Printing Processes

All printing processes produce lines and/or dots to form an image. Printing has made possible the production of multiple copies of graphic images. There are five types of processes which can be used for graphic reproduction. These processes are

Relief printing - Relief printing is the basis for the original printing press as invented by Johann Gutenberg. This is based on raised letter type. Gutenberg's characters were molded out of lead, a metal which was used until only recently.

This process came to be known as Letterpress and was the basis for all newspaper printing until only recently. Letterpress has evolved into Flexography, which uses a flexible plate which contains a raised image area of a cured photopolymer. The plate is wrapped around a cylinder and is used to put ink on the substrate. We'll describe the Flexo process in more detail later.

Intaglio printing - The reverse of relief printing.Intaglio printing as it is used in commercial printing is called Gravure or Rotogravure. The process is based on an image are consisting of a series of recessed cells which hold a liquid ink. The ink is transferred to the substrate from these cells with an image based on the the cell pattern. A gravure press generally employs a rotating metal cylinder consisting of these cells, an hence the roto in rotogravure. Again, we will return to this in more detail later.

Screen printing - transfers an image by allowing ink to pass through openings in a stencil that has been applied to a screen mesh. The process is often called silk screen printing, because silk was originally used to make the screens. Silk is not used industrially anymore. Screen printing is commonly used to print on textiles. We use screen printing to print on t-shirts for student groups which visit our building.

Lithography - Based on a process discovered by Alois Senfelder in 1789. It literally means "stone writing", because it was originally based on images created on limestone plates. The plates are neither raised nor recessed and the process is often referred to as planographic. The printing in based on wetability of different areas of the plate. The image area is oil (ink) wet and the nonimage area is water wet.

The process makes use of the non mixing of the an oily ink and water (actually an aqueous fountain solution). Lithography is the most chemical of the printing processes and Senfelder preferred the designation "chemical printing" instead of "stone printing".

Nonimpact printing - All of the conventional printing processes are impact methods. The plate and/or image carrier comes in direct contact with the substrate.

Nonimpact printing refers to series of computer driven devices that have evolved in a revolutionary fashion over the past 15-20 year. The developments in these devices have followed the dramatic improvement price and performance in desktop computers and the rapid reduction is cost of microelectronic devices. These printing devices include laser printers, ink-jet printers, dye sublimation printers, thermal transfer printers and others. Laser printers uses the same imaging method employed on office copiers.

Indeed, modern color copiers have a direct computer and/or network interface so they can be printed to using the appropriate printer driver. I like to refer to a color copier as a color laser printer with a built in scanner. These devices can be versatile and may sustain high throughputs. The printing process is known as Xerography and has formed the basis for the success of the Xerox company. We'll discuss these and other nonimpact processes in detail later.