Making Digital Magic

What tricks can help printers handle trapping's complexities?

By Terri McConnell

An in-house prepress and plating operation can provide more precise control over image reproduction and can significantly reduce turnaround times, while offering tremendous flexibility for coping with last-minute remakes and inevitable scheduling changes.

Some printers are electing to bring only the final "output" phase of the process in-house. They still rely on trade shops or color separators to perform all the magic required to transform a desktop packaging design into a plate-ready electronic job file that can be fed into a computer-controlled imaging device.

And it is magic; design files supplied by the customer or agency usually require extensive manipulation before they will image correctly for print. The checklist for prepping a design file is long, and it runs from adding a die line to applying a dot gain compensation curve. One item on the list, trapping, is particularly quality-critical and time consuming.

The term "trapping," as it applies to prepress, describes the precise spreading or choking of color areas to accommodate for inexact registration of inks on press. If and when the printer decides to take on all aspects of the prepress operation and stage his own show, he will find that trapping can be as big a challenge as sawing a lady in half and putting her back together again.

Trapping for packaging requires so much specialized knowledge, in fact, many printers are opting for a Facilities Management relationship with a qualified and experienced prepress service provider. In this relationship, the prepress house oversees the printer's prepress operations and may actually purchase, install, and even operate the film and/or plate output devices located at the printing plant. It is the trade shop's responsibility to feed the devices with production-ready electronic data.

But for the packaging printer who wants to bring prepress in-house via a different route than Facilities Management, the trapping issue remains one of the most important aspects of the prepress digital workflow.

Why so difficult?

Packaging graphic designers generally create new layouts using Macintosh or Windows-based Desk Top Publishing (DTP) software programs such as Illustrator, Freehand, Corel Draw, or Quark. These programs were originally developed for broad use by artists and content providers in the commercial printing and publishing markets. They were, and still are, attractively intuitive, but not necessarily functional for either packaging design or prepress production.

Ten years ago companies like Scitex, Linotype-Hell, Dianippon Screen, and Agfa raced to fill the need for high-performance electronic prepress systems that could accept a DTP file and make it ready for digital film output. But they too built their products around the needs of the much larger, and presumably more profitable, commercial printing industry. Pioneering packaging prepress operators had to significantly stretch the limits of the available tools to account for the nuances of packaging printing.

In packaging applications we frequently use special inks (spot colors) in text, backgrounds, logos, vignettes—even as substitutes for the yellow, magenta, or cyan within continuous tone photographic images. The further use of metallic inks, white underprints, support tints under black ink, and applied varnishes and special finishes all contribute to the higher complexity of trapping for packaging.

The craft of trapping for packaging is so specific, in fact, that it has its own language. Production operators throw around terms like centerlines, beveled corners, and holdbacks, the latter of which is used when white text reverses out of a four-color photographic image.

It is possible, while it may not be productive, to trap jobs in popular DTP programs using the stroke method. An object is selected, the "stroke box" is checked, and the color is made to be as an overprint. This method is suitable for simpler jobs, but it may be necessary to employ a higher-performance system when the stroking operation takes an hour or more.

Systems designed especially for packaging applications have been available for the last several years. Traditional commercial prepress vendors have expanded the tool sets of their publishing systems. As well, newer vendors, (like Barco Graphics and PCC Artwork Systems) started from scratch to create industrial-strength electronic packaging environments. These systems have evolved to where they can trap a complex six- or seven-color folding carton in forty minutes versus the fourteen hours it may take using a DTP program.

Top Flexible Packaging Converters Chart

Trapping before the RIP or after: Which method is best?

These prepress systems all take in design files, either in native form (such as Illustrator) or via an intermediate file format such as PostScript or PDF. What happens at this point is what distinguishes various vendors' offerings.

Raster image processing (RIPing) the job is a final, necessary step for converting electronic descriptions of the various objects in the file into the pixel data that is ultimately used by the imaging device. Several systems—such as Rampage, the Scitex Brisque, and the Contex ConSeps—perform trapping at the pixel level. This method is known as raster-based trapping. The trapping parameters are set according to differences in the color luminosity of adjacent pixels, or can be set according to the inks used in the job.

As the file is RIPed, it is automatically trapped. The operator can then view the trapped separations on a monitor to visually check the outcome of the decisions made by the RIP. "There are always occasions when we have to hand-adjust the traps due to special printing requirements or the complexity of the job," states Duane Bryant, prepress manager at L&E Packaging, Inc. in Greensboro, NC.

Another method of electronic trapping is known as vector-based trapping. Before the job is RIPed, trapping is applied to the individual objects of the file, and often the trap areas are isolated to a separate "layer" of job data. This method of trapping does not affect the underlying artwork. As in raster-based trapping, the results can be viewed and adjusted.

Vector-based trapping is acknowledged to be more flexible and editable than raster-based trapping, but both methods have strong advantages for particular situations. According to Bryant, "Raster-based trapping is better for creating sliding traps for vignettes and four-color process photographs, as well as for trapping line work to images where luminance is a factor."

Raster and vector trapping systems often have a fully automatic trapping option, where the operator presets the parameters and the file traps as a batch process on the computer. Bryant, who has a Barco system, states, "We use autotrapping every time, but we still need to review the decisions every time."

At Acorn Reproductions, the prepress arm of Oaktree Packaging in Versailles, CT, a recent switch to the PCC PackFlow system has resulted in faster job trapping. According to Dave Stuart, prepress supervisor, "Jobs that used to take three or four days to prepare for output now take less than a couple of hours. We use automatic trapping, view the trapped files onscreen, and also proof to an Iris or HP650 to see the traps. The productivity improvements we've made by moving to a packaging-specific solution are immeasurable."

Direct-to-plate imaging, direct imaging onto flexo sleeves, and advancements in registration at the level of the printing press may someday eliminate the need for trapping altogether. For now, however, trapping for packaging is still a magician's craft and while there are excellent electronic tools to speed up production, it's up to the operator to pull the proverbial rabbit out of the hat.