Laser marking on a coated substrate

A method of utilizing laser energy to mark on a coated substrate is disclosed, wherein the coating is dried at a temperature below that necessary to permanently affix the coating to the substrate prior to the coated substrate being exposed to said laser energy. The dried coating is selectively removed by the laser energy in precise patterns that can be quickly and widely varied. The method of the present invention can be incorporated as a component of time-critical manufacturing processes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is a method for marking on a coated, non-conductive substrate material utilizing laser energy. The non-conductive substrate may be a glass ceramic material, which is a devitried or crystallized form of glass. Preferably, the non-conductive substrate is a glass, defined as an amorphous, undercooled liquid of extremely high viscosity, which appears to be a solid, most preferably a soda-lime-silica glass suitable for a variety of uses, particularly the manufacture of high-volume automotive glass products, where cost efficiency is an important factor in the commercial viability of a given product. The coating to be applied to the non-conductive substrate may be any suitable enamel, containing a finely ground glass or ceramic frit, and finely powdered lead (although some lead-free enamels are now available, and could be used), such as are manufactured by Cerdec (n/k/a DMC 2 ), Ferro Corporation and others. The enamel may be applied to the non-conductive substrate by any conventional method, preferably by silk-screening. The desired thickness of the coating applied to the substrate is typically in the range of 1.6-2.2 mils, preferably, on the order of 1.8 mils. Certain ceramic enamels thus applied may be dried “naturally,” that is in the ambient atmosphere of the manufacturing facility, however, the varying temperature and humidity conditions encountered, along with the possibility of contamination of the coating by dust, or other airborne particulate matter, does not provide ideal conditions for creating high-quality coatings on the non-conductive substrate. In order to make drying time more predictable and to minimize contamination, it is desirable to accelerate drying of the coated substrate through the use of enclosed heating furnaces or radiant heat dryers. Even though providing accelerated drying, forced air dryers are not preferred as they create a greater opportunity for spreading airborne contaminants on the undried ceramic enamel. In order to integrate the process of the present invention into conventional high-volume manufacturing processes, it is desirable for the drying time to be from 10 to 80 seconds, preferably not more than 60 seconds. Similarly, the temperature necessary to dry the ceramic enamel should be closely controlled to minimize time spent in the drying process. It has been determined that temperatures above 200° F., but less than 1000° F., are desirable. The upper limit is also influenced by the fact that the temperature at which such enamels are “fired,” i.e., become permanently affixed to non-conductive substrate due to verification of the glass or ceramic frit in the enamel, may be as low as 1100° F. After drying of the ceramic enamel is complete, it creates a coating on the non-conductive substrate which may, relatively easily, be removed mechanically from the non-conductive substrate, e.g., by scratching with a metal object such as a nail. The coating is also susceptible to removal by other methods, such as contacting the dried ceramic enamel with laser energy. It has been found that contacting the dried coating with a Nd-YAG laser, such as is manufactured by GSI Lumonics, readily and quickly removes the dried coating, leaving no discernible residue. A Nd-YAG laser, connected to a computer programmed with suitable software, such as AMS 50 Lightwriter, also supplied by GSI Lumonics, enables the formation of intricate and very precise patterns such as bar codes, alpha-numeric codes, logos and the like. Particularly important for use in high-volume manufacturing processes is the ability of such a laser-computer system to produce readily alterable or variable patterns, such as serial number codes, date codes, etc. It has only been through substantial experimentation that the use of Nd-YAG type lasers has been found to be particularly useful for this purpose. While not wishing to be restricted to this theory, it is believed that the energy of the Nd-YAG laser successfully removes the coating from the glass substrate surface without damage to the substrate due to the particular wavelength of the energy produced by the Nd-YAG laser, which affects the structure of the coating, but passes through the glass substrate with no discernible effect. It has further been found that the method of the present invention may be advantageously utilized to mark on coatings on glass substrates which have been heat-strengthened or tempered. By heat strengthening is meant treatment of glass such that its surface compressive stress is greater than 3,500 pounds per square inch (psi), whereas, tempering may be defined as further heat-strengthening of such glass such that the surface compressive stress of such glass is greater than 6000 psi. Known methods of marking on coatings on heat strengthened or tempered glass substrates may pose problems such as weakening the physical structure of the glass. Marking using the present method of laser marking creates no such undesirable effects. Once the dried coating on the substrate material has been marked with laser energy it may be desirable to subject said marked, coated substrate to heating at a relatively high temperature in order to permanently affix said coating to the substrate. Preferably, when subjected to such heating, the non-conductive substrate will be glass, and the coating will be a ceramic enamel as has been previously described. When heated to temperatures in the range of600° F.-1500° F., preferably, 1100° F.-1350° F., most desirable, readily available ceramic enamels will be “fired” and thus, permanently affixed to the glass substrate. As previously alluded to, one of the benefits of the present method is that the drying, marking, and optional firing may be done at a rate compatible with high-volume manufacturing processes, for example, production of automotive glass products. Testing conducted to date has shown that time for the various process steps may be accomplished as follows: 1 Approximate elapsed time a) Application of coating 6 to 10 seconds b) Drying of coating 10 to 60 seconds c) Marking (with Nd-YAG laser) 2 to 60 seconds d) Firing (optional) 200 to 240 seconds Total cycle time (w/o firing) 18 to 130 seconds Total cycle time (w/ firing) 218 to 370 seconds Such cycle times allow the subject method to be performed “on-line” during automotive glass production, rather than as a separate process, or “off-line” which likely would require additional labor and potentially additional equipment for a stand-alone operation. It should be noted that preferred marking times are on the order of 2-10 seconds, which makes the process of the present invention more compatible with present-day on-line manufacturing processes. Marking of certain, highly complex patterns may require more than 10 seconds, up to 60 seconds. Some accommodation in the manufacturing process may have to be made to allow for marking times greater than 10 seconds. By contrast, known methods of marking require substantially longer cycle times, on the order of twice the amount of time described above in connection with the present invention, thus making these methods impractical for most on-line time-critical manufacturing processes. In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment.