Method and Apparatus for Multi-Color Laser Engraving

1. Technical Field

This invention relates to a method and apparatus for creating an engraved image and, more particularly, to a method and apparatus for creating a multi-color engraved image using a laser.

2. Background Art

Engraving of anodized aluminum panels by a spot of laser light is well known and is in common use for creating labels, instruments panels, artwork and other uses. Presently known laser engraving usually entails spot sizes between 0.002 and 0.010 inch at power levels between 5 and 50 Watts and writing speeds between about 0.5 and 5 feet per second. The spot is typically moved with a computer graphics system, scanning mirrors or x-y tables. Typically laser engraving has been a white-on-base color process, with the base color most often being black. Base colors other than the commonly used black are possible, particularly red, blue, gold and grey.

It would be desirable to have a system to create laser engraved panels that can provide the much wider range of colors available in paint and silk screening processes. Such a system would then combine the precision, flexibility and speed of laser engraving with the range of base and character colors available in paint-based systems, without the inflexibility and lead times involved in the tooling for paint-based systems.

In the disclosed embodiments, this invention is concerned with creating a multi-color engraved image in anodized aluminum panels using a carbon-dioxide laser, although the laser engraving methods and apparatus disclosed can be modified to be applied to other substrate materials, particularly other metals.

It is well known that an anodized aluminum surface consists of a porous surface having microscopic channels oriented at right angles to the surface. These porous channels can hold a dye and the pores can be sealed by application of heat and water to hydrate the Al.sub.2 O.sub.3. The hydration expands the surface material by the molecular inclusion of water so that the pores are essentially eliminated and any dye contained within the now glassy appearing surface is trapped. I have found that the laser beam not only vaporizes the dye but also removes the hydration, reestablishing the previous affinity for new dyes. The laser engraved areas can therefore be redyed and sealed in multitudes of secondary colors. This process can be repeated to not only create color-on-background but also color-on-color effects.

The preparation of anodized aluminum panels consists of a first step of anodizing using electric current in an acid bath. This creates a porous surface of a few tenths of a thousandth of an inch consisting of alumina, Al.sub.2 O.sub.3. The surface thus created consists of micro-channels running at right angles to the surface. When the surface is subsequently exposed to boiling water these micro-channels can be sealed by creating a hydrated form of Al.sub.2 O.sub.3 and a clear or natural color. This makes the surface less sensitive to contamination and environmentally stable. During the above sealing operation with boiling water, dyes can be introduced to create the familiar black, blue, red, gold or grey shades of anodized aluminum.

It has been found that exposing this surface to focussed intense laser radiation reverses the hydration and vaporizes any dye applied earlier, creating the original white state of Al.sub.2 O.sub.3 with its porous micro-channels and high affinity for dyes.

The surface areas thus converted by the laser can then be redyed selectively in any number of secondary colors by applying dye to individual areas within the image. The applied dye will essentially only be absorbed by areas touched by the laser beam, not the surrounding areas. The precision of the color image is therefore established by the laser marking process, not the dye application. More than one secondary color can be used simultaneously in different parts of the image, by selective manual or automatic application, to produce a multi-colored, color-on-background image, as long as the colored areas are not overlapping. The subsequent sealing can be accomplished when using water soluble dyes by exposure to heat. Overcoats can be used for further protection.

At this point the sealed selected secondary colors can also be reengraved and dyed as before to produce images in a set of tertiary colors as desired, having color on color image effect, rather than the simpler color on background effect as described above. This process can of course be extended to even higher order coloring processes.

It is an object of the present invention to provide a method for establishing a multi-color engraved image on a surface of a substrate material.

It is another object of the present invention to provide an apparatus for establishing a multi-color engraved image on a surface of a substrate material.

It is a further objective of the present invention to provide a process that can work on any type of surface that can be locally and selectively converted in its dye affinity by a laser. For example, one can visualize an absorbent paper type coating or film applied to a substrate. This coating may have a thin plastic protective coating applied to its surface which the laser can remove to expose areas of dye affinity, or the laser could remove the absorbent layer completely to create islands of dye affinity.

According to one aspect, the invention is an apparatus for establishing a multi-color engraved image by means of a laser on an oxidized metal surface. The apparatus comprises means for establishing a surface having high affinity to a dye, for optionally introducing the dye and sealing the surface so that it no longer has affinity for dyes, means for selectively engraving areas of the metal surface by means of a focussed laser beam to reestablish affinity for new dyes, means for introducing at least one secondary dye to the selectively engraved areas, and means for sealing the secondary dyes.

