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The starting point for printing onto ceramics and glass is the range of inks that can be used. Inks fall into categories: inks or enamels containing finely powdered glass or frit, inorganic pigments and fired in a lehr at a temperature exceeding 600°C; inks containing organic pigments and resins heated to 200°C; and inks containing organic pigments and resins cured by UV radiation.
When printed onto ceramics and glass subjected to automatic dishwashing, the ability to withstand this process is crucial. There are ISO standards but a good guide to resistance is provided by FERRO, a producer of components and finished enamels for this market. It considers an ink should withstand more than 500 washing cycles in professional dishwashers for catering use without significant deterioration of the printed film.
This is a fierce environment. Water mixed with detergent and other chemicals is an aggressive solution, far worse than many solvents. As the temperature of water increases so its surface tension decreases. Add detergent and the surface tension reduces further, effectively making the water wetter. The solution is alkali, hence providing a severe test for these inks. Other tests are applied to inks used in this application including: the effect of acids present in beverages and during washing; even the presence of sulphur in some corrugated packaging has to be accounted for.
Not all inks have to withstand dish washing. Surprisingly printing onto glass cosmetics containers can provide similar challenges. Cosmetics containers must cope with aggressive perfumes and other natural oils. One particular aftershave, normally held in a glass bottle, needed to be put into a plastic container. Conventional plastics melted in contact with the aftershave and a special moulded laminate was required to withstand the chemical attack.
For the highest resistance when printing onto glass the best solution is fired inks. However, this process is expensive and time consuming. UV cured inks on glass and ceramics are growing rapidly. UV curing uses far less energy than a lehr and the ink dries instantly. Adhesion can be very good if the glass is pre-treated and good if not. Claims it is as resistant as fired enamels are optimistic but it is a real alternative for most applications. The colour range of organic pigments is greater than inorganic (fired on), whose colour vibrancy is compromised by heavy metals, particularly lead.
The method of printing UV cured ink is the same as printing onto plastics and board. When printing thermally cured inks, two ink systems are used, one is cold printing the other thermoplastic.
Cold printing inks: enamels
The term ‘cold' means the ink is cold during printing. These inks (enamels) have oil based mediums that disperse the inorganic pigments, glass frit and additives so they can be printed through steel, polyester or nylon meshes. Nylon mesh is used if the printed surface is profiled. The squeegee would also need to be shaped to follow the profile. Once printed, the oils are burned off before the pigments and frit are fired into the glass or ceramic. The whole cycle takes 1 to 1.5 hours during which the temperature is raised to between 600 to 650°C for 10 minutes before the glass is allowed to cool. The lehr atmosphere should be oxidizing and ventilated to remove burned-off residue. Controlling conditions in the lehr is important in achieving satisfactory prints. The disadvantage of the cold printing inks is that inline multicolour printing is not practical as previously printed colours would be picked off by the screen. Thermoplastic colour (TP) allows multi-colour printing. Heat cured organic inks are applied but are baked up to 200°C.
TP inks are solid at room temperature and need heat to become printable. At temperatures below 50°C TP Inks are wax-like. First, they are pre-heated to 65 to 75°C. The molten enamel is then poured into a metal screen heated via an electric current or IR lamps. The molten ink acts like a conventional ink in its passage through the screen. Once the ink hits the colder glass it freezes and returns to its wax state. This eliminates drying between successive colours. TPs suit automatic multi-colour printing machines. Once printed, the multi-colour image is fragile and has to be fired like a cold colour. Multi-colour bottle printing lines run up to 9,000 bottles per hour and print as many colours as there are printing stations.
The big advantage of fired inks or enamels is that they fuse to, and become part of, the substrate. Also, metals can be used as pigments. Gold, silver, palladium and platinum can be applied to glass and ceramics. The application can be decoration or, increasingly, an electrically active film. The disadvantage is the complexity and cost of gas fired lehrs.
The most effective method of applying complex multi-colours is printing onto transfer paper and applying either as a water-slide or heat-applied transfer: image quality is second to none. Often up to 25 colours are used. Four colour process is also used for transfers and direct applications but the image must have a white background either as a printed raft or white substrate. Siak Transfers of Stoke on Trent are masters of the art.
Automotive and electronics applications are also important markets for glass and ceramic printing. As demand for greater visibility from the driving seat increases, so does the use of glass. Tones and filters have to be printed or electrical circuits such as heaters and antennae.
An area where this process is popular is architectural glass. As buildings use more external glass, screen printed effects are generating significant added value. Not just external glass but internal partitions with exotic designs can transform the working environment.
The photograph shows a continuous architectural glass screen printing line. This line has a maximum print area of 3.65 by 2.13m and features an automatic in-feed, printer and automatic out-feed. It can also be fed from the front of the printer when using small format glass. The printer is fitted with the patented Thieme pneumatic squeegee assembly. The machine is operated from a Mac control panel incorporating a machine set up memory. HG Kippax has recently taken orders for this machine model for Saudi Arabia, Canada, UK and India. The company demonstrates you don't have to produce equipment in China to be successful when innovation, dedication, experience and a manufacturing facility in Huddersfield Yorkshire will do.