How printing works
by Chris Woodford. Last updated: January 15, 2017.
It's amazing that you're sitting reading these words at your computer; back in the 15th century, it would have been just as amazing to be reading them in a book. That was when printing technology hit the big time and the invention of the modern printing press made it possible for books to be reproduced in their hundreds and thousands instead of being copied out laboriously, one at a time, by hand. Although newspapers, books, and all kinds of other printed materials are now shifting online, printing is just as important today as it's ever been. Look around your room right now and you'll see all kinds of printed things, from the stickers on your computer to the T-shirt on your back and the posters on your wall. So how exactly does printing work? Let's take a closer look!
Photo: Potato printing: This is printing the way most of us learn it. It's an example of relief printing in which the ink is applied to a raised surface (the parts of the potato surface that haven't been cut away) before the paper is pressed onto it.
What is printing?
Printing means reproducing words or images on paper, card, plastic, fabric, or another material. It can involve anything from making a single reproduction of a priceless painting to running off millions of copies of the latest Harry Potter. Why is it called printing? The word "printing" ultimately comes a Latin word, premĕre, which means to press; just about every type of printing involves pressing one thing against another.
Although there are many different variations, typically printing involves converting your original words or artwork into a printable form, called a printing plate, which is covered in ink and then pressed against pieces of paper, card, fabric, or whatever so they become faithful reproductions of the original. Some popular forms of printing, such as photocopying and inkjet and laser printing, work by transferring ink to paper using heat or static electricity and we won't discuss them here; the rest of this article is devoted to traditional printing with presses and ink.
Printing is hard, physical work so it's usually done with the help of a machine called a printing press. The simplest (and oldest) kind of press is a large table fitted with an overhead screw and lever mechanism that forces the printing plate firmly against the paper. Hand-operated presses like this are still occasionally used to produce small volumes of printed materials. At the other end of the scale, modern presses used to print books, newspapers, and magazines use cylinder mechanisms rotating at high-speed to produce thousands of copies an hour.
Types of printing
The three most common methods of printing are called relief (or letterpress), gravure (or intaglio), and offset. All three involve transferring ink from a printing plate to whatever is being printed, but each one works in a slightly different way. First, we'll compare the three methods with a quick overview and then we'll look at each one in much more detail.
- Relief is the most familiar kind of printing. If you've ever made a potato print or used an old-fashioned typewriter, you've used relief printing. The basic idea is that you make a reversed, sticking-up (relief) version of whatever you want to print on the surface of the printing plate and simply cover it with ink. Because the printing surface is above the rest of the plate, only this part (and not the background) picks up any ink. Push the inked plate against the paper (or whatever you're printing) and a right-way-round printed copy instantly appears.
- Gravure is the exact opposite of relief printing. Instead of making a raised printing area on the plate, you dig or scrape an image into it (a bit like digging a grave, hence the name gravure). When you want to print from the plate, you coat it with ink so the ink fills up the places you've dug out. Then you wipe the plate clean so the ink is removed from the surface but left in the depressions you've carved out. Finally, you press the plate hard against the paper (or other material you're printing) so the paper is pushed into the inky depressions, picking up a pattern only from those places.
- Offset printing also transfers ink from a printing plate onto paper (or another material), but instead of the plate pressing directly against the paper, there is an extra step involved. The inked plate presses onto a soft roller, transferring the printed image onto it, and then the roller presses against the printing surface—so instead of the press directly printing the surface, the printed image is first offset to the roller and only then transferred across. Offset printing stops the printing plate from wearing out through repeated impressions on the paper, and produces consistently higher quality prints.
Photo: The three most common types of printing: Left: Relief—Raised parts of the printing block (gray) transfer the ink (red) to the paper (white rectangle with black outline at the top) when the two are pressed together. Middle: Gravure—Grooves dug into the printing block transfer the ink to the paper when the paper is pressed tightly into them. Right: Offset—A rotating cylinder (blue) transfers ink from the printing plate to the paper without the two ever coming into contact.
For over 500 years now, most high-volume, low-quality printed material has been produced with letterpress machines, which are more or less sophisticated versions of the printing press Johannes Gutenberg invented back in the 15th century. In the simplest kind of letterpress, known as a flatbed press or platen press, the paper is supported on a flat metal plate called the platen, which sits underneath a second flat plate holding a relief version of the item to be printed (the printing plate, in other words). The printing plate is covered with ink (either by hand, with a brush or by an automated roller) before the paper is pressed tightly against it and then released. The process can be repeated any number of times.
Photo: The keys in an old-fashioned typewriter produce images on paper by relief printing. When you press a key, these metal type letters flip up and press a piece of inked fabric against the paper. The letters are cast in reverse so, when they hit the paper, the printed impression comes out the right way round. Typewriters like this are now largely obsolete, but great fun to use—if a little noisy—when you can find them!
