Page 66 - 3D Fibre Reinforced Polymer Composites

Preform Consolidation 55
resin system based upon factors such as mechanical properties, environmental
resistance, cost, etc. Although these are important criteria for any resin, they do not
directly effect the ability of the resin to be processed under liquid moulding conditions.
There are essentially two processing factors that are critical to know in selecting a resin
system for successful liquid moulding and these are the resin viscosity and pot life.
The viscosity of the resin must remain low enough during the entire moulding
process in order to enable the resin to successfully infuse the complete volume of
preform without the need for excessive driving pressures to be used. Within the three
types of liquid moulding processes described here, the driving pressure can range from
less than lOOkPa up to approximately 700kPa which is commonly used in rapid
injection processes within the automotive industry. The preform volume fraction and its
size also plays a part in determining the necessary resin viscosity, with low fibre volume
fractions having a greater permeability to the resin than high volume fraction preforms.
However, within the range of injection pressures, preform volume fractions and
component sizes, the general rule-of-thumb used is that resins suitable for liquid
moulding should have viscosities no higher than 500 cps (centipoise) during moulding.
This is particularly true for the high volume fraction preforms used in the aerospace
industry as the use of resin systems with viscosities higher than this tends to lead to
mould pressures that are difficult to handle and often produces composites with poor
fibre impregnation.
Given the critical influence of resin viscosity to the liquid moulding process, the
practical definition of resin pot life within liquid moulding is normally defined as the
time it takes for the resin system’s viscosity to reach a level which prevents further
liquid moulding from occurring (generally 500 cps). Depending upon the size and
complexity of the part, resin pot lives may be required to run from minutes, for the rapid
production of automotive parts, to hours for large marine structures. The time required
to fill a preform can be determined from Darcy’s Law which relates the flow rate of a
resin to parameters such as its viscosity and the preform permeability.

permeability x cross - sectional area pressure drop
Flow rate = X
resin viscosity unit length

As a significant proportion of liquid moulding processes occur with thermosetting
resins, the operator must be aware that the resin will generally be curing throughout the
process and thus its viscosity will be increasing with time. The temperature of the resin
during liquid moulding will also affect the resin viscosity. The initial viscosity will
decrease with increased temperature but the rate of cure will increase, therefore the
operator needs to obtain a balance between moulding temperature and pot life in order
to ensure that the preform is successfully consolidated. An illustration of how
temperature and time affects the resin viscosity for epoxy systems is shown in Figure
3.5
There are a wide variety of resin systems that can be used for liquid moulding and
more detailed information can be found in the references Kruckenberg and Paton