Gravure Printing Ink - Gravure Ink
- Food packaging
- Wall paper
- Wrapping paper
- Furniture laminates
- Greeting cards
Gravure printing is characteristically used for long run, high quality printing producing a sharp, fine image. The number of gravure printing plants in the U.S. is significantly lower than other printing processes. This is due, in part, to the cost of presses and components. While a lithographic press will cost in the range of $100,000 the cost of gravure press will be in the range of $1 million. Additionally a single gravure cylinder will cost around $5000 versus around $15 for a lithographic plate. Additionally, the gravure cylinder has a long service life and will yield a very large number of impressions without degradation.
The gravure process has its origins in the early seventeenth century when the intaglio printing process was developed to replace woodcuts in illustrating the best books of the time. In early intaglio printing, illustrations were etched on metal, inked, and pressed on paper. Gravure, still also known as intaglio printing, makes use of the ability of ink to adhere to a slight scratch or depression on a polished metal plate.
Currently, the dominant gravure printing process, referred to as rotogravure, employs web presses equipped with a cylindrical plates (image carrier). A number of other types of gravure presses are currently in use. Rotary sheet-fed gravure presses are used when high quality pictorial impressions are required. They find limited use, primarily in Europe. Intaglio plate printing presses are used in certain specialty applications such as printing currency and in fine arts printing. Offset gravure presses are used for printing substrates with irregular surfaces or on films and plastics.
Today almost all gravure printing is done using engraved copper cylinders protected from wear by the application of a thin electroplate of chromium. The cylinders (image carrier) used in rotogravure printing can be from three inches in diameter by two inch wide to three feet in diameter by 20 feet wide. Publication presses are from six to eight feet wide while presses used for printing packaging rarely exceed five feet. in width. Product gravure presses show great variation in size, ranging from presses with cylinders two inches wide, designed to print wood grain edge trim, to cylinders 20 feet wide, designed to print paper towels. The basics of Gravure printing is a fairly simple process which consists of a printing cylinder, a rubber covered impression roll, an ink fountain, a doctor blade, and a means of drying the ink.
Gravure printing is an example of intaglio printing. It uses a depressed or sunken surface for the image. The image areas consist of honey comb shaped cells or wells that are etched or engraved into a copper cylinder. The unetched areas of the cylinder represent the non-image or unprinted areas. The cylinder rotates in a bath of ink called the ink pan.
As the cylinder turns, the excess ink is wiped off the cylinder by a flexible steel doctor blade. The ink remaining in the recessed cells forms the image by direct transfer to the substrate (paper or other material) as it passes between the plate cylinder and the impression cylinder.
The major unit operations in a gravure printing operation are:
- Image preparation
- Cylinder preparation
Gravure Inks - Solvent Based, Water Based
Gravure inks are fluid inks with a very low viscosity that allows them to be drawn into the engraved cells in the cylinder then transferred onto the substrate. In order to dry the ink and drive off the solvents or water, which essentially replaces most of the solvent, the paper is run through Gas fired or electric fired driers. The ink will dry before the paper reaches the next printing station on the press. This is necessary because wet inks cannot be overprinted without smearing and smudging. Therefore, high volume air dryers are placed after each printing station.
The solvent-laden air from the dryers is passed through either a solvent recovery system or solvent vapor incinerator. A typical recovery system uses beds of activated carbon to absorb the solvent. Saturated beds are regenerated by steam. The solvent laden steam is then condensed and the water and solvent separate by gravity. Greater than 95 percent of the ink solvents can be recovered using this process (Buonicore). The solvents can either be reused or destroyed by incineration.
Water based inks, especially used for packaging and product gravure, require a higher temperature and longer drier exposure time in order to drive off the water and lower vapor pressure constituents. As mentioned subsequent sections, Flexo and Gravure inks are very similar and the constituents are essentially the same. Again, a pollution control device may be needed.
Gravure Press Design and Equipment
Web-fed gravure presses account for almost all publication, packaging, and product gravure printing. These presses are generally custom manufactured machines designed for a specific range of products. The typical press is highly automated and consists of multiple print units. The printing mechanism in a rotogravure press consists of a gravure cylinder and a smaller, rubber clad impression cylinder.
