TECHNICAL ARTICLES

Interferometric Anilox Volume Measurement

Some of the benefits of the process, as well as, comparisons to other measurement systems

By: The Harper GraphicSolutions Team

There has been a tremendous explosion of technical advancement in flexographic printing in recent years. What previously was very difficult to print is now routine, and what was impossible a few years ago is now cutting-edge technology. Technical advancements are being experienced not only by flexo printers and converters, but also by suppliers of flexo equipment and components.

In my opinion, the single most important tool introduced to the anilox engraving process since the invention of the laser engraving machine is interferometric volume measurement, which allows the characterization of anilox engraving at a level beyond human capabilities. Interferometry is the science of using state-of-the-art interferometric microscopy with advanced computer software to scan, record and analyze rough surfaces. This technology is applied to anilox engravings with a measurement resolution of less than 10 nanometers (1 nanometer = 0.01 microns). This resolution results in extremely accurate cell volume measurements.

THE BENEFITS OF INTERFEROMETRY
Since there have been numerous articles and publications describing the interferometric measurement process, I will spare the details of how this instrument takes measurements and instead focus on its benefits and comparison to other measurement technologies.

The benefits of interferometry to the engraving and printing industry include:

• Measurements are completely objective.
• Equipment and technology are standard throughout the industry and are reliable.
• Set-up tolerance in the engraving process can be much tighter than usual.
• Acceptance tolerances for anilox engravings can be cut in half compared to other typical industry standards, based on the accuracy of measurement.
• Measurement accuracy is independent of the cell’s characteristics, such as roughness of the cell walls, channeling wall width and cell shape.
• Interferometry can be as much as 15 times more accurate than other volume measurement processes.
• The time required to make an individual measurement can be five times faster than other methods.

Currently, there are approximately 25 interferometric roll scopes actively in use within the flexo industry. This number is growing every month. These instruments are used in applications ranging from the production and measurement of engravings by roll manufacturers to in-field measurements of anilox rolls at press side. Due to the widespread use of this technology by roll suppliers, it is now possible for printers and converters to communicate anilox volumes across supplier lines using a common language.

WHAT CAN INTERFEROMETRY MEASURE?
There are a number of measurements that can be made using interferometry, including:

• Cell volume
• Cell depth
• Line screen
• Engraving angle
• Cell wall width
• Depth of channels
• Surface roughness
• Wear curves
• Contour plots
• Perspective views
• Two-dimensional cross sections

Prior to the introduction of interferometry, there were two primary means of anilox volume measurement: theoretical calculations and liquid volume measurement. Both of these methods allow measurements of anilox volumes, but have fundamental flaws that never will be overcome.

THEORETICAL VOLUME MEASUREMENT
This remains the “standard method” for most engravers, but is sure to change. Theoretical volume measurement is a process in which one uses a conventional optical microscope to measure the dimensions of one cell (or an average of several cells) to theoretically calculate the volume-carrying capacity of larger areas of anilox engravings Although this practice is the accepted norm, it is by far the most subjective. It also is not as reliable as other methods. Theoretical volume measurement is affected by:

• A human’s ability to visually measure X, Y and Z axis dimensional characteristics of somewhat irregular shapes with measurements as small as ½ micron, or about 0.00002” (20 millionths of an inch).
• A mathematical formula of the shape of a cell. This formula assumes geometric cell shapes with consistent slopes that form walls of the cells. However, there is no way to develop a mathematical formula that accurately describes the exact shape of all laser-engraved anilox cells due to the irregular nature of the true cavity shape.
• Some cells have such characteristics as unlevel cell walls, wide post areas, unusually shaped cell bottoms and channeling. These characteristics are not accounted for in mathematical formulas; they lead, therefore, to improperly calculated cell volumes.
• The sampling rate for measurements is so small (compared to the actual population of cells on a roll), it is impossible to manually and objectively measure a representative number of cells to accurately characterize the true cell volume.

LIQUID VOLUME MEASUREMENT
This is an indirect measurement technique based on the following logical example: If one has exactly five gallons of water and is able to exactly fill five containers with absolutely no water spilled, left over or retained in the original container, then it is reasonable to conclude that each of the containers is exactly a one-gallon container. The accuracy of this measurement is somewhat questionable. Because the degree of exactness of each phase of this process has a direct impact on the validity of the final measurement, any variation in each step is compounded in the results.

To apply this logic to anilox engraving measurements the process must be capable of exact handling and measurements of very small characteristics. Accurate measurement depends on the following:

• Exact measurement and distribution, by hand, of fluid volumes as small as 0.001 liters without leaving any residue on any of the apparatus used to measure, handle or distribute the fluid.
• Exact metering or filling of cells with this fluid, by hand, on the anilox surface using a metal or plastic blade assembly without leaving any residue on the apparatus used to meter the fluid.
• Exact extraction by hand of the pattern or “spot” left after metering the anilox surface to a sheet of absorbent paper.
• Exact measurement of this “spot” size by hand to determine the area covered.
• Mathematical calculation to convert this information to anilox volumes.

Sounds impractical, but his method commonly is used. Other drawbacks of liquid volume measurement:

• Operators who engrave anilox rolls can not use liquid volume measurement to set up the engraving operation and must predict the liquid volume measurements using an optical microscope with varying degrees of success.
• Fluids change with time and temperature.
• Rapid evaporation of fluid can be experienced with low anilox volumes.
• People vary with respect to the technique, touch and feel of measuring.
• Accuracy varies according to line screen and cell pattern.
• The process is extremely time consuming.
• Equipment varies throughout the industry, with no accepted standards.

INTERFEROMETRY
This measurement process soon will be the standard throughout the anilox laser engraving industry. At Harper we refer to this method as echotopography because not only do we use the system to measure, but also to characterize and predict on-press performance.

Because interferometry introduces almost no variation in measurements, the manufacturers of anilox rolls can dramatically lower acceptance specifications. They no longer have to “build-in” acceptance tolerance to compensate for measurement error. Table 1 compares anilox acceptance tolerances:

These tolerances were developed through on-press testing to determine how much anilox variation is acceptable via densitometer and color measurement acceptance limits. Because of the tight tolerances and accurate objective measurements, interferometry allows converters to completely eliminate costly incoming inspection practices and equipment.

The quality inspection burden can be handled by the anilox supplier (as it should be), with the final acceptance of rolls based on printing performance. Interferometry can be linked directly to print performance.

CONVERTING TO INTERFEROMETRY
Converting from conventional measurement systems to interferometry is a little like converting form the English to the metric system. For example: one mile equals 1,609 kilometers. The physical distance is the same, but the numbers used to describe the distance are very different. The same relationship exists between interferometry and other volume measurements.

In general, the correlation of interferometric volume measurement to liquid volume measurement and theoretical volume measurement are described in Table 2 and Table 3, respectively.

Volume and line screen combinations using interferometry are available in the following ranges:

**MISSING CONTENT - WHERE ARE THE RANGES?**

Successfully converting to the interferometric measurement system does not require the purchase of equipment or the re-working of all rolls; there are two options for conversion:

• Because the interferometric measurement system units are portable, a complete inventory of anilox rolls can be measured at the printer’s plant and simply re-classified to interferometry measurement values. Future engravings would be duplications of the now re-classed volumes.
• If it is impractical to measure all rolls in an inventory, the best performing rolls in the categories of process, screen, lines and type, solid and heavy coverage, should be measured with interferometry to specify future volume specifications.

With the introduction of any new technology, there is always some degree of uncertainty regarding its applicability and long-term influence. However, the interferometry measurement process has been in use since 1994, and has been tested, developed and implemented.

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