To expose a screen with sun light you will need some special supplies:
1. 2 or 3 inch thick foam pad that fits inside your screen
2. Black cloth to cover your foam pad
3. A piece of glass the size of your vellum or film positive
4. Vellum or film positive with an image on it
5. Tape for vellum or film positive
Do this step inside or out of UV light.
Lay the screen over the foam pad with the cloth fabric side up (this prevents the light from scattering and exposing the back side of the screen). Place the vellum or the film positive on the screen so that is centered and square with the frame, it must be wrong reading as you look down at it. Tape it down and lay the glass over it.
If the sun is out, take the screen outside and expose it for 30 seconds, you might have to experiment with this time. After you do this a few times you will know. If it is cloudy or early or late in the day, you will have to expose it for a longer period. The ideal time is noon when the sun is as directly overhead as it gets.
After exposure take the screen to your washout sink and rinse it off with water as you would any other exposed screen.
I hope this helps and Good Luck on your exposures.
whats the best way to set it up, its soo beautiful out and sunny
Remember, every day, every hour and every weather condition will give you different UV exposure conditions.
Please invest $10 in a 10 or 21 step transmission gray scale and put it on EVERY screen you expose.
If step 7 remains in the mesh during washout, you have a well exposed screen and you have a benchmark to compare other screens to. If step 8 remains in the screen, you have more exposure than you need. If the image prints OK, and fine lines don't close up because of light undercutting - OK.
But if you only hold a step 6, you should expose longer. You get instant feedback on the amount of exposure.
When I took 20' flag screens into the alley because they didn't fit in the exposure unit, I attached the positive with Wisconsin's own water soluble KY Jelly. An ultra thin layer held the positive in intimate contact with the stencil and I rolled the positive in place with a 'brayer'. The KY dissolved and went down the drain with the unexposed stencil during wash-out. The positive was easy to clean.
Paper positives actually become more transparent as the KY is absorbed by the paper.
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20' flag screens are very big. You haven't lived until you go to the pharmacy and ask the clerk in a low voice, "Don't you have a larger container." and the clerk shouts to the back room, "Don't we carry a 5 pound jar of KY jelly?".
__________________
How are you measuring? retired Ulano Technical Support Screen printing since 1979 - SGIA Academy Member
Last edited by RichardGreaves; May 26th, 2007 at 11:25 AM.
so it is possible to do... i never try this before (never thought about it though). this technique is not familiar in my country, even my country is sunny for whole year!
Screenprint Answers by Bill Hood
for a natural sun exposure unit only 20 sec(summer) to harden 40 sec winter
There are instructions on using a Compression Exposure Unit at Screenprint Answers (Screenprint Answers by Bill Hood) but in brief they are:
1. Choose a place indoors, away from direct sunlight to prepare your exposure unit. Place a sheet of foam cut to fit into the screen on a table and cover it with the piece of black cloth.
2. Place the screenprint frame - coated with emulsion that’s dried – over the of the foam and cloth squeegee side down with the bottom of the screen facing upward. Because the emulsion is ‘slow’ you can do this indoors if you act quickly.
3. Place the film positive to be exposed on the emulsion surface “wrong reading” (i.e. backwards) in the center of the screen. Assure that the image is square with the frame by using a T-square or ruler to measure. Once in place, tape it down with the Scotch Brand 810 Magic Tape.
4. Place a clean plate glass (do not use tempered glass as it will inhibit the UV light) over the art, sandwiching the art between the glass and the screenprint mesh.
5. Expose the emulsion to UV energy from above. This unit can be carried out into the sun for 20 seconds on a bright day to expose the emulsion. Or you can use a single point light source situated above the unit. The distance from lamp to screen should be equal to the diagonal of the frame.
