Composite Materials Research Progress

124
W.H. Zhong, R.G. Maguire, S.S. Sangari et al.
RTM provides a lot of processing flexibility. Shrink control systems can be employed to
produce improved surfaces. The fibers and resins are utilized at their lowest cost plus resin
content can be controlled to a significant degree and reinforcements can be easily
incorporated. Cores and other insets can also be positioned prior to resin injection to yield
complex parts in one fabrication step. RTM can also be used to prototype parts for market
evaluation since the initial investment costs such as tooling and operating expenses are low.
In this case, RTM's short lead time and lower cost tooling is a real advantage. It also offers
production of near-net-shape parts which in turn leads to low material wastage. Closed RTM
molds release fewer volatile materials than open molds. Added benefits of this closed-mold
process are greatly reduced volatile organic compound (VOC) emissions.
VARTM can be used at room temperature with no heated mold and compared to RTM
more consistent and uniform coverage can be obtained. VARTM provides significant savings
in the tooling cost as it requires only a one-piece mold. The use of the vacuum bag eliminates
the need for making a precise matched metal mold as in the conventional RTM process and
thus reduces the cost and design difficulties associated with large metal tools. VARTM also
beats the 5-10% accuracy of hand lay-up.
For a given resin, mold filling using RFI is relatively faster than most other liquidcomposite-molding
processes since the RFI products are usually thin and the infusion
distance is short. In addition, RFI can provide parts with high mechanical properties due to
solid state of initial polymer material and elevated cure temperature.
Along with the advantages, RTM also has inevitable disadvantages including inability to
manufacture very large parts and permeability issues which lead to increased processing time.
It is an intermediate volume production process and its tooling and equipment costs are higher
than the hand lay-up and spray-up. Tooling design and fabrication to handle injection
pressures, clamping and sealing are more complex and manufacture of complex parts require
trial and error experimentations combined with flow simulation modeling.
VARTM also is a relatively complex process to perform well. The flexible nature of the
vacuum bag brings about the difficulty of controlling the final thickness of the preform, and
hence the fiber volume fraction of the composite. Due to the complex nature of VARTM, the
trial and error experimentation is not only inefficient but also expensive for the process design
and optimization.
RTM is being used for a number of products including aircraft components, recreational
vehicle, truck and sports car bodies, automobile panels, medical equipment, dish antenna,
storage tanks, electrical covers, windmill blades, plumbing parts, transportation seating,
chemical pumps, marine components such as hatches and small boats, bicycle frames and
doors.
Resin Infusion processes enable us to manufacture a wide range of complex as well as simple
fiber reinforced composite products. Improvements are being made on resins and tooling
developments which further expand market opportunities. The combination of pre-positioning
a variety of reinforcements and incorporating secondary reinforcements and other design
details with good surface control on both sides makes RTM a primary process candidate for
structural applications in the aerospace industry as well as other markets. Due to its
versatility, growing use in industry, and being environmentally friendly, experts believe that
the future of this useful composite process should continue to grow. In many applications its
replacement of hand layup/ prepreg material forms/autoclave processing production has