Separating Your Colour Layers for CMYK Screen Printing

CMYK screen printing is a great way of bringing both your photographic and coloured art images to life through colour separation. This is achieved by layering four colours (cyan, magenta, yellow, and black) on top of each other using only 4 screens.

Photoshop plays a key role in this printing process and is essential if you want to create the artwork yourself. We're using Adobe Photoshop CS5. (Other software may be suitable for CMYK colour separation too, but the method will vary from the below instructions).

To start, we want to set up our a-sized document. You’ll need to be working on A4 or larger. Make sure that your colour mode is set to CMYK, and your DPI (dots per inch) is set to 300.

Import your chosen image. You can do this by dragging and dropping the image onto the blank document you have just created.

It is important that the image you’re using is high quality (300dpi) and set at the size you want it to be when printed. We would suggest using an image that is A5 or larger. You may find that working smaller results in a lack of detail in your final print. If you find that the colours in your image are a little muted it would be worth upping the saturation. We would advise doing these steps before you start the colour separation process.

Once the image is imported, flatten the image (go to 'Layer' at the top, then 'Flatten Image' on the drop down).

Next, go to ‘Channels’ on the right hand side of the screen – the position of this may vary depending on the version of photoshop that you have. Underneath the master CMYK layer you will see a breakdown of the colours as Cyan, Magenta, Yellow and Black.

To be able to work on each layer individually, we need to separate the channels. To do this, go to the 3 line dropdown in the top corner of this box and select ‘split channels’.

This will open each colour channel into its own workspace. Each tab will have the initial of the colour channel in its title e.g. CMYK_C for Cyan so that you can see which one you’re working on.

Starting on the first channel, go to Image > Mode > Bitmap…

The ‘Bitmap’ box will appear. Here we want to make sure that our input and output are both at 300dpi and our method is set to ‘Halftone Screen’. Click ok.

To make the visual illusion work once all our layers have been screen printed, we need to make sure that each colour layer is set to a specific angle and that our frequency is set to the correct level for the mesh count of the screen we are going to be using.

The general rule for setting your frequency is to divide your mesh count by three e.g. 90T = 30. We’ve selected round as our shape, but you could experiment with the others that are available. Here are the angles needed for each layer:

Magenta – 75°

Cyan – 15°

Yellow – 0°

Black - 45°

Once you have filled out all the information select ‘OK’, and the halftone will be applied.

 

Note: When you have applied the halftone change the mode from ‘Bitmap…’ to ‘Grayscale’. To do this, go to Image > Mode > Grayscale

We now want to add some registration marks to our channels. This will help during the printing process and mean that each layer will line up. Depending on the size of your image you may need to make your document size larger to allow for this step. Ideally, you want a good few centimetres between your artwork and registration marks.

To do this without distorting the image, go to Image > Canvas Size and amend accordingly.

A4 = 8.267 x 11.692 inches

A3 = 11.69 x 16.53 inches 

A3+ = 12.99 x 19 inches

A2 = 16.5 x 23.4 inches

Using the Photoshop guides, find the centre of the image both vertically and horizontally. Do this by dragging from the ruler at the top and left hand side of the artboard (Press Ctrl-R to add rulers to your artboard if they're not already there). The line should snap to the centre when you drag it over. Apply the same guides to each of the other colour channels.

 Using the text tool add a ‘+’ sign on each side of the image at the centre point. When you have done this, merge the layers together. You may need to rasterize the text layers before you merge them. Select all four text layers (hold shift and click each one), then right click and select 'Rasterize Type'. The layers will remain isolated so, in the Layers window, select all four + layers and select 'Merge Layers' from the drop down menu.

Duplicate your merged registration marks layer onto each of the remaining colour channels, making sure they’re centred once they have been applied. To duplicate the layer, select it and right click. Select duplicate layer and select the document you want to duplicate it to e.g. CMYK_M

Note: You may also find it useful to add the initial of the colour layer next to one of the registration marks so that you know which colour you’re printing as it can be hard to tell once your screen is exposed!

When you have added the registration marks to each of your layers you’ll need to flatten the image. To do this, right click on a layer and select ‘flatten image’. We should now just have one locked, background layer.

Save each channel as a PDF file. You may find it useful to title the files accordingly e.g. CMYK_Cyan

Your artwork will then be ready to print out and expose onto a screen!

If you do not have access to a printer, we can print the films for you through our Custom Film Positive service and we can also make the screens. We can also split your image into CMYK for you if you prefer. Click here for more information about our Custom Screen Exposure service.