Screen Printing

As the name implies, the printing ink is pressed onto

the substrate through a screen or a stencil (fig. 3.1-54).

For this a squeegee is drawn across the printing form,

which forces the ink through the screen’s open mesh

(see sec. 2.4.1).

In the screen-making process the screen that serves as

an image carrier is opened in accordance with the image

to be printed. The main methods applied are as follows:

• In the direct process the screen is covered with a light sensitive stencil emulsion and exposed with a film

positive. The exposed parts of the coating then cure

and the unexposed parts of the coating are washed

off. This method is preferred for halftone jobs.

• In the indirect process a special screen printing film

is first exposed with a positive film and developed,

and only then transferred to the screen. After drying

the film base is removed; the printing form is ready.

• Finally, it is also possible to combine the above

methods into a direct-indirect process: the special

screen printing film is first applied to the screen,

additionally covered through the screen, and only

then exposed and developed.

• A special form of screen printing (rotary screen

printing) is applied when printing textiles or wallpaper.

Here cylindrical metal stencils are used that

have been produced – for example – in an electroplating

process (like the metal foils used in some

electric razors).

There are detailed recommendations for quality control

in halftone screen printing that may be considered as

standard. The quality of the print is not only affected by

the actual copying process, but also by the selection of

the screen frame, the screen fabric, and the preparation

of the process. Of absolute importance is the correct dimension of the screen frame considering the printing

format and the tension of the fabric. Checking the fabric

tension with a screen tension meter is important for

good register in multicolor printing. The evenness of the stencil thickness across the entire print format and the repeatability of the coating process are essential criteria for the quality. This is only achievable with an automatic coating machine or when direct films are applied. It is advisable to carry out routine checks of the roughness value of the printing side and of the thickness of the stencil. (The roughness value RZ [see e.g., ISO 4287]should be between 4 µm and 10 µm.)

The most important control device for the production

of screen printing forms is the UGRA Plate Control

Wedge 1982 (see fig. 3.1-51), well-known in offset

printing. If in the microline target the finest positive

lines reproduced on the printing plate are 10 µm, the

copy is correct. The tone value decrease in the middle

tones is then approximately 4%for frequencies of 24/cm

or 30/cm. This value corresponds to the circumstances in offset printing for 60/cm and positive platemaking.Once the copy has been calibrated together with the test wedge, that is, the optimal light sum has been determined for a certain type of stencil with a specified thickness, the contone wedge is a simple means of control for other copies, the solid step of the contone wedge on the ready-to-print screen forms must correspond to the value determined in the calibration. If a different step is shown a new calibration procedure must be initiated.

Дата добавления: 2015-11-16