Lithography and Quality. History of Lithography Sheetfed Offset Next Weeks Field Trip. Lithography is the most popular (static data) printing process and is useful—and best—for most jobs. Images printed by the process are sharp and clear. Plates are inexpensive and quick to make
History of Lithography
Next Weeks Field Trip
Lithography is a compound word formed from lithos (Greek for “stone”) and graphein (Latin for “to write”). Thus, lithography means “to write with stone.”
Alois Senefelder invented lithography in 1798.
Limestone cannot be bent around a cylinder! So, other forms of water-receptive image carriers became necessary.
Both zinc and aluminum were found to be appropriate as lithographic image carriers. Thin sheets of the metal were imaged and then attached to printing cylinders. Water and ink rollers formed the image in the same way as it had been done with limestones.
Today, common plate materials include aluminum and polyester.
Combination of photography and lithography called photolithography
Photography invented in 1826. Images are continuous tone and varying shades of grey, black, and white.
Photolithography uses photography to place an image on a lithographic plate. The process was invented in 1855 by Poitevin
Presses can print or not print. They cannot print varying tints of a solid color.
To give the illusion of tints, the halftone process was invented in 1852 and perfected in the 1880’s by Frederick Ives (of Currier and Ives). This process breaks down a photograph into varying sizes of dots to give the naked eye the illusion of tints. Large dots make dark areas and light dots make light areas.
To reproduce color images, process color printing was invented in 1868 by du Hauron. This process uses three halftone images printed using the primary colors of ink—yellow, magenta, and cyan—to simulate full color.
Images printed on paper directly from a stone or metal plate are somewhat broken because hard (and somewhat rough) paper is pressed against a hard plate.
Ira Rubel (1905) discovered that if the image from the hard plate was transferred first to a soft rubber “blanket” and then to the paper (offset) the softness of the blanket would fill in the nooks and crannies of the hard paper. This created a much smoother-looking impression and is why offset-lithography currently creates the sharpest and cleanest-looking images of any printing process.
Limestone…heavy, difficult to store, expensive, could not be bent around a cylinder
Metal plates…originally had to be coated with a photographic emulsion by the platemaker before being imaged photographically. Chemists had to be employed by lithographers to perform the exacting tasks of plate coating.
3M invented the first presensitized plate (already coated with light-sensitive material) on 1951. Derivatives of this presensitized plate are still in use today…even though they are exposed with computer-driven devices known as platesetters.
Feeding and register units (define register)
Printing units (define and show towers)
Inking systemDampening SystemPrinting unit (plate, blanket, impression cylinders)
Operating console (control most operations of the machine…register, ink flow…remotely from console)
Format of paperSheets…slower, suited for short runsRoll (web)…faster, but for long runs
Size of paper
Small presses (duplicators) print ± 12 X 18 or less
Large presses print larger than 12 X 18…up to 55 X 78 inches or so
Presses are generally named and/or described according to the largest sheet they can print…25”, 38”, 40”, etc.
Larger presses can also print multiple copies of the same image on a large sheet. This process is called up, gang, or step-and-repeat. For example, if somebody needs 10,000 8 ½ X 11 letterheads, a small press can print one or two at a time (10,000 or 5,000 impressions, respectively) while a larger press may be able to print eight copies at once (1,250 impressions)
Each printing unit can print one color of ink.
Standard color printing requires at least four printings…CMYK.
Printing color on both sides of a sheet requires eight printings.
One-sided CMYK would require four runs (per side) on a single color press, two runs on a two-color press, or one run on a four-color press. Thus, a four-color press is four times as productive as a single color press.
Presses often come with more than four towers to allow additional colors to be printed:
More accurate color printing using hexachrome (CMYKOG) or High-Fidelity (CCMMYYK)
Application of one or more clear varnishes or coatings to provide varying sheens within a single page or to protect the sheet from use or the elements (menus).
Coldset web presses allow ink to dry unaided (generally used only for uncoated stocks like newsprint)
Heatset web presses have drying tunnels to speed the drying process (used for shiny coated stocks like magazine paper)
Web presses often have several attachments to provide additional operations in-line.