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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
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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 • Costs are reasonable in comparison to other printing processes. • Process is fast—more than 10,000 sheets of paper can be printed per hour (as compared to ±720 per hour for a laser printer)
How lithography works… 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.
Problems with Lithography • Lithography was a slow and cumbersome process during printing because of its flatbed designTo speed up the process of printing, a rotary press was needed. Such presses use cylinders to hold the image carrier (plate) as well as impression cylinders. Thus, a turning motion can be used to print…much faster than an open-and-close flat bed press.
Possible Solutions 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.
Introduction of photography 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
Limitations of Presses 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.
Addition of “offset” 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.
Evolution of the lithographic plate 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.
In the 1990s, Toray, a Japanese firm, invented a lithographic plate that does not require water. It produces even sharper and more vibrant images than water-based lithography (show examples of plate and prints). But, the process somewhat fizzled due to the high price of the plates and expensive necessary modifications to presses to keep the ink chilled during the printing process.
Today’s offset-lithographic printing workflow • Need by customer to communicate • Graphic design…decisions about layout, photographs, illustrations, copy, color, substrate, size, number of copies • Page layout by graphic designer…scanning, color correction, placement of copy and images on page. • Creation of portable document file (PDF) or packaging of “native” page layout file and supporting fonts and graphics • Transmission of PDF or native files to printing company
Need by customer to communicate • Graphic design…decisions about layout, photographs, illustrations, copy, color, substrate, size, number of copies • Page layout by graphic designer…scanning, color correction, placement of copy and images on page. • Creation of portable document file (PDF) or packaging of “native” page layout file and supporting fonts and graphics • Transmission of PDF or native files to printing company
Today’s offset-lithographic printing press…components of the machine Feeding and register units (define register) Printing units (define and show towers) Inking systemDampening SystemPrinting unit (plate, blanket, impression cylinders) Delivery unit Operating console (control most operations of the machine…register, ink flow…remotely from console)
Today’s offset-lithographic printing press…types, sizes, features 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 print larger forms (groupings of pages for books, booklets, or brochures) than smaller presses. So, a job can be finished faster (fewer sheets need to be printed) on a large press than on a small press. However, large presses are more expensive to buy and run. 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)
Printing units 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:
Additional “spot” colors such as green, purple, brown. 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).
Special features 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. • Folders • Cutters • Perforators • Addressing • Hole punching • Cutting • Numbering
Quality expectations and measurement. • Quality” means meeting the customer’s expectations. But, in practice, a quality printing job is generally considered to be one that is within acceptable variation from perfect (nothing humans do is perfect…even less so in a custom-production printing environment) • Customer (ad agency, designer) and printer should agree in advance on an acceptable level of tolerance between “perfect” and “OK.”
Kenly recommends a very effective system to operationally define quality: • Basic quality printing doesn’t receive a great deal of attention at any stage of the job. Speed and legibility are all that count (copy shops, some newspapers) • Good quality jobs get more attention to preparation and proofs as well as more care during presswork and binding (novels, textbooks, magazines) • Premium quality requires increased attention. Pressroom and bindery operators are highly trained in quality control. Customers are sophisticated and are likely to be trained in the graphic arts. • Showcase quality…everybody strives for perfection.
Relating these quality levels to presswork variables: • The lower the expected quality level, the wider the tolerance between “OK” and perfect.
Variables include: • Register • Density • Screen percentages • Dot gain • Halftones • Separations • Color match • Minor flaws • Coatings • Finishing
The press check • Buyer attends the makeready of his/her job on the press • Before the press check: • Make sure everyone is introduced to each other • Insure check takes place under controlled white lighting (D50) • Make sure that original specifications are available • Remember to not look for flaws that you should have caught on the proof. It’s too late—too costly—to make changes that are not significant at this point.
Evaluate press sheets in sequence: • Is the correct paper being used? • Ask for a trimmed and folded sample. • Check for physical flaws • Check register. • Assess overall color • Examine areas of critical color (trademarked color, product colors) • Compare press sheet to OK’d proof • Examine trimmed and folded sample
Speak up if there’s a critical flaw that means you cannot accept the job. • Remember that minor flaws that a customer won’t notice are not worthy of holding up production and delaying the completion of your job.
When you’re satisfied with a press sheet • Ask for several copies to make sure they all match the one you like. • Request that density numbers be indicated on your sample. • Sign and date two sheets for the printers’ files and two additional copies for your files.
