Aerospace companies have been gravitating toward the use of more dielectric monitors in processing advanced composites, since the monitors shorten resin-cure cycles and cut auto-clave, press and oven processing costs. ``The aerospace industry has been, and will likely continue to be, the largest user of this technology for the next few years,'' said Nathaniel Smith, sales manager of Micromet Instruments Inc. in Newton Centre, Mass.
Micromet has sold more than100 monitors to the composites market since 1982. A subsidiary of Geo-Center Inc., Micromet dominates the niche.
Other instrumentation exists, however. David Kranbuehl, a professor at the College of William and Mary in Williamsburg, Va., sells a flexible, modular sensor system through his company, DekDyne Inc. Consisting of a Hewlett-Packard low-frequency impedance analyzer, a multiplexer and software, the system uses a single, disposable, patented sensor that measures dielectric properties at temperatures of more than 842§ F and pressures in excess of 1,000 pounds per square inch.
Signature Control Systems Inc. has patented its SmartCure intelligent process control system. Lockheed Martin Corp.'s Aeronautical Systems Co. unit acquired its first system to cure resin with extended cure cycles in an engineering-development autoclave in Marietta, Ga.
``The system looks friendly, and I believe in the technology. I want to see repeatability and quality,'' William Craig, operations engineer in manufacturing research.
Steve M. Binnix, president of Signature Control in Anaheim, Calif., said, ``We've reduced cycle times as much as 70 percent for complex chemistries such as polyimides, and by about 25 percent for simpler epoxy systems.''
Matthew Buczek, senior staff engineer with GE Aircraft Engines in Cincinnati, found a Micromet system helpful in monitoring advanced-composite curing on engine fan blades.
Buczek also is evaluating direct-current resistance as a low-cost alternative to dielectrics. Eventually, emerging technology in DC resistance and ultraviolet-or infrared-wavelength spectroscopy could compete with dielectrics.
``In many cases, you do not need to go as sophisticated as dielectrics,'' Buczek said.
The Army Research Laboratory's materials directorate in Watertown, Mass., is studying DC resistance as a way to achieve full-field-flow assessment on resin transfer molding applications. Research engineer Shawn Walsh said the department has demonstrated the system on a subscale prototype crew compartment for a composite armored vehicle being developed by United Defense L.P. of San Jose, Calif. The component will be fabricated with 50-55 percent S2 glass and 45-50 percent phenolic resin, Walsh said.
A process-design group at Wright Laboratory's materials directorate developed a powerful Qualitative Process Automation Language, commercialized the advanced-composite-curing controller in 1989, and seeks an improvement now. The laboratory is at Wright-Patterson Air Force Base in Dayton, Ohio.
QPAL can receive sensed dielectric data, but is ``too sophisticated for the average autoclave user,'' said Steven LeClair, technical leader.
Ten military and civilian sites use QPAL-II. Eventually, downloading QPAL's knowledge into a programmable logic controller will overcome the current need for a user to write computer software codes.
``When that occurs, QPAL will have more widespread appeal,'' LeClair said.
At least two versions of spectroscopy are under review. A team at the National Aeronautics and Space Administration's Marshall Space Flight Center in Alabama embeds one or more optical fibers in polymers and uses Fourier-transform infrared spectroscopy to monitor the curing process.
A Wright-funded group at Southwest Research Institute in San Antonio activates a laser to excite material that resonates at different frequencies as its cures.
Using ultraviolet or infrared wavelengths, the patented Raman spectroscopy system now monitors temperature, degree of cure and chemistry with a single sensor and, by the end of 1995, should be able to validate viscosity and pressure with one sensor.
LeClair envisions monitoring all five parameters and getting the Raman controller ``down to the size of a coffee mug for use in the field.''