As flexo technology has developed, there has been a concerted effort to print smaller highlight dots and achieve better solid ink densities. However, in order to compete head-on with offset and gravure printing it is the broadest color gamut, tonal range and image contrast that have often eluded flexo printers.
The quest for better ink transfer does not mean getting as much ink on the substrate as possible. Strong density and clean print yields a broader color gamut while too much density reduces the range of colors that can be reproduced and too much ink reduces the tonal range that can be printed. This diminishes contrast and muddies the appearance of halftone work. Therefore, the goal must be to print at target levels of high density or opacity while achieving the broadest color gamut and tonal range possible.
Implementing this successfully on press entails optimizing the entire mechanism for ink transfer, all the way from pulling the ink out of the anilox roller onto the flexo plate to transferring that ink effectively onto the substrate. Many factors influence this, including the shape and condition of the anilox cells; the plate material itself; the properties of the plate surface; ink and substrate dyne levels and environmental conditions.
Technology developments from a broad range of suppliers have contributed to refining and improving the process. Ink manufacturers have optimized pigment load, viscosity, pH and other properties of the ink. Anilox manufacturers offer a wide range of cell shapes and sizes, ceramic materials, and are constantly developing new lasers and engraving processes. New substrates and coatings have become available, and converters utilize flame and corona treating to improve ink laydown and adhesion.
The greatest contributor to improved ink transfer is flexographic plate technology. Plate suppliers offer solutions for plate surface texturization through imaging or mechanical means that are designed to spread ink out more efficiently rather than building up thicker ink films. Each approach brings improvements in ink density or opacity levels but they are not all equal in their ability to drive expanded color gamut, tonal range and contrast with maximum press throughput, or even in their ability to completely cover the substrate with ink.
10 pt text using Kodak’s Advanced Edge Definition technology to create a combination of ink barrier and optimized ink transfer.
Ink transfer problem
A magnified view of a solid black ink patch printed on clear film shows many gaps, pinholes or voids in the print. This reticulated look can be affected by dyne levels of the material and properties of the ink but it is primarily the result of the separation of the anilox cells by the cell walls. As the contents of the cells are deposited onto the plate surface, the ink is able to run in the machine direction due to the rotation of the press. The ink is not able to spread in the cross machine direction as easily unless print conditions are ideal. Typical actions to try and correct these issues are to add more ink volume and press harder on the plate.
This problem is solved through digitally applied micro surface texturization of the plate. The latest advanced plate patterning options include a set of six patterns with technology that plays a crucial role in controlling ink flow at the edge of objects and is the key to reducing impression.
This plate-patterning technology is digitally controlled and, unlike plates with a fixed textured surface, is employed in targeted areas and applied only where it is needed. The six patterns are expertly engineered to work with increasing anilox volumes and are selected based on the ink/ anilox/substrate/tape combination for optimum ink transfer. For process print, the finest surface patterns can be selected in accordance with the lower volume of the anilox roll. For spot colors, larger patterns can be selected where anilox volumes often carry twice the amount of ink as required for process work. There are larger patterns for whites, coatings and adhesive, which can use two to three times more volume.
With these technologies, plate users can create application-specific plates (for process print, spot colors, whites, adhesives, metallics, etc.), utilizing only one polymer type. These application-specific plates can be combined on one sheet (as size allows). This reduces the number of plate materials that have to be inventoried, eliminates the possibility of wrong plate type being used, and also reduces the process time.
These digitally imaged technologies utilize an ink carrying and application configuration to produce strong density in the solids and in text areas, and automatically apply a smaller pattern on the edges, to keep the ink only where it is desired. This helps keep print edges clear and sharp, and prevents reverse text and fine positive text from filling in while allowing trapped air to be released. The ink release properties, combined with a plate that has the appropriate flat top dot structure, allows for lighter impression pressure yielding finer highlights and enabling press operators to stop and clean the plates less often, reducing substrate waste and extending plate life.
This technology is unique in the industry and is the only solution that enables ink to be applied where it is needed and contained where it is not. The result is superior ink coverage with maximized color gamut and tonal range. The approach not only allows for the smoothest white ink foundation for vibrant overprinted colors, or for blocking product or substrate from showing through the print, but also opens up the mid-tones and ¾-tones to give halftones that realistic, continuous tone look that truly enhances shelf impact.
Without Advanced Edge Definition With Advanced Edge Definition
Effective ink-barrier technology requires advanced imaging and reproduction capabilities such as those utilized by KODAK FLEXCEL NX System ’16, which operates at 2400 dpi and creates each pixel in a 4×4 matrix, resulting in each pixel being comprised of effectively 16 laser spots. The ability of the imaging system to turn on or off these groups of laser spots selectively allows the accurate and precise creation of the micro-fine patterning features. A plate making process that enables 1:1 reproduction of the image file to the printing plate ensures that the imaged pattern is fully and accurately reproduced on the plate every time.
Effective ink transfer with minimal impression is the key to achieving offset and gravure-class flexo printing. This can only be achieved by utilizing a minimum amount of ink to completely cover the surface without filling in non-print areas or causing excessive dot gain.
The goal is to print strong, smooth colors on a solid foundation with whites that will block the product, or color of the substrate, so as not to detract from the graphics. It is essential to keep the color gamut and tonal range open to gain the maximum contrast and shelf impact.