Hi,
I have just built an exposure unit that has a 500w halogen bulb sitting approximately 40 inches away from the glass screen. The 500w bulb is only temporary until i get my 1000w metal halide wired up and ready to go. I use dual cure emulsion and was wondering how long it would take for me to expose using the 500w bulbs? A friend of mine has a similar setup and i'm sure he said that he can get screens exposed in 12 mins - does this sound right? Just thought i'd ask cause i don't want to go wasting emulsion!
Thanks.
The 500w bulb is only temporary until i get my 1000w metal halide wired up and ready to go. I use dual cure emulsion and was wondering how long it would take for me to expose using the 500w bulbs? A friend of mine has a similar setup and i'm sure he said that he can get screens exposed in 12 mins - does this sound right? Just thought i'd ask cause i don't want to go wasting emulsion!
Thanks.
You didn't share the name of your emulsion and almost every dual cure has a different speed, so exposure time is hard to predict.
40" would be the distance if you want to cover a 25" area, evenly.
Just do a screen test, cover your screen with a piece of card and every minute expose another couple of inches, see what happens when you wash out. You're looking for a solid stencil, plus a little bit for comfort.
Be aware that your light will age and you will need to increase your time.
Sort of a home made stepwedge.
how would one make there own step wedge...I was thinking about making one in photoshop....kinda like making a rectangle and have 100 black at one end and decrease the screen value by 10 until I have a clear or transparent end..would that work?
To calibrate a step wedge you'd need a spectrophotometer and too much time on your hands.
Basically you have to measure and control each step so that the light transmission declines at a given rate.
If your maximum exposure is, say, 10 minutes, you want each step reducing the light to 90/80/70% etc.
Plus you need a neutral grey so that you aren't filtering specific wavelengths.
how would one make there own step wedge...I was thinking about making one in photoshop....kinda like making a rectangle and have 100 black at one end and decrease the screen value by 10 until I have a clear or transparent end..would that work?
If you want to do a manual, stepped exposure test, it is easy to make several positive & negative lines 10 inches long.
Cover 9 inches of the positive, and expose an inch at a time for a 'stepped' exposure.
You will see how longer exposures 'can' choke the lines.
This doesn't really help that much to determine if the stencil is cured/hardened all the way through to the inside of the screen.
When you get bored with turning your vacuum on and off and moving the material that blocks UV energy in your stepped test and making sacrificial tests that can't be used in production, you will buy a US$10 Stouffer 21 Step Gray Scale that can go on every exposure you make for the rest of your life. Exposure FAQ Screen Making Products how to measure exposure
The Stouffer 21 Step Gray Scale is a small film positive with darker and darker filters next to each other in steps. You will get a simulation of 21 different exposures with one exposure. You get visual feedback that shows you how well your stencil is exposed. Start by aiming for a solid step 7.
Any printer you might have, can't make a continuous tone image.
You should make an halftone percentage pattern and use it on every stencil you expose though - to determine how much stencil choking or ink dot gain you have so you to calibrate your art.
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How are you measuring? retired Ulano Technical Support Screen printing since 1979 - SGIA Academy Member
500w Halogen Exposure Unit 
