What is Gravure Printing?
First of all, the full name of this method of printing is “rotogravure,” although “gravure” is the more common term.
Gravure is an intaglio process (the image is recessed into the printing plate, in contrast to relief processes in which the image rises above the printing plate—linoleum printing is a relief process, as is letterpress).
Gravure works in the following way: Text and/or photos are engraved (through an etching process or with a diamond tool or laser) onto an image carrier: the plate. This image carrier is a steel cylinder (plated with copper, for durability, and then chrome) because most presses that perform gravure work are rotary presses. Gravure can be used for printing magazines, promotional material, and corrugated board and other packaging (among other items). However, for the most part, due to the expense of engraving the printing cylinders, gravure is used primarily for ultra-long press runs (in the hundreds of thousands or millions of copies). Gravure can also be used for printing circuitry for electronics, and, unlike offset printing, gravure can be used to print on plastics and other thin films.
Here's how gravure works:
1. The printer subcontracts the engraving of the cylindrical plate. This takes time and is an expensive process. The halftones and even the type on a gravure cylinder are made up of little cells that accept ink. Therefore, even those elements that are solid on an offset printing plate (like letterforms in a line of type) will print as dots.
2. The cylinder is mounted on a press. It rests in a “fountain” or tray of printing ink.
3. As the printing cylinder rotates on press, the recessed cells absorb ink. The amount of ink the recessed cells on the printing cylinder contain determines the intensity of the ink color on the printing paper. That is, deeper cells yield more intense colors.
4. Then a doctor blade is brought across the surface of the cylinder to remove excess ink (i.e., ink on the surface of the cylinder, not in the recessed cells).
5. The printing substrate travels between the printing cylinder and the impression cylinder. The pressure of the rollers and the surface tension of the gravure ink draw the ink out of the cells and deposit it on the surface of the substrate.
6. After one color has been printed, the substrate travels through a dryer (because the ink must be completely dry before the next color can be printed).
7. After traveling through the dryer, the printing substrate travels through the next unit on the gravure press, and the second color is printed.
8. The paper (or other printing material) travels through all successive units of the CMYK press.
Gravure is a useful printing method for the following reasons:
1. Gravure provides a range of density (dark to light) that exceeds that of most other printing processes (such as offset lithography) because it can transfer more ink from the printing cylinder to the printing substrate. Due to this extended density range, it is ideal for fine art photography.
2. The set up charges are exceptionally high, so the press runs need to be especially long (often in the millions) to be cost effective. This technology is often ideal for newspaper circulars, catalogs, corrugated packaging, and such.
3. You can print on a wider range of substrates than those available for offset lithography, including polyethylene, polypropylene, polyester, and BOPP. This makes gravure ideal for packaging work. You can also print on thin films such as polyester and nylon.
4. A gravure press runs at 45 feet per second (or faster), yielding 7,000,000 4-color pages per hour. So even ultra-long runs can be completed quickly.
5. The gravure cylinder can be used for these ultra-long press runs without any image degradation (as would occur with an offset plate).
6. For ultra-long press runs, this process yields an exceptionally low unit cost.
Two Important Things to Do When Sending Files to the Printer
I received an email from a printer I work with saying that my client had uploaded low resolution files for her book and that she had forgotten to add bleeds.
First of all, I'm grateful for the close attention of a good printer, as well as for the error checking functions of preflight software. These two errors my client had made were easy to spot because so many clients miss them when distilling press-ready files to upload to their printer's FTP site.
Here are a few specifics noted in the PDF creation sheet this particular printer provides to clients. Not all printers have a sheet like this, and if you look closely at the PDF requirements of those printers that do provide such a list, you'll notice that different printers have different requirements.
This is what I had my client do to fix the problem:
I had her make sure that the bleeds (portions of the book pages where solid colors extended off the page) had been “pulled out.” This means that the InDesign text or image boxes (used to define the position of the solid colors) did in fact extend off the page, in the appropriate location, by 1/8 inch. My client noted that these were correct in the InDesign file.
The next step was to make sure the bleeds were correct in the PDF file. This does not happen automatically when distilling a PDF from an InDesign file, even when the InDesign file treats bleeds appropriately. There is still the step (which my client missed) of using the InDesign “marks and bleeds” tab for making sure the bleeds are .125”. Without this step, an InDesign file (with bleeds) distilled into a PDF will appear to have no bleeds, and the printer's only recourse will be to either send the file back to the client (which this printer did) or enlarge the content on the book page slightly to make the solid color (or photo, or whatever else bleeds) extend off the page.
High-Resolution PDF File
Since the printer had noted that my client's PDF files were low-resolution, I asked her to distill the new files using the PDF “press quality” option. This is because the particular printer the job was going to had requested this specific option. There are a number of other options for distilling PDF files from InDesign. My client had probably used “smallest file size” (specifically intended for images to be read on the Internet).
I asked my client to do two things. First, instead of creating new PDF files for all of the job elements she would be uploading, I asked her to correct and resubmit two files as a test. Once the printer had re-preflighted the new files and they had passed, my client could perform the same operations on the remaining files. This way, she would both ensure success and avoid extra work (if there were further problems).
I also asked her to remember that other commercial printers to whom we might take her future print jobs might have other requirements.
(After all, InDesign lets you also specify PDF/X-1a-, PDF/X-3-, and PDF/X-4-compliant PDFs. These are subsets of a standard PDF specifically destined for high-quality printing. They restrict PDF options to avoid errors in specific aspects of offset printing, such as color management. In addition to these presets, InDesign allows you to import printer-specific lists of PDF-creation options provided by commercial printers.)
I told my client that, going forward, it would be important to confirm with each commercial printer exactly what they would need to flawlessly print her work.
[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.]