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Ink 101: Part II

Last month we reviewed ink usage numbers as an input to converting, noting that as a percentage, ink purchases are small, especially when compared to the impact on process efficiency and ultimate package quality. This month we look at ink types.

Inks can be categorized by viscosity, as either liquid or paste. Liquid inks are used in flexo and gravure printing where low viscosity ensures release from anilox and gravure cells respectively. Paste inks are used in litho printing where high viscosity ensures the ink stays on the essentially flat ink image areas of the litho plate.

All inks contain three primary components. The carrier is the major component at 75%-85% by weight, followed by the pigment at 10%-20% and an additive package at 5%. The pigments used tend to be the same regardless of ink type, but where they do differ, significant color problems can occur.

The carrier defines the ink system type and determines the end-use characteristics such as dry rate, gloss, adhesion, and scuff resistance. The additive package consists of chemicals such as slip agents, adhesion promoters, and cross-linking agents to increase performance of the ink system further.

Liquid inks for flexo and gravure printing generally are water- or solvent-based. Water-based inks contain a balance of solution and emulsion resins, held together by a combination of amine and small amounts of solvent such as alcohol. The solution resin provides print quality, and the emulsion resin gives resistance properties and faster dry rate. Water-based inks dry by absorption and are used mainly for printing on absorbent substrates.

Solvent-based inks often have a combination of two resins dissolved in a solvent combination of alcohols and acetates. One resin may be for print quality and the other for adhesion and resistance properties. Solvent-based inks are used on non-absorbent substrates such as film and foil, and they dry by evaporation.

Drying liquid inks requires individual drying stations between print units and trapping wet ink on dry.

Paste inks for litho printing fall into two main chemical categories: oil-based and radiation-cured. Most paste ink is used on paper and board substrates. Oil-based inks contain drying oils, which when printed in a very thin film, react with atmospheric oxygen, catalyzed by metallic dryers in the ink, to form hard, resistant print. The initial drying to a tack-free state is by absorption into the paper or board substrate.

Trapping of oil-based colors is wet on wet, ensured by graduating ink tack from high to low through the press. Radiation-cured inks are based on acrylic monomers and prepolymers, which cure instantly when irradiated with UV or EB energy. These may be wet on wet or wet on dry trapped.

For each ink type, the formulator is faced with making the optimum selection and balance of raw materials to come up with viable ink formulations. In general, the industry does a good job of providing proven, fit-for-use ink systems.

However, a gray area remains on ink cost-of-use forecasting as it relates to mileage and converting efficiency. This can impact costing and pricing of packaging material. Inks are sold on price per pound but are consumed volumetrically. These topics will be explored in upcoming columns.

David Argent has 30+ years of experience in the converting industry. He specializes in process analysis and improvement with particular emphasis on ink and coating design and performance. Contact him at 636/391-8180; djvargent@sbcglobal.net


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