[RippingYarns]

This does not seem to be a 'life threatening' problem but as I am new to the screen printing thing and am striving for perfection I would like to hear your opinions on this observation. I am printing films through my epson R2400 using all black inks and getting nice dense films. I'm coating my screens and drying them for about 30mins in a darkroom with a fan going. They seem to be completely dry to the touch before I expose on a UV flourescent exposure unit from ASC. I did a test strip and worked out that the optimum exposure was about 3-4 mins and the screens are washing out fine and retaining good detail - no flaking and few pin holes. What I have noticed though is 2 things. 1. When I take the film off the screen right after exposure there is a slight color difference where the clear film has been - i.e. it is still slightly greener than the emulsion that has been directly exposed(which is now blue) 2. When I wash out the screen I notice that around the edges of the screen the emulsion is going tacky again in a few places normally right at the top edge. As I said it doesn't seem to be a major problem as after the screen has dried out the 'tacky' areas seem to have hardened up properly but I am wondering if my exposure is not enough or if i am not drying the emulsion enough before exposing? any thoughts appreciated.

 

[TYGERON]

The color difference is between cured and uncured emulsion.

The tacky emulsion is undercured.

 

[ScreenFoo]

Tygerons spot on--the film will play a big part of this too, if you're using cloudy film or vellum it's blocking UV.

You may go to no pinholes with the proper exposure--if you don't, you'll want to look into your screen prep as well.

 

[RippingYarns]

Thanks Tygeron and Screenfoo, Yes the film is the cloudy type so I guess I need to increase the exposure. Tygeron, I am assuming when you say undercured you mean not enough exposure to UV as opposed to not dry enough before exposing? thanks again

 

[TYGERON]

Tygeron, I am assuming when you say undercured you mean not enough exposure to UV as opposed to not dry enough before exposing? thanks again

Not enough exposure to UV.

 

[aaronc]

Yeah, I was going to say something about drying. How long do you let them dry? I used to not store coated screens and would coat and burn within like 2 hours, it will work but I have gotten remarkably better results just letting them dry up over 24 hours at least

 

[RippingYarns]

Thanks, Will definitely increase exposure time and drying time too I think. I was shooting my screens with in 1hr of coating which might be pushing it a bit.

 

[ScreenFoo]

A hygrometer (relative humidity) meter will pay for itself in the screen room, or wherever you store your screens.

Ulano lists humidity as up to a 50% time difference, on their tech sheet.

 

[RippingYarns]

Humidity could well be a factor considering we have just had the wettest week in Sydney since 1974! Do you use a dehumidifier in your screen room?

 

[TYGERON]

Humidity could well be a factor considering we have just had the wettest week in Sydney since 1974! Do you use a dehumidifier in your screen room?

I haven't but there have been some days where I needed one. Even had problems with films sitting around sticking together from the moisture. Doesn't take much.