreover, to exploit the consequent spraying of material for printing, displaying or gilding. The face (2) made of plastic is illuminated by a laser (1) pulse of very short duration and conveying sufficient energy density to cause heat-up of the layer at constant volume, which gives rise to a very intense shock-wave causing, upon reflection off the layer (3) to be ablated, the descaling and ejection of the latter, the support layer (2) remaining intact. The invention can be used both for demetallising flexible or rigid supports so as to produce capacitors, printed circuits or optical masks, and for spraying material so as to carry out printing or gilding or to produce large-area remotely-chargeable erasable display devices.

Description

- TECHNICAL DESCRIPTION
The present invention relates to methods for locally stripping a plastic film from a film of different nature coating it, and to marking methods in general.

More specifically, the invention makes it possible to strip a plastic film of a layer, for example a metallic layer, using a pattern, using a laser, and moreover to exploit the projection of - consecutive material to make printing, display or gilding.

Among the many known methods of stripping a surface of a layer of different nature coating it, there are those which only allow to strip large surfaces to the exclusion of any pattern (except if using a mask ), and those which make it possible to produce a real localized and repeatable pickling pattern without requiring masking, processes to which the invention is related.

The first processes can call upon a simple mechanical action consisting in shearing the layer to be eliminated using a cutting tool. It can advantageously be accompanied by a prior chemical attack which will in particular have the effect of detaching the layer to be eliminated from its support in order to facilitate its evacuation. The use of chemical attack is found in the so-called photochemical etching processes, which make it possible to produce pickling patterns when they are used in conjunction with masks, these are the well-known processes of screen printing and microlithography: the parts are protected not to be stripped by a resin, before carrying out the chemical treatment.

We finish by eliminating the resin, under which is then the layer of integral material. This process is qualified as photochemical for its principle of layer protection: a photosensitive resin is deposited over the entire surface of this layer which will polymerize under the action of ionizing radiation (visible light, UV, X- soft, electron beam). A mask is generally inserted in order to achieve selective illumination of the resin in order to obtain a pattern. Once the exposure has been carried out, the uncured resin is removed by dissolution.

Other processes exist, such as those which use a purely mechanical action, among which we find pickling by ultrasound and the more recent pickling by laser. Pickling by ultrasound is used in particular in certain industrial cleaning machines and consists in exposing the layer to be stripped to the compression waves generated by a transducer, by immersing it in a liquid coupling medium. The pickling by laser consists, for its part, in locally evacuating a layer of material by sending a pulse of high surface energy for a very short time, so as to cause on the surface of the layer to be scoured an extremely rapid heating at almost constant volume . The latter gives rise to a shock wave which will propagate in the layer, until it meets the surface of the support, endowed with a very different impedance. The reflection of this wave on the surface of the support causes a return wave which will have the effect of bursting the surface layer and therefore causing its ablation. The actual stripping is therefore a purely mechanical phenomenon and not thermal or chemical.

Unlike the ultrasonic process, it allows for pickling patterns. The process which is the subject of the present invention is similar to the latter in many respects (use of the laser under similar emission conditions without being identical, principle of mechanical ablation), it however differs therefrom. essential and in fact constitutes a radically different process, as we will see below.

All these methods have a certain number of drawbacks and impossibilities.

Among the disadvantages, let us quote the necessary recourse to masking for some of them (serigraphy, microlithography), which induces many difficulties, the heaviness of these same processes (numerous successive manipulations, importance of the equipment, duration of the process) , and the not very ecological character of the rejected products. Laser stripping also suffers from serious drawbacks, among which are the fouling of the focusing or beam deflection optics by projections of stripping products, which can cause problems in certain applications where there is only a little space at the right of the layer to be stripped. In the chapter of impossibilities, let us quote for the screen printing the incompatibility of certain supports (plastic films) with the chemicals of stripping and the fact that we must limit to simple part shapes (flat, cylindrical, ...). Pickling by ultrasound imperatively requires a liquid coupling medium, which in the best of cases requires the use of complex equipment, and which most often makes the process inadequate when the object to be stripped does not support water (or any other liquid) or that the process in which pickling takes place does not allow the object to come into contact with this liquid. Finally, all of these processes assume physical access from the face of the film to be stripped, which prohibits its use in cases where this access is impossible.

