Section
B – What is Screen Printing
Simply put, Screen Printing is a printing process whereby ink is forced through a stencil, which is supported by a mesh (or screen), directly onto the object or area to be imprinted. Originally the mesh material was silk, hence the term ‘Silkscreen printing’.
The number of threads per inch of the mesh is called the ‘thread count’. Higher numbers indicate that the mesh is more densely woven. Higher counts produce more detail, but at a price: less ink is deposited on the printed piece. Thus, the printer must balance ink density requirements against detail requirements. Screen printing is a balancing act – the printer must find a screen mesh that reproduces the substantial majority of the design’s detail while still depositing enough ink for a clear, dense image. For printing on textiles such as T-shirts, a good starting point is a 110 mesh screen. Printing process color designs on shirts will probably require a mesh count in the 200 to 300 range.
To screen print, one must first have the ‘artwork’ to be reproduced. This artwork is modified to make it work with the screen printing process, and then stencils are made of the various colors in the design, one stencil per color. Stencils for ‘underlays’ may also be made, but are not always required. Underlays are helpful at making the printing crisp, or at printing light inks over dark shirts. Underlays are printed first (when used) and the other inks are deposited on top of the underlay. Think of printing underlays as being the same as priming wood before painting it. White ink is most often used as the underlay.
The stencils are either made on ‘capilary film’ which is then applied to the screen, or they are made by coating the screen to be used with ‘emulsion’. Emulsion is a light-sensitive liquid that is applied to the screen and allowed to dry. Now the screen is ‘burned’, meaning the artwork is placed over the screen and a bright light is used to ‘expose’ the screen.
Wherever the light hits the emulsion, the emulsion hardens and becomes
resistant to water. Where the design is black, no light reaches the
emulsion, and it does not harden.
The burned (or exposed) screen is next ‘washed out’ with
a high pressure spray of water. Once dry again, the screen is ready
to use.
The screen is mounted into the press and positioned such that the design will be in the desired area when a shirt is placed on the press ‘platen’. Ink is placed on the screen, behind the image in the screen. Test sheets called ‘pelons’ are used to check the print position and quality – the pelons are placed on the platen, and a ‘squeegee’ is used to move the ink across the design under pressure. This pressure forces ink through the openings in the emulsion or film, and the object (shirt or pelon) is printed.
Textile inks vary by manufacturer, but generally they ‘cure’ or fully harden at around 320 to 350 degrees farenheit. Textile inks are known generically as ‘Plastisol’ inks, and represent a substantial improvement over the textile inks of just a few years ago. Plastisol inks are completely permanent once they are fully (read: correctly) cured. Anyone over 30 years old will remember having your favorite shirt ruined by repeated washings: the design came out, or big chunks of it did, anyway. Plastisol eliminates this problem (again, only if fully, properly cured!) by actually bonding to the fibers of the shirt. This is a chemical reaction that takes place when the correct temperature is reached, so it is very durable and consistent. Garments printed with plastisol will frequently wear out long before the design deteriorates!
