Emulsion washout issue

[dwdusharme]

Hi there
I am new to screen printing.
I am using Ulano Proclaim emulsion.
I am pretty certain I have my exposure time down. I include a Stouffer 21 step strip that reliably shows a 7 on washout.
Exposure time is 3 minutes.

I have started burning some largish screens, about 18 x 20.
Every now and then the entire stencil won't wash out. There will be an area or two (or three) that don't wash out.

I am trying to figure out why.

Because this doesn't happen all the time, and it doesn't happen in the same place on the screen each time, I don't think it is the light source, distance from light, etc.
It also isn't the stencil, because on some tries, an area that came out fine in one screen, won't wash out in another.
As far as I can tell, the stencil is in good contact with screen. I use a DIY method of a vacuum storage bag sucking the stencil to the screen. Seems to be holding it pretty tight.

One thought is that I can see my emulsion isn't always exactly even. I can see some areas that are a little heavier than other. Probably me not applying even pressure across the squeegee. Could that cause the problem? Would an area not washing out be because that area was too thick or too thin (which would it be)?

Your help appreciated.
Thanks
Dirk

[Catspit Productions]

It sounds like you are on to the cause of your problems. Are you using a scoop coater to coat the screens with emulsion? if not, that may be the issue. A squeegee is not ideal to coat the screen with emulsion.

[dwdusharme]

Yes. Using a scoop coater. I may have found problem with stencil contact but am still testing.
Meanwhile, for my information, would coating thickness exhibit the problem I am seeing? When I say there is some variation in thickness, I mean pretty minor. It's not like the coat is 1/4" in one spot and a 1/32" in another. It's just that if I hold coated screen up to light I can see that there are some areas that look less dense.
Thanks

[Catspit Productions]

It's possible for varying amounts of emulsion on the screen to expose unevenly since more emulsion will take more time to expose. But if you're noticing that the film is not sandwiched between the glass and screen, then that is a major problem as well. I would attempt to address both issues to resolve this problem.

[dwdusharme]

Thanks for your help

[Catspit Productions]

My pleasure to help out where I can. Good luck!