Exposure problem with home built exposure unit

[galmiklos]

Hi,

I have built an exposure unit, with 18 inch 15W filtered black light tubes (the dark purple ones) 2.5 inches apart (four of them). The 3/8 inch thick acrylic top is about 2.5 inches from the tubes (unit's side is of 1x4's), and I am pressing the screen down with a few books on top of a black foam board piece.

I print my positives on 3M inkjet trancparencies, with "heavy matt paper" setting, 1200 dpi, looks pretty dark (lots of ink). To increase the opacity of the mask, I print two, and tape them together.

I use Ulano Proclaim, and coat each side once with a scoop coater. I dry them in a dark closet for 24 hours, relative humidity is arouond 25-28%.

The problem is that I either overexpose the screen (about 10-15 minutes exposure time), or the squeegee side doesn't get exposed enough (about 5-10 minutes exposure time). I can only make a usable screen, if I kind of underexpose, and wash it very carefully, so that washing the underexposed squeegee side would not desolve too much emulsion.

I just ordered a new batch of emulsion, as well as a 21 step exposure guide, but I am not sure if my exposure unit is not the culprit. I've read on Ulano's tech sheet, that in case of "filtered black light fluorescent tubes" I have to double the exposure distance which they suggest to be 4-6 inches normally. Well, I am way too close to the tubes, I can see that, but before I rebuild the unit, I'd like to hear some expert opinions.

Thanks a lot in advance!

[Andymac]

what happens when you overexpose? nothing washes out? if it's only fine lines that don't, you are probably right about being to close - the light is undercutting the positive, exposing the black areas. this is the problem with fluorescent vs point light source, and is magnified by not having a vacuum and doubled up posis.

to start with, get some 1/4 glass. this will cut back your time.

try a single posi if it is black. better yet, for tests get something that is really black (a neg or positive made by imagesetter), or cut something on rubylith

I think your emulsion is confused.

what mesh? Lower UV output plus double thick posi plus thick acrylic plus really coarse mesh = I don't know.

[steve w]

I get 20 second exposure times with Ulano QTX and transparencies.
Before I got a vacuum blanket, I had all kinds of trouble getting good even contact.
Which caused problems.
I only coat one side. Maybe your coat is too thick?
There are a lot of variables which are hard to assess without being there and seeing your process

[Andymac]

Fly down there Steve and check it out. then eat a steak.

[galmiklos]

Thanks a lot for the amazingly fast response, Andymac!

When I overexpose, something does wash out, but it's not only fine lines that doesn't. Actually, hardly anything washes out. Eventually, I just keep washing, and even the exposed parts start peeling off. Of course, it points to inadequate positive, but I used the same film and printer the make positives for printed circuit boards, and it worked perfectly.

Glass? I thought glass kept more UV back than plastic. I can try glass, my only heartake will be that that acrylic top was the most expensive part of the whole unit. Home Depot sold it only in 2' x 4' for about $60. :-/

I am also going to pick up some KIMOTO inkjet films, maybe my 3M films are too old and yellow. I observed, that I can make out the contour of the film on my screen after wash out. I can actually make out even the tape I taped the two layer together. I surely will try using single layer. Once I actually tried it with aluminum foil, but of course it was perfect! I used the "overexposing" dosage of light, so all the exposed emulsion was rock solid, and of course the aluminum foil did not let one single photon through. I hope I will be able to lear more with the 21 step guide, when it arrives.

I use 305 and 230 dyed mesh. I am trying to print water slide decals for my very old B-24 Liberator kit, because those very old decals are totally useless. To be honest, this was the second reason I started to screen print at all. The first was that I always wanted to try it... Anyways, this will probably be the most expensive plastic model kit ever built. I even got a pressure wash, becasue the solvents harden the emulsion so much, there is no way of "gently" getting it out with any screen wash. Not to mension that those Nazdar 9700 solvent based inks, and all the additional chemicals (Nazdar thinner, IMS ink wash) don't come cheap either...

[galmiklos]

I think I found out what was wrong. In my opinion it's a simple case of undercut. Due to the nature of small water-slide decals for scale plastic models, all my lines are fine lines, I would say. So, I rebuilt my exposure unit and now the distance between the tubes and the screen is about 6 inches.

Last night I managed to make a fairly OK screen, but it was the one for the clear overprint (that holds the decal together), hence contained only relatively large spots. I still can see that the edges are kind of hazy, typical to light undercutting. So, I guess increasing the distance helped somewhat, but I suspect I don't press the screen nearly hard enough to the film.

The new glass top arrives today, and I hope I can put more weight on top of the black foam board. I didn't want to put more weight on the acrylic, I was affraid it would bend in too much, making it actually worse, but I hope glass is less flexible.

Oh, and another thing that the person at TeXsource told me. He said after wetting both sides, waiting a few minutes, wash the screen only from the print side, and just rinse the sqeegee side.

I'll let you know if the glass top + better film contact helped.

[galmiklos]

Status report, as promised.

It must have been the glass top, perhaps the better contact too. But I am voting for the glass top. The reason is that now the sqeegee side is properly exposed, too. There is no underexposed emulsion wash out on the sqeegee side, both sides are rock solid. Even the color of the screen is different.

I probably overexposed a bit this time, but only the really fine details were effected. And my 21 step guide is about to arrive, so I guess I am out of the woods.

Thanks again for the advice!

[huthbot]

loose those bulbs you want unfiltered UV, they are white, expensive and kind of a pain in the ass to get in over 24". That spencer's gifts black light blue lamp is going to cause your mom to think you are on acid but thats about the only benefit. underexposed on one side is definitely either too thick of emulsion or your lights aren't far enough away from your exposure surface...or both. Ink jet is also wrong. get some lasers they make everything easier, the movie Congo told me so. Inkjet is too light and it bleeds and is generally a CMYK build so its usually a green color which your black lights don't like. You're better off with laser on crap bond paper than you are with inkjet on transparencies.

[galmiklos]

My tubes are only 18" long, and as a matter of fact, I just got a few unfiltered ones. I am not sure how good they are, they were dirt chep. I got them from bulbster.com (F15T8 Fluorescent Tubes 18-inch : Halogen Light Bulbs at Bulbster.com: The Best Light Bulb Store for Halogens).

I am now using Ulano's honeycomb surface inkjet transparency (just got it from TexSource), setting my printer to use only black ink, and the maximum amount possible (in advanced settings you can do it). They really look good to me.

As I said, all these modifications (glass vs. plastic top, press harder on the screen, increase distance of bulbs from screen, new film) solved all my problems. I will try my new unfiltered tubes, let's see if it makes it even better.

Thanks for all the help!

[steve w]

I get great results with plain old black lights from Home Depot