Increasing Flexo Shadow and Solid Ink Density with High-Line Screens By Malcolm G. Keif

Transcription

1 Increasing Flexo Shadow and Solid Ink Density with High-Line Screens By Malcolm G. Keif Flexography is the predominant printing method for decorating packaging substrates. As a printing method, flexography has seen tremendous quality improvements in the past two or three decades. This ``rubber-stamp`` process is capable of printing on a widerange of substrates from corrugated liner to label stock, to films and foils. Advancements in ink metering systems predominantly anilox roll technology combined with improved plate, ink and press systems, have made high-quality flexo printing not only possible, but also common-place. What was once known as a low-quality printing process is now seen as comparable, and in some cases, superior to offset and gravure. The push to print photographic imagery on packaging substrates has required flexo suppliers to improve their contribution to the process and improve, they have. Significant advancements have been made on several fronts: ceramic anilox rolls offering more consistent ink deposit on plates; improved ink systems (water, solvent, and energycurable) with greater ink density and less dot gain; better tension and impression systems on press to print a smaller, more consist dot; improved polymers for plates, which are easily manufactured and offer good ink release; digital imaging systems improving highlight and shadow detail; the use of sleeve technology for faster changeovers; and improved doctoring systems to precisely meter ink films. Today, flexography is significantly better than it was 25 years ago. It is used to print beautiful full-color imagery on some of the best quality packages available. This increase in quality has inspired some to push the upper limits of halftone screen reproduction. Now, 150 line per inch (lpi) halftone screens are commonplace. High-end flexo printers in certain markets including high-end beverage, beauty, and pharmaceuticals are even printing 200 (or above) lpi halftones, requiring anilox rolls in the 1000 to 1200 line count range. Ceramic anilox roll manufacturing by laser can achieve these high-line counts with relative ease. A tiny, shallow cell is imaged into the ceramic roll, resulting in a cell count at or above the commonly accepted 4-to-1 ratio of cell count to screen lpi, even with fine-line halftones. The laser anilox engraving process creates a cup-shaped cell that releases ink more readily than older engraving methods. This provides benefit in getting the desired ink density. However, a high line-count anilox roll means reduced total cell volume. One can not make a cell with unlimited depth and expect good ink-release. As cell width is reduced to increasing cell count, its depth is likewise reduced. In fact, cell depth-to-width ratios are well-documented, typically in the neighborhood of 25%-30% for optimum ink release. One can not compensate for reducing cell volumes but simply increasing cell depth. As a cell s width is reduced (due to higher-line count), its depth must be reduced too. Lower cell volume means a thinner ink film delivered to the plate, compromising shadow and solid ink density. What can be done to increase shadow and solid ink density when printing high-line screens by flexography? Historically, two black plates have been used one for line work

2 using a high volume, low cell count anilox and one for tones with a high line anilox and low volume. Today, there are new strategies for combating poor shadow and solid ink density in flexo. Ink Pigmentation The use of highly-pigmented inks as well increasing ink laydown is critical for achieving high ink density. Ink pigmentation is increased by adding solid colorants to ink, notably pigment. As expected, increasing ink solids does change many of the working properties of the ink, particularly viscosity. When viscosity increases excessively, anilox starvation can occur, especially at high press speeds. Speeds are sacrificed in order to flood the anilox completely. Most ink manufacturers offer high-density ink lines and have been relatively successful with both water-based and solvent inks. Energy curable inks have become particularly popular in flexography offering a number of benefits. Both Ultra-violet (UV) and Electron-beam (EB) inks offer good ink holdout on the substrate and the ability to keep screens open, along with improved environmental emissions. However, one additional benefit of energy curable inks is their solid ink density. Energy curable inks contain 100% solids and the entire ink film remains after curing, maximizing ink density. Because all of the material applied to the substrate remains after curing, these inks achieve much higher density than conventional inks. UV and EB inks do have higher viscosities so care must be taken to ensure good running properties. However, a side benefit of high viscosity is reduced dot gain and therefore, more linear printing of color tones. Working with an ink partner will ensure a smooth transition to energy curable inks. Capped Plates The surface of a typical photopolymer plate is very smooth. When ink is deposited on this surface, it does not load well or disperse uniformly. The surface energy of the plate may hinder the ink from wetting out well on the plate s face. Color consistency and density suffer. This is problematic, particularly in solids, where density differences are particularly noticeable. The press operator is charged with the task of getting even impression on press, often over-impressing the plate to get good ink transfer in solid areas. Ink is ``squeezed`` during impression, compounding weak, non-uniform ink coverage, as the ink is squished across the substrate. Ink density is improved on plates with rough surfaces. Rubber plates are rougher than polymer and usually offer better ink transfer. But rubber technology is fading, mostly due to the hassles of making rubber plates. So how can one achieve the ink transfer of rubber with the convenience of photopolymer? Capped plates are one solution for reducing surface smoothness and have been effective in delivering more ink. Capped plates have a thin top layer on their surface. This surface results in a matte texture, which has an excellent affinity for ink. Offset plates are

