What are the basic principles of offset printing? Besides movable type (which I assume is obsolete) what other basic printing methods are there?
The main difference with offset is the ink travels from the type to the paper in two steps, not one.
It goes to a plate, then the paper.
Thus, the paper never gets into the type face, clogging it.
There’s Lithography (althought that too may be obsolete by know, I don’t know.) In lithography, instead of using raised type they use a smooth surface (originally marble, hence the “litho” part) covered with a water-repellent substance. The lettering is where there is no water-repellent, and so those areas take ink while the background doesn’t. (Unless the ink’s oil-based, in which case it’s the other way around.)
As noted, the ink is not put directly onto the page by the press. The original (and, I believe, still frequently used) method was to have a rubber drum that was rolled across the inked press, then rolled across the page to be printed. The ink is, thus, off set from the press to the page.
The Britannica gives a decent explanation and history at offset printing and Printing, Offset.
Ahh, printing. Step aside boys, amateur hour is over.
There are three types of printing: letterpress, planography and inaglio.
Offset printing is one type of planography. There is no “type,” as in “movable type.” Instead, it works on the principle that water and ink do not mix.
A metal plate – essentially a thin, flat (w/no raised surfaces at all, hence the name planography) sheet about the thickness of a sheet of paper – is created using a photographic process that chemically etches the print image on the plate’s surface. This image area is of a chemical nature that resists being wetted by water; the non-image area, on the other hand, will stay wet.
The plate is put on a press and spins around. A wetting roller wets the plate with water; the water only “sticks” to the non-image area. Then an ink-covered roller hits the plate with an oil-based ink. The ink “sticks” to the (non-wet) image area, and does not stick to the (wet) non-image area. So, now what you’ve got is a metal plate that is covered in ink only in the places that you want ink to be! Very smart.
Now comes the “offset” part. This plate does not print directly on the paper. Instead, it prints (offsets) onto another rubbercoated roller called a blanket. The blanket – which now has carries the inked image – then prints onto the paper.
More about the other processes to come…
(Continuing from previous post…)
Oops. There are two other printing methods besides planography, letterpress and intaglio. Sorry. There’s also stencil printing and xerography.
Planography (see my derscription of offset printing above) is distinguished from other printing methods in that the inked plate is essentially flat to the touch; there are no obvious raised (high) or carved (low) surfaces on the plate. Ink stays on the image area only by chemical, not physical, means. Classic stone lithography, offset printing, and spirit duplicating fall into this category.
Xerography is a very specialized type of planography. The infinately reusable plate is photosensitive, and holds an electric charge in the print area. The “ink” is dry, meltable powder (toner) that sticks to the charged areas of the plate. This is, in turn, deposited on a sheet of paper, which passes through a heating element which melt and fuses the toner to the sheet. Photocopiers and laser printers are examples of this method.
Letterpress is your classic “raised type” process. It’s used in old style (wine press) printing presses ala Guttenburg. Other examples include rubberstamps, linoleum block printing, and older rotary and sheetfed electric printing presses.
Stencil Printing is also pretty self apparent. A stencil is a thin, ink-proof barrier with open image areas. Ink is forced through the openings to the paper (or other substrate). Examples include silk screening and mimeography.
Intaglio is the “finest,” printing method and the one that allows the most copies before the printing plate begins to deteriorate. That is why currency, some stamps, bonds, etc. are printed this way. Essentially, a thick metal (steel or copper) plate is engraved with the reverse image of what the design will look like on paper; this is done mechanically (sometimes by hand) or chemically. Ink is applied to the plate then scraped off the top surface with a metal blade; but some ink lingers in the engraved lines/dots. Paper is then squeezed hard against this plate, and the ink in the engraved recesses transfers on to the paper. That is why items printed in this method have a “raised” feel: the print areas of the paper has been squeezed into the fine, inky nooks and crannies of the plate. Photogravure is a variation of intaglio that relies on capillary action, not pressure, to pull the ink out of the plate and on to the paper.
tomndebb and others, thanks for the links and explanation. Very especially to stuyguy. That is exactly the information I was looking for. Being of a curious nature I am always surprised when I look at something common (like printing) and realize how little I know about it.
Offset printing is really ingeneous.
Stuyguy
That was a very good explaination of the various types of printing,I could not have said it better and I have been a rotary offset litho press operator for over 20 years!
Are you in the graphic arts industry?
If you are ever in Houston and want the $0.10 tour of a medium - large forms shop drop me a line.
Peace
LIONsob
Ackkk! After a good night’s sleep, and a chance to calm down from the euphoria of the likelihood of a subway series, I noticed I made a typo in my last post. Photogravure should read rotogravure in the last sentence. Sorry.
Sailor, delighted I could be of help.
LIONsob, no, I’m not in the graphic arts industry any more. I’ve always been fascinated w/ printing and earned an Associates Degree in Graphic Arts Production. Soon afterward I shifted into graphic design, then into television. Thanks for the tour offer; I’m not in TX often, but I’ll keep it in mind.
I also found very interesting information in the britannica links supplied by tomndebb. it is a series of articles that cover all history and aspects of printing
Good answers, all. I just must point out one more thing regarding the ingeniousness of offset printing: It allows you an unreversed master. When the image is printed from the plate to the blanket, it is reversed. From the blanket to the paper, it reverses again.
When printing using the other methods, you must prepare a mirror image (backwards) plate of the copy to be printed.
So those big rotary presses you see in the movies printing newspapers when they say “stop the press!”, those are offset printing?
Sailor
To answer your last question , yes.
Almost all newspapers now use rotary wet or dry offset lithography.
Now if your talking about movies made 30+ years ago what they show and what was used by newspaper was rotary letterpress.
Rotary printing is where the press uses rolls of paper instead of stacks of cut sheets and imprints the paper in a unbroken stream called a web.
At the finishing end of the press the printed web can be rewound on to a roll,cut into sheets and/or folded as needed.
The press I operate is as large as an 18 wheel truck and has more horsepower than some cars.
Max speed is 1000+ Ft. per minute.
Peace
LIONsob