OUTPUT

Printing direct to glass isn't a new phenomenon. For many years screen-printing companies have been producing applications ranging from the decorative and architectural through to industrial jobs, all produced in a variety of run lengths. But, true to form, working with this medium has become an area where the convenience of one-offs and low volumes has attracted the benefits of ink-jet technology.

The composition of the inks needed for successful glass printing has not been particularly conducive to the process of making a smooth transition to digital, and this meant that manufacturers of wide-format digital engines needed to consider the most suitable way of getting successful and durable results without compromising quality, colour and overall longevity.

As well as opening up new markets where single pieces and short runs can enable the practical production of applications there are other advantages. One-offs largely would have been prohibitive in terms of time and cost for direct output; a popular alternative was to print onto film and apply this to the glass. Screen-printing's forgiving nature has made it a good candidate for applications but the necessity for screens, stencils and lengthy makeready has always been bound up with time-consuming processes and inherent costs.

In architectural terms, glass presents interior and exterior opportunities for decorative and functional applications. Not only can it be incorporated for aesthetic purposes, print can be used to diffuse unwanted light, add security and enhance privacy without relying on traditional frosted patterns. In office and domestic environments, too, glass has become an area where colour and design combine to brighten up what is, to all intents and purposes, just a sheet of clear material.

In the wide-format sector some manufacturers have separated glass printing from the other capabilities of their machines; others have been moderately successful using unmodified platforms. Key to the process is the efficacy of the UV-curable ink employed and its long-term adhesion; but glass is inorganic and ink is organic, and this makes a happy marriage tricky. It's complicated further because the surface of the glass isn't really conducive to ink acceptance because of its inherent smoothness and shine, plus there is a tendency to repel coatings and show up imperfections.

The two best-known manufacturers in the display sector offering glass printing have differing philosophies. For Polytype, its process relies heavily on pre-treatment so that output doesn't need any post-treatment in an oven or furnace. Conversely, Durst's option involves a production line which includes a drying system that heats the printed output to 200˚C for ten minutes. A typical configuration has a transport table which takes the material through a pre-clean unit and a Rho flat-bed printer, before moving it to an infra-red drier for intense heat treatment.

Glass is heavy; skew during feed and afterwards is undesirable. Thus handling needs to be exercised with caution and overall fragility needs to be factored into the process. White ink is advantageous not only in its own right but also as an underprint or overprint layer to give colours the additional opacity to make them stand out.

In terms of methodology, Durst has developed an organically pigmented product specifically for glass printing that's formulated to generate a strong bond once cured and then 'baked' so that chances of delamination are avoided. The company says that it doesn't need the application of time-consuming primers, although a pre-treatment washing and drying unit helps to automate the glass production line, along with special tables for loading and unloading the finished prints.

Polytype puts its eggs in the pre-treatment basket, using a two-step process to engender a strong bond between the ink and the glass. This consists first of a flame coating to encourage consistency in wettability and overall adhesion, before an adhesion promoter is applied to encourage stability of the ink layer. The company says that its Vetro inks have passed a wealth of tests to prove their durability and efficacy, despite the fact that the Virtu-based system doesn't require any post-treatment.

A dedicated glass production line won't come cheap and there are companies that happily print to this medium using existing technologies, and some display producers are producing stunning work using a standard UV-curable printer with a decent table. This takes away the myth that a combination printer doesn't have the stability to work with this material.

Specialists in printing direct to glass need to know their onions. But as designers and architects continue to find new ways of using this medium, so more interesting opportunities will surely arise.