Increasingly, buyers of packaging and other flexographically-printed products are demanding colorful, “high-impact” graphics to help make their products stand out and to increase brand recognition.  In response, most flexographic printing companies offer an almost infinite range of print colors, and many end up generating significant inventories of surplus inks of several to many different blended colors.  This is undesirable, as besides tying up cash, custom color blends may become obsolete while in storage.  Storing ink for months, only to later pay to have it disposed of, is not an efficient use of storage, or an efficient way of managing ink inventory.

As there is usually ink left over at the end of every print job, there will likely always be some amount of surplus custom blends in inventory.  To help minimize this amount, you’ll need to take a two pronged approach to the blending process:

1. Generate the ratios of the blend components as accurately
   as possible, so that I can duplicate the color accurately on
   subsequent blends with minimal color adjustments.
    
2. Work surplus ink blends into new ones, whenever possible.

While many color standards express color formulas as percentages, I suggest using ratios when working with ink formulas for a couple of reasons:

1. Ratios will allow you to calculate the base color requirements for any blend.  Whether making a large or small batch, you need only decide on how much ink is needed (target weight), then multiply that weight by the ratios for each of the base colors.  The calculated base weights should add up to your target weight.
    
2. Knowing the ratios of each base color in a blend will allow you to calculate the amount of each of the base colors in a blend.  This is essential when working blends into other blends with different base ratios.

Most color formula guides show formulas as percentages.  Fortunately, simply dividing the percent values by 100 converts percentages to ratios.  A formula whose base percentages are 75.0% color one, and 25.0% color two, converts to ratios of .750 color one, .250 color two, respectively.  A color whose formula is expressed as 81.2% green and 18.8% yellow, yields ratios of 0.812 and 0.188, respectively.

When working outside of the charts, ratios can be calculated directly from the weights of the base inks relative to the total weight of the blend.  To calculate the ratio of each base color in a blend, the weight of each base is divided by the total weight of the blend.

For example, if we blend 27 pounds of black ink and 32 pounds of white ink, the ratios for black and white in the blend are:

• Total ink = 59
• Black ratio = 27/59 = 0.458
• White ratio = 32/59 = 0.542

Therefore, if we would later wish to blend more of this gray color, we would simply decide how much of it we want to blend, based on estimated requirements.  Multiplying that amount by 0.458 determines the amount of black to add, and multiplying by 0.542 determines the amount of white to add.  So, if we want to determine how much black and white to blend to yield 95 pounds of this gray at a later time, we would perform the following calculation:

• Target weight = 95
• 95 * 0.458 = 43.51 pounds black
• 95 * 0.542 = 51.49 pounds white

Note:  If we include solvents and/or other additives, they must also be considered in the calculations (something not usually accounted for on color formula guides).

Working surplus blends into new ones is a little trickier, but uses much of the same math.  “Working-off” inks requires five basic steps:

1. Calculate the weight required for each base color in the new blend.
2. Identify a surplus blend compatible with the blend being mixed.
3. Calculate the amount of each base color in the blend being worked off.
4. Determine how much of the surplus blend can be used in the new blend.
5. Determine how much of each base color to add to the surplus blend to yield the target weight of the new blend.

To calculate the base requirements of the ink being blended, multiply the target weight by the ratio for each base, as in the 95-pound target example above.

Next, to find which surplus colors are compatible with a new blend, all we need do is study the base colors.  A surplus blend is compatible with a blend being mixed if every color present in the surplus blend is also present in the color being mixed.  However, not every color in the new blend has to be in represented in the surplus blend.

For example, let’s say we are making a color that has white, black, and yellow in it.  Blends with white, black and yellow, white and black, white and yellow, and black and yellow would all be compatible.  However, a surplus ink having white, black, yellow, and blue would not be.

Now, to determine the amount of each base color in the ink being worked off, multiply it’s weight by the ratio of each base color.  The process is similar to calculating the base quantities of black and white for the 95-pound target weight in the ratio example above, with the actual weight substituting for the target weight.

Determining how much of a surplus color can be used is a bit tricky.  First, compare each base color weight in the new and surplus ink blends.  If no base color in the surplus ink exceeds the base required in the new blend, all of the surplus ink can be used.  However, for each of the base weights in the surplus ink that exceeds the corresponding base weight in the new blend, we must divide the lower value by the greater.  Multiplying the total surplus weight by the least of these calculated values yields the maximum amount of surplus ink that can be used.  If this last step was required, we need to multiply the resultant maximum amount by the ratios of its base colors to calculate the amounts of each base that will be contributed to the new blend.

Once the amount of surplus ink that can be included in the new blend has been determined, we must calculate the amounts of each base required to complete the new blend.  Simply subtract the amount of each base being contributed to the blend by the surplus ink from the total amounts required for the blend.  The differences are the amounts that need to be added to the surplus ink to complete the mix.

A lot can be said about the process of ink blending.  Indeed, when writing this brief I had to resist the temptation to discuss many of the underlying sources of surplus ink and many of the difficulties associated with generating and storing surplus inks.  Errors in blending, repeated color adjustments, inconsistent base densities, and many other problems all can contribute to the creation of excess surplus ink.  Storage, disposal, and environment issues can present flexographers with many serious challenges.  However, for the sake of brevity, I have limited this discussion to broad points in the blending process.

If you have any questions or comments, or if you have specific questions as to applying the above concepts to your particular situation, I’d be happy to assist on a one-to-one basis.

Author:  Frank Burgos
E-Mail:   frankb@flexoexchange.com

©1997 Content of this article is original and may not be copied or reproduced without the express written consent of Frank Burgos, FlexoExchange.