Toolmaking for advanced composites applications is a necessarily exacting science. Tools must be every bit as dimensionally true as the parts that will be pulled from them, and they must possess sufficient surface quality to reproduce the desired appearance in the part surface, not to mention durable enough to withstand the rigors of multiple cure cycles. Historically, however, such tooling has been expensive to build, particularly for parts that undergo autoclave cure. Toolmakers and tooling materials suppliers, therefore, have sought, and as the following shows, continue to find, more affordable alternatives.
In and out of the autoclave
Toolmaker Weber Manufacturing Technologies Inc. (Midland, Ontario, Canada) has recently begun building nickel shell tools as an alternative to aluminum, epoxy, Invar and steel tools used to manufacture advanced composite parts that must be autoclaved. Nickel vapor deposition (NVD) is used to form a thin tool shell from a master mandrel (read more about NVD toolmaking in the article noted under “Related Content,” at left). The shell is subsequently mounted on a steel “egg crate” weldment that provides support and withstands thermal cycling in the autoclave environment. Although typical thickness is 6 mm to 10 mm (0.25 inch to 0.39 inch) to minimize cost, the shell can be built up to any thickness. Weber vice president Rob Sheppard says the shell can be built in as little as 24 hours, reducing tool manufacturing labor, material costs and lead time. Nickel also has three times the heat transfer capacity of P20 steel, offering substantial cycle-time reductions. To reduce cycle times further, Weber can integrate heating/cooling lines made of copper into the tool structure behind the nickel shell. Sheppard says the company has completed several nickel-shell molds for autoclaved parts, including a high-end automotive exterior application and another for a military project.
Although autoclave cure is still predominant in the aerospace industry, the need for large, one-piece advanced composite structures sometimes requires molds too large for existing autoclaves. The expense of constructing larger autoclaves has brought into sharp focus the need for out-of-autoclave processing both for parts and molds when composites are used to produce the latter. These tools, unlike parts, must withstand multiple cure cycling at 180°C/356°F, yet remain dimensionally and thermally stable and retain surface quality and vacuum integrity, says Dr. John Nixon, technical marketing manager for Advanced Composites Group Ltd. (ACG, Heanor, Derbyshire, U.K.). Such tooling, he says, requires several intermediate vacuum bag debulks to ensure full ply consolidation, plus autoclave cure at 3 bar to 5 bar (43.5 psi to 72.5 psi). Yet this process does not always yield expected surface quality, resulting in rework — and the length of time required to lay up and consolidate plies can push the envelope of freezer outlife. Moreover, this complex process has become increasingly impractical as delivery timescales tighten.
An example is the racing yacht hull, today built one-off from prepreg tooling to specifications every bit as exacting as those of aerospace parts. “How do you build a tool longer than 70 ft?” asks Nixon. Although resin infusion and wet-laid tooling has been considered, Nixon says prepreg is the preferred tooling material because a boatbuilder is already geared up to lay up the hull in prepreg. Unfortunately, standard tooling prepregs have not worked well for out-of-autoclave cure, where the attained consolidation pressure is about 1 bar/14.5 psi, which produces a poor-quality mold surface.
ACG says its ZPREG, a partially impregnated material (semipreg) originally developed for low-pressure molding of paint-ready automotive body panels, withstands thermal cycling and thus provides a high-quality tooling surface. “They can be readily modified and formatted to provide the heavy ply weights for rapid tool construction, even in a form that does not require the intermediate ‘debulking’ cycles,” says Nixon, who adds that the material also features a long outlife. The material was recently used to build a tool for the ABN AMRO Volvo 70 racing yacht hull, reportedly resulting in a tool with a “virtually faultless” surface area of 120m²/1,292 ft² (see middle image on this page).
The “secret” to surface quality and low void content, says Nixon, “is the provision of paths through the fiber architecture, allowing air to be drawn out and, during the curing process, allowing resin to be infused in.” During layup, he points out, the surface ply is extended outside the main laminate stack (see illustration, pwoer right). “This connects directly to the breather, ensuring all air is swept off the tool face, resulting in a pit-free surface,” he says. Initial cure takes place at 65°C/150°F, and further postcuring can result in a tool Tg as high as 180°C/356°F. Nixon says ZPREG and a similar layup strategy have been used by Cirrus Design Group (Duluth, Minn.) to construct a number of aircraft tools, some of which have successfully released more than 100 parts.
