Monolithic parts require innovative solutions
It’s getting harder to imagine any market that isn’t benefiting from the latest developments in parts manufactured from advanced composites. “Advanced composites will arguably dominate consumer and production products, especially in the near future,” says Bert Erdel, industry consultant and executive technology advisor, Morris Group Inc. (Windsor, CT), “as they have begun to gain wide acceptance in solving energy-related issues.”
The rationale for this energy-saving attribute isn’t difficult to see in the “lightweighting” and the strength that composites impart to structural components. Their use has taken hold in literally every form of transportation vehicle from rockets to commercial airliners. In fact, there is hardly a category of consumer or industrial product that hasn’t benefited from the flexibility in design and the durability in performance that advanced composites deliver.
We take for granted that snow skis, snow boards, and jet-powered personal water devices are made from plastic, or that products as diverse as riding mowers and prosthetic devices seem to have always been made from plastics. Nothing, of course, could be further from the truth.
The challenges facing advanced composites manufacturers fall into these general categories:
- Selecting the right material for the final part. Formulations are available for virtually any end-user environment of temperature, corrosiveness, and stress.
- Selecting the tooling material most compatible with the process. Successful processing depends on how closely tooling can match expansion of the part and maintain process integrity over many production cycles.
- Selecting the right manufacturing process.
The basic process for advanced composite manufacturing, Erdel explains, “involves combining resin, hardener, and reinforcing fiber under heat and pressure to shape and cure the mixture into finished parts. The resin holds the fiber together; the hardener is a catalyst helping to cure the resin to hard plastic. The reinforcement imparts the required properties of strength and flexibility to the composite.”
Erdel describes the various processes:
- “Prepregging involves the application of formulated resin products, in solution or molten form, to a reinforcement such as carbon, fiberglass, or aramid fiber or cloth. The reinforcement is saturated by dipping through the liquid resin. In an alternate method called a Hot Melt process, the low-solvent resin is impregnated through heat and pressure.
- “In the filament wet-winding process, continuous fiber reinforcement materials are drawn through a container of resin mixture and formed onto a rotating mandrel to achieve the desired shape. After winding, the part is cured in an oven. This process can also use preimpregnated fiber tows called towpregs.
- “In hand lay-up, prepreg material is trimmed and laid down in as many layers as needed over a mold where it’s formed to the desired shape. After forming, a vacuum bag is sealed around the lay-up. Vacuum is drawn on the raw prepreg to remove air, compact the part, and serve as a barrier when the assembly is placed in an autoclave for cure under heat and pressure.
- “In automated tape placement, the prepreg tape material is fed through an automated tape layer and applied across the surface of a mold in multiple layers.”
Other processes include resin transfer molding (RTM) and pultrusion, according to Erdel. RTM is recommended when parts with two smooth surfaces are required, or when a low-pressure molding process is advantageous. Pultrusion involves pulling continuous strands through a strand-tensioning device into a resin bath, and through a heated die where curing occurs. The result is a rod or similar profile with a high fiber loading.
Composites technology is as much art as science. The challenge is to match as nearly as possible the coefficient of thermal expansion (CTE) of materials used for tooling (molds and mandrels) with that of the composites used for parts. The rate of growth of the tool and material under heat affects cure rate, cycle time, and the strength, surface quality, and durability of the final part, as well as production efficiency and cost.
In the 1980s, the industry standard for aerospace applications were tools made from graphite-reinforced/epoxy (GR/EP) composite materials. Tom Sobcinski, tooling segment manager, Remmele Engineering Inc. (New Brighton, MN) explains: “Composite tools provided a close CTE match to the part. Composite tools were light in weight, and the cost of masters could be amortized across rate tools.”
However, composite tooling at the time suffered from substantial negatives. “They lacked durability for high volume runs, required costly repair and replacement, could fail during cure cycles for high-value parts, and had a limited supply-chain availability,” Sobcinski explains.
“In the late 1980s, the industry started to shift to metals-based production tooling. Invar, which has a CTE closely matching GR/EP parts, became the baseline for tools used to produce advanced composite parts,” Sobcinski says. The durability of Invar reduced total cost of ownership, and tooling would last the life of the program. In addition, a ready supply base of shops emerged that could form, weld and machine Invar, however difficult that might be.
Invar 36, a cast iron with 36% nickel, offers a minimal CTE, and was invented for applications that might best be described as delicate, like sensitive measuring devices, watch springs, clock pendulums, and such. Invar has a lower CTE than either aluminum or steel, but is much more difficult to machine, acting more like stainless and producing long, stringy chips. Invar is expensive and somewhat challenging to weld. Invar tooling becomes exceedingly heavy as molds and mandrels are designed for large parts.
