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, 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 — particular 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 suitable 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 integrity during cure. Shrinkage and thermal expansion of the tool and part will be very similar.
But most metal tool materials and composites are mismatched. C-20 steel and aluminum are less expensive and usually involve shorter lead times (the time between order placement and tool delivery) than high-performance alloys and, therefore, are common choices. But during heated cure, the CTE mismatch between the tool material 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.
An alternative to these 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.
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.