The Offset Report

By Kerry Pianoforte, Ink World Associate Editor | 09.12.05

Valued for its near-photographic quality, stochastic screening is becoming more widely used throughout the industry.

Although stochastic screening has been around for more than a decade, the largest growth appears to have occurred quite recently. Stochastic uses a special screening technology to create images that look more like photographs than printing. Normally in printing, the printer takes a finite number of dots and varies their size in order to create the illusion of images. In stochastic screening, all the dots are the same size, but they are much smaller and the number of dots is increased significantly.

According to Flint Ink, stochastic screening sparked a lot of interest when it was first introduced. “It was frequently associated with waterless offset since both technologies allowed for more photographic quality reproduction,” said a spokesperson for Flint Ink. “With recent improvements in digital technology, stochastic printing is undergoing a renaissance of interest, but has yet to gain wide use.”

“The concept of stochastic screening has been around more than 10 years. Only recently has there been a convergence of technology that has made the process more viable as a printing solution,” said Les Watkins, sheetfed product manager, Kohl & Madden. “First of all,” he said, “the complex algorithms that randomize the dots require a lot of computing power and sophisticated software to produce the screened images. Secondly, the process did not work well with systems using hard copy, films and plates because this frequently created uncontrolled dot gain. There was simply too much variability in the conventional process in terms of film and plate exposures, registration and vacuum problems during exposures. The introduction of computer-to-plate (CTP) systems eliminated most of these problems and allowed sharp enough dots that could make stochastic screening work well in the pressroom.”

“Because of this, stochastic screening is beginning to catch on,” added Don Sierzega, manager of Sun Chemical’s web offset national lab. He estimates that 50 percent of U.S. heatset printers have used it or experimented with it for at least some of their printed work.

Stochastic screening is frequency modulated (FM) screening, as opposed to amplitude modulated (AM) screening.

“We are seeing most major high quality printers move quickly to this process,” said Joseph Cichon, senior vice president, product and manufacturing technology, INX International Ink Company. “They continue to use AM screening, but virtually every large or high-end printer is now using or exploring stochastic screening.

“Stochastic screening has been around for well over 10 years, but computer power, costs and storage limitations have limited its growth,” said Mr. Cichon. “I think that the Y2K issue we faced a few years ago forced a lot of people to prematurely replace and upgrade computer systems, which in turn accelerated the cost reduction of computing power. In turn, this became an enabler for stochastic screening processes. We have seen recent shifts of large printing business from one printer to another simply based on the ability to produce printed material using stochastic screening. But even beyond that, it is important to use the new stochastic screening the correct way (proper process profiling and systems approach).”

Stochastic screening is currently being used in a wide variety of applications. Because of its ability to reproduce images with crisp, near-photographic quality, this process is gaining popularity for use in high-quality brochures and catalogs.

According to Flint Ink, stochastic is widely used in ink jet printing as well as offset and flexo. In both cases, the benefit is more photographic reproduction and a reduction in the potential for moiré patterns, especially when five, six or seven colors are used. Flint Ink reported that some newspaper printers have also seen improvements in quality by using stochastic screening.

“Stochastic screening can be used on a wide variety of applications from publications to commercial to packaging on offset presses,” said Attillio Paolilo, director of sheetfed and web offset sales and marketing, Sun Chemical Canada. “The process also is being used for packaging applications on flexo presses. The primary benefit of this type of revolutionary screening is enhanced resolution, including smoother tone transitions and better detail rendering,” he said. He added that the difference between FM and AM screening can be especially apparent when using flatbed scanning or lower quality substrates such as newsprint. The early adopters have included large retailers who use stochastic screens in their ads, as well as long-run magazines.

Mr. Paolilo said stochastic screens can produce fine details in electronics, fabrics and clothing, smooth flesh tones for glamour shots and a transition of tones that more closely resemble a photographic print.

“Currently the largest application area seems to be for high quality brochures and catalogs, but the usage is expanding rapidly,” said Mr. Cichon. “One of the key advantages to stochastic screening is its ability to reproduce and hold minute details in very small printed areas of diagrams or photographs. There is a shift towards stochastic screening for greeting cards as well.”

Controlling Dot Size
Inks that possess good flow are needed when using stochastic screening. Consistent ink lay down as indicated by stable curves is the most important factor in choosing the ink.

“Inks must provide good dot formation,” said David Sussman, marketing communication manager, Creo. “Dot gain is not an issue to be avoided, but it must be stabilized. The integrity of the dot shape is important with staccato, or any small dot printing. The higher perimeter to area ratio of the dots means that consistency in dot gain is important “In general, conventional sheetfed inks will perform well with stochastic screening, but we are finding certain characteristics such as roller stability and water emulsion characteristics that can be fine-tuned to assure stability and quality of dots if 10 micron resolution is being used,” noted Mr. Cichon.

INX is in the process of releasing a new sheetfed products called Vision Plus for sheetfed printing that has been designed around testing with stochastic screening. INX also recently introduced its ECOPure system, which shows excellent performance in stochastic screening according to the company.

