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Digital Printing is the term used to describe an emerging family of printing technologies which today are capable of short-run, variable printing, and in the future may also achieve long run printing. Traditional printing processes such as offset printing (lithography), flexo printing, gravure (intaglio) printing and screen printing require the production of an image forme - a plate, roller or screen - which contains the information to be printed. A separate forme is required for each colour ink which must be printed, and production of such plates, rollers or screens can be a costly and time consuming business. The image content of the forme is fixed, and cannot be easily changed. This means that traditional printing processes are typically good and very economic for printing longer runs of identical output, but prohibitively expensive for printing short runs or variable information content.

In contrast, digital printing does not require the production of any printing forme, and has the ability to write different images onto each successive piece of media / substrate as required. However, digital printing processes today tend to be slower than their traditional analogues, and often use more expensive materials. For this reason, digital printing techniques are generally finding and developing new niches. As digital printing technologies continue to develop, and the costs of consumable materials become more competitive, this situation is almost certain to change.

Analogue Printing

What is screen printing?

Known originally as Silk Screen Printing or Silk Screening, these are inappropriate terms today as silk fabric is never used. The more advanced and professional screen printers often object to use of the term silk screen. Another term is Serigraphy, nowadays commonly used to describe screen printing of fine art (Serigraphs), c.f. digital (Giclée).

The process is unique among "contact" printing processes, in that it is the only process where the ink actually passes through the printing forme or plate - actually known as a screen. This means that the screen has considerable influence on ink deposit. It is best not to consider screen printing simply as a printing process. Because of its ability, with the right mesh/stencil/ink combination, to print very thick ink or polymer layers onto many substrates, it can be considered as an industrial forming process in such applications.

In fact screen printing describes a versatile family of technologies based around the printing of a viscous ink usually through a woven mesh, or less commonly through an electrochemically-formed mesh roller or plate. Boundaries of the image to be printed are defined by a stencil. A wide range of inks, substrates/media and applications are involved.

Screen Stencil and Mesh technology
The key to flatbed screen printing is the Screen which comprises three elements - Frame, Mesh Fabric and Stencil. Today, other than for certain textile and garment printing, the frame is usually made of aluminium or painted stainless steel. Stretched or affixed to this frame is the mesh fabric, usually a fine woven mesh of polyester, or occasionally nylon or stainless steel. The thread diameter, thickness and mesh count (number of threads per cm or inch) are critical to the process, as the mesh has a strong role in determining the thickness of the ink deposit, and the maximum resolution and definition which can be printed.

The stencil (usually photostencil) is coated or applied onto the mesh fabric. Most commonly, multiple coats of a liquid photopolymer system (direct emulsion) are applied onto the mesh fabric with a coating trough, either manually or using an automatic coating machine. Alternatively for certain high performance applications, a specialist photopolymer film (capillary film) is applied with water, dried and the carrier film removed. [There are other less common or more esoteric methods which will not be described here.] After drying, the coated screen is exposed to UV light (described below) to cause photopolymerisation or cross-linking of non-image areas. Next step is development, where the exposed screen is washed out manually, or in an automatic stencil washer, to reveal the open image areas through which ink will be printed. After drying, the Screen is ready for printing.

The traditional method of UV exposure of the stencil is using a contact frame and film positive, similar to that for offset plate exposure. The only difference of significance is that the contact frame is often much bigger, and has a softer rubber blanket to accommodate the thick screen Frame. More recently, other methods for screen exposure include direct laser imaging (slow, costly and therefore rare); projection exposure (a large screen is exposed by projecting UV light through a small film positive; this reduces film costs); or ink jet screen imaging (opaque ink jet ink is printed directly onto the screen, eliminating the need for a film positive or contact vacuum frame). These latter systems work for lower tolerance (outdoor or textile applications), but do not yet generally achieve the requirements for the more demanding graphic applications. Contact films, produced by imagesetters in-house or from a repro-house, are still the most common method.

Although there are other factors, the finished Screen - frame/mesh/stencil - is the main determinant of ink deposit and image quality.

[A second major variant of screen printing is rotary screen printing. Here the flat screen is replaced with a rotary screen, actually a cylinder of electroformed nickel with regular cavities. Once again, a liquid emulsion is coated onto the screen prior to imaging. Rotary screen printing is widely used for the decoration of textile fabrics. It also finds minor use in reel-to-reel packaging and labelling, sometimes in combination presses with other technologies - flexo, offset, ink jet.]

Printing
After processing, the flatbed Screen is fixed or mounted into a screen printing press, which may be manual or semi-automatic (usually one-colour), 3/4 automatic or fully automatic (often 1,2,4 or 5 colours). Other variants are the so-called cylinder press - rapid single-colour printing onto paper and other flexible substrates.

