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Description | Applications
| Press Types | Press Components
Description
Offset lithography is the most widely used print process. About
40% of all print jobs are produced with offset printing. It is an indirect printing
process which means that an image is transferred, or offset, from one surface
to another. A printing plate mounted on a cylinder transfers the image to a
rubber blanket mounted on another cylinder. The image is then transferred from
the blanket cylinder to the substrate as the substrate passes between the blanket
cylinder and an impression cylinder. The image on the plate is "right reading"
and when the image is transferred to the blanket it becomes "wrong reading".
When the image is transferred to the printing surface it becomes right reading
again.
The image area and non-image area of the offset
plate are on the same plane and work on the principle that oil and water
do not mix. The non-image areas of the plate attract a wetting agent (fountain
solution) and repel ink made from an oil base. The image areas attract the ink
and repel the fountain solution.
Applications
The types of printed materials that can be produced
with offset lithography are numerous and varied. Some of the items include:
newspapers, magazines, books, continuous business forms, unit sets, advertising
pieces, brochures, posters, greeting cards, business cards, folders, mailers,
laser sheets, integrated products, coupons, and art reproductions.
Press Types
Offset presses can be put into two categories: sheet-fed and
web-fed.
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Sheet-fed: A sheet-fed press prints an image
on single sheets of paper as they are fed individually into the press.
The print quality and sheet to sheet registration is often better
than web-fed printing, but it is often more economical to produce
very large runs on web presses because of their higher running speeds.
Sheet-fed presses can be divided into three categories:
small, medium, and large sheet presses.
- Small Sheet-fed: The small sheet-fed press can print sheets
up to 14" x 17". They are used primarily for short runs
of one or two colors for such items as business forms, letterheads,
and business cards and are popular for instant print companies.
- Medium Sheet-fed: Sheet sizes of up to 25" x 38"
can be printed on a medium sheet-fed press. The presses are used
for runs up to 20,000 and are common equipment for many medium and
large printers. Products such as brochures, business forms, medium
press runs of color work are produced with the mid-size press.
- Large Sheet-fed: The largest runs (usually 100,000 or more)
and the most complex jobs are reserved for the large format sheet-fed
presses. They can accommodate a paper size of up to 49" x 74"
and they may have several printing towers so that multiple colors
can be printed with one pass.
Web-fed: A web-fed press prints images on
a continuous web of paper fed into the press from a large roll of
paper. The web of paper is then cut into individual sheets after printing
or as with continuous business form applications, it is left in web
form and is perforated for later separation into individual sheets.
Like sheet-fed presses, web-fed presses come in many
types and sizes. Some smaller web presses are capable of printing
only on narrow width paper rolls and can only print one or two colors
on the front side of the paper. Other web presses can handle large
width webs and can print on the front and the back side of the paper
in one pass through the press. There may be 8 or more printing
units so that applications requiring full color on the front and back
can be printed.
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Press Components
Offset presses (sheet-fed and web-fed) are made
up of some common components that work together to carry out the offset printing
function. Some of the common components include a device for feeding paper into
the press, a set of cylinders that create the printed impression on the paper,
a roller train for distributing ink and for dampening non-image areas of the
plate, and a system for removing the printed paper from the printing system.
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Feeding System: The feeding system
is the device that feeds the paper into the press. There are different
types of feeding systems for sheet-fed and web-fed presses.
- Sheet-fed: The paper is usually stacked in a tray at the
front end of the press where it is pulled into the press one sheet
at a time. Vacuum devices called "sucker feet" pick up each
sheet of paper from the stack. As paper is fed into the press, the
tray of paper automatically raises up so that there is no interruption
in the paper feed until the tray is empty.
- Web-fed: A mechanism called a "rollstand", which
accommodates a large continuous roll of paper, is used with the web-fed
system. As the paper is fed through the press, another system maintains
proper tension on the paper web as the roll of paper gets smaller
in the rollstand. Some presses have automatic roll changers which
splice in a second roll of paper as soon as the first roll is nearly
out of paper.
Printing System: The printing
system for offset presses contain 3 major components: the plate cylinder,
blanket cylinder, and the impression cylinder. The circumference of
the cylinders determine the size of the applications that can be printed
on the press. For example, a press with printing cylinders of 17"
in circumference is able to print applications with a depth of 17",
8 1/2", 4 1/4", and so on. For an 8 1/2" application,
there would be two separate 8 1/2 inch pieces printed per revolution
of the cylinders. Presses are often named for the circumference of
their cylinders, such as a "17 inch press", or a "22
inch press".
