What are they ?
How are they used ?
Halftone and Colour Printing
Most people think at the word "ink" about something to fill their fountain pen with, or, looking very far, at newspapers, where in the past they got black fingers from reading. But inks go much further !
Have you ever thought why your wallpaper has a pattern ? How you can get thursty of just looking at a sixpack ? Why your furniture looks like expensive wood where it's only cheap masonite ? Why your handbag looks like lizzard leather where in fact it's only ordinary plastic ?
(Rather dry stuff !) As a general rule, most of the modern printing inks are composed of a) a carrier composed of a resin and a solvent to dissolve this resin, b) a "pigment" to give it a color and c) some additives. The oldest resins are natural products like tree gum, tar pitch, derivates of boiled linseed oil etc. The first solvents were vegetable oils, still used frequently today in modern offset inks. Later came mineral spirits and for liquid inks organic solvents like alcohol (that's why some printers are always so happy), but the future for liquid inks is WATER ! The first pigments were natural ones like the mineral pigments earth yellow, iron oxyde red, ultramarine- and cobalt blue and - most important - lamp black. Nowadays, with the exception of vegetable oils in offset inks and tar pitch as a cost reducer for cheap newspaper inks all the ingredients are manmade, even the lamp black and specially the colored pigments. All this stuff is - simply said - mixed together in specially designed mills and whoop: the ink is ready ! Modern printing inks have to withstand a lot of attacks from different mediums without being damaged, just think about the banknote you forgot in your shirt and that went into the laundry or the inks used for the furniture industry where you can buy one table today, a second one after 5 years; and if they are printed with a good ink, you won't see the difference. But you also have the opposite side, for instance inks for checks which fade or change colors if you try to fiddle with the check.
It's a fascinating job working in this industry !
You can get a printed image in the classical way of "printing" in four different ways, which i'll name according to the main principles:
This is the oldest method of printing: The artist draws a pattern on any suitable surface, then cuts away a portion of the surface so that only the desired image remains on the surface. When inking the surface, only the parts which are not cut away can accept the ink and later on transfer it to the paper. Just think at the potato stamping we all did as kids. Look also on the net for "wood engraving", "wood cut" or "lino cut". You can cut letters (or drawings) out of wood, linoleum or do it the other way round: cut the desired image into a soft form and fill that up with hot, liquid lead. After cooling down and removing the lead from the form, you have a "letter", that's why this process is also called "letterpress"
That is the classical way with very "thick" inks, having a consistence somewhere between toothpaste and peanut butter and due to their consistence also called "paste inks" or "fatty inks". A more modern way is so called FLEXO printing, where the printing surface is made from rubber or photopolymer plastics and the image can be laser engraved or produced by applying a strong light source onto the photopolymers, washing away the non exposed parts and hardening the remains. In this case, the inks are much "thinner", between water and, let's say, salad dressing and quite logically are called "liquid inks".
Intaglio printing is the opposite of relief printing: the image to be printed is "cut" into a surface (mostly copper), the surface is covered with ink in a way to assure that all "holes" are filled with ink, then the surface is wiped* clean so that only the ink in the engraved parts is left over and then is transferred onto the paper. This "cutting" can be done really by cutting with a needle, (in modern gravure mostly with a diamond) or chemically by etching. On etching, you cover the plate or cylinder with an inert material and then use an etching needle to draw your pattern in this material. Then you put the plate into a bath filled with a strong acid which will attack the plate on the spots where it is not protected, but will not attack the protection layer. After that, you wash away the protection layer with a suitable solvent. The "hottest" new way in the real meaning of the word is burning in with a laser, but that's only in an experimental stage at the moment. After one of the above mentioned processes, the plate or cylinder theoretically can be used, but as copper is a very soft metal, normally it's covered with a very fine layer of some mils of the very hard metal chromium to assure a longer life of the copper image before being "erased" by the friction of the wiping* devices. Classical masters of etching are Leonardo da Vinci and Albrecht Duerer (who doesn't know his praying hands ?), but it's still today a very frequently used expression of art.
