Offset printing inks – Basic
Offset printing inks are compounded especially for use on offset presses. They must be able to withstand reaction with the press fountain solution it encounters on the dampened offset plate. Ideally, the ink on the ink roller should not absorb water (water-in-ink emulsification), nor should the ink break down and combine with the fountain solution on the non-printing areas of the plate (ink-in-water emulsification). Either of these emulsification problems will tend to impair the body, color, or drying qualities of the ink, or cause tinting on the non-printing areas of the plate and printed sheets.
The offset printing inks used on an offset printing press must be able to carry the full-intended color and covering power to the paper despite the split-film action. This occurs because the offset blanket picks up only a portion of the ink from the plate and delivers only a portion of that to the paper. The film of ink reaching the plate, then, is very thin, and the ink must be able to show its full color and opacity with this film.
COMPOSITION OF OFFSET PRINTING INK
Ink is made of three main ingredients: Pigment, which is the coloring material in the ink; Vehicle, which is the liquid that holds the particles of pigment; and Modifiers, which control the drying of the ink as well as other factors such as smell, scuff resistance, and fading.
- PIGMENT: There are two basic types of pigment used in offset printing inks.
Organic pigment, which is made from carbon, is used for making black ink. Inorganic pigments, which are made by mixing various chemicals together, are used for colored inks. For example, sulfur, silica, or china clay can be combined with either soda ash or sulfate salts to make ultramarine blue ink.
- VEHICLE: Vehicle is the liquid that holds the particles of pigment and carries them to the paper. There are two kinds of vehicles used in offset inks: oils such as soya oil or linseed oil (which is a yellowish oil made from flax); and synthetic vehicles, which are liquids resulting from the mixture of chemicals. For example, phenol and formaldehyde mixed together make phenolic resins, sometimes used in printing inks as a vehicle.
- MODIFIERS: Modifiers are ingredients added to the ink to control drying and other qualities such as smell and resistance to fading.
Visual properties of inks are a function of the colorant or pigment, in relation to the vehicle system used. They include color, transparency or opacity, and gloss. By far, the most widely used ink color is black. Then come cyan, magenta and yellow which are used in process printing to create the millions of colors so familiar to us in printed matter. While the physics of color is a highly sophisticated science, in simplest terms color comes from reflected light. White light contains the entire rainbow of colors. When that light passes through a filter or is separated by a prism or raindrop we see the individual colors in the light spectrum. An ink film acts as a filter on the light reflected from the printed surface, e.g., a red ink film allows the red segment of the reflected spectrum to pass through while blocking the rest of the colors. Because printed surfaces vary in color and in reflectance, they, too, will affect the reflected color. Thus, various ink colors printed individually or “trapped” one on top of the other create different filter effects resulting in different visible colors. Similarly, these same ink colors printed on different substrates will result in visible colors that are different yet.
When we refer to ink color, we are most often speaking of hue or shade—whether the ink is red or blue or green or purple. Secondarily, we might describe its strength or saturation, also termed chroma. Thirdly, we might indicate how light or dark it is—a reference to its purity or value. The amount of pigment used affects an ink’s color strength, and the type of vehicle used can affect both the hue and the value of the ink color. The color of the vehicle itself, its ability to wet the pigment articles, and even the chemical interaction between the vehicle and pigment can affect the shade or purity. Finally the color of the substrate, and its drying/absorption properties affect the printed color results.
Ink opacity- ability to hide the color beneath it. Sometimes, an ink with little opacity is needed, such as when overlapping two colors to create a third color. Other times, very opaque ink is needed to completely cover any color under it. The opacity must be suited in the use of the ink. Opacity is tested by spreading a sample of ink with an ink knife over a wide black line printed on a sheet of paper. The amount of covering is then compared to a standard to determine if the opacity is correct.
Ink transparency- refers to the opposite of opaque. A transparent ink does not hide the color beneath it, but mixes with it to create a third color. All inks used to print full color work must be transparent. The choice of colorant and the degree to which it is dispersed through the vehicle are the most important factors in determining the transparency or opacity of an ink.
Gloss – refers to an ink’s own ability to reflect light, and depends upon the lay or smoothness of the ink film on the substrate surface. Generally, the higher the ratio of vehicle to colorant, the smoother the lay, and the higher the gloss. Application of a thicker ink film tends to improve gloss while penetration into the substrate tends to reduce.
Runnability is a term unique to printing. It applies to the trouble-free interaction between the ink and the press, the paper and the press, and finally, the ink and paper. Body, temperature stability, length, tack, adhesion and drying all contribute to the runnability of an ink and are primarily a function of the vehicle system used in the ink.
Body – refers to the consistency, stiffness or softness of an ink. Viscosity is a related term that refers to the flow characteristics of soft or fluid inks. Ink body and viscosity requirements vary widely by printing process. In general, letterpress and offset lithographic inks are fairly thick or “viscous” (much like paste or honey). On press, they move through a series of rollers called the ink train where the action of the rollers spreads the ink into a thin film for transfer to the blanket and/or plate and onto the substrate.
Temperature stability – in an ink is important in allowing it to withstand the heat generated by the friction that occurs as the ink moves through the rotating rollers and cylinders. If an ink vehicle is not sufficiently stable, the increased temperature can have a deleterious effect on an ink’s body and therefore on its runnability.
