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4 colour process printing – what you need to know

4 colour process separation showing Cyan, Magenta, Yellow and Black separations

Full colour printing is achieved by using just four ink colours – Cyan, Magenta, Yellow and Black or CMYK (see box story below on what the K stands for). These are called process colours and full colour printing is also called 4 colour process printing. In theory, mixing Cyan, Magenta and Yellow inks should yield Black but in practice, we get an unsatisfactorily muddy dark colour due to the nature of the pigments used and hence Black was introduced as a fourth colour in 4 colour process printing to achieve better detail and contrast.

Halftones

Halftoning allows for less than full saturation of the primary colours. Tiny dots (screens) of the primary colours are printed so that when viewed from a normal distance without magnification, our eyes perceive the area of dots as a solid colour. For example, Magenta printed at a 20% screen will produce pink.

100% screen Magenta ink 20% screen Magenta ink

 

Colour Separation for 4 colour process printing

How is a full colour image printed with only four process colours? By using halftones of each colour, we are able to mix various percentages of all four process colours to print a huge spectrum of colours. To print a full colour image, the image is first colour-separated into four colours: Cyan, Magenta, Yellow and Black. Each single colour, consisting of halftone dots, is then printed separately, one on top of the other, to give the impression of a full colour image. If you take a magnifying glass to the full colour image, you will see that it is comprised of dots of various process colours.

4 colour process separation showing colour sequence

As Black ink is added to Cyan, Magenta and Yellow inks to improve better shadow details and produce a truer black, the amount of Black used to replace the other inks is variable and the choice depends on the technology, ink and paper in use. There are different processes called under colour removal (UCR), under colour addition (UCA) and gray component replacement (GCR) used to create different CMYK recipes. The advantages of using Black to replace the other colours are:

  • Black ink is cheaper than process colour inks.
  • By using Black ink to replace the other process colour inks, the total amount of ink used is less which helps with drying time and setoffs.
  • An image colour-separated using GCR is more stable (no colour shifts) during printing as you don’t need to balance CMY colours to maintain a neutral gray.
  • It is easier to achieve and maintain gray balance.

These 2 images below illustrate different colour-separation settings, yielding different CMYK plates to produce the same final composite image.

4 colour separation with no GCR 4 colour separation with GCR

Rich Black

Regular Black vs Rich BlackIf we add one or more of the other CMY colours to Black in 4 colour process printing, we get a darker, truer Black than just using Black ink alone. We call this colour Rich Black. A typical Rich Black mixture would be 50% Cyan, 40% Magenta, 40% Yellow and 100% Black. This produces a darker Black that is neutral in colour. Some other combinations of process colours can produce other looks like “Cool Black” or “Warm Black”. In theory, you can get the richest Black by using 100% of all four inks but in practice, you are limited by how much ink you can lay down on the paper (how wet the paper can get) and the technology used in the printing process. A safe rule of thumb is that total ink coverage should not exceed 240% on normal papers. Papers that are uncoated like newsprint should be even less.

Rich Black should never be used for small type, especially fonts with fine serifs. Also, be sure to trap any elements that knock out a rich Black background so that there are no registration issues.

 

Offset Printing, Digital Printing and Large Format Printing

At Samco, we offer exceptional offset printing, digital printing and large format printing. All these reproduction methods use 4 colour process printing to produce the final print. In the case of large format printing, we actually print using CcMmYK (CMYK plus light Cyan and light Magenta) to achieve an even greater colour gamut. When you send us your print files for printing, we meticulously check the files to ensure that there won’t be any surprises when the job reaches the press. Issues like missing fonts, low resolution graphics, wrong size, no bleeds etc are caught and you can elect to fix the file and send us a new one or we can fix the file here for you. We are experts in our field and we can help you get the most out of your print investment.

 

What does the K in CMYK stand for in 4 colour process printing?

 

If you thought it was the last letter in blacK, you’d be wrong. Many people suggest that “K” is used instead of “B” because it may be easily confused with “Blue”. This theory is a myth.

The story goes that old-style printing presses had only one colour station and printing a full colour job would require four separate passes through the press, laying down one colour on each pass. After each pass through the press, the plate would need to be removed, the ink train washed up and rollers cleaned and the next colour loaded up for the next pass through the press. This was very time consuming and the individual print jobs would take days to finish. After a long day of printing, a press apprentice would begin loading the last colour “B” into the press. If any confusion occurred and Blue ink was loaded into the press instead of Black, the entire job would be ruined and need to be started over again. So it seemed natural that printers would want to use an alternative letter for Black so that this confusion could be avoided.

The correct answer, however, is that the K in CMYK actually stands for Key, which is a traditional word for the Black printing plate. Text and image borders are typically printed in black, and also holds the detail in the images, so it makes sense that the other colours are positioned (aligned or registered) – or ‘keyed’ – to Black colour. The “key plate” was what the other colours are keyed to.

 


Samco Printers is a full service commercial printer providing exceptional offset, digital and large format printing in Vancouver, British Columbia, Canada using the latest pre-press, printing and finishing technologies. We also have a fully equipped art department to handle your graphic and web design needs.

Need help? Let us help you get started or put the finishing touches on your printed materials. Our Art Director will be happy to spend some time with you. Give us a call at 604.683.6991 or send an email to techsupport@samcoprinters.com

Don’t need help with design? Talk to us anyway to ensure that the files you’ll be supplying meet our specifications.

 

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