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FLEXO Magazine : November 2013
TeChNOLOGIES & TeChNIQUES Color Formulation Understanding relationships, Matching Theory, Sequence An Excerpt from Flexography: Principles & Practices 6.0 Non-process colors, typically made up of a combination of two or more pigments, created without screens or dots, are referred to in the printing industry as spot colors. Spot color formulation is a method of specifying and creating colors, in which each color is printed with its own ink. In contrast, process color printing uses four inks to produce all other colors: • Cyan • Magenta • Yellow • Black In flexography, spot color inks, when not consisting of pri- mary colors, are usually formulated and mixed prior to print- ing and are specified by the use of color matching systems, such as the Pantone Matching System (PMS). A color match- ing system, or series of printed color swatches, is used to match, specify, identify and display specific colors produced from a specific spot color formulation. There are many steps in formulating spot colors. The process begins with choosing the correct flexographic base ink system for a printing application. From there, specific spot colors can be formulated and blended. After formulation and blending, a spot color must be proofed and checked for ac- curacy. There are several ways to do this and the method cho- sen should be selected according to the printer ’s capabilities. CoLor mATCHing THeorY The most useful visual tool in color matching is the color wheel (Image 1), a slice of lightfastness, chroma and hue (L*C*h°) color space. The colors of the spectrum are arranged in a circle as shown in the diagram, reducing chroma to black at the cen- ter. When color matching, this wheel should be kept in mind. Mixing pigments, which are adjacent on the color wheel, re- sults in colors that are clean or bright. For instance, if green- shade cyan (GC blue) is mixed with green-shade yellow (GS yellow), the mixture will be a clean, bright green. Drawing a line on the color wheel between the two pigments shows how close to the center gray area the line will travel. If the same GC blue is mixed with red-shade (RS) yellow on the opposite side of the color wheel, the line on the color wheel joining the two pigments travels closer to the center gray area and the mixed ink will be a dull, dirty olive-green. The concept here is when color matching, the ingredients should be kept close in the color wheel to obtain clean, bright color matches and further apart to dirty up the match. It is important that color match formulas are constructed with individual pigments that are not too far apart. For ex- ample, a brown should not be matched using red, yellow and blue; preferably, it should be matched with red, yellow and black, the center point on the wheel. Formulas containing several pigments from distant areas of the color wheel will change hue very quickly with small viscosity changes. It is critical to be able to talk about color in a way that is intui- WONDEROUS WHEEL • Keep ingredients close in the color wheel to obtain clean, bright color matches and further apart to dirty a match • Construct color match formulas with individual pigments that are not too far apart • Formulas with pigments from distant areas of the color wheel will quickly change hue with small viscosity changes • A line on the color wheel between two pigments shows how close to the center gray area the line will travel tively under- stood. One way is to use the color wheel and remember the position of individual colors on the wheel. In this way, when the hue of a batch of red ink is compared against the standard red ink, the batch may be more yellow or blue in hue than the standard (See Image 2). If the batch is identical in hue it may, for example, be too strong or too dark. If this were the case, it would have a lower L value than the standard. An addition of extender or solvent to the batch would correct this. As the ink is weakened, all three color attributes—L*C*h°—are affected at the same time. Image 3 summarizes what happens to the L*C*h° numbers as various colors of ink are weakened. It should be noted that the ink can be weakened by adding solvent or extender, or by using a lower-volume anilox roller. CoLor mATCHing ProCedure A general flowchart for mixing special color ink is shown in Image 4. It shows the procedure for making a 100-g. batch to test and develop the specific formula. End-use requirements may dictate the choice of pigments available for a particular color formulation. Any colors that might be a problem, such as small amounts of rhodamine pigment in a white tint, should be avoided. The combination of rhodamine and titanium dioxide is unstable, due to chemical reactivity. If fade resistance or outside expo- sure is required, the pigments chosen should be suitable. When specified by the end-use requirements, the pigments used should be stable to aggressive products, such as milk, acids, alkalis, oils and solvents. Finally, the lowest cost combination of pigments should be used to achieve the color. Once the small test batch is made, the amount of material can be scaled up for the pressrun quantity. The initial formula can be obtained from a variety of sources, including historical data and experience. CoLor FormuLATion SequenCe Here are steps involved in color formulation: • Weigh Sample: A 100-g. sample of the initial formulation is weighed in the ink laboratory. Pigment selec- tion is based on the color being matched using the color wheel as a guideline. There are other con- siderations for the optimum color match. Use the fewest number of colors in the match since this makes weighing and control of the ink for the press much simpler and easier to adjust and control • Adjust Viscosity & Strength: This step is based very much on the experience and knowledge of both the ink system and the press components where it will be used. Actual pigment concentration and image 1: Color Wheel image 2: Coarse Color Wheel www.flexography.org NOVEMbEr 2013 FLeXO 47 46 FLeXO NOVEMbEr 2013 www.flexography.org © 2013 HARPER Yellows can be red or green to the standard Blues can be red or green to the standard Greens can be yellow or blue to the standard Reds can be yellow or blue to the standard Oranges can be yellow or red to the standard GS Yellow RS Yellow Orange GS Blue += += += L Value will: C Value will: H Value will: Yellows Go lighter to a higher number Go dirtier to a lower number Go greener to a higher number Oranges Go lighter to a higher number Go dirtier to a lower number Go yellower to a higher number Reds Go lighter to a higher number Go dirtier to a lower number Go bluer to a lower number Blues Go lighter to a higher number Go dirtier to a lower number Go greener to a lower number Greens Go lighter to a higher number Go dirtier to a lower number Stay about the same image 3: L*C*h° Variation Values