- Plate Cylinder: The plate cylinder contains a slot or "plate
gap" into which the lead edge of the plate is inserted. The plate
is wrapped around the cylinder and then the tail end of the plate
is inserted into the slot. The plate ends are then locked into the
slot.
Some sheet-fed presses utilize plates the are punched at both ends.
The plate cylinder gap contains two sets of pins that the punched
ends of the plate fit over. The pins are tightened do that that the
plate remains stationary on the cylinder.
- Blanket Cylinder: The blanket cylinder is much the same
as the plate cylinder except instead of holding a plate, a compressible
rubber blanket is mounted on it. The blankets vary in type and thickness
depending on the type of press on which it is used.
- Impression Cylinder: The impression cylinder is usually
a seamless, hardened steel cylinder that provides a surface for the
print impression to take place. The paper passes between the blanket
cylinder and impression cylinder where just the right amount of squeeze
between the two cylinders allows for the transfer of the image onto
the paper.
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Note: The gaps in the plate and blanket cylinders
are "non-printable" areas. Allowances must be made with
the overall image size so that the image on the plate does not extend
into the plate gap when the plate is installed. The slot in the blanket
cylinder, known as the "blanket gap" is usually wider than
the plate gap, so even though the image may look correct on the plate,
a sliver of the image may not be offset to the blanket because of
its wider gap. For this reason, the image allowance is usually based
on the non-printable area of the blanket cylinder. The non-printable
gap is also known as the "lock-up" dimension and it varies
between different types of presses.
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Note: Some applications may require that
the printed image be slightly larger than what can be actually printed
by the press. To accommodate the larger print size, the copy may have
to be split and printed on two separate printing units. This is known
as an "over image" job or a "split image" and
should be taken into consideration when planning a print job.
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Inking System: The inking system on offset presses consists
of a fountain which holds the ink and a set of rollers, known as the
roller train, which distribute the ink and carry it to the printing
plate. A roller within the fountain draws the ink from the fountain
into the roller train where it is milled into the proper thickness.
It is then brought to the final rollers in the system called the "form
rollers" which apply the ink to the plate.
The number and type of rollers in an inking system
varies widely between different types of offset presses. A small duplicator
press may have only a minimum number of rollers to supply the flow of
ink to the plate as most of the applications printed on a duplicator
press are very basic. A large web press used for printing complex applications
in full color requires a larger number of rollers to mill the ink and
several form rollers to apply the ink to the plate. The more rollers
there are in an inking system, the better the ink will be distributed
and the better the print quality will be.
Dampening System: The dampening system
consists of a set of rollers that distribute the fountain solution to
the plate. The fountain solution is necessary to keep the non-image
areas of the plate free of ink. As with the inking system, the dampening
system consists of a fountain which holds the dampening solution, a
roller within the fountain that carries the solution into the dampening
rollers, and form rollers that apply the dampening solution to the plate.
Like inking systems, the type of dampening system can vary greatly between
different types of presses.
Delivery System: Sheet-fed and web-fed presses
each have different types of delivery systems, which are described below:
- Sheet-fed: Printed sheets are carried from the printing units
of a sheet-fed press to a delivery tray. The tray has guides which
assist in delivering the sheets to the proper place on the tray. A
jogging system helps to keep the printed sheets in a neat stack. The
tray of the delivery system automatically lowers as it is filled with
printed sheets.
- Web-fed: The printed web is carried from the printing units
to one of two types of delivery systems. A "roll-to-sheet"
press has a mechanism for cutting the web into individual sheets.
The sheets are then carried a short distance on belts to a delivery
tray where they are automatically jogged and can be removed in predetermined
increments by the press operator.
Another type of web-fed delivery system is found on a "roll-to-roll"
press. The printed web is carried from the printing units to a "rewind"
unit where it is wound onto a spool. There are several reasons why
rewinding is necessary:
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- A multiple part business form may require off-line collating,
so each part of the form would be printed separately and wound on
rolls at the press. The rolls would then be mounted on a collating
machine where the individual parts would be attached together.
- The application may require additional features that cannot be
applied at the press, so they are handled off-line on other web-fed
equipment.
- Some types of continuous business forms are supplied to customers
on rolls so that the forms will work properly through certain types
of statement rendering equipment.
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Note: Besides the common components described
above, many offset presses have other components for applying additional
finishing functions that would otherwise have to be accomplished off-line.
The addition of perforations, scoring, punching, and consecutive numbering
are only a few of the additional functions that can be performed.
Many presses are modular in that additional printing units and finishing
units can be added to the basic press in order to provide added functionality.
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