Which stochastic screen

To make matters more complicated, there are of course different "stochastic screens" (or aquatint screens). In the end it will be up to you and your work on which you will decide. Stochastic screen are not cheap, certainly not the big ones. Handel your screens with care, they get scratched easily .

 Click for larger picture (Takach Press)

Resolution;

Fine: the fine pattern will produce a softer toned image, it produces a lower contrast.
Medium; the all around screen, average contrast. This is the one I use, certainly the best screen to start with.
Coarse; for high contrast results, use this one.
Most screens on offer seem to be around 80% density. You could get an aquatint screen (80%-1800dpi) produced, if you can still find a good imagesetter service. Some people print their own on an inkjet printer, but I have no clue if the quality of their work is up to standards.

You will have to do a test run for each screen you want to use. The screens come in different sizes.
There is more than one place selling the screens, here are a few:
Takach Press
Precision Digital Negatives
Polymetaal (only medium resolution)
Grafisk Eksperimentarium

Double exposure

Exposure, developing and positive film curves, all matter in getting a good end result. You should be consistent in every department, practice until you are comfortable enough to adjust your work on sight.

First we will tackle the exposure time or better times. I use a double exposure, because in my opinion the best results are reached this way. For the first exposure a stochastic screen is used, and for the second we use the positive film of the image we want to print. Some people combine the image and stochastic screen in photoshop (using the bitmap / diffuse dithering function) to end up with only one exposure. This will not give you the best results, you'll get a more grainy look.

The stochastic screen;
a halftone screen based on pseudo-random distribution of halftone dots, first used in the seventies. For a more technical explanation go here.

The stochastic screen eliminates the moiré effect in multiple colour printing. It allows us to print photographic images without the annoying effect of the dotted screen. If well performed you will end up with a print very close to the original picture, but with the advantage that you can digitally work with the image. You will be able to choose papers and colours, and when printed with traditional oil based inks on the right paper it will be an everlasting image.

The positive film;
The selected image will be printed on an inkjet printer. It will be printed as a positive image, and it will be the same size as the resulting print. Because it will be a contact exposure. There will be more on the positive film later.

Why we need the double exposure? The stochastic screen will prepare our polymer plate so it will be able to produce a wide tonal range, from deep black to subtle whites, without losing detail.

In the graphic below you will notice that the polymer plate exposed without the stochastic screen isn't able to produce the dark parts. The ink will only stick to the sides of the black volumes in your print. In the intaglio etching process the aquatint technique is applied to create this result, this results in holding the ink in  place as the plate is wiped.

 

Once you are familiar with the polymer process, you can try out different screens.

The vacuum frame, or not..

 Rotating vacuum frame, often used by screen printers.

Having a vacuum frame makes things easy, but it is not a necessity. Exposure units often come with a vacuum frame, they are very useful, certainly for the bigger format polymer plates. But there are other, and cheaper solutions.

Exposure unit with vacuum frame.

Small polymer plates can be exposed, fixed between two glass sheets using clamps.

Or you can use  a board, foam backing and a heavy glass sheet.

you can use a contact printing frame.

There a several diy projects out there to make your own vacuum frame. Here are a few:
https://www.youtube.com/watch?v=V22KWUwIn5A
https://www.flickr.com/photos/48428440@N04/sets/72157623645144972/
https://www.youtube.com/watch?v=8WJn5YFANxU


The one thing you should accomplish is a very good contact between your positive film and the polymer plate. Otherwise, you will lose the fine detail, and sharp contours. 

exposure units

Polymer plates are sensitive to UV light. The sun could do the trick, but it is not very accurate. Clouds, time of the year, hour of the day..... all will interfere with your exposure time.
So you will need access to a UV light unit.  There are different lighting units, you can buy one or build one. If you have access to different units, and you plan on using more than one, you have to test them all. If you are interested in more technical know how, read this article on the 'unblinking eye' website.

1.HPR lam;p;
The Philips HPR 125W bulb (HPR125 Mercury Discharge Reprographic Source). A lot of screen printers used this source. It delivers a powerful bluish-white light with strong actinic radiation. To operate it you need a transformer for each bulb.before exposure, the lamps needs to be on full power, this will take a couple off minutes. If you switch them off, let them cool down before switching on again

- How to connect a HPR bulb to a Philips BHL 125 L40 ballast.

2.Metal halide exposure units;
I use this one for the moment. The older ones need to heat up and completely cool down if switched off. Heating takes a few minutes.You can find second hand metal halide vacuum exposure units from a printing office. Check the internet for a decent priced one. Replacement bulbs are rather expensive.

3.UV fluorescent exposure units;

This one is easy to make if you like a little DIY.  Make sure to use the right UV fluorescent tubes. (e.g. Sylvania F25 T8 BL350 18)

These units produce less heat and you don't need a lot of distance between your unit and your plates. So it could be a table top unit.
DIY links:
http://www.atm-workshop.com/light-box.html
https://www.youtube.com/watch?v=_Zif0NBOeBc
https://www.youtube.com/watch?v=p-q2lkAMbdA

      Sylvania F25 T8 BL350 18. Click to enhance.
     

4. UV LED exposure units;
Low one energy consumption, and the life time of an LED bulb is between 50.000 and 100.000 hours. There is no waiting time, switch on the unit and it's ready to work.
A bit harder to build yourself, but you could.
DIY links:
https://www.youtube.com/watch?v=_2ZlrqMdTm8




DIY a few more;
http://www.screenprinting-aspa.com/how-to-articles/how-to-build-an-exposure-unit
https://www.pinterest.com/pin/184436547216133011
https://www.youtube.com/watch?v=D9IyQVFosFQ

You can find a lot of information on the net. Just find the one you would fancy. Print offices, screen printers, polymer plates... All of these units will work for you.

Protect yourself from UV light, and don't look into the UV light. UV exposure is not immediately felt . . . the user may not realize a hazard until after the damage is done.

The polymer plate

As mentioned, you can only use certain polymer plates.

If you want to use the Dan Welden solar plates, there is only one type of plate. You can order five different sizes; 4x5, 6x8, 8x10, 12x16 and 16x20 inches.

Toyobo Makes all kinds of polymer plates, called Printight. This is what's on offer:

The one I use is the KM95R plate, in the yellow section. Steel based. Also the one most intaglio printing shops sell. Comes in standard international paper sizes up, to A1.

Your polymer plates will arrive in a black light-blocking bag. Store your plates in constant humidity, avoid heat and keep them away from light. Exposed, store them in an air-tide plastic ziplock bag. Before storing, clean well. Keep exposed plates away from heat and UV light, the surface might crack.

Polymer plates will not keep forever. Some say one year if well stored, some say less. So it is a good idea to keep your images (film) for future use.

It should be easy enough to find a local supplier or order from the internet.
I mentioned Dan Welden solar plates before, you can buy toyobo and den Welden plates from Polymetaal if you live in Belgium or the Netherlands.