Flatbed presses are generally the slowest of all printing methods, because it takes time to keep lifting and inking the printing plate and loading and removing sheets of paper. That's why most letterpresses use rotating cylinders in place of one or both of the flat beds. In one type of machine, known as a flatbed cylinder press, the printing plate is mounted on a flat bed that shifts back and forth as a cylinder moves past it, inking it, pressing the paper against it, and then lifting the printed paper clear again. That speeds up printing considerably, but loading and removing the paper is still a slow process. The fastest letterpresses, known as rotary webfed presses, have curved printing plates wrapped around spinning metal cylinders, which they press against paper that feeds automatically from huge rolls called webs. Newspapers are printed on machines like this, which typically print both sides of the paper at once and can produce thousands of copies per hour.
The simplest kind of gravure printing is engraving, in which an artist draws a picture by scratching lightly on the surface of a copper plate that has been thoroughly coated with an acid-resistant chemical. Lines of shiny copper are revealed as the artist scrapes away. The plate is then dipped in acid so the exposed copper lines are etched (eaten much deeper into the metal) by the acid, while the rest of the plate remains unchanged. The acid-resistant chemical is then washed off leaving a copper printing plate, from which a number of copies, called etchings, can be printed. Traditional engraving and etching is quite a laborious process, so it's used mainly by artists to produce relatively small volumes of (originally) hand-drawn pictures.
A similar but much quicker and more efficient process called photogravure is used commercially to produce large volumes of high-quality prints. Instead of being slowly and painstakingly drawn, the image to be printed is transferred photographically onto the copper printing plate ("photo") and then etched into it ("gravure"). Once the plate has been produced, it's used to make prints on either a flatbed press (fed with single printed sheets) or a rotary web press known as a rotogravure machine. Glossy magazines and cardboard packaging containers are often printed this way.
The most common type of printing today uses a method called offset lithography (typically shortened to "offset litho"), which is a whole lot simpler than it sounds. As we've already seen, offset simply means that the printing plate doesn't directly touch the final printed surface (the paper or whatever it might be); instead, an intermediate roller is used to transfer the printed image from one to the other. But what about lithography?
Photo: A modern offset printing press used to produce small runs of a weekly newspaper. Note the final printed copy on the top roller and the offset cylinder in the middle just underneath it. Photo by Senior Airman Dilia DeGrego courtesy of US Air Force.
Lithography literally means "stone-writing," a method of printing from the surface of stones that was invented in 1798 by German actor and playwright Alois Senefelder. He took a large stone and drew a design on it with a wax crayon. Then he dipped the stone in water so the parts of the stone not covered in crayon became wet. Next, he dipped the design in ink, so the ink stuck only to the waxed parts of the stone and not the wet parts. So now he had an inked "printed plate" (or printing stone, if you prefer) that he could press against paper to make a copy. Lithography avoids the need to make a traditional printing plate, as you need for both relief and gravure printing.
Photo: A small offset printing press. Note the paper sheets feeding in from the left, the rollers that transfer the paper and copy the image, and the large bottle of ink in the center. Photo by J. Pond courtesy of Defense Imagery.
Modern offset lithography printing presses use an updated version of the same basic idea in which the stone is replaced with a thin metal printing plate. First, the image to be printed is transferred photographically to the plate. The parts of the plate from which the image is printed are coated with lacquer (clear varnish), so they attract ink, while the rest of the plate is coated with gum, so it attracts water. The metal plates are curved around a printing cylinder and press against a series of rollers, which dampen them with water and then brush them with ink. Only the lacquered parts of the plate (those that will print) pick up ink. The inked plate presses against a soft rubber (offset) cylinder, known as the blanket cylinder, and transfers its image across. The blanket cylinder then presses against the paper and makes the final print. High-speed offset lithography presses are web-fed (from paper cylinders) and can produce something like 20km (~12 miles) of printed material in an hour!
Other types of printing
Relief, gravure, and offset are used to print the overwhelming majority of books, magazines, posters, headed stationery, and other printed materials that surround us, but several other methods are used for printing other things. T-shirt designs, for example, are usually produced with a process called silk-screen printing (sometimes called serigraphy). This involves covering the article to be printed (something like a blank cotton shirt) with a mesh-screen and a stencil, then wiping ink over the mesh with a brush. Ink transfers through the mesh to the fabric below except where it's blocked from doing so by the pattern on the stencil. Collotype (also called photographic gelatin) is a less commonplace technique in which a gelatin-coated printing plate is made from a high-quality original using a kind of photographic method. It produces finely detailed reproductions and is still used for making high-quality prints of paintings.