Composites Offer More Opportunities
By Robert B. Aronson
Initially, high-strength composites gained their greatest acceptance in areas where their light weight and exceptional strength outweighed cost issues. This was chiefly in the aerospace industries. Now, improvements in materials and less labor-intensive manufacturing processes mean composites can be economically used in a growing number of applications.
- Lighter weight,
- Stronger and stiffer than metals in many applications,
- Corrosion resistant,
- Can be made conducting or non-conducting,
- Thermal and strength properties within the part can be tailored,
- Complex modular designs are possible.
The most commonly used composites are resin-based materials using carbon-fiber reinforcing. Honeycomb panels have long been used chiefly for nonstructural applications, and metal-matrix composites have made some inroads.
The manufacturing steps are about the same for many resin-based composites. Initially a "prepreg" tow, tape, or sheet of composite material is formed on a mandrel that closely matches the final shape of the product. Application of the material can be hand lay-up or automated. Then the composite part is cured either at room temperature or placed in an oven or autoclave.
Because the part at this stage is usually near net shape, the need for conventional machining is limited. Frequently this involves only hole drilling or trimming. More complex parts may be milled or ground to achieve special shapes. The abrasive nature of the reinforcing fibers can cause tool wear problems.
Aerospace projects have led to the use of composites, because that's the industry where the strength, light weight, and durability of these materials is most appreciated and cost effective.
"Weight is the prime focus of most composite applications," explains Steve Mitchell, who works in Advanced Mechanical Design for GE Aircraft Engines (Evendale, OH). "Since 1990 our work on composites has focused on making them not only affordable, but lower cost than previously used materials. Next, we stressed automation as a method of cost cutting. To support this work we developed cost models. This included the latest processes, not just hand lay-up.
"These models give the engineer access to the impact of costs on design. Often the tendency is to refer back to the old technologies, chiefly hand lay-up, instead of exploring all that is now available to them.
"There are two different philosophies when designing an aircraft: those used for the fuselage, and those used for the engine. The fuselage is rarely disassembled and its elements can function at a top temperature of 250ºF [121ºC]," Mitchell explains."The engine, on the other hand, is frequently disassembled for maintenance and refitting. This requires a design with a lot of bolted flanges, and part manufacture requires a lot of machining. Also, sections of the engine operate at much higher temperatures. The type of composite used varies greatly between the fuselage and engine.
"Our latest engine is the GEnx. With this design, our engineers came up with a structure that satisfies the requirement that the containment sections for this engine be as light as possible. And we did it with a 400-lb [181-kg] weight savings by using composites. All of the blades in the 'cool section' are composite where temperatures are around 250ºF. Ours are the only fan blades that are made of composites. For these blades, one of the main requirements was ensuring impact resistance. That's the force with which a large bird might hit the engine's blades." "Composite use at Boeing has been growing for some time," says Don Anderson, Integrated Defense Systems Engineering (Seattle, WA). "All commercial aircraft are going strongly to composites."
"Machining is still a significant part of aerospace composite production," says Keith Kuhlmann, Integrated Defense Systems Engineering (Seattle, WA). "We have pretty much learned the trick of modification of feeds, speeds, and the necessary machine tools. Most often trimming and drilling are the two major tasks. But an operator familiar only with metal removal processes should not assume that composites will behave the same way.
"We do no turning," says Kuhlmann, "If you want a round, you make your mold round. Waterjet trimming is the most common machining process for us. Laser cutting will not work in our applications. You have resin and carbon fiber, and a single beam is not capable of cutting both simultaneously. When you heat composites, this can generate toxic fumes because it's all resin based. We look at all the fibers and their weaves to determine part strength. What we choose depends on the properties needed. You can machine any composite with diamond abrasive. You can use carbide, but tool life is a concern.
"Generally composite cutting responds best to high feed rates and spindle speeds of around 20,000 to 30,000 rpm," Kuhlmann continues. "Burning and melting are not problems because of the high head speeds. A nice thing about composites is they behave just like wood. You have to worry about breakthrough, but good drill-point geometry or a backup material takes care of that problem.
