Resin Transfer Molding Process

Resin Transfer Molding

Resin transfer molding is a process where reinforcing material is manually placed into a mold. The mold is then closed and resin is introduced by injection, infusion, or vacuum pull.

Capabilities / Processes / Resin Transfer Molding
Resin Transfer Molding Process Volume Graph

Raw Material

The manual nature of the resin transfer molding process allows for almost any reinforcing material to be considered, chopped strand or mat. Similarly, the resin and catalyst blend can be manipulated to allow for ideal processing conditions.


A core and cavity are necessary for RTM. Tools can be made from composite materials or aluminum.


RTM parts can be made with very good surface finishes, on either side. Typically a gel coat will be introduced into the process to achieve improved finishes, or to provide a paint-ready surface.

  • Improved Part Tolerances
    Compared to the open mold processes of hand lay-up and spray-up, RTM processes result in higher dimensional control, especially in part thickness. This is the result of using matched-die, closed mold tooling.
  • Very Large Part Capability
    Part size is limited only by the ability of toolmakers to create molds. The RTM process is used for extremely large products such as wind turbine nacelles and yacht hulls. These would be difficult to produce using any other composite process.
  • Localized Material Properties
    The placement of reinforcing fibers prior to resin infusion allows part performance to be locally optimized. Additional (or varied forms of) fiber can be precisely positioned in areas likely to experience high stress, for example.
  • Process Flexibility
    There are many variations to manners in which resin can be introduced to the closed mold in the RTM process. High-pressure injection and vacuum infusion are two prominent methods. Vacuum infusion eliminates the need for high-tonnage assets (to keep the mold halves closed) and allows RTM parts to be made almost anywhere.
  • Improving Tool Life
    Use of aluminum in place of traditional composites for RTM molds can improve both part surface quality and greatly extend the useful life of the tools. CORE has experience with RTM tool life >20,000 cycles.
  • Mold in B-Side Datums
    For more complex part geometries that require it, the closed molds used in RTM allow part datums to be molded directly into both A- and B-Sides of a product. This improved tolerance control allows for better fits in downstream assembly operations.
  • Ability to Create Hollow Sections
    One variation of RTM molding is the use of a bladder (“bag”) that is inserted into a mold and inflated prior to resin infusion. The process is used to create hollow products such as fan blades.

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