The printing process usually comprises two steps. Step one is the inking of the screen, known in the industry as flood-stroke or floodcoating. Here, a metal blade (floodcoater, automatic) or rubber squeegee (manual) is used to apply ink across the flat screen and thereby fill the cavities in the stencil and mesh, ready for the print stroke. Step two is the actual print stroke, when a rubber squeegee traverses the screen in the opposite direction and causes ink to be printed through the mesh and onto the substrate.

After printing, the printed item is removed manually or automatically, and sent through a drier or stored on a drying rack. If necessary, further printing and drying operations may be necessary to add other colours, other effects (textured layers, fluorescent inks, etc) or overvarnishes (e.g. UV spot varnishing).

Final steps are usually guillotining or die-cutting of the finished print, possibly folding or packing.

Screen Inks and Drying Technologies
The range of screen printing inks is vast, differing in colour, performance, application and drying technology. Specialist inks are available for printing onto paper and board, textiles, garment transfers, CD's, circuit boards, signage and displays, carpets, membrane switches, ceramics, glass and other substrates.

Key types of screen inks include water-based, solvent-based, thermoplastic, thermoset, UV-curable, water-based UV curable, plastisol. Virtually all screen printing inks for non-textile applications are pigmented.

Water-based inks are used extensively throughout the textile industry, and most larger textile printers formulate their own inks from dyestuffs/pigments and emulsion binders. Water-based inks also find some use in graphics, for printing paper and board, but they are still a minority product in this sector.

Solvent-based inks are the mainstay of the screen printing industry, used in graphics, industrial applications, glass & ceramics, electronics, etc. Often the solvent partly dissolves or swells the substrate surface, providing a "key" to assist ink adhesion. Nonetheless, there are ongoing moves, particularly in developed countries, to substitute solvent-based inks with UV-curable or water-based for environmental reasons (reduction of solvent emissions).

Thermoplastic inks are relatively uncommon, but do find some use in, for example, bottle printing. Thermoset inks are similarly uncommon, used occasionally for specific applications on certain plastics or metals.

UV-curable inks are ~100% solid inks, where the ink comprises pigment dispersed in a viscous mixture of monomers and oligomers, with small amounts of photoinitiator and other additives. After printing, exposure to UV radiation causes photopolymerisation of the monomers and oligomers into a cross-linked structure. Virtually no solvent or additives evaporate, so the ink thickness after drying (curing) is the same as when printed. For various reasons, this means that some aspects of screen printing with UV curable inks are more difficult than conventional (solvent-based or water-based) inks. However, there are also several advantages, including elimination of solvent emissions, and greater screen stability. Provided that exposure to UV or direct sunlight is avoided, UV inks can be left unattended on the screen for hours or sometimes days, and will print well within one or two impressions. Whilst UV curable inks are more expensive than conventional screen inks, overall production costs usually work out cheaper.

Most importantly, UV-curing technology has enabled the design and build of fully-automatic multi-colour screen printing machines by companies such as Svecia, Sias and M&R. Such machines are expensive (typically $800K - $1.5M) but very much more productive and less labour-intensive than conventional methods.

Water-based UV inks comprise a water-emulsifiable UV-curable system, usually provided as a dispersion in water with a small amount of alcoholic solvent. These systems are not 100% solids, often nearer 50%. Currently, such products work best on card and board substrates. Drying occurs by a mixture of evaporation, absorption and UV-curing - many UV lamps are actually "hot", and act well as medium heat driers, in addition to their UV-curing capability. As yet, this technology has not seriously penetrated into plastics printing.

Plastisol inks comprise a dispersion of thermoplastic polymer in paraffinic oil, also some thinner such as white spirit. These inks appear to dry mainly by some rearrangement of the dispersion structure during heating, although there is also some solvent loss. These systems are used extensively for printing of sportswear (T-shirts, baseball caps, some sports bags), but find little use elsewhere.

As screen printing inks tend to be printed thicker than inks in other processes, the amount of solvent emission could be high. Hence there is a slow but steady trend towards development and adoption of UV curable or water-based systems.

Formats
Screen printing crosses a wide range of formats from narrow (mainly industrial) through wide (0.7m - 2.5m width) up to superwide (>2.5m format). However, the process is seeing a strong challenge in the superwide sector from digital (ink jet) printing, and an emerging challenge in wide format from digital (shorter run lengths) and (on paper/card) offset for longer run lengths.

Offset Litho Printing

Known more fully as Offset Lithography or Planographic Printing (sometimes Offset Litho; Litho Printing), offset printing is today the world's major printing process by volume. The process is particularly suited to the printing of paper, card and thin board.