"In the entire process, the biggest cost item is inspection, chiefly because it's so labor intensive. We use ultrasonic techniques. Inspection is necessary to ensure good laminate properties. There is a potential for voids, contamination, and foreign objects within the laminated composite material."
Composites are being used at sea. The Royal Swedish Navy has developed the "Visby" class of 73-m long corvettes. The hull is a carbon-fiber reinforced sandwich construction made of two composite layers with a foam filling. The fiber is T-700 woven into plies in configurations from 250 to 700 gr/m2. Core material is PVC Divinycell foam. Matrix material is a rubber-modified vinylester resin. Angular design gives the ships a stealth capability.
Lack of knowledge keeps some manufacturers from trying composites. One organization helping to present the positive side of composite use is the National Composite Center (NCC; Dayton, OH). It is a manufacturing process development house that focuses on advanced structural composites. "Our emphasis now is on developing more affordable composite manufacturing processes," says Lou Luedtke, president and CEO.
"Chopped fiber is one of our main research areas. We have found that discontinuous fibers 2 - 3" [51 - 76-mm] long provide a part-strength and stiffness close to that of continuous fibers.
"In this manufacturing process, you use volume of reinforcing as opposed to directionality to get strength," Luedtke explains. "It also uses a combination of fibers in the same part. Carbon fiber is used in highly stressed areas, and glass in more lightly loaded sections."
"First, dry reinforcing fiber is sprayed onto a mandrel, forming a mat in the shape of the final part. The mat or preform is placed in a mold, then a combination of catalyst and matrix resin is pumped in. The combined fluids infiltrate the preform, forming a composite. The parts may then be removed for an air cure or the mold can be heated for an accelerated cure.
"We generate income by developing actively developing processes internally or as suggested by our member companies, then we supply the technology for a fee, or royalty arrangement. In addition, we rent space and equipment and supply our knowledge to companies that want to do development work on their own."
Practical uses for composites are being funded at several government facilities. Some of the more leading-edge research into composites is being done under the Department of Energy and Oak Ridge National Laboratory (Oak Ridge, TN). This program involves a number of projects directly related to resolving industrial problems. In one recently instituted program, researchers will be looking first at the material properties of exotic intermetallic metal-matrix composites.
"We are looking at manufacturing issues such as machining characteristics, ease of material removal, tool wear, and surface finish," says Dr. Peter Blau, Leader, Metals and Ceramics Div. "All of these can be a problem in a material made up of hard reinforcing within a softer matrix. Then we will look at how to reduce manufacturing costs.
"The ultimate goal of the program is to develop durable materials for use in the manufacture of more energy-efficient diesel engines. This effort will also help industry meet the new diesel emissions standards.
"One major benefit of composites is 'made to order' thermal expansion," says Blau. "To develop more efficient fuel injectors, both the injector and plunger should have the same CTE to maintain the necessary tight clearance. Too much space and the injector will not hold high pressure. Too little and the injector may scuff or jam. Other components, like those in exhaust gas recirculation systems, must operate at high temperatures with little or no lubrication, and this presents another big materials engineering challenge."
"With composites, you make the material at the same time as you make the product," explains Gordon Bishop, Managing Director of NetComposites (Chesterfield, UK). "You get both the shape and structural properties in the manufacturing process, so usually only trimming and holemaking are needed."
Aerospace uses of composites get the majority of publicity, although according to Bishop, they makeup only about 5% of the total applications today.
"This company is, for example, doing a lot of work on polymer reinforcing fibers, instead of glass fiber, the most commonly used reinforcement. Parts made from these 'self-reinforced' materials offer the advantages of very low weight, high impact performance, and recyclability. One of the main application areas expected for these materials will be in nonstructural vehicle applications such as body panels," Bishop observes.
Special tools are sometimes needed to work with composites because they are so abrasive. "Because carbide does not hold an edge well, or give clean cuts with these materials, we recommend polycrystalline diamond [PCD]," explains John Bunting, CEO, Precorp (Spanish Forks, UT). "We use a special process to produce a PCD vein in the tool that lets us create the unique point geometries with PCD cutting edges needed to produce clean holes in composites. Clean holes are essential. They allow the manufacturer to drill and fasten in one operation. There is no magic to it. You need a very sharp edge, a diamond abrasive, and a low-thrust drill-point design and a point with high relief and rake angles.
