IMPOSITIONS AND LAYOUTS

1. INTRODUCTION

a. In printing, the word imposition refers to the placement and positioning of pages on a layout flat in such a manner that when the printed sheet is machine folded, all the page numbers will appear in the correct
sequence.

In addition, the orientation of images and the alignment and consistency of trim margins will appear correctly
after the book is bound and trimmed.

Some items to consider when planning an imposition scheme include :

- The printing press
- The binding method
- Maximum and minimum sizes
- Method of backing up in the press
- Print run quantities
- Spread page alignments
- Paper grammages
- Paper grain direction
- Folder and binder capabilities
- Potential printing problems

b. In preparing a layout / imposition for printing, it is necessary to understand the requirements of the Pressroom
and the Bindery.

In both web-offset and sheet-fed printing there is an allowance for plate bending margins.

These are the leading and trailing edges of the plate which will be slotted into reel rods or the plate clamps.

See Fig # 1.

Then there are the non-printing and non-usable areas of the sheet. On a sheet-fed press, this is the paper
gripper margin which is standardised at 25 mm from the lead edge of the sheet. It includes the non-printing
area created by the impression grippers and the space required for stripping in the quality control colour bars.

On a sheet-fed press, it is possible to accommodate various paper sizes within the constraints of its maximum
and minimum sheet specifications.

On a web press, the non-printing areas are determined by the cylinder gaps into which the plate and blanket
edges are slotted in. See Fig #2

Since paper is fed from a continuous roll, the cut-off, as determined by the circumference of the plate cylinder,
is fixed. In TP's quarto presses, this is 578mm.

This means that irrespective of the final trimmed height of the book, on a web press the paper cut-off will
remain constant at 578mm and smaller height quarto sized books may incur a higher level of paper wastage.

However the width of the web is variable from 420mm up to 965mm and this dimension can be controlled
when indenting paper to accommodate varying book widths to minimise wastage. See Fig#3.

The requirement for plate clamp and gripper allowances for sheet-fed presses is illustrated in Fig #4 .

c. The next consideration would be the binding method. In a saddle-stitched imposiition, each signature is
inserted into the other on the saddle in the stitcher's gathering chain. Wire staples are then punched through
the spine to hold the pages together before being trimmed to the final size.

In this case, the first half of the signature would carry low folio numbers while the back half would have the
high folios.

As a simplified example, in a 16 page brochure printed in multiples of 4 pp signatures, section # 1 would
have pages 1 and 2 in the first half of the signature and pages 15 and 16 in the back half.

This is illustrated in fig # 5.

A concern for saddle-stitched products is the compensation for creep or push-out inherent especially on the
inner section pages.

A trimmed book of 210mm width may end up with a page width of only 205 mm on the inner most pages.
See Fig.#6 .This may create uneven margins or even cause text to be trimmed off.

This is addressed by a process known as shingling whereby the inner pages are stripped in progressively
closer to the spine in millimetre increments to compensate for the push-out.

When done correctly, there will be minimal variations in margins between the first few pages and the centre
pages in the book.

Saddle stitching is a cost effective binding method but the maximum spine thickness is usually limited to
about 6mm.

d. In a perfect bound imposition, using the previous 16pp brochure as an example, each of the 4pp signatures
would be collated one on top of the other rather than inserted.

In this case, the first section would carry all the low folio numbers from pages 1 to 4 while sect 4 would
have the last four pages from page numbers 13 to 16. See Fig # 7.

In perfect binding and thread sewing, creep is not normally an issue unless the signature is more than 16pp
and/or is printed on thicker stocks.

e. A final consideration is to decide which type of folding method to use, either English fold or German fold.
For a 16pp signature, both are quite similar except that in a German 16pp signature, the open pages on the
fore-edge are below the closed fore-edge while in an English fold it is the reverse. See Fig. #9

All TP web presses use German folds with a slight variation between the Hantscho and Heidelberg presses.

For the sheetfed presses, the folders in TP are equipped to handle both English and German folds. However, a
point to note is that most outworkers in Singapore can only do English folds and this should be considered if
there is a possibility that the job may be outworked.

The default in-house practice is to impose all sheet-fed jobs in the English fold configuration.