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Manufacture of 30 Fibre Preforms 31

2.3.4 Multilayer Interlock Braiding
A different class of three-dimensional braiding does not rely upon the 2-step and 4-step
processes previously described, and is considered to be closer to the traditional process
of 2D braiding in its operation. This proprietary braiding process, called “multilayer
interlock braiding”, was developed at Albany International Research Corporation
(Brookstein, 1991; Brookstein et al., 1993) and the machinery is analogous to a number
of standard circular braiders being joined together to form a cylindrical braiding frame.
This frame has a number of parallel braiding tracks around the circumference of the
cylinder but the mechanism allows the transfer of yarn carriers between adjacent tracks
thus forming a multilayer braided fabric with yarns interlocking adjacent layers (see
Figure 2.20). The multilayer interlock braid differs from both the 4-step and 2-step
braids in that the interlocking yarns are primarily in the plane of the structure and thus
do not significantly reduce the in-plane properties of the preform. The 4-step and 2-step
processes produce a greater degree of interlinking as the braiding yarns travel through
the thickness of the preform, but therefore contribute less to the in-plane performance of
the preform.

Axials

Figure 2.20 Schematic of the multilayer interlock braiding process

A disadvantage of the multilayer interlock equipment is that due to the conventional
sinusoidal movement of the yarn carriers to form the preform, the equipment is not able
to have the density of yarn carriers that is possible with the 2-step and 4-step machines.
The consequence of this is that multilayer interlock braiders will be larger than 2-step
and 4-step machines for a comparable number of carriers and are considered to be less
versatile in the range of preform architectures produced (Kostar and Chou, 1999).
However the use of the traditional horn gear mechanisms offers improved braiding
speed over the 2-step and 4-step processes.
There are a number of disadvantages with all the 3D braiding processes described
here (Kostar and Chou, 1999). Firstly, compared to other textile processes, braiding can
only make preforms of small scale relative to the size of the machinery. Also, the