Composite Materials Research Progress

Major Trends in Polymeric Composites Technology 123
shorter time and provides the advantage of the lower temperature and faster cure time
combined with reduced thermal stress.
Resin film infusion, RFI, is a composite manufacturing process which has advanced from
earlier work on vacuum impregnation and RTM. In this process, a semi-cured resin film is
liquefied and absorbed throughout the fiber. The mold filling is further assisted by vacuum to
reduce the air voids remaining in the fabricated part. The resin and the fiber are generally
placed together into the mold but are not initially combined. In some applications the fiber
and the resin are placed in the mold in separate steps and are combined by applying pressure.
Computer simulations are commonly used to determine processing details such as resin
viscosity, preform permeability, resin/preform interactions, and the time to completely cure
the composite part. The major difference between RFI and RTM is that former uses a hot
melt resin film while the later utilizes a liquid resin. RFI does not require low minimum
viscosity as in the RTM process.
Orthophthalic, isophthalic polyesters and vinyl esters are primarily used in RTM
processes. A variety of polymers are being developed specifically for RTM application, e.g.
low-shrink and low-profile polyesters for improved surface appearance. New resins including
epoxies, acrylic/polyester hybrids, urethanes, bismaleimides (BMI), and phenolic resins are
also produced which require changes in the equipment and conditioning the resin prior to
injection. These systems offer a whole new range of cost and performance options to the
RTM process.
Reinforcements used in RTM are normally glass fibers, continuous fiber mats and
chopped strand preforms. Special mats that contain thermoplastic binders are heated and thermoformed
into perfect preforms. Both woven and non-woven glass fibers and biaxial and
triaxial mats have been produced for the RTM applications. Other high performance
reinforcements such as carbon fiber and aramid can be incorporated in RTM laminates either
alone or as part of a hybrid system.
A typical RTM mold features oil connections, injection runner system, and self
clamping/load devices. RTM surfaces offer high quality through a combination of
appropriate resin reinforcements, molds and process conditions. A combination of mechanical
clamping arrangements with special presses is necessary to secure the mold halves. This is
required for applying pressure uniformly to the mold when pressurized resin is employed.
RTM processing requires accurate and reliable injection of liquid resin. Resins must balance
low viscosity at processing temperatures and long pot life without sacrificing the mechanical
properties to alter the flow characteristics. The resin is injected until the mold is completely
filled. Motionless or non-mechanical mixers are normally utilized to blend resin and catalyst.
After curing for a required time the composite part is moved from the mold. A mixer flush
system is also incorporated when required to purge non-disposable mixers.
RTM is employed to reduce fill times and to fabricate large-scale composite structures
with substantial laminate thicknesses. It fills the gap between hand lay-up and compression
molding of sheet or bulk moldings in matched metal molds. In comparison with lay-up and
spray-up processes RTM provides two finished surfaces on parts which can be similar or
dissimilar, highly reproducible thickness, low monomer loss and higher output since it is less
labor and material intensive. Compared with matched-metal-die compression molding, RTM
enables the use of parts such as ribs and inserts, decreases lead times for molds, and has
lower-cost molds and molding equipment.