Ink
Basic Ingredients of Ink
Printing inks, in one form or another, have been around since long before Johann Gutenberg invented printing with movable type. Three hundred years before Christ, the Chinese were using inks to print with wooden blocks. One of the earliest ink formulas included lampblack, glue, and water. This was later improved somewhat by the addition of linseed oil, but it was not until the late 19th century, with the introduction of synthetic oils and man-made resins, that the quality of printing inks was greatly improved.
The specific ingredients used in the manufacture of printing inks are dictated by many factors-the printing process, the ink-drying system, the surface to be printed, etc. -however they can be broken down into three categories: pigment, vehicle, and miscellaneous ingredients (mainly driers and compounds).
Pigment. The fine, solid particles that give printing ink its color. In addition to determining color, pigment contributes to the ink's opacity and permanence. It is also the pigment that determines whether the printed page will bleed if wet. Pigments can be mineral, organic, or dyes.
Vehicle. The liquid ingredient into which the pigment and other ingredients are mixed. The function of the vehicle is to act as a carrier for the pigment, and as a binder to affix the pigment to the printed surface. It is also mainly responsible for the gloss and hardness of the dried ink film. To a great extent the vehicle determines the "body" of the ink; that is, the viscosity, consistency, and flow characteristics (fluidity) of an ink. Inks with rapid flow characteristics are called "long"; those with slow flow characteristics are called "short." The type of printing process and drying system determines the type of vehicle used. Some of the more popular vehicles are linseed Oil, petroleum oils, resin oils, and alcohol.
Miscellaneous Ingredients. One of the most important ingredients that all oxidizing inks must have is a drier. This is usually made from oil, a metallic salt, or a mineral compound and is added to the ink to help it dry more rapidly.
Waxes and compounds are added to prevent set-off and sheet sticking, and to improve scuff resistance. Greases, lubricants, reducing oils, and solvents are used to reduce tackiness and aid penetration and rapid setting. Body gums and binding varnishes help inks print more sharply, improve drying, and prevent chalking. Antiskinning agents reduce excessive drying and skinning on press and in storage. Cornstarch adds body to the ink and prevents setoff.
Because every pigment-vehicle combination behaves differently, the addition of these ingredients is carefully controlled; what is effective in one case may be harmful in another.
Ink and the Printing Process
Each printing process requires an ink with different characteristics. The ink used is also determined by the kind and speed of the press, the surface to be printed, and the end use of the printed piece. Let's examine some of the inks used for the more common printing processes, and the specific properties of each.
Letterpress Inks. Designed to print from a raised surface. Letterpress inks must be tacky and viscous enough to hold to the surface of the plate until printed. In addition to this, each type of letterpress printing press requires an ink with a different combination of ingredients. The platen press uses an ink that does not flow freely and is very tacky; cylinder press inks flow more freely and are less tacky; rotary press inks are the least tacky. Another reason for using longer inks on the rotary press is its high speed; the higher the speed of the press the thinner the inks must be. Letterpress inks can be dried by absorption, evaporation, or oxidation/polymerization.
Gravure Inks. Designed to print from a depressed surface. Gravure inks must be very fluid to fill the thousands of tiny wells, and at the same time have enough body and adhesion to be pulled from the wells onto the paper. Gravure inks should be totally free of hard particles that might scratch the engraved cylinder or plate. The consistency of the ink must be maintained to permit the doctor blade to properly clean the plate and ensure a proper transfer of the printed image to the paper. Gravure inks are quick-drying and are usually dried by evaporation. (They can also be dried by absorption or oxidation.) Because of the highly volatile solvents used in gravure inks, they must be handled with care.
Offset Lithography Inks. Designed to print from a flat surface. "Litho" inks are usually longer and more viscous and heavy bodied than letterpress inks. Offset inks must be resistant to the dampening action of the water used in offset printing. They must also be non-bleeding. Because the film of ink deposited on the surface of the paper is about half as thick as that of letterpress, the inks must be strong in color to compensate. To supply the proper ink for offset, the inkmaker must know if it is to be used on a one-color, two-color, or four-color press, and also the order in which the colors are to be printed. Offset inks are usually dried by evaporation, oxidation polymerization, or penetration.
Screen Printing Inks. Designed to be forced through a mesh screen onto a wide range of surfaces such as paper, cardboard, metal, ceramic, and glass. Screen printing inks are short and buttery. To ensure good adhesion, the binder must be changed to suit the surface being printed. The thickness of the ink film is controlled by the mesh size. To avoid clogging the screen, it is important that the solvents used in the ink do not evaporate too rapidly.
Flexography Inks. Another form of letterpress printing which employs rubber plates and aniline inks designed to print on a wide range of surfaces, including paper, cellophane, plastic, and metal foil. Flexography ("flexo") is extremely popular in the packaging industry; the ink colors are bright, strong, opaque, and can be made resistant to light and abrasion. Flexography inks are very fluid and fast-drying and can therefore be printed at high speeds. Most flexography inks have an alcohol base and are dried by evaporation; others have a water base and are dried by either absorption or evaporation.
Letterset Inks. Just as letterset is a combination of letterpress and offset lithography, letterset inks are a combination of letterpress and offset inks. Letterset inks are transferred from printing plate to blanket to paper on a special offset press that does not require the use of a fountain. Because no fountain solution is used, there is more latitude in the vehicle that can be used in the making of the ink. The thickness of the ink film is similar to that used in regular offset printing. Letterset inks are dried in the same way as letterpress and offset inks.
