(Written by N.R. Jayaraman)
ORIGIN OF INKS
Do you know the root of present day printing Ink ? When the era of displaying something in writing began, in 2500 BC some kind of writing inks were prepared in Egypt and China by mixing soot (material drawn out from black smoke) with natural gums which were made into the shape of sticks and rods and dried. Subsequently they were dipped into water and used as black colour pencil like medium.
3000 years or so when Chinese introduced some kind of printing, they used coloured earth material, colours derived from plants and vegetables and mixed them with gum to give a colour it and used them as ink to apply on the engraved wooden blocks for printing on some substance. However invention of actual printing ink came up only after Gutenberg invented the printing process in the year 1440 or so. First he too used only soot as the base material for making black coloured ink and mixed it with oil instead of gum and used them for inking the engraved blocks for taking prints. Later on as technological developments began to appear, the printing process itself progressed, and coloured inks began to be used with oils and varnishes as medium to carry coloured pigments.
Various types of printing Inks are in use today to suit the needs of several processes of printing carried out by different types of printing machines.
The printing Inks are formulated with specific qualities to suit each process on which they are deployed such as Letterpress Printing, Offset Printing, Intaglio Printing, Gravure Printing, Silk Screen Printing, Flexographic printing and finally Digital Printing. Yet the basic structure and composition of the ink remains same except change in material and % of composition, only the carrier vehicle or medium changed.
HOW DOES THE INK DRY ?
Any printing ink for that matter dry only by three means on any printed surface. The process of drying takes place by:
Whatever is the means the mode of drying of ink on a substance on which they are printed, the theory is that slight penetration by the solvents of the ink do place on the surface, lest they will be chalked away from the surface. The penetration would be to the extent that the base of the ink has some hold on the surface on which it is printed.
Absorption or penetration into the substance on which the printing takes place.
Oxidation- when the oxygen in the atmosphere reacts with the ink over a substance the layer is oxidized by natural forces thereby taking away the wetness from it to firmly stay over the surface.
Evaporation process, though similar but another kind in oxidation process, allow the solvents in the ink to quickly get released from the printed ink by evaporation to allow the particles to solidify and remain on the printed surface.It applies to inks with alcohol base.
Polymerization which is oxidation cum evaporation process. It applies to highly viscose oily inks.
- With special formulations, the ink is also made to dry by curing with UV light, hot air etc without getting their hues affected.
One crucial factor to be borne in mind in the process of drying is the Ph of the substance used. The Ph factor has direct relation to ink drying properties.
Based on the speed of drying required, the ink formulations are modified.
COMPOSITION OF THE INK
The composition of the ink generally confines to these components :-
1. The colorant or Pigment:- The Pigment is an important material in the formulation of ink. The Pigment is in the form of finely ground powders or granules and acts as the coloring agent. The amount of pigment used influences the color strength of the ink.
Two types of pigments are in use. They are Organic and Inorganic pigments. It is suffice if we know the basic difference between them as it is scientific subject.
The Organic Pigments are derived from chemical process and from plants or products of plants; they have smaller particle size; bright and transparent; have high degree of colour strength properties; usually safer to handle but are slightly expensive and therefore used on costly inks.
2. The Carrier Vehicle like varnishes and oils :- Different types of varnishes are prepared to suit different kinds of inks, but they are all made by mixing the resins, solvents and additives to form a homogeneous mixture. The Vehicle or carrier of the pigment is the liquid that holds the particles of pigment and other material together and facilitate free flow of ultimate ink on the ink rollers and ink fountains. Varnishes and vehicles play critical role in ink manufacturing as varnishes and vehicles also affect the final properties of the ink, such as printability, resistance properties and visual characteristics. While pigments provide the color, varnishes and vehicles are responsible for carrying the pigments to the substrate on which it is printed.
