The Secret to Print Consistency: Maintaining Anilox Roll Volume

The most important thing to any company is its identity. What makes its product different? What makes it unique? All successful businesses establish an identity. If maintained consistently, the result is a customer base that comes to expect a certain level of service and quality.

Because of this expectation, customers continue to be loyal to their brand rather than a competitor’s. This is known as brand loyalty, and it is the result of hard work and a relationship the company has built with its customers.

To some, their logo has become their identity. Coca-Cola is an iconic brand. It must maintain consistency in the way its logo is written and must match that “Coca-Cola red” exactly. Some companies have even been able to drop their company name from their logo and still get instantly recognized. The NBC peacock is one. Another example is Starbucks.

These companies want you to see their logo and not think twice about who they are or what they represent. The visual representation of that identity on packaging is paramount. Because of this, companies expect the color and printed image of their products to be unfalteringly accurate.

In order for the print industry to accomplish such a feat, there are hundreds of factors that must be properly aligned. Some of these factors include the type of press, the operation of the press and the environment in the pressroom. However, the three biggest variables in achieving consistency are inks, printing plates and anilox rolls. Maintaining the anilox roll volume is the most difficult component of the three and the most important. If the volume changes, the color changes and consistency is lost.

The Sweet Spot

So, what’s the secret to attaining that critical element of volume regularity? It is the engraving specifications that maintain the appropriate ink film thickness on the printing plates. Ink film thickness is the volume of ink that transfers to the printing plates and is what determines color strength and print quality. The strongest, thinnest film of ink (while maintaining color density), always prints best. Without enough volume, the color prints weak. On the other hand, an overload of ink results in dirty print or too much dot gain.

In order to find that sweet spot in the amount of ink transferred to the substrate, a few things must be addressed. The type and viscosity of the ink, the type of printing plates used and the types of substrates are key examples. However, in order to systematically control the color and print quality, anilox roll engravings with consistent depth-to-opening ratios should be chosen.

How shallow or deep the cells on an anilox roll are can be quantified by its depth-to-opening ratio. For example, if a cell is 30­-µ. deep and 100-µ. wide, its depth-to-opening ratio is 30 percent. On average, a 30 percent depth-to-opening ratio may transfer 20 percent of this volume to the web. If the same cell is 40-µ. deep, its depth-to-opening ratio is 40 percent. This cell may transfer 18 percent of what it holds to the web. In other words, at a given linescreen count, the deeper a cell gets, the percentage of ink transferred to the plate reduces. When possible, it is recommended to use depth-to-opening ratios in the 30 percent-to-35 percent range.

Cell Geometry

So, how does one determine which engraving to use on the anilox roll? As shown below, it is not solely the volume of the anilox roll. Rather, it is a function of the cell geometry on the anilox roll that dictates exactly how much ink transfers to the printing plate. This ink film thickness should be the focus when selecting an engraving. The chart below is an example in which the same ink film thickness was achieved using two different volumes. The result? Both the color strength and dot gain were almost identical.

How can a 2.6 volume produce almost identical results when compared to a 2.3 volume? The 900 is a deeper engraving and releases less of what it holds. This results in the same ink film thickness to the printing plate as the 750 with a lesser volume because the shallower cell releases more of what it holds. Essentially, the shallower cell has a more efficient ink transfer.

If a plant is using consistent depth-to-opening ratios for all volume requirements, the same percentage of ink will transfer to the plate. As a result, there will be greater ease in identifying which rolls to use with new colors and print jobs. For example, if a 4.0 volume at a 30 percent depth-to-opening ratio is used, it would be engraved to an lpi of 400. If this roll was used and found to be 10 percent light in color density, it would require a 4.4 volume to increase the color by 10 percent. Using the same 30 percent depth-to-opening ratio would result in an engraving using an lpi of 380. The outcome would be an exact increase in color strength of 10 percent.

On the other hand, using inconsistent or excessive depth-to-opening ratios will result in more adverse side effects than just color matching issues. Figure 1 is an example of an engraving with a depth-to-opening ratio of greater than 50 percent. Deep engravings are rough, form inconsistent cell walls and introduce many variables that greatly reduce the chance of consistency in your printing process.

However, choosing linescreen counts at proper depth-to-opening ratios do produce uniform cell geometries. The cell walls form more consistently and produce engravings that will last longer, print more consistently and be more resistant to damage. Furthermore, if cleaning anilox rolls is a challenge, keeping the depth-to-opening ratio at less than 40 percent will make rolls easier to maintain.

So far, it has been established that choosing the correct engraving specifications is a function of the ink film thickness achieved by considering the linescreen count, the volume and the depth-to-opening ratios. But what about the line count on the printing plate? The focus here is straightforward: Increasing the line count on the anilox too much will only decrease the ability to run consistently from job to job and affect the ease of keeping it clean. In addition, with today’s plate technologies utilizing supported dots, it may not be required to print with as high of a linescreen count on the anilox roll as one might think. This is a topic in itself, but there is plenty of data to support the idea that using uniform depth-to-opening ratios across nearly the entire range of print requirements is a benefit. And there are new cell geometries that can make it happen.

Figure 2 shows engravings at the same volume and depth-to-opening ratios. To the left is a standard 60-degree hex cell. If dots on the printing are smaller than the opening of this cell, new cell geometries (like the 75-degree hex cell on the right) can increase the plate support. The horizontal linescreen count has been increased by about 40 percent. However, to maintain a proper depth-to-opening ratio, the vertical line count has been reduced. About the same number of cells exist on the roll, but the affect is sufficient plate support for smaller dots.

Plugging & Wear

When it comes to cleanliness and maintenance, the volume of an anilox roll is dependent on two factors—plugging and wear. The first sign an anilox roll is plugged or worn is a drop in color strength. Loss in color strength can be the result of other factors, but the anilox roll is the most likely cause.

Because wear and plugging have such a high impact on consistency, it is important to know the volume of each anilox roll. It is highly recommended that a plant invests in a scope to measure volume or that it request an anilox roll audit be performed on site by the anilox supplier. This report will show the current condition of a roll in comparison to its original volume. If the roll is at an 80 percent effective volume, an audit will show if it is caused by plugging, wear or both. If the cause is wear and color strength is not to specification, the roll will need to be replaced. If plugged, there are several methods to restore the volumes of those rolls.

The ability to keep an anilox roll inventory completely clean is the best way to print consistently and to reduce downtime. Unfortunately, chemicals are still the most common way to clean anilox rolls when using the wash-up cycle on a printing press. There are hundreds of chemical companies, all with different cleaning products. However, chemicals are never entirely effective, and they are not good for the environment or operators.

There are two chemical-free methods that get anilox rolls clean, every time. The first is soda blasting, which can be used in-press to restore volume to top specifications. If the anilox roll can be removed from press, laser cleaning of the roll in a cabinet will also clean it completely. This technology is new to the print industry, but has already become a proven method that uses no consumables and only a small amount of power. It is the future of anilox roll cleaning.

Equally as important as cleaning is the handling of anilox rolls. Damage is a major reason anilox rolls need to be re-engraved. Whether caused by in-press damage from score lines or handling, there are many practical ways to reduce the risk of these occurrences. Proper storage, protective roll covers, a reduction in chemical cleaning, using filters and altering the type of doctor blades or metering pressures used are all options for lengthening the life of a roll.