Metallic inks have long been used to enhance the value and convey quality of the printed piece. As with any ink, it is necessary to measure the optical density of the ink with a densitometer or spectrodensitometer to ensure correct color and print quality. It had been determined that densitometer readings for metallic inks among several instruments out on the market did not correlate well if the ink film thicknesses were varied.
Understanding the correct readings help in preventing set off in the stack, insufficient drying of ink in web presses, better control of ink and water balance, and yield more consistent results between printed pieces. This study suggests that the employment of a polarized light filter in a reflection based densitometer or spectrodensitometer provides a better representation for comparing optical density values to ink film thicknesses than those instruments without such filters. Comparative test results of different makes of densitometers with and without polarized filters are discussed and shown via various graphic charts and pictures.
It is common understanding that the optical density of metallics cannot be measured with the same densitometers or spectro-densitometers used for conventional process colors. Printers rely on their eye to make judgments, but sometimes this does not reflect the reality. Film weights are often printed that are too high or low, which create various technical problems like incomplete drying, blocking, chalking or insufficient adhesion of over-coatings or laminates. The following describes ways to obtain correct ink density readings with commercially available instruments.
The Hypothesis I
Metallic pigments are by nature completely different than others used in printing. The pigments are flake-shaped, fully opaque and significantly larger. They consist of planar areas designed to reflect as much light as possible in order to create metallic sheen or luster. (see Fig. 1). The additional light reflection from the pigment surface versus substrate surface influences the accuracy of the densitometer reading (see Fig. 2). The higher the film weight printed the higher the degree of reflection leading to an overall reduced densitometer reading. At a critical level the readings become completely unreliable.
The illustrations shown here (see Fig. 3, Fig. 4, Fig. 5) indicate types of reflection expected for various surfaces. Perfectly specular reflection would result from a mirror-like or perfectly planar surface. Imperfect surfaces of coatings and coatings containing pigments of irregular or spherical shapes lean towards 100% scattering. Coatings containing metal leafing metallic pigments exhibit a combination of both reflection and scattering properties.
The table titled “Chemical Makeup of Metallic Pigments” illustrates the various types of metal alloys used in the graphic arts industry and a description of each alloy. The types of alloys focused on in this study were aluminum, rich gold, and pale gold. Each of these alloys was evaluated in a typical offset and gravure formulation.
Chemical Makeup of Metallic Pigments
Gold bronze pigments
Coppercopper = 100 %copper red
pale goldcopper:zinc = 90:10reddish yellow
rich pale goldcopper:zinc = 85:15yellow
rich goldcopper:zinc = 70:30greenish yellow
silveraluminum = 100%hueless
Metallic pigments are produced in a variety of particle sizes (See Fig. 6, Fig. 7, Fig. 8). The larger the flake size, the greater the degree of brilliance. Metallic flakes used in the graphic arts industry vary from 3 to 15 µm. In this study, pigment types of 3 µm (offset inks) and 7 µm (gravure inks) were studied.
Aluminum pigments can be produced by a variety of means (see Fig. 9, Fig. 10, Fig. 11). The conventional method of producing aluminum pigments found in the graphic arts industry is by the wet grinding method (Hall process).
Other means of production such as stamping and physical vapor deposition can produce flatter and therefore more brilliant products. The conventional method of producing bronze pigments in the graphic arts industry is by a dry grinding method (Hametag process). This study evaluated pigments from conventional processes.
The Hypothesis II
Reflection densitometers (Leach and Pierce, 1993) compare the light reflected from the unprinted substrate surface to the light reflected from the printed surface. The light extinguished by the pigment is responsible for the color density. (see Fig. 12, Fig. 13, Fig. 14). The measurement is only correct on less reflective ink surfaces.
Densitometer readings are calculated as: D = log10 Io/I1
Io = light intensity reflected from white substrate
I1 = light intensity reflected from ink
Note: Polarization filters in the densitometer should be suitable to suppress the surface reflection.
In a study conducted the following randomly selected commercially available instruments (see Fig 15) were tested measuring metallic prints of various film weights:
* GretagMacbeth Spectrolino (Spectrophotometer)
* GretagMacbeth D19C (Densitometer)
* X-Rite SP 60 series (Sphere Spectrophotometer)
* X-Rite 528 (Spectrodensitometer)
* Koeth Chameleon (Densitometer)
For each of the densitometer measurements, the black filter was selected when measuring silver colored inks and the yellow filter was selected for the rich and pale bronze inks. The black filter for silver inks and the yellow filter for gold bronze inks yields the best results in terms of which colored filter to use, but the addition of a polarized light filter was tested in order to determine if more accurate results were obtained.
Figures 16-20 present different density measurements for gold bronze and aluminum prints by comparing different metal alloys, various printing processes, and different calculation methods. Lightness values from spectrophotometer readings have also been tested in order to see the effect of polarized filters (see Fig 21).
It was determined that the measurement of optical densities for metallic inks had varied among several types of densitometer and spectro-densitometer instruments. In the process of investigating this phenomenon two hypothesizes were formed.
The first hypothesis claims that the higher the film weight of a metallic ink, the higher the degree of reflection. This idea is supported by results obtained from different instruments and the physics of metallic ink films.
The second hypothesis states in essence that polarized light filters are necessary to extinguish the high degree of reflected light caused by metallic inks in order to produce both accurate and precise readings.
This idea was supported by evaluating instruments of different manufacturers and geometries with and without the polarized light filter.
It is also seen that exclusion of specular reflectance cannot accurately display differences in film weight of a printed ink. The effect of calculation methods, light sources, and various metal alloys were also explored, and this preliminary evidence gathered also points toward the necessity of polarized light filters.
Further testing may be explored to determine any other possible effects on determining the color density of metallic printing inks, and for inks based on vacuum metallized aluminum pigments.
Color Control Systems, Paul Flaig, and X-Rite, David Benner.
Leach, R.H. and Pierce R.J., 1993, “The Printing Ink Manual” (Blueprint, an imprint of Chapman and Hall), 5th ed., pp. 107-110.