Stealing the CTP Scene?

Flexo printers can achieve outstanding results using patented screening techniques and imagesetter calibration.

By Terri McConnell

Flexo packaging printers have long been fighting the war on dot gain. Depending on the process variables (dot shape, plate material, substrate, inks, etc.), a 2 percent screen dot on a conventional film or plate could gain to around 15 percent on press. If not adequately compensated for, this characteristic can severely limit flexo's ability to reproduce a full, rich color gamut with brilliant, detailed highlights and clean open reverse areas.

The new photopolymer computer-to-plate technologies have certainly put an edge on flexo quality by conquering the dot gain menace. A good deal of space in this column has been devoted to explaining how CTP can reduce, even eliminate, the effects of dot gain. But CTP quality comes at a high price. The total investment for imaging and processing systems can easily hit the half-million-dollar mark.

What about the trade shops, printers, and converters who can't justify the capital equipment costs of CTP? Since it was introduced just a few years ago, more than 200 companies that produce flexo plates have opted for a CTP alternative called FlexoCal. The system was developed by PCC Artwork Systems, a well-known packaging front-end system provider. According to Mike Rottenborn, North American product manager, "FlexoCal is an imagesetter calibration technique which allows flexo trade shops and converters to approximate digital plate quality with their existing film-based assets."

The FlexoCal program is most often used in conjunction with PCC's Hybrid Screening, a program that combines conventional and stochastic screen patterns. Both are available as software modules on PCC raster image processors (RIPs), such as the Nexus. Formally introduced at DRUPA in May, Nexus controls step and repeat functions, screening applications, and the general flow of prepress image data to existing proofers and film output devices.

Nuts 'n bolts

The FlexoCal system involves three major process steps: intentionally overexposing on the imagesetter to produce smaller, harder, and smoother dots; calibrating the imagesetter to a curve rather than a line; and finally applying the calibration curve to the screening pattern instead of to the gray levels in the digital image file.

The net result of this technique is that the maximum number of gray levels can be preserved—especially in the highlight and midtone ranges. That means more potential print information to reproduce a wider, more saturated color gamut, fading vignettes, and brighter highlight details. The theory and methodology of this process might sound complicated, but the benefits were quite apparent on several print samples I reviewed.

The samples showed an even greater quality improvement when FlexoCal was used with Hybrid Screening. Hybrid specifies the use of stochastic or Frequency Modulated (FM) screening for highlight areas, and then transitions to conventional Amplitude Modulated (AM) screening throughout the rest of the density scale.

With conventional screening, density changes are effected by regulating (enlarging or reducing) the size of the printing dot. With stochastic screening, the dot size remains constant and the number of dots is regulated. Each method has distinct advantages under various printing conditions. In flexo, stochastic screen dots exhibit little gain in highlights, but can have a pronounced graininess in the midtones and shadows. Conventional screening gains too much in the highlights, but appears smoother in the midtones and shadows.

One might expect a distinct break at the point where the two types of screens combine. However, Hybrid manages the transition from one screening method to the other over a range of gray levels in such a way that it is not visible to the observer.

Putting the process to work

In 1998, FlexoCal and Hybrid Screening technology was recognized as a significant contribution to the industry's advancement when it was awarded the prestigious Technical Innovation Award by the Flexographic Technical Association (FTA). Today, the process is preferred and specified for a number of high-profile packaging applications by end-users like Frito-Lay, Kimberly Clark, and Coca-Cola. According to Rottenborn, FlexoCal is being used across the folding carton, flexible packaging and tag & labels segments.

The cost for adding the technology to a digital film-based workflow could range from $50,000 to $85,000—depending on the Nexus RIP configuration, system requirements, and software options. A significant investment, but still far less than the cost of a new CTP system. To assist new users

on implementing this specialized process, PCC has established the FlexoCal University at its Bristol, PA, corporate headquarters.

Two-day FlexoCal classes are held at the University every month. Taught by Mark Samworth, PCC's VP of Technology and the father of all this innovation, the classes feature the formalized instruction needed to understand the theory behind the process. In addition, students receive step-by-step practical training for successfully calibrating the process to their own (or their customers') printing presses.