Flow monitoring of microwave pre-heated resin in LCM processes

Liquid composite molding is manufacturing techniques that involve the injection or infusion of catalyzed liquid resin into a mold to impregnate a dry fiber preform. The challenges of LCM processes are related to the obtaining of a complete wetting of the reinforcement as well as a reduction of the void to obtain a final product with high mechanical properties. The heating of the resin prior the injection into the mold cavity has proven to be useful to improve the LCM processes. The increasing of temperature results in a reduction of resin viscosity and allows the resin to flow more easily through the reinforcement; the cure stage is also improved resulting in a reduction of global process time required. Besides the conventional solutions to heat up the resin based on the thermal conduction, in-line microwave heating is a suitable method to heat dielectric materials providing an even temperature distribution through the resin, thereby avoiding a thermal gradient between the surface and the core of liquid resin, which could result in a premature and uncontrolled cure. In the present work, an in-line microwave system, manually controlled, have been coupled with a VARTM apparatus to heat the resin before the infusion. In addition, parallel-plate dielectric sensors and pressure sensors, embedded into the mold, were employed to track the flow front through the fiber reinforcement in two distinct cases: unheated resin and pre-heated resin. The aim of work was to assess the effectiveness of microwave pre-heating to improve the macro and micro-impregnation of dry preform. The obtained results showed capability of in-line microwave heating to shorten the impregnation of dry fabric and provide a homogeneous wetting of fibers.