- February 01, 2002, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
Almost every day I receive questions relating to ink or coating thickness determination. You would think it would be a simple matter, but it is not.
Ink thickness can be described in several different terms, including reflective optical densities, microns, mills, grams/m2, lb/ream, etc.
It can be confusing. It's almost like everyone is speaking a different language about the same thing.
The amount of ink deposited on a substrate depends on the ink formulation, the printing process, and the substrate. You normally have to carry more ink on a porous substrate (compared to nonporous substrates) to get the same color density.
In practice, the actual ink thickness rarely is measured. Print reflective densities are used to put the desired ink level on the substrate. Since the amount of pigment may vary from as much as 10%-50% within a given ink formulation, the actual thickness of each ink applied is rarely the same.
A common measure of ink thickness is in units of microns. A micron is 1 millionth of a meter. In practice, however, it is not easy to determine the thickness of an ink in microns directly.
The amount of ink on a substrate often is expressed in terms of weight of ink per area of print. Commonly, this is expressed as pounds of ink/ream. A ream can be any size, but it is common in flexible packaging to use dimensions of 3 ft wide and 1,000 ft long, or 3,000 ft2.
The different printing processes deposit different amounts of ink. Lithography is estimated to deposit an ink film of approximately 1-2 microns, whereas flexography's deposit is estimated at 2-3 microns. Rotogravure can have a wide range from 1-20 microns, depending on the cylinder etching.
Achieving accurate ink thickness depends on the type of substrate. For instance, printing of paper presents some special problems. The paper surface can have significant surface variations and require more ink per area to assure it has the required reflective densities and surface uniformity. Ink penetration of the paper makes accurate ink thickness measurements difficult.
The reflective optical density is the most convenient method of estimating the amount of ink on a substrate, but it has accuracy limitations at high ink densities and with certain pigments.
Weight/area provides convenient measurement of ink thickness as long as you have the percent solids of the ink and the ink densities.
For coatings or inks with high transparency, optical methods can be used. One method depends on comparing the light reflection from the ink surface as well as the ink/substrate surface.
One of the more accurate ways to determine the amount of ink on a substrate is to look at it from a side profile using high magnification. The trick to this technique is to get a printed sample and then fracture it at a very low temperature (liquid nitrogen), which allows for a side profile that can be viewed.
In addition to these methods, a proven method is to “digest the ink” in a specific print area and then use analytical techniques such as atomic absorption to estimate ink thickness.
This method depends on having one or more detectable metal ions in the ink and knowing their concentration in the ink formulation.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; firstname.lastname@example.org.