In industrial printing, Intaglio can be diverted in two classes:
1) The real Intaglio that is mainly used for banknote printing: A very deep engraving and consequently a very thick layer of ink so that the blind can "touch" the banknote similar to braille and due to some specific spots on the note can recognize the value of it. Inks for this application are of the "paste ink" type and have to withstand all possible and impossible attacks.
2) The industrial printing for long runs: Where letterpress, flexo and offset are either to slow or the materials (rubber clichés in flexo, offset plates etc.) are not resistant enough, you go to gravure. Making a steel cylinder, covering it with copper, polishing it, engraving it and protecting it with chromium costs a lot of money and is too expensive for short runs, but if you want to print some millions of magazines, it's still the cheapest way ! The inks for industrial gravure printing are from the "liquid" type.
*Wiping: On real Intaglio banknote printing, the wiping is done either with a roll of tissue that runs against the rotation direction of the printing cylinder or with a roll of paper. The use in both cases is similar to the use of toilet paper: you wipe (and throw away) the surplus, in this case quite a lot of ink ! Is that one of the reasons why banknotes are so expensive ?
In industrial gravure, where liquid inks are used, you just let rotate the cylinder in a bath of ink and then clean the surface by the means of a so called "doctor blade". That's some kind of a very thin knife that scratches over the cylinder, and as the ink is liquid, it will drop down again into the bath.
The basic principle of lithographic (or planographic) printing is that a flat (plane) printing surface is devided into areas accepting the ink and others refusing the ink without cutting or etching the surface. The inventor, Alois Sennefelder from Bavaria is therefore called the father of the offset process. He found that if you draw on a polished limestone with a greasy pen or crayon, then rinse the limestone with water, roll over it with a greasy ink, the ink will adhere on the spots where the limestone was covered with the greasy crayon, but will be repelled on the spots where the limestone was wetted with water. After putting a paper over the inked limestone, you get a mirror image of the limestone on your paper. Artists still do it that way producing the so called "litho's" but for industrial printing the limestone is since long replaced by metallic "offset plates". Nevertheless, the principle is similar: You first dampen your (chemically treated) plate, then try to ink it. Where the plate is wet, it will refuse the ink. Theoretically, you can print from the plate, but to give a better print, you first transfer the contents on the surface of the plate (including the water film) onto a soft rubber blanket which then will give a better transfer onto the relatively rough surface of the paper. In doing so, you will also print a negative image with water, but this water is a) invisible, and b) first penetrating very quickly into the pores of the paper and later on evaporating. It sounds logical that this can be done only with greasy (fatty) inks. (The other way would be possible too:) Draw with a water based pen, then dampen the non printing areas with a volatile solvent e.g. white spirit and cover the printing areas with a water based ink, but this process would be far to dangerous for industrial use !!!
The principle of screen printing is that you have "a screen", in flatbed screen printing normally a fine synthetic fabric, in rotary screen some kind of a metallic drum with a lot of holes in it. A photopolymer coating is applied over the whole surface, then exposed and the surplus washed away. So you can "block" parts of the screen where you don't want to print. When you put the screen onto a paper, pour the ink over the screen and press it through the holes with a squeegee, you will get an impression of the non-blocked holes on your paper. This kind of printing is widely used under artists (think at the Marilyn series of Andy Warhol or the Austrian artist F. Hundertwasser) Industrial applications of flatbed screen are mostly for very small runs where it's not worth the money to use more refined technics.
Rotary screen is mainly used for textile printing and (in a minor amount) also for vinyl wall and floor covering.
A variation of screen printing is the so called "pad printing" where you first do a screen print on a very flexible surface and then press this surface as long as the ink is still fresh and wet against the stuff to be printed. This is generally used for irregular non-flat surfaces like plastic bottles, golf balls etc. A good simulation of pad printing can be found on one of the links.
Other applications of screen- and padprinting with VERY SPECIAL INKS which are no classic printing inks and I therefore don't go into the details are:
a)printing of integrated circuits e.g. motherboards for PC's and b) printing on glass, china or ceramics where the print then is burnt in.