The molds used for forming composites, also known as tools, can be made from virtually any material. But tooling costs and complexity increase as the part performance requirements, surface quality requirements and/or the number of parts to be produced increase.

The molds used for forming composites, also known as tools, can be made from virtually any material. For parts cured at ambient or low temperature, or for prototyping, where tight control of dimensional accuracy isn’t required, materials such as fiberglass, high-density foams, machinable epoxy boards or even clay or wood/plaster models often are suitable. Tooling costs and complexity increase as the part performance requirements and the number of parts to be produced increase. High-rate production tools are generally made of robust metals that can stand up to repeated cycles and maintain good surface finish and dimensional accuracy. The molds in which high-performance composite parts are formed can be made from carbon fiber/epoxy, monolithic graphite, castable graphite, ceramics or metals, which are typically aluminum or steel. Each material offers unique capabilities and drawbacks.

Sometimes called hard tooling, ceramic and metal tools, although relatively heavy and expensive, are able to withstand many thousands of production cycles. The most durable, but also most costly, are made of high-performance steel alloys, such as Invar. Further, few composites manufacturers have the equipment necessary to cut and polish metal tools — particularly Invar — so they often require the services of a tooling specialist.

Composite tools, sometimes called soft tooling, are more easily constructed and, because they are made from materials similar to those the composite manufacturer will use for the part, they can be made in-house. But as the moniker suggests, they are more vulnerable to wear and typically find service in low-volume production. However, several tools can be made with composite materials for less than the cost of a single hard tool, making somewhat larger volumes affordable.

Like those made on hard tooling, parts made on composite tools can be cured in an autoclave or oven, or by integral heating, in which individual heating elements are placed inside the tool. One product in this category is HexTOOL from Hexcel (Dublin, Calif.), a machinable carbon fiber/bismaleimide (BMI) composite tooling material. It comprises prepreg strips that are randomly distributed onto a release paper to form a larger mat. After layup and cure, it can be machined like metal, has a coefficient of thermal expansion that matches carbon/epoxy parts and can survive 500 autoclave cycles, all with a build time and cost comparable to existing alternatives.

A key issue with tooling for composites is the phenomenon of coefficient of thermal expansion (CTE) mismatch. Here, composite tooling has the advantage. Composite tools made from tooling prepregs have a CTE close to the part CTE, helping the part maintain dimensional accuracy during cure. Shrinkage and thermal expansion of the tool and part will be very similar.

Most metal tool materials and composites are mismatched. C-20 steel and aluminum are common choices because they are less expensive and usually involve shorter lead times before delivery vs. high-performance alloys. However, during heated cure, the CTE mismatch between the cheaper metal tool materials and the composite often is too extreme for use with close-tolerance composite parts. Only the higher-priced metal alloys, such as Invar, offer closer CTE matches. For example, Invar — alone among metals — offers a CTE very near to that of carbon fiber composites. For that reason, it has been the perennial choice for parts that must be manufactured to extremely tight tolerance. But Invar also is the most costly metallic tooling material and, especially for large parts, the sheer size and weight of the tools makes them difficult to handle. Aerospace industry manufacturers, in particular, have expressed a desire for carbon fiber tooling materials that can withstand thousands of autoclave cure cycles, like Invar does. To increase the durability of composite tooling, several suppliers offer hybrid tool designs that combine, for example, a thin Invar facesheet with a composite backup structure, or a carbon foam core with a composite facesheet.

New technologies have evolved to compete with Invar’s durability but at a fraction of its weight and cost. Janicki Industries (Sedro-Woolley, Wash.) has produced void-free, 300-ply BMI laminates using resin infusion, which enabled the company to produce billets of BMI/carbon composite that can be CNC-machined into precision tooling that is cheaper and one-fifth the density of an Invar equivalent. Janicki is using this technology for aerospace rib, spar and stringer tools, where the parts will be autoclave-cured high-temperature epoxy and carbon fiber. BMI greatly extends the tool’s service life compared to epoxy, which could not withstand cycling up to 350°F/177°C every three days for two years.

Other BMI technologies touting longer tool life include 3M Advanced Materials Division (St. Paul, Minn.) Fortified Tooling Prepreg (FTP) BMI and Airtech International Inc. (Huntington Beach, Calif.) Beta Prepreg tooling system using Henkel benzoxazine resin. Both are autoclave-cured, claiming lower resin shrinkage over epoxy and excellent machinability. Distributed by TenCate Advanced Composites (Nijverdal, The Netherlands and Morgan Hill, Calif.) 3M BMI TC-44 Si uses a 40 percent by weight loading of micoscopic silica particles to reduce through-thickness CTE by 15 percent, linear CTE by 40 percent and linear cure shrinkage by 50 percent compared to comparable non-filled autoclave-cure BMI prepreg. Together, these improvements reduce thermal stresses, part distortion and are expected to significantly limit springback. (Springback is a material’s tendency to deviate from the molded shape due to cure shrinkage in the composite.) Other benefits include better scratch resistance, fracture toughness, reduced microcracking, and 40 percent lower exotherm per mass for much improved thermal management, especially important for thick part fabrication. Airtech is claiming similar performance and production benefits for its Beta Prepreg, which uses benzoxazine’s unique chemistry instead of nanosilica, and has demonstrated a 70 percent improvement in springback. However, its 6-month ambient storage life basically eliminates frozen storage requirements for tooling projects and reduces shipping costs, while its in-service experience so far reports greater tack resulting in reduced layup time and number of debulks required. Airtech says BetaPrepreg is cost-competitive with BMI but contends its performance and fabrication efficiency can tip the scales in its favor.

