My first exposure unit consisted of a sheet of glass on sawhorses and a 500watt halogen lamp underneath.  10 years later, my exposure unit consists of a sheet of glass on a table built of 2x4's with black lights underneath.  They are remarkably similar in the structure.  Long and short is you don't need a fancy exposure unit to burn your own screens and you really shouldn't pay someone else to do it unless you're pressed for time or space.

Burn times vary widely based on your brand of photo emulsion as well as the type of lamp and number of lamps you are using.  My current exposure unit consists of 3-4' blacklight tubes and I am using an emulsion from Westix called Aquasol HV.  I use the Aquasol because it works with both Plastisol and Water-based discharge inks.  With the Aquasol my exposure time with the 3 lights is 3:30.  Using the Autotype Plus 2000 that I was getting from Tubelite, the exposure time with the same unit is 14 minutes.  The Autotype washes out a little faster, but i find that the difference in burn time more than makes up for the Aquasol's reluctance to rinse away.

I don't use a vacuum to hold the screen and film in postion, my setup includes a black tshirt (to absorb stray light rays after they pass through the screen), a sheet of MDF (medium density fiberboard) cut approximately to the size of the print area, and two 25lb dumbells to press the screen down to the glass.  Some folks will tell you that this method is not good for producing fine lines or small text, but i gotta say that it works great in my studio.  We do lots of tight linework on posters and shirts and I say those fools are crazy.

Here are some simple facts about light bulbs in relation to screen exposure:

• Halogen bulbs are expensive to operate and take a long time to burn screens because their UV output is low.  Very low. You can definitely use them but expect your burn time to hover around half an hour.  You'd think that you could decrease exposure time by using two lamps, doubling your wattage to 1000w, but using more than one s filament lamp will cause the edges of your image to be fuzzy.

• UV Florescent Tubes (blacklights) are the best low budget option for burning screens that I've found.  Using blacklights cut my burn time from 14 minutes down to 3.5 minutes.  The tubes are available at big box hardware stores for about $15 each.