Other types of gravure presses in commercial use today are sheet-fed, intaglio plate, and offset gravure. These types of presses are used primarily for special printing applications.
Web Fed Gravure
There are several types of web presses used in gravure printing, including publication presses, packaging presses, product presses, label presses, and folding carton presses. The printing process is basically the same regardless of which press is used.
Publication gravure is used primarily for very long press runs required to print mass-circulation periodicals, directories, inserts, and catalogs. Publication gravure maintains a competitive edge in the printing of mass-circulation magazines because the process offers high speed, high quality four color illustrations on less expensive paper, variable cut-off lengths, and flexible folding equipment. These presses can have as many as ten printing stations - four for color and one for monochrome text and illustration in each direction so that both sides of the web can be printed in one non-stop operation. They can handle web widths of up to 125 inches and are equipped to print most large format publications in circulation today. Publication gravure presses can also be fitted with cylinders of differing diameters to accommodate varying page sizes.
The major types of chemicals used in publication gravure include adhesives, metal plating solutions, inks, and cleaning solvents. In terms of chemicals, publication gravure differs from packaging and product gravure primarily in its heavy reliance on toluene-based ink (GATF 1992b). The publication gravure industry has had little success with water-based inks (Buonicore). The industry has found that in publication gravure where the substrate is always paper stock, water-based inks have not been capable of printing commercially acceptable quality productions runs of 2,000 to 3,000 feet per minute.
Packaging rotogravure presses are used for printing folding cartons as well as a variety of other flexible packaging materials. In addition to printing, packaging gravure presses are equipped to fold, cut, and crease paper boxes in a continuous process. Packages are usually printed on only one side, so the number of print stations is usually about half that required for publication gravure presses. However, in addition to printing stations for the four basic colors, packaging gravure presses may employ printing stations for the application of metallic inks and varnishes as well as laminating stations designed to apply foils to the paper substrate prior to printing.
Packaging gravure presses are designed with the accurate cutting and creasing needs of the packaging material in mind. However, image quality is generally less important in packaging printing than in most other types of printing and, subsequently, receives less emphasis.
The chemicals used in packaging gravure are similar to those used in publication gravure. However, the inks used in packaging gravure are largely alcohol- and not toluene-based (GATF 1992b). Water-based inks are being successfully used for lower quality, non-process printing on paper and paperboard packaging and for printing on non-absorbent packaging substrates such as plastics, aluminum, and laminates (Tyszka 1993). Use of water-based inks is expected to increase; however, problems still limit their use at press speeds above 1,000 feet per minute (Buonicore).
The continuous printing surface found on gravure press cylinders provides the "repeat" required to print the continuous patterns found on textiles and a variety of other products. In the textile industry, a gravure heat transfer process using subliming dyes is used to print images on paper. These images are then transferred from the paper to a fabric (usually polyester) through a combination of heat and pressure. The gravure process is also used to print continuous patterns on wallboard, wallpaper, floor coverings, and plastics.
The chemicals used in product gravure are similar to those used in both publication and packaging gravure. However, product gravure uses both water- and solvent-based inks (GATF 1992b). The industry has used water-based inks successfully on medium-weight papers and on nonabsorbent substrates such as plastics, aluminum, and laminates (Tyszka 1993). However, problems such as paper distortion and curl persist with lightweight papers (Buonicore).
Image preparation begins with camera-ready (mechanical) art/copy or electronically produced art supplied by the customer. Images are captured for printing by camera, scanner, or computer. Components of the image are manually assembled and positioned in a printing flat when a camera is used. This process is called stripping. When art/copy is scanned or digitally captured, the image is assembled by the computer with special software. A proof is prepared to check for position and accuracy. When color is involved, a color proof is submitted to the customer for approval.
The gravure cylinder is composed of a steel or aluminum base, is copper plated and then polished to a predetermined diameter. Precise diameter of gravure cylinders in a set is critical. Any variances in diameter, as little as 2 thousandths of an inch can significantly affect the print registration. These cylinders are extremely sensitive to scratches and abrasions. Extreme care is taken when handling and storing the cylinders.
Reprinted with permission from pneac.org/.