RTM research took center stage at composites conference.

Advances in on-line process control and in-mold cure monitoring for RTM were featured topics at the recent ASM/ESD International Advanced Composites Conference and Exhibition in Dearborn, Mich.

Among other presentations at the conference were reports on solid-state mixing and molding of thermoplastics, metal/plastic composites, electron-beam curing, and novel thermoset resins.

RTM CONTROL FILLS A VOID

Need for improved on-line process controls is often identified as one of the constraints on the ability of RTM to penetrate demanding, higher-volume application areas, such as automotive parts. To answer this need, Stevens Institute of Technology in Hoboken, N.J., and the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., are jointly developing new RTM control technology.

The program, funded through a NIST grant and directed by NIST project manager Richard Parnas, is aimed at developing a control methodology capable of relating processing parameters such as resin flow, viscosity, cure rates, temperature, and pressure to final part properties. Process data would be analyzed by computer to establish a process model and optimize cycle times.

The project's goal is a real-time process-monitoring and control system that utilizes in-mold cure sensors to track pressures at the resin inlet and outlet and monitor cure exotherm. The control will use these data to automatically make cycle-to-cycle adjustments.

Stevens associate professor Souran Manoochehri says a control-algorithm "contour map" is being developed to establish setpoint values for on-line cycle control. Formulation of the contour map involves mathematically transcribing relationships between process parameters and product performance.

MONITORING IN-MOLD CURE

Another RTM process-control R&D project presented at the conference involves fiber-optic monitoring of cure in the mold. So-called "evanescent-wave fluorescence monitoring" is being conducted at NIST under Parnas' supervision.

In this technique, a special fluorescent dye is dissolved in the resin at a low concentration. Dye molecules are excited by the electric field at the fiber/resin interface. The polar dye molecules attempt to orient themselves in the electric field. But as viscosity increases during cure, the dye molecules' freedom of molecular rotation is inhibited. Consequently, they dissipate energy in the form of fluorescent light, which can be picked up by a tiny embedded sensor.

NIST research associate Dara Woerdeman says that until now, application of fiber-optic sensor technology in process applications such as RTM has been limited by the low refractive index of the optical fiber relative to the matrix resin. However, a sensor has now been developed with a refractive index that permits optical monitoring of many RTM resins.

The initial phase of research demonstrated the ability to monitor cure rates with a 125-micron-diam. fiber sensor embedded in the glass preform of the RTM part. Variances in resin cure rates, based upon sensor readings, are downloaded to a process controller in real time, which then makes process adjustments.

NOVEL R&D TOPICS

Four unusual areas of research on composites and thermoplastics were explored in other papers at the meeting.

* Solid-state processing of thermoplastics: Virginia Polytechnic Institute and State University in Blacksburg is in the early stages of exploring thermoplastic powder processing in the solid phase. Sintering compacted polymer powders into a near-net shape may offer energy savings over melt processing, according to Ronald G. Kander, assistant professor of materials science and engineering. Reduced energy use and avoidance of solvents would also be beneficial in using powders to make solid-state polymer blends at near-ambient conditions. Finer, more homogeneous mixing could result.

* Metal/plastic composites: The Delco Chassis unit of General Motors Corp., Dayton, Ohio, has developed a process that solves problems of integrating metal attachments into plastic composite structures. Delco's "Litecast" process involves die-casting aluminum or magnesium alloys directly onto a thermoset composite part made by filament winding, SRIM, or wet braiding.

Melting points of the metals (1020-1200 F) are significantly higher than the degradation temperature of thermoset polyester, vinyl ester, and epoxy resins. But decomposition is limited to a thin surface layer of the composite by rapid cooling of the metal and the insulating effect of the glass fiber.

* Electron-beam curing: Researchers at the University of Maryland (College Park) are exploring EB curing of epoxy and glass or carbon fiber. Initial results demonstrate potential for more rapid curing of thick laminates at ambient temperature without generating significant levels of radiation in the work environment. EB curing reportedly also uses long-shelf-life resin systems, minimizes emissions of volatile monomers, and produces epoxy composites free of voids and delamination that can result from internal curing stresses. The French aerospace company Aerospatiale has EB cured rocket-booster casings up to 12 ft diam. x 30 ft long in 8 hr as compared with 100 hr by conventional means.

* Novel resins: According to research at the University of Detroit Mercy and the University of Technology in Eindhoven, The Netherlands, tough high-temperature thermosets with good processing characteristics can be prepared relatively inexpensively from polymeric isocyanate and standard epoxy resins. The resulting polyoxazolidone resins could be lower-cost alternatives to polyimides in composite applications with maximum operating temperatures in the range of 390 F.