Printing presses print an entire set of pages on a single large sheet
of paper to make the most efficient use of the paper and to shorten
the time required for printing a large number of different pages.
Imposition is the process of arranging the individual pages
on the sheet of paper so that after they are printed, folded and trimmed,
the resulting pages will back up correctly and be in the proper order.
The order in which the pages are placed on the large sheet of paper
is referred to as printers spreads, as opposed to readers
spreads, which is the order in which the pages will be read. The
pages are arranged on a large sheet called a flat, which is then used
to produce the plates. Two forms are printed back to back on a press
sheet. That sheet is then folded into a signature. Depending on the
size of the page and the size of the paper, signatures are usually
4-, 8-, or 16-pages. The signatures are combined, either by nesting
signatures inside each other or stacking one on top of the other,
which are then bound to create the publication.
order to set up your pages correctly, the first step is to create
a dummy. A dummy is a folded representation of the finished
job. To prepare a dummy, fold sheets of paper to obtain the desired
number and size of pages, then number them. The odd numbers are always
on the right-hand side and the even numbers on the left. Most printers
work in 16-page signatures, so the first spread would be pages 16
and 1. It is important to know which page numbers are on which side
of the sheet, so if you wish to print color on some pages, you could
save money by printing the color only on the pages that are on one
side of the sheet.
If the cover of a 16-page publication is to be printed in the same
stock as the rest of the publication, it is called a self-cover. The
first page will be the outside front cover (OFC), page 2 will be the
inside front cover (IFC), page 15 will be the inside back cover (IBC)
and page 16 will be the outside back cover (OBC). If the cover is
to be printed separately using a heavier stock, then it will be treated
as a four-page section. Printing the cover separately makes it 16
pages for the inside and 4 for the cover, making it a 20-page publication.
addition to placing the pages in the correct position and order, imposition
has to compensate for the behavior of the paper when it's folded.
The more pages there are in a publication, the more that the inner
pages move out from the binding edge compared to the outer pages.
The moving out of the inner pages is known as creep. To compensate
for the creep, the pages are shingled, which is the process
by which the inner margin or gutter is increased on the pages
working from the inside of the book to the outside. The gutter gets
successively wider page by page. The outside page has the widest gutter
and the inside page has the narrowest gutter. Increasing the gutter
moves the printed area closer to the outside margin. When the pages
are trimmed flush, the printed copy appears to cover the same portion
of each page.
Binding is the process of gathering folded signatures and assembling
them. Nested signatures are either left loose, like newspapers, or
stapled or saddle-stitched,
like magazines. Stacked signatures are usually stitched individually
into the spine of the book. In perfect
binding, the signatures are collated, ground off at the spine
of the book, and then bound with glue so that each page is individually
glued to the spine. The final step for stacked or nested signatures
is to trim the three outside edges of the page.
The direction of a sheet of paper as it goes through the printing
press can affect the layout. The gripper edge is the side of the paper
that the press "grabs" in order to pull it through the press.
The gripper area differs in width for various sizes and types of presses,
but is usually 3/8" to 1/2" deep. It is not possible to
print anything in the gripper area so this must be taken into consideration
when planning the sheet size. Opposite the gripper edge is the
leave edge, and the left side of the sheet as it passes through
the press is called the lay edge.
Work-and-Turn and Work-and-Tumble
Work-and-turn is a technique enabling the use of just one plate.
The copy for the pages on one side of a full sheet is put on one half
of the plate and the copy for the pages on the other side of the sheet
appears on the other half of the plate. The sheet is printed, and
then rotated 180° as well as turned over and printed again on
the other side. After printing, the sheets are cut in half to yield
a double quantity of sheets. If 1,000 sheets were required for the
job, only 500 pieces of paper would be required for the run. The gripper
edge remains the same.
Work-and-tumble works the same as work-and-turn except the paper
is flipped head over heels, and the gripper edge changes ends.
The work-and-turn and the work-and-tumble options are economical
for several reasons. Using only one plate saves both materials and
time. For smaller jobs, step-and-repeat (duplicating) can be used
to print two-up in one pass through the press to save press time.
Imposition was traditionally created manually, by stripping the negatives
for each page onto flats. Today, it can be created electronically
using special imposition software and large format imagesetters and