Production Page: Printing Preferences: Offset vs. Gravure

Among respondents to the most recent Catalog Age Benchmark Report on Print and Production (October 2000 issue), 83% printed their catalogs using offset lithography; 10% used rotogravure printing, and 6% used both. Let’s look at how the two printing methods stack up on several fronts.

Press planning and paper consumption

In offset printing, the plate does not come into contact with the paper. Instead, it transfers the image onto an intermediate rubber blanket that contacts the paper and prints the image. Offsetting the image onto the blanket allows for a more complete transfer of the ink onto the rough paper surface. Two common offset presses are Harris-Heidelberg’s M1000, which prints 32 standard-size pages at a time, and M3000, which prints 48 standard-size pages. Paper rolls for offset presses can’t exceed 56″.

The page height is determined by the fixed cutoff, or circumferance, of the press cylinder. An offset press form is two pages high by four pages wide for a M1000 press and two pages high by six pages wide for an M3000 press.

Because the cutoff size is fixed, you waste paper if the head-to-foot size is inefficient. For instance, if your book is 9-3/4″ tall on a 10-1/2″ cut-off press, you throw away a strip of paper 3/4″ wide for every page printed. But the width of the book can vary without a paper penalty because the roll width is not fixed.

The offset press is usually made up of at least four ink units, which print both sides of the paper web simultaneously. Unlike with the gravure press, ink colors are printed without any drying units between the inking units, so the ink is still wet when the next color is applied. The web goes through a gas-fired oven that dries the ink. Chilled drum rollers then cool the paper before the web is folded into signatures.

During the rotogravure process, the images are printed directly onto the paper rather than via a blanket. The impression cylinder is partially submerged in the ink, with excess ink removed by a thin metal edge called a doctor blade. The cylinder then makes contact with the web of paper, and an electric charge helps transfer the ink from the engraved wells in the cylinder onto the paper. After each color is printed, the paper web goes through a drying unit and is then turned; the colors are printed on the reverse side.

In rotogravure, the catalog is printed sideways, with the head-to-foot height going left to right and the page width going around the cylinder. The gravure press has about 16 cylinder circumferences that allow for different cutoff sizes. The book’s height determines the paper roll width; its width determines the cylinder circumference.

A gravure press can take a 96″ or even a 108″ paper roll, allowing for more pages to be printed per impression than a gravure press. While the offset M3000 prints 48 standard-size pages at a time, a gravure press can print up to 108 pages. And because gravure presses can accommodate both height and width variations due to the number of cylinder circumferences available, they are more efficient in paper consumption.

Print quality

Gravure color is generally deeper and richer than offset, in part because you can apply more ink to an area than with offset. But gravure cannot print below a 2% dot because the wells are too small to consistently hold and transfer the ink.

One downside to gravure printing is the reproduction of fine-line work or small lettering. The diamond shape of gravure dots does not handle small lettering, creating a jagged type, while offset’s round dot structure handles fine lines well. Serif type with thin edges can appear rough with gravure and smooth with offset, but technology is reducing the jaggedness of the type in the former.

Compromises are a matter of fact with offset. Ink color is applied in the direction the web is running through the press, so images that run in line with one another are forced to share color manipulations. Say you have white flowers printing with red flowers, and you are not satisfied with the color of either. If you take red out of the image of the white flowers, you hurt the image of the red flowers; if you put red in the red flowers, you hurt the white ones. You may need to compromise the color of one image for the sake of the other or both because offset color moves on press are not image-specific.

The greater the ink density, the more ink is put on the page. The maximum density of offset ink is about 260% for the four colors — cyan, magenta, yellow, and black — combined, while gravure can theoretically have a maximum density of 400%. Put another way, each color on an offset press averages 65% density, compared with a possible 100% for gravure.

