New equipment & processes for advanced composites.

That there's life yet in thermoplastic advanced composites would seem evident from the presentations of new processing techniques for these materials at the recent 38th International SAMPE Symposium and Exhibit in Anaheim, Calif. Technical papers at the meeting addressed ways to reduce the high cost that has inhibited growth of thermoplastic composites.

Other newsworthy topics at the meeting included a new laminate design for vibration-damping composites, improved hardware and software for 3-D braiding and complex-shape filament winding, microwave and electron-beam curing, new methods of real-time cure monitoring, five-axis routers, and a novel nondestructive test method.

(Last month, in Part I of our SAMPE report, we covered new resins and reinforcements.)

NEW THERMOPLASTIC TECHNIQUES

Feasibility of low-pressure consolidation of thermoplastic prepreg tapes was demonstrated in a paper from Polymer Composites Inc., Winona, Minn. Designed experiments with PP/glass unidirectional tapes produced by the company's "thermoplastic pultrusion" process showed that consolidation pressure as low as 12.25 psi and short dwell time (2 min) are sufficient at the proper temperature (347 F or higher) to achieve mechanical properties comparable to those obtained by high-pres-sure processing. The authors conclude that expensive large presses or autoclaves need not be required, and that less expensive, low-pressure methods like vacuum bagging could be used. (18)

Another paper on thermoplastic composites addressed the difficulties of "stamp forming" continuous-fiber prepregs without undesirable wrinkling or creasing of the sheet. Researchers at the German Plastics Processing Institute (IKV) in Aachen overcame this problem with a new prepreg guidance and tensioning device. The device consists of a frame with eight spring-loaded tensioning elements that clamp the periphery of sheet and maintain uniform tension during heating and forming. The frame also serves as a carrier for automated transport of the prepreg between heating, forming and demolding steps. Fig. 1 shows results with a PP/woven-glass prepreg. Avoidance of wrinkling is said to reduce prepreg waste by reducing the necessary prepreg size.

A third development for processing continuous-fiber-reinforced thermoplastics was presented by researchers at Delft University of Technology in the Netherlands together with Dutch press manufacturer Fontijne Holland B.V. They adapted a technique known as rubber forming in order to overcome a key drawback of more common matched-metal forming. The latter results in a high degree of detail but also is prone to nonuniform pressurization of the composite laminate, owing to the rigidity of the dies and perhaps to nonuniform thickness of the sheet. The Delft system replaces one of the rigid metal dies with a flexible rubber die, resulting in more uniform pressure distribution. The rubber die does not have to be perfectly matched to the steel half; sometimes it is advantageous to under- or over-dimension the rubber half. The Delft researchers experimented with a prototype of a fully automated rubber forming press with 220-ton force, closing speeds of 3.9 in./sec, and forming area of 5.75 x 3.3 ft.

Preliminary experiments at Concordia University, Montreal, Quebec, demonstrated that a C|O.sub.2~ laser at 60-watt power provided an effective heating method for continuous tape winding of PEEK/carbon-fiber composites (ICI Fiberite's APC-2). The laser beam was directed at the contact point between the mandrel and a consolidation roller. The highly localized melting and consolidation preserved prepreg quality, the authors says.

VIBRATION-DAMPING STRUCTURES

A new approach to preparing vibration-clamping laminates uses "zig-zag" fiber layup on the composite skins and a viscoelastic layer embedded in the center of the sandwich. According to researchers from the Advanced Composites Manufacturing and Engineering Center at Brigham Young University, Provo, Utah, this approach increases the vibration damping of composite structures without significant reductions in stiffness or increases in weight. Sacrifice of stiffness has been a particular disadvantage with other composites utilizing an embedded viscoelastic layer--so-called "constrained-layer damping."

NEWS IN BRAIDING & WINDING

New three-dimensional braiding technology was presented by Atlantic Research Corp. of Vienna, Va. ARC's 3-D braider is said to eliminate the low throughput and high cost of earlier 3-D braiding technology with production speeds five times faster than its predecessor. Until recently, applications for 3-D braiding were limited by size and cost. ARC's 3-D braider fabricates components of continuous length up to 30-in. diam. with wall thicknesses up to 3/8 in.

