Ink—that magical, mystical fluid—delights designers, annoys accountants, inspires inkies and P-O’s press operators! We use it in flexo presses every day, all day, across the country and around the world, yet so many people don’t understand how ink really works, what it can and can’t do, and how to use it best to maximize productivity in the pressroom.
Last year, FTA asked me to do an FTA Member Webinar on the basics of flexographic inks. We had good attendance, a lot of questions and a lot of follow-up communications. In this article, I’ll try to recap the information that was conveyed in the webinar, along with a little bit of further detail about some of the topics that were of most interest to folks. I’ll cover the basics of all three common flexographic ink chemistries—water based, solvent based and energy curable. We’ll explore the basic ingredients used in each chemistry, how they work in each type of ink and how they perform their functions on press. You’ll learn how ink manufacturers make these inks in both large bulk form and in small batches. Also covered will be typical quality control (QC) checks used, and the tools and equipment required to perform those checks, both in the QC lab and in your pressroom. Finally, we’ll cover some on-press variables that can affect ink performance and some basic suggestions for troubleshooting common ink problems.
Before we dig too deep, let me just offer three “qualifiers”:
- I’m not a chemist! I couldn’t formulate an ink if my life depended on it, but I do understand how inks work in the flexo printing process
- This is generalized information—There are many variables for specific ink systems designed for specific applications, whose information and details may vary from this article
- Always, always, always follow your ink supplier’s recommendations! They know their formulas and are the best guide for how to use their products
As previously mentioned, flexographic inks are divided into three main categories. Water-based inks are used on wide web flexo presses for applications like paper packaging, overwraps, bags, etc., and in narrow web flexo for products like pressure-sensitive labels, tags, envelopes and the like. Solvent-based inks are used primarily in wide web applications like film packaging, overwraps, bags, pouches, etc.; they see very little use in narrow web, but are occasionally the ink of choice for adhesives, coatings, etc. And lastly, the category of energy-curable inks covers three main energy curing methods—ultraviolet (UV), light-emitting diode (LED) and electron beam (EB). Both UV and LED are in widespread use in the narrow web portion of the flexo industry, with EB used to a much lower degree in both wide and narrow web.
Basic Ingredients & How They Work
The main components in any water-based ink system are the following:
- Colorant: Pigments or dyes, used to impart the color or shade to the finished ink
- Resin (vehicle): The backbone of the ink system, used to carry the colorant and bond it to the substrate, typically made of acrylic
- Amine: A chemical used to solubilize the resin and change it to a liquid state
- Diluent: Typically water, used to reduce viscosity (the relative thickness or thinness of the fluid ink)
- Surfactant: Often a defoamer or wetting agent is used to affect the surface energy of the ink
- Additives: Anything else the formulator needs to include to make the ink work for its intended use—products like waxes, silicones, etc.
For solvent-based systems, the components are similar, but slightly different:
- Colorant: Pigments or dyes, used to impart the color or shade to the finished ink
- Resin (vehicle): The backbone of the ink system, used to carry the colorant and bond it to the substrate, typically made of a variety of different waxes/lacquers, chosen for their intended use and end requirements
- Solvent: A chemical used to solubilize the resin and change it to a liquid state, chosen specifically to work with the resin that’s used; the solvent is also used as the diluent, to reduce the viscosity
- Surfactant: Often a defoamer or wetting agent is used to affect the surface energy of the ink
- Additives: Anything else the formulator needs to include to make the ink work for its intended use—products like waxes, silicones, etc.
For energy-curable inks, the components are once again similar, with some significant differences:
- Colorant: Pigments or dyes, used to impart the color or shade to the finished ink
- Resin (vehicle): The backbone of the ink system, used to carry the colorant and bond it to the substrate, typically made from one or more oligomers (a chemical combination of a few monomer molecules), chosen for their intended use and end requirements
- Monomer: A low molecular weight chemical used to reduce viscosity and affect the flow and print characteristics of the finished ink
- Photoinitiator: Used for UV and LED chemistries, this ingredient reacts to the application of intense UV light to begin the process of curing (polymerization) the ink film, changing it from a liquid to a solid
- Surfactant: Often a defoamer or wetting agent is used to affect the surface energy of the ink
- Additives: Anything else the formulator needs to include to make the ink work for its intended use—products like waxes, silicones, etc.
In summary, the basic ingredients of each ink chemistry vary mainly in the type of resin system used, and the method of drying or curing used. Each chemistry has its strengths and weaknesses, because each is utilized for different print applications, based on the suitability for the end product. Some colorants work better in one chemistry versus another, due to their relative ability to “wet out” those pigments.
Both water-based and solvent-based inks dry by evaporation, absorption or a combination of both. Energy-curable systems dry by being “cured” after exposure to energy; in the case of UV and LED inks, that energy is light, where for EB inks the energy is a stream of electrons.
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