Re: Ink Strength
CKL:
Three methods, whether sheetfed or web fed.
Tinctorial strength, or pigment loading, is a laboratory test. The ink is mixed 50 to one or 100 to one with a white ink, and a trained technician draws down the two mixtures and from experience, determines that the darker of the two is stronger by x amount. He adds x amount of white to the darker color, repeats the drawdown, and judges whether his estimate was correct. The result would be expressed as "magenta A is 20% stronger than magenta B." This takes about 30 minutes per color. Some ink companies try to do this estimate with a spectrophotometer and claim success, but you still have to weigh the stuff out and mix the inks until uniform. Advantages, it's fast, and doesn't tie up a press. Disadvantages: sometimes the process colors are slightly different shades, making the guessing a little difficult. This works for sheet fed or web inks.
Density versus ink film thickness curves. This is another laboratory test using a small laboratory printing press where you can weigh the ink amount that is printed on a sheet of paper. You don't weigh the paper, but the disc or printing plate that makes the print. You let the ink dry and plot status T density versus ink film thickness. The stronger ink will hit a typical density at a lower film thickness. Advantages: Brings the paper into the equation, and it's instructive to do this with coated and uncoated paper. Disadvantages: It might take four hours to do this for two sets of ink, or longer. This is similar to an ISO 2846 ink test. You need a careful technician to do this test right. Once again, you aren't tying up a printing press, but this isn't cheap to perform. As a printer, you should ask to see this data first, since the ink company may have already done it, to hedge their bet on what will happen out in the press room.
Finally, as a last resort, mileage tests on press. Should this be a live job or a test form? If you have CIP3 data, you can determine the exact coverage of CMYK on each form, to compare apples and apples, if it's not a test form. Is there temperature control on the press and in the press room? If temperature increases over the run, ink consumption will increase considerably as the viscosity drops. This has been documented by the print research geeks.
For sheetfed, it's convenient if you have one of those ink dispensing systems like Accel Graphics. If you run long enough, it can display how much ink was consumed. Otherwise, you need to weigh a few pounds of ink and scrape it all carefully into the ink fountain; then carefully scrape it all out again, to determine the precision and repeatability of the process. Then you need to run enough sheets to consume at least half of the ink in the fountain, probably 5000-10,000 copies. This can be done, but you can see that it takes time and care in scraping and weighing of ink.
Web presses are easier sometimes, I guess because they use a LOT more ink in a hurry. Again, you can scrape and weigh, only it's a lot more work. You might be lucky and have ink totes that are on scales that would give a direct read out of pounds of ink consumed. Again, hard to put Ink B into the same tote, but you get the idea. Another technology that is pretty cool is from LinkTech. This can continuously measure how much ink is added to the fountain. Again, the ink in the lines is Ink A, and to measure Ink B, you need to scrape and weigh.
It's a big project, but you will learn a lot about your equipment and your temperment as you go through the exercise. You could learn that less ink will be consumed when you print higher screen rulings, including stochastic. You will learn that uncoated or SCA or SCB papers suck up the ink at an alarming rate compared to coated paper.
John Lind
Cranberry Township, PA
724-776-4718