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FLEXO Magazine : March 2014
Today, doctor blade manufacturers offer a range of differ- ent materials and edge types, each tailored to meet specific pressroom applications including: • New high definition steels that feature new chemistry, including ingredients like nickel and chromium within the steel. This doesn’t require an expensive coating and is better able to tolerate a wider range of inks and coatings at high speeds • New treatments alter and refine micro structures, making them less abrasive against an anilox surface and extend a blade’s life two or three times because they are better able to tolerate temperatures of inks and coatings and resist wear. This is true even at speeds of faster than 1,500 fpm EDGE & THICKNESS In lamella or step edge doctor blades, originally a rotogra- vure design, the edge of the blade steps down to about half the full thickness—a 0.008 -in. thick blade steps down to 0.004- in. The idea was to reduce the contact area and achieve a thinner ink film. However, the lamella tip has a tendency to flex (a desirable feature in rotogravure but not in flexo) when pressure is applied, allowing more—not less—ink to pass beneath. Another problem of the thinner edge area is that it wears twice as fast, until the point that the tip wears complete- ly or in some cases snaps off. The lamella edge is prone to producing slivers of steel, which threaten the anilox. On a radius edge blade, which until recently was the most popular edge among wide web printing, the blade’s radi- us could seat to the anilox surface during the job’s startup process. The majority of printers chose a 0.008 -in. thick blade because it was thick enough to resist bending when pressure was applied and added life. Beveled edge blades were developed to assist a blade’s seating to the anilox. Originally, they were 45 degrees, then 30 degrees, 15 degrees and 8 degrees. New stable bevel designs arrived with the advent of high speed HD flexo machines and the use of an expanded color gamut. Printers require a blade to make and maintain a fine contact area for longer runs or for use on multiple jobs. Illustration 4 is an example of a new elon- gated 2 degree bevel, combined with a super alloyed micro refined steel. Coated blades were developed for the rotogravure printing markets. The ink repellent coatings with life enhancing properties reduce particle sticking—a major source of print defects in rotogravure. Recently, coated blades have found their way into flexo, initially through well intended OEMs. Their benefit in flexo—primarily longer life—can be offset by an increase in risk of anilox damage. To keep prices down, some coatings are applied to low quality steels. If considering such a blade, it’s important to know the quality of the steel beneath the coating, as it will eventually contact the anilox. When combined with a high quality, high performance steel, coated blades are proven winners in long run, high speed applications and in coatings and adhesive applications, especially when anilox line counts are coarse. BLADE BASICS Are doctor blades a key process element? Yes they are. So when does it make sense to buy a doctor blade on price? The answer is simple: Always! If you know how long you need a blade to run and if you know the cost of print defects, press stops, downtime and so on, you can find a cost effective blade that is ideally suited to meet your needs. How do you know which blades are best tailored for your pressroom applications? You must investigate what various blade suppliers offer and what services they provide. Ask for references. Your anilox roll or ink supplier may be a good initial source of this information. n About the Author: Paul Sharkey is the president and founder of FLXON Inc. He has been actively engaged in understand- ing and preventing the causes of waste in the flexo process since 1976. He can be reached at email@example.com. Illustration 4:is an example of a new, elongated 2 degree bevel, combined with a super alloyed micro refined steel. The stable edge is suited for high speed machines. Illustration 5:is a new HD steel with repellent micro enhanced coating. www.flexography.org MARCH 2014 FLEXO 31