Gravure allows for more precise registration of inks on press than offset because of the rigidity of the printing surfaces. The hard surfaces of gravure cylinders lay down ink in exact spots; there is no flex to control, as the cylinders do not “give.” And gravure inks are drier and therefore more solid than offset inks when applied to the paper. Adding water with offset printing — a frequent necessity — contributes moisture to the paper that, in turn, causes it to swell and change size. In this way, aligning ink colors is more challenging with offset printing than with gravure.

With gravure, the wells for each color are etched in the cylinder with elongated or condensed shapes. Because the wells are interpretations of the halftone dot pattern of the digital files, the chance of a moiré is reduced.

A moiré — a wavy pattern that appears in the dots making up an image — cannot be fixed without stopping the presses. The root of the problem is in the customer’s page files, so the printer will not bear the cost of the downtime. If the moiré is minor you may want to keep the presses running while the prepress house fixes the problem and the printer makes new plates. If the pattern is unsightly, though, you will need to stop the press and wait for new plates.

If the moiré does make it to the plate, the printer’s prepress department can sometimes reburn the plates at a slightly altered angle. The conventional 3M Matchprint and the digital Kodak Approval both have a dot pattern that will show the moiré as it will appear on the press. The stochastic Iris, however, will not show these patterns.

Costs

The cost benefit of using offset vs. gravure is often a matter of page counts and printing impressions. Generally speaking, the more pages you can produce with fewer impressions (turns of the cylinder), the cheaper your printing costs will be. Because offset presses produce fewer pages per impression than a gravure press, gravure should typically be less expensive. But other factors come into play as well.

Cost analysis will show that the point at which offset becomes less cost-efficient and gravure more economical for catalogs 48 pages and smaller is around 1.25 million-1.5 million catalogs, with qualifications such as page size, number of versions, and if the book uses the same paper throughout or a better grade of paper stock for the cover or outer wrap. For books with 52-76 pages, the cost-efficiency of gravure is more apparent, with breakeven of roughly 750,000-1 million impressions. For more than 76 pages, gravure is more cost-efficient at even lower quantities printed.

On a gravure press, a higher page-count book can be printed in one press run with one press make-ready, while offset would require the book to be printed as a number of press forms with multiple make-readies and press runs. For example, a 96-page book would require two M3000 48-page offset press forms, delivering four 24-page signatures. A gravure press run would require one press form and would deliver 96 pages as three 32-page signatures or two 48-page signatures.

Gravure: Pros and Cons
PROS CONS
Can print more pages per impression Cost-prohibitive for smaller quantities
Provides deeper, richer colors Cannot print below a 2% dot
Greater color gamut Poor reproduction of small point sizes, fine-line work
Color corrections can be isolated to one image Cost of engraving and changing cylinders is high ($10,000-$15,000), making versioning expensive
Precision registration of inks achieved quicker Color changes possible only across whole width of web
Requires less paper tension Few closed-loop color-matching systems available
Can run a lighter/cheaper paper Fewer vendors to choose from, although more are expected to come to the market soon
Averages fewer breaks than offset

If you factor versioning into the cost equation, however, you’ll find that using gravure for smaller quantities (fewer than 1 million-2 million impressions) is prohibitive because the cost of engraving and changing cylinders is relatively high — between $10,000 and $15,000. The press make-ready and cylinder versioning costs could be higher than the actual running charge.

Conversely, offset plate costs are minor — from $1,500 to $2,500 — when amortized across the complete press run, allowing you to make the most of versioning for different list segments. You could modify the black headline at the top of the page to welcome new customers, switch covers, change prices, or whatnot.

The offset plates are usually aluminum or similar water-friendly metals that are treated with a light-sensitive chemical that attracts oil, When exposed to light with the film overlay or the computer-to-plate (CTP) process and then developed, the image areas attract ink (which is an oil), and the non-printing area repels ink and attracts water.

The process is fairly quick, with less than a half-hour of actual exposure, developing, and baking of the plates. The inexpensive plates and the relative speed of processing make the offset printing make-ready more attractive for short and mid-size runs. But because the plates need replacing after about 1 million-2 million impressions, offset printing requires additional plates for longer runs.

