An article in the December issue of Flexo magazine ( http://www.flexomag.com/ ) entitled "Your Ruler or Mine?" describes the (FQC) Flexo Quality Consortium's latest endeavor, which is to perform gauge studies of the various methodologies used by anilox manufacturers to determine anilox roll cell volume. This study is vital to insuring that printers have good information to work with when trying to match color on press, run repeat print jobs or transfer a print job from one press to another.
Anilox cell volume is a major contributor to determining ink lay down and the consequent print density. It is critical to have accurate cell volume data for proper anilox selection. The problem is that there are multiple methods for determining cell volume and the methods do not always yield the same volume determinations. As a result, one manufacturer's rolls will not necessarily print like anothers, even if the stated cell volumes are identical.
Cell volume refers to the carrying capacity of the microscopic cells on the roll surface. The standard unit of measurement is BCM (Billion Cubic Microns/square inch). BCM essentially looks at how much ink a given cell can carry multiplied by the number of cells occupying a square inch of the roll's surface. The cells act like microscopic measuring cups, which contain a specific, precise amount of ink. The more accurate the measuring cup, the more predictable will be the amount of ink it carries.
Unfortunately, cells produced by lasers do not have perfect wall geometries. There are microscopic variations in wall peaks, contours and wall thicknesses. Even the cell bottoms have variations. This is not an indictment of the anilox manufacturers or the engraving technologies currently in use. It is simply a statement of fact.
Cells have rounded or somewhat rounded bottoms, which makes measurement difficult. Porosity in the ceramic material itself results in variations in depth. Variations in peak formation result from evacuation of debris from the laser engraving process. Additional variation comes as molten ceramic is recast during the formation of adjacent cells. Variation in wall contours come from a combination of porosity and minuscule variations in energy distribution within the laser beam itself, as well as in the delivery of that energy through optics that may have dust or microscratches on their surface. Wear and tear to electronics, as well as mechanical components of the laser and delivery system can also contribute to cell-to-cell variations.
Because of these variations, cell measurement is extremely challenging. Having an industry standard measurement technique would provide a common "language" for describing anilox carrying capacity.
Currently, there are 3 main types of measurement technologies used in anilox manufacture: liquid, optical, and interferometric. Impression tape used for most anilox audits these days makes a microscopic reverse mold of the cells, which can then be evaluated with one of the measurement technologies.
Disposable liquid volume measurement strips represent a variant on the liquid volume method. Liquid volume measurement involves applying a known volume of a liquid to the roll surface, spreading the liquid out across the roll surface, applying a special absorbent paper to the roll face and then measuring the surface area of the 'wet' surface or blot. Computer software then calculates how large the cells would have to be to create a blot of that size with the liquid applied.
Optical measurement involves using a high-powered optic microscope to measure the cell width and depth. Most microscopes currently in use utilize computer software to then calculate volume based on the measurements received.
Interferometry bounces light off the roll surface through special optics. This light creates an interference pattern that the computer software interprets to generate a volume reading.
Each of these systems utilizes a proprietary software package based on independently determined mathematical models. Each has advantages and disadvantages. Each of these systems has the potential to have measurement accuracy affected by system condition and vibration. Measurement0s from each method can be adversely affected by improper operation. Unfortunately, each system has a different level of Repeatability and Reproducibility. In other words, variation from operator to operator and variation in multiple readings produced by the same operator. Ascertaining which system offers the best R&R through gauge study analysis, will go a long way to helping the industry to develop standard measurement criteria.
The variability between systems was highlighted in a study conducted in 2008 inwhich a single banded anilox roll was set to each anilox manufacturer. The manufacturers then measured the roll using their technique of choice. The readings were then compiled by the company that initiated the study. The results were eye opening. Depending on linecount, the variation between manufacturers and methodologies was as much as 40%. When evaluating the results from multiple manufacturers using the same methodologies, evidence strongly suggested the highest R&R coming from interferometrtic measurement. The scientific analysis produced from the FQC study is vital to broadening the industry's understanding of anilox measurement and helping insure printers have good, reliable information to facilitate color-match on press. The FQC deserves a great deal of credit for taking on this daunting, but incredibly important initiative.
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