Biomedical Engineering Reference
In-Depth Information
In autoclave molding there is generally one metal
mold surface onto which the pre-preg stack is
assembled in preferred orientations. This is then
covered with a high-temperature-resistant plastic film
and sealed around the edges to make it airtight. Air is
then withdrawn from the bag through one, or more,
vac-line ports, which forces the pre-preg layers
together under atmospheric pressure. There may be
additional layers included between the pre-preg stack
and covering film to assist with drawing the vacuum
(breather layers) and there may be a peel-ply layer in
direct contact with the pre-preg stack to aid de-
molding and removal of the breather layers.
In the autoclave the temperatures and pressures are
increased in programmed steps such that polymer
melting and consolidation occur with minimum
entrapment of any residual air that would cause
voids. The temperature is then lowered to below the
melting temperature to solidify the polymer and then
further reduced to below
T
g
ready for de-molding.
3.5.2.2 Pultrusion
In some cases it is convenient to have the material
in the form of a rod with all of the constituent fibers
aligned along the axis (uniaxial).
Figure 3.24
shows
some ENDOLIGN rods made by pultrusion using
pre-preg tape as the starting material. This maxi-
mizes the strength and stiffness of the rod as all the
load-carrying components are aligned with the prin-
cipal loading direction. Tensile elements, such as
concrete reinforcing rods, have been made from other
materials in this way to achieve maximum strength.
Pultrusion is a process in which tapes of axially
aligned unidirectional continuous fiber pre-preg
material are pulled through a die to shape them into
a regular section. There is an oven section to heat and
melt the polymer, a shaping die to form the required
sectional shape, and a haul-off machine to provide
the tension needed to pull the material along. Cooling
and solidification of the polymer may be achieved by
just air cooling, or water cooling. In the case of
PEEK, this is more often an intermediate step and the
pultruded rod may be subjected to further processing
as will be described later. However, the rod may also
be used in its pultruded form for some medical
devices. During pultrusion it is also possible to co-
mix continuous metallic wires (e.g., tantalum) with
the pre-preg in appropriate amounts to achieve
a composite material with desired radiopacity in the
final part.
Figure 3.24
ENDOLIGN pultruded rods (unidirectional
carbon fibers). Photo courtesy of Invibio.
3.5.2.3 Filament Winding
Filament winding is a further method of making
parts from continuous fiber-reinforced composites.
In this process, heated pre-preg tape of a few mm
width is wrapped around a rotating metal mandrel
in an overlapping helical pattern a number of times
to create a layered cylinder with a preferred fiber
lay-up. The pre-preg is heated to above
T
m
at the
point of application, usually by using a hot air
blower, and is sufficiently heated to bond with the
underlying wrapped material when pressed to
consolidate using the applicator head. When the
prerequisite number of layers has been applied, the
mandrel is removed by pulling it out, leaving
a composite tube. It is also possible to use a pul-
truded rod as described earlier as mandrel and in
this case it may be retained as the core of the
device. The rod may also have a continuous bore
molded in at the pultrusion stage. Overwrapping
with pre-preg as a filament wound product creates
a level of reinforcement that gives torsional strength
and stiffness as well as preventing the bore from
splitting under adverse loading conditions. This
