Lasers are rapidly becoming the method of choice to mark everything from microchips and other electronic components, to auto and aircraft parts, over-the-counter and prescription drugs and packaging, food packaging, television tubes and catalytic converters to metal dog tags, cups, knives, trophies, and an almost infinite number and variety of promotional items.
Currently, the two most commonly used lasers are the Nd: YAG (neodium yterium aluminum garnet) and the CO2. A new solid-state-diode-pumped (SSDP) YAG laser has been developed and is rapidly gaining widespread acceptance. The SSDP YAG is smaller, more efficient and less costly than the traditional Nd: YAG lasers.
Nd: YAG lasers can mark on metal without the assistance of chemical compounds, and the new SSPD YAG can mark on many metal substrates. But neither YAG can mark on glass or ceramic materials. CO2 lasers can mark on glass, ceramics, paper, wood and plastic, but not on metal.
An evaluation of Cerdec's products for use in laser marking and market research revealed a vast potential for laser marking. This led to the formation of the CerMarkT Laser Marking Materials Business Unit.
The company subsequently developed various materials that permit direct, permanent, colored marks on glass and ceramic substrates, laser markable organic coatings for use on circuit boards, plastics, glass, metal or ceramics and laser markable mixed metal oxides and other pigments for use in coatings and plastics. These markings provide color change and high contrast marks that can be read by people or machines.
Marking materials are lead-free, cadmium-free and water-based for quick and convenient cleanup. Currently available in a liquid that can be applied by spraying, brushing, doctor blading, pad printing and roll coating, the water-based materials offer high resolution in addition to easy, convenient cleanup compared to other application methods (which may require the use of solvents).
A tape or sheet-like material is in development that will be dispersed in a resin and applied to a paper or plastic film backing. The material would be fused by the laser to the substrate and then applied and pressed to the substrate surface.
For the second method, the laser is used to remove materials from the substrate surface where it passes. In this process, known as a negative mark or ablation, the laser is operated in a pulsed manner. For short lengths of time, the laser cavity is closed off so that energy can build up. When the cavity is opened up again, the energy bursts out with high "peak power" and knocks the material off the surface.
Positive marks on glass can be made in black, blue, green, white and bronze using liquid materials. These sprayable materials consist of an active material, a rheology controlling material and the solvent (water). The rheological agent helps to prevent sagging of the coating. The best equipment for spraying these materials consists of a "pressure pot" that acts as the reservoir for the marking material and a pressure fed spray nozzle.
In a typical marking operation, the material is first sprayed onto the substrate and dried. Drying the marking material prevents expelling the material from the surface of the substrate due to rapid evaporation of the solvent during the laser firing. Next, the substrate is placed under the laser head for marking. Finally, the unsintered marking material is brushed and vacuumed off the surface or washed off with a water spray.
A glass marking material that fires out clear has been developed. It is applied to the bottoms of glass bottles and then dried. The laser is used to ablate material to form a negative mark. Upon firing of the bottles in the decoration step, the material melts to form a clear coating, leaving the ablated message visible on close examination-without damaging the bottle. Combined with the ability of the laser to easily assign unique numbers to each part, marks such as this can eliminate the need for changing the bottle mold and should reduce the amount of inventory subject to recalls.
Ablation marks can also be made in pigmented coatings, such as those used for beverage containers or auto glass. This method allows automotive windshields to be serialized easily.
The marks can also be formulated to be chemically resistant. Black marks made on glass have withstood concentrated sulfuric acid at 50°C for 5 minutes. The marks last well in basic solution, and as expected, hydrofluoric acid will attack the marks. Future tests are planned for dishwasher durability, weathering and survival in natural acids.
Post-fire marks on the back of floor tile can be made after spraying a marking formulation onto the surface. This marking formulation sinks into the tile and turns black upon irradiation with the laser. This mark, which is applied and marked after the tile glaze has been fired onto the top of the tile, allows the manufacturer to apply a glaze to the tile and mark a color and/or customer specific code on the reverse side of the tile. Marking tile this way eliminates the need for stick-on labels and may serve to help reduce inventory, since the numbering for different customers could be changed on the fly.
Marking on porous ceramic substrates is an ideal job for tape application. The tape or sheet-like materials consist of a marking material dispersed in a resin and applied to a backing material, such as paper or plastic film. The marking material is fused by the laser to the substrate, and the resin helps adhere the marking material to the backing material while providing flexibility to the tape. The tape-like materials can introduce a uniform depth of marking material to the surface for marking, without overspray or mess, especially in small jobs.
The tapes help to prevent marking materials from sinking into the pores of the ceramic surface, serving to increase the contrast by keeping the "white" areas more white. A prospective customer is presently testing tape marks on mullite and high alumina parts. Preliminary testing suggests that these marks survive 1200°C.
It is also possible to ablate material from the surface of ceramics, increasing the versatility of the marking process. A sprayable marking material is applied to the surface of the tile and ablated using the laser. These marks are also capable of surviving high temperature firing.