Using Photoshop to create color separations for use in screen printing?

How do I create a custom halftone separation for use in silk screen printing?

I am trying to create a custom halftone separation for use in screen printing. What I have done is use the Photoshop color halftone filter. I can then output each of the CMYK plates onto transparencies as halftone screens.

However, when using the color halftone feature, how can I use this to simulate an output screen? The color halftone dialog box only mentions Max Radius in pixels. But how would I use this to calculate, for example, a 45lpi line screen? Or am I going about this in completely the wrong way?

There are numerous approaches for producing color separations for screen printing. Especially as there are also a number of different applications of the process. Some implementations of screen printing are for one-off art projects, whilst others might `be for commercial T-Shirt printing for example. For commercial projects it is always advisable to check with the printer first as to their preferred method. Or even better, get them to do the prepress to match their systems, as this probably varies a lot depending on the equipment that they use and the substrate being printed on.

The techniques used in the above question might work, as it kind of simulates the CMYK separation process. But the color halftone filter is not really the correct way to create a halftone screen for screen printing. It is predominantly a Photoshop special effects filter rather than a prepress tool. For example, you can't really be precise about the resolution of the line screen. It also doesn't take account of screen angles and the color halftone filter will not produce different angles for each separated screen color.

The following is just one home brewed method and concentrates on achieving an adequate result by controlling the production of the halftone dots themselves. It is always worth experimenting with your own particular setup before commencing with an important screen printing project.

1. First we create our CMYK (or duotone for that matter) image at a normal print quality resolution. Let's assume 300dpi in this case.
2. We then copy each color layer (channel) into a grayscale Photoshop document and then convert that document to a bitmap single channel file (we usually choose 1200dpi at this stage, but your mileage may vary). When you get the Option window choose Method -> Halftone Screen.
   
   You then get to choose your halftone screen frequency, which may be around 65lpi for for commercial printing, or probably less for an art project. Once again, it is always best to check with the printer as to what is best.
3. To reduce the likelihood of a moire pattern, it is best to specify different screen angles for each color, which may be different for screen printing than it is for offset-litho printing (have we mentioned checking with the printer?). The following are two possible variations.
   • Yellow at 7.5%, Cyan at 22.5%, Magenta at 82.5% and Black at 52.5% or Yellow at 5% Cyan at 55% Magenta at 22% and Black at 80%.
4. For dot shape, the ellipse halftone dot is often recommended for screen printing.
5. Once you have converted each plate to Photoshop's bitmap format, you can save them as tiff files, import the file into a page layout program and then print each color plate onto your acetate or film.

A completely different approach would be to print separations directly from QuarkXpress, InDesign or a PDF file and specify the type of dots, line screen and screen angles that you wanted to use. This might save a certain amount of times, although arguable gives you less visual control.