What is Gravure Printing?
If you’ve read a copy of National Geographic, you have probably seen a sample of gravure (or rotogravure) printing. (At least back in the 1960s, when I had a subscription to the magazine, this is how it was printed, and articles I’ve read reference gravure printing of the magazine at least up through 2013.)
Gravure provides exceptional quality across huge press runs. However, it takes a lot of preparation, so it is the opposite of digital printing in terms of cost and workflow. That is, if you’re printing a million copies of a 4-color book, all of the preparation costs distributed over such a long run make the unit cost reasonable. But for a short run, the cost is prohibitive.
Here is a description of the gravure process.
The printing cylinder of the 4-color ink unit (on a multiple-unit press) is engraved with a diamond-tipped stylus or by chemical engraving (a process usually driven by digital layout information). The diamond stylus cuts wells of various depths and widths into the printing cylinder. On press, when the cylinder is rotated through an ink reservoir, the wells fill with ink. A doctor blade is then passed over the surface of the cylinder, removing any excess ink.
The ink has very little tack when compared to offset ink, so it does not pick off (remove the surface of) any of the printing stock traveling through the web press (gravure is a web printing rather than sheetfed printing process). Due to the nature of gravure, lower quality paper can still yield remarkable inking results, and the various depths and widths of the ink wells engraved into the printing cylinder allow for different amounts of ink to be applied to various portions of the printed image (unlike offset printing).
One thing to remember is that even the letterforms of typescript are composed of tiny wells (a bit like halftone dots). Another thing to consider is that since a gravure press is a web press (which operates at much higher speeds than a sheetfed press), and since the webs of paper are much wider than conventional web presses (up to 15 feet) compared to the width of a conventional heatset web roll (under 40"), a gravure printer can produce much larger press signatures (up to 48 or 64 pages) at speeds ranging from 65 feet per minute to more than 3,000 feet per minute.
What this means is that a huge amount of work can be produced very quickly when compared to offset lithography, using lower quality paper (softer and smoother than most offset paper) but still providing superior results because of the control of ink placement over the image area of the press signature. (After all, print quality doesn’t get better than the magazines National Geographic produces.)
The other benefit of gravure is that you can print on a wider variety of substrates than with offset lithography, including plastic films, cardboard, laminates, foils, vinyl, and nonporous materials. Therefore, gravure is ideally suited to printing floor laminates (due to the large press size and variety of printing substrates) and bags for loaves of bread (due to the thin and nonporous substrates that can be used). All of this is in addition to the high press speeds, wide web rolls, and large press signatures.
What is Burst Perfect Binding?
Another way to frame this question is, "What is the difference between regular perfect binding and burst perfect binding?"
First of all, what is perfect binding? Perfect binding involves first gathering a stack of press signatures. Let’s say these are 16-page, flat press sheets with eight pages on either side of the press form. Furthermore, let’s say these have been folded down (after printing) into little 16-page booklets with all pages in consecutive order.
So for a 96-page book you would have six 16-page signatures stacked one on top of the next. (Your printer may have a large enough press to produce 32-page signatures for this hypothetical book. If so, there would be three press runs for three 32-page signatures instead of six.)
If you’re doing regular perfect binding, you will use a perfect binder with multiple pockets, with each pocket holding a stack of the same press signature. A conveyor will carry these and stack them side by side to create a complete 96-page book text block. After this step, the spines of the 16- or 32-page folded signatures will be trimmed and ground to provide more surface area for the binding glue (liquefied hot melt glue) to seep into and hold the book pages in a sturdy grip so they won’t fall out of the binding. A paper cover will be wrapped around this book block. Then the three edges (not the bind edge) can be trimmed to complete the job.
Similar to traditional perfect binding, burst perfect binding differs in one main respect. Instead of grinding off the bind edge of the stacked signatures, the binder cuts notches into the spines of the stacked press signatures. Glue is then applied to the bind edge (as with traditional perfect binding), seeping into the notches and gripping the paper fibers.
What actually makes this stronger than traditional perfect binding is that the press signatures are still somewhat attached at the bind edge. That is, in traditional perfect binding the folds of the press signatures at which the edges of the pages are attached are cut off and ground, yielding a stack of (essentially) unattached pages. But on a burst perfect bound book, more of the (notched) signature fold edges are intact, so (even before the glue is applied) the "spines" of the press signatures still hold onto the pages.
This makes burst perfect binding stronger than traditional perfect binding. In fact, the only binding you can choose that is stronger than burst perfect binding is sewn binding, in which string has been used to stitch the press signatures together.
Burst binding and sewn binding cased into paper covers, rather than hard cases, are a particularly good option for soft-cover art books, which will get a lot of use and will thus benefit from an extra-strong binding. Moreover, casing these book blocks into paper covers instead of hard covers can save you a lot of money.
In order to print a photo on an offset press, you must first create a halftone. This is because offset inks print black (or any other color) or they don’t print at all. You can’t print a lighter or darker ink (like a gray rather than a black) in different parts of a photo.
To compensate, you have to turn a photograph into a grid of larger or smaller "halftone" dots. If these are small enough, they will be invisible to the human eye.
In traditional halftone screening (AM, or amplitude modulated screening), all of the halftone dots are equally spaced. To create darker or lighter tones, the halftone dots are made larger or smaller (larger for more dense areas of ink, like shadows, and smaller for less dense areas, like highlights).
If you scan an already screened halftone (perhaps you want to include a small version of a book or magazine page in your brochure) and then screen the images prior to offset printing, you might create unintended moire patterns. These are unsightly patterns in which the initial, preprinted halftone screen conflicts with the new screen added during prepress work.
To avoid this, you need to "descreen" the image. This involves blurring the image and then sharpening it again. The blurring obliterates the initial halftone screen. Then the sharpening process makes the photo somewhat crisper (but without a halftone screen). Finally, the photo can be properly screened prior to printing.
I have done this before from time to time. However, I will say that it doesn’t usually yield the best results. For clarity and crispness of images, starting with an original photo (or a PDF of a sample page) is a far better choice than scanning a book page with photos and then descreening and rescreening it for placement within another print project.
That said, sometimes all you have is a book or magazine page from a prior print run. If so, try to make it as small as possible within your new page design.
On a related note, let’s say you want to reprint a book. Maybe you designed it twenty years ago, and you no longer have the digital art files. All you have is a physical copy of the book. Instead of scanning, descreening, and rescreening each page, ask your offset printing supplier about dot-for-dot scanning or "copydot scanning." This is comparable to scanning an already printed book page and not adding a halftone screen. I would expect this to degrade the image somewhat, but if you need a quick and dirty solution and can’t afford to have a designer create the art files from scratch, this may well be an option.
The same option was used within the last twenty years to digitize ads provided to magazine publishers on film rather than disk. Publishers could copydot scan the films for an advertisement and then incorporate them into a
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]