Resin Transfer Molding (RTM)

How Resin Transfer Molding Works

RTM is a closed-mold process built around a matched, two-piece tool. Because both halves of the mold are rigid and machined to the finished part shape, the process gives a controlled, repeatable result on components that need a finished surface on both sides, not just the side facing the tool.

1. Reinforcement Placement

Our technicians cut and lay dry fiberglass reinforcement into the open lower half of the mold, positioning it to match the load paths and thickness the part design calls for. Inserts, mounting bosses or metal hardware can be placed in the tool at this stage so they mold in as a single piece.

2. Mold Closing and Clamping

The upper half of the mold is closed and clamped against the lower half, fully enclosing the reinforcement. This is what separates RTM from open-mold processes like hand lay-up and spray-up, where only one side of the part ever touches a tool surface.

3. Resin Injection

Liquid resin is injected into the closed mold under pressure, drawn through the reinforcement by a combination of injection pressure and vacuum assist. The resin wets out the fiberglass evenly before it has a chance to gel, which is what gives RTM parts their consistent fiber-to-resin ratio from one part to the next.

4. Cure

The part cures inside the closed mold. Because the tool controls both surfaces during cure, the finished part holds the shape of the mold on both sides rather than relying on hand work to set the exposed face.

5. Demolding, Trim and Inspection

Once cured, the mold opens and the part is removed, trimmed to its final profile, and inspected before it moves to secondary operations or shipping.

Tooling and Lead Time

RTM uses a matched two-part mold instead of the single-sided tool used for hand lay-up or spray-up. That two-sided tool costs more to build and takes longer to fabricate up front, but it is what makes RTM repeatable at higher part counts than an open-mold process and consistent from the first part to the last. We size and quote tooling per project once we have the part drawing, since mold complexity is what drives the timeline, not the process itself.

Tolerance and Finish

Because both faces of an RTM part are formed against a rigid tool, RTM holds tighter, more repeatable dimensions than open-mold processes, and it produces a finished surface on both sides of the part rather than one molded face and one hand-finished face. Exact tolerances depend on part geometry and are confirmed against your drawing during quoting.

Where RTM Fits Against Our Other Processes

RTM sits between our open-mold processes and our high-volume compression process. Hand lay-up and spray-up need no matched tooling and suit lower part counts or larger, simpler shapes. SMC compression molding uses a heated matched tool and suits the highest part counts, where the tooling investment pays off fastest. RTM is the fit when a project needs the two-sided finish and dimensional control of a closed mold, but the run size does not yet justify SMC’s compression tooling.

Materials

RTM parts are built from fiberglass reinforcement wetted out with a liquid thermoset resin, most commonly polyester, vinyl ester or epoxy depending on the strength, chemical resistance or heat performance the application calls for. We select reinforcement type and resin system together with you based on the part’s structural and environmental requirements.

Secondary Operations and Delivery

An RTM part rarely ships as-molded. Once a part is trimmed and inspected, it typically moves into the value-added work that finishes it for installation: painting, bonding, and fitting secondary components such as hardware, gaskets or PVC, metal and wood inserts. We handle this work in-house rather than sending parts out to a separate finisher, so an RTM order arrives ready to install rather than as a raw molding that still needs additional work elsewhere.

Every RTM part is checked against the part drawing before it ships. If your project needs a specific inspection step, tell us during quoting and we will build it into the run.