Printing Process And Inks

Because printing ink must be formulated to carry out specific jobs, we must pay some attention to the printing processes for which the ink is designed. This overview shows how the printing process and its typical products affect the ink requirements.

There are five major methods of printing: LETTERPRESS, LITHOGRAPHY, GRAVURE, FLEXOGRAPHY and SCREEN PRINTING. Although lithography is now the most important general commercial process, letterpress is considered first because it is the oldest method. The basic processes for printing are listed in the accompanying table together with few examples of methods. There are many other specialty processes that are used in some applications, to some extent.

 Letterpress
Letterpress is one of the oldest printing processes. Long before Gutenberg developed a system of movable type, man was using woodcut print to produce graphic designs. The letterepress printing from has raised characters and other image areas that, on modern presses, are inked by the from rollers of the press. The form is then pressed against a substrate such as paper or paperboard, or occasionally it may be trasferred to a blanket for printing of folding cartons.Flexography

Flexography
"flexo" uses flexible rubber or plastic plates. Only two or three rollers besides the plate cylinder are used, a fountain roller that picks up the ink, an engraved (anilix) roller that meters the ink (sometimes it also picks the inkup form the fountain) before it contacts the place cylinder, and a backup roller, or impression cylinder. Reverse doctor blades are being used on some of the newer presses to improve the metering of the ink. The used of so few roller permits use of a highly colatile ink that will dry quicky at the low temperatures required for drying plastic films. Flexography si widely used for printing flexible packaging, corrugated boxes, folding cartons, paper bags, and narrow web labels.

Lithography
Lithography printing was first done from blocks of imestone. Lithography now prints primarily from plates commanly made of alumimium, although plates for offset dupilcators are made of specially treated paper. Zinc, steel, and brass have also been used for making these plate. The nonimage area of the plate has an affinity for water (that is, it is hydrophilic) and, when wet, will not accept ink. The image area, on the other hand, dows not accept water (it is hydrophobic) but is wetted by ink, whick sticks to it. The nonimage area is usually aluminium or chromium, the image area can be diazo, copper, brass, or photopolymer. Sheetfed offset lithography is used in general commercial printing, short-run books and other publications, folding cartons, labels, and metal decorating. Wed offset lithography is used for longer-run books, magazines, and catalogs, newspaper, and business forms.

Gravure
Bank-notes, treasury bonds and some postage stamps are printed by copper plate or steel engraving, but most important of the intaglio processes is rotogravure (rotary-press gravure). Copper plates for intaglio may be hand-engraved, but steel plates are engraved from a hardened die. The plate is filled with ink, excess ink is cleaned from the surface, and the plate is pressed onto the paper to be printed. The process is cvapable of extremely high-quality line work. In gravure printing, the cylinder has a copper surface that is etched chemically or engraved mechanically to form the image. The size and depth of each halftone dot can be modified to give gravure the widesttonal range of all printing processes. The cylinder rotates in the fountain, the excess ink is wiped from the surface with a doctor blade made of metal or plastic. The cylinder is then pressed against the substrate, and the ink the cells transfers directly to the material. Products commonly printed by gravure are long-run magazines, catalogs, labels, flexible packaging and specialties like wallpaper and decorative panels.

Screen Printing
Once called "silk screen" because the screen were originally made of silk, the name screen printing is now prefered because the mesh is usually made of metal or plastic fabric. The screen can be coated with a phoyopolymer material that can be made insoluble by exposure to light through an imaged photographic film. After development, the nonimage areas are hardened and the image area whashed away. The ink is forced through the voids of the screen onto the substrate with a squeegee. Poster printing is a major use of screen process, but there are many other applications, such as clothing and other textiles, decalcomanias, ring binders, instrument panels and sials, preformed metal and plastic containers, and printing resists on circuit boards. Screen printing is ideal for printing irregulary shape objects.

Typical Ink Film Thickness

Although is might seem that buying ink ingredients and mixing them together does not require much skill, the making of a useful ink goes far beyond that. The inkmaker's task of selecting the right ingredients for the job to be printer and properly mixing and milling them requires a combination of scientific knowledge, experience, and skill. He must be familiar with pigment, vehicles, solvents, driers and additives, and know how to select and combine them to produce the required runnability and end-use properties. Inkmaker must know what properties are needed in the ink. If the printer does not provide the inkmaker with the necessary information like the job specifications: colour, printing process, press, paper or other subtrate to be used and other requirements, he may get an ink that is less than fully satisfactory.

Flexographic Inks
Procedures for manufacturing flexographic inks are similar to those for gravure inks. Flexographic inks, like gravure inks, can be produced from chips, which are dispersions og pigments in a dry resin. Since most of the work required to dispeerse the pigment was done in preparing the chips, only a high-speed mixer is required to make ink from chips.

Paste Inks
Paste inks maybe made by grinding the dry pigment into the varnish and dispersing the other additives required to modify flow, tack, and film properties.

Lithographic and letterpress Inks
The different between lithographic and letterpress inks are primarily in their formulations (pigment concentration, choice of pigment and varnish) and not in the method of manufacturing.

Screen Prinitng Inks
Screen printing inks usually dry by a combiantion of solvent evaporation and oxidation. The inks have a low tack and a thick, nonfluid body so that they can be forces by squeegee through the stencil screen. Screen inks are similar to paste ink that has been greatly reduced with mineral spirits.