Inks for Offset printing-Basic,Composition Visual and Runnability properties

Offset inks are compounded primarily for use on offset sheet feed machines.They must be capable to withstand reaction with the press fountain solution it encounters on the dampened offset plate. Ideally, the ink on the ink roller must not absorb water (water-in-ink emulsification),neither should the ink break down and mix with the fountain solution on the non-printing areas of the plate (ink-in-water emulsification.Either of these emulsification problems will have a tendency to impair the body, color, or drying qualities of the ink, or trigger tinting on the non-printing regions the plate and printed sheets. The ink being used on an offset press needs to be competent to carry the full-intended color and covering power to the paper even with the split-film action. This happens for the reason that offset blanket picks up only a small percentage the ink from the plate and delivers only a portion of that to the paper. The film of ink getting the plate, then,is very thin, and the ink must be competent to show its full color and opacity with this film.
Ink is made from three major substances: Pigment, the coloring material in the ink; Vehicle, the liquid that holds the particles of pigment; and Modifiers, which actually control the drying of the ink as well as additional factors such as smell, scuff resistance, and fading. PIGMENT: There are two basic kinds of pigment used in printing inks. Organic pigment,which happens to be produced from carbon, is used for making black ink. Inorganic pigments, which are produced by mixing various chemicals together,are used for colored inks. For instance, sulfur,silica, or china clay can be combined with either soda ash or sulfate salts to create 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 applied in offset inks: oils such as soya oil or linseed oil (that's a yellowish oil made from flax); and synthetic vehicles, that happen to be liquids resulting from the mixture of chemicals. For instance, phenol and formaldehyde mixed together make phenolic resins, in some cases 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 characteristics of inks are a function of the colorant or pigment, connected to the vehicle system used. They include color, transparency or opacity, and gloss. So far, the most frequently ink color is black. Then come cyan, magenta and yellow that happen to be used in processprinting to design the millions of colors so recognizable to us in printed matter. As the physics of color is a highly sophisticated science, in most basic conditions color comes from reflected light. White light contains the full 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 behaves 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 reducing the rest of the colors.Due to the fact that printed surfaces vary in color and in reflectance, they, too, will influence the reflected color.Thus, various ink colors printed individually or“trapped” one on top of the other create different filter effects leading to different visible colors. Alike,these same ink colors printed on different substrates will result in noticeable colors that are different yet. When we write about ink color, we are very often speaking of hue or shade—whether the ink is red or blue or green or purple. Secondarily, we might illustrate 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 quantity of pigment used affects an ink’s color strength, and also the type of vehicle used can affect both the hue and the value of the ink color. The color of the vehicle entirely, its ability to wet the pigment articles, and even the chemical interaction between the vehicle and pigment can affect the shade or purity. In the end the color of the substrate, and its drying/absorption properties have an impact on the printed color results. Ink opacity- skill to hide the color beneath it.In some cases, an ink who has little opacity is needed,for example when overlapping two colors to generate a third color. Other times, very opaque ink is needed to completely cover any color under it. The opacity needs to be suitable 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 level of covering is then compared to a standard to determine if the opacity is correct. Ink transparency- is defined as the opposite of opaque. A transparent ink does not hide the color beneath it, but mixes with it to produce a third color. All inks used to print full color work should be transparent. The choice of colorant and the level to which it is spread out 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. Normally, the upper the ratio of vehicle to colorant, the smoother the lay, and the higher the gloss. Application of a thicker ink film tends to maximize gloss while penetration into the substrate tends to reduce it.
Runnability is a phrase unique to printing. It indicates the problem-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 show the way the runnability of an ink and are primarily a function of the vehicle structure applied in the ink. Body refers to the consistency, stiffness or softness of an ink. Viscosity is a correlated term that refers to the flow characteristics of soft or fluid inks. Ink body and viscosity criteria vary widely by printing process. In general, letterpress and offset lithographic inks are quite thick or "sticky" (much like paste or honey). On press, they move via a series of rollers known as ink train where the movement of the rollers spreads the ink into a thin film for transport to the blanket and/or plate and onto the substrate. Temperature stability in an ink is useful in letting it to stand up to the heat created by the friction that occurs as the ink travels through the rotating rollers and cylinders. If an ink vehicle is not sufficiently stable, the increased temperature can have a deleterious result on an ink’s body and therefore on its runnability. Length, describes an ink’s tendency to create long threads when stretched or pulled. Long inks flow well but form long filaments that have a tendency to sling or mist, mostly 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 neither short. Tack refers to the stickiness of the ink, and it must be proper so that the ink will stick with 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 properites of an ink are vital 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 cut down air pollution, boost efficient use of energy, and even improve capacity in the pressroom by allowing faster printing and converting. Generally, the first phase of ink drying is setting;immediately upon being placed upon the stock, the liquid portion of the ink begins to vanish 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 relies on the relationship between the printing process itself, the ink vehicle system, and the substrate. Inks that are put on an absorbent substrate suchas newsprint or corrugated board dry by absorption.The liquid majority 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, elements in the ink’s oils chemically combine with oxygen in the surroundings to form a semisolid or solid ink film. It often appears in combination with absorption. Oxidation can be expanded by the use of driers in the ink formulation or by the use of heat or infrared radiation to the printed piece.Consider that non-porous substrates such as plastic films and glass are not able to absorb ink vehicles, they require inks that dry either through evaporation or by polymerization (e.g., radiation curing). In the former,vehicle solvents fade, leaving resins together with other materials behind to bind the pigments to the substrate. Evaporation that come from the inks should be rapid enough for complete drying, but not so speedy as to cause instability as the inks are still running on press.In
adiation curing, all of the elements in the ink continue to be on the surface of the substrate, but a repolymerized into a hard film by the use of ultra-violet light or electron beam energy to initiate a chemical reaction. UV-curable inks require the presence of a photo-initiator, while EB-curable formulations do not.