How to Determine the Correct Exposure when using Vellum(Or any other type of positive)
The vast majority of problems reported by printers using vellum come from incorrect screen exposure technique and exposure time. While we'd like to think that every printer is using the finest, most expensive and precise equipment available and producing the best possible work, the real world finds printers making due with the best they can afford. Emulsion manufacturers know that they are producing products for a wide range of printers, and for exposure units ranging from $80,000 direct projection units to $5 Halogen work lights. Modern emulsions made by most companies have been engineered to accommodate vellum use, a wide range of light sources, and even less than perfectly dense images produced by computer printers.
Determining optimum exposure times is not a difficult job. It just requires a little time, the right tool (which you can download from our site) and a screen that is freshly coated with emulsion and ready to expose.
|Step 1: You will need a fresh screen, properly coated & dried, using the brand and type of emulsion that you currently use. You will also need a step-wedge exposure calculator. A free step-wedge calculator document can be downloaded from our website by clicking here. This is a .PDF document. If your browser cannot display the document, click here to download the free "Reader" software offered by Adobe Software. Create the step-wedge exposure calculator by printing the document to an 8.5" x 11" piece of SMR Premium Vellum.|
|Step 2: Place
an unexposed screen in your exposure unit, using the step-wedge exposure
calculator as your artwork to be burned to the screen. Turn on the
vacuum, engage the compression, or do whatever you normally do to create
a secure contact between the vellum positive (the step-wedge calculator)
and the screen. Do not begin exposing!
Note: If your exposure unit is engineered such that light shines "up" toward a screen resting on a glass surface, tape your positive to the screen in such a manner as to ensure the best contact possible since your results will be based upon how well your positive contacts the screen, and slip a very thin cardboard between the screen and the glass.
|Step 3: Each line of the
step-wedge consists of:
Locate the line at the bottom of the step-wedge exposure calculator that says "8" near the bottom of the document. (the boxes represent the borders of each line) Cover all the lines except the "8" line with a piece of cardboard, cardstock, or other opaque sheet. Your cover should be outside the glass pane of your exposure unit so you can move it without disturbing the screen.
Estimate the amount of time (or units if you have an integrator) that
you think it should take to expose this screen. Multiply that amount of
time by 1.5, then divide the answer by 8.
Set your timer or integrator for the amount of seconds or units you arrived at. Expose the partially covered screen for the amount of time or units you have determined.
|Step 5: Once
your first exposure has been completed, Move the cardboard cover so that
the next line is now showing. Expose the new uncovered area (which is
both the original area + line 2) for the same exposure time / units.
Repeat the process for all lines. As you uncover the "1" line, remove
the cover altogether, and perform one last exposure.
When you finish, you will have a screen with 8 different exposure areas on it. Let's say that you used 30 seconds for each of these exposures. In that case the screen would have a 30 second exposure, a 60 second exposure, and so on, up to 240 seconds. (30 x 8)
|Step 6: Remove
the screen from the exposure unit, and wash as usual. As you wash, areas
that received short exposures should wash out easier, or even wash away
entirely, (both image and non-image areas) and in longer exposure areas
the image portions will be
difficult to wash out. When everything looks adequately washed out, use
a fresh paper towel to wipe down the "ink side" of the screen (the side
that holds ink - it should have been the side that was pointing away
from the exposing light) You'll likely notice a sizable residue of
unexposed emulsion will wipe away in lightly exposed areas. Wipe away
any unexposed emulsion, then give the screen a quick rinse on this side
to clean the mesh. If there is no residue anywhere to wipe away, the
screen is overexposed, even in the areas that only received short
exposures. Repeat the test with a new screen using an original
estimate of roughly 1/3 - 1/5 as much time.
If everything washes away completely, or no areas hold an image adequately, repeat the test using 3 - 5 times as much exposure time/units as your original figure.
Some printers use their pressure washer to rinse screens. Don't! You're destroying your emulsions' ability to create sharp edges! Use a steady spray in a fan pattern from a garden nozzle to rinse screens after exposure.
|Step 7: Examine
the image you have created. On the "Ink Side" of the screen, you should
see "stripes" where your different exposure times meet. Your optimum
exposure will be the minimum amount of time/units that produce a screen
where loose emulsion does not rub off on the paper towel profusely,
pinholes do not form on their own, and the design appears sharp. The
screen is overexposed where thin lines fill in. (Note: if your exposure
unit uses multiple bulbs, thin lines will fill in at normal exposure
levels due to undercutting)
If you use indirect emulsions, the time/units from this test will remain the same for all mesh counts. If you use direct emulsions, (liquid) you should perform this test for each mesh count you use.