[palomap]

Hi everyone,

Am new to the forum and thought it would be a good place to post a couple questions about screen printing! I'm also new to screen printing and have been having a lot of issues with my screen printing Would really appreciate some help!

Basically I am using the Speedball at home screen printing kit. I bought a 150 watt bulb and am exposing it at 12 inches about the screen. I have tested from between 45m and 1:15m (according to a tutorial I found online which used those timings for a 150 watt bulb). The transparency paper was printed at a t shirt printing company so that's all good and legit.

I found that initially the emulsion washed out straight away (using a shower head, nothing too powerful). So I exposed it longer. I did a test and the exposure at 1h15 didn't wash out quite so fast and I could see that it had that lighter tell tale sign of being lighter than the unexposed emulsion. However, after awhile under the shower, it washed out AGAIN! Very frustrating.

My question is whether the bulb is a) too weak, b) whether I should make a DIY reflector out of aluminium (keeping it safe of course and away from the naked bulb), c) whether I need both a 250 watt bulb AND a reflector (home-made or bought, but these look to be around £20 and I don't really want to spend that much!), or whether, d) I can avoid buying anything else and stick to 150 watt at a much longer exposure time...

Would really appreciate any help!

 

[Twisted Grafix]

The emulsion has to react to UV light in order to expose. I would say that your bulb is not the correct wavelength for the emulsion to react. That's why it doesn't matter how long it's under the light, it still washes out. I would suggest getting a 250w halogen light, remove the glass, and expose about 12-18 inches away from the screen and go about 10 minutes to start. See if that works for you. The halogen has a broader wavelength and should expose the screen.

Just be sure to remove the glass on the lamp - it's a UV filter. No use trying to expose without the needed UV!

If that works, then get an exposure calculator and dial in the times for the meshes.

Using foil as a reflector may create more harm than good. A surface that's not 100% smooth will bounce light around and can cause undercutting.

Wattage doesn't matter as far as the wavelength goes... I built an LED unit that uses low wattage LEDs, but exposes all my screens consistently.

 

[NoXid]

Yes, it can work, and that is how I did my first screen back in the early 1990s, but I do not recommend it.

As Twisted mentioned, I would go with a halogen worklight, a 500 watt. Take out the glass. FYI Don't stand around and stare at it while it exposes--that glass was there to protect your eyes from UV radiation. UV blocking sunglasses are a good idea if you will be in the room while it is exposing. Anyway, these worklights are the best low cost "exposure unit." But still slow compared to pro equipment.

Diazo emulsion is slow, and that is no doubt what is in your kit. Fine, use it up. BUt next time get a photo polymer type emulsion, like Saati PHU. They are much faster, and will give you a better result with a weak light source.

Do yourself a favor and keep the coat of emulsion on the thin side. Weak light, slow emulsion, thick emulsion ... all leads to a big mess for most people starting out.

And finally, do a Step Wedge Test (see the link in my sig). Never guess at exposure time, one test will dial it in for you.

 

[gardenhillemb]

NoXid is right on with his recommendation. I would like to add that if you move the light source further from the screen you'll get better edges. The idea is to get a point light source. By the way, the sun is the best. You can always expose your screens outside. Put a pad under the screen and put the film on it as you normally would. Lay a piece of glass over the film (to hold it flat against the screen) and set the screen in direct sunlight so it's perpendicular to the sun. We still use this method if we're waiting for a part for our exposure unit. Note: if it's cloudy outside, this will still work - just expose it longer.