In offset lithography a printing plate, which is most often made from aluminum, contains an image of the content that needs to be printed. When the plate is inked, only this image part holds ink. That inked image is subsequently transferred (or offset) from the plate 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.
Below is a picture of a 4 color sheetfed printing press. At the far end is the intake where individual sheets of paper are automatically fed into the press. The 4 towers or printing units each print one color, typically black get printed first, followed by cyan, magenta and yellow. The stack of printed sheets is visible on the front of the machine, underneath the press console & monitor which the press operator uses to control the press.
Digital printing can be done in various ways. Two technologies dominate the industry:
- Inkjet – In an inkjet printer the image that needs to be printed is created by small droplets of ink that are propelled from the nozzles of one or more print heads. Inkjet devices can print on a wide range of substrates such as paper, plastic, canvas or even doors and floor tiles. Inkjet printing is used a lot for posters and signage. It is also economical for short run publications such as photo books or small runs of books. In-line inkjet printers are sometimes combined with other types of presses to print variable data, such as the mailing addresses on direct mail pieces.
- Xerography – In xerographic printers, such as laser printers, the image that needs to be printed is formed by selectively applying a charge to a metal cylinder called a drum. The electrical charge is used to attract toner particles. These particles are transferred to the media that is being printed on. To make sure the toner is fixed properly, the substrate passes through a fuser that melts the toner into the medium. Laser printers are not only used in offices but also for small run printing of books, brochures and other types of document. These printers are also used for transactional printing (bills, bank documents, etc) and direct mail.
As its name implies, this printing technique relies on a screen, which is a woven piece of fabric. Certain areas of this mesh are coated with a non-permeable material. In the remaining open spaces ink can be pushed through the mesh onto a substrate. The advantage of screen printing is that the surface of the recipient does not have to be flat and that the ink can adhere to a wide range of materials, such as paper, textiles, glass, ceramics, wood, and metal.
The image below shows a screen printing press that is used to print t-shirts.
Also known as rotogravure, this is a technique in which an image is engraved into a printing cylinder. That cylinder is inked and this ink subsequently transfers to the paper. Gravure is used for high volume work such as newspapers, magazines, and packaging.
Flock printing or flocking is a printing process in which short fibers of rayon, cotton, wool or another natural or synthetic material are applied to an adhesive-coated surface. This adds a velvet or suede-like texture to the surface. Since the fibers can be dyed, flocking can also add a color to a printed area.
The fibers used in the process are known as flock. They can be applied to paper, plastic, metal, textiles and even glass.
Flock printing is an old technique which was already in use in China around 1000 BC. In Europe it became popular during the Middle Ages.
Thermography or thermographic printing is a printing process that causes text or graphics to be slightly raised from the substrate. The resulting visual effect is interesting, making this a technique that is commonly used on wedding invitations, letterheads, business cards, greetings cards, gift wrap, diplomas and packaging. Since it is also a tactile effect thermography can be used to print braille text.
Additional printing processes
- Letterpress – Once a dominant printing technique, letterpress is now used for business cards, wedding invitations,…
- Flocking – used to add a (colored) velvet-like texture to paper, textiles, etc.
- Pad printing – used to print on 3-dimensional surfaces.
- Intaglio – nowadays mainly used for used stamps and paper currency.
- Thermography – This is more of a finishing process than an actual printing process. It produces raised lettering on the printed side of the paper and is used for wedding invitations, letterheads, business cards,…