3 ``grained`` to provide texture. So too, flexo plates are capped to give the ink some tooth to adhere to. Capped plates deliver a more uniform ink film. They are available for both film and computer-to-plate (CTP) workflows from a number of suppliers. Capped plates are considerably more expensive than traditional polymer plates. Solid Screening One of the best ways to increase ink density is to screen solids. By simply changing solid areas to 90% screen tints, the effective ink density can be enhanced. This practice has been demonstrated to make solid areas darker, and in some cases, may allow tones and solids to be printed on the same plate. Solid screening provides the greatest benefit with traditional photopolymer plate materials printing on very smooth substrates like films or coated papers. In these situations, improvements are seen by replacing solids with a high percent screen tint. The ink gathers on each dot, including small amounts on the edges of the dot. The ink is loaded better on the plate and more ink is released to the substrate. Density increases, as does ink uniformity. Because less impression is needed, halos around type can be improved. And in many cases, because the ink is applied more smoothly to the substrate, less ink is actually used. Screening solids is achieved by either placing a dot in solid areas at the RIP, or by using one of three commercially-available products. Individuals can screen solids without purchasing additional hardware or software by simply calibrating their film or plate imager to render 100% areas as some lower percentage screen tint. One can also manually select solid areas and tint them in their vector application. However, this may impact other image areas so care is needed. Easier yet, and more reliable, is to use one of three commercial solutions: Artwork Systems Plate Cell Patterning, Creo`s DigiCap, or Esko-Graphics Groovy Screens. Artwork Systems` Plate Cell Patterning Artwork Systems is a provider of prepress hardware and software for packaging workflow, with a strong presence in flexography. They have developed a system called Plate Cell Patterning where the user places a repeating pattern of small cells in the plate surface of solid printing areas. These cells serve to break the smoothness of the plate and function like an anilox roll carrying ink in the plate cells to the substrate. The ink film thickness transferred to the substrate is increased, resulting in greater ink density. Artwork Systems claims that solid and tones can often be combined on the same plate, reducing the number of plates and stations for a given job. Plate Cell Patterning requires Artworks Systems` NexusRIP screening engine. It may be used in workflows using film or computer-to-plate imaging. Users select the cell pattern of choice. ``During the screening process, NexusRIP embeds cell patterns to be imaged in solid areas and normal halftone screens in tonal areas. Plate cell patterns differ from

4 normal halftone screens. First, plate cell patterns have frequencies similar to anilox rollers 400 to 1000lpi. Second, plate cells are shaped like anilox or gravure cells, containing a land area and a cell area. The land areas prevent the cells from connecting,`` according to Artwork Systems material. The plate cells can be imaged from film, via a laser ablative mask system, or by direct engraving. Although ArtPro`s object screening allows plate cells to be applied in only parts of the job, most users apply the plate cells globally in solid areas. Recently, Artworks Systems introduced their Cell Center screening. Cell Center screening places a plate cell in traditional halftones, allowing the screen to be applied across the tone scale. The result is a darker and smoother image, particularly in shadow areas. For additional information on Plate Cell Patterning, visit Artwork Systems at: Creo`s DigiCap Creo focuses on workflow systems and CTP hardware for several markets, including packaging, commercial and newspaper markets. Creo is particularly strong in offset printing but has also developed a presence in flexography. Their solid screening solution involves simulating a capped polymer plate in solid portions of the image area. This is achieved by imaging micro-dots in the surface of the plate. The dots are so small that they don t create a noticeable pattern but simply give texture to the plate, increasing solid ink density. DigiCap requires Creo`s Prinergy Powerpack workflow. Unlike Plate Cell Patterning, DigiCap can be applied over the entire halftone range if desired. Instead of only applying the algorithm to the solid areas, mid tones can also be capped. DigiCap is a post Rip process, controlled through Powerpack. This only provides benefit if a company has numerous prescreened files to process (i.e. 1-bit TIFFs) beneficial for salvaging the screening of repeat jobs. For additional information about Creo`s PowerPack and DigiCap, visit: EN_PrinergyPowerPack.pdf or Esko-Graphics` Groovy Screens Esko-Graphics` solution involves placing grooves in the solid areas of a polymer plate. Groovy Screens uses a lined patterned rather than a dot pattern. This pattern helps to break the smoothness of the plate surface and also serves to carry more ink in the grooves. While designed for solids, Groovy Screens can be applied throughout the entire tone scale. Like the two other technologies, Groovy Screens offers improved density and contrast. Colors appear more saturated as density is improved.

5 Groovy Screens uses Esko-Graphics` FlexRIP engine and ScreenManager application. Combined with Esko`s IntelliCurve calibration application, the precise transition point where Groovy screens take effect is controlled. Highlights and midtones are typically untouched or may use alternate screening technologies. For additional information about Esko-Graphics FlexRIP and Groovy screens, visit: Conclusion Recent development work in flexo has significantly improved the printing process. Finer screens are being used and richer, more elaborate images are being produced. And the need for deep, dense solids remains a vital part of improving the package converting process. The options presented above offer proven solutions to increase flexo shadow and solid ink density when printing with high-line screens. The solid screening solutions provide significant benefits on certain stocks, notably films and foils. But before settling for any one particularly technique, the astute converter considers and tests the rewards expected from the chosen solution. Complete a thorough series of trials on typical substrates to ensure noticeable differences are observed. Through investigation and experimentation, solid ink density can be dramatically improved. Additional articles on this topic: