Board is usually defined as paper above an agreed substance (220–
225g/m2 in UK, 250g/m2 in many other countries). It can be single-ply or
multi-ply. The multi-ply structure consists of top liner, under liner, middle
and back liner. Much of the making process is similar to paper. Points
to note include:
Basically the same as for paper, ranging from high-quality bleached pulps
down to recycled waste paper. Stock treatment is basically the same as for
paper. If waste paper is used, more cleaning and screening may be
required. Board is generally given a lesser degree of beating/refining in
order to ensure efficient drainage.
The liner plies are often given conventional beating and refining to
develop their strength while the middle stock is hardly given any mechanical
treatment, just simply broken then lightly refined.
In multi-ply board, the plies may be combined on-machine or off-machine.
In the case of on-machine boards, either Fourdrinier or vat process
machines may be used.
In the case of the Fourdrinier manufacturing process, the board is built
up on the wire from a series of Inverform units, each of which contains a
head box depositing successive layers on each other.
The vat boardmaker, on the other hand, consists of a number of vats or
cylinder units, in-line, in each of which revolves a large hollow cylinder.
Each cylinder picks up a layer of stock and deposits it on the underside of
a moving felt which carries the first ply of board. The layers are built up
progressively; and at the end of the wire the board is removed for drying.
In the case of off-line finishing, the resulting boards are known as
pasted boards to distinguish them from the homogenous product which
comes off a single machine. A number of webs of board are unreeled in
parallel to each other; adhesive is applied to the top surface of all but the
top sheet; and a pressing cylinder finally brings all the reels together, joining
them to create a single reel of thicker board. The edges are slit to provide
flush edges to the new reel.
The range of operations in finishing corresponds to those for paper and can
include: calendering, supercalendering, coating, slitting, sheeting, conditioning
and, finally, packing for despatch.