Many thanks to sdcolab.org for their classes and notes on screen printing and making screens!
Screen printing uses a mesh is to transfer ink onto a substrate, except for areas where a stencil blocks the ink. A blade or squeegee is moved across the screen to fill the open areas of the mesh with ink, and a reverse stroke with more downward pressure then causes the screen to touch the substrate. This causes the ink to wet the substrate and be pulled out of the mesh as the screen springs back after the blade has passed. One color is printed at a time, so several screens can be used to produce a multicoloured image or design.
Wear an apron.
Different mesh counts allow more or less ink through the screen and support more or less detail in the picture. 120 to 300 is the usual range, and 160 to 200 are commonly used.
Pellons are fibrous sheets that cost about a quarter used to test a screenprint.
To make a screen, emulsion is applied to both sides of the screen with a trought - scoop coater. It is flash dried, then it can be exposed with a mask in a light box, then the lose emulsion is washed out. The mask can be transparency film. If you print two, you can stack them to improve the opaque areas. It must be stretched over a frame. Or you can purchase a silkscreen in a frame for about $45.
The screen may have open areas, e.g. around the outside edge or for registration marks, which need to be closed off before printing. This can be done with painters tape, but it should not be left on the screen unless the screen will not be re-made. Areas that are to be perminantly masked off can be taped with pastic tapes.
Screen-printing inks are formulated for specific uses, e.g. paper, fabric, etc... Be careful which you order. For fabric inks, look for the t-shirt logo. Inks must be kept moving and thick or they will dry out and if they dry out on the screen, it can be impossible to remove them. For a waterbased ink, a simple water sprayer can keep the ink wet.
The press holds the screens in exact registration over the substrate and also rotate the substrate from the printing station to a flash drying station.
A special spray adhesive can hold fabric or other substrates to the platten so that they don't move at all between prints.
Squeegee's run $2 to $30. Take care to avoid denting the edge. Pull the ink down the screen to cover the pattern with a thin layer, then press down and push or pull the squeegee to make contact with the scree and the substrate, and allow some ink to pass through. Multiple passes can be made, and it's possible to check the print between passes by raising the screen; the press will keep registration.
The Flash Drying station heats the fabric above 350'F to cure the ink. This makes the ink flexible and avoids cracking. About 30 seconds to a minute is all that is needed. Too long and the fabric will burn, too short, and the ink will still be wet. Leaving the flash dryer on when you leave could burn down the building.
For PCB production: Etch resist
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