In most cases, the FRP composites are applied manually using hand-impregnation
technique. Also referred to as hand lay-up, this process involves placing (and working)
successive plies of resin-impregnated reinforcement in position by hand. Squeegees and
grooved rollers are used to densify the FRP structure and remove much of the
entrapped air.
To avoid potential delamination failures from occurring, a denser FRP must be
manufactured by removing nearly all air voids within the composite. Two methods that
are capable of accomplishing this task are vacuum-assisted impregnation (vacuum
bagging) and pressure bag molding (pressure bagging). Vacuum bags apply additional
pressure to the composite and aid in the removal of entrapped air, as shown in Figure 1.1.
Pressure bags also invoke the use of pressure but are considerably more complex and
expensive to operate. They apply additional pressure to the assembly through an
electrometric pressure bag or bladder contained within a clamshell cover, which fits over
a mold. However, only mild pressures can be applied with this system (May, 1987).