The originals of gravure printing were with the creative arts of the Italian Renaissance in the 1300s.
1446- First intaglio plate engraved in metal used for printing.
Fine engravings were cut by hand into soft copper.
The engraved surface consisted of channels of sunken areas.
1505- Invention of Chemical etching.
Etching- fixing image on a metal surface by mordant (acid).
Inventors credited with the invention of etching: Daniel Hopfer- Augsburg (1505), Franco Mazuolli (c.1504-1540).
*Draw on soft “resist” coating over the plate
*Scrape away the resist layer.
*Acid would penetrate the exposed copper – dissolve it.
Another metals: zinc, steel (later).
Cylinder vs. flat plate
Pressure- considerable force needed to press entire surface of paper against an image carrier evenly- intaglio.
Early presses- made entirely of wood- possible to print only small images without cracking.
Limit- how much force to apply through a screw (hand- foot lever)
First metal printing press- 1550- larger sheets, but still difficult to operate.
Putting paper or image (or both) on cylinder- less pressure- image carrier touch only a small “print zone”.
The Italian word INTAGLIO (in-tal-yo) means
engraved or cut in.
Intalio refers to a method of printing whose image carrier consists of lines or dots recessed below the surface.
Intaglio reproduces original design by pressing paper into the recesses.
The first intaglio plate was used for printing in Germany in 1446 about the same time as Gutenberg.
Unfortunately, intaglio was not compatible with Guttenberg’s letterpress, so it was not adopted by early printers.
The modern gravure printing press resulted from the invention of photography and the adoption of rotary printing from cylinders.
1818- first perfector- sheetfed press- took the sheet from the first form an printed the other side. Quadrupled the output- from 250 to more than 1,000 sheets /hour.
Invention of Photography
1826- Joseph Nicefore Niepce- while searching for way to transfer images from litho stone to intaglio plates- invented light sensitivity of some chemicals .
1838- principles of photography tested and proven.
Dichromated gelatin as the sensitive coating on copper or steel plate – Fox Talbot placed leaves, laces on the coating and exposed it to daylight. Gelatin hardened where exposed to light. Washed with cold water, dried. Gelatin served as resist for etching solution.
Etching solution penetrated the soft unexposed gelatin and etched underlying metal.
All printing processes benefited.
The use of photosensitivity to create new kinds of image carriers (faster to produce, easy to duplicate).
Printers learn how to reproduce the fine grain and tonal variations of photographic print.
The Halftone Screen
1860-Fox Talbot – principle of the halftone screen.
Black cloth between a light source and a photosensitive steel plate. He had imaged the plate with fine dots of varying size.
Placing screens between a projecting lens and a photosensitive material- broke up continuous tone into discrete dots of varying size- it reproduced tonal gradation.
This method is used to reproduce photographic images in all printing processes.
Auguste Godchaux received a patent for a reel-fed rotary gravure perfector press in 1860, press still in use in 1940.
1890 -Development of photoengraving (Samuel Fawcet).
1892 -Karel Klic- number of patents- father of rotary gravure.
Klic and Fawcet didn’t have patents of their process, so they tried to keep the process secret.
They sold prints from their press as “heliogravure” prints, even though they were really rotogravure as we know it today.
Their process remained a trade secret until an employee emigrated to the United States and made it public.
Gravure presses were used to print Jell-O cartons starting in 1938.
The process continued to improve with electromechanical engravers being introduced in 1968 with digital controls added in 1983.
Rotogravure printing has a significant advantage
relative to other printing processes for medium to long
Gravure can produce very high quality multicolor printing on a variety of substrates.
Its success results from the simplicity of the process.
Having fewer variables to control ensures consistent print quality throughout the run.
Each print unit has 4 basic components: an ink fountain, engraved cylinder, doctor blade and impression roller.
The Gravure Industry
Gravure is the process of choice for long-run high quality publication printing.
About 3 billion copies of gravure printed magazines are produced in a year.
Sunday supplement magazines such as Parade are printed with gravure (National Geographic, Better Homes, Cosmopolitan)
Newspaper inserts, catalogs, are printed gravure.
Gravure printing is used extensively in the packaging industry.
Gravure- folding cartons, flexible packaging made from paper, film, foils, laminated wrappers- Al foil. Labels are printed gravure.
