The more uniform the ink film, the better the adhesion, and the smoother the printing surface, thereby allowing for better reflectivity and better brilliance.
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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