Screenprinting
Screenprinting is a process used for both fine art printmaking and also in the textiles industry. It is particularly suited for use with photographic imagery, but there are also many non-photographic ways to use the process too. A screen consists of a frame, usually made from aluminiun, over which has been stretched a fine synthetic mesh which is glued in position. Ink can be forced through this mesh with a squeegee onto a surface underneath, which is typically paper or fabric. A squeegee consists of a rubber blade held in place within a wooden or aluminium handle.
Processing the screen
The screen is first treated with a chemical degreaser and dried, which cleans the mesh thoroughly. Assuming a photographic image is to be processed, a coating of photographic emulsion is then applied to the screen using an aluminium trough. This coating is sensitive to ultraviolet light, and the degreasing ensures the emulsion will adhere well to the mesh. The screen is now thoroughly dried. The artwork to be exposed onto the screen is now prepared. This is normally an image printed out on a laser printer on acetate or copy paper. Inkjet prints are not so suitable as the ink is far more transparent than the toner used in laser prints, and wouldn’t expose the image as well. If the image is printed on paper, it should be oiled with vegetable oil and blotted thoroughly to make the non-image areas transparent.
Exposing the screen
The artwork must now be exposed onto the screen using an ultraviolet exposure unit. The artwork is placed face up on the glass surface, the screen placed on top and then the vacuum pump should be activated. This ensures that the glass, mesh and artwork are clamped tightly together for an accurate exposure. The screen is now exposed with the UV light. The light passes through the glass and artwork and hits the emulsion. It hardens the emulsion where it gets to it, but the dark areas of the image prevent the light from passing through, and these areas are not hardened. The length of the exposure will vary depending on the result needed and the model of exposure unit. The screen is removed from the unit after exposure.
Washing out screen
The exposed screen is now brought to a wash-out booth and sprayed with a pressure washer. The pressure should not be set too high for this. This will remove the emulsion where it was not hardened by the UV light, forming the stencil for printing. A certain amount of control over the tones in the image can be had at this point by applying higher pressure water to areas where the image needs to be darker. The sreen is now dried and is ready to be printed.
Printing the screen
The screen is now either clamped onto a vacuum table for printing, or attached to a table top using jiffy clamps if one isn’t available. Assuming you are using a table, first attach spacers (a thin piece of wood or pieces of card) to the near edge of the screen so that the entire mesh is a few millimetres above the table’s surface. This spacing is called the snap and is important for good printing. Next tape off the areas of mesh around the image you are to print so the ink doesn’t flow into parts of the screen where you don’t want it. Apply a generous line of ink on the near side of the image. The consistency of the ink can be controlled with thinners. If the ink is too thin it will flow over the screen uncontrollably, and if it is too viscous it may dry too quickly and clog the screen. Holding the screen slightly up from the table top with one hand, push the ink gently across the printing area with the squeegee held in the other hand. The squeegee should be almost upright. This is called flooding the screen. Ensuring there is a piece of printing paper underneath, now print the image. With the screen placed fully down, grip the squeegee in both hands and place the blade down on the mesh behind the ink (now above the image.) Holding it at about a 45 degree angle, firmly pull the squeegee across the image towards you. This should have forced the ink from the mesh onto the paper below. Flooding and printing can be repeated with fresh paper as many times as needed. The position of the paper can be marked on the table top with masking tape if you wish, or a piece of acetate hinged onto the table with masking tape can be used to register the image. When finished printing, remove the excess ink from the screen with a piece of card and return it to a container. Wash the screen with water and a sponge to remove the rest, and dry the screen before printing again. The squeege should also be cleaned.
If a new stencil is to be applied to the screen, a stripping agent is used to remove the old emulsion as washing it with water won’t remove it. The screen is then degreased and the entire process repeated. Screenprinting is very suitable for printing images where many colours are layered over each other to create complex images.