There is a definite trend toward using composite materials or metal/composite combination materials for tooling, especially for the largest components. “As aerospace OEMs and tier one suppliers have begun producing larger integrated structures, they began to feel the limitations of Invar tooling. The weight of all-Invar tooling can exceed the capacities of handling and layup equipment, and additional weight reduces throughput due to slower cure cycles and reduced lay-down rate on automated fiber placement [AFP] machines,” says Sobcinski.
The Boeing 787, in particular, has called attention to the challenges of manufacturing composites for the largest structural components. Erdel explains: “In modern aircraft construction, large quantities of aluminum components are being replaced by more complex parts made of fiber-reinforced materials, primarily carbonfiber-reinforced plastics (CFRP). This reduces weight and greatly simplifies assembly and logistics.”
Erdel explains why graphite composite tooling is appropriate for the one-piece fuselage barrel for Boeing. “The graphite composite fixture can keep thermal expansion as low as possible during the process. In addition to the weight of the mold, all of the fixturing adds considerably to the mass that must be negotiated.”
New applications for composite materials are emerging and maturing almost daily. Hexcel Inc. (Stamford, CT) has broken ground in Windsor, CO, for a 100,000 ft2 (9290-m2) prepreg manufacturing plant to serve the North American wind-energy industry. HexPly prepregs are epoxy-resin formulations reinforced with glass and carbon fibers and supplied to customers on large rolls. When prepregs are cured under heat and pressure, they form exceptionally stiff structures with a high strength-to-weight ratio. They are well-suited for use in wind turbine blades. Composite blades are lighter and, therefore, easier to install, and are very durable.
Wind turbine blades, which can be up to 50-m long, join a familiar lineup of macro-sized parts and structures that have been employed for decades in the aerospace industry for space-based applications and flight. Wind energy is just one industry that is expected to grow its use of composites. Erdel pegs the growth of demand for composites in aerospace at 15% annually, with wind energy expected to grow by 25% annually. In ten years, he believes that wind energy will consume as much carbon fiber as did the entire world in 2008. Composite consumption in 2009 is estimated to be about 5750 tons.
The ability to machine tooling materials without distortion is essential for manufacturing tools with complex shapes and tight tolerances. HexTool tooling compound is fabricated from Hexcel’s BMI resin M61 for a lightweight, energy-efficient- cure carbon-fiber tooling. Fast heatup and cool-down rates are aimed at reducing production costs compared with tools made of steel or Invar. HexTool can be repaired and modified to accommodate design changes at far less cost compared to other tooling alternatives. The coefficient of thermal expansion (CTE) of HexTool matches that of carbon/epoxy, and is formulated to withstand several hundred autoclave cycles at curing temperature of 356°F (180°C).
“The biggest challenge today, especially for high-end applications like commercial aircraft and aerospace, is building tooling that is durable at elevated temperatures and can produce accurate parts. The tool designer has to be aware of the properties of tooling materials used for building molds. Molds are the first and foremost tools you deal with in building composite parts,” explains Louis C. Dorworth, division manager, Abaris Training Resources Inc. (Reno, NV).
Molds have to be able to provide a dimensionally accurate end part at the process temperature at which the composite material cures, typically 350°F (177°C). Because of the temperature change the choice of mold material is critical to the process. Dorworth explains: “Look at it this way, the composite material is a thermoset resin in a semi-tacky state, when it’s laid up. Then the part is vacuum-bagged on the mold and put in an autoclave. As it heats up, the viscosity of the resin drops and flows throughout the laminate. During this time, it is important to maintain the right heat rate, so that the resin viscosity will get low enough to move into any low-pressure areas between the weave of the fabric, or into the honeycomb core cells to provide filleting, and thus bonding, to the core.”
“In order to control the dimensions of the tool during the cure cycle, low expansion materials are chosen for the mold in order to maintain dimensional tolerance of the part itself. The idea is to have a part that is dimensionally accurate as it comes out of the mold. Because of this, tool design can become quite complex. This requires skilled tool designers who have background knowledge of both the materials and processes involved in making the part, and the materials used to make the mold,” Dorworth says.
“We’ve seen disastrous projects where, because of cost or because of a lack of knowledge or inexperience, the tool designers chose aluminum, a material with a high Coefficient of Thermal Expansion (CTE), perhaps because it’s easy to machine. Not taking into account that they’re trying to make parts using tooling material that grows at a rate about twelve times the rate of the carbon epoxy material used to make the part. Depending on the configuration, aluminum may not be the material of choice for making high-temperature curing parts.
