# Exposure issues



## MatBM (Jun 5, 2017)

Hi, this is my first post here having just started out screen printing and trying to get used to the whole process. Hopefully someone can help me out. 

I started using dual core emulsion, with a thin coat on each side. I managed to find a good exposure time with good screen results. I printed a few shirts with plasticol which came out well, but then subsequent prints didn't come out crisp with, for example, some line detail not appearing sharp on the shirt. 

I put this down to the emulsion coat being too thin. I also realised that to print white plasicol on dark shirts I needed thicker emulsion. So I tried using the rounder edge of the scooper to apply two coats print-side, one coat squeegee side. I also increased the exposure time by around 1.5 minutes, but the image would not wash out fully. I tried decreasing the exposure time, but it seemed I could not get all detail to wash out properly. 

I thought maybe the emulsion had been tainted through exposure to UV light, or was just too old, so I bought some brand new emulsion. This certainly seemed glossier & brighter on the screen. However, although most of the image seemed crisp and sharp, some of the smaller detailed lettering of the image was not perfectly clear - the centre of the 'A' or 'R' washing out for example. 

I thought maybe the positive wasn't opaque enough, so laid down two positives to make sure all light was blocked out, but I still got the same problem with the detailed lettering not fully clear. I coated another screen and increased exposure time by 30 seconds, but then the rest of the image didn't wash out fully, suggesting over-exposure. 

It's getting a little frustrating now. I really want to master screens with thicker coats, but I'm struggling to get the detail I need. Anyone have any ideas?


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## MatBM (Jun 5, 2017)

Ok so I got a very good screen again with a single coat of emulsion using the sharp edge of the scooper. And when it's a thicker coat (which, again, I believe I need for optimum opacity for white plasticol on black shirts), I cannot get fine detail in the stencil. 

So an easier question for experienced printers would be: is there a limit to the amount of detail you can achieve from thicker emulsion deposits on a screen? If not, then clearly I'm doing something wrong. 

My final guess is that I need to apply higher hose pressure than on a thinner coated screen?? Really appreciate any advice!


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## PatWibble (Mar 7, 2014)

What light source are you using? How far from the screen? What screen size and mesh are you using?
What is your exposure time?

A properly exposed screen will wash out very easily. I use about two gallons of water to wash out a well exposed 23x31" screen, using ordinary tap water pressure. An under exposed screen uses much more water and is much harder to wash out. You are much more likely to lose detail with an under exposed screen.

After exposure, soak the screen each side, and leave to stand for a few minutes. Wetting the screen serves two purposes - it stops the emulsion developing any further ( so the screen is now light safe) and it softens the image area, making washing out easier.

You should concentrate on getting exposure correct with a single coat of emulsion each side, before trying two coats.

If you are loosing print sharpness ( as in first part of your first post), it sounds like you are using too much ink and/or wrong off contact.


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## MatBM (Jun 5, 2017)

PatWibble said:


> What light source are you using? How far from the screen? What screen size and mesh are you using?
> What is your exposure time?
> 
> A properly exposed screen will wash out very easily. I use about two gallons of water to wash out a well exposed 23x31" screen, using ordinary tap water pressure. An under exposed screen uses much more water and is much harder to wash out. You are much more likely to lose detail with an under exposed screen.
> ...


Thanks for the reply! I'm using 110 mesh standard-size screen, exposure unit with eight 15 watt unfiltered UV black lights which normally exposes a one-coat dual-cure screen in 7 minutes. I don't have a vacuum lid, just a piece of black foam, and actually I wasn't given any weight with the unit at all, which just seems bizarre to me. In hindsight I'd much prefer a vacuum lid, but was advised because I was a 'beginner' I didn't need this. 

What I use is 4 floor tiles in the centre of the screen to make sure the image is weighed flat against the glass, plus the black foam on top. Not sure how I can improve this. Perhaps block light around the sides, but I don't think this is necessary.

Surely an OVER-exposed screen is much harder to wash out? An under-exposed screen would mean the emulsion hasn't hardened enough and washes easily, right? Over-exposed would mean the light has gotten in to the image. 

When I wet the screen after exposing a single-coat screen, the image appears ghost-like immediately and I can use a light hose pressure to wash out fully within 30 seconds. With 2 coats this doesn't happen, and it takes much longer for the ghost of the image to emerge, and wash out. So I kind of think there's a problem with the exposure method. I'm certain the transparency is opaque enough, as i'm using UV-blocking black ink, and 4 floor tiles are pretty heavy to weigh down on the image. The exposure unit is brand new, so I'm at a loss really. 