Length - describes an ink’s tendency to form long threads when stretched or pulled. Long inks flow well but form long filaments that have a tendency to sling or mist, especially on high-speed presses. Short inks have the consistency of butter and flow poorly. They tend to build up on rollers, plates or blankets. Inks with the best runnability are neither excessively long nor short.
Tack – refers to the stickiness of the ink, and it must be correct so that the ink will stick to the rollers of the press and not fly off, but still transfer from roller to roller, from roller to plate, from plate to blanket, and from blanket to paper.
Drying - properties of an ink are critical for a number of reasons. The most obvious is that a printed piece cannot be handled or used until the ink has developed film integrity. In addition, however, the way an ink dries can reduce air pollution, improve energy efficiency, and even improve productivity in the pressroom by allowing faster printing and converting. In most cases, the first phase of ink drying is setting; immediately upon being applied to the stock, the liquid portion of the ink begins to evaporate into the air or to penetrate the stock, causing the ink to thicken. Setting is followed by actual drying via one or more possible mechanisms: absorption, oxidation, evaporation, or polymerization. The specific mechanism is determined by the relationship between the printing process itself, the ink vehicle system, and the substrate. Inks that are applied to an absorbent substrate such as newsprint or corrugated board dry by absorption. The liquid portion of the ink penetrates the substrate, leaving an ink film on the surface. Depending upon the printing process, this ink film may undergo additional drying procedures.In oxidation, components in the ink’s oils chemically combine with oxygen in the atmosphere to form a semisolid or solid ink film. It often occurs in combination with absorption. Oxidation can be accelerated by the use of driers in the ink formulation or by the application of heat or infrared radiation to the printed piece. Since non-porous substrates such as plastic films and glass cannot absorb ink vehicles, they require inks that dry either through evaporation or by polymerization (e.g., radiation curing). In the former, vehicle solvents evaporate, leaving resins and other materials behind to bind the pigments to the substrate. Evaporation from the inks must be rapid enough for complete drying, but not so rapid as to cause instability while the inks are still running on press. In radiation curing, all of the components in the ink remain on the surface of the substrate, but are polymerized into a hard film by the use of ultra-violet light or electron beam energy to trigger a chemical reaction. UV-curable inks require the presence of a photo-initiator, while EB-curable formulations do not.
One of the most important qualities of offset printing inks is its color. If the customer orders black ink, there are few problems because black is essentially black. However, if the customer orders yellow-green, it is hard to know exactly what color yellow green she has in mind. To help solve the problem of mixing the exact color the customer wants, the ink manufacturing industry has developed a system of color classification know as the PANTONE MATCHING SYSTEM.
PANTONE MATCHING SYSTEM: The PANTONE MATCHING SYSTEM (PMS), consist of swatch books that contain samples of hundreds of different colors, each identified with a code number and the formulation for each color. In other words, the book tells exactly how much yellow and how much blue are required to make the specific color of yellowgreen ink the customer chose from the book. Printers can obtain these books so they can order the exact color the customer wants.The printer may also use this book to mix ink colors in the plant. The PANTONE MATCHING SYSTEM relys on ten basic colors (PMS black, PMS white, PMS rubine red, PMS rhodamine red, PMS warm red, PMS reflex blue, PMS process blue, PMS green,PMS purple and PMS yellow) which can be kept on hand and mixed using the roportions indicated in the PANTONE book to make the exact color desired. Beside quantities of the basic colors listed above, the printer must also have an accurate scale to measure the quantities of ink required for the color, ink mixing knives, clean-up solvent, and rags. To mix the inks, the exact amount of each required PMS basic color is measured out on the scale (use only PANTONE colors for acceptable results). Then the basic colors are mixed together with the ink knives. The ink is then tested for correct color and, if it is correct, it is placed in cans for use later or put directly into the press fountain. Finally, the mess from mixing is cleaned up with solvent and rags.
VISCOSITY: is the resistance to flow. If ink flows or pours readily, it has a low viscosity. If it refuses to pour at all, it has a high viscosity.Offset inks usually have a relatively high viscosity.
LENGTH: Ink may be “long” or “short.” Test it by tapping a puddle of ink with a corner of an ink knife and lift the knife up. The ink will follow the knife and stretch into a string. The farther it stretches without breaking,the longer the ink. Offset inks are generally long.
TRANSPARENCY: refers to the opposite of opaque. A transparent ink does not hide the color beneath it, but mixes with it to create a third color. All inks used to print full color work must be transparent.
PERMANENT: Permanent inks maintain their color and do not fade even though exposed to sunlight for long periods. They are especially suited to signs and posters. Permanent inks are also called “fast” inks.
FUGITIVE: A fugitive ink is one that tends to lose its color and fade when exposed to long periods of sunlight.
RESISTANT: Resistant inks are made to withstand the action of gases, chemicals, heat, moisture, or scuffing.
LAKES: Lakes are body colors—not particularly strong or bright.
TONERS: Toners are strong colors—highly concentrated.
JOB BLACK: Job black is an ordinary, inexpensive, black ink used for most jobs.
HALFTONE BLACK: Halftone black ink is made especially made for printing the small dots in halftones.
Metallic inks are made with metal powders. Bronze powder in an ink makes “gold.” Aluminum powder makes “silver” ink.
PROCESS INK COLORS: Process inks used to print full color work are: Process Yellow; Process Magenta (red); Process Cyan (blue); and Process Black.
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