Offset printing is based around a flat (planographic) yet flexible printing plate which is imaged to provide water-attracting/ink repellent and ink attracting/water-repellent areas. By applying oil-based ink and water-based fount solution to the plate, each material wets and adheres to different areas of the printing plate, thereby enabling printing of an image.

Substrates/media such as paper and thin board can be quite abrasive, so to avoid plate wear, and ensure good contact between ink and paper, a rubber offsetting roller is used. The plate transfers its ink image directly onto the offset roller, which then transfers the image onto the final substrate (paper or board). In this way, plate wear is reduced, and the rubber roller gives good contact even with slightly uneven substrate surfaces.

The heart of the process is the offset plate. Plates are usually based on an anodised aluminium carrier, although lower cost and shorter run plates can be produced using polyester film or even modified papers as the carrier. In fact, the earliest litho plates were made by waxing stone blocks with a crayon by hand, or wax/resign coating an absorbent stone then scraping all wax or resin from non-printing areas. Nowadays this process is restricted to highly creative fine art printing. There are two main types of offset printing plates in industrial use - photopolymer plates and thermal plates.

Photopolymer plates comprise a carrier (usually anodised aluminium) coated with a thin layer of photopolymerisable resins or diazo resin, normally sensitive to UV light. The plate is exposed to UV light in a contact frame, sandwiched between a sheet of glass and a flat rubber blanket, in contact with a film positive (or negative), the whole sandwich held under vacuum to ensure good contact. After UV exposure, the plate is developed usually with an aqueous developer by hand or in a processor. With a positive plate, the exposed (non-image) areas become or remain hydrophilic (water-loving), and the unexposed (image) areas become or remain oleophilic (oil-loving). For negative plates, the reverse occurs.

Whilst the concept of thermal plates has been around for many decades, it is only in the last ten years that the technology has matured. Thermal plates are directly imaged, digitally, in a thermal platesetter (c.f. imagesetter, used to produce films) by infra-red laser or LED. There are now over 50 brands of thermal platesetter on the market. This process eliminates the need and cost of intermediate films for contact exposure.

After processing, the plate is fixed or mounted onto the printing machine, which can be anything from single colour sheet-fed to multi (2,4 or 5) colour web-fed depending on cost and application.

A series of inking and fount rollers pick up ink or fount solution, and apply these materials to the litho plate surface. The oil-based ink easily wets image areas, but is repelled from non-image areas which are wetted by the fount solution (usually water admixed with some alcohol and surfactants).

The ink image is next transferred to the rubber offset roller, and thence onto the final substrate (paper, card or thin board).

Virtually all offset litho inks are viscous, pigmented inks. Most oil-based inks dry slowly on the substrate by a mixture of rapid absorption into the porous surface (hence paper/card/board), some slow evaporation or migration of component oils, and sometimes also oxidation (like gloss paint). Such oil-based inks cannot be printed effectively onto plastics and non-porous substrates, hence the relative failure of offset to penetrate the market for plastics packaging (dominated by flexo) and industrial printing on rigid plastics, vinyl and other substrates (dominated by screen printing).

Heat-set offset inks use heat to enhance the drying process and enable easier handling of printed product. There are also UV-set (i.e. UV-curable) offset inks for certain speciality applications.

The only litho printing process which can print effectively onto plastics and non-porous substrates is known as "Dry Offset", based around expensive silicone/photopolymer plates from Toyobo and possibly others. The dry offset process does not need any water or fount solution, as the ink repellency of the silicone resin is sufficient to be used dry. With correct ink formulation, such processes can be used for printing onto plastic packaging or CD's (polycarbonate), etc. At present, dry offset is still a niche process.

In terms of formats, litho printing is limited by the maximum size of available printing plates, the high cost of these, and the costs of wide format machinery. It is rare to find a multi-colour offset printing machine capable of printing formats bigger than A0 size, and these are very expensive. Hence the strong historical penetration of screen printing - and more recently wide and superwide format digital - into the larger format sector.

Common applications for offset printing are documents, brochures, catalogues, magazines, journals, paper packaging, labels, newspapers, letterheads, flyers, etc. Wide format applications are mainly outdoor posters, and usually slightly narrower format indoor point-of-sale.

So-called "bi-metal" plates actually allow the offset litho process to print onto tin plate for can decoration.

Flexo Printing

Also known as Flexography, this process is the fastest growing area among conventional (contact) printing processes today. The technology has emerged from the older concept of Letterpress, typified by newspapers and cold-metal type.

The process relies on printing from raised image areas on a plate or roller; recessed areas do not print. Modern flexo plates are based on modified nylon or acrylic-based photopolymer systems, which are UV imaged through a film positive in a contact frame. Alternatively, polymeric or rubber-coated rollers can be imaged or engraved directly by laser.

