The principle of screen printing is actually very simple: Colour in the form of ink is applied to a surface (paper or plastic, metal, textile etc.,) through a gauze. To make an image those parts of it which re not to be printed. These have to be blocked-off with a special medium which is applied to parts of the gauze weave which may have a fine or coarse mesh. The printing ink is then only allowed to be applied to the surfaces to be printed through the unblocked sections of the gauze.
Nowadays, gauze made from polyester is employed in the printing sector. The mesh of these synthetic gauzes are obtainable in various grades: From 10 threads per centimeter up to 200 threads per centimeter. In cases where the weave has a low fineness e g 30 threads per cm, the threads are relatively thick and the mesh gauge is large. This means that relatively more colour can be carried through the weave. A high application of colour such as described would be desirable for instance when printing textiles. However, because of low fineness, thin lines or fine half-tones cannot be printed.
In he case of a weave with high fineness e g 150 threads per cm, quite the opposite is obtained. The application of colour is low and thin lines and relatively fine screens can be printed because the fine gauge beds well to the medium to be printed on. It is important to know that in the silk screen printing process the application of colour can be varied dependent on what medium is to be printed on. Colour can also be applied to dark materials.
The making of a printing template is achieved by tightly stretching the gauze on to a medal frame. Nowadays the pattern or image to be applied to the gauze is achieved through photographic processes. Additionally, a thin layer of a light sensitive medium is applied to both sides of the gauze. This results in the gauze being blocked-off. Once this light sensitive layer has dried a transparent film carrying the image to be printed is laid over the whole of the whole gauze. It is then exposed to a very strong light source.
The image on the transparent film protects the light sensitive layer from the effects of the light. This causes the exposed parts to become waterproof as a result of being exposed. Those parts of the gauze which were not exposed to light are then freed-up by washing through with water. The result is a printing template.
The open mesh of the weave can now accept ink in whatever colour or quality which may be required. Ink is applied with a rubber sponge. There are many different types of ink which can be used without hindrance to a range of materials. There are inks for paper and board, for many types of plastics and synthetics, metal, glass, textiles etc., Shiny surfaces can also be matted and in some cases inks formulated for opalescent or highly reflective surfaces can be achieved. Additionally, special effects can be achieved in the silk screen printing process among them hammered silver, luminescent colours, pearl effect, ‘electric’ colours, colours which conduct electric current, rubber colours, perfumed colours, and many more.
Subject to the actual operations of an individual printer, silk screen printing techniques can be applied to make weatherproof, chemical-proof and washable products.
Silk screen printing inks which are concentrated are not print-ready. They will need to be diluted prior to use. Subject to the actual image required (thin lines, surface finish etc.,) additives to make an ink dry quicker or slower can be used. Ink can also be made jelly-like by a thickening agent. Use of additives is usually a skill developed through the experience of an individual printer.
Silk screen printing today is mostly undertaken on automatic machines although runs of limited quantity can be made by hand. In comparison to other printing processes silk screen printing speed is not high and the process is not, therefore, suitable for quantity runs.
The application is many sided. Small objects ie advertising material, business gifts etc., up to large objects can be printed on. Variable ink applications, the wide range of colours and colour finishes and the possibiity to have thick, heavy or formed images gives silk screen printing an advantage over other printing processes. Silk screen printing is distinctively an alternative form of printing - literally a coating technique using printing ink!
Typical articles for which silk screen printing is employed are: Adhesive tapes, T shirts, Posters, Shields, Equipment housings, CDs, Ceramic and glass products, Keyboards, Computer boards, Art prints Advertising panels, Plastic bottles, Foodstuff packaging (using vegetable based inks), Credit cards, Ceramic tiles, Textiles (incl bedclothes, clothing etc), Advertsing articles (lighters etc), Glass windows (cars, machines, furniture) … And many more.
