THE PRINTING PROCESS AND SUITABLE INK

 

THE PRINTING PROCESS AND SUITABLE INK

Printing inks, in one fore or another, has been around since long before Johan Gutenberg invented printing with moveable type. Three hundred years before Christ, the Chinese were using inks to print wooden blocks. One of the earliest ink formulas included Lampblack, glue and water. This was later improved somewhat by addition of linseed oil, but it was not until the last 19th century, with the introduction of synthetic oil and man-made resins, that the quality of printing ink was greatly improved printing inks are complicated mixture of chemical compounds. The composition varies by printing process, by whether printing is sheet fed or web fed, and by substrate.

Each printing process required 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 used for the most common printing processes, and the specific properties of each.

LETTERPRESS INKS: -Designed to print from a raised surface. Letterpress ink must be tacky and viscous enough to hold the surface of the plate until printed.

In addition to this, each type of letterpress printing press required an ink with a different combination of ingredient. The plate 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 speed of the press the thinner the ink must be letterpress inks can be dried by absorption, evaporation or oxidation.

 

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 on to 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 clean the plate and ensure a proper transfer of the printed image to the paper. Gravure inks are quick-drying 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 ink must be strong in colour to compensate. To supply proper ink for offset, the ink maker must know if it is to be used on a one-colour, four-colour press, and also the order in which the colours are to be printed. Offset inks are usually dried by evaporation, oxidation, polymerization, penetration.

 

SCREEN PRINTING INKS: -Designed to be forced through a mesh screen on to a wide range of surface such as paper, cardboard, meta, 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 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 from letterpress printing which employs rubber plates and aniline inks designed to print on a wide range of surface, including paper, cell phones, plastic and metal foil. Flexography is extremely popular in the packaging industry; the inks colours are bright, strong, opaque, and can therefore be printed at high speeds. Most flexography inks have an alcohol base and are dried by evaporation; other has a water base and is 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 transformed 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 film is similar to that used in regular offset printing. Letterset inks are dried in the same way as letterpress and offset inks.

 

INKS FOR VARIOUS PRINTING PROCESS: - Ink used in litho letterpress and screen-printing are necessarily more viscosity than for flexo and gravure, which required low viscosity inks. The printing surface in lithography process is palaeography meaning that the image and non-image area inks is that rolled across the plate surface and it’s expected by the image area but repeated by the dapped non-image area. For the ink to wet the image area the surface energy energy photo polymer coating must be greater than the surface tension of the ink, in the offset printing process the ink film thickness from the ink roller is splitting noise before reaching to substrate. The consequence of splitting the ink film is that its thickness delivered on the stock is 2-micron or less in order to produce strong image the ink has to be heavily pigment with high colour intensity. These heavily have high viscosity and geologies including thixotropy and show the viscosity of litho ink would be 100 poise compared to a liquid ink of viscosity centipoises. This is essentially required for with low absorption coating paper to avoid any set of problem in case of silk screen printing the thickness of ink film is much more higher and relogical balance should be maintained with adequate drying time to be allow white formulating the ink for the process.

 

SIMPLE LOW VISCOCITY INKS: - The simplest pigment vehicle combination in which the vehicle consist of a resin dissolved in a solvent can be used to produce very low viscosity inks although additives such as plasticizers may be required to impart flexibility to ink films particular for flexible packaging uses. These inks are of essentially simple liquid and this is reflected ink in their flow behaviours. The printing process that required such high mobility ink is gravure and flexography and so to some extent inkjet also.

 

The presence of two hydroxyl groups makes them take up water very quickly and once they do this they will no longer keep the resin in the solution but release it as solid precipitate