Pre-preg

Pre-preg also allows one to impregnate a bulk amount of fiber and then store it in a cooled area (below 20 °C) for an extended period of time to cure later.

Military aircraft mainly use 180 °C systems The prepreg matrix consists of a mixture of resin and hardener, in some cases an accelerator.

Since vinyl ester resins must be pre-accelerated with amine accelerator or cobalt, their processing time at room temperature is shorter than with epoxy-based prepregs.

The properties of the resin and fiber constituents influence the evolution of VBO (vacuum-bag-only) prepreg microstructures during cure.

[5] The dependence of microstructural evolution on resin properties, therefore, is critical to understand, and has been investigated by numerous authors.

However, Ridgard explains that VBO prepreg systems are designed to remain relatively viscous in the early stages of cure to impede infiltration and allow sufficient dry areas to persist for air evacuation to occur.

[6] However, the overall viscosity profile must also permit sufficient flow at cure temperature to fully impregnate the prepreg, lest pervasive dry areas remain in the final part.

[7] Furthermore, Boyd and Maskell[8] argue that to inhibit bubble formation and growth at low consolidation pressures, both the viscous and elastic characteristics of the prepreg must be tuned to the specific processing parameters encountered during cure, and ultimately ensure that a majority of the applied pressure is transferred to the resin.

Recent advances in out of autoclave (OOA)[11] processes hold promise for improving performance and lowering costs for composite structures.

Using vacuum-bag-only (VBO) for atmospheric pressures, the new OOA processes promise to deliver less than 1 percent void content required for aerospace primary structures.

Led by material scientists at Air Force Research Lab, the technique would save the costs of constructing and installing large structure autoclaves ($100M saved at NASA) and making small production runs of 100 aircraft economically viable.