Page 58 - 3D Fibre Reinforced Polymer Composites

Chapter 3

Preform Consolidation

3.1 INTRODUCTION

The 3D textile preform production techniques outlined in the previous chapter are only
the first stage in the production of a 3D fibre reinforced composite material. The use of
sophisticated equipment and the intelligent design of the preform will all be to no avail
if there is no adequate technology for consolidating the preform with polymer resin.
Some traditional methods of composite material production are simply not suited for
use with 3D textile preforms. Hand impregnation involves the use of brushes and rollers
to physically work the resin into the fibre preform, which can cause distortion of the
preform architecture. It is also not capable of removing all entrapped air from the
consolidated composite due to the process being performed at atmospheric pressure.
This would result in a component of low quality that would be unsuitable for the high
performance tasks normally associated with 3D textile preforms.
The pultrusion process involves a preform being pulled in a continuous fashion
through a resin bath in which it is fully wet out. It then travels into a heated die where
the resin is cured rapidly and a fully consolidated product emerges from the die where it
is cut to the required length. It is theoretically possible to consolidate 3D preforms via
the pultrusion process and there would be significant advantages to this as a single
source of fabric would be more cost efficient to set up and control compared with the
multitude of yarn and 2D fabrics sources that are currently used. However, the current
wet out process involves the fabric and yarn having to follow complex paths around
guide bars in order to work the resin fully into the fibres. This would severely distort the
fibre architecture of a 3D preform thus compromising the mechanical performance of
the final composite part.
The use of commingled yarns to produce the preform is another possible
consolidation route. These yarns consist of the reinforcement fibres intermingled with
fibres of thermoplastic resin or particles of partially cured thermosetting resin. These
commingled yarns can then be processed into textile preforms via the techniques
outlined in Chapter 2, although for commingled thermoset yarns it is more difficult as
the yarns often become less flexible through the commingling process. The application
of heat and pressure then causes the resin to melt and wet out the preform. The
difficulty arises in that the volume occupied by the resin relative to the total
unconsolidated preform volume is low. Therefore to ensure that the available resin
completely fills the fibre reinforcement and the volume fraction of reinforcement fibres
is structurally significant, the preform must be dramatically reduced in volume during
consolidation. This is generally not a problem for two dimensional fibre architectures as
the thickness can be reduced without disrupting the architecture however a three-
dimensional fibre architecture will be severely distorted through this consolidation thus
rendering this manufacturing route unsuitable.