Resin Transfer Molding

Resin Transfer Molding (RTM) forms composite parts by injecting resin into a closed mold containing dry fiber preforms, delivering good surface finish and repeatability.

Overview

Resin Transfer Molding (RTM) is a closed-mold composite process where a dry fiber preform (glass or carbon) is loaded into a matched tool, then resin is injected under pressure to fully wet-out the laminate and cure. The closed tool gives controlled thickness, two “tool-side” surfaces, and better dimensional repeatability than open-mold methods.

Choose RTM for medium-rate production where you need structural composites with clean surfaces on both sides, consistent fiber volume, and integrated features (ribs, bosses, inserts) without extensive post-processing. RTM can hit good cosmetics and stable dimensions when the preform and flow paths are designed correctly.

Tradeoffs: tooling and process development (preforms, flow media, gates/vents) drive upfront cost and lead time. Part size is limited by press/tool handling, and complex geometry can trap air or cause dry spots if venting and permeability aren’t managed. Trimming, drilling, and bonding prep are still common secondary ops.

Common Materials

  • Carbon fiber
  • Fiberglass
  • Epoxy
  • Vinyl ester
  • Polyurethane

Tolerances

±0.010" to ±0.030"

Applications

  • Automotive body panels
  • Aerospace fairings and access panels
  • Composite leaf springs
  • Structural brackets and stiffened panels
  • Medical imaging equipment covers
  • UAV fuselage shells

When to Choose Resin Transfer Molding

RTM fits medium volumes where matched-mold tooling is justified and you need controlled thickness, good cosmetics on both sides, and repeatable structural performance. It works best for parts that can be demolded from a two-sided tool and can be designed with predictable resin flow and venting.

vs Vacuum-Assisted Resin Transfer (VARTM)

Choose RTM when you need tighter thickness control, higher repeatability, and better two-side surface quality from a rigid matched tool. RTM also supports faster, more automated cycles once the tool and injection strategy are dialed in, at the cost of higher tooling complexity.

vs Prepreg Layup with Autoclave

Choose RTM when you want closed-mold dimensional control and good surface finish without autoclave-capable tooling, freezer storage, and prepreg handling. RTM is often a better fit for medium-rate production and thicker sections where resin-rich edges and layup time are major cost drivers.

vs Prepreg Out-of-Autoclave (OOA)

Choose RTM when you need two finished tool surfaces and more consistent thickness than vacuum-bag-only consolidation typically provides. RTM can reduce touch labor by using dry preforms and injection, but demands robust flow/vent design to avoid voids and dry spots.

vs Compression Molding (Composites)

Choose RTM when you want continuous-fiber laminates with engineered fiber orientation using dry fabrics or stitched preforms rather than chopped/SMC-style charge flow. RTM is also useful when resin system selection and controlled injection are critical to meet mechanical or cosmetic targets.

vs Hand Lay-Up

Choose RTM when repeatability, thickness control, and surface finish matter and you need a production process with less operator-to-operator variability. RTM’s closed mold reduces emissions and post-finishing compared with open-mold layup, but requires higher upfront tooling and process development.

Design Considerations

  • Design clear parting lines and draft that allow reliable demolding without distorting the laminate or damaging gel coat/paint-ready surfaces
  • Place gates and vents to promote short, uniform flow paths; avoid isolated pockets that can trap air and create dry spots
  • Keep wall thickness as uniform as possible; large thickness jumps increase risk of race-tracking, voids, and cure-induced distortion
  • Specify fiber architecture and areal weights early (stack, orientation, core details) so the supplier can model permeability and fill time for quoting
  • Use generous radii and avoid sharp internal corners to improve preform fit and reduce bridging and resin-rich zones
  • Define trim lines, hole locations, and datum features; plan for post-trim/drill and include tolerances appropriate for composite springback