A white ink underlayer in flexible packaging is of paramount importance. It forms the foundation for all the colors on the print.
A poor white underlayer will not give packaging the desired color saturation and pop that brands require.
One challenge commonly seen is that when white ink is deposited onto the substrate, it does not disperse uniformly. Depending on various interactions between the ink, the substrate, as well as the printing plates used, you typically get a white ink laydown that appears mottled, grainy or exhibiting pinholes.
To address this problem, printers:
- Use high-volume anilox rollers to reduce the voids in the white and improve laydown uniformity
- Employ multiple white stations, or to eliminate the problem altogether, use white substrates
- Print white on white substrates
All are costly choices which can be solved by just printing a better white in the first place.
No Trivial Issue
Why is measuring white ink difficult? Measuring the quality of white ink laydown is not a trivial issue. The industry standard to measure white ink is opacity, however, opacity alone doesn’t give the full picture.
Opacity measures the overall covering power of the white ink on a print, but it doesn’t provide any information on the smoothness, or uniformity, of the ink laydown. Imperfections in the laydown are often described as graininess, pinholing and mottle. These are overlooked when evaluating print samples for white ink quality, but they need to be included as part of any comprehensive white ink measurement because they affect print quality.
Consider the images in Figure 1 and Figure 2. Both have the same opacity of 52 percent, but are clearly different in uniformity—Figure 1 is preferred. Opacity alone will not quantify the obvious visual and quality difference. But if we also consider a gauge of uniformity, by measuring either the mottle or graininess, we can easily differentiate the two results.
Mottle is defined as macro imperfections, like pinholes. Graininess, on the other hand, is defined as micro imperfections. Based on ASTM testing procedures, both are calculated by print measurement devices which break up the image area into squares and calculate a standard deviation of the “white” level of each of those squares.
For mottle, the sample squares are bigger, and for graininess, they are smaller. The lower the standard deviation is between the level of these tiled squares, the better and more uniform the ink laydown. These measurements, in addition to opacity, are critical to the overall perception of white and together give us a better understanding of perceived brightness and print quality.
In managing and measuring white ink laydown, we can begin by taking a cue from color measurement. The field of colorimetry was developed to help accurately translate the subjective nature of human color perception into something measurable.
Routine color measurement for printing started off as one-color-at-a-time uni-dimensional density and dot percent measurements, until inexpensive and handheld spectrophotometers became available. These enabled the use of the multi-dimensional L*a*b* measurement system, which was quickly applied to the printing process, from design to press. It provided more perceptually accurate color measurements.
When combined with new applications and software tools, this led to a step change improvement in color management of print, leading to better print quality. A similar multi-dimensional challenge now exists with measuring the quality of white ink laydown.
Strategies in place rely in large part on the use of specialized screening technologies. One example, DuPont’s EASY BRITE screens and plates, promises brighter whites, higher print quality, improved consistency, faster press speeds and cost savings attributed to lower ink consumption rates. Average ink spend on white ink is $700,000 per press, per year.
A combination of screen and plate technologies designed to optimize ink laydown in high-volume printing, typically seen when printing a white undercoat, or solid areas with spot colors, delivers many benefits: elimination of trail edge voids; improved opacity, reduced mottle and graininess; establishment of a uniform ink layer and improved appearance of the finished print.
The approach entails utilization of a suite of screens shown to improve solid printing at volumes of 4.0 bcm and greater. There are five screens in total: two screens designed for customers without the HD Pixel+ imaging capability and three screens designed to take full advantage of HD Pixel+ imaging.
Putting It Together
In conjunction with the Cyrel EASY BRITE screens, DuPont has also developed the Cyrel EASY BRITE Index. This index uses multi-dimensional measurements, using both the opacity as well as a measurement of the uniformity to create a single number that can better quantify the overall quality of a white print.