It has become possible in recent years to print reasonably permanent colour images on toughened glass and more usefully to print colour images into the centre of laminated and toughened glass. The life span of these images varies greatly depending on exposure to direct uv light. However with best practice it is reasonable to expect up to 10 years in outdoor applications and considerably more indoors. This is very much in line with the alternative print technologies (silk screen or spraying.) The obvious advantage of UV print in this application is image quality but there can be considerable cost savings.
Typically the cost of UV is about 90 per square meter but there is no need for screens.

The most major difficulty is in ensuring adhesion of the printing ink to the glass. There are a number of steps which must be followed. There are a number of proprietary products available and these are usually a mix of isopropyl alcohol and silane. These in our test tend to leave a residue on the glass and while they assist with adhesion they are only part of the answer.

the areas which must be considered are
1. the cleanliness of the glass
2. use of adhesion promoter
3. surface tension of the glass
4. the quality of the uv light source
5. the quality of the inks

The glass must be cleaned with isopropyl immediately before printing.
Undiluted Silane should be wiped across the surface, always print the air side of the glass

The surface tension of the glass which can be easily measured with a dyne pen must be greater that the surface tension of the ink. Increasing the surface tension is done either by flame polishing or corona treating the glass.

The UV output of the light source is critical. It is possible to measure UV output with a suitable merer and this needs to be done regularly.

The inks are of course important although in testing all of the leading brands worked equally well.
The key to success is to use a proper cross hatch test system to evaluate the results. This consists of a tool that scratches a cross hatch and the a variety of adhesive tapes with known grip. It is important to use such a system as the process consists of a number of steps and each one only gives minor gains. It is therefore important that each change is tested and the effects of changes measured. Adhesion will also be affected by the colour being printed, white ink for example reflects the light and therefore minimises the effect of UV curing and so requires that this element of the process be carefully monitored. Black which comprises a mix of colours is more sensitive to ink quality (age for example)

Results of our latest testing are usually kept here http://www.larsendisplays.co.uk/printing-glass/