Screen Printing Technique
A screen is made of a piece of porous, finely woven fabric called mesh stretched over a frame of aluminum or wood. Originally human hair then silk was woven into screen mesh; currently most mesh is made of man-made materials such as steel, nylon, and polyester. Mesh is made in different thicknesses determined by threads per inch. A large blocky print is best printed through a low mesh count "81 treads per inch", a detailed print with halftones is best printed through a high mesh count "305 threads per inch". The screen mesh is coated on both sides with a non-permeable photo sensitive material "emulsion" to form a stencil, which is a negative of the image to be printed. That is, the open spaces are where the ink will appear. Once a screen is coated , the screen must be coated with emulsion and let to dry in the dark.
There are several ways to create a stencil for screen printing. An early method was to create it by hand in the desired shape, either by cutting the design from a non-porous material and attaching it to the bottom of the screen, or by painting a negative image directly on the screen with a filler material which became impermeable when it dried. For a more painterly technique, the artist would choose to paint the image with drawing fluid, wait for the image to dry, and then coat the entire screen with screen filler. After the filler had dried, water was used to spray out the screen, and only the areas that were painted by the drawing fluid would wash away, leaving a stencil around it. This process enabled the artist to incorporate their hand into the process to stay true to their drawing.
Today, the original image is created on a transparent overlay such as acetate, tracing paper, or a clear film positive. The image may be drawn or painted directly on the overlay, photocopied, or printed with a ink jet or laser printer, as long as the areas to be inked are opaque. A black-and-white negative may also be used (projected on to the screen). However, unlike traditional plate making, these screens are normally exposed by using film positives. The image to be printed is attached to the underside of the emulsion coated screen usually by clear tape, and placed under ultra-violet light in the 350-420 Nanometer spectrum. The UV light passes through the clear areas and create a polymerization (hardening) of the emulsion. Referred to as burning. The screen is removed from the light source, and the film is removed. Using a pressure washer the screen is coated with water. The emulsion that the light did not hit (the positive black on the film) will wash away from the screen. The emulsion that was hit by light stays on the screen leaving a negative stencil of the image on the mesh. The screen is placed to sit and the emulsion dries and hardens.
The screen is placed and attached to the screen printing press so that when it is atop the platen (the surface the tee shirt is placed over and attached to by spray glue) there should be about a sixteenth of an inch gap. Ink is placed on top of the screen, and a fill bar (also known as a flood bar) is used to fill the mesh openings with a layer of ink. The operator begins with the fill bar at the rear of the screen and behind a reservoir of ink. The operator lifts the screen to prevent contact with the substrate and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The operator then uses a squeegee (rubber blade) to move the mesh down to the substrate and pushes the squeegee to the rear of the screen. The ink that is in the mesh opening is pumped or squeezed by capillary action to the substrate in a controlled and prescribed amount; i.e. the wet ink deposit is equal to the thickness of the mesh and or stencil. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate (called snap-off) leaving the ink upon the substrate surface.
On a manual screen printing press the squeegee is used for flooding and printing by using a pushing stroke to flood and a pulling stroke to print. The thickness of the ink on the tee shirt is determined by the angle, pressure, and speed of the squeegee. A lower angle pulled slowly produces a thick layer of ink, and a higher angle with faster speed produces a lighter layer of ink. There are multiple types of screen printing presses. The "flat-bed", "cylinder", and "rotary". Textile items are printed in multi-color designs using a wet on wet or a flash technique. On dark tee shirts a white ink is printed first. If there is not white in the design the white under base film is "choked" or made smaller so the colored inks cover the white and the white ink does not show around the edges of the graphic. This is done in the graphic program the image was created in. The white ink is printed and then "flashed" with heat making the ink tacky but not dry or "cured" then the colored inks are printed on top of the white. That ensures the colors will be bright and vibrant on a dark surface.
The screen can be re-used after cleaning. However, if the design is no longer needed, the screen can be "reclaimed" (cleared of all emulsion and used again). The reclaiming process involves removing the ink from the screen then spraying on stencil remover to remove all emulsion. Stencil removers come in the form of liquids, gels, or powders. The powdered types have to be mixed with water before use and can be considered to belong to the liquid category. After applying the stencil remover by spray, letting set for a minute, the emulsion must be washed out using a pressure washer. The screen mesh is then washed or "de-greased" with a soap based liquid. If there is any impurities on the mesh the emulsion will not lay on the mesh properly and dry with bubbles or an uneven surface which will affect the burning of the screen. Most screens are ready for re-coating at this stage, but sometimes screens will have to undergo a further step in the reclaiming process called dehazing. This additional step removes haze or "ghost images" left behind in the screen from printing the ink through the screen mesh once the emulsion has been removed. Ghost images tend to faintly outline the open areas of previous stencils, hence the name.