Mr. Sierzega explained that inks being used to print stochastic screens must help control dot gain and prevent piling on the plates. Because the dots used in FM screening are so tiny, he said, even the slightest growth in size can throw off tone reproduction curves, reducing print contrast and hurting detail in both the highlight and three-quarter tones of an image. These inks must help the printer maintain good ink-water balance.

Sun Chemical has recently introduced two inks specifically designed for use with stochastic screening: High End for sheeted printing and a series called High Fashion FM for heatset web printing, which has formulas for magazines and cover stocks. For flexo package printing, Sun Chemical reports a high level of success when its Flexomax ink series is used with stochastic screens.

“Sun Chemical is a pioneer in releasing inks specially formulated to improve printing with stochastic screens using proprietary technology that builds on our experience in pressrooms around the world that are using this screening process,” said Mr. Watkins.

“Some printers have said they can run the same inks for FM or AM screens,” Mr. Sierzega said. “However, after researching and gaining experience in pressrooms using stochastic screening, we found there are things we can do better to optimize the process. We’ve adjusted the ink properties and components to improve the printing process.”

“When printers began to use stochastic screening, they found that they sometimes had problems achieving gray balance and controlling dot gain, and piling was becoming a recurring problem,” Mr. Sierzega said. “It appeared the piling was being caused by thinner ink films that do not provide enough protection to the plates. This resulted in frequent stops to wash the plates, creating more waste in the printing process.”

“As more publishers and consumer product companies want to use stochastic screening to improve the look of their publications or packages, the demand is increasing,” said Mr. Watkins. “Most printers are trying to make this change without changing their inks. They may be able to produce satisfactory results with standard inks, but if they use inks formulated and optimized for this type of screen, they can produce brilliant printed images that rival the best printing. They might not recognize this until they’ve seen the results themselves.”

One of the primary benefits of stochastic screening is that the use of many very small dots can create a fine-grained appearance more like a continuous-tone photograph than a conventional halftone, Mr. Cichon explained. “This is to some extent true, particularly in comparison with halftone screening at 150 lpi or less. A printer using conventional 150 lpi halftone screens who converts to 20-µ stochastic screening will encounter a significant overall improvement in apparent image resolution.

“In reality, 20µ stochastic is not directly comparable to 150 lpi halftone screening. The variable nature of stochastic dot positioning will necessarily place more dots in a given area than traditional screening; more dots equals more data, which in turn equals more detail. The dot density of 20µ to 25µ stochastic is nearly equivalent to the dot density created with traditional screening grids at 175 lpi to 200 lpi. It is therefore more valid to compare the image detail, that is, the perceived quality, of a 20µ or even 25µ stochastic screen with that of the same image reproduced at 175 lpi or 200 lpi.”

He said that in addition to providing, under some circumstances, a more detailed image, stochastic screening effectively eliminates most of the common causes of moiré, especially moiré between the different colors of a separation.

“Another advantage that is being discovered is an extended color gamut,” said Mr. Cichon. “Stochastic screening does seem to allow colors to be more cleanly printed with a higher chroma value in the tint areas.”

As more companies recognize the benefits of this type of process it will become even more widely used.

“These new markets are just now being discovered, but with the very high resolution of this type of printing and near to continuous tone appearance, you may expect to see new detail printing for specifications or possibly artwork reproductions,” said Mr. Cichon.

Printing with Small Dots - Dot Gain and Ink Choices
The entire purpose for the halftone dot is to move ink onto a substrate. The industry is moving towards finer line screens and smaller dots in order to benefit from increased detail and fidelity. It is important to note that screening patterns, be they AM or stochastic, do not dictate change in the press room. It is the size of the dot transferring the ink that matters.

Staccato screens range in size from 10- 40 microns or in AM equivalents 600 - 240 lpi. Ink needs to transfer through these small dots onto the various surfaces, plates, blankets and substrate.

The confusion surrounding printing with these smaller dots often stems (directly or indirectly) from an attempt to control dot gain. Inks have been modified over the years to do so, such as by increasing pigment loads or increasing tack. These measures are only effective within limited parameters. To address dot gain issues when making large changes in the press room (such as increasing line screen or decreasing dot size dramatically), workflow changes are needed - calibration curves that lay down a smaller dot to pre-compensate for anticipated dot gain.

The goal in printing ought not to be a reduction in dot gain, but a stabilization of it. Among other process control measures in the press room, this must include good ink practices - good ink laydown on both the solids and the halftone dots. Dot gain is not a problem as long as it is consistent. It is easier and more effective to compensate in the workflow for dot gain that it is to reformulate an ink.

There are other effects of small dot structures. Small dot structures have a greater number of exposed edges (perimeter), in relation to their area. Aggressive fountain solutions can have a greater impact on these small dot features (when compared to larger dot features) due to this greater edge to area ratio.

Small dots also use ink more efficiently. By dispersing the ink over many small dots, a great portion of the light is filtered through ink, rather than through surrounding substrate. This results in truer colors and an expanded midtone gamut. It also leads to a slower rate of replenishment which can have a chemical effect on ink/water balance.

Ink vendors do an excellent job of helping their customers navigate the complex chemical and physical processes involved in offset printing.

- Information provided by Creo

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