Tooling roundup: New materials, new methods
"Ingenuity" describes new tooling approaches for composites manufacturing.
By Sara Black, Technical Editor | September 2004
Once dubbed by an industry writer the "Rodney Dangerfield" of composites, the tooling trade has definitely moved up to new levels of sophistication and earned the industry's respect. The growth of advanced composites applications -- particularly parts with complex contours -- has heightened the need for tooling that affords design flexibility as well as repeatable and accurate performance. No single, ideal material exists that's right for all projects. Every tooling material has its advantages and disadvantages, which must be assessed on a project-specific basis. Certainly, large steel, aluminum and Invar tools are still selected for many critical aerospace projects because of their durability and predictable temperature profile. But, tooling manufacturers continue to push the envelope, demonstrating ingenuity and innovation in the quest for more cost-effective alternatives. As a result, high-performance composites manufacturers have a host of recently introduced tooling materials and design concepts from which to choose.
Source: Airtech
A conical master model, prepared with Airtech's Infusioncoat surfacing material, is layed up with new Toolfusion material, which produces cost-effective composite tools with very low void content, via vacuum infusion.
Carbon prepreg alternatives
Carbon fiber/epoxy prepreg tools are entirely appropriate for limited-production or prototype parts with complex contours. Like the specialty metallic alloys, carbon offers coefficient of thermal expansion (CTE) compatibility with part materials, but results in a lighter, easier-to-handle and less costly tool. Carbon tools also have lower overall mass, resulting in faster heat cycling and less energy use. High-temperature heat cycle durability and propensity for damage can be issues in autoclave manufacturing, and as a result, many projects are moving out of the autoclave. To keep pace, toolmakers are responding with some lower-temperature and less-labor-intensive alternatives to traditional tooling prepregs.
Dry fiber costs less than prepreg, and vacuum infusion, once mastered, requires less labor than hand layup. With an eye to those realities, Airtech International Inc. (Huntington Beach, Calif., U.S.A.), a long-time supplier of traditional composite materials, recently introduced a combination of fabrics and resins called Toolfusion, for vacuum-assisted resin transfer molded (VARTM) tools. Toolfusion has been demonstrated as suitable for aerospace-grade parts, with service temperatures in the >150°C/300°F range.
Guy Schindler, Airtech's technical director, notes two key advantages in infused tools. First, they do not require autoclave cure, which means that a wider variety of room-temperature master models can be used to create them. Second, infusion enables more reverse engineering, because tools can be taken from parts without exposing them to damaging heat.
Source: SP Systems
A marine tool made with SPRINT materials from SP Systems is shown with its steel backup structure. SPRINT uses resin film infusion as an alternative to traditional prepreg tooling materials.
Airtech offers two epoxy systems reinforced with either glass or carbon fabrics: Toolfusion 1A/1B is cured at room temperature, with postcure at 191°C/375°F. For higher performance, toolmakers may chose Toolfusion 2A/2B, which is cured at 55°C/130°F, with freestanding postcure at 200°C/392°F, achieving a Tg of 183°C/360°F. The low-viscosity, two-part resins are formulated in-house by Airtech and require no refrigeration, which means long shelf life and unlimited out time. Fabrics are optimized for resin dispersion.
To make a Toolfusion tool, an Airtech resin called Infusioncoat is first hand applied over the model or plug surface. The black, non-sagging resin essentially acts as a gel coat or hard surfacing layer, and provides sufficient tack to hold the first fabric ply in place. (Airtac 2 spray tackifier or Tac-Strip tacking mesh can be used to position subsequent plies.) A resin-permeable peel ply is placed over the dry fabric stack, followed by a resin distribution mesh. The layup is then vacuum bagged and prepared for resin infusion.
