Problems with exposing times.

[Jacinta1986]

Hi, I am having problems establishing an exposure time for my print. I am exposing my screen by coating it in Diazo photoemulsion, leaving it to dry, and then laying it on the floor with my image then placed over it and then a piece of glass over both. I am exposing it using three 250W bulbs suspended over the frame.

Initially I did a test run, covering up sections every 5 minutes for 90 minutes, and 40 minutes looked best. I then exposed my screen at 40 minutes again, everything remaining the same (or so I thought) and my image washed out after exposure. So I increased the time to 50 minutes, at which point it was very overexposed. I then went back down to 40 minutes and still it was overexposed. At this point, I decided it was est to do a test run again(but this time every 5 mins for 40 mins), after which the best result was at 40 minutes- but if anything it needed a bit longer.

My image, the glass and the height of the lights from the screen are all remaining constant. My emulsion was mixed yesterday.

If anyone could give me any idea on what may be causing these different times for exposure I would really appreciate it- it's quite frustrating! (and a massive waste of emulsion).

Thanks.

[squeegeethree]

How far away from the screen are your weak bulbs?

[Jacinta1986]

The lights are about 1.2 metres above the screen, and always at the same height and same space apart.

[paul204]

I started by exposing with 100w incandescent bulbs with speedball diazo. I was only doing small images (maybe 8"x8" max) and my exposure was about 45m with 2 bulbs six inches from the glass. Metres may be too high.

Diazo chemistry is not capricious, though. If you expose once at 40m and get a different result than another time at 40m, it's because light has crept in at some other point during the process, not because the screen printing gods are playing tricks on you.

[Andymac]

or your coating is inconsistent. are you using a coater? is your coating even when you hold it up to a backlight?

[BigmanJames]

Well your time is somewhere in between 40 and 50 obviously so just start at 40 and move your cardboard an inch a minute.

I use the same emulsion as you and go for 40 minutes with a 500 watt bulb about two and half feet up.

[squeegeethree]

I think your lights are too far away the begin with. 40 mins is a long time to be exposing.

[paul204]

fyi, to get an even exposure the lights need to be placed at a distance from your screen 1.5x the diagonal of the image you're trying to print.

[Jacinta1986]

Yes- it must be daylight getting in and affecting it. I think I shall have to construct a light proof box. I thought I had blocked out the light in the room sufficiently but I guess not. Just wanted to check there was no other factor that may be affecting it. Thanks.

[Jacinta1986]

My screens quite long, it's 170cm x 70cm, which is why my lights are quite high. Thanks to everyone for getting back to me with your advice, I really appreciate it!