The device according to the invention makes it possible to avoid these drawbacks and impossibilities since it does not require a mask, heavy equipment or complex manipulations. It also does not reject toxic products and does not require a coupling medium. Finally, it allows intervention through the layer which supports the film to be stripped, therefore by transparency. This feature in fact solves the problem of the interaction of material projections from the pickled layer with the energy source. The device according to the invention also makes it possible to project material for marking purposes.

The invention relates to a pickling and material spraying device characterized in that it comprises:
.a laser (1) suitable for periodically sending a pulse of given geometrical, energy and time parameters,
.a semi-absorbent material (2) for the wavelength of the laser (1) receiving the pulse and in which a shock wave will arise, without this material being affected,
.a material (3) intimately linked to the material (2), and undergoing the effects of the shock wave in the form of flaking and ejection of material.

According to another characteristic, the materials (2) and (3) are in the form of sheets or layers.

According to another characteristic, the laser (1) or the ray coming from the laser (1) can be controlled in position by any means (4) such as for example galvanometric mirrors controlled by computer, so that the beam strikes the material (2) in particular places.

According to another characteristic, the material (3) is a metal layer deposited by any means such as for example vacuum evaporation, on the layer (2), itself made of low-ductile plastic material, making it possible to use the device for obtaining a surface of material (2) pickled in places.

According to another characteristic, the layer of material (2) is made of polyester and the layer of material (3) is of aluminum, so as to obtain, after passage of the laser beam, a polyester-aluminum strip desaluminized in places which can be used in the capacitors .

According to another characteristic, the layer of material (2) is made of resin such as advantageously epoxy, and the layer of material (3) is made of a metal that is a good electrical conductor such as advantageously copper, so as to obtain, after passage of the laser a resin plate supporting metal tracks constituting a printed circuit.

According to another characteristic, the thickness of the material layer (2) is of the order of a micron, and in that the laser used emits radiation of short wavelength, such as advantageously an excimer laser, of so as to obtain, after application of the laser pulses, a very precise mask which can be used in the integrated circuit industry.

 According to another characteristic, the material (3) is a layer of any material which is sufficiently adherent to the layer (2), itself made of plastic material which is not very ductile, and in that it also optionally comprises a device for driving this two-layer assembly in front of the laser (6), making it possible to use the device for projecting material (3), in order to make the coating or the marking.

According to another characteristic, the material (3) is a colored substance deposited by any route on the layer (2), and in that it also comprises:
any support (5), such as advantageously a sheet of paper or a plastic film, placed in line with the layer (3) so as to receive the splashes of colored substance,
.suitable means (7) such as rollers for starting the support (5) while scrolling in front of the two-layer assembly (2 & 3),
. possibly heating means (6) such as advantageously infrared lamps or heating resistors, making it possible to fix the colored substance on its new substrate, for example by fusion, the whole constituting a printer.

According to another characteristic, the layer of material (3) is made of gold, making it possible to use the device to make precise, localized gilding, of adjustable thickness, on any objects, even non-metallic.

According to another characteristic, the layer of material (3) consists of pigments, such as for example colored oxides, and in that it also comprises a heating means such as for example an oven or an oven, making it possible to melt these coloring substances after application and make them adhere to the support on which they have been deposited, the whole constituting a means of precisely decorating any objects.

According to another characteristic, the layer of material (3) is metallic, and in that it further comprises means (8), such as for example an arc maintained between two electrodes, making it possible to carry the projected particles of material ( 3) at their melting point in the interval separating the layer (3) from the object (9) on which these particles will be deposited, in order to metallize the surface of the object.

According to another characteristic, the layer (3) is made of magnetic material (for example based on chromium oxide Cr203), and in that it also comprises:
any light-colored support (10) reflecting light well, for example white, such as advantageously a painted metallic film - a metal sensitive to the magnetic field is chosen - on which the magnetic particles will be fixed,
possibly means (11) allowing the support (10) to circulate in front of the bi-layer assembly (2 & 3), such as for example rollers,
.means (12) for cleaning the support (10), such as advantageously electromagnets, all constituting an urban display means for drawing in real time images or texts, for example advertising or advertising municipal information, of large dimension, and to erase them before posting the following image.

According to another characteristic, the device further comprises means such as for example a vacuum pump and a sealed enclosure in order to achieve a relative vacuum in the space separating the layer (3) from the support (5, 8, 9 or 10 ).

