Is anilox scoring and damage becoming an expensive and frustrating problem? Take heart. Many printers have resolved the problem, and this article discusses how.

I. DEFINING AND IDENTIFYING ANILOX SCORE LINES

When a hard particle becomes trapped between the doctor blade and the anilox roll, the particle can ride there and effectively destroy rows of anilox cells. These rows of cells appear as thin bands running the circumference of an anilox and are commonly called score lines. There are two types of anilox scoring:

1. Deep gouging score lines which show as dark streaks in the printed image.
2. Light polishing score lines which are more common and show as light streaks in the printed image.

Fortunately, a few easy precautions can eliminate or at least minimize the problem.

Viewing Anilox Scoring Lines:

Figures 1 & 2: Deep Gouging Score Lines

Figure1

Figure 2

The two photos above show deep, gouging score lines. Notice the unharmed cells surrounding the damage. This type of score line is caused by large particles lodged between the contact tip of the doctor blade and the roll’s surface. The area damaged is always multiple cells wide and normally runs around the entire circumference of the roll.

This type of score line tends to cause a continuous dark streak in the printed graphic area.

Figure 3: Light Polishing Score Lines

The photo at left shows light polishing score lines. Notice that the cell wall is the only area damaged. The “polished looking” surface of the land area is the light score line.

These score lines reduce ink delivery volume of the anilox just enough to show as a continuous, lighter line within a graphic.

Note: A continuous, lighter line within a graphic is not necessarily caused by scoring. A plugged group of cells in need of cleaning can also print a continuous, light line. In this case, particles of metal, dried ink, or the like, actually grind into the anilox roll cells filling them in. This causes loss of ink transfer or a light streak in the printed graphic.

II. THE PRIMARY CULPRIT FOR ANILOX SCORING: BLADE TIP SLIVERS

Large pieces of metal caused by improper doctor blade use are the primary cause of anilox scoring (Figure 4).

These large steel particles break off of doctor blades as they wear.

Figure 4

Improper Doctor Blade Use

When installing new blades, the shorter, worn blade is replaced by a new longer blade. If the chamber is not readjusted for the new, longer blade, over pressure occurs when engaged with the anilox. Press crews also over pressure to get more color to the plate without changing anilox.

Normal blade shavings do not damage a roll because the micro-slivers of metal are too small to get lodged between the blade edge and anilox cell walls.

Figure 5: Incorrect Blade Pressure

When over pressured, the doctor blade bends back causing wear on the side rather than the tip (Figure 5, shown above). As the side of the blade wears through, the tip breaks away in the form of a long metal sliver.

Ideally, only the very top of the doctor blade contacts the anilox as seen in the drawing below (Figure 6).

Figure 6: Correct Blade Pressure

When adjustment is correct, the best metering of ink, print quality and blade wear occurs. Over pressured blades from chamber alignment problems, operator error, trying to stop chamber leaking, rippled blades, or using blades for too long, create large slivers and pieces of metal which contaminate your ink system.

Viewing the Primary Culprit

The photo below (Fig.7) shows metal, hair-like tips caused by over pressing doctor blades.

Figure 7: Blade Tip Slivers

The photo below shows blade tips taken from a filter magnet. (These can range from 1/32″ to several feet in length.)

Proper blade chamber alignment (both horizontal and vertical alignment in relationship to the anilox) and maintenance of the blade chamber (cleaning, proper seal setting and blade setting) are the first and best steps toward prevention of score lines.

The amount of ink that is transferred from a cell depends upon the carrying capacity of the cell or the cell’s volume. Anilox volume must be accurately measured and maintained and is THE primary controlling factor in flexo printing.

Impact on Quality Printing

Remember, you print with a wet and fluid film of ink and it is beneficial to print with the thinnest ink film possible. Therefore, you want to make sure your anilox volume, or cell carrying capacity, suits the specific printing needs of your operation.

Thinner films of wet ink are easier to control, reduce dot gain, and increase graphic capability. If you are trying to print with excess volumes, you will have a hard time reaching the optimum flexographic process in your facility.

Why are thinner ink films important? Because dot gain is decreased with thinner ink films, simply because there is less ink to transfer from the printing plate dot.

