4-color
offset printing is the most common form of commercial printing used around
the world today. Offset color lithography using cyan, magenta, yellow and black
(CMYK) inks in combination has been the standard for mass quantity print
production for nearly 100 years. Nearly every full-color illustration created
for print is reproduced using CMYK based technology, whether it be printed
using offset lithography or digital printing processes.
The Press
The Offset Process
The term offset
lithography not only is used to classify a form of printing it actually
describes how the printing operation occurs. Essentially, an image is offset
from a printing plate to a flexible blanket and then offset from the blanket to
a paper surface. The plate never directly contacts the paper itself. Offset
differs from older, contact printing technologies such as letterpress and
gravure, which were based on a printing plate contacting a paper surface
directly. A major advantage to offset lithography is that an image on a
printing plate is “right reading” as opposed to having to be backward as in
contact printing.
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| Diagram of CMYK inline printing units. © 2014 Don Arday. |
The Resist Process
Another
difference between offset lithography and gravure or letterpress is that offset
does not require a printing plate to contain either raised or recessed areas to
hold ink. Flat offset plates work on the principle that ink and water don't
mix. An image is formed in an emulsion. The emulsion hardens wherever there is
image content. Areas without image content are left blank. Ink that is oil
based adheres to the image areas on a plate while water or silicone compounds
coat the blank areas to form a resist to the ink.
The Image
The Separation Process
Since offset
lithography uses 4-process colors (CMYK), and each color is applied to the
paper separately, the process requires that original full-color images be
translated or “separated” for the production process to be reconstructed using
the four colors. Hence the term, color
separation. To facilitate translating a “continuous tone” image, i.e., an
original illustration, it is broken down into four sheets of film with the
image converted to a screen-pattern of tiny dots. The sheet of film corresponds
to a process color of ink–cyan, magenta, yellow and black. The dot screen
patterns are then applied to individual plates resulting in four separate printing
plates.
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| Separated CMYK color plates. © 2014 Don Arday. |
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| CMYK dot screens in angled allignment. © 2014 Don Arday. |
The Reconstruction Process
The four
separate printing plates that were created in the separation process are
mounted on a 4-color printing press. The press has separate units, each
containing a set of plate and impression cylinders, water and ink rollers, a
blanket, and an ink fountain. The printing units are “registered” sequentially,
each applying a color to the paper to reconstruct the image by combining the
individual pattern of tiny dots. The effect of the small dots being overlaid on
one and other creates the illusion of continuous tone color.
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| Wine & Cheese. Reconstructed CMYK image using150-line screen. © 2014 Don Arday. |
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Detail from above illustration using CMYK offset 33-line
screen dot pattern. © 2014 Don Arday.
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Types of Offset Presses
Sheet Fed Presses
Sheet-fed
offset presses print one sheet of paper at a time. Preferable for small to
medium quantity printing jobs, sheet fed presses permit extreme
fine-tuning of color for optimal image reproduction. For two-sided printing,
the paper must be turned over and run through the press a second time.
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| 4-color sheet fed inline offset press for one sided printing. |
Perfecting Sheet Fed Presses
Essentially,
a perfecting press is a sheet fed press that can print on both sides of a piece
of paper at the same time. Each printing unit contains plate and impression
cylinders, water and ink rollers, a blanket, and an ink fountain above and below
the paper. Perfecting presses are highly efficient for producing prints like mailers,
brochures, booklets and other materials with front and back content. The
paper only needs to be put through the press one time.
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| 4-color sheet fed inline perfecting offset press for two-sided printing with inking units above and below the paper path. |
Work & Roll Sheet Fed Presses
Also called
work and turn presses, these offset presses contain a mechanism that can turn
the paper over midway through the print run so both sides of the paper can be
printed on. A work and roll press differs from a perfecting press in that each
printing unit can only print on one side of the paper.
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| 4-color sheet fed inline work and roll offset press with a central paper turning unit for two-sided printing. |
Web Offset Presses
Web offset
presses print on rolls of paper that move through the press at a very high speed.
Like perfecting presses, web offset presses print front and back
simultaneously. They are best suited to high quantity print runs. Web presses
can make up to 12,000 print impressions per hour making them unmatched when it
comes to cost per unit efficiency.
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| 4+1 color roll fed inline web offset press with two-sided printing with inking units above and below the paper path. |
Offset Vs. Digital Printing
Digital
printing differs from offset lithography in that no plates are used to transfer
ink to the paper. Images are digitally
transferred to the paper using either lasers, positive and negative static
charges, or heat using either dry, liquid, or solid toner that is distributed
to the paper surface in the form of particles. Regarding paper delivery, digital
printing presses have the capability to operate like sheet fed, perfecting, or
roll fed presses. Depending on the particular press, ink can be deposited on
the paper as separate CMYK colors in a sequence, or all colors simultaneously. Digital
and offset presses both produce similar results with 4-color process inks. Most
printers feel that offset lithography produces higher quality results. Below is a comparison of inkjet and offset printing.
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Detail from Wine & Cheese illustration showing CMYK digital
inkjet print dot pattern. © 2014 Don Arday.
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Detail from Wine & Cheese illustration showing CMYK offset
250-line screen dot pattern. © 2014 Don Arday.
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For more
information on digital printing technologies refer to:
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