Screen Printing

Screen printing is a process in which ink is forced through a screen. The screen printing stencil serves as a printing plate. More often than not, the screen is a fine fabric made of natural silk, plastic, or metal fibers/threads. Plastic or metal fabric is generally

used nowadays. Ink is imprinted/transferred through the image-specific, open mesh that is not covered by the stencil. The screen printing plate is therefore a combination of screen and stencil.

It is the material, the fineness of the screen (the number

of screen threads per centimeter of fabric length), the thickness of the screen, the distance between the top and bottom sides of the screen, and the degree of

opening of the screen (the degree of screen opening areas as a percentage describes the ratio of the total of all mesh openings to the entire surface of the fabric) that determine the printing properties and quality of the

fabric (screen).

Fabrics can be obtained in levels of fineness from 10

to 200 fibers/cm. The most frequently used fabrics are

those between 90 and 120 fibers/cm. The notches on the edges of the screen dots come about due to the screen structure.

The screen work and printing of very detailed illustrations necessitate the use of very high levels of fabric fineness that are matched to the resolution requirements of print image reproduction. For screen work, fabric fineness (threads/cm) should be around three to four times greater than the screening of the print image (lines/cm) – therefore nine to sixteen different screen dot area surfaces per screen cell.

The stencil on the fabric defines the actual print image.

The stencil is on the side of the screen opposite the

side on which the squeegee (blade) works, to avoid

damage and wear to the stencil. Manual stencils, which

can be produced as drawn or cut stencils and transferred to the underside of the screen, are used for simple, solid-area print work.

For sophisticated printed products (halftone prints,

multicolor printing) it is almost exclusively screen

emulsions with diazo sensitizing 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 lacquer (screen filler).

In practice, three methods are used for screen printing:

• The flat-to-flat method (flatbed). 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.

• The flat-to-round method and body printing.

– 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 squeegee.

– 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.

• Round-to-round method (rotary printing). 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.

The actual printing process can be sub-divided into

four different sub-areas.

The screen is held by a screen frame. The substrate lies flat on the base plate, which forms the printing body, and is held there securely during the printing process. The volume of ink on the screen is moved like a tidal wave by the squeegee. The ink beneath the tidal wave penetrates the screen. This area is known as the filling zone. Once again the ink in front of the squeegee edge in the contact zone passes through the fabric and makes contact with the printing substrate. Behind the edge of the squeegee, in the adhesion zone, the ink ensures that the printing form adheres to the substrate.

The screen’s pulling strength pulls ink fibers out of the ink film in the “release zone,” meaning that residual ink

remains in the mesh of the screen and an even layer of

ink remains on the substrate.

It is possible to apply a very thick layer of ink in the

screen printing process (normal values are around

20–100 mm, offset printing values are typically around

0.5–2 mm). The thickness of the stencil (the distance that the stencil stands above the screen) determines the

thickness of the layer of ink.

The most varied types of ink with the most varied properties are available to the screen printer, depending

on the print job and the substrate. There is a larger selection of inks for screen printing than any other

printing technology.

Typical screen printing products are:

• textiles/materials,

• printed T-shirts,

• printed toys,

• fronts of televisions, radios etc.,

• automobile dashboards, measuring equipment, etc.,

• packaging (plastic bags),

• printed circuit boards,

• large-format advertising posters.

Дата добавления: 2015-11-16