Gravure is used to print gift wraps, vinyl plastics- including wall coverings, shower curtains, tablecloth, ceiling tiles, decorative laminates, wood grains, floor coverings, paper towels.
Sheedfeed gravure with plates rather than cylinders is used to print postage stamps, securities, currency, bank notes. It uses extremely high impression pressures, intaglio plate has continuous engraving, (not engraved discrete dots), ink is very pasty, heavy.
Gravure transfers ink from small wells or cells that are engraved into the surface of the cylinder.
The cylinder rotates through a fountain of ink.
The ink is wiped out from the surface – non-image areas by a doctor blade.
The inverted pyramid- shape or cup-like shape of each cell holds the ink in place as the cylinder turns past the doctor blade.
Laser engraved cells. Cells can be chemically etched or electro-mechanically engraved.
The latest technology- electron beam engraving 150,000 cells /sec.
(Electromechanical- up to 5,000 cells/sec.).
The formation of perfect cells is accomplished by the gravure engraver.
The gravure cell is characterized by 4 variables, depth, bottom, opening, cell wall.
The depth of the cell is measured from the bottom to the cylinder surface.
The opening is described by the shape and cross sectional area.
Bottom can be flat- or nearly flat – chemically etched, laser engraved or inverted pyramid shaped- electromechanical engraver with diamond stylus- 90-130 deg diamond.
Cell wall- bridge is the surface of the cylinder between cells.
GRAVURE CYLINDER ENGRAVING
Chemical etching- historically three types of chemically etched cells-
Conventional- same area, different depth,
Lateral hard dot-variable size, variable depth and
Direct transfer- variable area, same depth.
The chemical etching of lines and dots through photo resist was the beginning of rotogravure.
The carbon tissue method of image photo-transfer to a gelatin resist was discovered and perfected in the later quarter of 19th century.
Traditional Chemical Etching - The same
cell area, variable depth.
1940-1960-conventional gravure etching –combination of 2 glass photo positives exposed to carbon tissue.
Carbon tissue is a water sensitive fibrous paper coated with smooth gelatin resist.
Tissue was etched by FeCl3 ferric chloride- depth 45 micron, various screen counts.
The resist is sensitized to light by submerging it into a potassium bichromate solution. The sensitized carbon tissue can be dried, and refrigerated until used in the photo-transfer process.
Tissue is formed into a sheet sizes and wrapped around copper plated cylinder.
A glass positive was used to expose the light sensitive gelatin. One positive is a continuous tone variable density image.
The second positive is a gravure screen of specified count 100-200 lpi (lines per inch).
The two positives are exposed consecutively to UV light on the same sensitized surface.
After double exposure, the carbon tissue is wrapped around the cylinder and all the backing is removed.
The exposed areas are hardened with alcohol and forced air dried. (This is done in amber light to prevent UV exposure).
Variable thickness membrane is formed- depending upon the amount of light passed through the two positives.
The membrane is thick in the highlights and thin in the solids.
This controls how quickly the etchant will break through the membrane and begin to etch the metal.
This process forms cells of variable depth, but uniform cross sectional area.
Ferric chloride-reliable for copper removal at a controlled rate
2 FeCl3 + Cu -->2 FeCl2 + CuCl2
Ferric chloride +copper -->ferrous chloride +cupric chloride
Supplied at spec. gravity 48 deg Baume, carefully diluted for use (42
deg Baume), tested with hydrometers sensitive to .05 deg change. Lower
concentrations tend to break down the gelatin and speed the process, while
high concentrations pass more slowly through the gelatin.
Temperature- constant, etching at constant conditions ? micron/minute. Impurities in HCl, Fe2(SO4)3 - would cause problems.
DIRECT TRANSFER (SINGLE POSITIVE)
Produces cells of uniform depth, but variable
1950s- photographic polymer films introduced to industry
New transfer method- image transfer from film directly to metal, single positive or direct transfer system.
Exposure through film to a resist coated cylinder.
Wrap-around photographic film and light sensitive photographic emulsion
Single film with combination of halftone and screened solids.
150 line screen engraved to a depth 42-44 microns.
Finished etched wall is about 12 microns wide.
Also other screens used: 175, 20, and 250 line hex-shaped screens.
Normal etching time-3-4 minutes.
Photopolymer emulsion- high contrast, high resolution emulsion, forming tough rigid resist layer when properly exposed and developed.
