Hi,
I've built my own Screen Printing equipment at home, and am exposing with 4x 20W blacklight bulbs
about 4 inches away from the screen.
I coat, dry with a heater for about 1 hour.
The problem is no matter how long i expose the emulsion (not only in the unexposed area) rinses out with a fairly weak water stream.
I switched to the BL bulbs from regular fluorescent and thought that would solve the problem.
The most I exposed with BL is 15 minutes, and 90 minutes with fluorescent bulbs.
for the first few minutes nothing washes out, and then everything (unexposed and exposed screen areas) start washing out all at once.
I would be happy for any ideas on what is going wrong (drying to much, bad emulsion, etc)
Re: Problems exposing screen, Emulsion rinsing out
I would suspect the light source. I think that you are not getting a suficient amount of UV to the emulsion. Did you install true blacklight bulbs or simply grow light bulbs? Is the glass between the screen and light source of a UV blocking type? 90 minutes with flourescent lamps is way too long. Something is not right. To identify the culprit I suggest this. Arrange a setup to use the sun as your UV source. Expose a screen for about 45 seconds but let some of the image be uncovered by the glass you use. The results should lead you to a definite path to the source of your problem. - Scotty
Re: Problems exposing screen, Emulsion rinsing out
Thanks for the quick reply.
I installed 4x20w true blacklight bulbs
I too suspected something was blocking the UV, I should have written that earlier
I did a test with 12 minutes blacklight bulbs with no glass at all, and the emulsion still rinsed out.
I'll try to expose in the sun, does it matter that during these months of winter its pretty cloudy outside? or is the UV about the same
Re: Problems exposing screen, Emulsion rinsing out
With this additional information I'm leaning toward another cause of your problem other than the light source but I think you should do the sunlight exposure to eliminate some possibilities. What emulsion are you using? - Scotty
Re: Problems exposing screen, Emulsion rinsing out
Yes indeed.
Also something I didn't mention.
Post-exposure (before washing), the stencil can be seen very clearly on the screen, so in some matter it is being exposed somewhat correctly.
I think I might just have to try a different emulsion.
Just throwing this out there, It isn't "wrong" to dry the screens with a heat source is it?
When I started I had problems removing emulsion, till I found out it wasn't completely drying in the middle although I let it dry for 24hrs.
And only since I started drying it with a heater in the room for 1-2 hours the emulsion remover works properly.
Re: Problems exposing screen, Emulsion rinsing out
Did you remember to degrease the screens? I know that I've forgotten that step before and all of the emulsion just falls right off just like you are describing.
Re: Problems exposing screen, Emulsion rinsing out
I must admit, I don't do any action to degrease before coating.
None of the tutorials I read/watched said anything significant about such a thing, aswell as the local screen printing suppliers.
I will try to look around for a degreaser, Thanks for all the help, I will post back when I have results
Re: Problems exposing screen, Emulsion rinsing out
The only time I degrease is when the screen is new or if it's been sittin' around for a while, uncoated. Otherwise: reclaim, dry, recoat.
In reference to the heat affecting the emulsion/ washout problem, my, not completely dependable, memory is telling me that from reading previous posts in various screen printing forums, the problem of excessive heat goes the other direction, in that screens will not wash out as if overexposed. - Scotty
Last edited by scottyjr; November 18th, 2009 at 06:31 PM.
Re: Problems exposing screen, Emulsion rinsing out
Water will start breaking down the emulsion, so if you are siting there trying to wash it out for a while then it will cause all of it to wash out. It happens when the output films isnt really dark enough and the light shines through and will expose the area then when you go to wash it out you are sitting there washing and washing.
Also the decreasing is a must.
__________________
I Run An Epson 1400 with and ALL Black CIS system and it WORKS!
Re: Problems exposing screen, Emulsion rinsing out
I was having the same problem with exposing time. If i were you id put an index card or playing card below the screen and see if it washes out. i also had the std 8 bulbs, 20watts at 5 inches and stated at 6minutes. i used a photo test strip and found out i only get details at 2.30 seconds. emulsion gets wet on back "furthest from light source" but when i rinse i just dry under light. im running another test now, but im using 110 mesh and ulano
Problems exposing screen, Emulsion rinsing out 