As shown in the photographs below, lower anilox volumes provide cleaner, sharper graphics, as opposed to the heavy print produced by the higher anilox volume below.

Thinner Ink Films Effect:

• Dot Gain/Tonal Range
• Contrast/Fidelity
• Trap and Press Speed
• Ink Mileage

Thinner Ink Films Yield:

• Predictable Dot Gain
• Clean Printing
• Smaller Dots
• Open Reverses
• Clean Type
• Sharp Edges

However, you still need to achieve acceptable densities from your inks.

Therefore, you must evaluate the ink you are using. If you are not getting adequate color strength, you may need to re-think your current ink choice or talk to your ink supplier to see if they have a solution. Ink manufacturers have responded to the demand for higher pigment inks. They may cost a bit more in the beginning, but improved graphics will more than offset this cost. Also, because you are transferring less ink, handling less ink, and wasting less ink, you will be buying less ink.

Reduced VOC emissions are another benefit of using lower volume anilox rolls. It is the same concept: less ink gives off less VOC emissions. It is that simple. Line screen is chosen in direct correlation to anilox volume. For example, an anilox volume of 3.2 BCMs, requires a line screen of approximately 500. If an anilox volume of 3.2 BCM was engraved at 1000 line screen, cells would be much too deep. In correlation, a 3.2 BCM anilox at 120 line screen anilox would result in cells being much too shallow.

With increased ink strength and lower anilox volumes, higher line screen anilox rolls can now be used. These higher line screen rolls give printers the opportunity to reach for higher graphics with finer vignettes, line and type, and process work. Following is a rough outline of printing applications matched with appropriate line screens and cell volumes:

Most importantly, a printer must choose the best volume and line screen combination for their anilox roll to meet the needs of the specific job he or she is printing. One helpful tool for determining the line screen needed is a banded roll.

A banded roll contains actual bands of different line screen and volume combinations. A banded roll test helps determine the correct line screen and volume combination for printing the thinnest ink film possible and still delivering the color strength you need. Your Harper Technical Account Manager can help you run and interpret a banded roll test at your facility if you are interested.

Measuring Anilox Volume

Anilox volumes are measured in BCMs (Billions of Cubic Microns). Keep in mind that we are dealing with very small measurements and that every micron is critical. How big is a micron? A human hair is approximately 70 microns in diameter.

At Harper, we measure our anilox rolls using Echotopography™. This measurement system uses reflected light to gather height information of microscopic surface patterns with accuracy as fine as 10 nanometers. This advanced technology allows Harper to maintain the tightest manufacturing tolerances in the industry.

With over one million dollars invested, we now bring Echotopography™ to you as our standard measurement system on every Echocel ET2000™ roll. You’ll experience the benefits in every Echocel ET2000™ roll, and at no additional cost.

The Echotopography™ measuring system provides the following benefits of other anilox measuring techniques:

• Correlates to Press Performance
• Standardized Volume Specifications
• Precise Cell Volume Measurement
• Repeatable Cell Volume Measurement
• Non-Subjective Analysis
• Reduced Measurement Error
• Statistically Valid Measurement Process

Other common anilox cell measuring techniques include liquid volume measurement and microscopic measurement. Both of these techniques have inherent flaws.

Liquid Volume Measurement:

• Does Not Address Print Quality
• Does Not Address Cell Quality
• Not Repeatable
• Not Available At Set Up
• Subjective Based On Technique

Microscope Measurement:

• Developed for Mechanical Engravings
• Based On Depth Measurement
• Assumes A Particular Cell Shape
• Wall & Opening Measurement
• Theoretical Volume Calculation

Conclusion

Remember that anilox volume is THE Primary Controlling Factor In Flexo Printing, and it works in correlation to all other anilox properties. In addition to an understanding of anilox volume, you need a basic understanding of the line screen and cell geometry of your anilox rolls.

Knowledge of these three components will give you the ability to choose the anilox inventory best suited to your printing operation.

If you have further questions, please do not hesitate to e-mail our technical service department, or call Harper at 800-438-3111 / 704-588-3371.