Many people are familiar with digital, inkjet or laser printing, perhaps through a colour printer at home or in the office. Industrial printing introduces other processes, including offset lithography, flexography, screen printing and gravure.
Printing methods can broadly be grouped into three categories: direct printing, indirect printing and non-contact printing.
Direct printing
In direct printing, the surface being printed comes into direct contact with the printing form. Relief printing and intaglio printing are examples. Making a printing block is one of the oldest approaches; the stamps used in shops and organisations are familiar examples.
Relief printing
The image areas stand above the non-image areas on the printing form. These raised areas receive ink and transfer it to paper or another substrate under pressure. Both the inking rollers and the substrate contact only the raised areas. Letterpress and flexography are relief-printing methods.
Letterpress
Letterpress is one of the oldest foundational printing processes and remained widely used into the early 1970s. Its printing form is a rigid metal or photopolymer surface whose image areas are raised. A high-viscosity ink is applied to the plate, and pressure transfers the image to the substrate. Modern letterpress differs from flexography in plate hardness, ink properties and the inking system.
Flexography
Flexography uses a flexible printing plate, typically made from photopolymer or elastomer. It is a modern development of letterpress and can print on a wide range of substrates, including plastic films, foil for food and pharmaceutical packaging, cellophane, metallised films and paper. It is widely used for food-packaging films, cartons, self-adhesive labels and wallpaper, including non-porous substrates.
Intaglio printing: gravure
The article traces gravure’s origins to Italian artists around 1300 and the Renaissance tradition of engraving fine designs by hand into soft copper plates. The engraved grooves or recesses held ink for printing.
In gravure, the image areas are recessed below the non-image surface. The recesses are filled with ink and the excess ink is wiped from the cylinder surface. The cylinder then contacts the paper or other substrate under pressure and transfers the ink. Deeper recesses hold more ink and produce greater colour intensity. On a gravure cylinder, these recesses are separate cells; even fine lettering is reproduced through this cell structure.
The source article describes how technical developments strengthened gravure’s position in packaging after years of preference for flexography. A principal advantage it identifies is consistent print production over long runs.
Screen printing
Screen printing has been used since the early twentieth century on textiles, wood, glass, advertising panels and other materials. An image is formed as a stencil on a mesh, with the non-image areas blocked. Ink is pushed through the open mesh onto the substrate. Polyester, nylon or stainless-steel mesh is stretched tightly over a rectangular wooden or aluminium frame. Silk was originally used, hence the name silk-screen printing, but is now uncommon. The stencil can be prepared separately and transferred to the mesh, or produced directly on it using a light-sensitive liquid emulsion.
Choosing meshes with different opening sizes changes the amount of ink deposited. The range can extend from a very thick wet ink layer of about 150 micrometres to a comparatively thin dry deposit of 4–5 micrometres.
Screen printing’s principal advantage is the flexibility of its applications.
Indirect printing
In indirect printing, the image first moves from the printing form to an intermediate surface, and from there to the final substrate, such as paper. Offset is a well-known example: the plate transfers the image to a rubber blanket, which transfers it to the substrate.
Offset lithography
Offset lithography is widely used for high-volume commercial printing. The word lithography comes from the Greek lithos, meaning stone, and graphein, meaning to write. In early lithography, the image was drawn on a stone slab, which was treated with water and gum arabic. Before printing, the stone was dampened. The greasy image areas repelled water and accepted ink, while the wet non-image areas repelled ink. Paper placed against the stone received the image under light pressure.
In offset printing, image and non-image areas are essentially on the same plane. The process relies on the difference between water and oil: image areas accept ink and repel water, while non-image areas accept water and repel ink. The plate receives dampening water followed by ink, then transfers the inked image to the blanket.
For four-colour offset printing, the artwork is separated into cyan, magenta, yellow and black (CMYK), and the corresponding aluminium printing plates are prepared.
Sheetfed offset
Sheetfed offset presses print individual sheets. Feeding the paper one sheet at a time helps position text and images consistently on each page and suits smaller and medium-format work.
Sheetfed presses come in different sizes, often close to the international B-series paper formats, which are slightly larger than the corresponding A-series formats. B2 is 707 × 500 mm, B1 is 1,000 × 707 mm, and B3 is 353 × 500 mm. The source relates these to the press-size descriptions commonly used in its local market.
Sheetfed offset presses are available with one, two, four, six or more colour units. Some can be changed from single-sided printing to perfecting, allowing both sides of the sheet to be printed in the same pass.
Web offset
Web presses are fed with a continuous paper reel, which the source describes as commonly weighing around ten tonnes. Beyond the feeding method, web and sheetfed printing differ in speed, cost and output characteristics.
Large web offset presses have a fixed format determined by the cylinder dimensions. Advanced folders at the delivery end can fold the printed web in different ways and pass it to connected binding equipment. Alternatively, the web can be cut into sheets of a specified size for finishing on separate machines.
Large web offset presses fall into two main groups: heatset and coldset.
Heatset web offset
As its name suggests, heatset uses heat to dry the ink on paper. It is used for high-quality, high-volume products such as magazines, catalogues and commercial brochures.
Coldset web offset
Coldset has no heat dryer: the ink sets by penetrating the paper. It is commonly used for newspapers and similar products, printed in black and white, with an additional spot colour, or in four colours.
Non-contact printing
In this group, printing does not rely on contact with a conventional printing form. Ink or toner is applied through other mechanisms. The source includes inkjet printers, laser printers and photocopiers in this category.
Inkjet
Inkjet turns digital information into an image by depositing liquid ink directly onto paper. Tiny nozzles produce small droplets of ink to build the image.
The desired image is programmed in the printer’s control system. In charged-droplet systems, electrical charges help guide ink particles into the planned image. Letters and pictures are formed as patterns or matrices of dots.
Inkjet is used for serial numbers, coding and date marking on packaging. Colour inkjet has also developed considerably; examples include addressing magazines and newspapers. Large-format inkjet printers produce posters, advertising graphics, plots and banners, with high-quality output.
Electrophotography
The article distinguishes toner-based and ink-based digital electrophotographic processes.
In the toner process, coloured powder forms the image. The source describes an electrically charged roller, with a charge of around 600 volts, and an exposure stage that prepares the image areas. Toner is then transferred and fixed to the substrate using pressure and heat. This is a fast process with industrial applications.
In the ink-based process, coloured ink is used instead of powdered toner. This approach also supports high printing speeds and high output volumes.
Dye sublimation
Dye sublimation is a thermal printing process. Colourants carried on a film ribbon are vaporised by heat and transferred onto the receiving material to form the image.
Laser marking
The source describes a laser process in which focused heat changes the colour at an individual point. Although treating an image point by point might seem slow, each change happens very quickly, allowing high operating speeds.