Lithography and Quality

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Transcript Lithography and Quality

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.
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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 design
To 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
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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
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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 system
Dampening System
Printing 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 paper
Sheets…slower, suited for short runs
Roll (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.
Be thankful to those who
made the process
possible.