Holding the Dot

Anilox Rolls

Combination printing continues to evolve due to technological advancements in the fast-paced flexographic environment. However, there are a handful of issues our industry faces on a daily basis.

  • Why is combination printing an issue?
  • What are the concerns from a printer’s perspective?
  • How should the industry address this issue?
  • What should be done if the screen prints dirty when the line color is a match?
  • What should be done when the screen prints clean and the color doesn’t match?
First we must start by understanding the amount of ink film thickness required to achieve the desired line or spot color your customer is requesting. There are a few variables to consider.
  • Is the production run on a standard substrate or similar substrate as the standard color swatch that was approved?
  • Is the production run on a semi-gloss, metallized film, or boardstock substrate that is different than the standard used to establish the approved color?
  • Is your facility primarily printing with water-based, solvent-based, or ultraviolet ink chemistries?
  • What is your current anilox inventory condition?
  • Are your press technicians aware of the current anilox volume or BCM at this point in time?
  • Are your press technicians confident the anilox condition, including BCM, is the same as the most recent anilox verification performed?
“Ink on paper,” as they say, is not a clear-cut statement that reflects what printers repeatedly try to do from one press run to the next. We must position our printing processes strategically to deliver a consistent, repeatable product to our customers.
Terms and Definitions
Combination printing A line or spot color used in a single print station that includes a solid portion and a screen or tonal scale on the same printing plate.
BCM Billion cubic microns.
LPI Lines per square inch referring to the printing plate.
DPI Dots per square inch on the film.
PPI Pixels per square inch in the digital file.
CPI Cells per linear inch (a new term the author suggests should be recognized as an industry standard to decrease confusion when describing anilox cells).

How to Determine LPI
Determining the desired plate screen value LPI for your facility will depend upon many variables, some of which are listed above. For narrow web, the industry standard for combination plates averages 120 LPI to as high as 175 LPI. The flexible packaging industry average is 85–133 LPI, and all other flexographic printing industries fall within 65–120 LPI.

The next options for consideration are the plate technology, dot gain, and acceptable ratio from the plate screen (which will vary depending upon plate technology) to anilox cell count. When attempting to determine the LPI, don’t lose sight of the minimum dot you have qualified as your standard. Consider that the digital dot structure is much different than the analog or conventional plate dot structure in that the digital dot does not grow. The CPI to LPI ratio of 6:1 has been an industry-accepted ratio that eliminates over-inking or, as some have described it, dot dipping. The printed result of dot dipping is dot bridging in the screen area of the printed piece.

For clarification, if the plate LPI is 100 and the minimum dot is 2%, than a 600-line screen anilox roll would satisfy that theory or eliminate the possibility of the smallest dot having less diameter than the opening of one cell on the anilox.

A 2% dot on a 100-LPI plate is 41 microns, and the opening of one cell on a 600-line anilox roll is 38 microns. By utilizing this ratio, you eliminate the possibility that the phenomenon of dot dipping will occur.

But putting dot dipping aside, you always want as many cells on the anilox to contact the plate dot for the most uniformed ink film thickness and thus print fidelity.

Likewise if the minimum dot being maintained on the plate is 3%, then a 5:1 anilox to LPI plate ratio can be utilized as a standard and so on, with the minimum dot being the controlling factor. You can see why it is critical to know and maintain a standard minimum dot size for combination printing—and process printing for that matter.

Volume Controls Ink Film Thickness
The next critical factor to have quantified is the volume or BCM required to achieve the non-process line color you are considering for a combination printing plate.

Volume is the controlling factor to achieve the desired ink film thickness assuming all the other variables have been maximized, including mounting tape, plate materials, dyne level of substrate, viscosity, etc.

Today an industry standard thrown around is the ability to hold a 21-micron dot and maintain consistent dot structure on a digital plate. What does that mean? For example, a 175-LPI, 2% dot diameter on a digital plate has a 22-micron dot diameter. To keep consistent with our 6:1 anilox-to-plate ratio would require a minimum 1,000 CPI, which has a cell opening of 22 microns. With that information, you should be able to run a 1,000 CPI with a 175-LPI plate and print our job cleanly without a problem. Unfortunately, at this point you must go back to the volume needed to achieve the desired color your customer is requesting.

Today’s ink technology has come a long way with much greater strength at lower volumes than we thought would be possible as recent as three years ago. A variety of new screening technologies for highlight dots and solids has seen great advancement, depending upon your product line.

