Resin Transfer Molding

Resin Transfer Molding injects resin into a closed mold with dry fiber preforms to produce repeatable, two-sided composite parts at medium to high volumes.

Overview

Resin Transfer Molding (RTM) is a closed-mold composites process where a dry fiber preform is placed in a rigid tool and impregnated by injected resin under pressure. The mold is then heated to cure the part, producing good surface finish on both sides and consistent fiber volume fraction. RTM suits structural components that need controlled thickness, embedded inserts, and repeatable performance.

You typically choose RTM for medium to high production volumes, where investing in matched tooling is justified and part size is small to moderately large. It works well for complex shapes with ribs, bosses, and stiffeners that are difficult to achieve with open-mold methods. Tradeoffs: higher tooling and upfront engineering effort than hand lay-up, more sensitivity to flow path design, and limits on extremely thick or highly variable section thicknesses. RTM delivers better dimensional control and cycle times than manual lay-up, but you must design for resin flow and venting to avoid dry spots and porosity.

Common Materials

  • E-glass fiber with epoxy
  • E-glass fiber with polyester
  • Carbon fiber with epoxy
  • Carbon fiber with vinyl ester
  • Aramid fiber with epoxy

Tolerances

±0.010" to ±0.020" on molded dimensions

Applications

  • Automotive body panels and closures
  • Aircraft interior panels and fairings
  • Wind turbine nacelle and hub covers
  • Rail and bus exterior body panels
  • Industrial equipment and medical housings
  • Sporting goods shells and protective gear

When to Choose Resin Transfer Molding

Choose Resin Transfer Molding for structural composite parts that need good two-sided surface finish, moderate to tight dimensional control, and repeatability across medium to high production volumes. It fits best when you can standardize thicknesses, design for resin flow paths, and justify matched-metal or composite tooling investment.

vs Vacuum-Assisted Resin Transfer (VARTM)

Pick RTM when you need tighter dimensional control, higher fiber volume fraction, and more consistent two-sided surface finish than typical VARTM setups. Closed, rigid tools and pressure-driven injection give better repeatability and shorter, more controlled cycle times on medium- to high-volume parts.

vs Prepreg Layup with Autoclave

Choose RTM when you want lower material cost, shorter layup labor, and higher throughput without autoclave capital expense. RTM is better suited to larger production runs of moderately loaded structures where ultra-high fiber volume and autoclave-level void content are not critical.

vs Prepreg Out-of-Autoclave (OOA)

Select RTM when you need higher automation potential and more repeatable fiber wet-out than manual OOA prepreg layup. RTM is more efficient for medium- to high-volume production with consistent geometries, while still achieving structural performance adequate for many transportation and industrial parts.

vs Filament Winding

Use RTM instead when you need complex, non-axisymmetric shapes, integrated ribs, or molded-in inserts instead of primarily cylindrical or rotationally symmetric parts. RTM handles housings, panels, and shells where filament winding cannot practically form the geometry.

vs Compression Molding (Composites)

Choose RTM over compression molding when continuous fabric reinforcements and more complex laminate architectures are important, rather than chopped or sheet molding compounds. RTM offers better control of fiber orientation for structural parts with directional load paths, at the cost of longer fill times and more complex flow design.

Design Considerations

  • Maintain relatively uniform wall thickness and avoid abrupt section changes to promote even resin flow and reduce dry spots
  • Plan resin inlet, runner, and vent locations early; run flow analyses on complex parts to size gates and ensure complete wet-out
  • Include draft (typically 1–3°) on vertical walls and generous radii at internal corners to ease demolding and reduce fiber bridging
  • Define critical-to-function dimensions and surfaces so the shop knows what may require secondary machining rather than relying solely on mold accuracy
  • Design in accessible locations and flat pads for mechanical fasteners or inserts; specify insert type, temperature limits, and load paths
  • Specify allowable porosity, cosmetic requirements, and surface class for both A- and B-sides so tooling, venting, and process controls can be aligned with expectations