The characteristics and advantages of the invention will emerge more clearly from the description which follows, referenced in the appended drawings, in which:
FIG. 1 represents the absorption profile of the incident energy in the materials (2) and (3) in the case where the latter is reflective,
FIG. 2 represents the absorption profile of the incident energy in the materials (2) and (3) in the case where the latter is non-reflective and for a material (2) which is not very absorbent.

FIG. 3 represents the absorption profile of the incident energy in the materials (2) and (3) in the case where the latter is non-reflective but where the material (2) is very absorbent,
FIG. 4 represents the absorption profile of the incident energy in the materials (2) and (3) in the case where the latter is non-reflective but where the incident power is reduced.

FIG. 5 represents the basic device used in pickling in which the beam deflection device has not been detailed for the clarity of the drawing,
FIG. 6 represents the basic device used in projecting material for printing purposes,
FIG. 7 represents a section of the material projection device, coupled to heating means during flight of the projected material,
FIG. 8 represents a display device by projection of magnetic particles, and the principle of erasure.

These figures are given for information only, in order to better understand the principle of the device, they are in fact in no way limiting and therefore do not detract from the generality of the invention. In addition, the various elements have not always been represented according to their actual proportions when this does not interfere with the understanding of the figures.

We will use one of the applications of the device to describe its underlying physical principles, which again does not detract from the generality of the invention.

Let us therefore consider a two-layer assembly formed, to fix ideas, of a polyester film on which we have come to deposit by evaporation under vacuum a thin film of any metal which can be for example aluminum. This assembly is illuminated on the polyester layer side by a laser beam - for example a solid YAG laser, or, better, a CO2 laser - generating very short pulses (a few nanoseconds). The choice of laser is guided by the fact that the layer (2) - polyester in this case - must be semi-absorbent for the wavelength considered. If the specific energy deposit E following partial absorption of the beam by the layer (2) is produced for a sufficiently short time T, this layer does not have time to expand (on the scale of the durations considered) and the material (2) is almost in conditions of constant volume heating. For this, the duration of the pulse must be much less - for example ten times less - than the time it takes for the expansion wave to go from the surface of the layer (2) to the interface separating the two layers. :
e
this being the thickness of the layer (2) and c the speed of sound in this same layer. If for example the thickness of the film is 10 ym, and the speed of sound of 1000 m / s, we must have 2 10 ns. Take for example T = 2 ns. Under these conditions, the layer (2) is heated to an almost constant volume and is compressed at very high pressure. The value of the pressure obtained on the interface is given by the expression:
P = pyE where p is the density of the layer (2), y the Gruneisen coefficient (approximately 2 in MKS units) and E the specific energy absorbed by the layer. Consider for example that the latter absorbs only 10% of a pulse transporting 0.1 mJ, and that this interaction takes place on a surface of 0.1 × 0.1 mm2 and over the entire depth of the layer (2), or 10 clam, that is to say in a mass of material of 10-10 kg, the pressure will then be 2.108 Pa, or 2.000 bars. This is more than enough to take off the layer of the polyester layer and to tear it off.

soit 25e dans l'hypothèse (pessimiste) où la totalité de l'énergie est convertie en énergie mécanique, ce qui est tout à fait tolérable.The heating of the polyester layer must be reduced in order to avoid its destruction (in particular the glass transition temperature must not be exceeded). This heating is worth:

ie 25th in the (pessimistic) hypothesis where all of the energy is converted into mechanical energy, which is completely tolerable.

The four figures in Plate 1 make it possible to understand the mechanism of energy deposition. In the case where the layer (3) is metallic, the total absorption by reflection of the wave on the interface is increased, which amounts to doubling the energy deposit in the layer (2). It is therefore possible to accommodate a material (2) which is not very absorbent, or a beam of lower power. This situation is described in Figure 1. The minimum energy E required in the beam is:

Pmin being the pressure from which the layer (3) flakes, has the absorption coefficient of the layer (2), S the interaction surface and e the thickness of the layer (2), the whole expressed in MKS units. The maximum admissible energy is obtained when the pressure in the layer (2) is high enough to cause the destruction of this same layer by a process analogous to that, classic, of direct pickling by laser. This pressure is generally greater than Pmin, but it is still all the more quickly reached when the support film (2) is less ductile and therefore the specific energy is less dissipated by plastic deformation mechanisms.

