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This is a highly skilled science, and nowadays inks are often tailor-made rather than to any set formula. This is because of:
1._The large variety of stocks now available.
2._The speed of printing machines varies considerably.
3._The machinists handle their machines and inks in various ways.
Other factors an inkmaker needs to know are:
a._Is the ink for a platen or cylinder?
b._Is it to be opaque or transparent?
c._What type of forme is to be printed? (type, halftone or solid)
d._Must the ink be acid or alkali, light fast resistant etc?
e._What finishing processes are to be used after printing, such as embossing, varn-ishing, plastic sheet or spray laminating etc?
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The trend today is for quicksetting inks. These have the advantage of staying open on the press but setting quickly on the stock. These inks do not need driers added to them and care should be taken when adding reducers. The vehicle of a quick-setting ink consists of two varnishes, a heavy strong varnish made from a resin base oil, the other is a thin varnish either vegetable or mineral oil. If an incorrect reducer is used the balance of these vehicles may be upset resulting in either chalking (from mineral oil) or slowing down the quicksetting by adding too much vegetable oil. Other advantages of these quicksetting inks, they reduce set-off and because of no need for extra driers, crystalization is eliminated allowing a job to be overprinted at any time.
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Inorganic Pigment is made by precipitation of two chemicals mixed together. The residue when the solution is poured off becomes the pigment. The pigment is then washed, filtered, dried and poured into drums. Different shades are obtained by varying proportions and to conditions during manufacture.
White Pigment is made up of two main types, opaque and transparent. Listed in order of decreasing opacity, they are:
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(1) Titanium Oxide - Opaque White, (2) Zinc Sulphide - Cover White, (3) Litho Tones (zinc oxides) - mixing whites.
Organic Pigment is made by a long, complicated chemical procedure from raw materials obtained from coal tar. The chemistry of these are very complex, coal tar dyes are produced in very strong colour brilliance that makes them ideal for high gloss inks.
Lake Inks are precipitated onto colourless bases such as alumina hydrate, blanc fix, magnesia carbonate etc.
Transparent White Pigments
Listed in order of decreasing transparency, they are: alumina hydrate, magnesia carbonate, calcium carbonate and blanc fix. Used to reduce colour strength of ink, to aid dispersion of some colour pigments and to assist in carrying some heavy pigments making tints.
Moisture Set Inks
Pigmented resin dissolves in glycols. The principle, a small amount of moisture (7%) precipitates or forces the resin out of solution and changes it to a solid form on the paper. Advantages: odourless, fast drying, can be handled within an hour, no set-off.
Zip Dri Inks
Instantaneous drying on all uncoated stocks.
Carbon Inks
Two kinds of carbon, Cold Carbon and Hot-Spot. (For Hot-Spot, the ink duct has a special heater inserted)
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Manufacturing Process
The pigment, or colouring matter, is mixed in with the vehicle by stirring or churning. The mixture is then ground in an ink mill in which a thin film is forced between steel rollers turning at various speeds and clearances. These rollers break the pigment into minute particles that become saturated with the vehicle. Driers and other modifiers are added during this process also.
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GLOSSARY OF INK TERMS
ABSORPTION - The penetration of ink into the surface of the paper stock by capillary attraction between the ink vehicle and paper fibres or coating.
BINDER - Part of the ink vehicle that binds the pigment to the paper, usually a heavy bodied drying oil or resin.
BLEEDING - The spreading or running of a pigment by the action of a solvent, e.g. water, oil, etc.
BODIED LINSEED OIL - Linseed oil, thickened by heating until the desired viscosity has been obtained.
BRONZING - Printing with a stick size and dusting same with finely powdered metal particles to give the appearance of metal finishing.
CREEPING - Flowing of an ink beyond the limits of the design or image area.
DRIERS - Substances which accelerate the rate of oxygen absorption or drying of oils, varnishes or vehicles. These usually comprise organic salts of lead, manganese or cobalt.
FLOW - The ability of ink to spread over a surface or into a thin film.
LENGTH - The property of an ink or varnish whereby it may be drawn into strings before breaking. The longer the strings the longer is the ink or varnish.
LINSEED OIL - Oil pressed, or extracted from flax seeds. It is the most widely used vegetable drying oil and is main ingredient of printing ink and paint vehicles.
LITHO VARNISH - Linseed oil, or other drying oil that has been bodied by heating to produce a vehicle of proper viscosity for use in litho ink.
LIVERING - The solidifying of printing inks over time caused by chemical reactions between the ingredients.
OPACITY - An ink termed opaque will cover the body cover of the stock being printed, or any matter which it is overprinting. The degree varies considerably with the colour and type of ink, even those described as opaque.
PICKING - The lifting or pulling of fibres or coating off the stock due to tack of ink.
REDUCERS - Liquid materials added to ink to reduce its consistency or tack.
TRANSPARENCY - Inks which allow light or colour to pass through them, often changing the base colour/s.
SHORT INK - An ink which possesses a buttery consistency and breaks off with short strings when an attempt is made to draw it out between fingers. Prone to spin off rollers at high speed.
TACK - The adhesiveness of an ink for another surface. It is the stickiness or pull observed when tapping the ink out onto a piece of paper.
TRAPPING - The ability of a printed film to accept overprinting with another colour.
VEHICLE - The liquid portion of an ink that holds and carries the pigment and provides workability or press drying properties and binds the pigment to the paper after the ink has dried.
VISCOSITY - The resistance to flow of an oil, varnish or ink caused by internal friction between the particles comprising it. In simple terms it may be considered as a thickness or thinness of oil, varnish or ink from the standpoint of flow properties.
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