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Building an Exposure Unit

         Building an Exposure Unit

The exposure unit must have one and ONLY one bulb as small as possible.  If
you have more than one bulb, you are eliminated from the market where half
tone and find detail images are required. Think of the problem this way.

You shine a flashlight at a vase on a table next to the wall in a dark room.  The
vase creates a shadow on the wall.  In screen printing, you want the shadow
dimensions to match the vase dimensions so that the mage you print on a shirt
matches the image in the screen.  If instead of one light you have two
lights each shining at the vase at an angle to the vase, some of that light
will hit the wall behind the vase, and the shadow will be smaller than the
vase.  This is called undercutting. 

If you are trying to expose the dot over the letter I and light is shining
at that dot from different angles, then the emulsion or capillary film under
the dot might be fully exposed to light and not wash out.  If you are
attempting to create images composed of dots from programs like Photoshop while
experiencing undercutting, then the dots will be smaller in the screen than
intended.  Colors you print will be diminished or skewed.  Fine lines
in images might disappear, because like the dot over the I the
emulsion under the image in the positive has been fully exposed.

The exposure light has to provide equal light to all areas of the image to be exposed.  Screen
printed images on shirts, for example, are typically 12 x 12 and
sometimes 14 x 14.  The math of an exposure unit is the
light should be at a distance from the screen of 1.5 times the diagonal of
the image.  So layout a square 12 x 12, and then measure
the diagonal.  Then multiply that diagonal distance by 1.5 times.  This
typically works out to be 21.  Most professional exposure units
costing $3-4000 are set up with 21.

`When images to be exposed are larger the light needs to be moved farther away than 21.  The
light does not emit more light to compensate.  So the light reaching the
screen is less intense, and the exposure time will increase dramatically.  The
solution is to increase the wattage of the light.  At 21 we recommend
1000 watts.  So a greater distance, depending on the distance, could be
1500 watts, 5000 watts or even more.

Often people ask if they can use two lights 500 watts to get to the 1000 watts.  The
answer is clearly NO for the reasons stated above.

The light can be metal halide, halogen or quartz.  Metal halide are very difficult
to find being sold commercially.  Halogen and quartz are commonly sold.  The
difference is the metal halide converts electricity to light waves more efficiently
saving you a few seconds of exposure time.  The less efficient halogen
and quartz bulbs also produce more heat waves.  So when you construct
your exposure unit, point the light up to the screen and leave the sides of
the unit open to release the heat.  If the glass the screen is resting
on gets hot, the emulsion or capillary film will be baked into the mesh of
the screen and be difficult later to remove during the reclaiming process.  If
necessary, point a household fan at the underside of the glass to keep the
glass cool.

1000 watt bulbs in fixtures are sold by some photographic shops at 110 volts, but are
typically expensive.  1000 watts and more are available at 220 volts in
metal fixtures with a safety glass cover from retailers who serve commercial
electricians.  That is NOT Lowes or Home Depot.  Commercial electricians
go to some store where they walk up to a counter, and the clerk goes into the
back room to get the product.  The 220 volt light can be plugged into
the outlet for the clothes washer.  The plug on the light might have to
be changed.  Just be sure the voltage of the light and receptacle match.

For more information on this subject, see the video:

Screen Printing; Best Exposure Unit

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