S. Devaraju, M. Alagar, in Unsaturated Polyester Resins, 2019 2.9.1.6 Resin transfer molding. Resin transfer molding (RTM) is a closed-molding fabrication process where UPR is transferred over already-placed reinforcement fibers (glass fiber, carbon fiber, aramid fiber, and natural plant fibers such as sisal, banana, nettle, hemp, and flax) which are then placed on the surface of a mold.