Printing processes

Nowadays, there is a wealth of different printing processes. From offset printing to digital printing to letterpress or 3D printing - it is possible to print on a wide variety of materials, from the smallest paper greeting card to a banner for outdoor advertising or even a 3D model. However, not every printing process is suitable for the best possible print result. In this blog post you will learn about the different printing processes in general, how they work and for which applications they are suitable.

The basics: What are printing processes?

Printing or printing technology is a process in which printing ink is transferred to a substrate. Among printing processes, a distinction can be made between analogue and digital processes. In the former, a fixed printing plate is needed to transfer the ink to the medium to be printed. This is not necessary for digital printing, where the printing information is sent to the press electronically or digitally.

The four main printing processes according to DIN 16500

The DIN 16500 standard distinguishes between the following four main printing processes: Letterpress, gravure, through-printing and flat printing. Digital printing is added to these. The printing processes differ in the surface structure of the printing formes and the associated form of ink transfer. Likewise, the type of transfer (direct versus indirect) and the production of the printing forms (chemical, manual, electrostatic) are differentiated.

1. Letterpress printing

Letterpress belongs to the analogue printing process. It was originally invented by Johannes Gutenberg in 1450 and is thus the oldest printing process. However, stamp printing was already known in antiquity as a form of letterpress printing. The letterpress with movable metal type that Gutenberg developed revolutionised modern letterpress printing and marked the beginning of the printing industry. Originally, letterpress printing involved printing entire lines of type directly onto the printed object, but today this complex process has been replaced by digital printing or offset printing.

In letterpress printing, the printing elements are raised from the printing form both haptically and visually, with the areas to be printed being higher than the areas not to be printed. Thus, the areas to be printed are raised, while the non-printing areas are recessed. Letterpress is a direct printing process, which means that the ink is transferred directly from the inked printing elements, which are mirror-inverted, to the substrate without an intermediate stage. The printing form can be composed of individual letters, in the form of clichés or a combination of both. For time, space and financial reasons, which are comparatively high with letterpress printing, the printing process is now only used by individual printers for commercial purposes. It is particularly represented in the artistic fields in the form of linocut or woodcut. The result of letterpress printing is usually characterised by a squeeze margin and a slightly raised print image on the reverse side of the substrate. The process is also used in further processing for embossing, perforating and punching.

1.1 Flexographic printing

Flexographic printing is a further development of letterpress printing and is a rotary printing process. It can be used in many ways because of the flexible printing form, which is made of rubber or a photopolymer. Flexo printing is also more elastic and requires less contact pressure than letterpress. The special feature of flexographic printing is that it is the only printing process that can compensate for run-out and thickness variations of the substrate due to the flexible printing form. Accordingly, everything from thin foils and all types of paper and cardboard to rough and flexible materials and textile fabrics can be printed, and clean halftone printing can be guaranteed.

2. Gravure printing

Rotogravure is also one of the oldest printing processes, its origins go back to the 19th century and can be found in the copperplate engraving technique. In intaglio printing as a direct printing process, the areas to be printed, which are also called cells, are recessed in the printing forme/metal plate, in contrast to letterpress printing. The cells are formed by engraving, etching or laser. The entire printing forme is then inked and the non-recessed area is squeegeed ink-free again. A squeegee, i.e. a metal band, is used for this, which is why gravure printing is also known as squeegee printing. The printing ink remains only in the cells and is transferred directly to the printed object by the contact pressure with two cylinders (printing forme and printing cylinder).

Printed products that have been produced by the gravure printing process can be recognised by the raised area on the back of the paper. The raised area is the result of printing into the damp paper, which creates a plate edge that in turn imprints itself into the paper. The prints made must then dry for a certain period of time, during which they shrink somewhat. This sometimes leads to dimensional differences of one to two percent.