RE: Print sharpness, the image I'm using has a box with clear thin lines. I notice that the stencil on the screen comes out very slightly jagged on these lines. I'm assuming light is somehow preventing clear crisp lines?


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## PatWibble (Mar 7, 2014)

MatBM said:


> Surely an OVER-exposed screen is much harder to wash out? An under-exposed screen would mean the emulsion hasn't hardened enough and washes easily, right? Over-exposed would mean the light has gotten in to the image.
> 
> When I wet the screen after exposing a single-coat screen, the image appears ghost-like immediately and I can use a light hose pressure to wash out fully within 30 seconds. With 2 coats this doesn't happen, and it takes much longer for the ghost of the image to emerge, and wash out. So I kind of think there's a problem with the exposure method. I'm certain the transparency is opaque enough, as i'm using UV-blocking black ink, and 4 floor tiles are pretty heavy to weigh down on the image. The exposure unit is brand new, so I'm at a loss really.
> 
> RE: Print sharpness, the image I'm using has a box with clear thin lines. I notice that the stencil on the screen comes out very slightly jagged on these lines. I'm assuming light is somehow preventing clear crisp lines?


You are confusing over exposure with correct exposure, and assuming that an under exposed screen is over exposed because it is hard to wash out. 
When you wash out after you double coat the screen, the under layer is sticking to the mesh, making washing out harder. This is because the emulsion is under exposed.

The outline (ghost) of the image should be very apparent as soon as you remove the screen from the exposure unit. On green emulsion, it will show up as a distinct yellow outline. If it doesn't, then the screen is under exposed.
When you wash out, the back of the screen should not be slimy. If it is, then your emulsion is under exposed. Your water should have no more than a slight tint of the emulsion colour. If it is running green ( or whatever colour your emulsion is) then your screen is under exposed.

The jagged edge around the detail is another sign of under exposure. Water is penetrating the under exposed emulsion around the outline of the image, causing the jagged edges. Under exposed emulsion is very soft and very susceptible to water ingress.

Your black foam should be against the screen. The reason it is black is to stop light reflecting back onto the rear of the screen, effecting exposure.


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## MatBM (Jun 5, 2017)

Ok I only went for 7 minutes initially because it was recommended by the guy who delivered / set up my equipment and also the supplier. I already did a wedge test up to 9 mins and found I got no detail, but perhaps that's because I didn't have good positive contact all this time. 

I just tried exposing a single coat screen for 7 min 30, and also cut out a board of MDF to fit the inside of the screen. I wrapped it in a black shirt and placed it in the screen, with one floor tile to weigh it down, and just to be sure, placed the black foam on top of that, weighed down by another 3 floor tiles. If there was an issue with the image not being flat against the exposure unit, it should be fine now.

Unfortunately I found it much more difficult to wash out, fine detail appearing blurred as the image begins to appear. 

So I'm now thinking a possible cause of the lines being slightly jagged is I spent too long hosing the initial screen, causing emulsion to become soft around the edges.

I've got one final coated screen left to burn, so I just burned it for 7min 15. I can wash this much easier, but do not get the same clarity from the detail as when I burned for 7mins. I washed for at least a minute. It looks ok, but not perfect. 

I'll now clean these screens, re-coat and come back Sunday evening to try for 7 mins but hosing more lightly, for a shorter amount of time. 

However, I also now have another problem. I saved one ok-looking screen to practise printing white plasticol on dark shirts but there appears to be a small transparent mark in the stencil that is blocking the ink. It looks like dried water. They're brand new screens so perhaps I didn't de-grease them properly. It appeared in two screens. 

Ah great fun! Thanks again for your advice


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## MatBM (Jun 5, 2017)

So.. it looks like I found the reason why I had exposure issues. I was drying the screens the wrong way up. And coating the outside last when I was doing multiple coats. I dried the screens the correct way, and coated the inside last after multiple coats.. and now I have no problems exposing and washing out. 

Oh well.. you live and you learn.


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## Wickedweb (Apr 20, 2017)

Hello I'm new to screen printing and I'm having trouble washing out the screen after it's been exposed. I'm exposing on a 110 mesh with a 500 watt halogen light at 13 1/2 inches from the screen for 12.5 minutes. I've watched multiple videos and it looks like everyone has the screens washed out in 5 or less minutes. Please help!!!

Craig


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## Appjetty (Oct 5, 2017)

p { margin-bottom: 0.25cm; line-height: 120%; } It all depends on the screen you use, And if you want to use its better you first try with a single coated emulsion for each side before you go for double.
And if you are getting off clearity or bluntness then its a result of too much ink on it so use it or change it accordingly.
If the screen is properly exposed then it can get washed off very easily and use detail for the screen which is under exposed. And you dont face any more problems.


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