"There can be problems when the composite is stacked with aluminum or titanium in a component," Bunting continues. "Chips from the metal can interfere with the drilling of the composites. The chips can degrade the cut.
"Lubrication can help give a really clean exit hole. We have recently begun offering through-the-tool cooling. It's a minimal amount of coolant applied as a mist. You don't need much to make a tremendous difference in the cut.
"We don't recommend coated tools. Coatings round the drill edge, and this is a problem.
"Temperature control is critical. You cannot melt or even soften the composite's resin. Otherwise, the fibers will move and you will get pull-out in the hole," he concludes.
Lasers are used to cut composites, chiefly on very thin material. But there are problems. Lasers can char and burn certain materials. They can be used for holemaking if the initial penetration damage is not critical. Abrasive waterjet is a good process for cutting carbon fiber and other laminates, producing a very high quality edge with no heat effects.
"With abrasive waterjet, you have to be careful of delamination if internal cutouts or holes are required," says David Krump, LAI International (Westminster, MD). "If you try to penetrate a composite section directly, the water is forced between the laminates, causing the composite to separate. The size of the affected area varies with different material compositions and thickness. Delamination can be avoided by starting with a pilot hole. For larger holes, or internal cutouts, the delamination zone may be small enough that the affected area is not a factor when penetration is kept away from the part edge. For perimeter cuts on composites or laminates, the best practice is to lead in from an edge to avoid any damage to the material.
"Waterjet does well when cutting composites reinforced with carbon fiber, glass fiber, or Kevlar. We have also worked with metal-matrix composites, rubber, plastics, ceramics, and many other materials. Abrasive waterjet will cut almost anything.
"Waterjet also offers the advantage of no heat transfer to the part, unlike other cutting processes, which can be very important with composite materials. Another advantage of waterjet is it puts very little load on the part, and there are no lateral forces, so part holding is not a problem, eliminating the cost of expensive fixtures."
To eliminate the cost and inefficiencies of hand lay-up, Cincinnati Machine LLC (Hebron, KY) has been supplying composite lay-up machines for more than 20 years. According to Ron Hennies, composites product manager, "We initially developed an automatic tap layer by drawing on our experience with specialized machine tools and industrial robots.
"The first system applied 3" [76 mm] wide tapes made of carbon fiber in a thermoset epoxy resin. It was used chiefly for developing military aircraft. As tape-laying gained acceptance, it was adopted by the commercial aircraft industry. Emphasis on increased productivity resulted in the introduction of tape layers that could dispense 6 and 12" (152 and 305-mm) tape on contoured surfaces," Hennies says.
"To take on the problems of composite lay-up on more complex shapes, we developed our Viper fiber-placement system that dispenses a ribbon-like form of composite material. This ribbon, called a tow, was typically 1/8" [3-mm] wide. The machine would automatically combine up to 32 tows into tapes from 1/8 to 4" [3 - 102-mm] wide. Variable tape width combined with the ability to independently dispense each tow allows the fiber-placement machine to make contoured parts that cannot be made on a tape layer. Independent tow control also provides the advantage that little scrap is created. You lay down only what you need.
"As with the tape-laying machine, the demand for increased productivity increased the need to dispense wider tows. Machines now available lay 32 tows from 1/4 to 1/2" [6 - 13-mm] wide. This capability can provide tapes from 8 to 16" [203 - 406-mm] wide.
"Today, our customers work primarily with thermoset composites. They are delivered frozen to prevent the resin from starting to cure, then thawed to room temperature and loaded into the machine for lay-up and subsequent curing. The resulting composite is usually limited to a top temperature of 250ºF. For higher temperature applications, we are developing the ability to automatically lay thermoplastic composites that can withstand operating temperatures to 500ºF [260ºC]."
This article was first published in the August 2005 edition of Manufacturing Engineering magazine.