Removing a key barrier to 'fixed palette' print success
Whatever name you know it by – fixed palette, Expanded Color Gamut (ECG), extended process, N-color, or something else; the objective is the same: To simulate spot colors using screen tint builds of 4 or more colors (e.g. CMYK + OGV) to provide more choice to brand owners while increasing production throughput and reducing costs.
Replacing spot colors with process builds has been possible for more than 20 years; however, the print process has struggled to provide the stable foundation necessary to maintain consistent color throughout a production run with the required level of repeatability to meet the needs and wishes of brand owners and retailers.
The market pressures of shorter runs have more recently generated renewed interest in the potential such techniques provide, primarily with label, packaging, and folding carton printers in both offset and flexo markets. However, there are still some significant barriers to achieving the required success, most notably:
- Halftone screening
- Controlled ink transfer
The problems with halftone screening
The conventional AM halftone screening, as well as hybrids such as XM, used by most printers, has several inherent problems which limit the likelihood of success with ECG printing:
- Screen angles / moiré / angle clashes
- Rosettes and color shifts
- Large mid-tone dots
- Grainy or noisy flat colors
To eliminate visible screening moiré, the angles of the halftone screen for each color must be 30° apart; this is only possible for three screens in a CMYK ink set. One of the colors, typically the Yellow, will always produce moiré. With yellow, moiré is not as visible to the naked eye because it is so light in color. The problem arises when there are more than four process colors used to build screen tints: as there are no more screen angles available to use, this can result in moiré ruining the integrity of the simulated spot color.
A bi-product of screen angles is a rosette: a high-frequency moiré pattern. Rosettes have a frequency, an lpi, which is half the lpi of the screen, for instance; a 200 lpi AM/XM screen has rosettes with an lpi of 100. An lpi of 100 is low enough to be easily visible and results in a grainy (or noisy) patch, instead of rendering a smooth flat color build. Additionally, rosettes are affected by minor shifts in registration causing color shifts in both tone and hue.
A further characteristic of conventional screening is that the halftone dots vary in size through the tone scale. This variation results in variable dot gain on press, solid ink densities fluctuate over the course of a print run, which in turn, creates variable tone and color in the presswork. These variations happen with regular presswork but become increasingly difficult to control when additional colors are used, and so it is not ideal for simulating spot colors.
FM screening – a possible solution?
FM screening is often proposed as an alternative to AM/XM screening for ECG printing. While FM screening does eliminate some of the problems associated with AM/XM, such as screen angles and rosettes, it comes with its own set of drawbacks: Implementing FM screening requires an almost laboratory standard of plate-making and printing environment to achieve satisfactory results on press.
FM can be challenging to use in an offset environment, but it’s extremely difficult, if not impossible, to implement in a flexo one. For many printers, the size of the dots, as well as the patterns created in FM, can contribute to an unacceptable grainy, noisy, appearance in flat screen tint builds. Overprinting of FM screens can also result in secondary patterns which make some flat screen tint builds look blotchy in color and tone instead of being smooth and even.
DMS – going beyond FM screening to deliver fixed palette printing success
Superficially, Digitally Modulated Screening (DMS) (Auraia DMS for offset and Bellissima DMS for flexo) might appear to be just another form of FM screening because, in common with FM screening, DMS eliminates screen angles and rosettes, and the dot sizes are uniformly distributed in size through the tone scale.
However, looks can be deceiving! DMS is carefully matched to the plate-making and press conditions, precisely controlling the dots in each color and between the colors. DMS achieves the high-level of image detail associated with FM stochastic screening while producing ultra-smooth, noise-free, flat tints which are as smooth as, or smoother than, AM/XM screening.
By maximizing the lithographic or flexographic plate and press properties of halftone screening, DMS dramatically enhances reproduction quality and stability, while offering ink savings over conventional technologies.
Digitally Modulated Screening is a catalyst for change in the flexographic printing industry; changing the expectations a printer can have related to the print reproduction quality they can achieve, the restrictions caused by problems with moiré, misregistration, rosette drift, color shifts, banding, dot gain, dot loss, shadow loss, etc. DMS removes the critical barriers to success with fixed palette and conventional printing.
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