The screen-printing process, used today worldwide for printing on garments, was initially developed in China over 1000 years ago and in essence has changed very little. Developments in materials, inks and chemicals have refined the process but the basic method of passing inks or dyes through a mesh screen on to the printable surface remains the same.
It is possible to get similar quality prints whether you are using homemade manual equipment in your garage or operating large automated machines in a factory – one thing that will vary greatly though is production times. The following explains the basic ‘spot colour’ process from somewhere in between, ie working with a small-scale professional set-up using a manual printing carousel:
The first step is to separate the colours in the design using standard software such as Adobe Illustrator or Corel Draw – or to employ the services of any one of a number of graphic designers advertising on the internet. For each colour a screen has to be produced which will be used in turn with all the others to print the image (if printing on to dark garments an extra screen to produce a ‘flash’ undercoat of white ink will be needed to make the colours in the design more vivid). Once the colours have been separated they are printed to scale in black on to acetate sheets, together with registration marks, using a standard ink-jet printer. The blacked our artwork then has to be transferred to the prepared screens.
The standard screens used have a metal or wooden frame with a tightly woven mesh, usually made of nylon or polyester, stretched across them. Screens with different mesh counts are used depending on the intricacy of detail in the design (generally speaking higher mesh counts are used to reproduce more detail as less ink can pass through the screen). Screens are prepared by coating them with a photo-reactive (light sensitive) emulsion and then left to dry flat in an area free from UV light. When this solution is dry the screens are ready to have the artwork ‘burned’ or exposed on to them.
A light box is now used which is basically a box containing UV light emitting tubes, a clear glass surface above and a material lid. Each acetate is placed upon the glass area and a prepared screen laid on top – mesh side down. The material lid is closed and drawn tight over the screen using a vacuum pump – a timer is then set while the image is developed on to the screen. The emulsion on the screen now reacts and hardens where the UV light hits it and stays soft where the areas of opaque black areas on the acetate block it. When the timer has stopped (anywhere between 1 and 20 minutes depending on the intensity of UV light) the screen is ready to be washed off with water using a hose or low powered pressure washer. The water washes away the areas of emulsion that have not developed, creating areas on the mesh for the inks to pass through.
After the developed screens have dried masking tape is applied around the edges to prevent ink seeping through areas that it should not. Each screen is then clamped onto an arm of a carousel, mesh side down, and special ‘plastisol’ or water-based ink in each corresponding colour applied on top. To register, or line up, the screens a scrap piece of material is placed over the platens (boards which hold the garments to be printed) and a test print is produced. This is done by forcing the ink through the clear spaces in the mesh using a hand-held rubber ‘squeegee’, printing the lighter colours first. If printing on to dark coloured garments the flash coat of white is printed and then passed under a heat lamp to touch dry before the colours are applied. The registration marks that are printed onto the scrap material are lined up in turn until the finished, registered image is produced. To cure the ink the printed garment is placed on the conveyor belt of a tunnel dryer and heated to 320 degrees F (160 C). This will prevent the ink from washing away ensure the image will remain for as long as the garment lasts.
If a number of garments are to be printed time is saved by working with a colleague, one t shirt printing and one loading/unloading. To be sure of getting the best reproduction of your designs and logos always use the best quality T-shirts and inks that your budget can accommodate.