When a large sheet is folded several times, the inner pages of the signature will be slightly rotated or skewed. This effect, as well as its solution, are both called bottling. It is caused by the large amount of paper in the back margin.
The amount of bottling is determined by:
- the number of cross-folds (folds along both the horizontal and vertical axis). This means that bottling tends to occur more with either large press sheets since these contain more pages and require more folding, or small page sizes since more pages fit on a given press sheet size if they are smaller (once again requiring more folding).
- the thickness of the paper. With a thicker paper stock, pages will bottle more.
- the bindery equipment. Older or less precise folding machines may increase the amount of bottling.
Since bottling occurs with press sheets that are folded a lot, it typically is an issue with jobs printed on large sheetfed or web presses such as books, magazines or catalogs. Overall it is a niche problem, only affecting a small number of printing companies whose setup and quality requirements require a fix.
Bottling causes the page content to move slightly in relation to the trimmed page edges. To compensate for this content shift, pages are rotated slightly in the opposite direction within an imposition program.
Kodak Preps and Heidelberg Signastation are among the imposition applications that offer control over bottling.
- Ideally the software counter-rotates the inner pages of each signature the most and then lessens the rotation gradually towards the outer pages. The rotation is subtle, for example 0.5 degrees for the inner pages. On a portrait A4 size page that means graphic elements shift around 2.6 millimeters horizontally from top to bottom. In Preps the maximum inclination is 5 degrees. It can be either a positive or negative angle.
- Most applications offer control over the pivot point, the point around which the pages are rotated, such as the top left corner or the page center. The screen capture below shows the five options in Signastation.
Bottling is one of the mechanisms that imposition software offers to transform pages on a signature. The others ones are shingling (or creep) and web growth (also called web stretch or fan out).