by DPS Magazine Staff
Commercial printers are tasked with producing a variety of products day in and day out. With so many substrate, ink, and varnishing combinations, coatings help optimize the surface for digital print as well as serve as a protective layer post-printing.
Coatings are often designed to meet the needs of specific applications, Jan Denies, manager, global account – HP Indigo, Windhof, Luxembourg, Michelman, says solution recommendations take into account product end use, type of substrate, and printing method.
Pre-Press: Priming
Jim Kedenburg, principal, Ontario Specialty Coatings Corp., suggests that most substrates cannot be printed on water-based inkjet printer very well without some type of inkjet receptive coating applied to the substrate prior to printing.
Bob Leidlein, VP sales and marketing, Alliance Technology Corporation, admits that some aqueous inks do not work well with adhesives and UV coating almost always prevents an adhesive to adhere to its surface. “Folding carton, corrugated printed boards, and commercial printed work that required an adhesive would ‘cut out glue flap areas’ to prevent coating from being applied in that area. As special effects became more important, spot coating was used to create those special effects.”
“Substrate manufacturers, including paper mills, apply our coatings to substrates to optimize surfaces for digital printing,” comments Denies. These coatings, including DigiPrime Icon and DigiPrime Edge, help improve the uniformity of substrate surfaces for liquid electrophotographic (LEP) printing. “Uniform substrate surfaces promote improved ink receptivity, rub resistance, and image quality. By enhancing the uniformity of substrate surfaces, Michelman primers enable commercial printers to utilize a broader range of substrates in many different applications.”
Depending on the specific coating used, it should not affect recyclability. “Our inkjet receptive coatings offer the same recyclability as any other commercially available inkjet media for water-based inkjet printers,” states Kedenburg.
He points out that water-based, inkjet receptive coatings can be “spot” applied but this is a very uncommon application. Usually the entire surface is treated.
Post-Coating
After digitally printed output, flood and spot post coating is an option. Flood coating provides complete coverage of the entire substrate while spot coating provides targeted coverage of specific areas of the substrate.
Flood coating was developed first to provide full coverage over offset printed materials in order to protect substrates in the packaging and commercial print applications and to provide some special effects, offers Leidlein.
“One of the applications is protection of printed items like postcard mailers that could otherwise be damaged during mail handling. For self mailers, the addresses are printed on the items themselves, so post-coating prevents them from becoming unreadable,” shares Pim Cornelissen, product manager, Contiweb.
Cornelissen says post-coating can also boost quality by adding a gloss or matte coating to the printed items.
Another flood coating application is adding a barrier coating for direct food delivery or paper cup application to provide protection against external elements, says Cornelissen.
One growing area for post-printing involves spot coating, which puts down a coating on the substrate where you want it to go. “This became necessary when adhesives had to be applied,” offers Leidlein.
Increasingly, spot coating is used as a way to embellish printed output.
Coating Types
Popular coating types for digital print include aqueous and UV.
Leidlein points out that aqueous coatings came first but these coatings would dry up on the application rollers. “The aqueous coating would become hard making it nearly impossible to clean up and would result in damaged rollers. Also, some aqueous inks did not work well with adhesives and UV coating almost always prevents an adhesive from adhering to its surface. UV coatings followed with increased opportunity for special effects.”
A few benefits of aqueous-based coatings are they pose no safety issues, making them easier to handle and eliminating health risks for staff. Additionally, these coatings have minimal environmental impact, as they are easy to degrade. “The cost-effectiveness of aqueous-based coatings is a significant advantage. They are generally more economical than UV-based coatings, resulting in lower operational costs,” comments Cornelissen.
One notable limitation of aqueous-based coatings is the lower maximum gloss rate compared to UV-based coatings, according to Cornelissen. “It’s important to note that this limitation is attributed to the current available coatings and not directly related to the technology used in the Contiweb Variable Coater.”
Finding the Right Solution
When selecting what coating solution to use, it is important to take full advantage of the capabilities of digital printing, which requires proactively managing a complex set of variables including substrates, primers, inks, overprint varnishes (OPVs), and application methods. “Commercial printers benefit from working with suppliers with a deep bench of digital printing application experts who can help meet the requirements of each unique job, including providing guidance on selecting the most appropriate primers and OPVs,” shares Denies. When applying OPVs, commercial printers should make sure that the chemistries of the pre-optimized substrate are compatible with the chemistry of the OPV. “Working with knowledgeable suppliers can help you manage matching substrate chemistry with OPV chemistry,” says Denies.
Customer end use, customer requirements, and in-house coating application/digital printing capabilities are all essential considerations when choosing a coating solution, concludes Kedenburg. dps
Mar2024, DPS Magazine