Janicki has developed composite trim and drill fixture production techniques that save cost versus metals. Most companies use large aluminum trim fixtures, apply vacuum to affix the parts and then stick the whole assembly into the CNC machine. Janicki achieves the same tolerances using its low-cost tooling technology — wood structure with machined putty and fiberglass — but at a much lower cost. This is possible because of the way it “clocks in” the milling head’s positional accuracy (see “Big machines, small tolerances” under "Editor's Picks"). Janicki makes the pattern in this way, takes one splash off of it to make the tool and then uses the pattern again to make the trim-and-drill fixture. Thus, the means to make the fixture is paid for in the tool fabrication, which results in significant savings.

Another alternative to traditional metals is a toolmaking process that uses a nickel vapor deposition (NVD) process to produce relatively thin nickel-shell tool faces. Mounted on backing structures, the nickel tool surface can achieve high dimensional fidelity, and offers low CTE, long life and, because it is far less bulky than machined metal tools, it weighs less and facilitates faster mold heating and cooling. NVD specialist Weber Manufacturing Technologies Inc. (Midland, Ontario, Canada) produces nickel-shell tools for applications ranging from automotive body panels to aircraft interior storage bins.

EireComposites Teo. (Irish Composites) (County Galway, Ireland) has developed patented technology to build integrally heated tools with embedded electrical heating elements using an aluminosilicate-type ceramic cement, which offers low CTE, density, thermal mass and electrical conductivity. Carbon fiber reinforcement bolsters the low tensile strength of the ceramic using polyetheretherketone (PEEK) high-performance thermoplastic polymer as an adhesive between the carbon fiber and the ceramic. The tooling can be built on inexpensive patterns because the ceramic becomes rigid at 60°C/140°F. The tooling is removed from the pattern after this initial lower-temperature cure and then processed to full temperature (200°C to 400°C or 392°F to 752°F) via a freestanding postcure. The in-plane CTE measured for a tool at least 15 mm/0.6 inch thick is less than 5.0 x 10-6/°C, which, according to ÉireComposites, is generally accepted as a “matching value” for most glass and carbon fiber composites. Using an electrical wattage density of 10 KW/m2, this type of tool surface can be brought to 200°C in less than 10 minutes.

Embedded resistance wiring or other electrical means are not the only way to integrally heat tooling. Tooling with built-in ducts that heat and cool using forced air has been used in the aerospace industry for decades. The wind industry has infused layups in heated molds for years, commonly using fluid conduits. Using heated and cooled liquids for molding composites first started gaining traction with VEC Technology LLC (Greenville, Pa.) Floating Mold technology patent application in 1999, and more recently with Quickstep Holdings Ltd. (North Coogee, Western Australia), “balanced pressure fluid molding” (patent application in 2002). Other systems include Techni Modul Engineering’s (Coudes, France) self-heated tooling that features a fluid circulation system for oil, water or metal-based fluids. Although VEC describes its Floating Mold processing as resin transfer molding (RTM), and QuickStep markets its process as being adaptable to liquid resin infusion, resin film infusion or light RTM, all three companies can use a variety of fluids to impart temperature control and each describes its composite tooling shells as thin, lightweight and less costly than the traditional metal tools used in RTM.

Commercialized tooling design software is reducing the time it takes to model and manufacture a tool — including backup structure — by 80 percent in some cases. New inspection systems give tooling suppliers and fabricators a way to verify a tool’s dimensional accuracy prior to and during production. In recent years, a variety of low-cost modeling materials that maintain dimensional stability at higher temperatures have made inroads into traditional toolmaking.

No matter the tooling material, the importance of mold release agents cannot be over-emphasized. Releases create a barrier between the mold and part, preventing part/mold adhesion and facilitating part removal. For open molding, most releases are waxes or are based on polymer chemistry. Of these, most are polymers in solvent-based carrier solutions, such as an aliphatic hydrocarbon blend. Some manufacturers prefer naphtha-based releases, which have longer shelf life and faster evaporation rates and are considered to be less damaging to composite tool surfaces. Increasingly strict emissions regulations have encouraged development of water-based releases, which produce no VOCs and clean up more easily, with less skin irritation.

Semipermanent polymer mold release systems enable multiple parts to be molded and released with a single application, in contrast to traditional paste waxes that need to be reapplied for each part. Semipermanent releases, which are preferred for better control over VOC emissions, have been formulated specifically to meet the needs of resin transfer molding (RTM) and other closed mold processes.

Internal release agents, added to the resin or gel coat and used instead of or in addition to external agents on the mold surface, further reduce emissions, and have negligible effects on a part’s physical properties and surface finish. Internal release formulations are required for pultrusion processing because the part is pulled continuously through the die, allowing no opportunity for intermittent application of external releases to the die surface.

Nova-Tech Engineering

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