Flexography is a printing process used for a large number of substrates and applications, as such the process colors can differ based on the pigments based on in their properties. Although there are color specifications, there is no general norm for density values leaving a number of open questions... What is the correct density for a printed job? Why do we find different values from company to company? Who is right? The reality is that in recent years a popular belief has emerged: the higher the density, the better the result. Ideally, an attainable, repeatable, and consistent standard should be established maintaining a correlation closest to the color proof.
|Image 1. Density values for different |
levels of reflectance.
Source: A Guide to Understanding Graphic
Arts Densitometry by X Right
To optimize solids we must first review the basics...what is density, or to be more accurate, reflection density? According to the Flexographic Image Reproduction Standards & Tolerances (FIRST), and Flexographic Principles & Practices, reflection density is a measure of the proportion of light reflected from the substrate and ink, in our case would be a printed ink film. This measure is established as a logarithmic equation of the reciprocal of the reflected light (Image 1). This means, the less light transmitted from an ink film, the greater absorption of the original light source resulting in a higher density. Reflection densitometers in the industry use this principle which corresponds to how the human eye sees.
Why is it important to measure density? Controlling color and maintaining a consistent printed product is fundamental to guarantee a correct amount of ink applied to the printed substrate (thickness of the ink film) is one of the most important factors for the printing of halftones and polychromies. Theoretically, the more ink applied, the more saturated the printed sample, because there is less reflected light resulting in a higher density. However, when the thickness of the ink film approaches a certain point, there is no longer a considerable increase in density. (Image 2)
Establishing standards will depend on factors that affect the printed ink film, such as:
- Ink: viscosity, quality, and quantity of pigment
- Anilox: volume and its variation in the useful life
- Plate: hardness, surface tension, and texturing
- Stickyback: density and resilience
- Substrate: surface tension, treatment, coating
An optimization process will identify the best combination of these printing variables, maintaining an adequate balance of quality and stability.
Let’s stop at another fundamental: how we reproduce images on a substrate. The subtractive printing method uses as a light source a white surface reflecting all wavelengths of the visible spectrum in equal proportion. Using inks as filters for the absorption of the primary components red, green, and blue should reproduce as many colors as we see. However, the pigments used in cyan, magenta, and yellow inks are not completely pure and while we expect each of them to completely absorb a third of the visible spectrum, what we find is inferior behavior.
Image 3. Subtractive printing method and comparison between ideal and real inks.
Source: Flexography Principles & Practices. 5th Edition
- Real inks do not completely absorb the component of the light they should, but instead reflect a portion showing dirty or contaminated; red light in the cyan and green light in the magenta
- The reflected light peaks for real inks are not as high as those of the ideal ones, so the colors are less saturated
The combination of these factors result in a reduction in the range of colors that a printer is capable of reproducing. This introduces two properties of a process color ink which are known as the hue error (deviation from the ideal color) and the grayness (gray or dirt component). The hue error is clearly evidenced when similar percentages of the three base inks do not produce a neutral gray but a dirty brown tone. A hue error requires a pre-press adjustment to find the ideal gray balance.
Maximizing the gamut of a printing system is critical to produce bright and vivid colors that optimally represent the final product on the supermarket shelf. To increase the achievable color space, the key is to increase the chroma/saturation, maintaining the hue and luminosity of each of the process colors. A technology that increases the density of solid ink (the amount of ink provided) will allow the user to increase the gamut of a press machine while not affecting other attributes of color; this can be seen in the following image (Image 4).
As the density increases, the color space achieved is expanded thanks to the increase in chroma/saturation (the distance of the color from the center) (Image 4). However, beyond a certain value, the color hue begins to vary dramatically with no additional increase in chroma. This is mainly illustrated by magenta and cyan inks which move to redder and bluer colors respectively. Additionally, although the graph does not show it, the lightness values are affected for the three colors resulting in darker colors. This is because process inks are not perfect light filters, so we could consider that by increasing the thickness of these we will also be increasing these errors that they present. John Seymour in his article “Why does my cyan have blues?,” describes a very simple example using Beer's law which predicts that the reflectance is multiplied if the amount of ink in the substrate is doubled: by having an ink film magenta that has an assumed reflectance in the red component of 90%, in the blue component of 40% and in the green component of 10% when applying another layer of the same ink we will obtain a reflectance of 81% (90% x 90%) for the red portion, 16% (40% x 40%) for the blue portion and 1% (10% x 10%) for the green, so the resulting color is more reddish. Remember the ideal magenta ink is one that fully reflects the red and blue light, so this result is different than expected in hue but also darker by reducing the amplitude of the red light; It can practically be a different color from the intended one. (Image 5)
Image 5. Comparison between a Magenta under an ISO specification and the same ink at a higher density. The color is close to a Rubine Red. Source: Esko.
