Offset printing is the dominant industrial printing technique – used for printing a wide range of products such as cards, stationery, leaflets, brochures, magazines, and books. It can also be used for packaging, such as boxes or cartons. This page provides more information about this technique. For an overview of other printing processes, check this page.
How offset printing works
The full name of the offset printing process is offset lithography. Both terms each describe part of the process:
- Lithography is a printing process in which the image area and the non-image area co-exist on the same plane. That means the surface from which you print is completely flat. This is feasible because of a very useful chemical principle: offset printing ink is an oily substance, which means it repels water. If you can create a surface on which some parts contain a thin layer of water, those areas will repel ink. The image areas need to be lipophilic (or oleophilic) – they accept ink – while the non-printing areas need to be hydrophilic – they are repellent to oil and receptive to water.
Some offset litho presses do not use a water-based dampening system but instead use printing plates on which a silicone layer repulses the ink. These are called dry offset or waterless offset presses.
- Offset refers to the fact that the image is not transferred from a lithographic printing plate to a sheet of paper. Instead, the inked image is transferred (or offset) from the printing surface to a rubber blanket and then to the printing surface. The process can be used to print on paper, cardboard, plastic, or other materials, but these have to have a flat surface.
Anatomy of an offset printing press
A sheetfed offset press consist of the following components:
- The sheet control system
This mechanism transfers the sheets of paper that will be printed on through the press.
- In the feeder section sheets are picked up from a paper pile. The stack of sheets is placed on an adjustable pile table. Jets of air and/or a vacuum make sure the feeder mechanism only picks up a single sheet of paper each time. That sheet is then transferred to a feedboard where it is properly positioned before being transferred to the printing unit.
- In the infeed section each sheet of paper is transferred to grippers on the impression cylinder.
- If the press has multiple printing units, the transfer section assures that the sheet moves to the next impression cylinder. This is typically done using some kind of chain mechanism.
- Once the printed sheet leaves the final impression cylinder, the delivery section carries the sheet to the delivery pile. This is a table on which the printed sheets pile up in a stack.
- One or more printing units
A printing unit contains everything needed to print one color on the sheet of paper that is moved around by the sheet control system. Offset presses typically contain 1 to 10 printing units. A single-color press obviously contains one single unit to transfer ink to one side of the printing sheet. To print on the other side you have to wait until the ink has dried, then rotate the pile of paper 180 degrees and print on the other side. A press with 4 printing units can print full-color text and images using cyan, magenta, yellow and black ink. Often a fifth printing unit is added for varnish or maybe adding a special metallic ink or so. Such a press with 5 units still only prints on one side of the paper. By adding a reversing cylinder and another 5 printing units, you end up with a very long and expensive press with 10 printing units that can print up to 5 colors on both sides of the press sheet. Such a press that prints on both sides of the sheet simultaneously is called a convertible perfector press.
A printing unit consists of the following components:
- The inking system
The ink fountain is a reservoir that holds ink. Offset ink is not a fluid ink, it looks more like a kind of thick paste. From the ink fountain, this ink needs to be transferred to the printing plate or cylinder. From there that ink will be transferred to the paper or another substrate. The inking system is responsible for this transfer. It needs to break the thick, viscous ink down into a thinner, more workable, and uniform ink film. This is done using a set of rollers. Presses can have up to 10 (or even more) rollers in their roller train. The amount of ink is usually controlled by so-called fountain keys that control the gap between the ink fountain and the first roller. The wider open this gap is, the more ink can be picked up by the roller. Controlling that gap is done manually with a screw on some presses but nowadays those keys are often motorized. The fountain keys are lined up in a series across the width of the ink fountain so that more ink can be transferred to the left part of a page if there are more images or solid tints on that side of the paper. Some presses do not use fountain keys: they periodically dip the first ductor roll in the ink fountain and control the amount of ink that is picked up by the duration of that dip. The inking system assures that a thin layer of ink that is typically 0.2 to 0.4 mils thick is transferred to the printing plate.
- The dampening system
The dampening system makes sure the non-image areas of the printing plate are moistened so that they will repel ink. This is mainly done using water, but additives are needed for long print runs to improve the ink repellency, lower the surface tension, desensitize the non-image plate regions and make sure corrosion, mildew, bacteria, and fungi don’t cause issues. That’s why gum, alcohol, a fungicide, and other agents are added to the water of the fountain solution. To make sure an even layer of water is put down on the non-printing parts of the printing plate, a mechanism similar to the inking system is used. The dampening system is, however, less complicated and requires fewer rollers.
- The plate, blanket and impression cylinders
The plate cylinder is a large roll to which the printing plate is attached. The plate is usually made of aluminum. Its non-imaging parts will be covered by a thin layer of water that is applied by the dampening system. This means the ink which is fed by the inking system will only adhese on all the other areas. During printing, this image created by ink is transferred to a rubber blanket that is attached to the blanket cylinder. From there is the image is transferred to the press sheet. An impression cylinder carries the paper through the printing unit and provides a hard backing against which the blanket can impress the image on the paper. Not all presses use impression cylinders: there are for example perfecting presses that print blanket-to-blanket: the impression cylinder is replaced by a second blanket cylinder, printing both sides of the press sheet simultaneously in a single printing unit.
- The inking system
Now let’s see an offset press in action:
Offset presses are fast. Modern presses can print up to 18000 sheets per hour. Such a sheet can contain up to 48 A4 or Letter-sized pages. Most common are presses that can print 8 pages in one go on a press sheet. That means such a press running at 15000 sheets per hour can print up to 120000 pages per hour.
- Depending on the page content, quality requirements and a lot of other parameters presses do not necessarily run at their full speed. One of the key parameters is the speed at which the inks dry.
- Presses also do not run all of the time – when switching to another set of pages that need to be printed, the printing plates need to be replaced, and first a number of pages are printed to check if the quality is optimal. This is called the setup or makeready time. It can range from 3 minutes to 15 minutes, depending on the complexity of the job and the automation level of the press. On a single-color press switching to printing another color includes completely cleaning the inking system, which is quite some work!
History of offset printing
The offset press was accidentally invented by Ira Rubel, a paper mill owner in New Jersey, at the beginning of the 20th century. He used a lithographic stone press to print bank deposit slips and his press operators would occasionally miss a sheet, causing the press to print an image on the rubber surface of the impression roller. Consequently, the next page fed to the press would get printed on both sides: on one side by the image from the lithographic stone and on the other side by the ink from the rubber blanket of the impression roller. Rubel noticed that the image transferred by the blanket was clearer and sharper than the one transferred by the stone plate. He joined forces with another printer to build a press that would always transfer the image first to a rubber blanket and then have it ‘offset’ on a sheet of paper. Their first offset press was soon followed by others and nowadays this is the most commonly used industrial printing technique.
For more information check this page on the history of printing from 1900 onwards.