The Inorganic Pigments are derived from metals or minerals; are larger in particle size; more opaque in nature; often dull in shades with low colour strength properties; not fully safer to handle, but are not expensive. They get easily dispersed in carrier.
Though there are many kinds of vehicles available in the form of Oils and Varnishes, generally processed varnish is used for mixing with the pigment.
- The type of vehicle used can affect both the shade and the value of the ink color. The color of the vehicle itself, its ability to wet the pigment articles, and even the chemical interaction between the vehicle and pigment can affect the shade or purity.
3. The Modifier or Driers: - The modifiers also called driers are added to control the drying properties of the ink to suit specific process for which the ink is intended. They also play significant role in other factors such as smell, scuff resistance, and fading.
Sometimes in combination with certain solvents they are also added to adjust the viscosity of the ink.
4. Resins:- Resins are primarily binders that bind all the ingredients of ink together so that while printing on a substance it binds the ink to the substance. They also contribute to glossiness, resistance to heat, chemicals etc.
5. Wax:- This promotes rub resistance.
MACHINERY AND EQUIPMENT FOR MANUFACTURING INKS:-
How are the final inks made ? Only limited machineries and equipments are available for manufacturing the printing ink. Once the Varnish is manufactured, the process of ink making begins by mixing pigment with the varnish. The pigment added with the varnish in a big drum is mixed well by mixers. While mixing the pigment does not get dispersed well with the varnish and remain in clump form. Those clumps must be broken up and the pigment dispersed evenly through the resin into the varnish to make it an effective paste. Therefore the Pigment cum Varnish clumps must be broken up in such a way that there is complete dispersal of pigments into the Varnish. This process is done on ink grinding machines called Roll mills.
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.
Three roll mill is especially effective for pigment dispersion in high viscosity paste for offset and Intaglio base inks. However it will be difficult to mill liquid ink on a three roll mill since the solvent will evaporate in the course of running.
TYPES OF PRINTING PROCESSES AND PROPERTIES OF INK USED:-
Letterpress printing is one of the oldest printing techniques by which the copies are produced by repeated, direct impression from inked block or composed type matter. Most of the presses earlier were flat bed type till the Rotary printing came.
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.
Inks used in Xerography or Electrostatic printing is called toner which consists of a fine, dry powder coated with the desired color imparted by a colored resin binder. No varnish or oil base is used for dispensing the pigments as it is dry printing process and ink gets fused by slight heat. The important consideration is not only the particle size, but electrical properties as the electrostatically charged plate hold the image which gets transferred on to the surface of the paper.
REQUIREMENTS OF INKS
1) Visual properties of inks are result of pigment in relation to the type of carrier or vehicle used. The properties include appearance, transparency or opacity, and gloss. The amount of pigment affects color strength of the ink, and the type of vehicle used can affect both the hue and the value of the ink color. The color of the vehicle itself, its ability to wet the pigment, and even the chemical reaction between the vehicle and pigment can affect the shade. Therefore the proper selection of vehicle and pigment is important to suit the specific process of printing for which the ink is made.
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.
5) Runnability refers to trouble free running of the ink on press, i.e smooth flow of ink on rollers, transfer on to the the paper without losing the characteristic like body, temperature stability, length, tack, adhesion and drying etc.
6) Viscosity refers to the characteristics of flow on inking rollers. Some of the inks used in different processes of printing are highly viscose and some moderate depending upon the press on which they are used. However whether they are highly viscose or moderately viscose they should flow evenly on the chain of inking rollers which carry a thin layer of ink to apply on the plate which transfer it to the substance on which prints are made.
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.
9) Tack refers to the stickiness of the ink. The ink should have sufficient tack to stick on the inking rollers but not to fly away from them and fall on blankets or plates as sprays of ink. The tackiness of the ink also play an important role in plucking away the fibbers from the paper surface. Too high a tack may pull out the fibbers from the surface of paper. In multi-color printing, the first ink printed must have greater tack than the ink that will be printed on top of it subsequently.