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Description | Applications
| Cells | Printing Process
Description
Gravure is a high quality printing process capable of producing
printed images which have a continuous tone effect similar to a photograph.
The gravure process utilizes a metal printing cylinder onto which the image
is etched. The gravure cylinder can be created with analog
or digital plating
processes.
Applications
Gravure is used for many packaging applications, magazines,
and pressure sensitive labels. Gravure is the preferred method of producing
magazines and catalogs that have large circulations. An example of a high volume,
high quality publication that utilizes rotogravure is the "National Geographic"
magazine.
There are also many specialty items that are created with rotogravure
including gift wrap, wallpaper, plastic laminates, printed upholstery, imitation
wood grain finishes, and vinyl flooring. Many of the specialty items are printed
on very wide presses. Some of them, such as those that print patterns on floor
coverings, are up to 150 inches wide.
A gravure sheet-fed process is used for smaller runs for such
items as limited edition prints and other artwork, photographic books, high
denomination postage stamps, stock certificates, and some advertising pieces.
Cells
The printing impression produced by the gravure process is
accomplished by the transfer of ink from cells of various sizes and depths that
are etched into a copper covered steel cylinder. The cells are different sizes
and depths and the cylinder may contain as many as 22,500 cells per square inch.
The various sizes and depths of the depressions create the different densities
of the image. A larger or deeper depression, transfers more ink to the printing
surface creating a larger and/or darker area. The areas of the cylinder that
are not etched become the non-image areas.
Originally, the cells in a gravure cylinder were all equal
in area but they were different in depth. Today, the cells that are engraved
in the cylinders are different in area and depth or they can be the same depth
but different in area. This allows for greater flexibility in producing high
quality work for different types of applications. Cells that vary in area but
are of equal depth are often used on gravure cylinders for printing packaging
applications. The gravure cylinders with cells that vary in area and depth are
reserved for the highest quality printing.
Printed images produced with gravure are of the highest quality
because the thousands of ink cells appear to merge into a continuous tone image.
One drawback with the cells is that the quality of small typefaces, although
good, may not be as sharp as type printed with a process such as offset lithography.
This is because the type is created with individual cells just like the rest
of the image, instead of being printed as a continuous solid shape.
Printing Process
Because of the expense of the cylinders, gravure is largely
performed as a rotary web process (rotogravure). It is most often used for very
long runs of up to a million and many times the press runs are greater than
that. For runs of a million or more, the cylinders are plated with chromium
to provide extra durability. If the chromium begins to wear, it is removed from
the cylinder and a new coating is applied.
During the printing process, the gravure cylinder revolves
in an ink fountain where it is coated with a very fluid ink. A stainless steel
blade (doctor blade) clears the ink from the unwanted areas, leaving the ink
in the depressions of the cylinder. The substrate passes between the gravure
cylinder and an impression cylinder covered in rubber. The substrate passes
between the two cylinders and the ink from the cells is deposited onto the substrate.
Besides being very thin and fluid, the ink colors used with
process color applications differ in hue than the inks used with other processes.
Instead of the usual cyan, magenta, yellow, and black used with offset lithography,
blue, red, yellow, and black are used. Standards have been established by the
Gravure Association of America for the correct ink types and colors that should
be used for different types of substrates and printing applications.
Gravure is a direct printing method so there is no need to
utilize fountain solution to keep the non-image areas clean. Eliminating this
variable allows for better print quality control and jobs can be run at higher
speeds. Some applications can be run as high as 3,000 feet per minute. The microscopic
depressions on the gravure cylinder create an almost continuous tone image on
the printed surface, which is why it is often used for high quality image reproduction.
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