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Welcome to  ‘Ask The Expert’.  This page is moderated by industry consultant, Steve Wilkinson.  This page features the answers to your questions. Our visitors can also post comments to these questions/answers as well.  You can learn more about Steve’s background and our ‘Ask The Expert’ page by Clicking Here.

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June 28, 2010

Can you help me better understand the differences between CMYK, Hexachrome TM and Opaltone TM color models.

Let’s start with CMYK, which is the original color system that believe it or not was invented more than 100 years ago.

This is a subtractive color model that is generally used in process printing. Most people use CMYK to describe the process itself and is an abbreviation of the process color inks used, that being Cyan, Magenta, Yellow and key (black).

Generally the colors are printed in the order as it is described but this can be varied dependent on the requirement of individual jobs.

This technique works by partially or entirely masking colors on a lighter background which is usually white. This in turn reduces the amount of light that will be reflected off a printed image, other color models use just RGB, but these are additive techniques that use a combination of primary colors. Black is achieved by a combination of these colors and is generally used when deeper black tones are wanted.

This color model is still by far the most widely used method of achieving multiple colors in most markets.

Now HexachromeTM, often referred to as the CMYKOG, was introduced in 1996 and is limited by trademark, patent and is a licensed process for those wanting to use it. Hexachome is a sophisticated enhanced version of the subtractive color model that uses much purer ink set of Cyan, yellow, magenta, black inks, plus a very strong orange and green.

With this combination of six colors over 90% of all solid colors can be created, which is almost twice as much as with the traditional 4col CMYK model.

The HexachromeTM inks also have brighteners in them that also add to cleaner colors being achieved, which in turn helps to produce a much larger color gamut that is close to a RBG model. This system is quite complex and needs RIP systems that can handle these colors and is challenging for those new to this technology.

If 6 colors are good, then 7 colors should be even better, which is the claim for the OpaltoneTM color model. This technology uses standard CMYK process inks, plus  unique OpaltoneTM red, green and blue additive primaries. With these colors it is possible to create more than 2880 digital hues that produce very vibrant color and density. Thus the need for a black ink is completely eliminated with black being produced using RGB that saves on a lot of make ready and generally gives improved gray balance and richer shadows. Again this system is trademarked and licensed; of course you need to remember you need up to seven color stations if you want to take advantage of this system although to be fair many jobs can be printed with just six colors due to its wide color gamut range. Again you will need compatible workflow systems with appropriate software to handle this 7-color process.

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June 1, 2010

Why is it so important to control ink viscosity, and how best do I control it?

An inks’ viscosity has the ability to affect almost every aspect of your printed product from color strength, graphic quality, ink thickness, density, ink usage, press speed, drying time, need I go on?
The real problem with viscosity is that it is a constantly changing factor and if not monitored and controlled will have an effect on most of the factors listed above, all of which can have a dramatic affect on your bottom line, as well as the obvious quality of your print.
Viscosity is checked by hand by most printers and for best possible control should be checked at least every hour or even more which is what most ink manufacturers would recommend.
Too high a viscosity and dirty or less sharp print is often the result but if adjustments are made to the ink and no improvement is seen it may mean that you need to change your anilox.
Low viscosity will generally be seen as mottling and inconsistent thickness and uneven laydown.In the case of water-based inks, viscosity is strongly connected to its pH, which must be adjusted before any attempt is made to adjust the viscosity. Both water- and solvent-based inks are thixotropic, meaning that when they are agitated or well mixed they will give a lower viscosity measurement. This means you should only be measuring from a bucket of ink that is being pumped through the ink system if you want to get an accurate reading.
Most inks are affected by heat, which decreases as the atmospheric temperature increases and as the ink gets warmer during the run and the exact opposite is true as an ink gets cooler.
In terms of measurement two measuring devices are used for manual measurement: this is a Zahn cup and a Shell cup.  Both are metal cups with a specific size of hole at the bottom to allow the ink to drain out. The most common cup used in flexo is the zahn cup but there has been some move toward shell cups, which are used more commonly in the gravure industry and are seen as being slightly more accurate.
The main difference between the two cups is that the shell cup has a narrow tube at the bottom that must be kept clean at all times.
The Zahn cup comes in a range of sizes that are used in flexo from #2 to #5, although the #2 and #3 are the most commonly used size.
All cups are used by dipping them into the ink and measuring the time it takes for the ink to drain through the hole in the bottom of the cup with a stop watch. Your ink supplier will recommend what viscosity should work best for your ink system and print requirements.
Now because of the frequency of checking the viscosity many printers have actually switched to automated viscometers that make automatic adjustments and give much more consistent results than manual adjustments can ever do no matter how careful the operator may be.

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