- October 31, 2001, Teresa Koltzenburg, Senior Editor
CNF, MNF, CTP, LAMS — Flexography's imaging material acronyms are growing. For the proper fit, the methodology continuously tries on sleeves and much more.
Flexography is catching up (some say now is equal) to gravure and offset/litho quality. Increasingly, digital workflows (like computer-to-plate or CTP) are replacing conventional.
These are a couple of things we've been hearing for a few years. With groups like the High Definition Flexo Consortium (HDFC) voicing flexo's qualifications, and with the increasing CTP buzz, it sounds as if hearing is believing.
Flexo printers: If you want to keep up on flexo quality, then you'll want to hear what European flexo expert Martin White has to say about CTP (read below). In addition, you'll want to attend Ipex 2002 (in Birmingham, U.K.). Slated for April 9-17 at the National Exhibition Centre, the event will highlight flexography in Hall 19. For more Ipex information, visit ipex.org.
Sleeve It to CTP
Platemaking is at the heart of flexo quality, and Martin White (an apprenticed printer in his former life) has definite ideas about flexo platemaking's future. Some of it centers on sleeve technology and — what else? — digital technology (as well as the combo of the two). (See Table 1)
|•||Continuous photopolymer printing sleeves are (CPPS) expensive to produce|
|•||Digital seamless sleeves: coated with a laser-sensitive mask|
|•||Least sophisticated technology: “plate-on-sleeve”; tape-mounted plate to sleeve|
|•||All three sleeve types processed “in-the-round”|
While CTP has helped flexo in its quest for quality, flexo printers cannot let digital prepress costs price the methodology out of the market, White warns. A more expensive process at the onset, he says CTP's benefits (faster makeready and running times) offset the initial weighty investment. “This is one area of flexo prepress that, if reduced in price, would put it way above other processes in plate production costs.”
In the following, White provides us with a comparison and evaluation of the various imaging processes used in flexo platemaking.
CNF: Conventional Negative Film
CNF has been used for many years in photopolymer flexo plate production. Gradually, manufacturers have optimized the film. “Today,” reports White, “these materials have reached quite a high standard.” However, he adds, it's not likely there'll be any additional revolutionary innovation.
White believes the only further film advances will be in thermal film. Designed for fully dry processing, this material still is limited in its detail reproduction and resolution. But, he says, manufacturers report it should reach the standard of chemically developed films within the foreseeable future. (See Table 2)
|•||Vacuum utilized (film to raw photopolymer)|
|•||Imaging source: UV light|
|•||Emulsion side matted (avoids side-lighting effects caused by trapped air)|
MNF: Modified Negative Film
Compensating for dot gain seems to be the most serious problem in CNF flexo-plate imaging. “The negative's necessary tone-value reduction results in the loss of distinguishable values in highlight areas,” White explains. To solve this problem, modified negative film was developed.
The MNF-based process is assisted by the filmsetter's ability to manipulate the imaging-light intensity, White says. “It makes use of the slightly larger-sized laser spot.” (See Table 3)
|•||MNF target: to write dots as in CTP; keep tonal gradation range on plate wide|
|•||MNF imaging: transparent-area reduction, primarily affecting screen dots|
Mask-Based CTP (Or LAMS CTP)
Today, the CTP processes widely used in flexo platemaking utilize mask layers. The plate's photopolymer material is coated with an opaque, black top layer referred to as the “LAMS” (Laser Ablation Mask System) layer. (See Table 4)
|•||Thermal lasers perform microscopically fine engravings; layer is partially removed in platesetter|
|•||Separate second step: UV light polymerizes relief|
|•||LAMS CTP-imaged impression: “steeper shoulders and reverses” (compared to conventionally imaged plates)|
|•||Two-step process advantage: mask optimized to platesetter; photopolymer optimized for print characteristics|
LAMS CTP systems have been used successfully in flexo platemaking for several years, says White, adding that the disadvantage of a somewhat more complex raw material structure, combined with higher price, is balanced by considerable benefits.
“The tiniest halftone dots can be generated in the highlight areas, which is especially advantageous in flexo printing,” White points out. In fact, he says, this methodology paved CTP's way into flexo printing.
Direct Laser Engraving
Direct laser engraving images with a laser beam, causing excess plate material to evaporate in selected areas. Because the process is dry, the photopolymer “washout” step is not a factor. This, says White, can reap a measurable economic benefit for the technology's users. (See Table 5)
|•||One-step, dry process using rubber, though recently, photopolymer & polymer utilized|
|•||Elimination of “washout” step in process can offer savings|
DuPont Cyrel FAST also offers dry processing but in photopolymer platemaking. “But,” cautions White, “the actual savings [using either method] largely depends on the engraving machine used, which also determines the achievable productivity and quality levels.”
This brings us back to the engraving machines used.
The majority of direct laser engraving systems used work with what's known as “CO2” lasers. “These high-watt lasers are required for productivity, and they keep costs at bay,” says White.
The CO2 laser beam has a wavelength that's about ten times that of lasers used for LAMS CTP, White reports. “According to an optical rule, its beam, consequently, only can be bundled — at best — down to ten times the focal dot size of the LAMS CTP laser.
“In this case, the cost benefits will be reduced, if not neutralized, by the higher investment costs and longer engraving times that come with higher resolutions,” he adds.
Direct laser engraving is oddly paradoxical in its purpose, White admits. “On one hand, the plate material has to be eroded easily and quickly, while on the other, it has to be extremely robust for printing.”
The solution: a “butter-titanium alloy,” says White. “But this is most certainly extremely difficult to develop,” he notes, “if at all possible.”
So, he suggests, “dividing the imaging procedure into two sub-processes [defining image position; creating relief], as in the LAMS CTP process, allows for a considerably enhanced optimization for all requirements.”Direct Digital UV
At Drupa 2000, direct digital photopolymer imaging with ultraviolet extended the range of imaging processes in flexo platemaking, says White. In this process, the imaging is achieved with finely bundled UV rays, which can be modulated digitally; the intermediate mask-imaging step is eliminated.
According to White, Global Graphics is the only manufacturer using the direct digital UV process for flexo plates today. Though it's not a one-step process, it's an extremely promising alternative.When You Want the Best
White believes CTP LAMS is the process that best meets the highest standards in quality and productivity. He adds, “Yet dry-plate processing provides photopolymer-based processes with considerable advancement potential. How efficiently this potential is developed remains to be seen.”
DuPont Cyrel Wilmington, DE, USA; 800/345-9999; dupont.com/cyrel.
Global Graphics Software Inc. Waltham, MA, USA; 781/392-1600; globalgraphics.com.