Ink is transferred onto the plate or roller from a so-called anilox roller, a roller engraved with a regular pattern of carefully designed cavities which hold the ink. This enables the plate to be inked, without causing surplus ink to flow over plate image edges. Much of the development of quality in flexo has been due to developments in anilox roller technology.

As no water is involved, and considerable flexibility in ink formulation is allowed, flexo can print onto a wider range of substrates than offset litho; albeit not at quite the same quality.

Major applications include packaging printing onto paper, plastics and even metals.

Most inks are solvent based, although water-based and UV-curable (UV-set) systems are also available for certain applications.

Gravure Printing

Described sometimes as rotogravure or intaglio printing, gravure prints using recessed cavities or cells, engraved mechanically or photochemically into (usually) a copper roller. Ink is applied to the print roller from an inking roller, and is then transferred directly onto the final substrate, often low cost paper or card/board. [In a sense, the process is the exact opposite of flexo or letterpress, where the ink is printed from raised image areas on the plate.]

A unique ability of gravure - although not always used today - is the ability to deposit different actual thickness of ink across the image areas, according to the depth of etched or engraved cells. Gravure is greyscale capable, without the need to rasterise the image. In this embodiment, there are no dots or pixels to spread, and ink can only spread at image edge areas.

Gravure can therefore print with good quality onto much cheaper papers or substrates than can offset litho. However, the set-up costs (cylinder engraving and plating; reclaiming) are very high.

Together, these factors conspire to restrict gravure printing to long run colour printing on cheaper paper (magazines, colour supplements, mail order catalogues), or packaging applications.

Textile Printing

Textile printing is a process whereby a coloured pattern or image is reproduced onto a textile material or substrate. Of total global textile production, between 11 and 15% is decorated by printing, with the remainder being dyed, woven, textured or embroidered if colouration or patterning is required. It would be a mistake to assume that the amount of printed textiles is constant, or constantly growing – rather, the amount of textile printing tends to be cyclical. It is at the mercy of fashion trends and economic cycles, so when fashion leans towards simple plain colours or recessions occur, the global meterage (yardage) of printed textiles will usually decline, albeit temporarily. As a macro-trend, textile printing grows in line with population.

There are many techniques for textile printing, among them screen printing, roller printing, transfer printing and inkjet printing. Today, screen printing is the dominant process by far, accounting for over 80% of printed textile production. There are two main types of textile screen printing, flatbed and rotary screen printing, with roughly equal shares of the global textile market.

In screen printing, coloured printing ink is forced through a screen onto the textile fabric. Screens can be flat (flatbed screen printing or table printing) or cylindrical (rotary screen printing). The pattern to be printed is defined by a stencil (usually made from a photohardened liquid emulsion) which is supported on a mesh carrier which may either be a woven mesh fabric, usually polyester, stretched onto a metal or wooden frame (flatbed); or an electroformed nickel roller (rotary). Roller printing is an older and declining process, where an etched metal or patterned rubber roller is used for printing.

Traditionally, every colour to be printed requires a separate screen or roller, which means if the design requires fifteen spot colours, fifteen screens or rollers must be prepared. This means that traditional textile printing methods must print appreciable volumes (minimum run lengths) to be profitable.

In transfer printing, the multicoloured design is printed onto an intermediate carrier layer – usually a transfer paper – and then subsequently heat transferred onto a textile substrate. In total, transfer printing accounts for considerably less than 5% of global printed textile production.

Inkjet printing is a relative newcomer to the textile industry, and today accounts for less than 1% of global production. As inkjet printing does not require production of patterned films, screens or rollers, it can be very cost effective for production of proofs, samples and shorter run lengths. Whilst inkjet should have a bright future in textile printing, there are many technical, economic and structural roadblocks which restrict its adoption today, and it will take time, investment and more in-depth industry knowledge to overcome these. In particular, digital vendors must design machines which textile printers want, rather than simply adapting wide format printers and plotters.

The textile printing industry is heavily segmented by printer type and application, with different technical requirements for each sector. Different textile fabrics also require different dyestuffs or colorants, and different post-print fixing processes, which make the industry a complex one to serve.

Key exhibitions include ITMA and ATME (in USA 2004). Web Consulting's own Digital Textile Conference will take place during March 2005 in Berlin. Further details will be available on this website in due course.

Other

A number of Web Consulting's personnel or associates have specialist knowledge of the printing or decoration of a wide range of products and materials, and the industry sectors where such technologies are used.

The following is a non-exclusive list of sectors where Web Consulting has or can simply access market, technology and competitor information:

As is our norm, we have little publicly published information in most of these sectors, but often considerable in-house data and knowledge. In addition, our knowledge of Internet and library searching, database management and data mining can be invaluable for your other information requirements.