Removing a key barrier to ‘fixed palette’ print success
Whatever name you know it by – fixed palette, expanded gamut, extended process, N-color, and others – the concept is the same: reduce production costs while delivering more options for brand owners by simulating spot and brand colors using screen tint builds instead of custom blended inks.
It’s a print production method which has evolved and become more reliable and effective since its inception some 15 years ago. As a result, it has recently begun to gain wide interest – primarily with label, packaging, and folding carton printers – in both offset and flexo camps. However, a barrier to complete success has remained: halftone screening.
The problems with halftone screening
The conventional halftone screening – AM, and hybrids like XM – that most printers use has several inherent problems that can limit complete success with fixed palette printing.
The key problems with conventional screening from a fixed palette point of view are:
• Screen angles
• Large midtone dots
To eliminate visible screening moiré the angles of the halftone screen for each ink have to be 30° apart. That is only possible for three screens in a CMYK ink set. So one ink – typically the Yellow – always produces a moiré. The visibility of the moiré is reduced because the Yellow is so light in color. However, if more than four process colors are used to build screen tints there are no more screen angles available to use and the result can be moiré ruining the integrity of the spot color being simulated.
A byproduct of those screen angles is a rosette, basically a high-frequency moiré pattern. Rosettes have a frequency – an lpi – that’s half the lpi of the screen itself. Put another way, a 200 lpi AM/XM screen has rosettes with an lpi of 100 – low enough to be easily visible. So instead of rendering a smooth flat color build, the patch can appear grainy or noisy. Also, rosettes are affected by minor shifts in registration. The result is a color patch that shifts in both tone and hue.
In addition to screen angles and rosettes, a characteristic of conventional screening is that the halftone dots vary in size through the tone scale. That variation in dot size results in variable dot gain on press as solid ink densities naturally vary through the run. That, in turn, can result in variable tone and color in the presswork. That happens with normal presswork but becomes increasingly more difficult to control when more colors are added as with fixed palette printing – not ideal for simulating spot colors.
FM screening – a possible solution?
FM screening has been proposed as a solution to using AM/XM screening for fixed palette printing. Although it eliminates some of the problems associated with AM/XM screening, like screen angles and rosettes, it comes with its own set of drawbacks. Typically, implementing FM screening requires an almost laboratory plate-making and printing environment or the screening will produce less than satisfactory results. FM can not only be difficult in an offset environment, it’s virtually impossible to implement in a flexo one. For many, the dot patterns used in FM and their size can contribute to an unacceptable grainy, noisy, look in flat screen tint builds. And finally, the overprinting of FM screens can result in secondary patterns that make some flat screen tint builds look blotchy rather than even in color and tone.
DM screening – going beyond FM screening to deliver fixed palette printing success
Superficially DM (Digitally Modulated) screening – Auraia DMS for offset and Bellissima DMS for flexo – appears to be just another FM screen. Like FM screening it eliminates screen angles and rosettes. Its dot sizes are uniformly distributed in size through the tone scale. But looks can be deceiving. DM screening is custom tuned for the plate-making and press conditions. DM screening precisely controls not only the dots in each separation but also between the separations. It not only achieves the high-level of image detail long associated with FM stochastic screening, but also produces ultra-smooth noise-free flat tints that are as smooth as, or usually smoother than AM/XM screening. By maximising the lithographic plate and press properties of halftone screening, it dramatically enhances reproduction quality, stability, and gamut, while offering ink savings over conventional technologies.
DM screening represents a fundamental change in the expectation a printer should have on the reproduction quality of print that is achievable. No longer restricted by problems with moiré, misregistration, rosette drift, color shifts, banding, dot gain, dot loss, shadow loss, etc… a key barrier to success with fixed palette, as well as conventional, printing is removed.
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