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Closed-loop color-control update

Apr 1, 2002 12:00 AM


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The first new generation closed-loop color-control system was introduced at Drupa 95. Since then, an estimated 332 systems have been sold for web presses. About 62 percent are retrofits; the remaining 38 percent are installed on new presses. Publication printers account for 49 percent of the total installations, followed by commercial printers (29 percent). (See “Closed-loop color today,” on p. 36.)

A closed-loop system scans and measures a color bar on the moving web, continually feeding data to the press ink-control console for ink-film thickness compensation. In addition to maintaining consistent color quality, these systems provide an efficient means of real-time data collection for statistical process control and customer quality assurance.

Closed-loop color-control systems help control a process with built-in variation. Continual adjustment is required to maintain color on web — press operators can't control factors such as plate and blanket wear, changes in paper and ink, lint buildup and temperature fluctuations. Other variables, such as ink-fountain levels, fountain solution pH, ink-train temperature, roller stripes and durometer, blanket height and tension, can be controlled but aren't always, due to poor maintenance or inadequate systems or training.

While closed-loop color is not cheap — an installation costs about $200,000 per web — some pundits claim its benefits far outweigh the expense. Potential closed-loop color-system benefits include:

  • More consistent quality
  • Reduced crewing/role changes
  • Reduced makeready waste and time
  • Increased customer satisfaction
  • Reduced ink consumption
  • Reduced ink/water chasing
  • Printing using SWOP, GRACoL, etc.

CLOSED-LOOP SYSTEM OPTIONS

Currently, four vendors offer closed-loop color technology for web presses: Graphics Microsystems Inc. (GMI) (Sunnyvale, CA), Perretta Graphics Corp. (Poughkeepsie, NY), QTI (Sussex, WI) and Web Printing Controls (WPC) (Lake Barrington, IL.). Perretta, QTI and WPC offer online densitometer-based CCD video-camera systems; GMI offers a spectrophotometer-based system.

  • GMI's ColorQuick features a true spectrophotometer for color measurements and density conforming to ISO/ANSI Status-T and CIE L*a*b* standards. It measures color gamut, density, dot gain and contrast, and uses a video camera for location and inspection. ColorQuick also offers a segmented blade and preset ink system.

  • Perretta's Dynascan offers video-based densitometry. The company claims 20 percent of the color patch provides accurate data; accurate density readings can be produced before registration. Dynascan offers a segmented blade and preset ink system. The company is reportedly beta-testing a closed-loop register system.

  • QTI's Color Control System (CCS) incorporates video-based densitometry. QTI has a partnership with Switzerland-based System Brunner to provide three-color balance and midtone control and analysis of proof, plate and printing. The company also offers closed-loop register and compensation controls.

  • WPC's closed-loop color control offers video-based densitometry available with one camera for register and color control. It measures and reports on print contrast, dot gain and ink trapping. WPC also can supply segmented ink blades and offer separate closed-loop register and compensation controls.

These systems measure and control solid-ink density — some also handle color gamut (GMI), as well as contrast, dot gain and three-color gray balance (QTI). This is a huge step forward for color consistency through automatic process control and improved operator knowledge.

Combining these systems with register and compensator control moves printers closer to full-process control. Using a single camera and color bar will further reduce the cost and complexity of these systems. Full press process control, combined with signature automation, significantly reduce labor and facilitate re-engineering of pressroom manning.

WHAT CLOSED-LOOP SYSTEMS CAN'T FIX

While a closed-loop color-control system can dramatically boost printers' productivity, other factors must be considered, including ink-train response time and an individual operation's overall color-managed workflow. Corrections made on press produce lots of waste — depending on ink coverage, it may take 400 to 1,000-plus cutoffs for a web-offset ink train to respond. This slow response limits the variables for which a closed-loop color system can compensate: A closed-loop color system can't correct doubles, slurs or gap streaks caused by bad bearings or bearers. Maintenance is essential.

A primary objective of a color-managed workflow is to minimize startup waste by matching customer color requirements with the minimum number of adjustments on a running press. Essential ingredients include color-managed proofs that match press conditions, layouts that can be reproduced on press and prepress allowances for web fanout.

Major components of a color-managed workflow include:

  • Content and graphics creation — digitizing the process is faster, cheaper, more accurate and flexible

  • Prepress (preflight, separations, digital proofs adjusted for printing conditions, imposition and CTP) — CTP is more accurate than film and includes algorithms that compensate for web fanout.

  • Pressroom — CIP3/4 for color presets, inks that meet color standards, closed-loop register and color controls.

Closed-loop color systems are especially attractive to printers with high makeready waste caused by lack of process control or inexperienced press operators. Printers with complicated web paths or frequent folder changes also are good closed-loop color candidates. Since press operators don't have to spend as much time on color issues, they can concentrate on web control and folder adjustments. One cautionary note: While closed-loop color-control systems can reduce makeready waste, you can't ignore other waste factors, such as setting folders or ribbon control, which may exceed the time to set color.

Preset systems, when fine-tuned and coupled with good color and process management, can average 2,000 to 2,500 cutoffs for makereadies. Where these results are achieved, the incremental benefit of reducing makeready cutoffs may be small. MAN Roland (Westmont, IL) and Komori (Rolling Meadows, IL) are said to offer 1,000 to 1,500-cutoff makereadies with algorithms to reduce inker response times.

