Given the complex balance of commercial printing ink and water (plus all the other variables in ink formulation), plus paper choices, it’s amazing that anything gets printed or looks good.
First of all, let’s step back a moment and discuss ink on paper, or more specifically offset ink on paper, since ink varies considerably from offset printing to gravure printing to flexography.
Offset printing is a planographic printing process. That means the custom printing plate is flat. Image areas are not raised (as in letterpress) or recessed below the surface of the plate (as in such intaglio processes as engraving).
Offset ink (which is greasy) and water don’t mix. Therefore, when a printing plate on a press cylinder rotates through the ink and water units (like troughs along the width of the press), the ink and water stay separate. Image areas on the printing plate that have had the plate surface removed (by selective exposure to a laser) attract the ink, while non-image areas (unexposed areas on the plate) attract the water and avoid the ink.
This general concept explains how the fine letter forms of type, plus halftone images, plus solids receive and hold the ink on the plate, and then release the ink onto the offset press blanket and from there onto the paper.
Keep in mind that all of this is usually happening for four process inks that are laid on top of each other, and the press is operating at 10,000 to 20,000 impressions per hour. So getting the ink just right is a major challenge.
Properties of Ink
Here are four properties of ink to consider: color, body, length, tack, and drying capability. Moreover, the printer has to understand how specific inks and papers work together, since mixing ink for offset printing depends heavily on the paper (or other commercial printing substrate) for how the ink behaves, dries, and appears when all is said and done.
Ink Color
Printing ink is composed of pigment (usually organic, but some non-organic) particles within a fluid mixture of solvent (which controls the body of the ink), vehicle (which gives the ink its fluidity), plus other additives such as drying agents.
The various hues of offset printing ink your printer uses have the aforementioned physical characteristics, but they also have optical properties. These range from their opacity to their transparency, and also their ability to create (when mixed) the greater portion of visible colors. Cyan, magenta, yellow, and black inks printed over one another only match most of the PMS colors. For other colors, your printer adds specific PMS hues (also referred to as match colors).
To read the color, printers use a spectrophotometer (a computer device) and agreed-upon standards like SWOP (Specifications for Web Offset Publications). Only if color can be communicated, and only with adherence to color standards (using closed-loop color reading instruments on press), does the printer have the ability to successfully print colors in a way that will match your expectations.
Ink Body
“Body” pertains to the stiffness or softness of the ink. Offset ink straight out of the can is thick and must be softened through the action of the multiple roller systems in each inking unit of a commercial printing press. In contrast, gravure and flexographic inks are much more fluid. (These are also usually water-based, unlike oil-based offset printing inks.) And screen printing inks have the consistency of thick paint, which is why the application of custom screen printing ink on a messenger bag or hat looks thick and opulent.
To put ink body in context, you would use a much less fluid ink for newsprint offset printing than for offset printing on high-quality coated stock.
Ink Length
“Length” pertains to the ability of an ink formulation to form filaments. (Picture an ink knife lifting ink from a blob on a printing plate.) The filament will be longer for a long ink and shorter for a short ink before it beaks apart. You don’t want the ink to be too long or too short. If an ink is too long it will spray (“fly” or “mist”) when the press is operating at 10,000 to 20,000 impressions per hour. If an ink is too short (like the consistency of butter, with a lesser ability to flow), it will pile up on the rollers and press blankets. (As with the characteristic of ink body noted above, an ink for offset printing newspapers will be longer than one for printing on a gloss coated press sheet.)
Ink Tack
This is one of the most important characteristics of ink specifically used for offset commercial printing. Tack is the stickiness of ink as an ink film is being pulled off the plate and applied to the blanket (i.e., as the ink film is split between two surfaces) and then transferred from the blanket to the printing paper. The first ink laid down must have a higher tack than the second, third, and fourth (CMYK), or the extra PMS inks on press.
If an ink is not tacky enough, one layer of ink will not adhere to the prior layer (known as “trapping to the color”). If it’s too tacky, the stickiness of the ink will peel off pieces of the press sheet (known as “picking”). So, clearly, the nature of the printing paper plus the ink tack and the order of color ink application must be taken into consideration during the custom printing process. Otherwise you will create a mess. During this process, the printer must often make compromises, since preferred ink tack (measured with a tackoscope or inkometer) is different for text, halftone screens, and solids. (That is, tacky ink does not print smooth solid areas of color.)
Ink Dryers
Ink will dry in a number of ways, including absorption, oxidation, evaporation, and polymerization, just to name a few.
Absorption pertains to the ink’s going into the fibers of the paper and leaving the pigment on the top of the paper. You might use this for newsprint or other non-heatset (or cold set) web press work.
Oxidation involves the outside air being absorbed into the ink. The chemical reaction between the outside air and the ink causes the ink to harden on top of the paper surface.
Evaporation involves the use of heat to cause the vehicle and solvents in an ink to turn into a gas and be released into the atmosphere, thus hardening the ink film. You might find this kind of drying technology on a heatset web press (using ovens just past the inking units) or a drying unit at the delivery end of a sheetfed offset press.
Polymerization refers to the process of exposing printed UV ink to UV lamps as the press sheets travel thtrough the press. UV light will instantly turn the UV inks into a solid, sitting up nicely on the surface of the paper (called good “holdout”). UV inks treated with UV or LED UV lights will allow you to use less ink, print crisper type and brighter halftones, and either use the printed product as soon as it exits the press (or the finishing equipment) or immediately print the opposite side of the press sheets without needing to wait for the ink to dry.
What You Can Learn from This Technical Information
Even though there’s an abundance of educational material out there on properties and characteristics of paper, it behooves you to learn about commercial printing inks as well. Offset printing, or any other kind of printing, depends on the successful pairing (like pairing wine with a specific kind of fish) of the right paper with the right ink.
The aforementioned information will be well known to your printer. However, it always helps you to understand the fundamentals of your craft as well. And as a designer or print buyer, your knowledge of ink and paper will also help you clearly see any printing problems and identify their causes if something goes awry in the commercial printing process. It will also make you appreciate the Herculean task of getting just the right mix of ink, water, printing plates, and paper to create a gorgeous publication.
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