Screens are Screaming for Attention

Innovative screening technologies are providing package printers with new tools to improve graphic quality.

CREATIVE PROBLEM SOLVING is a beautiful thing. When technical process limitations are overcome resulting in tangible quality and productivity improvements, it truly is rewarding for both the developers of the solutions and the people that use them.

In the case of packaging screening technologies, suppliers have been hard at work improving and developing innovative techniques and technologies that provide real improvements for package printers and their customers. These solutions have made positive impacts in both offset and flexographic printing applications. And for the most part, the solutions are applicable to both computer-to-film and computer-to-plate systems.

A brief overview

Four primary variables are manipulated in the screening technologies that are used to produce the dot patterns formed on offset and flexo printing plates. These are dot size, dot frequency (number of dots in a given area), dot formation, and most recently, dot shape.

Conventional AM (amplitude modulated) screening changes the dot size through the entire tonal range, while maintaining a fixed center spacing on the dots, denoting the lineature. A smaller overall dot size can provide a higher line per inch (lpi) screen, while lower lpi screens use larger dot sizes. The choice of which to use is determined by several printing and substrate factors. AM screening generally provides good reproduction in the mid-tones, but runs into problems in the highlights (below 10 percent) and shadows (above 90 percent), especially in flexo.

FM (frequency modulated) screening (also called random or stochastic) uses a small dot that remains constant in size. Color density is achieved by randomly varying the spacing of the dots to achieve a different number of dots in a given area. This method can improve highlights and shadows, and eliminates moires and rosettes that can result from the set line-pattern of AM screening. In the mid-tone areas, however, reproductions can appear grainy.

Recent developments have come from modifying the shape of the dots themselves. These include Esko-Graphics' Groovy Screens, that form variable lines or grooves; Creo's SQUAREspot™ that essentially "squares off" the round dot to provide a better fit with the digital pixel; and NuDot™ from Phototype that uses three different and unique dot patterns in different segments of the tonal range.

Working with each of these screening methods is not mutually exclusive and recent product innovations have come from combination approaches to get the best out of each technique. Let's take a look at some of the recent developments screening-technology suppliers have to offer.

Agfa (Wilmington, Mass.) can lay claim to development of the first FM screening product in the early 1990s, called :CristalRaster. The objective of this new approach was to address various drawbacks with existing AM screening technology. According to Frances Cicogna, Agfa's product marketing manager for packaging, the new FM technology was very good at delivering fine detail and addressed other AM screening limitations, but it also tended to appear grainy or sandy, especially in areas of flat tints. "Over the course of the next decade," says Cicogna, "both technologies had their best fit—FM for greater detail and AM for smoother flat tints."

Agfa's answer to this tradeoff was introduced this year. :Sublima for Packaging takes a hybrid approach to capture the strengths of each method and is available for both offset and flexo printing. It uses FM techniques in the extreme highlights and shadows, and uses AM screening throughout the majority of the mid-tonal range.

Cicogna points out that all the dots in :Sublima are aligned along the established AM paths, even in the FM portions. This is done to eliminate "crossover noise or technology intersections that were found in earlier hybrid approaches."

With this technology, :Sublima can achieve line screens up to 340 lpi, and by "taking press characteristics into account, it will never produce a dot that the press can't hold," says Cicogna. Furthermore, he states that flexo line screens can be significantly improved without having to make changes in the anilox roll. "A converter, for example, printing an 85 line screen can increase resolution to 120-plus line screen without changing out the anilox roll."

Creo (Billerica, Mass.) offered its first FM screening technology some years ago, introducing Staccato in 1998 for offset printing using a 25-micron dot size. Its benefits included fine image detail, improved color registration, larger color gamut, and greater ink mileage.

The use of Creo's SQUAREspot thermal imaging technology with Staccato now provides the capability to image and print with a 10-micron dot. According to Creo, this is the equivalent to a 1 per cent dot from a 418-line AM screen. SQUAREspot is designed to consistently produce the dot shape and size on the plate, and can reduce the grainy appearance of reproductions from FM screens.

