The more uniform the ink film, the better the adhesion, and the smoother the printing surface, thereby allowing for better reflectivity and better brilliance.
Metallic screen-printing inks are typically formulated using two components: metallic pigment and varnish. They are either sold as separate components or as ready-mixed, one-part products. Pigments start in copper, zinc, or bauxite mines. Harvested ores are atomized in fine specks, ground, polished in mills, and classified according to end usage.
As a general rule, the coarser the particle size, the more reflection or brilliance—and the harder the transfer. Coarse pigments offer more surface exposure, which increase light reflection. Conversely, finer particles are easier to transfer, but they offer less brilliance. A coarse flake is preferable for garment printing, while the finest pigments are ideal for printing optical discs.
By using finer pigments, the screen printer sacrifices some of the brilliance but gets better coverage. The challenge for the pigment manufacturer is to make a pigment as fine as possible with high coverage, high brilliance, and a tight particle distribution.
Safety can be jeopardized when silver/aluminum, as professionals call it, is in powder form. Aluminum reacts with water/humidity and releases hydrogen, which explodes. As a matter of fact, a common application for coarse aluminum pigment includes explosives for slurry mines, rocket fuel, and fireworks. It is extremely dangerous in the powder form and should be kept in a temperature-controlled, dry location to prevent explosion.
Fortunately, aluminum/silver is safer in a paste or ink form, and all producers of aluminum powders also have their product available in paste form, whether it is for solvent, water, UV or any other type of screen-printing ink. It is strongly recommended, for safety reasons, that screen printers not have any silver/aluminum powder in the shop, but instead keep silver/aluminum paste.
Chemistry and end results
The chemistry of gold and silver flakes is critical to understanding the results when printed. The pigments are usually coated with some type of fatty acid for processing purposes and to allow the pigment to rise to the surface of the ink film, thus making a nice, flat, shiny, reflective surface. However, flakes that set at the surface of the ink film will flake or rub off easily. A tape test confirms this.
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