Resin transfer moulding

Resin Transfer Moulding (RTM) fits into the broad fabrication category of closed moulding processes, where composite manufacturing is conducted within an enclosed cavity.

Since it can be automated and is capable of rapid production cycle times, RTM is suitable for medium to high volume production, producing relatively low cost parts. Higher production rates can be achieved by heating the moulds, using preform reinforcements and resins with short gel and cure times. RTM involves a capital investment in tooling and infrastructure that is higher than vacuum infusion but lower than compression moulding.

RTM uses two matched moulds - a bottom mould and a top mould - brought together to produce parts with two finished surfaces.


  1. The top and bottom moulds are firstly gelcoated.
  2. Dry fibreglass reinforcements (and core if required) are placed by hand or robot into the bottom mould.  The use of preforms will reduce loading times.1
  3. The two moulds are brought together and clamped.
  4. The catalysed resin is injected under pressure into the cavity between the moulds until this is full and resin comes through the air vents in the bottom mould.
  5. Vacuum can be used to enhance the resin flow as it wet’s out the reinforcements.
  6. The part is cured in the mould.
  7. The two moulds are separated and the completed part removed.


1Preforms are reinforcements that have been pre-shaped in a separate pressing process to speed up positioning in the mould.

Hints & Comments

  1. Moulds for RTM range from low cost composite moulds to higher cost temperature controlled metal moulds.
  2. To withstand the pressures of RTM, moulds must be relatively heavy.
  3. Part thickness in RTM is determined by the size of the cavity between the moulds, with relatively high tolerance and high repeatability achievable.
  4. Being a closed mould process, the resin is enclosed and emission of volatiles is minimised, resulting in improved workshop conditions.
  5. Lower labour costs become more apparent as the volume increases.
  6. Epoxy, polyester, vinyl ester and other resins can be used in RTM.
  7. Most reinforcements can be used with this process and most cores with the exception of honeycombs.
  8. RTM produces parts with very low void content, and high fibre volume can be achieved.