Lithography and Quality

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Lithography and Quality

• History of Lithography
• Sheetfed Offset
• Next Weeks Field Trip

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Lithography is the most popular (static data)
printing process and is usefuland bestfor 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 fastmore than 10,000 sheets of paper
  can be printed per hour (as compared to 720 per
  hour for a laser printer)

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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.

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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
  printmuch faster than an open-and-close flat bed
  press.

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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.

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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

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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
  1880s 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
  inkyellow, magenta, and cyanto simulate full
  color.

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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.

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Evolution of the lithographic plate

• Limestoneheavy, difficult to store, expensive,
  could not be bent around a cylinder
• Metal platesoriginally 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 todayeven though they are exposed
  with computer-driven devices known as
  platesetters.

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• 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.
  

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Todays offset-lithographic printing workflow

• Need by customer to communicate
• Graphic designdecisions about layout,
  photographs, illustrations, copy, color,
  substrate, size, number of copies
• Page layout by graphic designerscanning, 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

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• Need by customer to communicate
• Graphic designdecisions about layout,
  photographs, illustrations, copy, color,
  substrate, size, number of copies
• Page layout by graphic designerscanning, 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

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Todays offset-lithographic printing
presscomponents 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
  machineregister, ink flowremotely from console)

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Todays offset-lithographic printing presstypes,
sizes, features

• Format of paperSheetsslower, 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 18up to 55
  X 78 inches or so
• Presses are generally named and/or described
  according to the largest sheet they can
  print25, 38, 40, etc.

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• 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)

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Printing units

• Each printing unit can print one color of ink.
• Standard color printing requires at least four
  printingsCMYK.
• 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

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• 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).

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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

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Quality expectations and measurement.

• Quality means meeting the customers
  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 perfecteven 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.

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Kenly recommends a very effective system to
operationally define quality

• Basic quality printing doesnt 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 qualityeverybody strives for perfection.

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Relating these quality levels to presswork
variables

• The lower the expected quality level, the wider
  the tolerance between OK and perfect.

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Variables include

• Register
• Density
• Screen percentages
• Dot gain
• Halftones
• Separations
• Color match
• Minor flaws
• Coatings
• Finishing

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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. Its too latetoo
  costlyto make changes that are not significant
  at this point.

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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 OKd proof
• Examine trimmed and folded sample

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• Speak up if theres a critical flaw that means
  you cannot accept the job.
• Remember that minor flaws that a customer wont
  notice are not worthy of holding up production
  and delaying the completion of your job.

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When youre 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.

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Be thankful to those who made the process
possible.