Screen Printing Metallic Inks

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.

User safety
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.

The size of the particle influences how much or how easily it will flake off a finished garment or graphic. The finer the particle, the greater the adhesion and the less flaking or rubbing off. In the silver, non-leafing pigments are available and have been used extensively. These particles sink to the bottom of the ink film, which improves adhesion but sacrifices brilliance, as the pigment is not allowed to neatly align itself at the surface of the ink film and reflect light. Some of the golds are also available in nonleafing, but they haven't been as popular or as successful.

Outdoor applications pose a challenge in the form of oxidation and discoloration. There are pigments for screen printing in gold and silver that are specifically treated to resist tarnishing and oxidation. They’re more expensive and, in some cases, are similar to those in the automotive industry and can therefore be exposed to sunlight, salt spray, and more. However, the golds typically are not as tarnish resistant as the silver and, in general, are not leafing. Testing for suitability is an absolute necessity to ensure that the final product will meet customer specifications.

The copper and zinc in gold metallics will oxidize, losing brilliance and, in certain cases, turning green. Aluminum/silver tends to become dull and whiter. This is why color consistency can be hard to achieve. Because the metal flakes are sitting next to each other, on top of the ink film, this leaves small gaps between the flakes—gaps that are bigger when the flakes are coarser. After printing, the presence of these little gaps will cause the substrate or any ink under the metallic layer to affect the resulting color of the print. For example, a blue underneath a silver will give a bluish cast to the silver; a yellow or green underneath a gold will change the shade of the gold.

Another factor influencing brilliance is the purity of the resin used by the ink manufacturer. Purer or clearer ink provides more brilliance. In addition, looking at a metallic print objectively can be very difficult because of the flip-flop effect. Here, looking at the metallic print from different angles can produce results that may not be as visually appealing or desirable as what is seen when looking at the print straight-on.

Substrate selection
Selection of the substrate is critical when screen printing metallic inks. An absorbent substrate will absorb the clear, especially if the ink has low viscosity, leaving the pigment on the surface of the substrate and making the metallic silver look more like a gray and the gold more like a brown. If you absolutely must use an absorbent substrate when printing metallics, then consider printing an underbase to seal the surface. In a similar manner, a topcoat can be used to prevent the metallic ink from rubbing or flaking off; however, this process will reduce reflectivity or brilliance.

Color creation and matching
Purposely oxidized pigments can be used, for example, to create yellow/lemon or reddish-orange in gold metallics. The golds also come in tinted shades such as green or purple. Other inks are used to tint silvers to make imitation golds—making silver look like gold without the oxidation problem. Similarly, imitation golds can be made more easily and are most cost efficient when a transparent color—yellow, red, green, or blue ink—is used on top of a silver substrate, like a silver foil or a silver CD-ROM. This can give the appearance of a metallic without having a real metallic ink, a process also used by many beverage companies to create gold cans, which are really aluminum with a transparent yellow ink.

Color-matching systems feature a range of metallics. They’re made by using the base colors rich, pale, and silver and tinting them with red, yellow, or orange ink. The challenge is that some of the red, yellow, and orange pigments used in the inks to match the selected color system attack the metallic pigments and react negatively. Therefore, the ink manufacturer has to be extremely careful in formulating these shades.

By using a color-matching system, you will increase coverage but sacrifice brilliance because of the colors added to the base pigment to create a match. The same is applicable when using a metallic selected from a color-matching system, as those books use only a few colors to make all of the metallic colors. Consider staying with the rich, rich pale, and pale golds and the silver to maximize brilliance.

If you purchase the metallic ink in two parts, there are other variables to take into consideration. First, mix the two parts carefully and slowly. A high-speed blender or mixer will create heat, which will oxidize the gold and break the metal flakes, turning the silver to gray and the gold to brown—and ruining brilliance and reflection.

Properties when printed
The proportion or ratio of pigment in relation to clear varies according to pigment sizes of the flakes and properties sought. Follow recommendations from the ink manufacturer. Expect some trial and error. Cost, adhesion/rub, and transfer properties make using the minimum amount of pigment in the mix attractive. Putting in more paste/powder does not automatically create more brilliance. Actually, the smaller the amount of pigment, the more uniform the ink film, the better the adhesion, and the smoother the printing surface, thereby allowing for better reflectivity and better brilliance.

Screen printing layer upon layer of metallic inks does not make the finished graphic more reflective. You want to find the balance between pigment load and optimum coverage. A little pigment goes a long way. Ready-mixed, one-part inks alleviate this issue.
 

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