Emulsion sprayed on cylinder surface with precise control – volume/inch square.
Emulsion- light sensitive photopolymer – chemically altered after
Exposure – UV light through photographic film as the film emulsion and polymer emulsion are in direct contact and rotated around circumference of gravure cylinder.
Several techniques to bring the positive emulsion in contact with
MYLAR drive film- continuous belt around 3 rollers,
One- drive roller, above it idler roll. Third- movable back and forth- Tensioning roll.
Film belt acts as a drive and overall contact impression device to hold the positive firmly in alignment and contact.
Positive film is aligned around the cylinder, the cylinder rotates
against the drive film, in front of mercury vapor exposure light.
DIRECT TRANSFER PROCEDURES
Photo resist direct transfer methods and developing of exposed resist- similar in most processes. Photo resist- colorless when applied.
During developing process dark blue, green or black dye applied to
Developer- water, rinsed from surface-carry away unexposed material, leaving rigid resist.
Cells develop cleanly leave exposed copper metal when resist developed away.
Etchant application- roller applied, rotated in bath, paddle splashed
and sprayed. Time to etch 2-4 minutes.
Direct system- very accurate in color to color register, cell specifications. Not affected by humidity. Engraving can be interrupted, adjusted again and again.
Chemical etching- are quick, provide wider variety of cell configurations.
Electromechanical engraving is recognized as the most common method of cylinder imaging today. There are, however, many applications that still require chemically etched cylinders. Some of these uses include unique packaging products and specialty products such as wall coverings and flooring patterns. The ratio of electromechanical to chemical cylinder making is about five to one.
Cylinder Cell Geometry
The cylinder cells are the most important part of the gravure printing process. The quality of the printed image is dependent on the size, shape and depth of the cell.
The width of the cell refers to how wide the cell is in the cross direction. The depth is how far below the surface the cell extends. The wall is the barrier between the cells and is used to support the doctor blade. The top of the cell wall and the unengraved areas of the cylinder are sometimes referred to as the land.
The electromechanical method of engraving
is a direct result of advances in electronic technology. There are two
common types of mechanical engravers. One is a Helio-Klischograph and the
other is an Ohio engraver. The Helio and Ohio are the industry leaders.
Image Processing. There are two methods for image processing on these machines. One method is from a direct positive photographic film image. This film image is mounted on a synchronized rotating drum and the varying density information is read by an electronic scanner. The scanned information is digitized and processed for the engraving section.
The other method of image input is through direct electronic transfer. The image information is scanned at another site and transmitted to the engraving machine by disks, tapes, phone lines or satellite.
Engraving. Once the image information has been scanned and digitized it is processed for the engraving section of the machine. The objective of the engraving process is to produce cells which, when printed, will duplicate the density of the desired image. The very small volume of ink must be controlled within the engraved cell volume. A typical cell volume is less than the size of a single grain of sand. All of these tiny cells, when printed together, produce a solid area of color.
The tool used for engraving these cells is a diamond stylus of triangular cross section that engraves an inverted pyramid. The digital processed image information is converted to an electronic vibration that produces a mechanical motion in the diamond stylus. The darker the desired image the deeper the diamond penetrates into the copper. The large cell will carry more ink and produce more density. Conversely, if a light tone is desired, the diamond makes only a slight cut into the copper.
The cells are cut at a rate of 4000 per second. The average processing time for a packaging cylinder is 30 to 40 minutes depending on how much copy needs to be engraved. The copper chips or debris from the engraving operation are collected through a vacuum system and recycled.
Electromechanical Cell Configuration. There are four basic cell structures formed during electro- mechanical engraving. They are compressed, elongated, normal and fine. By using these alternately shaped cells, color process printing becomes possible. The size and position of the cells begin to form a line screen image. This screening effect allows for the successful combination of the four process colors.
Corrections. The electromechanical process should begin with accurate film positives. Any color correction (or adjustment) should be made during the color separation (pre-press) stage.
Cost. A diamond stylus may cost between $500 and $800 and can engrave
600 to 800 hours. The diamond stylus can be resharpened at a cost of about
$90. The cost of the cylinder base can range between $800 and $1,200. The
engraving, depending on type and difficulty, can cost between $1,000
to $10,000. As you can see, the finished cylinder is a very expensive and
significant part of the gravure process.