Creo has also recently introduced several screening and plate-performance-enhancement products that address some of the problems stemming from the flexible plates that are inherent in flexographic printing. These issues are especially evident in flexo's ability to resolve microdots in the highlights and shadows, says Jonathan Agger, packaging segment manager for Creo. These new offerings include Maxtone™ and HyperFlex™.

Maxtone is a hybrid screen that uses a high-lpi AM screen throughout roughly 80 percent of the main part of the tonal range. Below 10 percent and above 90 percent, very small dots are avoided by using FM techniques to vary the number of dots in these areas. "Maxtone allows the operator to set a minimum dot size in order to prevent the formation of dots that are too small for the flexo medium," says Agger. Creo applies imaging algorithms to smooth the transition between the AM and FM interfaces.

One of Creo's plate-enhancement technologies, HyperFlex, is designed to improve the quality of Maxtone screening by enabling smaller dots to be formed and held throughout the flexo printing process. This is achieved by creating a thicker, firmer foundation for a given dot size, and effectively allows an increased resolution of the flexo plate and improves reproductions in highlight areas. Agger also says that with Maxtone and HyperFlex, a printer can hold print quality at higher speeds.

Different dots

A brand new technology brought out by Esko-Graphics (Vandalia, Ohio), called Groovy Screens, is designed to improve the performance of flexo printing in the shadow areas of the screen. This screening technique uses variable lines or grooves—at operator-chosen angles and starting at any part of the tonal range—through to a point determined as "solid", where the maximum amount of ink density can be carried, retaining a smooth, even ink laydown. "The transition between the normal screen pattern and the line pattern is smooth, leading to a smooth gradation on print between the lower density of the non-groovy print (highlights and mid-tones) to the higher density (shadows) of the groovy print," he says.

One benefit of Groovy Screens includes improved solid rendering due to a more even ink layer. Tests have shown that solids of 1.1 density without grooves can increase to 1.3 density.

Harrell also says that larger ink volumes can be accommodated with Groovy Screens, which could allow presses to be run at higher speeds due to better ink transfer. Although Groovy Screens have their best performance on foils, the technology can also print well on paper and other metallic substrates.

Phototype's (Cincinnati, Ohio) NuDot™ screening technology is unique in that it incorporates three different dot shapes throughout the range of tonal values. According to Marketing Director Chris Deye, a standard round dot is used from about 1 to 6 percent. From this point through the mid-tones, the shape of the dot resembles a cross with arrowheads on the end of each arm. In the shadow areas, the dot shape changes to a honeycomb-like structure.

The theory behind this technology is to "exploit the natural rivering and beading characteristics of liquid flexo inks on high-holdout [non-porous] materials," says Deye. "The net effect is that ink is deposited more uniformly throughout the entire tonal range; dot gain is more consistent; and less plate impression is required to reach desirable densities. Because a printer doesn't have to 'overpress' the plates, better densities, tonal range, and highlights can be achieved.

"When press profile information is available for a specific ink/substrate/anilox combination, the separations and plates can be optimized for even greater quality improvements. Density increases as high as 20 percent have been verified, with the tonal range expanded by as much as 18 percent. The visual results of flexo printing with this level of ink control are such that color saturation is higher and process imagery is more vibrant and realistic," says Deye.

Other bottom line results can be seen in both improved makeready times and faster press speeds. One plant using NuDot technology reportedly increased press speed from 525 to 820 fpm.

NuDot is commercially available for computer-to-film applications, but Deye reports that Digital NuDot (for CtP) is in beta testing and is expected to be available in the first or second quarter of 2004.

These screening technologies, and others that are available, have the potential to make significant improvements in quality and productivity, especially for flexographic printing. However, this discussion has only touched on the highlights, and there is much more that must be investigated relative to each technology's impact on print quality and process robustness.

By: Tom Polischuk