The artwork is arguably the most important part of the screen printing process. In order for a design to be a screen printed the artwork should be created in a vector format. Bitmap artwork can be used to screen print, but vector is a much easier format for screen printers to handle. Vector artwork scales better than bitmap, and vector prints better because it groups and constraints colors. The color is less consistent in bitmap images as they are composed of thousands of tiny pixels. Once you have suitable vector artwork, the colors in the artwork must be separated. This is because most custom screen printing is “spot color,” which means that each color must be printed individually rather than blended. Once the artwork is color separated, each color must be printed onto its own transparent film, called a film positive. This film positive is used to expose the artwork to the screen.
The correct screen must be chosen for the job before the artwork can be exposed. There are many options when it comes to the screens themselves, and it all boils down to the mesh count. The higher the mesh count, the more detail you can print in. Lower mesh counts allow more ink through the screen, which creates more opaque prints. The vast majority of screen printing is done between 80 and 310 mesh, though there are higher and lower mesh counts available. The numbers indicate how many holes are in one square inch of the screen. So if there are 80 holes in a square inch vs 310, then the 80 mesh screen will have much larger holes since there are fewer, but still in the same amount of space. These larger holes allow for more ink to easily be pulled through the screen. White ink is by far the thickest type of screen printing ink, which is necessary for the ink to be opaque enough. Black ink, on the other hand, is typically the thinnest and easiest type of ink to work with because it easily delivers the opacity that you need when printing. The press operator must consider the type and color of ink when selecting the right screen for the job.
Once the right film has been selected for the job it must be taken into a light-safe darkroom. The film positive must then be taped to a screen that has been coated with a photosensitive emulsion. Emulsion is essentially glue that binds to the mesh of the screen in order to create a solid stencil. Different inks will require different emulsions. For instance, water-sensitive emulsion is used with plastisol inks. However, if you were to use water-based or discharge inks, the same emulsion would break down due to the water content of those inks. Most screen print shops clean and coat their screens in batches so that they always have some that are ready to be exposed at any moment. Coating a screen is done by simply dragging a scoop coater filled with emulsion across the both sides of the screen and then placing the screen on a shelf to dry. A well-coated screen will make the exposing process much easier.
Once the film has been taped to the coated screen, it is then exposed to UV light for approximately 30 seconds on average. Exposing the artwork is what burns the design into the screen that will become the stencil for printing. The exact exposure time is dependent on the equipment, mesh count, and type of emulsion being used. Exposing the design for too long or too short will result in a poor, unusable stencil.
Finally, the screen printer will remove the film positives from the screen. The screens are then power washed to remove the emulsion that was covered by the film positive, and thus leaving a stencil for ink to be pulled through. When the screens have dried, they are ready to print. The stencil should last for thousands of prints before it wears down. Now you are ready to load the screens onto a screen printing machine and get started!
