Exposing Screens

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The Exposure Table-


At the heart of screen printing is the creation of the screen. One of the earlier techniques, drawing fluid and filler,  featured artwork created directly on the surface of the screen .  When the printing is complete, the artwork is removed from the screen and lost during screen reclamation.  Exposing a master image to a photosensitive emulsion on a screen allows the reclamation of screens without loss of the artwork.  Now the screens are simply tools of the craft rather than artwork originals.

Exposure tables are a nice tool to have although screens can certainly be "burned" using halogen shop lights and a watch.  Screens can even be exposed using the sun!  At it's core, an exposure table consists of a bright, non-point light source and a way to hold the artwork master tightly to the emulsion on the screen.  A tight fit reduces light bleed and allows detail to be transferred to the emulsion.  Several types of masters can be used including printed film, Rubylith or printouts from a Laser Printer.  Simply adjust the exposure time to suit the media.




Exposure times for a range of media

Using the Exposure Table-

Using the exposure table is a straightforward task.  Simply open the top and place your artwork master on the glass.  Place it as it would normally be placed viewing or reading.  Then place the screen, with the emulsion in contact with the master, on the glass. Taking care to center the image in the screen now will make things easier to register later!  A rough placement on center is OK, precision is not needed at this stage.  When using a  a vacuum table, check for a bit of rope and lead this from the outside to the inside of the screen frame.  In the image below, the rope is in the upper left of the screen.  This allows vacuum to draw down in the center of the screen for the best contact between art and emulsion.  Good contact is equivalent to good detail.




Initial Set up.


Now that things are positioned, it's time to burn the screen!   Shut the top and secure the latches on each side.  Set the exposure time for five minutes (this is a laser print master) and start the vacuum pump.  It will take a few minutes to draw down full vacuum.




Exposure table control panel.




Starting to draw down.




About half way done.




Vacuum complete and ready to expose!


Once the vacuum has everything tight, flip the exposure switch.  The timer, set previously, will shut the machine off when it's done.  If things have gone as planned, the master image should be exposed to the emulsion, creating the stencil needed for printing.  Most screen printing processes use art "positives". Where there is black on the master- or red in the case of Rubylith, will be where the ink will print. This exposure table shuts off automatically by using it's built in timer.  Other exposure units will require watching a timer and will need to be shut off by hand.  Like film, emulsion can be over-exposed so watch and double check the exposure times!  A final note NEVER place any objects on the top of the rubber vacuum sheet.  Doing this wears out the rubber sheet prematurely.  The rubber is under tension and a small poke leads instantly to a large hole, and a large replacement expense!


As soon as the screen is exposed it can be retrieved and washed out. Don't forget to retrieve the master in case another screen needs to be made.


If it washes out cleanly, it's ready to print after it dries.  If there are areas that need a little help washing out, a Magic Eraser can help but use it lightly.




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