Monitoring of Resin-Transfer Molding Using Laser-Based Ultrasound

Abstract

In both the aerospace and automotive industries, polymer-based composite materials are increasingly being used to replace metallic structures. To reduce the cost of fabricating complexly contoured polymer composite structures, manufacturers are actively developing low-cost alternatives to hand lay-up/autoclave curing and other labor and capital-intensive processes. One technique, that has proven to be an economical method for fabricating polymer composite structures containing complex contours, is resin-transfer molding (RTM). In the RTM process a dry fiber preform is placed between two faces of a mold. Resin is injected into the mold at low pressure and fills the mold cavity permeating the fiber reinforcement. After the part is fully cured it is removed from the mold. The key to efficient and cost-effective resin-transfer molding is to have an automated process in which the infusion of resin is controlled so that the flow front rapidly and evenly permeates the fiber preform. Several problems can occur during the transfer of the resin into the mold such as lack of penetration into all regions of the mold or incomplete infiltration of the fiber preform. Thus, in situ sensors are required to monitor the RTM process and ensure that pumping of resin is not stopped prematurely. Laser-based ultrasound (LBU) is well suited for this monitoring application since it can work with heated molds having nonplanar surfaces and can acquire ultrasonic C-scan images in times that are short compared to the total transfer time of the resin. These ultrasonic images can be used to enable rapid assessment of degree-of-completion of resin transfer, and indicate problems associated with the resin transfer or with defects in the mold. In this paper, results are presented which demonstrate the capability of laser-based ultrasound to monitor the resin flow front along the inner surface of the mold during the RTM process.