By Dwight Zilinskas
Computer-to-plate has been part of the printing lexicon for well over five years, but for many sheetfed printers, CTP is more a concept than reality. Web printers dominate CTP usage with fully half already utilizing the technology, while only one-tenth of domestic sheetfed, four-up commercial printers have made the switch, according to the Printing Industries of America Vision 21 Study.
But that’s changing. CTP hardware and software products are more affordable as providers offer their wares down to the thousands of middle market printers. And unlike the mid-1990s, the available workforce is more technically savvy.
But broad-based acceptance among sheetfeds is still in its infancy. Partially, it’s because of the mind-numbing availability of so many systems. Sheetfed printers remain CTP fence sitters because, as the Vision 21 study also pointed out, they fear being saddled with a system that could fall from favor in a market shakeout.
In mulling over when and how to adopt CTP, there are basic operational procedures and equipment options to consider. Luckily, web trailblazers have left an easier path for their sheetfed brethren to travel.
With all its promise of streamlined workflow, enhanced flexibility and greater throughput, the transition to CTP proved problematic for numerous web printers. Initial bottlenecks didn’t have to do so much with hardware, but by a lack of familiarity with digital work flow, employee training and available resources.
Unfamiliarity among a printer’s customer base about CTP contributed to a sluggish changeover for some. Customers were slow to provide electronic files, either because they are unacquainted with CTP or did not have the capability to go digital. This forced printers to function with dual output for conventional and digital files.
Training was also a factor. Simultaneously instructing employees on CTP and developing new work flow routines, while keeping a conventional prepress area in full production at the same time, was a difficult balancing act. Some printers had to restrict spending time and money for training on a low-volume activity, especially when it took away from conventional business.
Equipment also plays a roll in accomplishing an easy transition and everyday effectiveness. Even with the multiple equipment options, one type of equipment that most CTP solutions require is the baking oven. While ubiquitous, the need to prebake and postbake thermal CTP polymer plates allow little room for error. If conditions are not optimal, printers who use ovens could process more than their share of substandard plates, which can impact productivity.
Prebaking most thermal CTP plates is necessary to fully form the image. Portions of the plate exposed to a platesetter laser form a strong acid in the coating matrix. Heat catalyzes the reaction between the acid and polymer to either cause crosslinking or de-protection of the polymer system to create an image. Difficulties associated with prebaking, however, can impact efficient makeready.
The speed of the conveyor and oven temperature should not deviate. Variation in both will cause problems with the development of the plate, such as image loss and/or fogging or banding from underexposure.
Sending plates of varying sizes through the same oven can also complicate matters. Larger plates with more mass will require higher temperatures to get to the proper preheat (or postbake) temperature compared to smaller plates that have less mass. This creates an environment in the oven in which the operating window is very narrow to ensure proper pre-baking (or postbaking).
Fortunately, quality control catches most plates that develop problems during pre-baking. However, compromised on-press efficiency can be attributed to potential problems associated with post-baking.
After developing, the postbake oven hardens the plate so it is more durable and can withstand longer run lengths. This process is very similar to using positive plates. Again, consistency is the key; overbaking or underbaking can cause problems at the last place printers want, on press.
Overly aggressive oven temperatures can weaken aluminum. Some recommended temperatures for post baking are close to the annealing temperature of aluminum, the temperature at which the aluminum begins to change its physical characteristics. At this temperature, the aluminum will actually soften. This can lead to warping, which will cause problems in bending, resulting in problems during mounting. The end result is usually plate cracking.
Depending on a printer’s operations, several on-press factors can affect image durability and plate life due to underbaking. Polymer plates are susceptible to chemical breakdown. Harsh chemicals (very alkaline washes) and ultra-violet and electron beam inks, known for their chemically aggressive tendencies, can weaken the plate’s polymer and lead to early replacement.
Working with thermal CTP plates, however, doesn’t necessarily mean slaving over a hot oven. The established technology of bimetal plates combined with thermal CTP imaging allows high-quality printing without baking with significant cost savings. Both conventional and CTP bimetal products use the same processor and chemistry. Without the baking steps, processing is fast and flexible.
In thermal CTP plates that require baking, the imaged area rests on an aluminum base and carries the ink on press. After imaging and prebaking, a developer strips away unexposed photopolymer. At this point, the photopolymer dots are roughly equal to or slightly larger than those within the file.
A bimetal thermal plate, on the other hand, is processed through the developer and the laser-exposed polymer is removed revealing the copper layer. The nonimaged copper is removed in the etching process allowing for exact dot control with no undercut (no sharpening). The final step is removal of the stencil to reveal the copper image on the substrate.
Once on press, the bimetal plates can stand up to aggressive chemicals, abrasive papers, old presses and a host of other on-press challenges. What’s more, they can help reduce pressroom downtime and waste, while increasing productivity.
Those who intend to introduce CTP into their operations should consider all aspects of available technologies and how long the transition to full digital work flow will take. Baking could add to the cost of conversion, and equipment that is not quickly maximized won’t help deliver a quicker return on the investment.
Dwight Zilinskas is director, International Sales and Marketing, Printing Developments Inc.