01 Jun Predetermination of Ink Color
No printer sets out to print an image with the incorrect color match, to make two or more squeegee strokes to achieve opacity, or in the case of printing on a textile substrate having to use a print/flash/print technique. This article explains how to predetermine the correctness of ink to avoid problems on the press.
by Bill Hood, ASDPT Fellow
Any printer can take the guesswork out of printing the correct color, i.e. hue, value, and chroma of ink. And, with simple tests the correct mesh, thread count, tension level, off contact, and opacity. But first, we need to warn you of three things that make color matching difficult.
1. Pantone Color
There is no Pantone Book that will inform you how to mix colors for screenprinting inks using the Pantone recipe. Pantone is a proprietary ink system for offset that uses colors not available in screenprinting inks. For example, there is no true Rubine Red for screenprinting, nor do other colors of screenprint inks match Pantone. Each screenprinting ink manufacturer makes a slightly different hue of Cyan, Magenta, and Yellow. Offset inks are made to work on white paper, not textile products, and especially not colored materials.
The Pantone Color System does allow a client to specify a Pantone color that can be matched with your Pantone Book so that we don’t get a specification like banana yellow, mauve, or gold, each of which can be confusing. However, this only works well when both parties have the same model, issue, and dated Pantone Book, which is rare. Pantone Books age over time, and exposure to ordinary room light. Pantone recommends that they should be replaced yearly, or earlier if not cared for properly, i.e., exposed to light.
2. Computer Monitors
Computers are back-lit electronic displays that emit light, which is different for every monitor. If you build a design on your art department computer and send it to someone by email, or load it onto your website for their approval, the colors that they see will vary from what you are seeing on your computer. Again, every monitor is different. You can adjust a monitor with precision equipment, but unless both monitors are calibrated precisely the same, the colors will appear different.
The use of Pantone to describe a color means nothing if observed by a trichromatic human eye, with a varying sensitivity of different cells in the retina perceiving light of different wavelengths, is never going to work as a means of describing color.
No human can mix color by eye, and expect that another human is going to “see” the color in the same way. Unless your ink technologist is female, the color of the mixed color will not likely match the expectations of the client. This is due to the difference in chromosomes in humans. Each person normally has one pair of sex chromosomes in each cell. Males have only one X chromosome that allows humans to see colors more accurately, but females have two X chromosomes. When damage is done to the chromosomes (and it happens regularly), it only affects one of the X chromosomes in females, allowing them to still see colors more accurately. If you want to excel at meeting your client’s expectations hire only females, who are better spectroscopists, in your ink department.
Densitometers and Spectrophotometers
Invest in a color densitometer and spectrophotometer, to achieve perfect color readings. Yes, there is an investment cost, they offer a great return on your investment to assure color matches.
The mixing of inks to match a particular color for printing can either be done entirely or primarily done by the ink manufacturer, with a modest amount of in-house adjustment or completely performed in-house. When mixing colors in-house, base colors are matched according to color charts and systems provided by ink manufacturers and other color experts. The important factor in color mixing is identifying in which particular color direction a base ink has to go to match the desired color.
For example, yellow ink can range from a greenish-yellow to a greenish-orange; blue can range from purple to green; brown, commonly a dark orange, can either be reddish or yellowish; purple can range from red to blue, and green can range from blue to yellow.
It is also important for the ink mixer to keep on hand a sample of the mixed ink (in case more is needed), a detailed record of how the final color was arrived at, and a press proof, an example of how the color prints on the intended substrate.
In matching a color, a color chart (such as a Munsell or Ostwald chart) will indicate the key color or the color which will form the primary component of the ink mix. A small test sample should be mixed, and a dab of the final ink placed on the intended substrate, to ensure that the ink will retain its color after printing and drying.
Careful measurements should be made throughout the test sample mixing, to ensure that when the time comes to make the final batch, the proportions of the mixed colors will produce the same result.
The primary ingredients for color mixing are the strong base colors, white inks (transparent inks and opaque inks), black ink, and a neutral gray. By utilizing color charts, the three color measurements:
- Hue (the specific wavelength or shade of the color)
- Value (its degree of darkness or lightness)
- Chroma – its strength
These three color measurements can be determined and each of these three color aspects can be adjusted by the color mixer. The hue can be adjusted by adding the color that will bring it back in the direction it needs to go. If a blue is too purple, yellow or green may need to be added.
If the value is incorrect, the ink will need to be darkened or lightened, using either black (which has the tendency to dirty an ink, however) or a suitable dark color, or white or suitable light color.
If the chroma is incorrect, adding neutral gray can help bring the color purity where it needs to go. Should too much gray be added, the original base color can be added to reverse the graying trend.
Another important consideration in color mixing is to ensure that all the ingredients added are compatible chemically and mechanically: inks that dry by evaporation should not be mixed with inks that dry by oxidation, heat-set inks should not be mixed with moisture-set inks, inks designed for screenprinting should not be mixed with inks designed for other printing processes, such as letterpress, offset lithography or other processes.
Some mixtures can also possess a lesser degree of permanence than the constituent inks; a problem not typically noticed until well after the material is printed. For example, some types of yellow pigment will react with iron blue, causing the desired shade of green to become increasingly yellow over time. Incompatible inks may also become altered in other ways, such as in body and chemical resistance.
Testing of Color
Testing of the ink should check the myriad aspects: accurate color, flow rate congruent with the printing method to be used, the press speed at which it is to run, the tension of the mesh, the fill stroke, the squeegee considerations (durometer, angle, pressure, and downstop), the substrate on which it is to be printed, correct drying or cure, and the desired end-use characteristics of the printed material.
Inks will behave differently on different types of substrates. Testing can be accomplished by depositing a sample of the mixed ink on a sample of the actual substrate to be used. A Pat-Out Method, or a Drawdown Method may be used for testing on the actual substrate.
A better method to accurately gauge an ink’s performance on the press is to actually duplicate the final print run in as exact a manner as possible. For this reason, many screenprinters will have a single station (manual or semi-automatic) in the ink department to perform the tests. This testing in the ink department avoids the non-production downtime on the press.
NOTE: Testing of color should be performed on the exact color of the substrate to be used.
Proper ink drying is commonly evaluated after an appropriate length of time (such as overnight), and the dried ink is examined for proper ink penetration (or holdout), gloss, opacity, and color.
- Pat-Out or Tap-Out is a means of testing color-mixed ink for proper hue, substrate compatibility, and drying performance by dabbing a spot of it on the intended substrate with the finger, and distributing it to approximate the actual thickness with which it will be printed.
- A drawdown, sometimes referred to as pull-down is a means of evaluating the color mixing of a printing ink by depositing a layer of the mixed ink on the surface of a substrate using a smooth-edged knife. Drawdown is one of three basic tests used to determine the accuracy of color matching and mixing processes, the compatibility of the various inks combined, the performance of the ink on the substrate, and the drying characteristics of the ink.