Meanwhile, composite parts man-ufacturer AAR Composites’ Clearwater, Fla., facility is using a “trapped rubber molding” process to make hollow parts for customers in an out-of-autoclave process that, the company claims, yields parts more void-free and dimensionally accurate than autoclaved parts. The process employs a two-part closed mold for the visible exterior (“A”) surface of the part and an internal mandrel that defines the inside (“B”) surface. The latter features a rubber lining that expands under heat to provide autoclave-like consolidation pressure. Moldmaking starts with a “pour mold” for the “B” surface tool — built in aluminum or steel from CAD data — into which an extremely temperature-sensitive, two-part rubber compound is flowed and cured to mimic the inner mold line of the part. Because the rubber exhibits predictable expansion at a given temperature, the pour mold is slightly undersized to achieve final part dimensions. Joe DeCillis, director of business development, says the key is precise calculation of the undersizing ratio. The tools for the “A” (outside) surface are machined conventionally from P20 or 4140 steel.
The rubber-coated “B” tool is mounted on a steel base to form a precast, rigid mandrel. Prepreg materials are layed up on the mandrel, which then is enclosed within the “A” tools and can be mounted in a clamshell closure assembly. The mold is heated to prepreg cure temperature, increasing the pressure inside the mold to about 300 psi/20.68 bar. The expanding rubber forces the prepreg outward against the “A” mold surface.
DeCillis says the process works very well for hollow parts with thinner laminates (0.060 inch/1.52 mm and less) that are made with lighter, plain-weave 3K fabrics. He says AAR has made several electronics enclosures with the process. “If you’re looking for tight geometries, it’s a very good process. It yields a really great surface on the rubber/mandrel side.”
Low-pressure RTM proposed
In recent years, resin transfer molding (RTM) has provided an out-of-autoclave option to aerospace manufacturers, but it typically requires matched-metal steel or Invar molds to handle high resin injection pressures (up to 300 psig holding pressure). North Coast Composites (Cleveland, Ohio) has developed a low-pressure RTM process that permits use of lighter cast aluminum tools, yet it is said to produce parts structurally equivalent to components molded using traditional RTM tooling. The main benefits of low-pressure RTM are lower tooling costs and faster cycles times achievable with lighter tools. The company estimates cycle time improvements in the range of 15 to 40 percent.
In recent process feasibility tests, North Coast Composites and the Cleveland-based NASA Glenn Research Center molded flat panels on an existing steel mold using the low-pressure process to compare its performance with that of conventional RTM. The panels were fabricated using braided Toray T700 carbon fiber fabric from Toray Carbon Fibers America Inc. (Flower Mound, Texas) and two grades of commercially available epoxy, EPON 862 from Hexion Specialty Chemicals (Houston, Texas) and Cycom 5208, supplied by Cytec Engineered Materials Inc. (Tempe, Ariz.). Panels made from the 862 resin were low-pressure molded at 15 psi/0.07 bar of hydrostatic pressure; panels made from 5208 were molded at 41 psi/2.8 bar. Subsequent testing found less than 1 percent void content in all panels molded with low pressure. North Coast says the conversion from steel tooling to aluminum is practical given the low pressures of the process and the fact that for most aerospace production, limited runs eliminate concern about tool wear. The company has calculated a potential mold weight savings of more than 900 lb/408 kg by converting from a steel mold used at 300 psi/20.7 bar to an aluminum mold operated at 75 psi/5.2 bar. Additional mass could be eliminated by using cast tools with a grid stiffener pattern on the back of the mold surface to maintain the same support as wrought plate materials.
Dan Davenport, North Coast’s RTM technology manager, says processes such as VARTM have demonstrated that low-pressure molding is technologically feasible, but that a standard was set for high pressure in RTM molding during the EMD phase of F-22 fighter jet development in the mid-1990s. During that project, manufacturers experienced problems with voids and porosity in parts, and the command was given to maximize mold pressure — without fully analyzing the nature of the problem, says Davenport. “No one did a study to see what the voids were caused by,” he says. “They didn’t overcome it, they just hit it as hard as they could.”
Davenport believes high pressure still is the key to packing a mold with high fiber content and admits that he also has doubts about aluminum tooling. “With aluminum tooling, it’s not so much a strength issue in RTM but a durability issue due to the abrasive effects of local high fiber volume in certain areas of the mold,” he explains, adding that such effects are compounded in tools with deep draws and shallow draft angles.
Nonetheless, North Coast is currently low-pressure RTM’ing a series of structural aircraft parts for an unspecified aerospace company. “We intend to introduce the process to our customer base as new opportunities arise,” Davenport says. “We expect it might take awhile to gain full acceptance [and might] have to be validated on a case-by-case basis.”
