by Shane Weber
Flexo printing has evolved significantly in the last decade. Mechanical improvements now allow for high end process printing to be run at speeds in excess of 1500FPM. The exponential advancements in digital technology have paved the way for printing plates that can hold dots down to quarter % at 240 LPI and beyond. Despite these advances, we still have not reached a point of “one size fits all” Flexo. Each system is unique and must be optimized with regards to inks, substrates, anilox, photopolymer plates, etc. This is certainly a good thing
as it allows for the Flexo industry to remain competitive. Companies that are willing to innovate and develop efficiency can use these advantages to remain profitable. Optimization of the relationships between the printing plate and anilox roll are critical for print to proof color reproduction.
Let’s begin with the anilox roll. I think it is important to review some of the pertinent terms and definitions to properly understand the relationships discussed throughout the article. To get started, I define the anilox roll as an Engraved Ink delivery roll designed to apply a specific I.F.T. (ink film thickness) to the printing plate.
- Cells: microscopic cups that carry ink.
- Cell Count: Number of cells per linear inch on the angle of engraving.
- Volume: Amount of ink contained per square inch of cell area. Specified as Billion Cubic Micron per square inch.
- Angle: Angle at which the cells are engrave to horizontal roll axis. (45,60,30 degrees)
The cell count and, more importantly, the cell volume of the anilox roll will determine the ink film thickness that is applied to the plate. There is a wide range of options for cell count and volume combinations.
In addition to cell count and volume combinations there are new and advanced cell geometries available that offer benefits.
Now, let’s briefly cover the various styles of anilox. The current standard for anilox engravings is what’s referred to as a 60 degree laser engraved ceramic anilox. Different suppliers have various product names for their specific technology, but all are an arrangement of cells engraved at a 60 degree angle to the horizontal axis. The end result is similar to a honeycomb pattern. This technology has been available since the late 80’s and continues to be the workhorse for laser engraved anilox rolls.
There are some newer styles of engravings available in the market place today. One of these engravings is an open cell geometry engraved in the pattern of an “S” and the other is an elongated variation of the 60 degree cell pattern. (See photo) They are designed to increase the range of printable graphic capabilities with-in the same anilox. The idea being to increase density delivery while maintaining fine detail in the highlights and mid-tones. Improvements of this kind can in fact increase the printable color space. More importantly, they can print better levels of contrast giving the image more “POP”.
Regardless of anilox cell geometry, the ink film thickness delivered by the anilox rolls needs to be predictable, repeatable, and manageable in order to print consistent brand colors. Minor shifts in the ink film thickness will add time to on press color matching and create headaches for the ink suppliers.
How do we select the correct anilox cell count and volume combination?
First, to determine what ink film thickness (cell volume) we will need to answer the following questions:
- What are printing on?
Each substrate accepts ink differently. Even substrates within the same family accept ink differently and will change the characteristics of the print job. , i.e.: Paper films, foils, tyvecs…etc.
- What are the graphic requirements?
Are we printing line, type and solid colors or expanded color gamut process printing, or a combination thereof?
Next, what should the anilox cell count be?
Based on conventional wisdom, a good starting point for selection of the proper anilox in relation to plate screen count, is to multiply the plate screen count by four. For a 133 line plate separation: (4) x (133) =532. Therefore we would select a minimum of 550 cell count for the anilox. As digital plate materials have become more common the multiplier increased by 2 to a 6-1 anilox-plate screen. This accommodated for 1% dots at higher plate screen counts, and the newer digital dot structures. (round top vs. flat top) The reasoning behind this method is to insure that there is a minimum of four anilox cells applying ink to each individual dot. If the dots at the highlight section of the tone range become smaller than the anilox cell it may cause a “dipping” effect that can lead to bridging dots and dirty print.
Now select a cell volume that is in the achievable list of volumes for that particular cell count. (See volume range chart) A 550 cell count can have a volume ranging from 2.0-4.5.
Let’s run the first test: The banded anilox roll test.
We have now gotten in the ballpark with the anilox cell count and volume. It would then be necessary to optimize the process. A banded anilox test is highly recommended at this juncture to establish the anilox cell count and volume combination that will achieve the minimum target density and provide the best graphic reproduction. A banded anilox is an anilox that has several different combinations of engraving on a single cylinder. It can be engraved onto any existing anilox from inventory. A test plate is made to match the widths of the anilox engravings. The graphics in each lane will be identical to each other. This process isolates the anilox and allows for critical anilization of the I.F.T. delivered by the various engraving specifications.
If you do not currently have established density targets I suggest using the FTA FIRST densities as a guide. Run each color on your banded anilox roll to determine what volume achieves the density target.
Courtesy of the Flexographic Technical association FIRST
In conjunction with anilox selection we should determine what kind of printing plate materials we will be using and the ink transfer characteristics of each. There is wide range of photopolymer materials available in a range of durometers and thickness’s. Each will affect how the ink film thickness is transferred to the substrate thus impacting the final images. The primary selection criteria for plate materials are consideration of the substrate and graphic requirements. Many questions arise that need to be answered in order to select the proper photopolymer material:
- Substrate- is it rough or smooth. Will it quickly wear out my plates?
- How does it accept ink? High vs. Low holdout
- Graphic requirements? Not all materials can hold fine dots at high screen counts.
- Thickness – what is my cylinder undercut? .125”or .085”? Is the material I want available in the thickness needed for my undercut?
Also to be considered along with photopolymer material is the processing method. Will the print job be processed using analog or digital processing? There are new methods that exist for both, each offering unique advantages. Due to the many variables with regards to plate materials and processing methods, for the purposes of this discussion, we’ll cover the primary factors; solid ink transfer and dot structure.
From the data collected during the single color banded roll test we will select the anilox cell count and volume combination to have four process anilox’s engraved to. Data collection should be done in partnership with your pre-press supplier. Generally the anilox that achieves target density while providing the highest level of contrast throughout the tonal scale is the specification selected.
After we select the anilox that yielded the best results from a density and dot gain perspective we will now be able to run a four color characterization to determine the printable color gamut. Another set of testing plates will be made and on-press test run. (See sample characterization target) The greater the level printable contrast for each process color, the larger the printable color gamut will be. Therefore minimizing dot gain and maximizing density is a primary goal for selecting the correct anilox. Keeping the tone ranges from 1%-99% allows for the largest possible color space. We should be able to print a neutral grey if the CMY process colors are in balance with each other.
Running the press characterization target allows the color separator to establish the color gamut for a printing press. The best control over registration and dot gain allows for the largest color space.
Courtesy of the Flexographic Technical association FIRST
Once we have optimized the anilox to plate relationship the process will need to be maintained. We will have to manage anilox volume loss and plate wear. As anilox’s wear or become plugged they will inevitably lose density. As plates wear dots print larger. Both of these issues will throw the neutral gray out of balance. Constant monitoring and measurement of the printing process will keep this potential problem under control and manageable. In order to track changes and catch issues quickly, use some form of printed target on every job you run. This will assist and notify the printer when the system is moving away from center. In order to manage the vast number of variables it is necessary to use some guidelines to bring the process more in focus. A characterization will need to be run, the process re-calibrated, and monitored regularly.
These are very general concepts outlined here. It is always important to work closely with industry professionals who can bring a wide range of expertise and experience to the process. Through continued process improvement, education, and expansion of capability your printing operation will remain competitive and marketable.