Ink-Substrate Interactions...

Broadly speaking, there are two kinds of ink-substrate interactions: desirable and undesirable. Desirable ink-substrate interactions are defined by their outcome: satisfactory ink adhesion; appropriate scuff-, weather-, or moisture-resistance; light-fastness, color-fastness, printability, etc. Undesirable interactions fail on one or more counts. This is true for all forms of printing, but is especially so in package printing, given the vast array of substrates used in packaging.

All of which is to say, somewhat axiomatically, that a product’s end use will—or should—determine the materials and the processes used to produce it, as well as a determination of the post-press finishing and handling it will undergo. Often overlooked, says Russ Barton, director of operations, demo rooms for Heidelberg USA (www.heidelberg.com), is that package printing, per se, is just an early chapter in the production cycle.

“We forget that printing isn’t the biggest part of package production,” Barton says, observing that additional finishing processes such as stamping, embossing, folding, and gluing, etc. also will affect the initial choice of substrate and ink. In other words, “If it looks good on press, it’s only part of the story.”

“Ideally,” says Barton, “your choice of substrate and ink would start with a discussion between the package designer and the end-user, taking into account not only the graphics and the layout, but also a determination of how the box is to be printed, processed, handled, shipped, used, and displayed.” Physical properties such as adhesion and scratch resistance of the final package are determined by a combination of factors, including ink, substrate, and surface treatments, all keyed to end-use requirements.

So the question becomes, how to optimize ink-substrate performance during printing while meeting all customer requirements for the end use of a product. For example, Barton explains, there are food-level inks that have to be edible if they come in contact with the product to prevent toxins from leaching into the food. By the same token, he says, inks and materials used to create a box of fish sticks or the label on a bottle of wine destined for the ice bucket will have different requirements than those used to create a cereal box, based on color-fastness and moisture-resistance.

Think ink

Basic incompatibilities between substrates and inks are often overlooked due to a lack of communication among the designer, printer, and ink supplier. In reality, experts say, ink is often treated as an afterthought.

“Few people think about the ink early enough in the process,” says George Varughese, vice president, product development for Superior Printing Ink (www.superiorink.com). As a result, “there are too many assumptions made along the line,” creating multiple opportunities for error.

As with the choice of substrate, multiple considerations come into play in the selection of appropriate inks (and coatings), including the choice of substrate itself, and all should proceed from a clear understanding of the product’s end use. Will it be used indoors or out? If it comes into contact with food, will it be printed with ink derived from an FDA list of approved ingredients and pigments? Other factors include run-length and budget. Varughese emphasizes that the sooner an ink supplier becomes involved in helping to sort out these issues, the more likely it is that problems can be avoided when a job goes to press.

Make it stick

More commonly, problems arise with respect to ink adhesion on coated paper stocks or non-porous plastic, as well as an appropriate level of scuff- or rub-resistance that will stand up to post-press handling of the product. Poor ink adhesion means that the substrate in question has low surface energy, which can short-circuit print and post-print operations. Simply put, if the surface energy of a substrate is lower than the surface tension of the ink, the ink won’t stick.

Non-porous substrates, such as films and foils, must have their surface energies altered in order for proper ink adhesion to occur. Plastic substrates can be modified to increase the surface energy of the material. Corona treatment alters the static charge of a material’s surface, making it more receptive to the application of inks, coatings, and adhesives.

While fast-drying UV inks are particularly good for printing on plastics and foils, conventional inks may bead up and refuse to adhere. For this reason, Superior Ink usually makes recommendations on the tricky business of using conventional inks to print on non-porous substrates.

See me, feel me

Among the reasons for adding a coating over a printed piece are to increase its durability. Chemical-resistant UV coatings are made to order in this regard, and the high-end look of specialty UV coatings is highly prized by packagers whose products must stand out on retail shelves.

Beyond their obvious aesthetic appeal, however, UV inks and coatings have some very practical uses, which account for their popularity in packaging applications. In UV printing, specially formulated inks are exposed to ultraviolet radiation, which causes them to harden instantly on top of the substrate. The result produces high levels of gloss or dull coating, vivid color, and vibrant detail with superior rub resistance and no post-cure dryback—even on soft, uncoated materials.

Good communication between the prepress department and the pressroom is essential when working with UV inks and coatings, and can avoid many lurking disasters. Not only must screening curves be adjusted to account for the additional dot gain that occurs with UV inks, but jobs also must be proofed on the actual substrates on which they will be printed.

The nature of packaging projects is necessarily collaborative, with the various supply-chain partners cooperating to develop the proper specifications for a given job before it goes to press—because by then, it may be too late. pP