Many times ink, paper, and other consumables are bought on price without regard for the lowest total cost of manufacturing after factoring in their impact on productivity, waste, and defects. Many companies also ignore the value of developing specifications and testing incoming consumables as an element of their process control program. Too often the answer is, "we will deal with issues as they arise."
When it comes to ink, we recommend testing be done routinely (weekly, monthly, quarterly, or bi-annually) to ensure that it continues to meet your quality standards and that your supplier is providing a consistent product. This makes your printing more consistent, predictable, and reduces makeready times. It’s certainly not unknown that a disruption to production is traced to a consumable supplier that has reformulated its product, which then inadvertently or intentionally changes its print properties.
Obviously, an important consideration to controlling color on press, and successfully using a methodology like G7, is to have consistent ink color. Ink color is a combination of undertone (color of thin ink film on a white surface) and masstone (color of thick ink film unaffected by paper). It can be determined using a printability tester or proof press, as we do in our paper and ink lab, or with an ink drawdown test. The former is used when an ink is being verified to conform to the ISO 2846 ink standard for suitability meeting ISO or G7 color printing requirements and provides an exacting evaluation of both the ink color and transparency. Color on the test samples can also be measured using a colorimeter and defined by L*a*b* values.
Note: if you are following either printing methodology, you should be using an ISO 2846-certified ink or having your ink tested to see if its color is within the standard’s tolerance. More information can be found here.
Testing this ink quality is probably the most important test for a printer because it uncovers the amount of pigment in an ink. In general, the more pigment, the thinner the ink film that can be run and the greater the ink mileage. However, as the ink strength changes, so does the performance of the ink on the press. Tinctorial (color) strength is determined when a standard amount of ink and tint base are mixed together, drawn down on a test paper, and then compared to another ink or a previous sample. The test report will tell you the percentage that one ink is weaker or stronger versus another, as well as its opacity. This difference in strength (mileage) can be financially significant for large-volume users. Our recommendation is that ink strength should vary no more than +/– 5% when testing the same ink over time, with a 10% change in strength normally being visible.
Controlling ink tack prevents picking of paper and ensures proper trapping in color printing. The best way to test is to use an inkometer that simulates the rollers on a press and measures the force required to split an ink film at press speed. The test report shows the tack of the ink, revealing its stability at 90° (32° C) for 10 minutes. To have the same performance on press, tack variations should be kept to +/– 1.0 unit from one sample to the next. In addition, the inks should be tack rated according to print sequence, with the highest tack being the first ink down and the lowest for the last ink down.
This test indicates an ink’s ability to accept water. Inks that do not emulsify enough fountain solution have a tendency to scum on press. Similarly, inks that over-emulsify provide poor quality, snowflaking, and excessive dot gain. In this test an increasing amount of water is mixed with a standard amount of ink, with samples of the mixture taken every minute. The water content of each sample is determined, and a graph of water content versus time is drawn and compared with an ideal plot. A specification should be worked out between you and your ink supplier probably including maximum and minimum water content throughout the test.
Fineness of Grind
Inks that have poorly ground pigments contribute to accelerated plate wear. This characteristic is easily tested using a fineness-of-grind gauge, which determines the largest size of pigment particles. A specification can be set so that any change in the courseness of pigments is easily detected.
While the properties above are satisfactory for most printers starting a specification and testing program, there are other characteristics that can be specified, such as drying time, viscosity, and surface tension (when printing on film or foil). Remember that specifications are of doubtful value if you’re not going to test to see if those specifications are being met.
Printing Industries of America’s Paper and Ink Lab can conduct these and other tests for you, eliminating the need to invest in testing apparatus and know-how. Even large-volume purchasers that have the clout to request these tests from their ink supplier would be wise to have a neutral organization confirm the results. The above tests can be done in a day or two and are relatively low cost, particularly for member firms.
To learn more about ink testing, contact Lindsay Ferrari at firstname.lastname@example.org or 412-259-1785.