Screen printing is most commonly used on T shirts, flatbed, and windshields. It is also used for outdoor advertising, bumper stickers and industrial applications like surface printing on electronics panels, car parts and similar surfaces. There exists a digital flatbed printer which is also used for flatbed prints, perhaps the screen printer is also in use. When printing on the flatbed, you can choose any of them.
Screen printing is a process that can be applied to almost any surface, laying down an amazing amount of ink. And, unlike other printing processes, that ink can be structural, becoming an integral part of a product like the screen printing on the inside of car windshields. That ink is there to provide a bonding point for windshield adhesive which will otherwise not stick to glass. What’s the difference between the two printers? Ink is one of the components.
Screen printed garments are so popular that many of my college-age students don’t own a shirt that does not have screen printing on it. Attend any event anywhere and you see screen printing on the shirts. In the poorest corners of the world I have seen peasants wearing fancy screen-printed garments.
Screen printing falls into the category of porous printing, along with mimeographing and pad printing (more on that later). The ink flows through a polyester fabric screen, the image on which is created by a photosensitive emulsion exposed through a sheet of litho film (some producers use toner-imaged vellum). The latest technologies for exposure use UV light on very high resolution imagesetters which travel over the surface of the stretched screen and make the exposure directly onto the emulsion. One machine uses ink-jet technology to put a water-soluble black image onto the emulsion. Exposure to UV light is blocked by the black emulsion, leaving the image unexposed.
Screens are then processed by wash-out with warm water, leaving the image area open, and the non-image area opaque. A squeegee pushes the ink through the screen onto the substrate to complete the process. Depending on the ink used, drying can be by evaporation of ink solvent, or by curing in an oven.
Screen printing on fabric creates an amazingly strong surface where the ink is essentially impregnated into the fibers, and then hardened in place. I own one shirt which I wore so much (and washed so many times) it wore-through in the non-image areas, making holes where there is no ink to hold the fabric together. It’s a testament to the combined strength of screen printing ink and cotton fabric.
On hard surfaces, screen printing sits on top of the substrate and is bonded to it at the contact surface. A few years ago I worked with a major audio dic manufacturer to color-manage their screen printing processes for imprinting music CDs and DVDs. We printed a 108-patch color target on the surface of the disc, and I read the color patches to make an ICC profile of their machines. The result was a surprisingly successful profile which they used to prepare artwork for printing on their multi-color presses.
Those presses used screens made of platinum wire, tensioned on aluminum frames. The press was easily capable of holding 5 pt. type, reversed-out of a single color background image. It’s extraordinary how nicely the process works.
Though there are many “garage” screen printers (and entire industry of small-scale printers got their start that way), most of the professional shops today use automated presses, scientifically-prepared screens, precision register systems, and color processes so precise that they can produce in-spec printing on T shirts. It’s an amazing part of our industry.