Photo emulsion is what makes themodern day screen printing process possible. The emulsion is a substance that reacts to ultraviolet light. Once it has been exposed to the correct ultraviolet wavelength, the emulsion hardens and becomes impermeable. Thereare many different types of emulsion in the market, as well as manydifferent methods of application. In a later post, I’ll dive into the two most popular methods for coating the screen mesh with emulsion.
A squeegee is a flat, rubber orurethane blade set into a handle.Some people may argue that this is not necessary and thatbrushes or sponges can be used to push the ink through the screen. They are correct and it can be done, but the results and time consumption can lead to very mixed results. Almost all professional screen printers use squeegees to produce consistent, high quality results.
This is a tool that is used to coat the screen with a thin, uniform layer of liquid photo emulsion. Without the scoop coater it ispossible to use a squeegee to spread the photo emulsion but this can sometimes lead to uneven thickness and poor exposure results.
The film positive refers to the artwork which is typically printed or drawn onto on a transparent medium. The artwork must be opaque in order to produceproper results when exposing the screen. One of the cheapest ways to create the film positive is to print directly onto transparency film from your laser or ink jet printer. Be sure to have the printer settings set to produce the darkest black possible for your artwork. As analternative, vellum can also be used as a film positive. If you are using vellum add approximately 20% to the exposure time due to the reduced transparency of the vellum.
In order to properly exposethe emulsion and “burn” the screen, a light source is required that contains Ultraviolet Radiation in the 350-420 Nanometer spectrum. Fluorescent unfiltered tubes, metal halide lamps and even the sun can be used to expose a screen.
These are essential if you plan on screen printing paper goods or other flat objects. The hinge clamps you to keep your screen inthe same position, allowing for easy registration and consistent prints.
There are kinds of tape that I use on a regular basis. The first ispainter’s tape which use for easy registration ofscreens, positioning acetate, etc. The second type of tape that I use is a tape made for screen printing which is water and solvent resistant. This one is used for taping my screens. I have also used plastic packing tape with great results.
During the coating and exposure process it is important to create light safe conditions. Premature exposure to UV light (sunlight, regular household lights, etc) can pre-expose the coated screens resulting in poorresults. Yellow bulbs filter out thespectrum of light that can expose the emulsion, making it “safe” to work under those conditions.
There are two main types of inks that are used for screen printing, each with their own advantages and disadvantages. Water-based inks utilize either dyes or pigments in a suspension with water as the solvent. The evaporation of the water is necessary to set or cure the ink. Plastisol is a thermoplastic ink. It is composed of PVC (polyvinyl chloride) particles suspended in a plasticizer. High temperatures are required to cause the molecules of PVC resin and plasticizer to cross-link and fully cure.
Water-based inks tend to be a little more environmentally friendly and give printed fabrics a soft “hand”. A soft hand means that your hand cannot easily feel the ink when it passes across the surface of the fabric. Water based inks printed on fabric can also be ironed (Plastisol will melt and smear if heated up to it’s curing temperature). For paper screen printing applications water-based inks are ideal as they will air dry as the water content in the ink evaporates. Clean up is easy and can be done without much impact to the environment.
Water-based ink is much more difficult to cure than plastisol for fabric applications. With water-based ink, the curing temperature must be reached and then held until all of the solvent (water) is removed. If you don’t fully cure the ink (for example on a T- shirt), the ink will fade with repeated washings. If water-based ink is left in open mesh for even a short period of time, it can clog the mesh and ruin the screen. You don’t really have to worry with regards to paper applications however. The other thing is that water-based inks will break down regular emulsion very quickly and even break down water resistant emulsions over time leading to screen breakdown for longer print runs.
Plastisolinks are not water-soluble and the ink will not dry if left in the screen for extended periods of time. It can be left in the screen for extended periods of time without worrying about clogging the mesh. Platisol will not break down the screen emulsion like water-based inks. They also happen to be very opaque and great for applications on colored fabric
Plastisol inks will not air dry and need to be cured (heated) as a result. Curing the inks can be done with a flash dryer, or more inexpensively, any home oven. Be careful not to burn your house down! Most plastisols need to reach a temperature of about 350 Fahrenheit before being fully cured. Plastisol tends to sit on top of the threads instead of soaking into them, which typically results in a raised, plasticized texture. There are, however, plastisol additives that can give the ink a softer “hand”. If the ink is under-cured, the print will crack and peel over time. Plastisol inks are generally considered harsher on the environment. Fortunately, there are a number of plastisol inks on the market that contain less toxins, and there are also eco-friendly solvents that are available for clean up.
The screen frame is a structurethat the woven mesh is stretchedupon and adhered to. Screenframes come in a variety ofshapes and sizes, depending onthe size of the art work andprinting surface. The mesh iseither stapled or glued to theframe with sufficient tension toprovide a flat printing surface that
Typically, frames are made out of wood or aluminum. Personally I recommend aluminum frames over wood because they are easily cleaned and can be reused indefinitely without losing their shape.
Wood screens will be fine for awhile but repeated waterexposure tends to warp the frameover time. Over time, withextended use, both wood andaluminum screen meshes will losesome tension which may result inpoorer quality printing results. Ifyou start to notice this occurringit’s best to have the screens re-
Always choose a screen frame that is larger than the area you are attempting to print by at least 2 inches on every side. This will make your life easier when applying emulsion, exposing artwork and printing.
Woven mesh is the “silk” in silkscreen. Typically this mesh is made out of mono-filament polyester fabric which is stretched taut over a screen frame. The spaces between the mesh make the fabric porous which allows ink to pass through. Screen fabric come in a wide variety of mesh counts. Lower mesh counts means that the fibers of the mesh are spaced farther apart, allowing more ink to pass through. Higher mesh counts deposit smaller amounts of ink and are generally better for fine details and line work. A general rule of thumb is 110 – 160 mesh count for fabrics and 230+ if you are printing on paper. If you are on a budget and only want one screen, 150 should do the trick for both fabric and paper. Screen mesh is available in either white or dyed mesh (typically yellow). Dyed mesh cuts down the amount of light diffusion during the exposure process which leads to a sharper print. For lower mesh counts around 110 it doesn’t really make a difference if the color is white or dyed.