The Secret to Print Consistency
As first seen in Flexo Magazine, February 2018
The most important thing to any company is their identity. What makes their product different? What makes them 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 their customers.
To some, their logo has become their identity – even iconic. Coca-Cola is an iconic brand. They must maintain consistency in the way their logo is written and they must match the “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 press room. However, the three biggest variables in achieving consistency are the 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.
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 the anilox rolls are can be quantified by its depth to opening ratio. For example, if a cell is 30 microns deep and 100 microns wide, its depth to opening ratio is 30%. On average, a 30% depth to opening ratio may transfer 20% of this volume to the web. If the same cell is 40 microns deep, its depth to opening ratio is 40%. This cell may transfer 18% of what it holds to the web. In other words, at a given line screen 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 to 35% range.
Below is a pictorial of different depth to opening ratios at the same line count. These represent depth to opening ratios of 25%, 35%, 55% and 80%.
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% depth to opening ratio is used, it would be engraved to an l.p.i. of 400. If this roll was used and found to be 10% light in color density, it would require a 4.4 volume to increase the color by 10%. Using the same 30% depth to opening ratio would result in an engraving using an l.p.i. of 380. The outcome would be an exact increase in color strength of 10%.
On the other hand, using inconsistent or excessive depth to opening ratios will result in more adverse side effects than just color matching issues. Below is an example of an engraving with a depth to opening ratio more than 50%. 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 line screen 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 below 40% 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 line screen 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 line screen count on the anilox roll as one might think. This is a topic in itself, but there is plenty of data to support 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.
Above are 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 one on the right, can increase the plate support. The horizontal line screen count has been increased by about 40%. 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.
When it comes to cleanliness and maintenance, the volume of an anilox roll is dependent on two factors – plugging and wear. The first sign that 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 they 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% 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 volume 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 properties. 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.
In summary, companies must promote their brands and packaging in the most consistent way possible – both in matching their brand color and in print quality. Knowing your individual process is the best way to achieve that goal for your customer. By mastering ink film thickness, honing in on anilox roll maintenance, and getting suppliers involved, your ability to be successful will increase. Consistency in operation equals consistency in print, and a thorough understanding of your anilox roll inventory is a major contributor to this success.
By Mike Poppen,
Of Bingham Flexo Services, Inc.
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