Controlling Ink Film Thickness at 3,000fpm

Quicker is Better and Safer! In Flexographic Printing

By Johnny Stamey
Top Gun, one of the most popular movies from the summer of 1986, produced a widely recognisable and often repeated quote delivered by Tom Cruise's character, Maverick, and his faithful sidekick Goose, "I feel the need, the need for speed." Of course, their reference was to traveling at Mach 2 (approximately 1,800mph) in an F-14 Tomcat designed to defend the U.S.; so obviously, quicker is better and safer! In flexographic printing, anything you can do faster saves time, and time is money, right? Everybody likes to save money. But in the printing industry, what is the price of saving money?
For the majority of pressrooms in existence, the goal has always been to achieve faster press speed. Looking back to the late 1800s and "Bibby's Folly," some of the very first flexo printing crawled along at a snail's pace. When the first aniline press came along in 1908, more speed came with it, producing up to 150fpm. With the introduction of chrome anilox rollers and polyamide resins and inks in the 1930's and 1940's, the capability to increase up to 750fpm was achieved.
Then the ceramic anilox roller brought with it the ability for higher line screens, new photopolymer plates, and inks that could print better and dry faster. Although these presses were equipped with the latest technologies and offered improvements with registration and web tensions, central-impression (CI) press speeds began to settle around 1,200 and 1,500fpm. During the 1990s, the introduction of servo-driven flexo presses brought speeds that were never before thought to be possible. Suddenly, printers gained more control over the process and began recognizing improvements in the overall print quality.
The flexo market has achieved tremendous growth in quality and market share during the last 20 years. To meet the increasing needs of flexo printers, industry suppliers have been challenged to provide advancements within their product lines. Flexo CI presses are being manufactured to run at speeds close to 3,000fpm; something that only publication gravure printers were able to achieve in the past. Anilox roll, plate, ink, and doctor blade suppliers have all stepped up to the challenge and now offer more durable products capable of running on these higher speed presses. All of these advancements combined have contributed to flexo printing's overall success and growth.

Deflection

Flexo printers strive to achieve their goal of providing the printing process of choice for their customers. Given today's newly developed press speeds, they now face the task of producing and maintaining the same high-quality print their customers have grown accustomed to while continuing to make a profit running at those speeds for hours or even days. An important variable for completing this task is ink film thickness and the ability to control it.
Doctor blades are a key to this particular issue and the question is often asked, "Why don't doctor blades meter as well at higher press speeds?" A major reason is because of doctor blade deflection. A tremendous amount of turbulence is created by the high speed of the anilox roller inside the small cavity chamber, putting pressure on the body of the blade and causing the tip to hydroplane on the anilox roller. When this happens, the effectiveness of the blade to meter the anilox clean is limited, thereby allowing an excessive amount of ink to remain on the anilox roller, resulting in an inconsistent ink transfer to the plate. Typically, when this happens, color and density is hard to achieve and sustain, therefore dot gain and dirty plates result in poor, unacceptable print quality.
In typical wide-web flexo presses, commonly used doctor blades have thicknesses of .006in. and .008in. with a tip machined to a radius, bevel, or lamella tip (see Figures 1, 2, and 3). Usually these blades provide the adequate components for even ink film metering. However, as press speeds approach 3,000fpm, these blades are limited in their effectiveness. Most press manufacturers have recognised this and have begun to recommend a thicker blade to address the problem. Although thicker blades provide less blade deflection, they generally come with thicker blade tips which ultimately result in larger contact areas on the anilox roller. Since larger doctor blade contact areas result in less ink metering, this solution essentially resolves one problem but creates another one with the same ink metering issue.
It is important for printers to make blade selections carefully. Blade suppliers should provide a more specialised blade for individual press types and printing situations. While a blade with less resistance to deflection (normally thicker) shows an improvement, a radius edge tip is not recommended. A better choice would be to use blade thicknesses of .010in. or .012in. to address deflection; however, the metering tip should have a reduced thickness in order to meter the anilox in a sufficient manner. For example, a .010in. blade thickness with a pre-ground lamella tip or a 10° to 15° bevel (see Figures 3, 4 and 5).

Non-Metallic Blades

Deflection also causes ink to leak from the chamber. Excessive amounts of ink film not removed by the plate can "back doctor" from the stiffer trailing/containment blade. In most cases, end seals take the blame for the leaking ink when it is actually caused by another problem. Upon seeing a leaking chamber, a pressman will usually increase the doctor blade pressure in order to stop the leaking ink, adding to the problem rather than resolving one. When this happens, ink accumulates in the drip pan over a period of a day and can be very costly in lost ink to the printer.
Printers try to fix this kind of issue by using a plastic or mylar containment blade. Historically, a .007in. or .015in. thickness has worked well, always performing an adequate job (see Figure 6). The breakdown in this method happens because the thinner blades cannot withstand the pressure that builds up inside the enclosed blade chamber during the press' wash-up cycle. Blades begin to fail and cover the press in dirty solvent. Printers now run .020in. to .030in. thick plastic containment blades to help resolve this issue and in general experience very good results (see Figure 7). Alternatively, some printers are not fond of running a steel and plastic blade combination. In that situation, the best option is to apply a thinner steel blade. A .004in. or .006in. thickness is typically sufficient. The important thing to keep in mind is an optimal outcome of reaching doctor blade stability at high speeds.
Wavy blades also cause improper ink metering due to the accumulation of dried ink between the holder and doctor blade. Regular press cleaning and maintenance is extremely important for achieving appropriate metering performance, yet it is frequently overlooked. Because the majority of presses are now manufactured with on press wash-up systems, it is often assumed that the need for supplemental cleaning no longer exists. These systems perform well at cleaning anilox rolls, the inside of the chamber and all ink lines, but they simply cannot replace what hand cleaning or independent part washers can achieve for the doctor blade holders. It is not necessary to do so prior to every make-ready, but they should be taken apart and cleaned on a regular basis.
It is also important to remember that the bolt tightening sequence is another way to eliminate wavy blades. During blade set-up, start in the center and alternate sides working toward the end of the holder. If a more precise tension is desired, use a torque wrench.
Other Factors
There are a couple of other points to keep in mind about high-speed presses. Doctor blades are wearing at a faster rate due to the added heat being generated between the anilox and the blade during pressruns. Thicker doctor blades will help improve blade stability and life, but a thicker coated doctor blade will help extend it considerably more. End seals, the second consumable item of your chamber, are also wearing at a faster rate and should be addressed during the blade evaluation process. The rubber-molded seals being provided by many press manufacturers are generally successful; however, they tend to be very expensive and sometimes require long lead times when they are shipped from Europe.
A wide range of end-seal materials are available in the market today, but a high grade, pre-soaked felt seal can provide the best results on high-speed presses if rubber-molded seals are not working. Pre-soaked seals decrease the high amount of friction that may build-up between the anilox roller and end seal and don't allow as much ink absorption as a foam seal typically does.
Addressing each of these issues with a doctor blade and end-seal supplier will make it possible for printers to control ink film thickness and enable their presses to run much more efficiently. The quality standards that have evolved over the past 20 years within the flexo market will be easily attainable and printers will continue to make "good impressions" for their customers.