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Resin Transfer Molding

Resin Transfer Molding (RTM) is a method of composite material production. The main driving force for the continued research and development of the RTM process is the need for easy production of complex parts. The aerospace in particular is in need of complex composite parts and RTM gives them a means of creating them consistently and in a timely manner. RTM arose from further development of the Marco Process, which was a method for creating reinforced plastics back in the 1930's through the 1960's. In the 1960's more research was done and a method was created for pressure injecting high performance resin into an organized preform.

Process

Resin Transfer Molding is a process that relies on a 2 sided mold to create a simple or complex part. The process is begun by adding a release agent to both sides of the mold to ensure that the part can be easily removed upon completion .Next is hand laying up the reinforcement phase of the composite part. The reinforcement phase can be a variety of fibrous materials such as glass fibers and can be in the form of a woven mat or roving. The reinforcing material is placed into the mold to create the desired shape, then the mold is tightly closed. Once the mold is closed, pressurized resin, the matrix phase, is injected into the mold and dispersed throughout the fibrous reinforcement. The air inside the mold is displaced by the injected resin at the edges and this continues until the fabric is completely saturated and the mold is filled. The resin injection process can be sped up with an increase in injection points/gates. At this point resin is stopped from being injected, and the part is left to cure for minutes to hours depending on the matrix material used. Once the resin is fully cured the mold is opened up and the part is removed.

Processing Parameters

Significant Parameters: Pressure, temperature, viscosity, permeability, volume fraction and filling time of the process, angle of the attack nozzle, orientation of the fibers, paths of flow, shear rates

The injection velocity of the resin into the mold is determined by the process pressure. This injection pressure also affects the holding and closing forces of the mold as well as the filling time. If the filling time is too long, premature gelation of the resin can occur. On the other hand, if the filling time is too short, there may not be enough time for the fibers to be adequately impregnated. The filling time and pressure also affect the formation of air voids in the composite. These void defects change the surface appearance and mechanical properties but can be avoided by using the correct pressure to ensure an even distribution of the resin.

Temperature, viscosity, and pressure also have significant interactions. As the temperature decreases, the viscosity increases. With an increased viscosity, higher pressures are needed to transfer the resin.

Advantages and Applications

●    Complex shapes and parts can be made with RTM

●    Lower defect rates for aerospace parts than in other molding processes

●    Controlled fiber volume fractions, which results in consistent mechanical properties

●    Less money is spent on storage costs since prepreg is not used

●    3D wovens, braids, etc. can all be molded

Companies That Utilize RTM

●    Composite Integration Ltd - Manufacture and Design tooling for RTM processes.

●    JHM Technologies - Offers three time of RTM injection equipment with a high level of process control

●    Owens Corning - RTM products

●    Apogee Products - Hand laminating and fiberglass repair specialist that produce mold sets for fiberglass components

●    FormaShape - Helped advance the Light RTM process with their product range