Nanoenhanced tooling concepts
For all its much-touted potential, nanotechnology often appears to be in a permanent state of near commercialization. This situation might change with a promising nanoenhanced tooling innovation developed by Boyce Components LLC (Phoenix, Ariz.). The system consists of carbon nanofibers dispersed into a vinyl ester resin, which is then sprayed onto the surface of a mold or plug. The carbon nanofiber, supplied by Pyrograf Products Inc. (Cedarville, Ohio), is mixed into the resin at a loading of about 2.5 percent. The nanofiber-filled resin is applied to a thickness of about 0.030 inch/0.76 mm, making the tool face electrically conductive. A series of aluminum electrodes are embedded, spaced about 2 inches/51 mm apart (on x and y axes) across the mold surface. Copper main lines are attached to every other aluminum electrode, creating a series of anodes and cathodes. The electrodes are buried and insulated under another layer of fiberglass and resin. Holes are drilled to connect wires to the copper leads. When voltage is applied to the electrodes, current is generated and passes through the coating via the nanofibers to heat the tool face, thereby reducing the curing time of the parts being molded.
The technology is being marketed as the K-Factor Composite Heating System. Company owner Darren Boyce says the technology evolved out of his efforts to track filled resin, as it flowed through the mold, to eliminate entrapped air. Because conventional fillers require high loadings, which negatively impact resin flow, Boyce experimented with high-aspect-ratio fullerene nanotubes and discovered the cured resins became electrically conductive at less than 1 percent dispersion.
The cost of the fullerene nanotubes, however, was prohibitively high for practical applications. Pyrograf’s nanofibers, on the other hand, reportedly cost more than 50 percent less than fullerene nanotubes, but they offer similar electrical properties at slightly higher loadings. The company’s Pyrograf-III carbon nanofiber has a tubular structure with a sidewall composed of angled graphite sheets. This morphology, often termed “stacked cup,” generates a fiber with exposed edge planes along its entire surface. Boyce uses grade PR-24, which comes as a dry powder with an average particle diameter of 100 nm.
Although the coating can be applied to either surface of the mold shell, Boyce says it is typically used behind the mold’s work surface, protected beneath the gel coat. Tests show the coating is stable during repeated cycles, but users are advised to ramp temperatures gradually because heavy current applied too quickly might cause the gel coat to delaminate.
Boyce says he envisioned that the in-mold heating technique could be most useful to boatbuilders who simply want to keep their molds at 77°F/25°C in the winter, eliminating the expense of space heaters and reducing cure time from as many as three days to as few as one. But he says the technology will effectively elevate mold temperatures well above ambient levels, accelerating cure in any molding process that involves composite tooling, including light RTM. “If you get the gel coat of a part up to its exotherm temperature, you can save the four to five hours it takes to cure at room temperature,” Boyce says. “This provides a huge boost in productivity.”
Nanotechnology also has found use in coatings designed to harden and prolong the life of composite tooling. Integran Technologies Inc., a nanostructured materials supplier based in Toronto, Ontario, Canada, is targeting its metal tool facing, Nanovar, to makers of carbon fiber-reinforced composite tooling. Nanovar is a low-thermal-expansion, nickel-iron alloy with a Vickers HV hardness of 460, reportedly about five times harder than Invar. Compatible with composite tools made from prepregged or resin-infused carbon/epoxy and carbon/bismaleimide, the material is applied to the tool surface at 0.006-inch to 0.008-inch (0.15 mm to 0.20 mm) thickness for most applications by “a proprietary immersion process.” The coating hardens the tool surface by capitalizing on the “Hall-Petch” effect, a metallurgical axiom that correlates decreasing grain size with increasing strength and hardness: Typical metals have crystalline structures with average grain sizes of 10 to 100 microns, but the patented process used to produce Nanovar reportedly creates grains of 20 nm or less, or about 1,000 times smaller. The result, according to product development program manager Rich Emrich, is a fully dense (effectively nonporous), extremely durable and void-free metal layer.
The product was launched about nine months ago, and, according to Emrich, Integran is currently conducting validation and demonstration projects with several customers. “Our goal is to extend the life of carbon-fiber tooling by two to three times,” he says, adding that nanoenhanced performance enabled his company to succeed where others have failed. Attempts to apply a traditional nickel vapor deposition coating to CFRP tools have not worked, Emrich maintains, because nickel has a much higher coefficient of thermal expansion (CTE) than the tooling resins and, therefore, will delaminate during thermal cycling. Nanovar’s CTE of about 3 ppm/°C — less than one quarter that of nickel — enables a Nanovar-faced composite tool to withstand aggressive thermal quench testing with liquid nitrogen without delamination. Emrich says the company plans to apply Nanovar to customer tooling at its own factories but does not intend to enter the tooling market directly.