SAVE ON INK, LABOR

In addition to saving paper, closed-loop color-control systems may help printers save ink. Many press operators tend to add ink for higher contrast and densities. Printing “by the numbers,” whether with closed-loop color or offline densitometry, gives operators greater control of the process. The result is better-quality jobs with less ink consumption.

Labor savings is the key benefit of closed-loop color vs. an offline system. Typical manning on an eight-unit press is one lead operator, two second press operators, plus joggers and a roll tender. With closed-loop color and register controls, manning generally includes a first and second press operator — one operator handles register and color for both webs. A three-shift operation that can reduce crewing by one operator will quickly recoup its closed-loop color investment.

Finally, closed-loop color-control systems may offer printers a quality advantage. Web-offset printers that can provide customers with reports that illustrate the range of density variation over an entire run are demonstrating a new level of professionalism, especially given the industry practice of furnishing “office copies.” This factual documentation is also valuable if a customer questions job quality.

Real-world experience

Makeready cutoff claims for closed-loop color-control systems range from 750 to 9,000. Learn how closed-loop systems have boosted productivity at Courier Graphics, Berlin Industries, Williamson Printing, California Offset Printers, Inc., Dome Printing, Sells Printing and St. Ives Co. See “Closing the color-control loop,” April 2001, p. 34,” or go to www.americanprinter.com.

Closed-loop color-control systems

Here is an overview of popular closed-loop color-control options:

Graphics Microsystem Inc.'s (GMI) (Sunnyvale, CA) ColorQuick combines a 2-mm-high color bar with built-in recognition features and a proprietary high-speed pattern-recognition feature. The spectrophotometer-based system features advanced optics, new CCD cameras and industrial vision software. Only a tiny reference mark — the size of a 120-lpi dot — reportedly is required. Once register targets are acquired, the system stays locked on because of the CCD camera's expanded field of view. Speed changes and tension variations don't induce errors or cause renewed searching.

Perretta's (Poughkeepsie, NY) DynaScan II is said to operate at any press speed, even those exceeding 3,000 fpm. DynaScan II continuously scans the web (both sides simultaneously), reading color bars with less than 2 mm wide. Its register control marks can be positioned within the color bar.

DynaScan automatically adjusts and records ink-key settings, provides SPC output to existing data-management networks, and maintains a comprehensive color-attribute database.

QTI's (Sussex, WI) Color Control System (CCS) first measures densities, brings them into a specified range and then maintains color throughout the run. QTI and System Brunner's video-densitometer-based CCS system provides closed-loop color control by tracking and maintaining solid ink density on press. System Brunner extends the CCS' abilities further by providing three-color gray balance and midtone control. Optional Instrument Flight software analyzes more than 30 process variables — including gray balance — that affect color. It calculates optimal inking recommendations, signaling CCS to adjust the ink keys to meet these parameters.

The system can track color bars during lateral and circumferential web upsets, read patches with minor defects, and handle a 1.6-mm color bar with full bleed up to both sides. A new feature, OptiGuard, reduces the detrimental effects of ink misting and paper dust on the camera lens.

Web Printing Control's (WPC) (Lake Barrington, IL) CLC system features one air-purged sensor per web to locate the color bar, understand targets, take measurements and issue simultaneous ink-key corrections. When printing begins, the CLC system automatically searches for and locates the color bar. The CLC system monitors the color bar and waits for inking to stabilize. Once inking has stabilized, the system issues corrections to each inking zone to bring the color density within predetermined tolerances. The system then waits for new ink levels to stabilize. If additional moves are necessary to bring inking into tolerance, the CLC system issues another correction and the cycle repeats. The system conforms to Status-T, ±2 d — the same rating as most handheld densitometers.

More pressroom advantages

Some printers that have installed closed-loop color-control systems are achieving 50 percent less cutoffs on makereadies.

Closed-loop color can:

  • ELIMINATE SUBJECTIVITY. No two operators see color the same way, but closed-loop color-control systems compare readings to a target density, ensuring consistent ink-key adjustments and color quality.

  • REDUCE VARIATIONS IN SETTINGS caused by different levels of operator skills as well as operators' tendency to play with keys during a run.

  • OVERCOME THE EFFECT OF “TIRED” EYES. After running a press for eight to 12 hours, press operators are desensitized to color changes.

  • REDUCE COLOR VARIATION between forms printed on different presses.

  • ALLOW ONE OPERATOR TO MANAGE color and register on a two-web press, since it eliminates the pulling of signatures and adjustment of color throughout the run, or the scanning of signatures, if offline densitometers are used.

  • ENABLE OPERATORS TO DEAL with color on an “exception” basis. Closed-loop systems alert operators when acceptable variation is exceeded. When operators know the system is struggling to maintain color, they can check water balance, in-roller problems, etc. Until then, they can concentrate on folders, tension and other performance issues.

Finally, closed-loop color-control systems generate automatic reports that provide feedback to:

  • Prepress for improving the color-management process
  • Material suppliers for measuring density, dot gain, etc.
  • Technical services and engineering for improving press settings
  • Maintenance and engineering for tracking changes in press condition.