What, then, is the ideal density value that should be used in the press? Although a process for this analysis can consume time and resources on the machine, the ideal is to use market references as starting points since they should have sought this review. FIRST in its most recent edition not only suggests density values but also accompanies them by the hue angle specifications. (Image 6 & 7) Each company must define what values they are capable of reproducing in a consistent and repeatable manner, as well as the tolerances that are specific to each system.
|Image 6. Table of densities and tolerances. Source: FIRST|
Lightfast Hue Angle
Image 7. ISO 12647-6: 2012 specifications for process colors hue angle. Source: FIRST
When talking about specifications for printing process color solids, FIRST establishes that the protocol must first specify the value of the hue of each ink achieving a tolerance preferably within +/- 2°. Subsequently, this should reach the highest possible saturation, and finally, confirm the density value reached. This is important to maintain an optimal gray balance throughout the tonal range.
The same FIRST methodology confirms that during the optimization stage the printing variables that produce the desired results must be identified. The plate, in combination with the surface pattern, is a fundamental piece to establish appropriate density values. There are two approaches in the market that depend on the preference of each user and the level of investment in technology that is had:
- Digitally controlled surface patterning (Image 8).
- Surface patterning included in the raw material.
|Image 8. Example of various surface patterns. |
Regarding software to create different structures on the surface of the plate, customization for each printing condition is possible through its respective optimization test commercially called Benchmark. In combination with the selected anilox and ink, it will provide a range of results that serve as an evaluation to determine, not only, the achievable densities, but also, the coverage obtained for each patch. The surface of the plate now has some cells and therefore a volume, which, in combination with those same characteristics of the anilox, plays a crucial role in achieving the best results (Image 9). These surface patterns must be optimized independently considering whether they are process colors, spots, or white so that a suitable pattern for one printing condition will not necessarily produce optimal results for another.
We can’t forget about coverage, an important aspect of the solid and possibly more important than density. A high-density value does not always translate into a homogeneous solid. The way in which the ink is distributed in the substrate depends on a complex interaction of variables where the cohesion force of the particles of the surface of the ink (which becomes surface tension) converges, together with the surface energy of the substrate (Image 10). The smoother the ink film is to the eye; the higher quality for the observer.
Image 10. Density values and coverage obtained using different surface patterns on the plate. Source: MacDermid
Tools are available to measure the uniformity of the ink film in the solid area, each with different correlations of how the human eye distinguishes these defects.
We hope this article broadens the concept that while density is a value to consider when evaluating a solid area of a process color, the colorimetric values (hue, saturation/chroma and lightness), as well as the coverage, can, in fact, be of greater importance than just relying on the higher the density value the better. Although the density is used as a control parameter of the printing process, the final product on the store shelves must meet the brand’s identity with an image that consistent and repeatable. Each printer must determine its own printing conditions and the variables to achieve consistent results under established tolerances.
MacDermid offers a product line that optimizes printed results in solid areas. Physical properties such as resilience and our patented clean plate technology, but also the near 1 to 1 imaging capabilities present in our LUX ITP™ platform. This guarantees that smooth surface plates such as LUX ITP 60 and LUX ITP M have greater fidelity compared to other products on the market when reproducing advanced surface patterns such as Pixel +. (Image 11)
Image 11. Surface patterns in LUX ITP-60 plate (left) versus another product of the flat point market (right). Source: MacDermid
On the other hand, LUX ITP EPIC® incorporates a micro rough surface eliminating the need to apply surface patterns; this reduces rip times digital file sizes. In comparison to other products in the market, the roughness is homogeneous, ensuring greater consistency run after run. (Image 12)
Image 12. Comparison of different surface patterns technologies and LUX ITP EPIC
Beyond our plates, our technical service team is prepared to elevate your printing of solids to the next level through an accurate evaluation of the complete workflow from pre-press to printing.
Written By: Ivan Rozo, Business Development Manager, MGS Latin America
Ivan Rozo is the Business Development Manager for Latin America. Ivan is a Chemical Engineer with a Master Degree in Business Administration with an emphasis in marketing. He has more than ten years of experience in the Flexographic industry working in roles of sales and technical support in which he has led optimization projects. Ivan's is responsible for attracting new business and consolidating strategic accounts at MacDermid in Latin America.