Get Set 2-Up 4-Up

Computer-to-plate technology has made tremendous inroads, simplifying the way we produce plates. Aside from newspaper printing, where film-based exposure is still the norm, platesetters dominate all major categories of prepress for books, commercial and publication work.

The last arena for CTP adoption is among smaller operations, where firms still using analog-based camera platemakers can choose from a host of offerings, including desktop lasersetters, integrated polyester platemaking systems, as well as small-format metal CTP in thermal energy and violet light formats.

With the shift to CTP, the quality control systems for making plates have also changed. CTP has reduced many plate problems, eliminating (along with the film) numerous variables in the exposure and finishing process. So how do people approach plate quality control in a real-world setting? When we talked to our peers, we found discrepancies between manufacturers' recommended practices and what actually occurs in the trenches.

For example, suppliers' standard operating procedure is to linearize platesetters. The idea is that a 50% dot in the file should equal a 50% dot delivered by the plate when it runs on press. Because dots on presses swell, i.e., “gain” in size, a curve is created to make the original dot smaller to compensate for this gain.

The linearization process provides a baseline for press conditions. Humidity, temperature, packaging and other variables will also impact press gain, so it makes sense to have a known starting point that you can always get back to, no matter what other variables may affect the printing process. But Pat Berger, co-owner of Mercer Color in Coldwater, OH, never follows that procedure.

“Why would you anyway?” asks Berger. “It's usually an exercise in futility. Once you know what dot the setter will put on the plate without any curves at your exposure setting, that is all you need to know.” Berger believes that because of the inherent consistency of the overall CTP process, controlling the plate emulsion is a better way to keep his system in line. “We order plates 1,000 at a time with all the same emulsion number,” he says. How does he get a handle on plate quality? “Use common sense on exposure settings, and completely ignore what the manufacturer suggests.”

Mind you, we would not advise ignoring manufacturers' suggestions. And Berger's view is not widely held. Doing so could void warranties, to say nothing of making troubleshooting a challenge should problems arise.

But even manufacturers will note that CTP plate quality control can become needlessly complicated—if procedures are added that have no appreciable effect on the outcome.

“Over-diagnosis can compound problems that can morph into unnecessary downtime, and that hurts production,” notes Ken Parker, VP engineering for Xanté, a manufacturer of small-format metal and laser plate systems. He says the firm's Z7 Technology simplifies the quality control process, guiding users through a calibration process at a Web interface. Its “More X-Act” technology corrects for dimensional inaccuracies introduced in the process of manufacturing gears, motors, drums and rollers. Browser-based software uses the data from measuring targets imaged on the plates with a supplied ruler. The software computes corrections horizontal and vertical alignment, and also makes corrections for skewing.

Most of the printers we talked with do indeed believe the first step in the quality control process is to “linearize” the platesetter. The linearization process is essentially to confirm the stability of the overall process and provide a baseline for establishing press conditions. But even more important than this starting point are establishing the consistency of the quality control process itself—and verifying the stability of the CTP system.

Thankfully, these days, the basic system quality and consistency across the market is excellent. John Brink, prepress manager of LT Litho, Colorado Springs, CO, uses Kodak Sword plates imaged with a Kodak Trendsetter, ripped using a Xitron Xenith Xtreme. Plates are read with an Xrite/GretagMacbeth ICPlate 2. “The plates are near perfect without linearization straight from the Trendsetter,” says Brink. “But we do linearize them with the screen sets on the RIP.”

Many targets are used to measure grayscale; but it is simple enough to change the results of those measurements by altering exposure times or laser strength. Instead of addressing what might be changing in the process, exposure is continuously tweaked, to the point where plates “suddenly are not running long enough,” notes Franz Sigg, research associate, Test Targets at the Rochester (NY) Institute of Technology. “Manufacturers basically sell a whole package including targets and instructions, and generally they work well. But some of these are not really optimal,” he says.

Sigg and his fellow RIT plate specialists have developed several targets for plate quality control. His favorite is a deceptively simple checkerboard. The system images a checkerboard pattern and other target components on the gripper edge as part of the basic information on every plate. Using this test pattern, says Sigg, the operator can see any potential problems—the checkerboard pattern will fill in with fairly minor fluctuations in the process. Users of very high-resolution imaging systems don't even need a measurement tool to read image fluctuations. (See sidebar on next page.)

