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 evaporationor 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.