Hey, guys and gals! I'm trying to expose on some new screens and am having a hell of a time. They're 200 yellow mesh, and I've never experimented with yellow mesh before. I'm guessing a need to expose for longer because the problem I'm running in to is most definitely under exposure. It's like the well side of the screen isn't getting exposed at all! I'm running a 500 watt halogen bulb placed 16" over the top of my screens, with the positive being taped in place and a piece of glass placed over it. Screen is coated on both sides. It's the same set up I've been using for almost a year with no problems. Normally I expose for between 8 and 9 minutes on a white mesh, but I bumped it up to 11 minutes because I remember reading somewhere that yellow needed longer. Apparently that's not long enough. On a set up like mine, how long would you expose your screens? 16 minutes? Would that be too long?
Here's some tips re exposure. these have to do with point light sources. They all affect exposure times. Understand how and you will be able to dial in your times quickly, and never have a shitty stencil ever again....
1. Distance from the screen....the longer the distance the longer the time. If you double the distance, you quadrupal the time (inverse square formula) So, find the shortest distance, that (a) gives an even spread over the entire stencil, and (b) does not produce a hot spot in the centre, and soft (under exposed area) to the outsides....Also depending on the light, too close and you will crack the glass. A rule of thumb (which may be broken in some cases) is to make the distance equal to 1.5 times the diagonal of the area to be exposed.
2, Intensity of the light.... the bigger the wattage, the more intense the light, and the faster it will burn.
3. type of light....It is UV that makes the stencil harden, so light sources with high amounts of UV work faster and make harder stencils. A regular lightbulb gives off almost no UV...
it goes something like this....Sun>metal Halide>mercury vapour>quartz Halogen>photo flood>
(UV tubes are different than point light....see below)
4. Glass...Plate(float) glass is usually optically clear and allows transmission of UV rays, but the thicker the glass, the slower the exposure. Using tempered glass is acceptable, but some tempered glass blocks UV to varying degrees, and sometimes have surface marks from the heating process. Glass needs to be thick enough to stand up to heat and getting smacked with metal frames. Oh yeah, it breaks from time to time.
4. Stencil material - Stencils are not all created equal. For our purposes on GP (mostly waterbased ink printing), there are 3 main types people use, classed as 'direct stencils'. they have different characteristics - if you look at a manufacturer's chart (not your local dealer's stock) you will see there are 20 or more types formulated for different applications, and different price points
a) diazo based - the old standby. diazo gives waterbased ink resistance.
b) SBQ or 'One Pot' photopolymer - way faster than diazo, sensitive to low levels of UV light, creates crispy edges on your stencil...only one problem, it has little waterbased in resistance
c) Dual cure - the best of both. fine detail, water resistance
5. Coating....the thicker the coating, the longer the exposure. if the coating is uneven (thick and thin), you are going to have trouble getting a good stencil, because the thick parts will be underexposed, and fall off during washout. Even if you have an even coating, if it is underexposed and doesn't cure all the way through, it will come apart or fall off. The light passes from the bulb side of the emulsion through to the far side. the bulb side is always the first bit to be fully exposed, the away side takes longer. If it is sticky after a good washout, your stencil is still underexposed.
6. Mesh colour and count. white mesh is faster, yellow slower, and the higher mesh counts take longer. But yellow gives finer detail.
There are a lot of other things that can affect times, but these are some basics. What you need to do is standardize all the above, so they are repeatable each time. then the only variable will be the time, and you can modify this as you go, eventually settling on a fixed time for your particular setup/stencil comb that remains consistent and give perfect results. Change something, and you will need to recalc your time.
Andy, you might be the greatest person on the planet. In all seriousness I have learned more about printing from your posts on here than any other source I've encountered. If I didn't already want to buy the book I would now.
greatest person?....you need to get out more and meet more people....
Seriously, pass it on once you figure it out. this ain't oooga booga say a prayer and hope it works. there is a science and technology at work in screenprinting, the worst thing anyone can do trying to learn it is think "Oh screenprinting, what could be simpler?"
SkinnyD, I just build my first exposure box with 18W blacklight tubes.. and I will NOT go back to 500W halogen
Got normal 18W / 60cm tubes from local hardware store and installed 5 of them on my initial text box. Earlier with same emulsion I did use 12-15min expo times but now 2m30sec is more than enough also no heating problems or anything.
On that post there are links to two tubes that are similar. Do not use those real "disco" blacklights as they have UV filters.. No good.
"Insect" blacklights seems to be great and by tubes are giving 365nm UV light which seems to be perfect for my emulsion.
At first when I assembled my box and put powers on, I looked lights and were thinking.. Hmm let's see if this is gonna work or not as lights were not strong looking. Rather mild light coming out but yeah immediately when I burned my first silk I was really pleased with those.
And as they don't have any UV filters... Do not stare their light long or you might get sore eyes or something....
A little something about UV tubes...I have been using them for about 8 years now as my primary exposure.
So there are some distinct advantages to using a single point system. Expense IS NOT one of them. They are really expensive to obtain, and service. But for a lot of super high detail work, they are critical.
A single point light source (the sun, one bulb at a distance) creates an angle that is from the center of the stencil out to the edges.
UV tubes (CFL) light sources create a situation where the light reaches your screen at multiple angles, specifically from reflection from the edges of the exposure unit, and curves in the tubes themselves. This creates angles from the edges towards the center, reverse of single point. This can cause the light to reach the screen under the edge of the positive. This is known as UNDERCUTTING. It can cause the positive to shrink. On high detail work this can be a problem.
In general UV tubes work well, but make sure you have a decent distance from tube to glass 4-7 inches. The distance reduces the number of waves that reach the screen at a sharp horizontal angle. Thinner glass helps too. I recommend 1/8 for smaller exposure areas and 1/4 inch for larger units; less bow in the glass. Lastly, use better positive sources! Longer exposures with say paper positives, is when you really notice issues with undercutting, when the light has a long time to eat away at the emulsion you are trying to keep covered!
UV lights are great for 95% of the work that I come in contact with. In the other 5% almost always UNDERCUTTING is the culprit
Last edited by IndependencePrintage; 09-15-2014 at 08:36 AM.
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