In a further aspect, the invention is a method for establishing a multi-color engraved image having a first color and a second color on an oxidized metal surface having an affinity to a dye. The method comprises the steps of a) applying a first dye having the first color to the surface, b) sealing the surface where the first dye was applied, thereby substantially reducing the affinity of the surface to a dye, c) selectively engraving areas of the metal surface to which the first dye is applied by a laser beam that removes the first dye and substantially reestablishes the affinity, and d) applying secondary dyes to the selectively engraved areas of the metal surface.

FIG. 1 is a schematic diagram of a first embodiment of the laser engraving apparatus of the invention.

FIG. 2 is a schematic diagram of a second embodiment of the laser engraving apparatus of the invention.

It has been observed experimentally that conventional laser engraving of anodized aluminum panels will restore the original affinity of the surface of the metal panel to new dyes introduced selectively to the engraved areas. Surrounding areas not touched by the laser will not absorb the new dye. High precision in the secondary dye process is therefore not required, since laser engraving is a highly precise process. After application of the secondary dyes the panel can be oversprayed using a transparent lacquer to protect and seal the dyes. Because of the extremely small pore sizes of the anodization, a low viscosity dye is necessary. For manual dyeing, a type of water soluble dye found in felt tipped pens has been found useful. Note that the precision of the color image is primarily established by the precision of the laser image--not the precision or skill of the dye application. In this way the invention differs fundamentally from a "painting" process.

FIG. 1 is a schematic diagram of a first embodiment of the laser engraving apparatus 10 of the invention. The laser engraving apparatus 10 is used to engrave a surface 12 of a substrate 14 with any desired characters or other image 16. The laser engraving apparatus 10 includes a laser source 20 producing a beam 22 of laser energy, focusing lens 23, optic means 24 for directing the beam 22 from the laser source 20 onto the substrate surface 12, and control means 26 for controlling the laser source 20 and the optic means 24.

The substrate 14 can be any suitable material, and the surface 12 can be made from any material which has a first affinity for colored dyes before being subjected to the beam 22 of laser energy and a different second affinity for colored dyes after being subjected to the beam 22 of laser energy. It is particularly suitable for the surface 12 to be made from a metal, such as aluminum, having an oxide, such as alumina (Al.sub.2 O.sub.3).

Further improvements in the technique can be obtained by overspraying the anodized and hydrated surface with, for example, an acrylic coating. The laser will vaporize the coating and any dye below it to expose the unhydrated Al.sub.2 O.sub.3. By this method it is possible to create background colors other than those available with anodizing. By this method a white background for subsequent engraving can be achieved. A clear overcoat prior to engraving can be used to further reduce affinity and adhesion of secondary dyes to the surface where they are not wanted.

The optic means 24 may comprise an optical element 30, such as a mirror, for deflecting the beam 22 that passes from the laser source 20, through the focusing lens 23, toward the surface 12. The optical element 30 can be rotated independently about x- and y-axes by stepper motors 32 and 34, respectively, as directed by signals received through the respective cables 36 and 38. The stepper motors are under the control of the control means 26, which may be a computer programmed in accordance with principles well-known to those skilled in the programming art. The control means 26 also controls the laser source 20 through the signal cable 39 by appropriately modulating the intensity of the laser light in the beam 22 between intensities which will not affect the affinity of the surface 12 and intensities which will. If desired, the laser source 20 can be turned off and on by the control means 26. For example, when it is desired to define discrete areas, such as letters on the surface 12, the control means 26 can turn on the laser source 20 when it is forming each area using appropriate manipulation of the mirror 30 by the stepper motors 32 and 34. It can also turn the laser source 20 off when the beam 22 is to be directed from one area to another area of the surface 12 without engraving the space between the two areas.

FIG. 2 is a schematic diagram of a second embodiment of the laser engraving apparatus 10' of the invention. Those features of FIG. 2 which are the same as those in FIG. 1 are given the same reference numerals in the two figures. In the second embodiment, the substrate 14 having the surface 12 is placed on an x-y table 40 which is capable of independent translations in the directions of the x- and y-axes under the control of the control means 26 through the cable 42. The laser source 20 and focusing lens 23 are held in fixed position while the x-y table 40 moves the substrate 14 to change the point at which the beam 22 strikes the surface 12. The control means 26 controls the laser source 20 through the cable 39.

While the detailed description above has been expressed in terms of a specific examples, those skilled in the art will appreciate that many other methods could be used to accomplish the purpose of the disclosed inventive apparatus. Accordingly, it can be appreciated that various modifications of the above-described embodiments may be made without departing from the spirit and the scope of the invention. Therefore, the present invention is to be limited only by the following claims.