By Olivia Cahoon
Part 1 of 2
Coatings offer several advantages for digitally printing to corrugated packaging, including better adhesion, a smoother surface, and special finishes. Pre-coatings are often used for adhesion purposes and improving print quality while post-coatings provide special effects and durability.
Improving the Print Process
When printing to corrugated board, a coating helps improve the print process. Depending on the digital ink and substrate—coated or uncoated—coatings manage ink absorption and adhesion.
While a coating helps create a uniform and smooth surface for printing, it specifically enables digital printing onto corrugated packaging by providing receptivity to digital inks while adhering to the substrate, says Suhas Chavannavar, market segment manager, printing and packaging, BASF Corporation. “It improves the appearance of graphics or the durability of the print by protecting against abrasion, chemicals, oil, grease, and water.”
Ink types for digitally printing corrugated material include water based and UV-curable. From a coating perspective, each digital ink approach poses different challenges, which also varies depending on the substrate, explains Marcos Valdezate, product manager, SunEvo Digital Coatings, Sun Chemical.
Ink behavior is not the same when printing on highly absorbent uncoated substrates compared to printing on glossy coated boards. While uncoated substrates help with water absorption, Valdezate admits they do not stop the pigment from diving into the substrate—resulting in low image density print outs. On the other hand, inkjet inks tend to sit on top of coated papers rather than penetrating the surface. “Primers are an effective way to manage ink absorption on both coated and uncoated substrates,” he adds.
The Pre-Printing Technique
Pre-coatings and primers are used before digital printing, typically to improve print quality. The type of pre-coating or primer depends on the substrate and digital ink.
“They help provide a smooth surface for printing by leveling out the substrate,” suggests Chavannavar. This improves ink vibrancy and receptivity, which are later reflected in the corrugated packaging after printing.
Primers not only provide ink receptivity but also adhesion. In the case of UV-cured inks, Chavannavar says primers prevent inks from penetrating the fibers, thereby remaining uncured after the drying process. Because adhesion can be challenging, he suggests a primer with two-way compatibility to provide adhesion and receptivity.
Another important primer requirement is the ability to manage dot gain. According to Chavnnavar, the proper primer ensures the dot gain is controlled and doesn’t spread excessively on relatively more porous substrates such as corrugated board.
Before selecting a primer, the substrate and digital ink need to be considered. For substrates, Chavannavar says thickness, composition, recycled content, and whether or not it is fluted will determine the correct primer.
For ink, solvent, water, or energy cured are options. Typically, corrugated packaging uses water-based or UV-cured. UV cured inks require a primer to prevent the ink from penetrating the fibers and from not drying completely. “The same concern doesn’t exist with water-based inks and these can be printed directly onto the corrugated packaging,” offers Chavannavar.
Water-based technology is common in primers as it enhances ink adhesion and overall durability. According to Kelly Lawrence, global marketing manager, digital print, The Lubrizol Corporation, A Berkshire Hathaway Company, printer manufacturers often work with chemistry and substrate suppliers to develop optimized print systems for a given end-use application.
The Post-Printing Technique
Post coatings and overprint varnishes give digitally printed corrugated packaging special effects while also protecting the graphic.
Post-coatings and overprint varnishes may be water-, solvent-, or UV-cured. Regardless of type, Lawrence suggests each be optimized to perform to the desired end-use requirements and enhance the overall print systems. “Application method can impact the aesthetics, the product resistance and environmental storage resistance, and the total cost of the finished packaging.”
High-end, bright white corrugated sheets are commonly for display printing or point of purchase signage. In this market, coatings accomplish high gloss, dry rub gloss for erasure notices, a matte or satin appearance to minimize reflection, and a pearlescent or sparkled appearance to draw attention, says John P. Vogel, president, Chesapeake Digital Consultants, Alliance Technology Corporation consultant.
For liner board coatings, a top coating with good gloss, excellent wet rub, and fast drying qualities is necessary for the secondary process of attaching the liner board to the corrugated board, explains Vogel. “This process is highly specialized and requires very special coatings.”
Water-based and UV post-coatings provide ink protection and a COF—or gloss—to the finished products. “Coatings may also help to protect the printed surface in post-print processing,” says Jim Wegerner, director national accounts, INX International Ink Co.
He suggests selecting the right combination of coating application weight, drying capability, and desired finished appearance to successfully use a post coating. Coating roll stock that is corrugated later also needs to meet the corrugation process’ demands. “Coatings are designed for the ink type, coater used, application parameters, and finished properties. No one coating works for all.”
A major challenge for top coating corrugated board is that the coating equipment’s pressure may show the fluting. “If the board has been scored for its glue flaps, the coating may take on a very thick appearance at the spot where the scoring has taken place,” warns Vogel. Therefore, it’s typically best to coat inline or offline before the scoring process to protect from this effect.
Additionally, if surfaces are too porous the coating may also feature a strike-through special effect. “The ink will look glossy and the board will have a matte look,” says Vogel. Unless that is the desired final appearance, he suggests learning about the stock, print, and coating interaction prior to manufacturing.
Pre- and post-coatings enable digital printing onto corrugated materials. Depending on the digital ink and substrate, coatings offer a variety of advantages for packaging such as ink absorption and adhesion. Part two of this series provides a selection of available coatings used in corrugated packaging applications.
Oct2018, Industrial Print Magazine