“Another factor is that you have to deal with in-process, is in regard to pressure. If I’m going into a pressure vessel with a vacuum bag, I have to have a tool that doesn’t leak. Once I seal my vacuum bag around the part, the vacuum integrity of that tool has to be such that it prevents pressure from driving in through leak paths in the tool and into the part. By allowing air or gas into the part during processing, you create voids in the laminate, which will diminish the part’s shear strength and compressive properties,” Dorworth explains.
Tooling for monolithic composite structures has become extremely large. Just how large can be seen in tooling from ATK Aerospace Structures. The low-CTE fiber-placement layup mandrels for the JSF upper wing skin that measures 37 x 13′ (11 x 4-m) wide required a welded Invar 36 structure that weighed in at 55,000 lb (24,948 kg). The low-CTE hand layup Invar 36 mandrel for the Delta IV thermal shield measured 5-m in diam and weighed 30,000 lb (13,608 kg), with vacuum-ported end manifolds in a complex contour and 3-D shape.
For example, tooling for a wing of the Boeing 787 is so heavy that special air bearings and tugs have to transport the tooling over reinforced floors and support fixturing has to be designed to support the weight of the tooling. In addition, the autoclave has to put out enough energy to heat the heavy metal mass at a rate that is compatible with the requirement to make the epoxy cure properly and flow properly to get the right resin/fiber ratio in the laminate.
ATK is working with an industry collaboration team to design a break-down and composite layup mandrel to meet requirements for the largest composite parts. For the 787 Section 43 fuselage, ATK and a team including Alcore, Cytec, Hexcel, GrafTech, Odyssey, and, of course, Boeing have developed a light weight masterless carbon foam/BMI lay up mandrel architecture. The mandrel design is said to be 57 metric tons lighter than one of traditional Invar design, reducing the capacity required for cranes, handling equipment, tooling, and fiber-placement machines.
Autoclave curing has traditionally been necessary to produce the surface and laminate quality acceptable for durability of service. Autoclaves can be quite large and expensive. Units ranging from 12–15′ (3.7–4.8 m) ID x 30–50′ (9–15-m) long are not unusual for the largest structural components. However, autoclave cure limitations include the capital investment required, size limitations, and limited availability in the supplier base.
Increasingly, there is a trend toward out-of-autoclave (OOA) processing. Advanced Composite Group Ltd., a member of the Composites Division of Umeco plc, specializes in manufacturing high-performance prepreg advanced fiber reinforced composites for process technologies, including autoclave, vacuum bag, OOA, and press molding.
In October 2008, ACG successfully demonstrated the capability of its MTM 4401 OOA-toughened structure prepreg resin system on the first sub-scale wing box demonstrator produced for the collaborative research ALCAS program (Advanced Low Cost Aircraft Structure). ACG is partner in the Business Jet Platform of this Airbus and Dassault Aviation-led, EU-funded program.
ACG’s main focus in the program is to design and manufacture the lower cover of a structural wing-box demonstrator. This cover would then be used by four other partners (Alenia, Dassault Aviation, SAAB, and Stork Fokker AESP) to complete the structure using different materials, designs, and manufacturing options. The four structures would subsequently be tested to validate and compare the technologies.
In OOA applications, carbon fiber and glass-fiber reinforcement are preimpregnated using specially formulated epoxy, cyanate ester, or bismaleimide (BMI) resin matrixes which are used in woven and stitched fabrics or as unidirectional tape formats in tape placement or tape-winding operations. ACG’s OOA technology has successfully been processed using only atmospheric pressure and significantly lower temperatures of 140°F (60°C).
Remmele Engineering is developing its Invalite hybrid tooling system for advanced composites as an alternative to all-Invar or all-composite tooling. Objectives of the Invalite tooling include weight reduction, durability and stability, reduced cure-cycle time, lower total cost of ownership, and increased capacity for tool manufacture in the supply chain. The hybrid Invar/composite tool features a reduced thickness Invar face sheet and interlocking GR/BMI composite substructure.
The Invalite hybrid tooling system is said to reduce tool weight by more than 50%. It’s cost competitive with GR/BMI composite tooling, and eliminates the cost for Invar masters. “One of the things we had to do was reduce the weight of the Invar face sheet. A standard face sheet is 0.5″ [12.7-mm] thick. Depending on the configuration of the tool we have reduced the thickness as low as a 0.25″ [6.4 mm] using special processing,” says Sobcinski. Machining is limited to the Invar face sheet and minor composite machining at the interface joint. The substructure can be produced by waterjet cutting. A 4′ (1.2-m) Invalite hybrid tool is currently in production testing.
This article was first published in the April 2009 edition of Manufacturing Engineering magazine.