What is resin transfer molding (RTM) in composite manufacturing

Resin Transfer Molding (RTM) is a closed-mold composite process in which a dry fiber preform is placed in a matched, rigid two-part mold (aluminum or steel), the mold is closed, and liquid resin is injected under low-to-moderate pressure (roughly 1 to 10 bar) to impregnate the fibers and cure in situ. Flow-front sensors and exit-vent observation monitor fill.

RTM produces near-net-shape parts with dual-sided smooth finishes, good thickness control, and high dimensional stability. It is suited to medium-volume production (hundreds to thousands per year) of aerospace brackets, blade spars, and automotive structures. The principal cost is the expensive matched rigid tooling, and the process demands careful mold sealing to avoid resin leakage during injection.

How to recognize RTM in practice#

A part made by standard RTM has three features.

It has matched smooth surfaces on both sides, because both mold faces are rigid tool steel or aluminum. This is the defining surface signature versus autoclave or VARTM, which have one rough bag-side face.

It has the geometric envelope of the matched tooling, so the part is dimensionally precise on both faces with no draft taper beyond what the mold imposes. RTM parts can carry integrated ribs, bosses, and inserts molded in during fill.

It is a closed-mold process: the preform is laid up and consolidated dry, then the mold is closed and resin is injected. This is the operational difference from wet layup or prepreg processes, which start with the resin already on or in the fibers.

A complete RTM tooling setup includes the two-piece matched mold, resin and hardener mixing and metering equipment, injection ports and exit vents (with flow-front sensors), a clamping press or hydraulic actuator to keep the mold closed under injection pressure, and a heated platen or external oven for cure. The capital cost of the tool is the dominant economic factor, which is why RTM lives at medium-volume scales between low-volume hand layup and high-volume HP-RTM.

The variants: HP-RTM and VARTM#

Two variants extend the basic RTM process to different volume and part-size regimes.

High-Pressure RTM (HP-RTM) is the fast-cycle high-volume variant. The chemistry is fast-reacting resin (typically a high-reactivity polyurethane or fast-cure epoxy), the injection pressure is tens to over 100 bar, and the tooling is heated steel held in a compression press. Cycle times drop to 2 to 5 minutes, and the surface finish reaches class-A on both faces. HP-RTM is the method of choice for high-volume automotive carbon chassis components, including the BMW i3 and i8 passenger cells that demonstrated the process at scale.

At HP-RTM pressures the preform must be firmly clamped or fiber washing (resin pushing fibers out of position) results. Process control is critical: fiber placement, preform binding, resin viscosity at injection temperature, and clamp pressure all interact. The high tooling cost (heated steel plus press) sets a higher minimum production volume than standard RTM.

Vacuum Infusion / VARTM (Vacuum-Assisted Resin Transfer Molding) uses a single rigid tool face and a flexible vacuum bag to form the cavity over a dry preform. Vacuum is drawn and atmospheric pressure draws resin in from one edge through the fibers, often via flow media that distribute resin laterally before allowing it to wick through the laminate thickness. The process is watched through the transparent bag and monitored by vacuum transducers.

VARTM has low tooling cost (only one rigid face is required) and excels at very large, complex shapes that no autoclave can hold: wind-turbine blades, yacht hulls, large structural panels for marine and infrastructure use. The trade-offs are that VARTM does not reach autoclave fiber volumes (atmospheric pressure consolidates less than 6 to 7 bar autoclave pressure), cure times are longer, and the bag-side face lacks precise dimensional control.

Confusion points#

RTM versus autoclave cure. Autoclave cure starts with prepreg (fibers already wet with B-stage resin) and cures the laminate in a sealed pressure vessel. RTM starts with dry preform and injects liquid resin in a closed mold. The autoclave cure produces the highest consolidation quality (under 1 percent void, 60 to 65 percent fiber volume); RTM produces lower fiber volume (typically 50 to 55 percent for standard RTM, higher for HP-RTM) but enables dual-sided smooth finish, near-net shape, and lower per-part labor at medium-to-high volumes.

RTM versus VARTM. Same family, different driving pressure and tooling. RTM has two matched rigid molds and injects resin at 1 to 10 bar; VARTM has one rigid face plus a flexible vacuum bag and uses the atmospheric pressure differential (about 1 bar) to draw resin. RTM is medium-volume with class-A finish on both sides; VARTM scales to very large parts with one smooth tool-side and one textured bag-side.

RTM versus HP-RTM. Same closed-mold injection principle, different chemistry and pressure regime. RTM uses lower injection pressure (1 to 10 bar) and slow-to-moderate-cure resin in aluminum or steel tooling. HP-RTM uses tens to 100+ bar injection of fast-cure resin in heated steel tooling under press, dropping cycle time from 30 to 90 minutes to 2 to 5 minutes. The choice is volume-driven: HP-RTM tooling and press capital amortizes only at high volumes.

RTM versus liquid composite molding (LCM). RTM is one method within the broader LCM family. Other LCM variants include resin film infusion (RFI, in which resin film is laid on the preform and infused by heat), Seemann Composites Resin Infusion Molding Process (SCRIMP, a VARTM variant with engineered flow distribution media), and Light RTM (a low-pressure variant using a semi-rigid composite upper mold and vacuum). Each variant trades tooling cost against fiber volume and part-size envelope.

Preform versus stacked plies. The "preform" in RTM is the dry reinforcement, shaped to the part geometry before mold close. It can be a stitched multi-axial fabric, a 3D woven preform, a braided tube, or a stack of cut layers. Preform handling is its own subspecialty; binder chemistry holds the preform shape during transport and closure.

Related terms#

• Autoclave cure cycle: the contrasting prepreg-based cure process that produces the highest consolidation quality.
• Vacuum infusion (VARTM): the single-side rigid mold variant that scales to wind blades and yacht hulls.
• Fiber volume fraction: the consolidation metric that distinguishes RTM (50 to 55 percent) from autoclave (60 to 65 percent) and VARTM (lower).
• Prepreg: the alternative material format that bypasses the injection step entirely.