Advanced / CMYK Process Halftone Printing

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CMYK Halftone Magic-

A majority of screen print work is printed using inks mixed for a specific color.  Qualities such as color and transparency are controlled on a per-batch method. The color in the bucket is what will print on the page. This is "spot color"  Another prevalent color methodology is CMYK process color.  CMYK is printed using Cyan, Magenta, Yellow and Black. (Black is noted as "Key" in CMYK.) The combination of these four colors along with the variable dot size of halftone, can replicate a wide color gamut. Interestingly, if only solid color inks are overprinted CMYK only yields seven colors.  It's the halftone that provides the color range. Rather than being mixed in the bucket, the colors are mixed in your eye!  From color laser printer pages to vehicle wraps, CMYK halftone printing is everywhere!



Printmakers can lever CMYK technologies to get full color prints with minimum stock of ink colors.  Nothing comes for free though.  What is saved in ink variety is often spent on graphics software that can generate color halftone separations.

Mixing Ink-

The four colors of ink used in CMYK need to be mixed with transparent base in a defined ratio of transparent base to ink.  This allows inks to blend on the printed stock and produce a full range of color.  Good results can be had by roughly mixing the ratios together.  For example one scoop yellow to one scoop base but more precision in the recipe mix would yield better results on the print.


CMYK Recipe

What makes it work-

Halftone is simply a matrix of discreet dots of varying sizes.  The dots of ink are printed in patterns and each screen's pattern is created to have  a unique angle in relation to the other halftones in the print.   The size of an individual dot regulates the intensity of color.  In the image below the text is shown from the black screen of a CMYK print.  No hard edges can be found, just dots with more intense black at the center of the text.  If your CMYK project needs some hard edges, for example to get crisp text, an additional, non-halftone screen would be prepared.




Moire Pattern Interference-

Moire (pronounced 'more-ray') Patterns can show up in your final print if there are conflicts in the way a color separation was prepared or when the color separation has a conflict with the screen mesh count.

Moire Pattern Demo:

Moire Pattern Showing on a Print:




The tricky part is that in process printing, the individual dots on each screen need to print in a way that allows them to show.  This is accomplished by changing the angle of the dot pattern on a separation relative to the other screens being printed.  Fortunately, there are "safe" screen angles that can be selected in software that reduce moire pattern interference.  Enough about moire patterns being bad though.  It's a effect and as such needs to be controlled rather than avoided.  If a moire pattern is desired as part of the work then introduce one!  This is art, there really are no "no go" techniques.  Moire is only bad if it's unexpected or not part of the artists final vision.

** One note, the tan lines in the "April 22" photo above are not a moire pattern. rather they originate from the fluorescent tubes in the light table used in the picture. That table uses an older magnetic ballast that creates an electrical "ripple" in the light it makes.  Cameras can see and capture the ripple effect, the human eye can't!

Preparing the screens in software-

there are some ground rules in basic process screen printing but they are few. First, the original artwork has to be created in in it's final size.  Second, don't change the resolution after you've created the halftone separations.  Changing these values after the start if the project can yield unpredictable results!   If scanning an image 200 DPI will do nicely, set image size to be what the final print size will be.  Increasing the canvas size in Photoshop while leaving the image size unchanged will allow room for the registration marks to print on the laser.  Make sure, however, that there is room on the lasers printable area for them!

For our example Photoshop was used to create the separations.  To start a, portion of a photo was scanned at 200 DPI.




The photo was then edited to produce what I wanted to print.  Final size and resolution need to be set at this stage. Changing these values after the halftone is generated will mess things up.

To create the separations in Photoshop is just a quick few steps and will result in printer output that can be put in the exposure unit. There are many screen angle "recipes" on the Internet  These work in the shop I print in.

  1. Set the image to CMYK mode.  Go to IMAGE-MODE-CMYK. Accept the change to the U.S. Coated (SWOP) v2 profile. 

  2. Convert the image to color halftone. Go to FILTER-PIXELATE-COLOR HALFTONE.

    1. Set max radius to anywhere between 8 -12 pixels  4px is the minimum and dot radius settings above 12 add more texture.

    2. Set the screen angles to:

      1. CYAN/Channel One - 23 degrees

      2. Magenta / Channel Two - 83 degrees

      3. Yellow / Channel Three - 98 Degrees

      4. Black / Channel Four - 53 Degrees

Use the channels tab to activate each individual channel to print  Each channel should preview as a black and white image.  If there are grays in the image use IMAGE-ADJUSTMENTS-LEVELS to get the channel to black and white only.  Grey values don't expose well!

When printing each channel, don't forget to add labels and registration marks, once the color separations are printed in black and white for the exposure, the text label is the only way accurately ID the screens.

Creating the final print-

After burning each separation to an individual screen, or a half screen each to save materials, it's time to print.  Each single print is comprised of four individual prints each printed over the top of the ones below it.  After the previous color is dry, use the registration marks to accurately align the next color and print the run.

In CMYK the order of printed color goes from light to dark.  That is, Yellow, Magenta, Cyan, and Black.  In the photo below, the magenta separation has been printed over the yellow.  The magenta overprint created orange and red areas. The difference between the red and orange areas are the result of the different dot sizes in the halftones.  The red area, having larger magenta dots than the orange area make a more intense representation of magenta, pulling the color seen to red. It's still simple yellow and magenta dots arranged to fool the eye into seeing another color.  There is no raw yellow or magenta in the final print.




In the rack are the final CMYK halftone prints along with a test print on cardboard.  There are several colors in the final print and none of them are the colors the print is made of.  CMYK process halftoning is a lot like magic!



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