# How do u adjust exposure time for halftones



## Albie1 (Sep 2, 2009)

Hey guys got another question.. How much do u adjust your exposur time when u print halftones? On my exposure unit I usually expose for 5 min 30 sec and I get good results and for half tone I usually drop down to 3 and a half to 4 minutes.. But I loose some half tones in certain areas the outcome looks wiered cause I loose some halftones during washout and the back of the screen is very slimy should I increase my exposure time or? I'm using swr 3 emulsion with 180 watt uv bulb exposure unit. Albie


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## PositiveDave (Dec 1, 2008)

There should be no difference in exposure times. On the scale of the wavelength of light (365nm) any halftone is HUGE. If you find that you need to change your exposure:-
Are you using a vacuum frame - it makes a big difference?
Try a coloured mesh to reduce scattering.
Increase the exposure distance.


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## Albie1 (Sep 2, 2009)

thanks for the reply i cant change the distance its a box with 6 30 watt uv black lights. no its not a vaccum blanket unit but i put foam wrapped with a black garbage bag inside the screen and i put a metal plate with a 50 lb weight slab on top. as for my film positives there from an r1800 with fast ink on waterproof film. i have a strp wedge the 21 step. it is funny cause with 3 and a half minutes it washed out to the 8 the 7 down to 1 was solid but the back was slimy like always when im down at the 3 min mark at 5-6 min i still get slim on the back of the screen but it is very little.....


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## in2infinity (Dec 8, 2009)

I'd follow the step wedge. If you washout and 7 stays solid then you've found your exposure.

If you want better half tones then go to a vacuum blanket unit (or build one on the box you have). Vacuum blankets work much better then the weight method you have there. The next step up is to go to a single point light source with lots of power. Also, make sure your emulsion is meant for half-tones.

I have a single point 800 watt metal halide exposure unit with vacuum hold down and I can get excellent dots with 120 LPI films on 460 mesh at an exposure setting that gives me a solid 7 on the wedge. I use Saati Cap Film or Saati HGR graphic emulsion and my films come out of an Agfa 800 imagesetter (3.95+ density).


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## PositiveDave (Dec 1, 2008)

It's probably the lack of vacuum, read this:
http://www.t-shirtforums.com/t-shirt-articles/t108270.html


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## scottyjr (Sep 12, 2009)

50 LPI halftones or no halftones, my exposure time is the same. - Scotty


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## RichardGreaves (Nov 7, 2006)

Beware that if you don't wash the inside of your screen, dissolved image area "tears of emulsion" could be the slime you feel. Remember to rinse and blot with newspaper. Check to see if there is color on the paper. If there is, you aren't finished washing.

If you cure the stencil film so it adheres to the mesh, but the open image areas are choked by under cutting or light scatter - you have to calibrate larger dots in the art room so the stencil prints the dots you want.

PositiveDave showed you an excellent example under cutting.

You have a bank of low energy diffused fluorescent lamps. 6 lamps, 6 points of light, 6 shadows made by your positive.

So like a sniper, you make aim, then see what you actually print because of the "choke" that is built into your exposure unit that isn't designed for easy reproduction of halftones.

*Make a Control Guide*
Make a standardized pattern of *known *halftone dots.
5-10-20-30-40-50-60-70-80-90-95-100% in your favorite line counts.

Print several of these, or even save the image as a symbol and print it *on every image you make a positive for.*

Make a screen. When you develop the stencil you have a clue about which dots are washing out well. Remember, the stencil is swollen during development and needs to dry.

Print & cure the control guide.

Lay your positive on the print and use a magnifying glass to compare your print to the positive. 

Decide what you need to do to change the printed image and make the changes in the art.

Those 2% 5% 10% dots will be casualties of the equipment you have.

Now someone will tell you under expose the stencil so it's not completely cured. OK. You may get dots, but you may get a stencil that doesn't survive.

You have a 21 Step Stouffer scale so you know you have a solid step 7. Something you can monitor on every exposure you make. If you choose to under expose, you will be able to monitor how much you can under expose and still survive a print run - or not.

Beware that your Epson R1800 makes halftone "*spots*" with lots of ultra tiny drops of ink jet ink. At 720 dpi that's 518,400 drops in every square inch. Look at them under magnification and see if they are good shields that stop UV energy from reaction with your stencil.


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