Signs that included text were translated into eight languages and the explanatory leaflets explaining the complex fact you cannot smoke in any enclosed public places or building used by the general public were translated into twenty four, yes twenty four, languages. Why you may ask? How our esteemed Government spends our taxes is not for discussion in this article. Thank goodness so much is spent on print, particularly the screen and digital variety.

The Statutory Instruments 2007 No 923 Pubic Health, England The Smoke Free Signs Regulations 2007 takes up less than a page. The BS EN ISO 7010:2003 standard details the signs' specifications. Between them these two documents have created the need for tens, possibly hundreds, of millions of signs. What a delight for us printers.  It is not just ‘no smoking' but every other hazard that needs explaining to us.

As well as the message, there is the substrate onto which it is printed. Common ones are coated paper and vinyl, then polycarbonate, acrylic and metals. The most widely used metal is aluminium both non-anodised and anodised. So what is the difference?

Non-anodised is plain or lacquered aluminium where an ink, normally two component, is printed onto the surface. No matter how good the ink it can be removed by scratching or chemical attack. Two component ink uses a catalyst to affect cure, while the addition of heat can improve adhesion and chemical resistance. However, it is still a surface print.

Printing onto anodised aluminium is quite different. Firstly, it uses dye rather than pigment to provide colour. Secondly, when the production process is completed the image is formed inside the material. So how does it work?

Hard as sapphire

Aluminium naturally forms a hard surface oxide which helps protect the metal from mechanical and chemical attack. In the anodising process this protective coating is thickened by combining aluminium with oxygen to form a protective surface film of aluminium oxide (AL2O3). The oxide film has a hardness similar to ruby or sapphire. The anodising process is normally carried in a specialised facility: it requires plenty of space, a series of immersion tanks and large amounts of electricity. The process is called anodising because the part to be treated forms the anode of an electrical circuit. The process can create up to 200µm of aluminium oxide coating. Companies could  to do their own anodising but the investment and inconvenience normally prevents it. Thus, printers buy pre-anodised sheets.

Images are normally screen printed although a photomechanical method can be used or, increasingly, digital printing. Look at an anodised surface through a microscope and you see an open cell structure. It is the coating's porous property that lends itself to retaining the printed image.

The secret is filling the cells with dye. So, you simply print the special dyes onto the surface and they enter the open cells in the oxide coating.

Anodised sheets must be kept clean before printing with dye-based ink. Colours are limited but with careful selection a good range is achievable. Printed dye must be air dried before additional colours are applied. At this stage the image is unstable and can be wiped off. To seal it into the porous anodised surface, printed sheets must be immersed in boiling water for about 30 minutes. This seals the once open cells and the dye is trapped inside, resulting in an under-surface print. It is a laborious process but the finished result is impressive.

This results in a hard, chemically resistant surface with the image completely protected. The technique is ideal for aggressive environments where image permanence is paramount. Regarding safety signs, it may not be possible to match the precise colour stated in the standard but the image will be in the material indefinitely.

Probably the most challenging application in the production of safety signs is printing illuminated emergency exit signs. These are ones with running figures in white on a green background. To the uninformed it looks easy: a white background with a simple green print on top. How wrong they can be, it is actually one of the most challenging screen printing applications. Many printers have tried and most failed.

Without blemish

The difficulties of this application are triggered by the fact the sign is backlit and can often be viewed from both sides. When you backlight a solid colour, what looks fine when viewed in reflected light can look streaky and miscoloured when backlit. The ink film has to be consistent across the print without blemish. In some cases it is necessary to compensate for the light intensity over an area by deliberately altering the printed image's density.

Experts in this sector are Chesterfield-based MTM Labels. The company operates a dust free environment at every step of the process and adopts rigorous production control systems, basing its techniques on lean manufacturing.

Managing director, Ian Greenaway, said: "We are not printers, we are an engineering company who uses print as one of its main production processes. We also have to understand lighting technology and work closely with lighting technologists at the design stage."

The disciplines learnt in producing illuminated signs have been reflected across other products, particularly fast turnaround orders via the company's Express Order Service.

For companies whose market place is the safety sign catalogue the emphasis moves from high quality to fit for purpose. The challenge here is to manage the stockholding of signs and respond instantly to orders placed that day, fulfilling where possible from stock and producing the shortfall in the day. As an industry we are used to quick turnaround as print is often responding to market fluctuations. However, in the safety sign business afternoons on the stencil room and print production floor can be controlled and sometimes uncontrolled chaos, where there is as much adrenaline as ink.

You may ask why screen print? Why not use digital printing for these very short runs? The answer is the line colours have to match and must have a light stability foreign to digital printing. This doesn't exclude digital print and vinyl cutting from the safety sign jungle. They all have their place but for some time to come screen printing will be king of this particular jungle.

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