3. TYPES OF IMPOSITION

There are basically 3 main types of impositions used in TP ;
- Sheetwork
- Work and turn
- Work and tumble

The main difference amongst these is the way the sheet is handled and turned over for printing the
reverse side of the sheet.

a) SHEETWORK IMPOSITION
This is also known as sheetwise in other companies and uses different sets of plates to print the upper
and lower sides of the sheet. It uses the same front lay edge but switches the side-lay alignment to the
opposite edge of the sheet for backing up. See Fig # 11.

This is a common imposition in TP.

A quick method to determine the position and orientation of each page on the layout flat is to produce a layout
dummy as shown in Fig. #12.

First fold a blank sheet of paper as per the folding scheme of the press. For illustration we will use a 16pp
signature.

Place it on a cutting mat in the correct orientation, i.e. with the head fold on top and the spine fold on the left.
Cut an inverted "V" on this dummy ensuring that all 16 pages are cut. The sharp point of the die-cut should
point towards the head fold.

Peel back each inverted "V" sheet and write down the page number until all 16 pages are numbered.
Open up the dummy signature and lay it flat.

The number written on the page will be where each corresponding page should be stripped in with the sharp
point of the "V" denoting the head. After printing and binding, if done correctly, the page numbers should fall
in the correct number sequence.

b) WORK AND TURN IMPOSITION
This imposition scheme uses a single set of plates to print both the upper and lower sides of the sheet.
After the first side is printed, the sheet is turned over left to right maintaining the same gripper edge but
again using the opposite side lay edge.

This imposition saves the cost of one set of plates and make-ready time and wastage is minimal.
It is useful for producing two-ups of a similar section with half the page extent in each copy.

As an example, on a 16pp press, two-ups of a similar 8pp signature can be printed simultaneously.

The full layout would consist of 4pp of the upper pages on one half of the sheet and another 4pp of the
lower pages on the other half, all printing on the same side.

When the printed sheet is turned over to print the reverse side, page 1 will print on the back of page 2,
page 4 on the back of page 3 and so on.

The sheet is then cut in half along the vertical axis and folded as two separate 8pp signatures.

The diagram below illustrates a work and turn imposition.

Note that the same set of plates are used to print both the upper and lower sides but when the sheet is
backed up, the page numbers end up correctly e.g. page 5 backs up over page 6, page 4 over page 3, etc.
This sheet is then cut in half and folded to produce 2 x 8pp signatures.

Some benefits of this imposition include :
- Reduced production time
For a print quantity of 50,000 x 8pp signatures ,only 25,000 sheets need be printed as each sheet
carries two signatures.
- Only one set of plates are required to print both sides of the sheet.
- Make-ready time for the back-up is minimised as colours and registration has been stabilised and
only the back-up alignment need to be adjusted.

c) WORK AND TUMBLE IMPOSITION
This imposition is quite similar to the work and turn imposition in that it also uses the same set of plates
to print both the upper and lower sides of the sheet. Similarly, it also outputs two signatures from each
sheet.

It is usually used for gatefolds. The main differences between this and a work and turn imposition are :

- The work and tumble imposition uses the same side-lay edge for both sides of the sheet and does not
switch to the opposite side-lay.
- When backing-up, the sheet is flipped gripper over tail edge.This means that the gripper edge for the
first pass now becomes the tail edge on backing up. There are now two gripper edges, one for each side
of the sheet.
- Work and tumble impositions require sheets to be trimmed perfectly square before printing.
- It requires the image to be centralised on the sheet during printing as otherwise the back up alignment
will be affected. See Fig. # 14

The printed sheet is then cut along the horizontal axis and folded into two separate products.
Other considerations include;

a) Spread page alignments
Ideally, spread pages or butt-joints should be planned to fall within the same signature for better control
of alignment and colour matching.
If this is not possible then the spread page must be highlighted to the Pressroom, and both sections
should be printed close together in terms of timing to ensure quality reproduction of cross-over colours
and alignment.

b) Paper grammage
When thick paper stocks are subjected to multiple folding, there is a tendency for creasing and crow-feet
to appear. For illustration, a 150gsm bulky stock may be planned to fold as 2 x 16pp signatures instead
of a single 32pp signature. However , other factors like additional costs incurred and production time
frames must be considered.

c) Paper grain
Paper folds best along the grain direction. Folding across the grain may cause the fibres to rupture
resulting in unsightly cracking especially along the spine. This may in some cases be resolved by using
channel scoring or by laminating the covers if possible.

d) Machine capabilities
Some impositions may be easy to print but may be problematic to process efficiently in the Bindery.
These non-standard specials should be discussed at the planning stage with the specialists in the
Bindery . Examples are fold-out posters, complex map folds, gate-folds, irregular die-cut patterns, etc.

e) Potential print problems
Some images and colours are prone to ghosting and streak marks. These can in some instances be minimised
by changing the position or orientation of the image at the planning and layout stage if possible.