There's a reason it's called Screen Printing!
The screen is critically important to the process. In fact, this is the weak link in the chain for many printers,
cutting corners and “making it work” when the screen is your most
important tool. Get your screens right, and you’re halfway home to a
great finished shirt.
Once we have the art and we have output a film positive, we need to expose this film to an emulsion coated screen (watch the video: How to Coat a Screen). In this industry, you’ll hear much debate about whether to use wood frames, static aluminum frames or retensionable frames.
As with many things in screen printing, all will accomplish the task at
hand. The differences will be in efficiency, repeatability and your own
You will start with a frame stretched with polyester mesh. There are a variety of mesh counts (number of threads per inch) with a variety of screen openings. The greater the size of the screen opening, the more ink flows through.
Lower numbers, for example an 86 mesh, will allow more ink through than
higher numbers, 305 mesh, as another example. 86 mesh has fewer threads
per inch so the opening are bigger, allowing more ink to flow through.
Your ink and your artwork will determine the proper mesh count (Here's how to use your artwork to find the proper mesh). Add to that, the tension of the mesh on the frame. For this overview, we will assume a frame stretched with proper tension.
Coating the Screen
To make a screen ready for the press, we need to apply a photo-sensitive emulsion to the mesh after the screen has been degreased.
This degreasing step is very important, as a clean screen before
coating will be more durable throughout the print run. Basically, degreasing the screen will wash away oil from your hands, dust and lint from your shop, and any other contaminants on the mesh. (Read the article Coating a Screen, or Watch the Video. Also, check out the article on The Care and Handling of Screens)
There are two ways to accomplish the coating step. The first is to apply a liquid emulsion to both sides of the screen using a scoop coater.
This hand-held device is filled with emulsion, placed against the
screen mesh and slowly pulled bottom to top, depositing emulsion as we
go. The screen is then turned and the same process is repeated on the
other side. A scoop coater should fit inside your screen with at least
1” clearance from the frame on each side. Just as with squeegees, scoop
coaters are purchased by the inch.
One surprising part of screen coating for students who are already printing shirts, is that we will coat our screens in full room light. Coated screens need to be stored in a dark room or light-tight container as they dry and as we wait to use them, but the initial coating can be accomplished under normal room light. Now, bear in mind we will not coat our screens near windows with direct sunlight, but standard room lights will be perfectly fine.
The second method of applying emulsion to the mesh is by using capillary film.
This emulsion is a dry product on an acetate carrier sheet. Capillary
film is applied to a wet screen to activate the emulsion and cause it to
stick to the mesh. A squeegee
is used to press the capillary film against the wet mesh. After drying,
the carrier sheet is peeled away leaving the emulsion on the print side
of the screen mesh. A screen coated with capillary film will not be as
durable as a wet emulsion coated screen since the emulsion is only
applied to one side, so best suited for short runs.
Exposing the Screen
The reason we do not coat our screens close to windows with direct sunlight is because ultraviolet light (UV) is what exposes (hardens) our emulsion.
Sunlight is the perfect source of this UV light. That’s why we wear
sunscreen! There are a variety of sources of UV light available for
screen printers, from unfiltered black lights to metal halide bulbs.
The process in a nutshell is this:
We place our film positive against the coated screen, and then expose
it to UV light. Where the light touches the screen, the emulsion
hardens. Where the light does not touch the coated screen (behind the
black parts of our film positive), the emulsion stays soft. After
exposing the screen to UV light, we remove the film positive and take
the screen to our washout sink. (Watch the video: How to Expose a Screen, or read the article: How to Find the Right Exposure Time)
After removing the film from the exposed screen, we should see a slightly different color of emulsion where the artwork blocked the light.
Under low water pressure, with preferably warm water, we will rinse
both sides of our screen, and then run water across the artwork area of
our screen. Within 15-30 seconds, we should see the artwork area of our screen begin to wash out and be clear.
The screen is then dried using a blotting technique, vacuum or forced air. The emulsion remaining on the screen should be allowed to dry completely before we proceed.
At this point, the screen is ready to be taped up, loaded into our press, and filled with the proper ink color for the job. We’re ready to print!