What a printer should know about ink ?

The printing inks are generally ground through the three roll mill, a heavy duty ink grinding machine where three cylinders running in opposite directions grind the heavy clump of  pigments already mixed up along with other components and varnish  to break down the clump of agglomerates to the desired pigment particle size. With selective pressure between rollers, selective speeds and selective temperature maintained in the cylinders of the Triple roll mill,  the clumps of pigments are evenly ground to make necessary thickness of pigments (in microns) to suit the printing machine requirement.

Of course though other than Triple roll mill, there are no of other types of machineries and equipments in many sizes are available to do this process of grinding and mixing Pigments with Varnish, only the triple roll mill is used by the major ink manufacturing units.

Since Letterpress process involves printing  by relief imaged blocks or composed metal types, the ink properties should be such that it stick on to the surface of the metallic images and then transfer the same amount of ink on to the surface on which they are printed. Therefore the ink should neither be very hard, nor very soft.

Since the blocks or type matter directly contact the paper surface, the inks formulated are such that they dry on paper without pulling away the paper fibbers from surface as the inks are slightly more viscose (thicker) in nature than the Offset inks. Also the inks will have to quickly dry on the coated surface of the paper without causing back side impression called set off. Whether the Letterpress printing is done trough Rotary or the Flat bed machine, the basic properties of the ink used in them remain the same.

Letterpress uses moderate thick (paste) inks whose tack varies according to the speed of the machine deployed. The type of substance used in this process is both coated and non coated paper and therefore the ink required for this process is to dry by absorption (penetration), oxidation, or by polymerization through evaporation depending upon the kind of stocks being printed.  Since the highly clay coated art paper is preferred to be printed by this process, the ink used has to dry by oxidation cum polymerization.

Almost all letterpress inks are translucent which means that when one color over another is overprinted it creates a third color. It also means on large inked areas, you’ll see a little paper show-through. The technologist say that the letterpress ink generally consist of 15 to 25 % pigments rest being varnish, driers, resins etc.

Offset Printing inks are more viscous than other types of inks, and since the ink film is thinner with offset printing, the pigment content must be higher which is up to 25 % as the technologists say. Offset presses deposit ink films that are about half the thickness of films deposited in letterpress process. And since offset printing is based on the fact that oil and water do not mix, inks formulated for the process contain significant amounts of water-repellent materials. The inks may have to withstand reaction from the fountain solutions which may sometimes be used in some of the wet Offset machines and therefore the inks should not emulsify and impair the body of ink, its shade or drying properties of the ink or cause tinting on the non printing areas of the plate and printed sheets.

Unlike Letterpress process which print on the substances by relief plates, the Offset process has two divisions in printing techniques – one printing on surfaces by relief image plates and the other plane surface plates. The Dry Offset printing machines have no water fountain to wet the plate before inking as the plate have print areas in relief format. The ink from the rollers get directly charged on to the relief plates and are therefore different from the ink used on wet Offset printing machine. The ink used on the Dry Offset machine however has different qualities as against the inks used on Letterpress and wet Offset inks. The quality of ink used on Dry Offset is such that a thin layer of ink from the plate is transferred on to the blanket surface without smudging and set off.

Sheet fed offset presses primarily use partial quick drying inks, which dry rapidly without the drying apparatus. However some of the modern Offset machines are equipped with radiation curing devices, as is needed for quick-set infrared ink, ultraviolet curing ink, and electron beam curing ink and therefore the composition of the inks are entirely different than conventional Offset inks.

The Web offset inks tend to be less viscose and have less tack than sheet fed Offset machine inks.

In Intaglio printing, highly viscose ink is used and the composition is made in such a manner that the pigmentation is very heavy, and the varnish used have higher tackiness compared to other processes. The qualities of the ink used in Intaglio will be such that it tends to slightly loosen while inking the plate due to heat generated apparatus fixed in the ink fountain and the plate cylinders. At the same time, once the ink is transferred on to the substance on which it is printed, the ink has to slowly dry on the substance by slight penetration cum oxidation cum polymerization process to ensure that the relief effect of the ink required is also not lost.

In respect of Screen printing inks, the composition has higher Pigmentation (say 30 % as technologists say) rest being Binders, solvents and additives. Screen process printing requires paste inks that are thicker, but able to pass through the small holes in the screen and leave sharp prints on the substance. They must also perform well under the action of a squeeze which is used for pushing the ink into the screen holes. The binder added to screen printing ink must be compatible with the surface on which it will be printed. The solvents used should also not be overly volatile, as early evaporation would cause the remaining ink dry non the screen.

In Flexography Printing Process liquid ink used which  has to dry very fast as this process is used to print on corrugated boxes, multi-wall sacks, paper sacks, plastic bags made up of various materials such as plastic, paper and others. The printing plates are flexible and made of rubber or plastic. The  ink  used is mostly water based and the process of drying of ink is by evaporation. The solvent (vehicle) is made up of an alcohol, ethyl, propyl, or isopropyl etc. The vehicle is generally alcohol based ones as it facilitates quick drying by evaporation. Typical resins used in flexographic inks include acrylics, cellulose esters, nitrocellulose, poly amides, modified rosins, and ketone etc.

The principle components of the ink used in Gravure printing is also alcohol based and materials like esters, ketone, glycols and water are used for the solvent part and cellulose, vinyl, acrylic and polymers and organic and mineral pigments are added for coloring and other properties. The selection of solvent should be such that the pigments dissolve in the binder and eventually evaporate during drying process. The ink used on this process should have higher colour value and being liquid, it has to stay inside the pits on the cylinder and also transfer the ink on to the paper surface in true tone and strength without causing smudging on the edges.

The inks used for Ink jet printing consist of dyes mixed with a highly fluid vehicle or carrier that form very small drops by picking up an electrical charge, and can be deflected properly to fall in the right place for the formation of a printed character or image.

2) Since in multi colour printing, the colour overprinted on the previous colour printed below should be partially opaque but not to completely mask the color beneath it, the ink should have certain amount of transparency  to show the original colour printed below or to merge with them to form another colour wherever such colour changes are required to match the shades of the original. Generally on patch colour work the fully opaque ink is used and not when the printing is multi coloured reproduction having varying shades.

3) On the other side semi  transparent ink is also required in multi colour work. A semi transparent ink does not hide the color beneath it, but mix with them to create a third color. Therefore for the multi colour reproduction work printed by colour separation, the quality of the inks should be semi 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 of an ink.

4) Another quality of the ink is glossiness. It refers to the ability of the ink to glow i.e reflect the shade as though it is shining. The extent of gloss depends upon the composition of the ink, thickness or layer of the ink film deposited on the surface of the substrate.

7) The ink has to withstand the heat generated by the friction  chain of rotating rollers. Normally the ink is manufactured taking into consideration the optimum temperature to which they will not lose their properties.  If however the ink reacts to the heat, then it will have deleterious effect on an ink’s body affecting the runnability.

8)  Length describes the tendency of the ink to form long or short threads when stretched or pulled from the ink fountain while mixing them on the running press. Length of the ink is very important factor for flow of ink. The best Inks are those which are neither excessively long nor short as excess of either one affects the runnability.