For the 2005 Flexographic Technical Assn. (FTA) Forum in Orlando, a quantifiable test was performed to evaluate most of the screening technologies commercially available. A non-biased evaluation of the technologies was delivered at the Flexible Packaging Technical Session.

If alternative screening is an avenue you want to look into, contact the FTA and samples of the actual press run can be sent to you for comparison of today’s technologies for FM screening in the highlight dots.

Platemaking processes keep changing to make plates faster and more consistent. The anilox manufacturing industry has the ability to produce volumes higher than in the past, optimizing the ink delivery system. Standard depth-to-opening ratios in the flexographic industry in years past focused on 28% as being the optimum ratio for transfer efficiencies of inks and coatings. Due to advancements in laser technologies, product tolerances, and increased cell cavity smoothness, some can stretch the depth-to-opening ratio and still optimize transfer efficiencies to 35% and 40% or higher.

In order to print combination plates successfully, you must have a complete understanding of the operational workflow and have the ability to define and quantify the materials used in the process. When all is in order, the decision becomes more scientific versus trial and error.

What Are Your Options?
So how can you address the combination printing concerns everyone is faced with today? Fortunately we have a few options. The option exists of splitting the screen and the solid into two different plates or print stations. The obvious problem with this option is the added cost due to the extra plate material and set-up time for the additional station. The added cost usually cannot be passed along to the customer.

Another option is to run six- or seven-color process, building the Pantone color and screen into the process plates and eliminating the need to run a combination plate. This option requires an additional fingerprint (colorimetric) to map the potential color gamut to determine what additional colors will be available above and beyond the gamut of your standard CMYK color space achieved on press. When six- or seven-color process is considered, keep in mind the knowledge of your press technicians and the condition of your press. If you have minimal success and are challenged with four-color process, your challenges will be two-fold with additional process colors—registration being one of the more critical aspects that must be under control for successful six- or seven-color process printing.

Don’t take this the wrong way; this is a brilliant cost-savings option for numerous reasons, including less ink inventory, less set-up time, and less clean-up time, and color matching becomes more of a prepress function than a press function.

The most popular option today is running the color as a combination plate with the Pantone line color or custom color on the plate as the tonal screen. To accomplish this task, you must know what anilox volume is required to achieve a match to the Pantone color or custom color approved by the customer.

For example, a 3.5 BCM requirement will achieve the proper color strength. Today’s technology will allow you to utilize a 1,000 CPI. Knowing the anilox selection, you are now ready to choose a maximum standard DPI for the plate screen value for the combination plate, assuming you have established a minimum dot requirement for a standard round dot screening of 2%.

Based on the data, you easily will have the ability to run a 175-LPI plate screen on the combination plate (see Table I).

Table I
Anilox Roll Volume Examples
(cells/in.)
Screen Minimum Volume Maximum Volume Extended Volume
600 2.3 4.5 5.5
700 2 4 4.9
800 1.5 3.5 4.3
900 1.3 3 3.8
1000 1.1 2.2 3.6
1200 1 2 3.1
1400 0.9 1.9 2.5
1600 0.8 1.5 1.9
1800 0.7 1.2 1.5
2000 0.6 0.9 1.3
Based on past experience, if you can satisfy your customer’s requirements with a 150 LPI, then run the 150 LPI. If you can print 150 LPI well and 175 LPI marginally, stick with 150 LPI. Your end product will print cleaner dots in the mid-tones and run longer without having to clean the plate. With any volume above a 3.5 BCM, the potential for dirty print exists regardless of the industry-standard 6:1 anilox-to-plate ratio. At this point, the ink film simply is too thick for the small dots.

Anilox manufacturers can produce volumes far higher than in years past. The benefit is cleaner print (provided the cells are sized properly) and the ability to run combination plates much more successfully than ever before. Without all the improvements in inks, plates, substrates, and presses, the increase in anilox volume would not be of much use. Consult your team of industry experts to discuss the possibilities for improvements with combination printing if this is a problem in your facility. By getting the right experts involved to support your graphic improvement needs, everyone wins.


SUPPLIER INFO:
Harper Corp. of America—harperimage.com



David Brewer has 20 years of experience in the flexographic printing and flexible packaging industries, including eight years as director of Harper GraphicSolutions, Charlotte, NC. Brewer is now chief operating officer of DLBrewer & Assoc., Green Bay, WI, a supplier of print process services for the flexo industry. He can be reached at 920/497-9752; dbrewer5130@sbcglobal.net.



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