Page 40 - 3D Fibre Reinforced Polymer Composites

Manufacture of 30 Fibre Preforms 29
2.3.3 Two-step 3D Braiding

The second style of flat bed braiding is referred to as 2-step (Popper and McConnell,
1987; KO et al., 1988; McConnell and Popper, 1988). Unlike the 4-step process, the 2-
step includes a large number of yarns fixed in the axial direction and a smaller number
of braiding yarns. The arrangement of axial carriers defines the shape of the preform to
be braided (see Figure 2.17) and the braiding carriers are distributed around the
perimeter of the axial carrier array. The process consists of two steps in which the
braiding carriers move completely though the structure between the axial carriers. This
relatively simple sequence of motions is capable of forming preforms of essentially any
shape, including circular and hollow. The motion also allows the braid to be pulled tight
by yarn tension alone and thus the 2-step process does not require mechanical
compaction, unlike the 4-step process.
-
Carriers

000 0
v
..eo. Po .Io 0
b
0 000
f--ot--------o
0
w

Figure 2.17 Schematic of the 2-Step braiding process

Both the 4-step and the 2-step braiding processes are capable of forming quite intricate
shapes as shown schematically in Figure 2.18 (KO, 1989b) and have been successfully
used with a range of fibre materials; glass, carbon, aramid, ceramic and metal. It is
possible to braid inserts or holes into the structure that have a greater degree of stability
than holes that have been machined. The braid pattern can be varied during operation
so that a change in cross-sectional shape is possible, including introducing a taper to the
preform. Thick-walled tubular structures can also be made by suitable arrangement of
the carriers. Flat preforms can be made from tubular preforms by braiding splits or
bifurcations into the preform then cutting and opening it out to the required shape
(Brown and Crow, 1992). A bend is also possible as well as a bifurcation, which will
allow junctions to be produced and these processes even allow 90" yarns to be laid into
the preform during manufacture. Further development of the 2-step and 4-step braiding
techniques have concentrated primarily on computer-aided design of the braided
preform and improving the process of controlling the transfer of the yam carrier across
the bed (Huey, 1994; Roberts and Douglas, 1995). This includes the use of computer