Also developed by ARC is a Through-the-Thickness braiding method for producing a single-piece, fully integrated preform shape for structural composites. Fibers are fully interlocked and travel completely through the preform, allowing greater design complexity and flexibility, the company says. (19)

A new mathematical programming technique using B-spline interpolation has been shown to provide more accurate placement of fibers, smoother machine control, and higher performance during filament winding of complex shapes, according to a paper presented by Shaiq A. Haque of the Dept. of Mechanical Engineering at the Univ. of Nottingham, England. During filament winding, fiber payout is at some distance from the mandrel, so that accuracy of the payout-eye path does not necessarily result in fiber-placement accuracy. B-spline (or Basis-spline) mathematical treatment is said to yield smoother curves than any other method, at the price that the curves will not pass through the given points. Experimental work with a three-axis winder and an IBM-compatible PC included development of techniques to control the deviation of the B-spline interpolated path from the desired payout-eye path.

B-spline interpolation is said to give a clear advantage in filament winding where high speed and smoothness of carriage control are important. In case of non-geodesic winding patterns, more accurate placement of the fibers is achieved because the effect of the pay-out movement on previous fiber positions is controlled, due to the look-ahead capability.

NEW CURING TECHNIQUES

Experiments at the Dept. of Chemical Engineering, Michigan State University, East Lansing, indicate that microwave energy can be used as the sole method of curing composites during pultrusion. Although microwave energy has the potential for faster and more efficient heating in pultrusion, it has been limited primarily to preheating or post-curing, partly because the dimensions of waveguide devices must be altered for processing different materials. The MSU researchers overcame this problem by using a tuneable, "resonant microwave cavity" instead of a waveguide. This cavity consisted of a box with adjustable height that surrounded the pultrusion die. The die was made of ceramic so as to be transparent to microwaves, and its surface was treated with a release agent to prevent it from being scratched by the material being pulled through it. Results showed that microwave energy was absorbed strongly and distributed evenly in curing a prepreg of glass, vinyl ester resin, and vinyl toluene monomer.

Electron-beam curing of fiber-rein-forced composites has been the focus of research at AECL Research-Whiteshell Laboratories in Pinawa, Manitoba, Canada. Electron-beam curing is said to offer several advantages over thermal curing, including reduced stresses, shorter curing times, and fewer volatiles. Electron curing at ambient temperatures has reduced the stresses in unidirectional composites by at least 75%, compared with an equivalent composite heat cured at 302 F. Carbon-fiber composites up to 170-plies thick can be uniformly cured using single-sided electron-beam treatment. And electron curing reduces the amount of volatiles generated, as a result of curing at near-ambient temperatures at greatly increased speed.

A new approach using artificial "neural networks" to interpret data from dielectric monitoring of epoxy cure has been developed at the Laboratory for Intelligent Automated Systems at the Colorado School of Mines in Golden. Dielectric cure monitoring typically requires considerable interpretation of the sensor measurements. A technique has been developed for on-line monitoring of the degree of cure using neural networks to fuse the data from several channels and output a single "cure index." Neural networks are a branch of artificial intelligence that has the capacity to "learn." The neural network was shown to be successful at learning the complex nonlinear relationship between the dielectric sensor data and the cure index, so that it could predict the cure index directly from the raw frequency measurements--obtained in this case from a Micromet Instruments microdielectrometer.

A fiber-optic, tool-mounted sensor has been developed at the University of Dayton Research Institute, Dayton, Ohio, to monitor several aspects of composite resin curing. The sensor uses a commonly available silica optical fiber and an off-the-shelf spectrometer with a diode-array detector. Broad-band light is transmitted through a resin-filled gap between two fiber ends and is monitored for intensity changes. This sensor can detect the time of resin flow (when the gap fills with resin) and the extent of cure by monitoring a shift in absorption. The absorption shift or "darkening" of the resin that occurs during the cure can be correlated with the degree of cure or glass-transition temperature of the final composite.

FIVE-AXIS ROUTER

Soon to be introduced to the market are two new five-axis machining systems from American GFM, Chesapeake, Va. One is the US-50 ultrasonic core-carving system. It's said to be especially suitable for providing detailed cores and beveled cuts on honeycomb materials. The other is an RM-50 trim router that uses either an ultrasonic cutter or a traditional routing spindle, which are interchangeable as needed. The new machine reportedly can handle blocks of core material up to 48 in. high or large curved composite panels. Universal fixturing is possible with vacuum pods to hold the workpiece in place. This eliminates the need for hard tooling to hold odd-shaped workpieces. This feature was introduced on GFM's RM-30 three-axis router late last year.(20)