Ink-Drying Processes
Before inks dry they "set." Setting is the initial stage of the drying process in which the printed sheets, though not fully dry, can be handled without smudging. The ink is considered dry when the ink film is converted to a solid state and is absolutely dry to the touch.
Perhaps the best way to understand the ink-drying process is to think of the ink as a fresh coat of paint on a board. Part of the paint is absorbed into the wood, part of it gels (polymerization), and the rest dries by evaporation.
Drying is brought about by any one or more usually by a combination-of the following: absorption, evaporation, oxidation/polymerization, and precipitation.
Absorption. Also called penetration. The ink used is usually quite thin, and the vehicle, which is non-drying, is absorbed by the inner fibers of the paper, where it stays damp for some time. Meanwhile, the pigment remains on the surface. This is the ink-drying process used by most newspapers, which explains why one's fingers are always so dirty after reading a newspaper.
Evaporation. Also called volatization. Used in gravure and flexography: the ink is dried principally by the evaporation of the vehicle from the ink, leaving behind a solid film of pigment. The evaporation drying process is used with or without heat. Screen printing and die stamping inks, as well as heatset ink, are also dried by evaporation.
Oxidation/Polymerization. A two-step ink-drying method used for most offset and letterpress inks. Oxygen is absorbed by the drying, oil, portion of the vehicle (oxidation), followed by a cross-linking of molecules (polymerization), causing the ink film to gel and then harden.
Precipitation. A drying method in which the printed piece is subjected to water in the form of steam, a fine mist, or merely the moisture in the atmosphere. The water moisture causes the binder to be thrown out of the solution (precipitated), binding the pigment firmly to the paper. Commonly used for steam-set and waxset inks.
Specialty Inks
Specialty inks are the result of the ever increasing demand for brighter colors, higher speeds, and new printing techniques. The following are some of the more commonly used specialty inks.
Cold Set. Unlike all other inks, cold-set inks are solid at room temperature, but melt at 1500 to 2000. The printing plate and the press must first be heated to melt the ink. When printed on the relatively cold paper, the ink immediately reverts to its solid state. There is no smudging, no set-off and the printing is almost tackfree. Coldset inks produce sharp printing results, however they require a complicated heating and cooling system.
Fluorescent. Available in a rather limited range of a few reds, yellows, blues, and greens. Fluorescent inks are bright and vibrant, but they require two passes to achieve full color. Day-Glo is the brand of fluorescent pigment whose formula is the most widely used today. Fluorescent inks are most widely used for screen printing signs.
Heat Set. A fast-drying ink that permits large, high-speed, high-quality runs. To use heat-set inks, the press must be equipped with a heating unit, cooling rolls, and an exhaust system. As the printed sheet passes over the heating unit the solvent is evaporated, leaving only an ink film, which dries almost immediately. Used for printing almost any job involving long runs.
High Gloss. Made possible by the development of a vehicle that permits only a minimal penetration of the paper's surface and ensures a maximum gloss, For the best results and highest possible gloss, the paper used should be coated or cast-coated stock with a surface resistant to the penetration of the vehicle. The drying of highgloss inks should be done with the least amount of heat in order not to reduce the gloss. Available for both letterpress and offset.
Magnetic. These inks are made with special pigments which can be magnetized after printing. The printed characters can then be recognized by electronic reading equipment. In order to be successful, the printing must be of high enough quality to meet the rigid requirements of the equipment. Magnetic inks are used a great deal for such jobs as bank checks and business forms.
Metallic. Metallic powders such as bronze, gold, aluminum, and copper are suspended in the vehicle, which also serves to bind the powders to the surface of the paper. When preparing gold inks (the pigment is actually bronze powder), the bronze powder and vehicle must be mixed just before printing because the bronze has a tendency to tarnish rapidly once the ink is mixed. Metallic inks are also available in a metallic paste, which makes far less of a mess.
Moisture Set. These inks are created in such a way that when the printed surface is subjected to water, usually in the form of steam or a fine spray, the binder is thrown out of the solution (precipitated), binding the pigment to the paper. Moisture-set inks are relatively odor free and are popular with the food-packaging industry for such things as bread wrappers, milk containers, and paper cups. Moisture-set inks dry principally by precipitation.
News. Generally used on web presses for printing newspapers, these inks have a fluid consistency that must be adjusted to the type of press, press speed, and stock. News inks are made of inexpensive raw materials (mineral oils and carbon blacks) and are generally dried by absorption. As newspaper is uncoated and has a rough surface, it permits the ink to penetrate readily.
Ouick Setting. Primarily designed to prevent set-off. After the ink has been printed the vehicle rapidly penetrates the paper, leaving a film of ink that dries almost immediately on the surface. This permits printing at high speeds and also reduces the time required before printing the reverse side of the printing surface. Quicksetting inks, which have a high gloss, are most effective when printed on enamel or cast-coated stocks, and they may be used for both letterpress and offset.
Scuffproof or Rubproof. Hard-drying inks made particularly for labels, cartons, box wraps, or any printed pieces that can be marred by rubbing against one another or against the sides of a shipping container.
Watercolor. Generally used to print wallpaper, greeting cards, and novelties. The vehicle permits the use of pigments or dyes in the preparation of this type of ink. Watercolor inks require special rollers on press and can be washed with water after printing.
Wax Set. A specially prepared vehicle permits this ink to set and dry immediately when immersed in a bath of molten wax. It was developed to fulfill the need for faster printing and handling of bread wrappers and other wax papers.