Offset: Pros and Cons
PROS CONS
Cheaper for smaller books and print runs Fixed cutoff size can waste paper
Plates are cheap and easy to make, which allows for versioning Larger books must be printed as a number of forms
Handles fine line work and thin edges well Greater risk of moirés
Plates can be changed in two hours Maximum density of ink is only 260%
Several offset vendors using cutting-edge closed-loop color-matching systems Aligning ink colors is challenging; more tradeoffs required to match colors
More vendors with high-speed, large-format offset presses from which to choose More tension means you must print on heavier paper stock
Requires additional plates for longer runs

The speed with which offset plates can be changed results in less than two hours press downtime. This can also be an advantage when a color move cannot be made by manipulating the ink fountain but instead requires adjustment to the digital file and new plates made. Gravure cylinder engraving takes hours per cylinder, and four-color changes can take all day.

Other miscellaneous benefits

The paper web tension required for gravure is about 1 lb. per square inch, while offset requires 4.5 lbs. per square inch. With less tension, gravure presses can run a lighter — and less expensive — paper. Gravure can print on coated supercalendered paper that is lighter than 28 lb. Offset, on the other hand, is usually limited to printing on 34-lb. to 38-lb. stock.

Less paper tension also means that the paper web will not break as often. The gravure press averages fewer web breaks than an offset press. This added offset downtime increases the cost and decreases the efficiency.

With gravure, you may have fewer signatures to bind. As we mentioned earlier, the gravure press form will produce more pages for each form than the offset press: With an M3000 48-page press, you will have two 24-page signatures; a gravure press can produce up to 108 pages as three 36-page signatures. More pages per signature translates to fewer pages to bind.

For example, you can produce a 76-page catalog on offset as three 24-page forms and a four-page cover. In gravure it can be produced as two 36-page froms with a four-page cover. Beside the printing savings, this could eliminate one pocket in the bindery, saving about $1/M-$1.20/M catalogs printed.

But the key benefit belongs to offset: more vendors from which to choose. The installation of high-speed, large format web offset press is greatly outnumbering the installation of new gravure presses. In the U.S., many commercial printers have both high-speed 32- or 48-page presses, but only three also have gravure: RR Donnelley & Sons Co., Quebecor World, and Quad/Graphics.

In the end, the differences between gravure and offset are becoming less marked. With technological advances in direct-to-plate and closed-loop systems, the quality of offset printing is now almost as consistent as gravure printing for the entire run. On the gravure side, the electrical assist has improved the quality of fine-line work. The prices of both plates and cylinders are going down, making gravure more competitive in short runs and offset more cost-efficient in long runs.


Tim Gable is director of print production for Plow & Hearth in Madison, VA. Russell Goin is production manager for Chicago-based cataloger/retailer Hammacher Schlemmer.

Of CTP and Closed-Loop Color

Among the printing industry’s most significant breakthroughs of the past 10 years is the elimination of film, thanks to the direct-to-plate or computer-to-plate (CTP) systems. Removing the intermediate step of creating film and then photographically developing the plate from the film has increased plate qualities and reduced start-up times because you don’t have to translate the computer source file into a different, intermediate medium. Also, film had a tendency to pick up dust and stray hairs or scratches. (Direct-to-cylinder for gravure was available before CTP for offset because the gravure etching process was already computer-driven.)

Another evolving technological innovation is the closed-loop color system on offset presses. Previously, the color fountain keys were locked once the color was matched, with the operators performing periodic checks to ensure that the color was not moving. Now specialized color bars are printed on the press sheet in the area that will be trimmed off. A computer reads the color bars and adjusts the ink supply to maintain the color.

The operator also uses this system to start the press by having the computer dial up the correct coverage; the operator can then make minor adjustments to match the color proofs. This is a better system because it adjusts based on the output (the printed piece), not the input (the ink fountain settings). While cutting-edge closed-loop color-matching systems are beginning to appear on offset presses, installation on gravure presses is lagging.
TG/RG

For the most up-to-date industry information, visit www.CatalogAgemag.com.

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