Vacuum-Assisted Resin Transfer (VARTM)

Vacuum-Assisted Resin Transfer (VARTM) infuses dry fiber layups with resin under vacuum, enabling large, high-fiber-fraction composite parts with moderate tooling cost.

Overview

Vacuum-Assisted Resin Transfer (VARTM) is a closed-mold composite process where dry fabric or reinforcements are laid into a mold, covered with a vacuum bag, and infused with resin under vacuum. The vacuum pulls resin through the laminate, delivering relatively high fiber volume fractions, good consolidation, and two reasonably good surfaces when a matched tool is used. It scales well to large parts and complex shapes with significantly lower tooling cost than hard steel molds.

You use VARTM when you need structural composite parts, medium to large sizes, and moderate volumes without autoclave or high-pressure equipment. It excels for parts where weight, stiffness, and consistent laminate quality matter more than tight dimensional tolerances or automotive-class cosmetics. Tradeoffs: cycle times are longer than high-pressure molding, process control is more operator- and setup-dependent, and local thickness control is looser than machined or compression-molded parts. Expect to design for generous draft, consistent wall thickness, and well-planned resin flow paths to avoid dry spots and voids.

Common Materials

  • E-glass fabric
  • Carbon fiber fabric
  • Aramid fiber fabric
  • Epoxy resin systems
  • Vinyl ester resin
  • Polyester resin

Tolerances

±0.010" to ±0.030" on critical dimensions with good tooling; laminate thickness ±5–10% typical

Applications

  • Boat hulls and decks
  • Wind turbine blades and spars
  • Automotive and truck body panels
  • UAV and light aircraft fuselages and wings
  • Radar radomes and fairings
  • Large composite covers and enclosures

When to Choose Vacuum-Assisted Resin Transfer (VARTM)

Choose VARTM for medium to large composite parts where weight, stiffness, and laminate quality matter more than very tight tolerances or Class A surfaces. It fits low to medium production volumes, especially for boats, wind, transportation, and structural covers where you want better repeatability and fiber fraction than hand lay-up without investing in high-pressure or autoclave equipment. It is also a strong fit when you can accept longer cycle times in exchange for lower tooling cost and flexible part geometry.

vs Resin Transfer Molding

Pick VARTM over high-pressure Resin Transfer Molding when you want lower tooling and equipment cost and can tolerate longer fill times and slightly looser tolerances. VARTM suits larger parts and lower volumes where simple composite or aluminum tooling and vacuum equipment are sufficient, and where you want easier process changes and lower risk if design iterations are likely.

vs Prepreg Layup with Autoclave

Choose VARTM when you cannot justify autoclave capital and operating cost or need parts too large for an autoclave. VARTM delivers good structural laminates with simpler equipment, at the expense of slightly higher void content, less precise resin content, and lower surface perfection compared to autoclaved prepreg.

vs Prepreg Out-of-Autoclave (OOA)

Select VARTM over OOA prepreg when resin cost and material shelf life are concerns, or when you want more flexible resin selection and in-shop mixing. VARTM can be more economical for thick or very large parts, where controlling exotherm and avoiding porosity with OOA prepreg is challenging, provided you can engineer reliable resin flow paths.

vs Filament Winding

Use VARTM instead of filament winding for complex, non-axisymmetric shapes, sandwich panels, or integrated stiffeners that cannot be made on a rotating mandrel. VARTM handles flat, singly, and moderately doubly curved geometries more easily, although it will not match winding for highly optimized axisymmetric pressure vessels.

vs Compression Molding (Composites)

Favor VARTM when parts are large, volumes are modest, or you want to avoid expensive matched metal tooling and presses. You give up cycle time and tight thickness control, but gain much lower tooling cost, easier design changes, and the ability to build significantly larger single-piece structures.

Design Considerations

  • Keep laminate thickness as uniform as possible and avoid abrupt thickness jumps that create flow hesitations and resin-rich spots
  • Design clear resin flow paths with provision for flow media, strategically placed inlets and vents, and adequate vacuum channels to avoid dry areas
  • Include generous draft angles and smooth internal radii to simplify demolding and reduce risk of bridging or voids at corners
  • Provide ample, flat flange area around the part for vacuum bag sealing, peel ply, and edge breather placement
  • Specify realistic tolerances and surface requirements for each side of the part so the shop can choose appropriate tool materials and bagging strategy
  • Call out core details (material, vent paths, edge closeouts, inserts) clearly so the manufacturer can plan infusion, degassing, and leak prevention up front