Conventional decorating methods use a variety of solvents to assist in applying the decorative ink materials. These solvents control the viscosity and rheology of the wet inks and keep them in liquid form during processing. The curing process evaporates the solvent, leaving a stable film adhered to the surface. Heat, an important part of the curing process, accelerates the reaction and increases the adhesion of the ink film to the glass or ceramic substrate.
Aside from visual appearance, adhesion is probably the most important property demanded of a decorative coating on a glass or ceramic surface—and it can also be the most difficult property to achieve. TherMark Corp., headquartered in Los Angeles, Calif., in cooperation with CERAM Research Ltd. and Keele University’s Postgraduate Training Partnership Scheme, both located in the UK, has been working on the development of laser-applied organometallic coatings for decorating tableware. Using new analytical tools to determine what fundamental forces govern the bonding of organometallic and inorganic materials, researchers in these organizations are developing new laser-applied materials to meet both the visual and adhesion demands of the glass and ceramic industry.
The new laser marking materials (LMMs)* being developed through this research are available in liquid form, as a dry transfer tape or label product, and as a dry powder for electrostatic deposition. The liquid LMMs are applied to the substrate surface, dried and then fused to the substrate using laser energy. The laser is operated in a continuous wave mode so that the laser energy is absorbed by the marking material and converted to heat, causing it to permanently adhere to the substrate surface. The laser accomplishes in seconds what would normally require hours in a conventional oven or kiln and consumes only a small fraction of the total energy. Drying the LMM prevents the pigmented material from migrating along the surface of the substrate due to the rapid evaporation of the water-based solution during the laser firing. Finally, the remaining LMM can be brushed or vacuumed off the surface or washed off with a water spray. The liquid materials can be applied using an industrial spray gun, airbrush, brush, roller, pad printer, inkjet printer or screen printing device. The entire process can be automated using standard, off-the-shelf industrial application and cleaning components.
The dry transfer tape or label LMMs consist of a marking material coated onto a backing such as paper or a plastic film. The coating is slightly self-adhesive and is placed in contact with the substrate surface. When laser energy is passed over the tape, the LMM is transferred and permanently bonded to the substrate. Marking on porous ceramic substrates is an ideal application for these dry transfer tape products because the tapes keep the marking material from soaking into the pores of the ceramic surface. This results in increased contrast by keeping the surrounding area clean, without the overspray associated with liquid LMMs, and also eliminates the need to remove any unfired marking material. This type of LMM is especially useful for small production lots because it’s much quicker than liquid LMMs and doesn’t require preparation or cleaning of industrial spraying equipment.
Although the LMMs can be used with most commercially available CO2 and Nd:YAG laser marking systems, a solid-state, diode-pumped fiber laser offers the most durability and flexibility for industrial marking and decoration due to its simplified design and lack of critical optical alignments. Using fiber-delivered laser energy means that only the beam steering head has to be mounted near the product, greatly simplifying the installation of the laser marking equipment in the industrial environment. Laser marking systems allow the decorator to change the image as needed to perform personalized decorations or to serialize individual items.
Post-fire marks can be made on the back of floor tiles after the glaze has been fired onto the front side of the tile, allowing the manufacturer to apply a glaze and mark a color and/or customer specific code on the reverse side of the tile. Marking tiles in this way eliminates the need for stick-on labels, facilitates manufacturing and distribution, and also serves to better control inventory at the retail level.
Post-fire marking is also currently being used by a major manufacturer of catalytic converters for automobiles to provide permanent product identification and production information markings on the surface of the ceramic cores. The LMM tape is contained in a cassette similar to a typewriter ribbon. The individual ceramic cores are brought into contact with the LMM tape by a robotic handling system, and a message is sent to the laser marking system to write the specific product identification and production information on the product. The robot then removes the ceramic core from the tape surface, the tape advances, and the system is ready for the next part to be presented.
As ceramic and glass manufacturers and decorators seek to make their processes more efficient and environmentally friendly, new decorating and marking techniques will be needed that meet these demands. The new laser marking materials, with their enhanced application speed and performance, will undoubtedly play an important role in this new era.