Above: Rick Ferreira, of GS Incorporated in Rhode Island, with the Esko-Graphics CDI Spark digital plate imager |
Only recently have changes come about in the way plates are produced, but those changes have been significant. Though gradual in its acceptance, the digital method of plate production is moving forward. Flexographic plate production today is more exacting than ever, and the resulting printed products have catapulted flexo into the competitive arena once occupied by letterpress, offset and gravure.
The conventional process by which flexographic plates are made involves production of a piece of film containing the image to be printed. The film is placed atop a piece of photopolymer in an exposure unit, which contains a bank of lamps (or, recently, one lamp). The ultraviolet frequency in the light passes through the clear sections of the film and causes a chemical reaction in the plate material, which then bonds it into the image that later will be transferred by ink to the substrate. After exposure, the timing of which is determined by the image and the plate itself, the exposed plate is placed into a machine which then washes out the material from the unexposed portion of the plate.
Solvent wash plate processors still dominate the narrow web market. These utilize a specific bath of solvents, which later must be carefully disposed of. After the unwanted material is scrubbed out of the plate, the finished product must dry thoroughly in air. Drying time can be an hour, possibly more.
DuPont developed a system that uses water, plus a non-solvent detergent, to replace the solvent wash, but the system took years to bring to market successfully. It was operational in the 1990s, and several companies market the aqueous processors today. They are in use among narrow web printers today, though not in large numbers. “The water wash system still has its niche,” says Laurie Bryant, industry and product manager for Macdermid Printing Solutions, Atlanta. “It’s especially popular with envelope users. The plates are very durable, with a high durometer, and they carry a ton of ink. Those who are interested in the water wash systems usually have graphics that are not that demanding.”
Creo ThermaFlex 2630 |
When it comes to making plates, flexographers are ever in pursuit of high exposure latitudes. A high exposure latitude lets the prepress person expose the plate and hold image detail without compromising the exposure traits, such as filling in shadows and reverses. In the days before improvements in plates and exposure units, operators would mask (cover) parts of film and plate surfaces during exposure to alter the effect of the lamps. Though this practice still occurs, higher and wider exposure latitudes make it less necessary.
One light
Traditional plate processing units feature a bank of long fluorescent tubes for exposure. Two years ago, Cortron, of Fridley, MN, introduced its eXact plate exposure system, which utilizes only one lamp. The Cortron eXact 3040 (which holds plates up to 30" x 40"), which has been growing in popularity, is soon to be joined by newer models with advanced features.
“The problem with fluorescent tubes is that they degrade in differing variations,” says Marc Fioravanti, Cortron’s vice president of sales and marketing. “One could be half the intensity of the one next to it, or three quarters. You’re never really sure of the quality of the exposure. A good shop will constantly test its lamps to find the spots where the light isn’t even. With our unit you don’t need to do that. Our one lamp produces equal light all over the area.” The Cortron exposure unit uses a fixed reflector, which is a bit larger than the plate area.
“Exposure time can be faster, up to 15 percent,” he adds. “The lamp lasts for 1,000 hours of real platemaking life, depending on how much power you have.”
Cortron is introducing a new development, a system called eXact Scanning Exposure. “We have taken the light source and put it in a reflector not much bigger than the light source. We hang the light about a foot from the plate and scan it across, moving in the same way an inkjet printer will print. It exposes a swath of the plate, steps over and repeats.” The company also is producing an ITR — “In The Round” — version, which exposes images on plates that are formed as sleeves.
Digital advantages
Digital platemaking is something far different from the conventional. It still requires light exposure (True computer-to-plate manufacturing, which employs laser engraving of the actual plate, has not penetrated the narrow web field), but the imagesetter and film output has been replaced by the digital film/plate substrate.
Esko-Graphics, based in Belgium, is a manufacturer of a digital plate imaging systems, particularly the Cyrel Digital Imager (CDI) Spark (25" x 30") and the Spark XT (35" x 47"). Ian Hole, director of market development , describes the plate imaging process: “Digital and conventional plates use the same polymer substrate, but the digital plate is coated with a thin layer of carbon compound (black). The plate is held by a clamp system and vacuum assist on a drum/cylinder that rotates. A laser, moving across the path of the drum surface in the axel direction, (black sensitive) then writes the data from a graphics file into this black layer. The process is called ablation, because the imaging is achieved by thermally burning away the carbon compound, producing ‘smoke’, which is evacuated away by suction into a filter system.”
The resulting plate “enables a lot of benefits to the people who are using the plate,” says Jonathan Agger, packaging segment marketing manager for Creo Americas, Billerica, MA. Creo’s product for the narrow web market is the ThermoFlex 2630 (26" x 30").
Cortron’s eXact ITR unit |
“It allows greater control over the imaging by and large. The nature of the laser we use and the characteristics of the laser itself allow for very uniform dots that you can’t get from a photographic process. When you expose film to light by imaging, it still requires an amount of the photographic process to occur that will distort the dot itself, and it will affect some more than others, depending on the nature of the dots. During each of the steps in the process — exposure processing and subsequent re-exposure — there is the introduction of a degree of distortion.
“When we image directly through a controlled laser,” Agger continues, “we can get very precise screening patterns with very high predictability, allowing us to compensate for characteristics that are germane to the plate materials themselves, or to the press itself.”
In its digital imager, Creo utilizes what it terms MaxTone, a combination of amplitude modulation (AM: changing the size of the dots to adjust the amount of ink that will be applied) and frequency modulation (FM: keeping dot size uniform but producing more of them). “For example,” says Agger, “we can use 90 percent conventional screening in the midtones, but in the highlight areas we will image the minimum size dot but have fewer of them. If the minimum is a 9 percent dot, what we’ll do is image 9 percent dots but apply a random pattern to remove those dots as we get into the lower size dots. We hold the size of the dot we know will carry the ink, but reduce the frequency of them to simulate those lower percentages. It’s not unique to us, but it is unique from the standpoint that the laser doing the imaging has the ability to render those dots accurately.” This Agger refers to as “the first next step.”
The “next next step”, he says, is called HyperFlex: “If we know that a very small dot is going to collapse, we make a dot that might collapse under its own weight and raise the base for those dots so there is support and structure to them. If, for example, 9 percent is the minimum dot we can render on press, we can go to a 4 percent dot, and when combined with MaxTone we can actually image 1 percent dots on press.” That, he adds, will enable the flexographer to eliminate traditional flexo challenges, such as breaks in vignettes and smoother transitions.
Digital plates are then exposed to light to produce the image in the plate, but avoid the washing process altogether. The DuPont Cyrel FAST system eliminates the wet process and produces a plate much faster. (See Photopolymer Plates, page 46.)
Advantages of digital plate over the conventional kind carry over into the press room beyond the improvement in the actual image, says Ian Hole. “Converters vary in their appreciation of the differences between the two types of plates, mainly due to whether they are running older or newer presses. There is less substrate wastage (paper/carton board) due to faster press set-up time; less ink usage due to running a more controlled job to match the run; less press downtime due to faster set-up and less stoppage for plate cleaning on the run; faster press speeds due to cleaner, open reverses in plate shadow image areas; less press downtime due to match-to-proof achievement time increase.”
“Our objective,” says Agger, “is to turn the process of platemaking into more of a manufacturing process, to eliminate the steps and variability that do not contribute to an actual end product. We’re trying to mechanize this process as much as possible.”