In the case where the layer (3) is absorbent, for example non-metallic, it risks destroying itself before the shock wave strikes it, which can have the consequence on the one hand of causing the heating of the layer (2) by contact, therefore destroying the support, and on the other hand volatilizing the material (3), making it unusable for making projection of material as our device provides. This situation is described in FIG. 2, in which the illumination parameters are the same as those in FIG. 1. It can be seen that the energy deposition in the layer (3) is greater than that which it can accept without vaporize (hatched area) .This application therefore requires that very little energy reaches the absorbent layer (3), therefore that the material (2) is very absorbent (Figure 3), or that the laser pulse transports a small amount of energy (Figure 4), knowing that the minimum energy required is worth:

As for the maximum admissible energy, it is the same as for a metallic material (3), with the additional constraint that this layer does not sublime by absorption of the residual energy.

An interesting feature of this pickling process is that it does not destroy the chemical structure of the layer (3): the layer is transformed into particles propelled at high speed, but these particles have the same structure as the layer itself, it are neither fused nor dissociated. It will be noted that the conventional laser etching process does not have this feature and that the material projections have undergone a transformation. The advantage is that one can use this process to carry out an impression by coating a support (2) with a layer of the material which must form the impression, which one will come to strip with the help of a laser .

Experience shows that the materials are projected in a very narrow cone, of slight divergence, which makes it possible to project the material for marking, printing and coating purposes, very precisely. It will again be noted that the known method of laser pickling in which the layer to be destroyed is directly attacked by its free face does not make it possible to make a coating, since the material is projected towards the optics, which has the consequence to require the illumination of the layer (2) under oblique incidence, so as not to foul the optics, thereby reducing the coupling and inducing great design difficulties.

The applications of the device according to the invention are numerous, they are encountered in the field of pickling or engraving as in that of projection of material and printing. The following list gives a simple overview of the potential of the invention, it is therefore in no way limiting:
pickling and etching:
The electronic industry is the first to benefit from this process. Let us cite for example the local demetallization of polyester films coated with aluminum, in order to produce capacitors, or the demetallization of a resin plate (for example epoxy) coated with copper, in order to produce printed circuits. This principle can also be used to manufacture optical masks for the lithography and microlithography industry.

.projection of material and printing:
Figure 2 illustrates what a desktop printer using this principle could be schematically. A support (2) previously inked circulates in front of the print head formed by the laser scanning device. The latter can advantageously be a laser diode. A sheet of absorbent paper is entrained in front of this assembly and is impressed by the projections of ink (then optionally dried using any means 7). A microprocessor manages the synchronization of the mechanism (6) for advancing the support with that (4) for scanning the beam and with the advance of the sheet (5), in order to optimize the ink consumption of the support (2) The support film (2) can either be consumable, that is to say that the ribbon is discarded after use, or else recyclable, that is to say re-inked in the machine itself. The first solution seems to be simpler if not more economical.

We can also take advantage of the principle of metal projection to deposit gilding. This chemical element lends itself better than another to the metallic coating since it is very ductile and will take the form of the object to be covered. Two devices are possible: the first, which implements a scanning (4) and a film advance (6), and the other which would appear in the form of a small hand tool, without scanning, which would project a metal spray on demand in the desired direction. This latter device would allow for touch-ups on site. With this principle, it is possible to deposit gilding on objects of all kinds, both metallic and non-metallic. This projection process is not limited to gold, it can also be used to make tin, silver, platinum or cadmium coatings.

In another application, the particles projected are melted “during flight” using for example an electric arc maintained between two electrodes (8).

The advantage of this device is that it allows deposits of very diverse materials, even very little ductile, to be produced.

It is finally possible to create erasable displays, used in municipal information panels found in major cities. One of the methods consists in depositing on a screen (10) of a clear surface a magnetic and opaque material (3) according to the desired pattern, this material (iron or chromium oxides for example) previously coating the support layer (2), molten by the means (6) already encountered. So that the screen retains these magnetic particles, it can for example be metallized or drown a metal strip therein. The cleaning of the screen is obtained by scanning its surface by an electromagnet (12) which will collect the magnetic particles. -The screen (10), animated by the means (11), scrolls successively in front of the transfer station, the display front and the erasing station. In addition to the display of municipal information, this device can be used in the field of advertising display of complete messages containing photographs, with a resolution at least equal to that of conventional four-color processes, and with the addition of total versatility.

All these methods using the projection of material probably require a vacuum pump and a vacuum enclosure around the free space between the layer (3) and the object to be marked in order to increase the range of the jet and to decrease its divergence.

