BOARD MACHINES ' 247
Except in the case of very thin papers, high speeds cannot be attained, and
for this reason better prices are usually obtained for M.G. papers than for
similar qualities supercalendered or unglazed.
THE BOARD OR VAT MACHINE
This important paper-making machine is very different from the Fourdrinier
machine in the manner in which it makes or forms the web of paper, but it is
the same as the Fourdrinier machine from the presses to the calenders.
The simplest type is the single-vat machine, which has no drying cylinders
and is used for making very thick boards. It works as follows:
The pulp is pumped from the stuff chests to the stuff box and thence to
the strainers; it then flows to a vat or trough, in which revolves a bronze cylinder,
hollow in the centre and open at the ends, and with a covering of fine-mesh
machine wire cloth over a stout backing wire.
The construction of the cylinder is much the same as that of a wove dandy
roll. As the cylinder revolves about two-thirds immersed in the stuff, the
water in which the fibres are suspended rushes through the meshes of the wire
in the same way as it runs through the wire on the wet end of a Fourdrinier
machine. As, however, the meshes are too fine to allow the passage of the
fibres, the latter cling to the surface of the wire drum, and thus a thin film of
fibres is formed on the revolving mould.
As the drum moves round towards the top, the film of stuff is lifted clear
of the stuff and water in the vat, and more water drains away. When the wet
film reaches the highest part of the circumference of the roll it comes in con-
tact with an endless travelling wet felt. As the web touches the wet felt it
is couched by means of a felt-covered wooden roll, which presses the travelling
felt against the surface of the drum. In this way the wet film of pulp is picked
up from the wire surface and adheres to the felt.
The felt carries it along to squeezing or press rolls, where it is relieved of
as much water as possible. The web sticks to and is wound round and round
the top press roll until a sufficient thickness has been formed. This is indicated
automatically by the ringing of a bell. It is then cut off by hand, laid in a pile,
pressed and finally dried by hot air.
The resulting paper is known as a board, and such boards are used for
boxes, panels, suitcases, etc. Larger machines are provided with drying
cylinders and calenders for drying and glazing the board as it is made* By
using two stuff chests, two sets of strainers and two vats, a board can be made
of two different qualities of colours—that is to say, it may consist of common
brown paper stuff on one side, and have a white top side made of sulphite