Fluorescent effects are often combined with an optical brightener, which then glows with a high level of colour saturation under black light. These effects might be used in a disco setting or haunted house.
In this type of application, an optical brightener is first added to a clear ink. When it is printed, it is almost invisible, but then when a black light is present, the chosen area glows. For this reason, optical brighteners are often used in security printing, such as hidden marks in identification (ID) documents.
Optical brighteners are also used in special window graphics that appear clear to people, but can be seen by birds, thus helping prevent them from colliding into buildings. And white inks use optical brightener to appear either whiter or bluer.
Metallic powders and pastes
The most common special effect additives are essentially metals made into powders and pastes. They vary in shade, surface sheen and sparkle.
Metallics tend to be added to clear inks, rather than intermixed or tinted. The particle size helps determine the mesh count for screenprinting. Effective mesh openings range from less than two times and up to three times the size of the flake. A larger opening provides a better transfer and less filtering.
Metallics include aluminum/silver, copper, rich gold, pale gold and rich-pale gold products. Those with the highest level of sheen tend to be leafing; i.e. the flake comes to the surface of the ink, providing a shinier appearance. Unfortunately, this tends to inhibit adhesion to overprinted inks.
Metals react to oxygen, slowly tarnishing or changing colour as the powder or paste ages. A new container of metallic can therefore be expected to exhibit a different colour than an older can. The same is true for prints.
Metallic powders also tend to separate from ink easily, either floating or sinking during the printing process. This can result in uneven printing, which is especially noticeable in flood coats or large areas.
Another disadvantage is metallic powders and pastes react with the ink. Some can have a pot life of weeks or months, but typically, they are added to ink at the time of use, much like fluorescent powders. Also, metallic flakes are opaque, so they can block UV light from penetrating the ink film, which may result in incomplete curing.
As metallic powders are classified as combustible, they face potential hazardous shipping requirements. Despite these disadvantages, metallics remain very popular. Indeed, prices are rising with increases in demand, so it can be worthwhile to consider alternatives.
Large-flake powders added to screen inks are considered ‘glitter.’ They exhibit their highest level of sparkle in sunlight or high-power directional lighting. These powders can provide metallic, holographic, transparent, sparkling, pearlescent, interference and multi-chromatic effects.
The larger flakes mean printing consistently can be difficult. A coarse mesh limits the degree of detail. Print shops that use extremely large glitter tend to resort to printing an adhesive or glue first, then shaking the glitter onto that surface, rather than using a stencil.
If a sign shop is accustomed to fine meshes and detail, then sourcing coarse, wide meshes and developing and printing with thick stencils can be a challenge. Also, cleaning all of the glitter flakes out of a screen is very difficult, even with solvents. A shop may well end up with ‘glitter only’ screens and squeegees.
Metallic/holographic glitter powders come in a range of colours, from gold, silver and copper to blue, green, purple and red, and are mainly made from polyester. Some are fluorescent. The more chemically resistant the flake, the more stable it will be in the ink.