Is scoring damage to your anilox becoming an expensive and frustrating problem? Take heart, many printers have resolved the problem, and this article discusses how.
I. DEFINING AND IDENTIFYING 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 score lines:
- Deep gouging score lines which show as dark streaks in the printed image.
- 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 Roll Score Lines:
Figures 1 & 2: Deep Gouging Score Lines
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: 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.
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.
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