Leaving exam topic Number
Screen printing - equipment
Equipment
- stencil
- wooden or metal frame
- screen emulsion
- positive film
- light
- printing ink
- printing paper or another materials
- squeegee
After using we must clean equipment by special materials and chemicals.
Stencil
Screen printing stencils serves as a printing plate. More often than not, the screen is a fine fabric made of natural silk, plastic or metal fibres. Plastic or metal fabric is generally used nowadays.
The stencil on the fabric defines the actual print image. The stencils is one side of the screen opposite the side on which the squeegee works, to avoid damage and wear to the stencil.
Manual stencils, which can be produced as drown or cut stencils and transferred to the underside of the screen, are used for simple, solid-area print work.
Kinds of stencils for screen printing
a)mechanical stencils – knife-cut stencils, uses for arts works , called as serigraphy
b) fotomechanical stencils – direct
- indirect
- combination of direct and indirect stencils
the most uses stencils are direct stencils, becouse of economical reasons
Frame
There are diverse types of frames for screen printing.
Wooden, steel and more often aluminium is used. Frames with clamping devices for the fabric are used as are designs where specific clamping arrangement using several clamping devices on aech side of the frame is employed to avoid disttortions to the image structure. The stability of the frame is also vital with respect to the quality of the printing. Low frame weight is expedient for ease of handling, especially in the craft work sector.
Screen emulsion
For sophisticated printed products ( halftone prints, multicolor printing) it is almost exclusively screen emulsions with diazo sensiting that are used ( to produce stencils). After coating and drying a right-reading positive film copy is exposed on the coated side (the side not facing the squeegee) using UV light. The UV light hardens the film emulsion on all non-image areas (transparent areas of the film copy). The image areas themselves are not hardened and can be removed by a jet of water during developing.
This is followed by the drying process. Possible defects can be eliminated with masking screen filter.
In practise, three methods are used for screen printing:
- flatbed method
- flat-to-round method
- rotary printing method
Flatbed method
The printing plate and the printing substrate are both flat. The ink is transferred through the mesh apertures and onto the printing substrate by the movements of a squeegee.
Flat-to-round method
The printing plate is flat, printing onto the printing substrate is done via a rotating cylinder. Printing plate and impression cylinder move synchronously in one direction in the course of which the ink is transferred through the mesh apertures and onto the printing substrate by a stationary squegee.
Printing form and squeegee are adapted to the shape of the printing substrate ( curved, arched, round). Printing plate and printing substrate run synchronously in one direction and the squeegee is in a fixed position. This procedure is used for printing on cans and balls that is on curved surfaces.
Rotary printing method
The printing screen is cylindrical. Printing plate, printing substrate and impression cylinder move synchronously, ink is transferred from inside through the cylindrical printing plate and onto the substrate.
Printing process sections –(frame, squeegee, stencil, printing ink, - printing plate )
Printing ink
The most varied types of ink with most varied properties are available to the screen printer, depending on the print job and the substrate. There is a largeer selection of inks for screen printing than any other printing technology.
It is possible to apply a very thick layer of ink in the screen printing process (normal values are around 20-100um. The thickness of the stencil(the distance that the stencil stands above the screen) determines the thickness of the layer of ink.
Kinds of screen printing ink in the industry
In comparsion with order printing technologies screen printing has the most varied range of application with regard to possible substrates. It is divided into four production areas:
- commercial screen printing for differnt advertising media
- silk screen printing, reffered to as serigraphy
- industrial screen printing, for instance, for bottles, tubes, cups
- special processes such as textile printing or printed circuit boards for electronic circuits and so forth
along with paper and card, possible substrates are plastics, glass, metal, textiles and so forth. The very varied chemical and physical properties of these materials are the reason why a very large variety of ink types are available for screen printing.
When printing on plastic material, screen printing inks are similar to gravure inks and flexographic inks in their composition.
The viskozity must be matched to the desired ink, layer thickness and the fineness of the mesh. Drying takes place by evaporating the volatile solvent and is speeded up by supplying warm air.
For paper and card, however, matte and glossy printing inks of an oil or varnish basis are used. These inks mostly dry in an oxidative process. UV inks are sometimes also used in screen printing.
It is characteristic for screen printing that it is possible to transfer a thick ink layer of up to 12um and more.
Light
The screen printing plate consists of the screen, the frame covered with the screen fabric and the stencil containing the printed information, than we must the prepare screen plate lighted.
We can use light:
- light-press rtuť light by PHILIPS firm, uses UV light
- high-press rtuť light, uses UV light
- Xenon lamps
- Methyl-halogen lamps
- uHlíková lamps
Screen printing machines
In this sector of printing exist very big amount of equipment and machines.
We can choose it during nároku on formats, nákladu and during print paper or another mateials
Printing machines:
- screen printing table
- half aotomat
- ¾ automat