More than just dot %... or is it dot area?
So often when we are speaking of dot quality and measurement we default to dot %. But is there more we can measure to understand what aspects of the printed dot are affecting dot size, quality, and even longevity? Even if we focus only on dot size there are different formulas used to calculate dot coverage, including the well known Murray Davies equation. This is a formula for calculating dot area based on density measurements. This measurement approximates the total of physical dot size plus optical dot gain due to insufficient light absorption of the substrate, thus the term "apparent dot area". This is not the same as dot %. Let's start with that and then expand;
Terms and Definitions of Flexographic Dot Measurement
- Dot % - This is an actual measurement of dot percentage. It is based on the outer edge of the printed dot and does not take ink density, coverage, or transfer within the outer boundaries of the dot's radius. In other words, if the outside diameter of the dot measures such that it would equate to a 30% dot, then the fact that the inside of the dot may be fish-eyed or lacking ink transfer.
- Dot Area - This is most commonly measured as the mean of the area of the dot in square microns. On some image analysis devices, this same feature can also be used to measure the openness of characters when printing reverse type or registered trademarks.
- Now that we know the difference in dot % and dot area we can more easily understand more about the ink transfer of the printed dot. If dot area and dot % both measure the same value, then we know that ink is effectively delivered to all areas inside the perimeter of the dot.
- If the dot area value is less than that of the dot % value, we can determine that ink is NOT effectively delivered to all areas inside the perimeter of the dot.
- Threshold % - Affects both dot % and dot area. This Is a setting used while collecting data (absolute or relative). In laymen's terms it determines where it considers what is dot and what is not as the ink fades or bleeds into the surrounding substrate. But there's a catch;
- While there is an ISO procedure determining that 60% is the correct threshold % for a printed line, the same cannot be said for the printed dot, and therefore the 60% value is most commonly carried over.
- Diameter Mean - You would think if we knew the values of dot area and dot % that would be enough to acquire the desired information regarding dot size. Well, it is not. The dot diameter = the sq. root of 4A(area) divided by pi. Typically this takes into account the average of individual diameter of dots. In doing so it gives us value that we are forced to assume all dots are printing the same size. In order to verify the size consistency of dots measured within a tonal patch, we must look to the standard deviation.
- Diameter SD- How much that average diameter differs (standard deviation) between the total number of dots analyzed. A deviation of more than 10% indicates the dots are irregular in size. The most common causes of this scenario include broken/damaged dots on the printing plate and dirty print (caused by a variety of reasons).
- Dot Circularity - Circularity of the dot can be determined using C= p2/(4pA), where p= the perimeter length and A= area. As flat-top dots have gained popularity this aspect of the printed has become less of a focus. Why? Using flat-top photopolymer plates the dots may be more pixelated on the plate because of the near 1:1 reproduction from the digital file to the printing plate. That said, once the ink is transferred from the plate to the substrate, the radius of these dots contour and often still present a value of near 1.00.
- Note that for a perfectly circular dot, circularity = 1.00. For dots of any other shape, circularity > 1. For very small dots the circularity measurement accuracy may be compromised therefore have a value of < 1.
- Dot Perimeter Mean- Is a measurement in microns of the dot perimeter length. Often a “Fail” prompt will appear on a reading device if the dot is too small to measure the perimeter.
- I must admit, I personally have not found this measurement to be helpful. I'm sure the developers may have had inkjet in mind or even dot size uniformity, but I personally would always gravitate to the more simplistic Diameter SD. I could stand to learn more and if you have the answers please reach out to me at [email protected].
- Box R Mean- This is to measure the slur or elongation of dots. Dots falling within .9 – 1.1 would be considered to have minimal slur. Whereas dots falling outside that range could indicate issues with plate relief, impression, or even a speed match problem in press. Box ratio, the ratio of the vertical dimension to the horizontal dimension of the minimum rectangle bounding box that can be fit around the dot. In other words, this is the maximum vertical height of the dot divided by the maximum horizontal width of the dot.
- Screen Mean - This is simply the measured LPI or LPCM.
- Screen Angle Mean- Screen angle of which the dots were taken. Keep in mind this is most often relative to the positioning of your measuring device and may require you to adjust the position of the printed sample to obtain useful information
- Number- Last but not least, the number of dots analyzed to obtain data. The more dots measured in a tonal patch will yield more valuable information. Focus on a few dots just doesn't give a true representation of the printed sample. This is not unlike measuring an anilox roll under high magnification and focusing on a couple of microscopic cells to determine the overall roll quality. In this case, LESS is NOT MORE.
If you want to learn more about this subject, please feel free to reach out to us by calling 1-800-445-4017, or by e-mail at [email protected].
All Printing Resources has formed our Technical Solutions Group (TSG) to encompass our full range of expertise in all critical areas of the flexo process. This team is made up of industry professionals dedicated to being up-to-date on new technologies along with best practices. They are armed with the latest in diagnostic tools and are experienced in problem-solving that can achieve sustainable results. The TSG has walked in your shoes, and have felt your pain.