June 4, 2019
Thanks to their lightweight nature, folding cartons are witnessing an increased customer demand globally. This means quality assurance is not only important for the physical carton, but also for the final package print.
To address this growth and meet demand, printers are looking at multisite printing operations. In such a multisite operation, a critical challenge comes into play: print consistency.
Ready to set about tackling the issue and gaining control? Let’s examine the criteria to ensure a uniform print, as well as the variables in the print package allowing a printer to gain this consistency. Outlined here are key areas to watch for to ensure you are on the right track for folding cartons.
Design Considerations
Consistency is the key word here, meaning work closely with your designers and suppliers to develop good, repeatable artwork that can be printed to your specifications.
Some things to consider with artwork for folding cartons include:
- Keeping ink coverage to a minimum to keep screens open and print clean.
- Watching for moiré in screening more than CMYK. Some designs tend to have many colors that overlap screens and won’t be visible until print reproduction.
- Reviewing all components within the artwork to ensure they meet the specifications set by the printer (minimum dots, blends, images, text, bar codes, etc.).
- Combo plates (images with large solids with minimum dots all on the same plate) that can be difficult to reproduce, and special screening may need to be applied to the flexo plate.
- The Flexographic Technical Association’s (FTA) Flexographic Image Reproduction Specifications & Tolerances (FIRST) 6.0 has a very comprehensive and detailed design guide you can download for free and follow for artwork elements. Download it at www.flexography.org/first.
Measuring is key! Have a system in place to manage and monitor print density, dot gain, ink pH, and ink viscosity.
Screening
Screening of the plate image has drastically changed over the years, and there are now many different options. Currently:
- The most widely used screening in folding carton is circular, in which the dots maintain a round/circular shape in the entire halftone.
- Most folding carton printers are running between 120 lpi and 150 lpi for solid bleached sulphate (SBS) board.
- Hybrid screening is also a common way to keep the highlight dots from having a harsh tone break when transitioning off to zero (white).
- In the last few years, companies have been testing and using flat-top dot technology, along with screening modifications to the surface of the plate, to create improved surface ink transfer to the paper. This has been widely used in flexible packaging with film substrates, but not as widely used in printing on paper substrates.
- Surface screening utilizes higher-resolution images (high-definition imaging at greater than 4,000 ppi) to create the patterns on the surface of the plate. You will need to test and validate surface modification techniques based on your anilox and ink system.
Plate Materials
Selecting the best possible plate for paper substrates is a never-ending discussion among printers. It’s important to understand that paper materials have varied ink absorption, coatings, and textures. It is critical to monitor substrates throughout the press run. Pay close attention to the amount of ink carried to the plate and the amount of latitude you have when impressing the plate to the substrate.
For folding carton paper substrates:
- There is a decent amount of absorption needed for the paper stock to impress a good, solid image.
- A medium durometer (58-65 Shore A) plate works very well.
- Plate thickness will generally be determined by initial plate undercut of the cylinder/sleeve. The most common plate thicknesses in the folding carton segment are between 0.045 inches and 0.067 inches.
- Photopolymer plate relief specifications will vary based on substrate, plate thickness, and type of material.
- FTA recommends a relief thickness between 0.018 inches and 0.022 inches for folding carton printing on SBS.
- It is important to work closely with your plate manufacturers for the tolerances recommended and to use what works best in your environment for the specific type of plate.
Newer plate materials with flat-top dot technology are now in use to get a more even surface area, to increase ink density, and to improve ink transfer. This is being driven primarily by increased press speeds and demand for higher line screen graphics.
Plate Processing and Storage
Plate imaging is a critical process. For optimal registration and consistency, it’s best to image all plates in the same direction on a digital imager. Optimize your plate room to enhance your exposure, processing, handling, cleaning, and storage variables. Prolonged exposure to UV light or the environment can lead to poor plate longevity, higher dot gain, and decreased density.
When it comes to storage, plates should be cleaned with a compatible plate cleaner, dried properly, and light-protected after each press run. Storing plates in an area with high UV nonfiltered lighting will reduce shelf life and reusability.
Work closely with your designers and suppliers to develop good, repeatable artwork that can be printed to your specifications.
Sleeves
The majority of the folding carton business requires a larger repeat sleeve and sleeve width. It is critical that print sleeves be consistent in TIR (total indicated runout) and have even surfaces. Your sleeve supplier can provide you with a certificate of analysis. Also, if you see a print void in the sleeve, it could have a low spot, which may indicate the sleeve has been damaged. Most damage is visible, but some can hide inside the sleeve.
Print sleeves for folding cartons come in various materials, including fiberglass composite, urethane, and nickel (thin). There are also cushion sleeves available for special applications.
Sleeve manufacturers have been working to reduce the overall weight of larger sleeves by improving raw materials in order to make them more durable and lighter. In addition, there are sleeve-handling systems to assist operators and virtually eliminate handling. Consult with your sleeve manufacturers to determine what type of sleeve will work best for your facility.
Mounting Tape
When selecting an optimal cushion/plate combination, you will want to use the firmest tape possible that offers the least amount of impression, without influencing dot gain and print slur.
This will enhance plate longevity and prevent failure on long press runs. In addition, it is important to optimize the cylinder and plate-size adhesion to prevent plates from lifting on edges or allowing air bubbles to form during the run. These factors are all very common in folding carton printing.
Inks
Water-based inks are more common in folding carton printing; viscosity and pH are key to consistency on paper, along with print sequence.
Print sequence is generally dark to light, improving the wet trapping of the yellow ink. Automatic ink pH/viscosity control systems work to improve management during runs. Inks also need to be filtered due to paper dust residue getting into the ink system.
Coarse papers can damage the plate dot surface throughout the print run, so check periodically. Some printers struggle with static discharge from paperboard. There are systems to vacuum and reduce static prior to print impression, and they can keep ink cleaner during a run.
Quality Control
Most importantly, measure! Measuring is key! Have a system in place to manage and monitor print density, dot gain, ink pH, and ink viscosity. It’s always a good idea to keep these records in a system that can be referenced for quality control for each item you’ve run, as well as the history if there are multiple runs.
Quality control can also be moved upstream into the platemaking areas to include laser ablation checks on your digital imager, as well as final plate inspection using a plate analyzer.
In closing, I hope these key areas identified will help you keep on the right track for folding cartons. I understand not all the information here may be fully applicable in your environment, but it might help you see opportunities that could be applied to your processes.
Editor’s note: This article originally ran in the October 2018 issue of FLEXO magazine, the FTA’s flagship member publication, and has been edited to fit BoxScore’s style. It represents the first in a series designed to expand technical coverage via partnership between FTA and AICC and their respective magazines. Collaboration between the membership of both organizations is encouraged.
Greg Wishon is a technical service consultant at DuPont Advanced Printing.
