Blacklights vs flouro's in exposure unit.

[Wildcard]

After reading some info about how important a good uv light is for getting proper emulsion exposure, I decided to upgrade my home made unit from regular (6x 36w) fluorescent tubes to (6x 36w) UV blacklights. Images of reducing my exposure time from 9min to 4min and washing out with no slime were flashing through my head getting me pretty excited. I ran a step test ranging from 4-8min and it pretty much gave me the same result as before with the 8min exposure being about right.

Why would the blacklights not work better/faster? Did I just waste my money?


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[tonyt79]

I do not know the answer to your question. I run home made unit with unfiltered blacklights. Not real easy to find. My exposure times are 30 to 40 seconds depending on mesh counts. The lights are 40w f40BL

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[jvanick]

what kind of glass did you use?  If you used something other than plate glass, it may have a UV-blocking coating on/in it.

[3Deep]

Also how far are your tubes from the glass? emulsion type plays a big factor in this where pure pre-mixed polymer is more forgiving with weaker lights sources when it comes to faster exposure times, and dual cure work best at longer exposure times like your 8 minutes.

D

[mimosatexas]

way too many variables at play here to give you a true answer.

Major things to consider:
-type of emulsion?
-how thick are you coating and on what mesh counts?
-type of glass and how thick?
-type of blacklight bulb (unfiltered or not)?
-distance of the bulbs from the glass?
-spacing between bulbs?
-type of film used?
-how truly black are your films?

ambient temp, humidity, how you're drying your emulsion, how you're washing out the stencils, and a whole host of other things may be coming into play as well when it comes to determining proper exposure and issues with slime and the stouffer strip readings.

My daylight flo bulb unit exposed in 3 minutes, but I had optimized to a certain extent the glass, distance to emulsion, coating technique, etc

[Shanarchy]

Are you using "party style" black lights that look black?

Or the type that would come from an actual light supply store that look white?

Is your glass "optically clear"?

Do you have a vacuum hold down? If not what are you using in place of that?

And the big question, what are you using for film positives?

[tonyt79]

Are you using "party style" black lights that look black?

Or the type that would come from an actual light supply store that look white?

Is your glass "optically clear"?

Do you have a vacuum hold down? If not what are you using in place of that?

And the big question, what are you using for film positives?

What's a vacuum hold down? Haha jk. You should see mine. I don't use that unnecessary stuff. I'm like playing tetris with paper boxes and catalogs to hold my screens down! Been meaning to do something about that but it works pretty good.

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[mimosatexas]

vacuum tops are seriously simple and cheap to make and AWESOME!  Get to it!

[Frog]

What's a vacuum hold down? Haha jk. You should see mine. I don't use that unnecessary stuff. I'm like playing tetris with paper boxes and catalogs to hold my screens down! Been meaning to do something about that but it works pretty good.

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At the very least, add a good compression style hold down to your unit

[tonyt79]

What's a vacuum hold down? Haha jk. You should see mine. I don't use that unnecessary stuff. I'm like playing tetris with paper boxes and catalogs to hold my screens down! Been meaning to do something about that but it works pretty good.

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At the very least, add a good compression style hold down to your unit

That seems like a lot of work! Nah that is the plan unless I pull the trigger on real unit.

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[kingscreen]

Regular fluorescent bulbs, Black/Purple coated (party-style) Blacklight bulbs, and standard plate glass all have UV blocking properties.
For best results in a homemade unit, switch to unfiltered Blacklight bulbs (see: topbulb.com) and call your local glass shop for what's called "low iron float glass".  This type of glass has no UV blocking properties.  Rule of thumb, if the edge of your glass is green, it has UV blocking properties.
With those updates, you will definitely see shorter exposure times.

[StinkyDaddy]

With my homemade unit, consisting of 2, 2 bulb 4' shop lights (4 bulbs total) with consumer black lights from Lowes I expose at 45 seconds using QTX on most mesh counts. The lights are about 6" from the glass. Hope that helps.

[Maxie]

I used black lights for years, with a vacuum table.
Exposure time about 6 min and always had some slime inside the screen.
Just bought a 300 W LED from Saati.
Best thing I could have done, with Diazo 4.5 min exposure and with photopolymer (PHU) 45sec.
Great detail and no slime.

[Wildcard]

Okay, I had a feeling the discussion would need to get technical but I thought I would try for the simple answer first. My details based on the post from mimosatexas and are estimates because I'm not at the shop right now to measure the unit.

Major things to consider:
-type of emulsion?
>Kiwo Poly-Plus SWR Red, I believe its a dual cure since it comes with a sensitiser. Considering trying the Ulano Orange which apparently is pretty forgiving and good in high humidity.
-how thick are you coating and on what mesh counts?
>Coating 2+1 (sub side then ink side) and this particular test was on 55T (metric) mesh since the higher meshes use less emulsion and give me less trouble.
-type of glass and how thick?
>Glass would be about 6-8mm thick, unsure if it has filtering properties since I didn't make the unit myself.
-type of blacklight bulb (unfiltered or not)?
>Can't find specifications on the bulbs to say if they are filtered. I did some research on whether the blacklight blue (party type) were giving off more UV than the fishtank blacklights and chose the blue type: Philips 36w  BLB 4' fluorescent.
-distance of the bulbs from the glass?
>approx. 20-25cm
-spacing between bulbs?
>6 bulbs spaced approx. 2-3cm apart
-type of film used?
>laser film with a matt/frosty finish
-how truly black are your films?
>laser printed at a local print shop, so not perfectly opaque, but they work.
-ambient temp
>approx 18C at the moment (winter) and sits around 30C in the summer.
-humidity
>70% outside of the drying room on the day I tested, about 50% inside. Gets up to 90% here in summer.
-how you're drying your emulsion
>dry overnight (ink side up) in a small, dark room (toilet) and then run a fan heater in there for 30min or so before exposure
-how you're washing out the stencils
>in an old bathtub, using a garden hose with a multi spray nozzle on a soft/misty setting.

I also don't have a vacuum top, just place a board inside the frame with weights to press the mesh and film flat on the glass. Feel free to share any 'simple and cheap' vacuum top plans.

Apologies for the metric measurements, I figure you guys will get the idea. My set up is a little rough, inherited from the previous owner of the business, but it does work - I just want to keep improving on it. I thought the move to the blacklights was going to be an affordable upgrade to the exposure unit (they were not cheap, but more affordable than buying a new unit) that would get a full exposure through the emulsion and rid me of the slime.
The exposure issues have definitely come up since I have been working on my coating technique to get thicker stencils. My long to-do list includes building a drying cabinet for screens and putting a small bathroom dehumidifier in the cabinet to try offset the high humidity in my area. I'm disappointed that I didn't get a remarkable improvement from the UV lamps, so I'm considering returning them and putting that money into some other improvement. Maybe there has been some improvement that I'm not aware of after 1 test exposure though.

Thanks for the feedback so far.

[Maxie]

Why are your lamps so far from the glass?
I would raise them to a maximum of 10cm for the glass, even less if you can get an even exposure.
You are obviously not doing big runs so why have thicker emulsion.
Coat 1:1 on the thin edge.
The thinner coating will be easier to expose.