Reformable tools for big parts
2Phase Technologies Inc.’s (Dayton, Ohio) president, John Crowley, reports that there have been several new developments in its reformable tooling technology. The company now has three completely commercialized, sealed tool rectangular tool beds sized from 6 ft to 9 ft (1.83m to 2.74m) long for resins systems curing at 400°F (204°C) or less. He says several systems have been sold for repair, replication and design applications. The reformable tools use material that combines ceramic microspheres and an inorganic, water-soluble refractory binder. The tool bed, filled with microspheres, binder and a small amount of water, is covered with a robust elastomeric membrane. A master model is placed against the membrane, covered with a vacuum bag or temporary vacuum cap and drawn down into the liquid-like material. The water is removed via vacuum, and the material solidifies. The master is removed, and applied heat stabilizes/hardens the tool for use. A prototype tool can be prepared in as little as five minutes and can be repeatedly transformed back to a liquid state and reshaped. Moreover the material can be tailored with a CTE to match a specific resin and can be used under vacuum or under pressure as high as 100 psi. Crowley says the technology can be used with VARTM, autoclave and hand layup processes, among others.
2Phase also has developed a reformable tooling system capable of withstanding 800°F/427°C processing temperatures. Linda Clements, director of materials R&D, says they are still working on an effective barrier system for these high temperatures and conducting tests to understand the material’s dimensional stability. “This system will allow us to fabricate all the organic, thermoset matrix composites systems, including cyanate esters, polyimides and some exotic bismaleimides,” Clements says.
Autoclavable prototyping tools
In just five years, the ability to convert 3-D CAD models to machine code for CNC milling machines has opened the door wide, says Matrix Composites’ (Rockledge, Fla.) Dave Nesbitt, to expedient use of high-temperature epoxy tooling board for short-run autoclave tooling. Gene Curnow, technical sales representative with CASS Polymers of Michigan (Madison Heights, Mich.), says he is seeing a similar trend for light RTM applications, noting that “epoxy board is good for parts requiring tight tolerances.” Epoxy board suppliers have responded with tooling board capable of enduring autoclave cycling. Among the products is CASS Polymers’ EP-300 epoxy plank, which has a heat deflection temperature of 325°F/163°C and “bowling ball-like” Shore D hardness of 78. Huntsman Advanced Materials (The Woodlands, Texas) offers RenShape 5008 Syntactic Intermediate Temperature epoxy modeling board designed for use up to 250°F/121°C with a glass transition temperature of 287°F/142°C. Epoxy board, unlike urethane tooling board, does not inhibit the cure of epoxy resins, says George Wise, product manager at Freeman Manufacturing & Supply (Avon, Ohio), adding that in high-temp curing, board life can be extended by slowly ramping the temperature up and down.
To avoid part imperfections caused by bond lines in glued stacks of machined epoxy board, however, Huntsman suggests the use of aluminum honeycomb with a tool face CNC machined from RenPaste, a high-temperature, two-part epoxy seamless modeling paste (SMP). RenPaste SMP is used as the honeycomb adhesive and as the machinable layer on the tool/model surface. Reportedly, the highly thixotropic SMP can be applied with extrusion equipment in a continuous ribbon to 40-mm/1.57-inch thickness on vertical surfaces without slumping. The honeycomb structure mitigates the effect of interfacial stress as a result of the difference in CTE between the aluminum and SMP, and the cured SMP has a heat deflection temperature of 400°F/205°C and a Shore D hardness of 80, making the tool suitable for curing of aerospace prepregs, although the current CTE of the epoxy paste limits tool design to “simple” shapes for which expansion/contraction correction values can be calculated and then incorporated into tool design.
Special Section: Automated, on-machine tool inspection
Given the large molds required today for some aerospace parts, mold inspection has become a time-consuming and difficult task. Toolmakers that are looking for ways to simplify inspection are automating the process. Reno Machine Co. (Newington, Conn.), for example, recently installed the PowerINSPECT OMV system developed by Delcam Plc (Birmingham, U.K.) on its 5-axis vertical gantry mill, supplied by Henri Liné Machine Tools Co. (Granby, Quebec, Canada). The CAD-based inspection system allows Reno to measure and verify the geometry of tooling online, generating "hard point"data with the aid of a Renishaw probe (Renishaw Plc, Gloucestershire, U.K.) preinstalled on the CNC machine in place of the cutting tool. Delcam's system uses the probe to trace tool dimensions and compare them to the CATIA data file, eliminating the need to make discrete measurements offline with coordinate measurement machines (CMMs), laser trackers or hand-held devices. "We were using portable CMM roamer arms to inspect, which is a two-person job,"says sales director Jay Mulligan. The PowerINSPECT system "automatically creates a red/green point cloud of data off the surface we've just cut,"he reports. "We can do a rough cut during the day, inspect at night, then come in and evaluate the data in the morning and touch up the red areas. This is absolutely revolutionary for the output of large molds."(The software also can be used with CMMs and optical measuring devices.) Reno manufactures large molds primarily for the aerospace industry. Mulligan says the system allows the company to build a 35-ft mold without having to shut down production. Mulligan says the company is already seeing an increase in workflow and likely will install PowerINSPECT on its second Henri-Liné 5-axis vertical mill, used mainly to machine high-strength alloys, such as Invar.