Page 63 - 3D Fibre Reinforced Polymer Composites

52 30 Fibre Reinforced Polymer Composites
The SCRIMP process has generally become associated with the production of non-
aerospace components such as yacht hulls, bus body-shells, refrigerated rail cars, wind
turbine blades, etc, as the use of only vacuum pressure to consolidate the preform
generally produces components with lower fibre volume fractions than RTM or RFI.
Through the careful selection of the resin systems, cure times can be lengthened to the
point that very large structures can be economically produced via the SCRIMP
technique and yacht hulls of up to 37.5 metres (123 ft) have been manufactured (Stewart
2001).
The SCRIMP process has also been described under a number of other acronyms,
VIP (Vacuum Infusion Process) and VBRI (Vacuum Bag Resin Infusion) to name just
two. The only apparent differences between all the SCRIMP-based processes appear to
relate to the techniques or materials used to distribute the resin rapidly across the
surface area of the preform.

3.3 INJECTION EQUIPMENT
Out of the three primary techniques of liquid moulding, two of the processes (RFI and
SCRIMP) do not require specialised injection equipment to introduce the resin into the
preform. The selection and use of resin injection equipment, as described in this section,
is therefore related specifically to the RTM process.
All injection equipment consists of three basic components: the resin storage area,
the resin feed apparatus and the delivery hose (an example of an RTM injection
machine is shown in Figure 3.4). There are many variations in style and operation of
these components that are available through the numerous manufacturers of injection
equipment, however one of the first equipment choices that has to be made is influenced
by the choice of resin and its handling. Essentially, resins can be handled as either one-
part, pre-mixed resinhardener systems that are injected into the mould via a single
valve, or with the resin and hardener kept separate in individual reservoirs and mixed
during the injection process in a multi-valve machine.
Both options have their advantages and disadvantages. In the one-part, single valve
process, uneven mixing can be eliminated as a!! the resin components are pre-mixed
prior to use. The cure process can also be easier to control as all the resin components
have been mixed together at the same time. There are generally less moving parts on
single valve machines therefore maintenance can be reduced and the system heating is
simplified as only one reservoir is used. Cleaning of the system is generally simpler
than multi-valve machines therefore the use of single valve machines is more suited to
low production volumes or when a variety of different resin systems are to be used. This
is generally seen in the aerospace industry or for research and development. The main
disadvantage is that as the resin is pre-mixed it can be curing within the reservoir.
Therefore, if too much resin is mixed or delays occur in production, there is a risk that
the usable life of the resin will be exceeded and the excess will be wasted.
The main advantage of multi-valve machines is due to the fact that the resin
components are kept separate and thus unmixed. This means that the usable life of the
resin system is extended and therefore larger volumes of materials can be stored in the
reservoirs. As mixing and injection of only the required amount of resin is
accomplished, waste is generally reduced. This equipment is most often used in a
production-line format where a limited number of resin types are used and the