SCREEN Glass and ceramics decoration


Meticulous process control at Royal Crown Derby (Courtesy Richard Dennis)


sheet. This is not suitable for printing onto glass or ceramic. For true metallics on glass or ceramic, one must use inorganic enamel.


Think thermoplastic


As well as the inks above, when using inorganic inks/enamels there is the option of using thermoplastic ink. This is an ink system that at ambient temperatures is a wax-like solid. At 65 to 70°C, the wax liquefies and depending on its composition, will have a viscosity between 800 and 2,000Cp. Thermoplastic ink is particularly useful when printing multi-colours because when the warm ink contacts the cold ceramic or glass it hardens quickly so more colours can be overprinted.


The normal method of heating the ink is to pre-melt it and pour it onto a stencil made from steel mesh. A controlled electric current is passed through the mesh to heat up and maintain the ink in a fluid printing state. This makes it possible to print multiple colours on glass bottles at rates up to 7,000 per hour. This method is also used on ceramics. In the case of glass, the printed item has to pass through a lehr and for ceramic, a kiln. Both a lehr and a kiln must be carefully controlled regarding speed, temperature profile and internal atmosphere, otherwise colours will change and images distort. The use of enamels that are either thermoplastic or liquid can use a great deal of gas or electrical energy. Alternatives take the form of organic pigment based inks that are not fused by glass frit onto the surface of the glass or ceramic or under the glaze. Organic pigments would not withstand the temperatures to melt the glass frit in the ink. The alternatives are: reactive inks that use a


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catalyst to achieve a hard chemical resistant surface; baking inks that need elevated temperatures of 120 to 150°C to trigger a chemical reaction; or ultra violet curing ink systems. The latter is gaining a foothold in glass decoration since instant UV curing means multi-colours are possible with resistance to repeated dishwashing.


Total transfer


When printing directly onto glass and ceramics, both screen printing and pad printing can be used as well as a combination of both processes called 'total transfer.' This is where the image is screen printed onto a flat plate and then picked off by a silicone pad and transferred to the article. This allows thick films of ink to be applied to uneven objects. Pad printing with heated plates works well, but there are pros and cons for both systems. Four-colour process is possible with inorganic cyan, magenta, process yellow and process black, but this is not for the inexperienced, as colours can easily change in the printing and firing process.


The simpler alternative to direct printing with ceramic colour is to produce waterslide or heat applied transfers. These transfers are produced by screen printing and lithographic printing. Waterslide uses water soluble glue that allows the image to lift off special paper before being positioned on the substrate. Heat applied transfers come off the carrier paper or film when applied with a heated blanket to the substrate. In both cases the applied image has to be fired at elevated temperatures. Royal Crown Derby is at the pinnacle of waterslide transfer decoration. Their screen printing facility maintains the skills evolved over many years to produce transfers used on their fine bone china products.


Thousands of different designs using inorganic pigments and precious metals can be stored and recalled from a massive database for the screen printing department to meet the exacting quality of collectors around the world. With its beginnings before 1750 Royal Crown Derby has demonstrated its durability by consistently producing top quality table ware, giftware and collectables on their current site since 1890. This is an excellent example of where a British private limited company has moved with times to continuously develop a top quality product that it sells worldwide.


Finally, none of these methods should be confused with sublimation transfers that appear to print onto ceramics and glass but are in fact producing an image in a polyester lacquer that has been sprayed or dip coated on to the ceramic. Although attractive, this is not as resistant as fired enamels or even UV cured inks. Sublimation inks are not particularly colour safe in sunlight or elevated temperatures but it is still a big market, particularly in personalisation and business gifts.


Digital printing is used in this area and there are some ceramic applications where it works well, such as direct tile printing and the production of transfers, but low speed is always a drawback on long runs. For printers who don’t have a kiln or lehr, the production of transfers can be a good business and there are still some companies making a living from this market. As the globalisation business model demonstrates its shortcomings, such opportunities are increasing leading some to ponder whether the 'made in Britain' stamp may start to re- emerge.


June 2010 l SCREEN PROCESS AND DIGITAL IMAGING 15


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