Tooling Update: New dimensions in tooling
Nanoenhancements, out-of-autoclave strategies and low-pressure RTM headline efforts to increase mold quality and productivity and cut tool cost.
By Michael LeGault, Contributing Writer | January 2008
Source: ACG
ZPREG semipreg, from Advanced Composites Group Ltd. (Heanor, Derbyshire, U.K.) was recently used to build a tool for out-of-autoclave processing of the ABN AMRO Volvo 70 carbon/epoxy racing yacht hull.
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.
Source: ACG
ACG says that the tool for the Volvo 70 hull (viewed, here, down the hull’s length from one end) had a “virtually faultless” surface area of 120m²/1,292 ft².
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.
Source: ACG
In this suggested ZPREG layup for an aircraft-suitable, out-of-autoclave tool, the surface ply (bottom) is extended and connected to a breather ply to help evacuate air and admit resin. Red indicates the semipreg’s unimpregnated fabric layers while blue represents the initial position of the resin. The two broken layers are ZPREG bulk plies. The stack includes a syntactic ply behind the surface layer, and ”balance” is achieved with a final ply of syntactic and (top) a simple prepreg.
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.
