Understanding the challenge of achieving high opaque white in flexography requires the printer getting the best balance and performance from many variables, including the anilox, ink, doctor blade, tape, plates and substrates.
- Doing so is essential to hitting the 80+ percent mark in opacity, like rotary screen
- Managing the surface tension of substrate at speed is a given, but the relationship between the anilox and doctor blade can be overlooked when trying to maintain a desired level of opacity and transfer of opaque inks
For narrow web UV inks, doctor blades between 0.008-in. and 0.012-in. in thickness will be needed to successfully meter these heavy volumes. Wide web ink systems typically require 0.008-in. or 0.010-in. thick blades to achieve a similar effect.
All photos courtesy of Sandon Global
The appropriate anilox engraving specification, however, will influence these measurements. Combined trials are recommended to determine the best engraving design and doctor blade partnership to achieve the required level of opacity. Set your target and ensure you have a calibrated system in place to measure opacity accurately. Taking your time to test and measure results will save countless press hours in the long run.
As innovation in flexographic print technology moves forward, with press speeds increasing and smarter inks, advancements in plate and prepress are likewise occurring at a stratospheric pace. Hybrid anilox geometry and next-generation doctor blades continue to improve ink delivery. It is always the basics that make the real difference. Controlling white ink on a substrate is no exception. In many ways, it is the most critical.
You can have the best expanded gamut (EG) workflow, using the most innovative DM screening; however, if the white ink does not have enough opacity and good laydown, then you have fallen at the first fence.
It is estimated in flexible packaging, a minimum 50 percent of a printer’s ink spend is purely on white ink. Now with the increasing price of titanium oxide, it is more important than ever to use what you have got to best effect.
When discussing white ink, you cannot look at opacity alone. Some printers use a “second hit” of white ink to mask the problem. However they don’t increase the opacity, just fill the pinholes. Using a second deck that provides 50 percent more ink than a single print deck also brings the challenge of drying it fast enough. Ultimately, it means slowing the press down to do so. And, time is money.
Sandon Global has created two engravings to overcome this issue. First is the High Opacity White (HOW) for narrow web customers and the High-Volume Process (HVP) for flexible packaging and pre/postprint markets.
Label KnowHOW
The HOW engraving was developed in partnership with a plate, ink and tape manufacturer with the aim to achieve a rotary screen-quality white.
Screen printing whites is a slow process, especially on long printruns or for quick turnaround work. Cleaning can be a major issue if not handled in a timely fashion, which was a key consideration when developing the solution.
In flexography, the standard engraving style for depositing heavy amounts of ink is a conventional 30-degree engraving. The HOW cell structure outperforms the format and is specifically designed in order to deal with the highly viscous nature of UV white ink, used widely by label and narrow web printers.
Pinholing and reticulation on the surface of the substrate are often issues, particularly when printers attempt to deposit large amounts of ink using conventional engravings. The flowing nature of the HOW engraving allows difficult UV white ink to evacuate from the cells with ease, allowing a smooth lay of ink across the printing plate—thus, removing the inherent issues of pinholing that we have come to associate with printing heavy white designs.
Furthermore, the cell structure of the HOW engraving (which is protected) retains a good control element, which allows the ink to displace itself. The key benefits experienced by narrow web and label printers are reported to include screen-quality opaque white, successfully maintaining press speeds and reducing operational costs. Interestingly, this engraving has evolved to offer a range of designs to suit a variety of finishes as driven by the printers.
Versatility & Performance
In the world of wide web flexibles and corrugated board printing, the challenges are slightly different in that versatility and performance go hand in hand. To achieve this, an elongated cell design allows increased print latitude in comparison to a conventional specification.
Indeed, the Sandon Global HVP engraving evacuates more ink, which helps eliminate pinholing issues and reduces ink starvation, maintaining good optical density at up to 1,968.5 fpm—a key requirement when printing on absorbent substrates, particularly in the corrugated market.
The quality of the laydown is far improved by the increased line counts utilized by HVP. Increasing the number of cells in a square inch creates a greater resolution, as the ink releases from the cells in a more controlled manner, leading to less reticulation and an improved all over laydown. Also, a key characteristic here is the ink collection. With an elongated cell, the correct cell-to-depth ratio has more opportunity to collect as well as transfer ink to the plate.
This factor of depositing more ink, but with greater control, is why HVP has proven to be suitable for both white ink and combination printing. Therefore, wear resistance is never far away from a printer’s mind.
Now let’s discuss the link between anilox quality and longevity, and doctor blade settings and materials. Given that flexographic plate screens are matched to the needed graphics and anilox rollers are matched to plate screens, why not match doctor blades to anilox roller linescreens?
It just makes sense to make the connection across all parts. Many printers still regard the doctor blade as a consumable only, and not an important tool for high-quality print. Printers tend to struggle with the overall price versus quality of doctor blades.
When the topic of white inks is brought up in a conversation, there is never a shortage of issues to discuss. No. 1 on that list is typically press downtime—the costliest of all. White ink performance is directly related to how aggressively it wears doctor blades, end seals and anilox rollers. All of these are very intricate parts of keeping the press running.
What’s that worth to you per hour? If you can alleviate two press stops a week, you can more than cover the cost of a specialty blade for the white inks. The ink savings alone will more than cover blade cost.
Longer-life blades for white inks typically have a single- or multi-layered hardened coating applied to the doctor blade tip, whether it be radius or rounded, lamella or bevel edge. The coating helps protect the integrity of the steel for improved blade performance and extended durability, when compared to traditional carbon steel materials.
The coating also increases lubrication and offers much lower friction values. The second part of decision making is what tip will work for the application. A round or radius tip blade is most often the choice, but always consult with your doctor blade supplier and do monitored, controlled data collecting trials to confirm the proper blade tip configuration for your press.
Always take into consideration the anilox linescreen and volume you want to use to hit the targeted opacity. The lower the linescreen and bigger volumes always wear blades at a faster rate. Combine in the aggressive nature of white inks from the Titanium Dioxide (TiO2) and you have the components to lower blade life and cause problems. Work within these parameters and you will never again walk away with your fingers crossed and hoping this setup performs well, you will know it is going to perform great.
The final polishing stage of the manufacturing process is paramount to achieving an extended life to both anilox volume and doctor blade. The HVP engraving is proving to outlast conventional 60-degree geometry on wear tests, due to the enhanced release characteristics of the tri-axial ratios—staying cleaner for longer.
Other benefits of this approach to anilox cell design include:
- Improved solids
- Reduced pinholing
- Improved lay
- Greater release characteristics helping keep print cleaner on the run
- An increased lifespan on wear against conventional 60-degree engravings
- Easier to clean
Housekeeping
Often overlooked when the white ink topic is discussed are cleaning systems, all of which have good and bad points. However, you must consider this when working with any ink system. White is an aggressive product and over time can damage tooling, including both anilox rollers and doctor blades, as it naturally acts as a cutting compound. To keep the anilox in the best condition, a strict cleaning regime is also key to trouble-free print.
Ink transfer is more consistent if you are printing with a full cell, not a partial cell. Repeatability is fundamental to print across all sectors. We can all print great once; the question is: “Can you do it again and again?” Cleaning systems are becoming more of an automated function, rather than a manual task, giving time back to the printer.
Many aspects of printing white ink need to be considered. Clearly the anilox is a fundamental component to achieving better print. It is the relationship and balance achieved by the printer between so many variables that makes print a highly skilled industry.
If technology can help printers manage that balance consistently, day-in and day-out, then anilox and doctor blade suppliers alike will have contributed to our ever-improving standards.
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