# definition trouble with new home-built exposure unit



## brent (Nov 3, 2006)

hi all
I built an exposure unit last week and have been trying it out the last couple of days. Today I exposed for 20 minutes and it washed out well, it's just that I'm not getting definition in the center of the design, which is an american flag. some of the stars come out very rounded. The exposure unit is a wooden box with 4 20" blacklight tubes, two per light strip, which are placed about 5" apart and the tubes are about 4" below the plexiglass lid. Any input would be appreciated. I'll get new bulbs if necessary and/or change the placement of the strips.


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## Fluid (Jun 20, 2005)

sounds like you may need one more set of bulbs in the middle. If your getting a good burn on the outside there isnt enough light in the middle.


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## brent (Nov 3, 2006)

Fluid said:


> sounds like you may need one more set of bulbs in the middle. If your getting a good burn on the outside there isnt enough light in the middle.


hey fluid. If i'm only burning designs in that are 8"x11" should I try just moving the two strips closer together?


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## Fluid (Jun 20, 2005)

That ight work. Honestly youll just have to test asn its a homemade unit. Blacklight bulds will work yet are not the best.

Set aside an afternoon to test a couple burns and document your findings so you can get good time and position for future burns.

good luck


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## brent (Nov 3, 2006)

Fluid said:


> That ight work. Honestly youll just have to test asn its a homemade unit. Blacklight bulds will work yet are not the best.
> 
> Set aside an afternoon to test a couple burns and document your findings so you can get good time and position for future burns.
> 
> good luck


So if I decide to try different bulbs, what kind should i get?
Also, what if i added another strip in the middle, of different/better bulbs? Would the blacklights on the side help round it out ?


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## RichardGreaves (Nov 7, 2006)

brent said:


> hi all
> I built an exposure unit last week and have been trying it out the last couple of days. Today I exposed for 20 minutes and it washed out well, it's just that I'm not getting definition in the center of the design, which is an american flag. some of the stars come out very rounded. The exposure unit is a wooden box with 4 20" blacklight tubes, two per light strip, which are placed about 5" apart and the tubes are about 4" below the plexiglass lid. Any input would be appreciated. I'll get new bulbs if necessary and/or change the placement of the strips.


I put up a picture of a commercial fluorescent exposure unit:
http://www.t-shirtforums.com/screen-printing/t14577.html#post94694
an odd question under that subject.

Notice the spacing - tight.

Replace the Plexiglass as soon as possible.

In the 400 nanometer to 350 nanometer UV light range, glass transmits 78% to 93% of the UV light, BUT plexiglass only transmits 9% to 69%.

Exposure speeds will improve if you use an SBQ presensitized emulsion.


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## brent (Nov 3, 2006)

RichardGreaves said:


> I put up a picture of a commercial fluorescent exposure unit:
> http://www.t-shirtforums.com/screen-printing/t14577.html#post94694
> an odd question under that subject.
> 
> ...


Would acrylic work or does it have to be glass? I'm only trying to stay away from glass because it's so expensive in what I would think would be necessary thickness.


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## RichardGreaves (Nov 7, 2006)

brent said:


> Would acrylic work or does it have to be glass?



You know it works, you've made screens with it.

My point is that when you replace it, your exposures will be faster.


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## oldscot (Nov 18, 2006)

If you are experiencing a light distribution problem due to the lamp placement, you might try covering underneath the lamp area with 12"x12"
mirror tiles. This will reflect additional light upward and also distribute the light more evenly. I have also noted that plexiglass tends to bow under heat and this could also cause a problem depending on what method you are using to cause the emulsioned screen to be pressed against the positive. I replaced the plexiglas with glass on my homemade exposure unit and did get better definition in the middle of the positive altho the mirrored tile may have also improved the situation. I also use daylight bulbs and expose for 20 minutes and get great screens. Hope this helps


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## mtmob (Apr 21, 2007)

oldscot said:


> If you are experiencing a light distribution problem due to the lamp placement, you might try covering underneath the lamp area with 12"x12"
> mirror tiles. This will reflect additional light upward and also distribute the light more evenly. I have also noted that plexiglass tends to bow under heat and this could also cause a problem depending on what method you are using to cause the emulsioned screen to be pressed against the positive. I replaced the plexiglas with glass on my homemade exposure unit and did get better definition in the middle of the positive altho the mirrored tile may have also improved the situation. I also use daylight bulbs and expose for 20 minutes and get great screens. Hope this helps


 
Im new to this but wouldnt the tile mirror cause undercutting? i mean isnt that the reason they sometimes say to paint the box black in the inside to prevent shadowing of light?


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## ftembroidery (Nov 25, 2006)

Brent,
I have a commercially built florescent exposure unit that uses 4 blacklight tubes. You can see it at Brown Manuf.'s website. It's the smallest, cheapest one. The tubes are less than 2" below the bottom of the glass. I expose screens with pure photopolymer emulsion with excellent detail in about 22 seconds. I think your tubes are too far away from your glass. And BUY A SHEET OF GLASS...your t-shirts aren't going to look better than your screens. It's like painting a car...if the car is all wavy and bumpy before you paint it, it may come out "shiney" with new paint, but it's still going to look like crud.


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## willyindep (May 19, 2007)

RichardGreaves said:


> I put up a picture of a commercial fluorescent exposure unit:
> http://www.t-shirtforums.com/screen-printing/t14577.html#post94694
> an odd question under that subject.
> 
> ...


where do you get those flourescent tubes at? are those the daylight tubes you can get at lowes?


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## RichardGreaves (Nov 7, 2006)

willyindep said:


> where do you get those flourescent tubes at? are those the daylight tubes you can get at lowes?


GE Daylight Deluxe lamp
GE Consumer & Industrial Lighting: Redirect to NA Splash Page
F40DX

Search the GE site for UVA. They have a large selection of 'blacklight' lamps.

I also like Ultraviolet Light Bulbs and Lamps (UVA, UVB, UVC))

both of the ranges listed peak at 350nm which is OK.

Don't waste our time with 20watt bulbs, get the 40w bulbs and of course, 40 watts in, only yields 9 watts out!


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## willyindep (May 19, 2007)

RichardGreaves said:


> GE Daylight Deluxe lamp
> GE Consumer & Industrial Lighting: Redirect to NA Splash Page
> F40DX
> 
> ...


thanks for the advice! im gonna try it!


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## altype (Jul 1, 2008)

Not enought pressure in the center of the screen. Always use glass not plexiglass.


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## brent (Nov 3, 2006)

Welcome to the forums, Todd.

And yeah, last summer I replaced the acrylic with glass and got much better results. Then a few months ago I got a HIX tt180 table top exposure unit and it's awesome. The only time a screen hasn't come out right was when I thought I was exposing it and I wasn't. Everything washed out. Whoops.


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## xbrandon408x (Jan 24, 2008)

yea get glass its not that expensive and it doesn't have to be anything special i just went to a glass cutting shop and they cut out the exact size for like 40 bucks 

the only times iv had problems is when i wasn't pushing down my screen evenly so not everything burned well  so now i have a blanket and a piece of wood i put on top to push it down evenly


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