Rotogravure printing is preferred in the industrial sector because it is suitable for producing large print runs. For example, up to 60,000 prints can be produced in one hour with rotogravure or rotary printing. These printing processes involve printing cylinder against cylinder, which means that the printing formes are not made of plates.

2.1 Pad printing

Pad printing is a special form of gravure printing and has existed since 1968. It is also known as indirect gravure printing. As with the original process, ink is filled into a recessed printing form and then squeegeed off. The ink is then picked up by a flexible pad, which is made of silicone rubber, and applied to the printing material with almost no loss.

Pad printing has several advantages. It is also suitable for printing on non-planar objects, and the printed images can be designed with a high level of detail. Pad printing is mainly used as a printing process in the advertising industry, for printing on toys or in the medical sector such as for syringes. It is also used in the automotive industry or for printing tableware. The printing process is profitable for runs of 1,000 or more, but runs of 6,000 to 3,000,000 are also possible. However, a pad has to be changed every 100,000 pieces.

3. Through printing

Through-printing is a universal printing process. The ink is applied to the printed object through recesses in a screen or textile fabric. For this purpose, the screen or textile fabric is stretched in a frame. A so-called rubber squeegee (elastomer) presses the ink against the recess. If certain areas are not to be printed, they are covered with a stencil. The advantage of through-printing is that the technique is independent of the nature of the printed object. The colour result depends on the structure, fineness and elasticity of the fabric. This makes it possible to print on a wide variety of print objects.

Through-printing is mainly used for printing on plastic, pressure-sensitive or inflexible objects, such as bottles. However, advertising banners, stickers, bags or tableware as well as textiles can also be printed.

3.1 Screen printing

Screen printing is a special form of through-printing and is used especially for objects that are difficult to print on. The printing ink is pressed through a fine-meshed fabric onto the printed object using a rubber squeegee. Areas that are not to be printed are covered with a stencil and thus made impermeable to ink. Screen printing is mainly used for textile printing and sign printing. Wood, ceramics, glass, metal and fabrics can all be printed.

4. Planographic printing

The special feature of flat printing is that both the areas to be printed and the areas not to be printed are almost on the same level, so there is no difference in height. Planographic printing is based on the lithographic printing process (direct printing process) and has developed into an indirect printing process. The flat printing process is based on the repulsion principle of water and grease, which is used for ink transfer. The printing block/ink carrier is chemically pre-treated - the greasy ink adheres to greasy surfaces but is repelled by the water-containing surfaces. A cylinder stretched on a rubber blanket indirectly transfers the areas to be printed onto the printed object. Flatbed printing allows the use of several colours and also double-sided printing within one printing process.

4.1 Offset printing

Offset printing is the most widespread flat printing process in the world today. The printing plate, which is mounted on a cylinder, does not come into contact with the printing substrate. Instead, the ink is first transferred to the impression cylinder and from there to the substrate. Corresponding machines for offset printing exist as sheetfed and web presses (heatset or coldset).

5. Digital printing

Since 1990 digital printing has conquered the market, which is not surprising since it has hardly any competition, especially for short runs. Digital printing does not require a physically fixed print body onto which the ink is applied, so there is no need for preparatory work or necessary equipment. Accordingly, it is possible to realise even the smallest print runs from one copy with digital printing with little effort.

5.1 The laser printing process

Laser printing is a form of digital printing and is also called electrophotographic printing. If a copy is to be made, an electrically negatively charged roller is partially discharged by the influence of light (laser or LED). The positively charged toner, i.e. dust-fine powder, is then attracted by the remaining charge and drawn onto a paper that is also electrically charged. The powder, or rather the microscopically small pigments coated with plastic, are melted and squeezed open by heat and pressure and thus fixed in place.

5.2 Inkjet/Injection Printing

In inkjet printing, tiny drops of ink are applied to the paper. This can be done either by heating or by applying voltage to ceramic piezo crystals. However, the colours used in the CMYK spectrum are usually not waterproof or lightfast.

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