Claims (14)

- CLAIMS - 1. Pickling and material spraying device characterized in that it comprises:  .a laser (1) suitable for periodically sending a pulse of given geometrical, energy and time parameters,  .a semi-absorbent material (2) for the wavelength of the laser (1) receiving the pulse and in which a shock wave will arise, without this material being affected,  .a material (3) intimately linked to the material (2), and undergoing the effects of the shock wave in the form of flaking and ejection of material. 2. Device according to claim I, characterized in that the materials (2) and (3) are in the form of sheets or layers. 3. Device according to claims I and 2, characterized in that the laser (1) or the ray coming from the laser (1) can be controlled in position by any means (4) such as for example galvanometric mirrors controlled by computer , so that the beam strikes the material (2) in particular places. 4. Device according to claims l, 2 and 3, characterized in that the material (3) is a metal layer deposited by any route such as for example vacuum evaporation, on the layer (2), itself made of slightly ductile plastic material, allowing the device to be used to obtain a surface of material (2) pickled in places. 5. Device according to claims 1,2,3 and 4, characterized in that the material layer (2) is polyester and the material layer (3) aluminum, so as to obtain after passage of the laser beam a ribbon location-de-aluminized polyester-aluminum that can be used in capacitors. 6. Device according to claims 1, 2, 3 and 4, characterized in that the layer of material (2) is made of resin such as advantageously epoxy, and the layer of material (3) is made of metal which is a good electrical conductor such advantageously copper, so as to obtain after passage of the laser a resin plate supporting metal tracks constituting a printed circuit. 7. Device according to claims 1, 2, 3 and 4, characterized in that the thickness of the layer of material (2) is of the order of a micron, and in that the laser used emits radiation of short length wave, such as advantageously an excimer laser, so as to obtain, after application of the laser pulses, a very precise mask which can be used in the integrated circuit industry. 8. Device according to claims 1, 2 and 3, characterized in that the material (3) is a layer of any material sufficiently adherent to the layer (2), itself of little ductile plastic material, and in that it also optionally includes a device for driving this two-layer assembly in front of the laser (6), making it possible to use the device for projecting material (3), in order to make the coating or the marking. 9. Device according to claims 1, 2, 3 and 8, characterized in that the material (3) is a colored substance deposited by any route on the layer (2), and in that it further comprises:  any support (5), such as advantageously a sheet of paper or a plastic film, placed in line with the layer (3) so as to receive the splashes of colored substance,  .suitable means (7) such as rollers for driving the support (5) in movement in front of the two-layer assembly (2 & 3),  . possibly heating means (6) such as advantageously infrared lamps or heating resistors, making it possible to fix the colored substance on its new substrate, for example by fusion, the whole constituting a printer. 10. Device according to claims 1, 2, 3 and 8, characterized in that the material layer (3) is gold, allowing the device to be used to make precise, localized gilding, of adjustable thickness, on any objects, even non-metallic. 11. Device according to claims 1, 2, 3 and 8, characterized in that the material layer (3) consists of pigments, such as for example colored oxides, and in that it further comprises a means of heating such as for example an oven or an oven, allowing these coloring substances to be melted after application and to make them adhere to the support on which they have been deposited, the whole constituting a means of decorating any objects precisely. 12. Device according to claims 1,2,3 and 8, characterized in that the layer of material (3) is metallic, and in that it further comprises means (7), such as for example a maintained arc between two electrodes, allowing the projected particles of material (3) to be brought to their melting point in the interval separating the layer (3) from the object (9) on which these particles will be deposited, in order to metallize the surface of the object. 13. Device according to claims l, 2, 3 and 8, characterized in that the layer (3) is made of magnetic material (for example based on chromium oxide Cr203), and in that it further comprises:  any light-colored support (10) reflecting light well, for example white, such as advantageously a painted metallic film - a metal sensitive to the magnetic field is chosen - on which the magnetic particles will be fixed,  possibly means (11) allowing the support (10) to circulate in front of the two-layer assembly (2) & (3), such as for example rollers,  .means (12) for cleaning the support (10), such as advantageously electromagnets, all constituting an urban display means for drawing in real time images or texts, for example advertising or advertising municipal information, of large dimension, and to erase them before posting the following image. 14. Device according to claims 1,2,3,8, and 9 or 10 or he or 12 or 13, characterized in that it further comprises means such as for example a vacuum pump and a sealed enclosure in order to create a relative vacuum in the space separating the layer (3) from the support (5, 8, 9 or 10).