The essence of QC in CTP is consistency. “What we are truly concerned with is repeatability,” Sigg says. RIT produces a number of targets to evaluate tone reproduction evenness, registration, calibration, process control and plate life control. (For more, contact Sigg at fxsppr@rit.edu.)

Holding even tone reproduction is a challenge for plant manager Bill Chegwidden of Arrow Printing, Salinas KS. “The biggest problem we have is balancing the 16 laser zones across the plate. They must be constantly monitored if you are doing continuous tone blends or flat tints.” The only way to do that, he says, is “run a test plate and visually inspect the tints,” he says. Arrow uses a Presstek 800 DI platesetter.

There can be a significant difference in handling, disposal and water treatment in different CTP systems. And those factors can also affect the quality control process because filtering, cleaning and other maintenance can have an impact on quality as well as cost. It should also be noted a most common reason for plate failure is skipping required maintenance.

There is also a quality control issue peculiar to some no-process CTP systems. While “process-free” plates such as Presstek's Anthem or Heidelberg's Saphira non-ablative thermal plates have sufficient contrast to be read with a densitometer, those that “develop on press,” like Kodak Thermal Direct or Fujifilm Brillia Pro-T, don't provide a latent image of sufficient strength to be read by densitometers. They must be inked up to be evaluated. But users we spoke to don't consider that a huge issue. The lower cost (due to no processing) of these plates is a primary selling point. As long as there is enough visible image to tell the pressman which color they are mounting, most do not consider it a problem. But for those uncomfortable mounting plates without first reading the image, a potential solution is noted below.

Manufacturers of no-process plates suggest maintaining proper production controls will avoid potential problems at press time. David Maestas, Kodak national technology manager, notes one of the most common problems associated with Kodak non-process plates “can be attributed to the plate seeing excessive white light prior to press roll up or not enough pre-damp revolutions on the press start up.”

Generally, though, risk found in using plates without visually analyzing to make sure they have received proper exposure, is balanced by the elimination of chemical and mechanical processing variables. With fewer variables in the system, the argument goes, comes more consistent output. That's a point of widespread agreement among users on the front lines.

Other issues that can arise in CTP environments include chemical contamination or expiration (both the developer and finisher), and laser malfunction or failure. Such problems may sound easier to avoid than they are in practice. Chemistry and laser fall-off can occur in stages and be difficult to detect without quality control verification tools. Heidelberg says its systemservice 36 can monitor platesetters remotely, to warn in advance of imaging diode degradation or impending failure.

Plate readers come in several forms, such as Tobias PlateCheck and X-Rite's iC Plate II and PlateScope. Readers range from handheld units to plate scanning systems that not only evaluate plate quality but convert data into press ink-key presets. Newer versions offer automated calibration and spectral selection for easier operation.

In some respects, one can say evaluation remains part of the quality control process: Conventional plates get evaluated before press; no-process plates as press sheets.

There is a plate reader for low-visibility plates before they are put on press. The Techkon SpectroPlate from GripDigital uses two spectrally balanced lights to eliminate errors common in older technologies. Embedded imaging standards (from Europe's Fogra Graphic Technology Research Assn.) are set into the portable plate microscope to assure ISO compliance for all AM & FM plate imaging calibrations. “We are the only company approved by Fujifilm Graphic Systems to read the invisible plates,” says GripDigital president Greg Imhoff. “Agfa's Azura and other plates are no problem.”

Among users we spoke with, most have quality control measures in place. The consensus seems to be that quality control is only an issue when things get out of control, which doesn't happen very often.

More important than the system used, however, is the ability of the employees in the plate department to understand and consistently evaluate the data the system makes available to them. That means keeping plate quality control simple and easy to evaluate and maintain.

ONLINE: www.ecrm.com, fujifilmgs.com, graphics.kodak.com, gripdigital.com, mitsubishiimaging.com, presstek.com, us.heidelberg.com, xante.com, xitron.com, xrite.com and fogra.org