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Getting into Ink

It's not an exaggeration to say that ink is the lifeblood of the screenprinting industry. So, once you've mastered fundamentals like getting ink to stick to your substrate, it's really worth your while to get to know a little more about ink itself.

By Bill Stephens

Fortunately, there is a lot of information available. Every ink manufacturer provides some sort of brochure about their products, and discussions about ink turn up in almost every screenprinting trade journal and forum. Within this free library you'll find information that can not only help you improve your present screenprinting operation, but can provide you with ideas about additional screenprinting opportunities you might want to consider.

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  • You will, however, be encountering a few unfamiliar ink-related terms. Information sources are often aimed at experienced audiences so they sometimes overlook the need to supply basic definitions. That's what we're going to do here.

    Adhesion and Cohesion: Factors that make ink work
    A screenprinting ink has to stick to two things: First of all, it has to stick together enough to make it a printable substance, and after being printed, it has to stick to the surface of the substrate. Cohesion describes the ink's ability to hold together and adhesion refers to its power to stick to a different material like a substrate. Printing is possible because the ink wants to adhere to a different material more than it wants to stick to itself.

    Ink has only two basic components: pigment, which provides color and a vehicle, which provides the ink's cohesive and adhesive properties. Pigments are finely ground powders derived from various, mostly inorganic, compounds. They are not dyes. Dyes, which are also occasionally used in screenprinting inks, dissolve in the vehicle; pigments do not. Pigments remain in solid form both in the ink and on the substrate.

    Supporting the pigments in a printable medium and getting them to adhere to the substrate is the job of the vehicle, the liquid portion of the ink. Sometimes referred to as the carrier, or the base, the vehicle is a combination of binders and solvents. Solvents not only have to keep the ink liquid enough to be printable, but they also have to control how the ink dries. Solvents must evaporate slowly so that the ink does not dry in the screen, but once the ink is printed, those solvents must dry quickly so that drying time doesn't slow down production. Almost all solvents used in inks that dry by evaporation are volatile substances and flammable to some degree. Water-based inks are the exception, but they also take the longest to dry.

    The binder is a solid, or heavy liquid that provides body to the ink. Its most important job is getting the pigment to adhere to the substrate. Binders are film-forming resins like ethyl cellulose and nitro cellulose, which are used in most screenprinting inks. Some product literature will call them film-formers rather than binders.

    The vehicle undergoes changes as the ink dries. Solvents evaporate, leaving only resins and pigment on the surface of the substrate.

    According to the dictionary, rheology is the study of the properties and behavior of fluids, specifically how they flow and react to pressure. When screenprinters use the term, it applies only to ink, and what they are usually talking about is an ink's viscosity.

    Viscosity is how a liquid flows. Liquids with low viscosity flow quite easily; liquids with high viscosity are thick and tend to stay put. Viscosity can change. Inks, for example, react to a number of factors from outside temperature to the force of the squeegee.

    Probably no other single characteristic influences how ink prints as much as its viscosity. If you work with different types of screenprinting inks, you will soon discover that the viscosities of screenprinting inks can vary widely.

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    The long and short of it
    Screenprinters often refer to inks as being either short or long. A short ink tends to drop in a blob like a spoonful of gravy. A long ink is stringy like honey. All screenprinting inks will seem short when compared to the inks used in other types of printing, but screenprinting inks evolved not from ink but from sign paints and still retain some paint-like qualities. Generally, a short ink is what you want for screenprinting.

    Thixotropy refers to the tendency of some liquids to run more freely after being shaken or stirred. The viscosity of a thixotropic liquid is lowered when it is agitated. The more you stir it the thinner it gets. In screenprinting this term refers almost exclusively to ink. The plastisols used in printing T-shirts and other fabrics are thixotropic inks.

    Shear is force applied to a material. When referring to ink, shear describes the force applied to an ink either by stirring or by the movement of the squeegee. Shear will be of greatest concern to printers printing with a thixotropic ink, because the force of the squeegee can reduce the viscosity of the ink and change its printing characteristics.

    II. Additives
    Manufacturers know that sometimes a screenprinter will have to make adjustments to the ink to adapt it to special conditions, so they supply a range of compatible additives designed to alter an ink's basic characteristics. Many of these additives are actually ingredients already present in the ink. All the printer is doing is modifying their proportions.

    But caution should be the rule when you begin to modify your ink. The additives that can be used with a particular type of ink and the proportions that can be safely added will be detailed in the product information supplied by the manufacturer.

    The proportions for additives are often given as a range rather than a single figure and usually in the form of a percentage of the total mix by weight. The purchase of an accurate electronic scale is very useful for obtaining consistent results in mixing in additives, and is essential when you get into mixing colors. Color formulas -- for matching Pantone colors, for example -- are given by weight.

    Once an ink has been modified, any leftovers must be carefully labeled with the additives used and the percentages. You now have separate versions of that ink and you don't want to mix them up. Mistakes are easy to make and unfortunately are common.

    Manufacturers tend to supply inks at their highest printable viscosity, because it is easier to thin an ink than it is to make it thicker. For this reason thinners are probably the most commonly used type of ink additive.

    A thinner's job is simply to lower an ink's viscosity. But you have to be careful. By altering an ink's viscosity you can change almost everything about it: how it flows through the mesh, how it dries, and how well it adheres to the substrate. The addition of significant amounts of thinner can make an ink more transparent or make it dull. Add too much and the ink begins to break down.

    Thinners often include several different solvents, and manufacturers have developed specialized thinners to match every kind of ink they produce. When purchasing thinner, it is important to get the right one. To be on the safe side use only the specified thinner and never add more than the recommended proportions. Thinners from other manufacturers may not be compatible, and using inexpensive thinners from a paint store is definitely not a good idea.

    Fast thinners and slow thinners
    Inks that dry by evaporation depend on thinners already in the mix to control their drying speed. But the temperature of the surrounding air can profoundly affect their evaporation rate. In summer, when outside temperatures rise, inks can dry so quickly that they clog the screen.

    One solution is to by mix up a thinned-down version of your ink before you start printing, and add that when you have to replenish your supply. The thinned ink will blend in with the ink already in your screen and the extra thinner will help replace the thinner it has lost to evaporation. When temperatures rise especially high, an even better solution is to replace your normal thinner with one specifically designed to evaporate more slowly. These are called slow thinners or retarders. Some printers automatically add retarders to their ink anytime they will be printing fine details or when they expect to be printing at a slower speed.

    Transparent bases
    Almost all types of ink incorporate a transparent base in their mix, but transparent base is also available as a separate additive. Transparent base makes an ink less opaque without greatly lowering its viscosity, useful whenever you want more of a background color to show through. In addition to making inks more transparent, however, they can also make a glossy ink dull and affect an ink's leveling ability, leaving it with a rough surface. Some caution is advised.

    Thickening agents
    Thickening agents are most often used when printing on highly absorbent substrates. Thickening agents act like transparent base, but do not reduce the density of the ink. They usually require some form of machine stirring and are probably best used by experienced printers. Thickening agents can also affect an ink's gloss and it's leveling ability, and can make an outdoor ink less weather- resistant.

    Now that you're equipped with some of these basic definitions, you're ready to tackle some of those technical ink-related articles. Remember that there are many types of ink out there, but the ones you need to focus on are the lacquers, enamels, or epoxies. Those are the ones most often used in sign printing.

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