Reference Manual

17−4
fabric producing a mat of fiber on the fabric
surface. The jet velocity at which the stock is
deposited onto the fabric is very important. This
process controls fiber alignment and affects the
strength properties in the direction of web travel.
This wire will continuously travel the length of the
wet-end of the fourdrinier machine providing time
for sufficient water removal.
Forming Board
This is the first static element under the wire used
to remove additional water. This element supports
the wire at the point of jet impingement. This
device is needed to prevent wrinkles in the forming
fabric. This is accomplished by correctly spacing
the blades of the forming board at the correct
angle so jet delivery can be optimized for best
sheet formation. This element also serves to
retard initial drainage so fines and fillers are not
washed through the sheet.
Foil Units
Following the forming board, water is drained from
the sheet over foil units. These foil units have a
blade with a high slope or angle towards the rear
of the blade. This creates a small vacuum which
pulls more water through the mat. The foils also
create turbulence to help break up any cellulose
flocs that are beginning to form. The higher the foil
angle, the greater the vacuum created thus more
turbulence and water drainage.
Flatboxes
The sheet on the forming fabric is still very wet as
the action of the foils is not enough to remove any
more water. A flatbox, or vacuum box, is a narrow
box positioned under the forming fabric, across
the width on the paper machine and is connected
to large vacuum pumps that provide a differential
pressure or vacuum that is needed for further
removal of water. These vacuums are capable of
increasing the sheet to around 15% solids.
Couch Roll
The final device used to remove water on the
forming section, the couch roll, consists of a
hollow outer shell that rotates with the wire and a
stationary inner vacuum box. The vacuum box is
connected to a large vacuum pump. The holes in
the shell allow the vacuum inside to remove water
from the sheet. This removes water to make the
consistency 20 - 25% solids. The overall goal of
this final section is to get the sheet as dry as
possible to improve the strength of the sheet
before it is peeled, or couched, from the wire and
sent for further de-watering to the press section of
the paper machine.
Fourdrinier Multi-Ply Process
This process is exactly the same as the single-ply
process except multiple layers or plies are
eventually combined into a single sheet. This can
be accomplished in a multitude of ways:
D Stratified Headbox — This device is
capable of depositing two or three different layers
of stock on the fourdrinier wire at the same time
(figure 17-7).
D Secondary Headbox — This device is
simply a common paper machine with a second
headbox to put a secondary layer on top the
preexisting layer.
The above two options still have an issue; all
drainage occurs through the bottom layer thus,
causing some mixing of the fibers. To alleviate this
problem, papermakers have devised a few other
options.
D Multiple Fourdriniers — These machines
actually have multiple fourdrinier machines running
on top of one another. The sheets all wind into a
central area where the sheets are pressed
together to make a multi-layer paper.
D Cylinder Former — This device consists of
a vat and a large diameter wire-covered cylinder
(figure 17-8). The sheet is formed on the wire as
the cylinder rotates through the dilute stock slurry.
The water is then drained into the cylinder. The
wet sheet is then consolidated and couched off
onto a wet felt. This is most effective in the
multi-ply process as each cylinder lays down an
individual ply of paper, which are each individually
couched to the previous ply, thus, making a
heavyweight paperboard.
Twin Wire Formers
This device now holds the speed and production
record for the majority of paper grades. This
technology is becoming more popular than the
fourdrinier machine because:
D Water can drain from both sides of the sheet
rather than one, leading to the top and bottom of
the sheet being more alike.