Types of Print
The main types of print you may encounter commercially are:
- "Fast print"
- Intaglio or gravure
Also of relevance is the difference between sheet fed and web presses.
The most common kind of printing, it is technically called offset lithography.
First, a "plate" is made which represents the image to be printed as areas that attract ink and areas that repel ink. Originally this was done with a greasy crayon applied to porous stone; today it is done electronically on metal, plastic or even paper plates. The plate is inked and the image transferred to an intermediate rubber-covered blanket cylinder, which then "sets off" the image onto paper it is rolled over. Set-up costs per print job are relatively high (they include making plates and adjustments specific to each job) but unit costs are fairly low once the press gets started. This makes small quantities expensive (due to the start up cost) but means you can increase the quantity for only a small extra cost. Offset is part of a group of printing processes referred to technically as planographic, and it includes variants such as waterless printing and collotype, both quite rare in Australia.
Digital print uses the same principle as an office laser printer: toner is adhered to the page according to a pattern of electro-magnetic charges set up by a laser and then melted on to the page to make the image. Digital presses have low set up costs so it is practical to print very small numbers, even one or two. The unit cost however is high compared with offset printing. When the print run gets around 300 it often becomes cheaper to print offset - and you'll get ten times as many copies for the same price. Often with small runs however unit cost does not matter, it is the total cost for a few items that matters. Getting thousands of extra copies you eventually throw away is no bargain at any price. Digital print has a couple of other advantages: it's faster and it offers "variable data" where each item is different in some way (for example, it may include the recipient's name). Digital print is generally good quality and it is improving all the time, but designs must still be setup with digital in mind, to avoid a few pitfalls.
The basic principle is to rub ink through silk screens which have been made impermeable to ink in certain areas by the application of wax or some other blocker, though new methods have improved significantly on the technology.
Screen printing is fairly low resolution, fairly slow and not so good at full colour images, but it is very suited to non-paper printing such as vinyl, china, cloth and metal.
Ink jet printing
Just like the ink jet printer you may have at home, commercial ink jet printers squirt ink onto the surface to be printed. Their main commercial uses are for very short run, high quality and large format images (eg, large photographs) and for signs and banners. Ink jet printing on vinyl and metal is much better quality than screen printing and is starting to take over for this kind of work. Ink jet printing can print very large sizes, compared with digital print (currently limited to A3+) but it is higher cost and suited only to very short runs.
Printing from a plate of some kind which has raised surfaces of type, lines and dots that are inked. The plate is pressed directly against the paper. This process is in principle unchanged from the first printing over 400 years ago. Types of relief printing include letterpress and flexography; they are still sometimes used to make raised-print business cards.
A euphemism for low quality, high volume, photocopy-style print, increasingly rare as general print costs come down.
Intaglio or gravure
This class of print includes gravure, steel-die and copper-plate engraving.
A plate is made by cutting, engraving or etching minute cells (called wells) of various depths, that are flooded with ink and then applied direct to the paper. High volumes of ink can be applied, creating rich, saturated colours. This is an expensive and thus little used process.
Sheet-fed versus web-fed
Sheet fed presses use pieces of paper cut to predefined sizes and fed into the press one at a time.
This is the most common way of feeding a press. It gives the best quality.
Web-fed presses draw their paper in from a large roll in one continuous piece. Newspapers are printed this way and some magazines. It's fast and cheap for long print runs, though quality is usually less.