6. You can use the inexpensive Step Wedge Exposure Calculator (Products and Services for Screenprinters) for determining your exposure times.
will make sharper images and finer lines
I am a newbie to both this forum and to screen printing in general. I have tried to expose Diazo photo emulsion 3 times now, and today I exposed it, in 5 minute increments, from 10 to 90 minutes. I did not get a single image of the numbers. There was no image burned in at all, nothing was visible. I am trying to limit my investment in this adventure right now if I can so I don't want to buy anything else like lights for a while if I can. Money is a little hard to come by, but if I have to I will. I have seen alot of people with pics and video showing that it can be done, I just haven't been able to do it. I have exposed screens for much less time in the past. I believe I started at about a minute or 2, then 20 minutes then today's experiment. I have tried a total of three times with no results.
Help! I have been at this forever and am really getting frustrated, and it is frustrating hubby too. he is my business partner and wants result LOL
I am a newbie to both this forum and to screen printing in general. I have tried to expose Diazo photo emulsion 3 times now, and today I exposed it, in 5 minute increments, from 10 to 90 minutes. I did not get a single image of the numbers. There was no image burned in at all, nothing was visible. I am trying to limit my investment in this adventure right now if I can so I don't want to buy anything else like lights for a while if I can. Money is a little hard to come by, but if I have to I will. I have seen alot of people with pics and video showing that it can be done, I just haven't been able to do it. I have exposed screens for much less time in the past. I believe I started at about a minute or 2, then 20 minutes then today's experiment. I have tried a total of three times with no results.
Help! I have been at this forever and am really getting frustrated, and it is frustrating hubby too. he is my business partner and wants result LOL
Exposing a screen in the sun takes less than 2 minutes in direct sunlight.
I started at 2 minutes on my 1st screen and nothing happened. Then I read more and exposed it longer on the 2nd and 3rd screens, again nothing happened. I will try a short time at midday again but I am getting discouraged with this.
some informative tip posted why don't u just buy or borrow a cheap halogen work light.and make a stand. your local hardware should have it. this is the cheapest exposure unit available.and i can be more controlled. u need a dark room. u can you tube search (explore units) spend a hour or two and watch some videos. u might learn some thing.
Last edited by 1leonchen; September 20th, 2010 at 12:08 AM.
I have tried to expose Diazo photo emulsion 3 times now, and today I exposed it, in 5 minute increments, from 10 to 90 minutes.
I did not get a single image of the numbers.
There was no image burned in at all, nothing was visible.
I believe I started at about a minute or 2, then 20 minutes then today's experiment. I have tried a total of three times with no results.
Try to be more specific about "nothing happened". Do you mean no color change, or the entire stencil dissolved with water?
The purpose of exposure
The purpose of UV exposure is to change the stencil from something that will dissolve with water, into something that won't dissolve or breakdown with water. Keep this in mind and your solution is clear.
Exposure is easy. If the image area you blocked from UV exposure doesn't dissolve and holds in the mesh, something cross linked it. Your positive failed you.
If the entire stencil resists dissolving, the sensitizer did it's job and hardened.
Measure Exposure
To measure stencil hardness (resistance to developing or cleanup solvents), I suggest a US$10 Stouffer 21 step gray scale to simulate 21 different exposures. This is a standard photographic darkroom test positive that's been used since the 1930's.
Positive comparison
Next time you expose any screen, test if your positive completely stops UV energy. Tape a dime (or any thin coin), or a piece of aluminum foil to the outside edge of your stencil. No light will penetrate the dime and that area should wash out like a dream compared to your positive.
__________________
How are you measuring? retired Ulano Technical Support Screen printing since 1979 - SGIA Academy Member
Mr Greaves is indicating that the difficulty with Sun exposure is this:It is extremely difficult to predict your result. Depending on factors like the time of day, time of the year, cloudiness, etc, you are quite likely to get either under or over exposure using the Sun. This may not be critical for large block images, and sign printers have used the method for years when they need to expose very large screens. But relying on the Sun for multiple exposures throughout the day and using an exposure calculator for each exposure would be far closer to a science project than a production process, particularly if you are trying to hold any sort of fine detail or halftone dots.
You really can't beat a good point light exposure unit for predictability and repeatability, and that's what process control is all about. And without process control, well...it's just a hobby.
Exposure using the sun!??! 
