Composite Materials Research Progress

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W.H. Zhong, R.G. Maguire, S.S. Sangari et al.
In-situ compaction and curing is another approach that will have great value for lean and
rapid production of composites using unidirectional composite prepreg tape or tow forms.
The uni-prepreg is wound onto a mandrel and heat applied from several IR sources to the
material as it is placed. There is also work being done at NASA on E-Beam in-situ curing
using low energy processing [22].
Resin Infusion Processes: Resin infusion although it is a generally an out-of-autoclave set of
processes, is itself a highly significant trend in composites processing. It covers many
processes such as RTM, VARTM, RFI and each of these major subsections has many
variations (including some which go back into the autoclave), but the common factor among
all of these is that the resins and the fibers are marketed separately and the customer fulfills
the impregnation process within their own manufacturing planning. This can offer
opportunity for customization and optimization for specific user needs and as well a
significant cost savings in materials since the costly impregnation process is now done inhouse.
Several of the major composite material suppliers forecast growing markets for the
infusion resins and dry fiber preforms as compared to the more historical prepreg material
forms.
Resin infusion is a closed low pressure process for the manufacture of complex-shape,
high-strength and lightweight composite parts for a wide range of aerospace, automotive,
marine and satellite applications. In this process, a resin system is drawn into a dry fiber
laminate in a mold where it can cure to form the finished part. RTM is an infusion process
that employs an injection system to transfer a mixture of liquid resin and catalyst into a closed
mold containing a preform, which is preset fiber mats. The resin is injected under a controlled
pressure by a carefully designed pattern of inlet ports and vent holes. This guarantees that the
fibers become fully wet and produce a low void content and high fiber-volume composite
part. Fiber volumes approaching the 65% values, which is typical for prepreg lay-up
techniques, have been achieved with RTM. Subsequent curing of the resin forms a net-shape
part with good dimensional tolerances. The size and complexity of the part significantly
influences the cycle times.
VARTM is an adaptation of the RTM process and is generally used to manufacture parts
for marine, ground transportation and infrastructure applications. The process uses an open
mold cavity which is laid up with a preform and covered with a vacuum bag made of air
impervious films such as nylon or silicone film. The air is expelled from the preform
assembly using a vacuum pump. A liquid resin is allowed to infuse into the mold from an
external reservoir after all air leaks are eliminated and the system is equilibrated. A high
permeability resin distribution medium is placed on top of the preform to facilitate the resin
flow over the lateral extent of the part. The system is kept under vacuum until the resin is
completely gelled. The part may then be cured at room temperature or in an oven. Bag leaks
and bridging are common problems in VARTM. Bag leaks take place at the sealant-bag-film
interface or as a result of film failure due to improper handling. Bridging is the failure of the
sealing bag to conform to the shape of the mold. This leads to the part failing to receive
uniform pressure during the cure cycle.
Matrix viscosity and process time are the two main differences between RTM and
VARTM. For RTM, the resin must travel through the "X" and "Y" directions while for
VARTM it travels on top and only needs to impregnate the "T" or "Z" direction. This requires