Folding involves
the use of a tool or mechanical pressure to produce a sharply
defined fold. The pressure required for folding may be applied
manually with the use of a bone folder, however, this reference
will only address the mechanical aspects of folding for production.
In mechanical folding
the sheet is doubled between rollers while pressure appropriate
to the thickness of the paper is exerted. The end result is
a sharp fold that virtually eliminates the paper's natural tendency
to revert to its original shape.
What looks like a
folding mistake by the bindery is often designer error. Remember
that if a panel is to fold into another, it must be slightly
smaller or there will be what is called "telescoping",
or the inability for the folded sheet to lie flat. It will have
a roundish profile because the panels are too long and push
against each other for lack of anywhere else to go. The bindery
will adjust the panels so there is no telescoping, but margins
and color breaks will shift noticeably. Looking through the
diagrams for the individual folding styles in FOLD: The Professional's
Guide to Folding and, notice that there are slight adjustments
in panel widths where appropriate. Pay attention to the adjustments
and apply them consistently for good, predictable results.
In the process of
mechanical folding, there are two commonly used techniques—knife
folding and buckle plate folding. One type of folding may be
used alone or both techniques can be combined.
Knife Folding
Knife folding uses a vertically moving knife and two rollers
rotating in opposite directions. The sheet is carried from the
feed to the folding station until it makes contact with the
sheet stop. At this point, the knife descends vertically, plunging
the sheet between the rollers that have been set to the thickness
of the sheet going through them. As the sheet passes through
the rollers, it is pinched and the fold is formed. This folding
process is dependent upon the repetitive action of the knife.
Only one folding
knife is to be found in any one folding station. For every subsequent
fold, it is necessary to have a further knife folding station
at right angles to the preceding one. Printers and binders specializing
in folding large sheets usually use knife folders.
Buckle Folding
A buckle folding station consists of three rollers and a buckle
plate. The first two rollers are arranged vertically above one
another and their job is to carry the incoming sheet into the
buckle plate until it reaches an adjustable feed guide stop.
The sheet is delivered
into the buckle plate at a speed determined by the characteristics
of the paper. As the lead edge of the sheet strikes the feed
guide stop, the sheet continues to be fed into the buckle plate,
creating a buckle in the space between the three rollers. As
the excess paper drives downwards it is grabbed by the contra-rotating
rollers and the fold is formed as the sheet passes through them.
Unlike knife folding,
buckle folding is not restricted to any cyclical movement, which
makes it ideal for high-speed folding. Each folding station
can fit up to six buckle plates, arranged alternately above
and below each other, allowing for a large number of fold variations.