Schindler stresses that toolmakers need to understand VARTM to ensure good resin flow and complete wetout of the reinforcement under vacuum pressure. Vacuum integrity is extremely critical because any leaks will introduce air into the laminate, causing a loss of compaction and increased void content. He recommends that full vacuum be maintained for a minimum of 24 hours at 22°C/72°F to allow the system to cure to a stable condition. Exposure to mild heat significantly reduces cure time.
Airtech also sells backup structures, including Masterflex S-Series, a spiral-cut, flexible square tube made of carbon or glass cloth stiffened with a binder. The tubing can be applied to the back of the tool as a hat stiffener, using tooling prepreg or wet layup plies. Masterflex stiffens the tool face for better dimensional control during cycling, but unlike tooling boards, it doesn't interfere with uniform heat distribution, says Schindler. Backup structures can be built with Masterflex pultruded carbon fiber tubing.
"Toolfusion tools have an optimal fiber-to-resin ratio and less than 1 percent voids, which defines a good laminate for high-quality carbon composite tooling," concludes Schindler.
Another tooling prepreg alternative, normally used for producing parts, is SP Resin Infusion Technology, or SPRINT, from SP Systems (Newport, Isle of Wight, U.K.). SP interleaves pre-catalyzed resin film and dry carbon or glass fiber fabrics to create tooling SPRINT ("T-SPRINT") material. T-SPRINT allows entrapped air to escape more easily during compaction and cure, for exceptionally low void content, says SP's tooling products manager Andy Harvey. "You don't need to debulk as often during layup and you can achieve a 0.5 percent void content or less," he says. (Voids are the bane of tooling designers because they tend to act as point sources for microcracks during heat cycling, leading to tool damage.) Essentially a resin film infusion material that is sandwiched between fabric layers, SPRINT's resin must travel at most a few millimeters to fully wetout the reinforcement. Therefore, SPRINT can incorporate higher-performance, toughened prepreg-quality resins that have viscosities too high to be workable with wet layup or other infusion methods. But like prepreg, adds Harvey, "SPRINT has excellent drapability and is easy to lay up and reposition on the pattern or plug."
Although it can be cured at room temperature under a vacuum bag, T-SPRINT has a relatively high service temperature of 160°C/320°F. Target markets are large molds for the wind turbine and marine industries. A typical wind blade tool laminate on a male plug consists of a gel coat surface followed by eight plies of T-SPRINT. A lightweight 35 g/m² tooling tack film allows the SPRINT materials to be tacked to vertical surfaces or around complex curvatures. The materials are cured at 65°C/149°F for 16 hours, and Harvey recommends a freestanding postcure at 80°C/176°F to achieve the 160°C/320°F temperature performance.
While a variety of rigid carbon foams are currently made from petroleum-based mesophase pitch or carbonization of polymer materials, Touchstone Research Laboratory Ltd. (Triadelphia, W. Va., U.S.A.) produces CFOAM carbon foam from coal, in a process of thermal decomposition termed "coking" in the coal industry. The new material is an alternative to traditional tool structures.
"The finished foam has mechanical properties dictated by the thickness of the pore walls and the final density," says Touchstone program manager Andy Guth. CFOAM has a compressive strength of more than 5 MPa/800 psi at a density of 0.27 g/cm³ (17 lb/ft³), which gives it plenty of strength for autoclave pressures and dimensional stability at temperatures in excess of 260°C/500°F, reports Guth.
CFOAM's porous open-cell structure lacks vacuum integrity, so the material requires a vacuum-tight tool face formed with an appropriate surface coating. (One alternative is RENShape high-temperature epoxy-based tooling paste from Huntsman Advanced Materials of East Lansing, Mich., U.S.A.) With the surface in place, CFOAM can be machined quickly to create a tool shape with significantly lower mass than metallic tooling for easier handling and good CTE match with carbon prepreg laminates. Based on extensive cycling tests, CFOAM is durable and robust enough to play the role of production tooling says Guth. And at a cost of about $400/ft³, he claims, "We can produce a production tool as durable as Invar at half the price."
