by Cassandra Balentine
Digital print technologies are permeating the corrugated scene. However, corrugated materials and liners are notoriously difficult to print to depending on the print process.
Lloyd Kent, VP, sales and operations, Kento North America, admits that the variability of corrugated boards is an industry norm, but when it becomes extreme it can have a huge impact on the productivity of many single-pass machines.
“There are typically two main challenges with digitally printing corrugated board—first is quality of the liner and how well the ink performs. If the liner is very porous it can create challenges with digital print. The second challenge is warp on lower quality board,” offers Steve Lynn, director, labels and packaging, Durst North America.
Solutions
Digital print technologies advance to address common issues with printing to corrugated boards and liners.
For warp, Lynn says it is best to work with an equipment manufacturer with experience in handling corrugated board using feeders/stackers and material delivery to manage some warp. “There are options in ink technology and/or priming solutions that can help with print quality on lower grade liner,” shares Lynn.
Because most boards are warped to some degree, utilizing mechanical holddowns and maximizing vacuum efficiency are critical to productivity and to a quality print. “Material thickness variances from pallet to pallet can be detrimental to print quality. To prevent printhead strikes, the throw distance or ‘gap’ is increased. This increased gap causes print blurring and continuous print defects. This is especially noticeable when using higher resolution printheads—600 to 1,200 dpi—where the printhead to liner gap is minimal to allow for the precise dot placement necessary to produce the highest quality print,” explains Kent.
One option is to use a larger pixel size dot, which Kent explains has more mass and provides stability when the printhead gap needs to be increased due to board deficiencies.
“Kento’s approach is to provide a true corrugated machine with a digital print section. This allows the Kento Hybrid to feed and transport boards as they have been for decades. For added protection, productivity, and print quality, we can utilize mechanical holddowns, and a patented process in our vacuum system to ensure the boards are held flat through the digital print section. In the event a board is out of tolerance, the Kento Hybrid’s sensing system determines whether to stop the machine or raise the print bars to allow the defective board to pass through without stopping production. In the early days of single-pass, digital print machine stoppages killed productivity,” attests Kent.
Further, Kento’s open ink system allows for many ink and primer options. “Most ink suppliers match their ink to their primer. This normally gives the best adhesion on coated and uncoated and also allows for optimal ink ‘flow’ on coated materials, giving the best print results. Some machines are known to have ink adherence problems so it is important to test your preferred materials with any machine or ink and primer supplier you are considering,” suggests Kent.
“All paper suppliers have their own recipes so how the ink adheres will vary to some degree, more so on coated liners where absorption is minimal,” shares Kent. “Any approved supplier of ink and primer for the Kento Hybrid must pass rigid testing prior to installing ink in the Kento Hybrid. Our testing includes adhesion, abrasion protection, and print quality.”
Lynn doesn’t feel that ink adhesion is a problem on corrugated board. “Historically for inkjet, it is not a problem to get adhesion to paper-based products as the material surface is receptive to inkjet print.”
Lynn explains that pretreatments are used on corrugated board, but not generally for adhesion. “It is typical to seal highly porous paper liner to allow ink to stay on top of the board and create a good looking image. Without a primer on lower grade board or direct to kraft, the ink can bleed into the liner and create a dull appearance.”
Kent adds that almost every digital printer has a priming station or system with most of these being flood coating systems. “Anilox roll systems are used on some machines and some machines have primer applied utilizing printheads.”
For the highest productivity, Lynn feels that it is best to apply any pretreatment inline.
Necessity or Not
Pretreatments are not always necessary when digitally printing to corrugated materials, since almost every machine supplier has inline priming. “Primers are not usually needed on the Kento Hybrid on uncoated boards. Primers are almost always needed on coated boards,” shares Kent.
Lynn adds that pretreatments are not always necessary with Durst inks, “due to some clever ink technology, however we do see some inkjet systems in the market that require 100 percent prime.”
Kent feels that pretreatment from the mill might be a solution in some cases, but with so many variables throughout the entire corrugation process priming on a single-pass printer will almost always still be necessary. “Typically, the expense for true ‘pretreated’ liners is costly and many times very difficult to get. With digital production being such a small part of the corrugated industry output it is difficult for paper suppliers to justify making costly enhancements for a small amount of production.”
Ink Advancements
Advancements in digital ink chemistry make a difference in the need for the pretreatment of corrugated boards prior to digital printing.
Durst has developed inks over time designed to perform well on corrugated board without a need for priming, while maintaining good color and ink performance. “We do have a digital priming option that can be used for special effects like spot prime to enhance an area of print, or it can be used to prime very low-grade liner to maintain quality and color,” shares Lynn.
Factors that affect the need to use primer on uncoated are usually related to how the ink cures, how much absorption happens prior to curing, and the desired print look or finish. “At Kento, we have found that without primer digital ink cures and has a ‘flexographic-like’ matte finish. Using primer on many uncoated liners can promote a ‘sparkly’ effect throughout the printed area. This can be especially noticeable on lower quality liners such as mottled white,” adds Kent.
For high porosity liners like brown, Kent says the Kento Hybrid can utilize white flexographic ink to act as a barrier, which gives a more robust print than the machine printing purely digital on brown. “We have also found that the ink’s vibrancy, or saturated color on brown is much higher than our customers expected, allowing them to offer a more premium brown box than their competitors can offer while being more competitive.”
Corrugate Considerations
Advancements in primers and ink chemistries support better performance in the growing digital print space for corrugate materials.
Nov2024, DPS Magazine
