Ceramic Salt Mill

Tooling for composites: Evolutionary trajectory

A Carbovar tool, on display at the recent 2011 JEC Composites Show, combines a nano-structured metallic coating by Integran Technologies (Toronto, Ontario, Canada) over a carbon prepreg composite tool from Advanced Composites Group (Tulsa, Okla.). The result is a low-thermal-mass, lightweight tool with the surface durability of metal tooling.

Tooling is the vital but often underappreciated silent partner behind every well-designed composite part. It also can be a very costly aspect of that part’s production. The manager for a Tier 1 aerospace facility recently revealed that the tool inventory in the plant cost more than the building and all its production equipment combined! Given that level of investment, molders want to get the most out of their tooling dollars. Toward that end, HPC Self-heating tools save time

A tooling material that has attracted considerable attention since its commercial introduction seven years ago is carbon foam. This low-density, machinable material has a low coefficient of thermal expansion (CTE) compatible with carbon composites (see “Carbon/BMI and carbon foam form Invar alternative,” under "Editor's Picks"). Now, engineers at Touchstone Research Laboratory Ltd. (Triadelphia, W.V.) have introduced Electrically Heated Tooling (EHT), a self-heated foam tool for OOA processing.

Touchstone’s Jesse Blacker explains that its CFOAM is produced from powdered bituminous coal, which is pressed into tray molds and processed under high heat and pressure in an autoclave. Foam panels are bonded together to form a near net-shape billet, which is machined to the desired shape. A tool face is formed by applying a composite material, such as Hexcel’s (Stamford, Conn.) HexTool mat material. After cure of the tool face, the CFOAM tool is CNC-machined to the final profile, wet sanded, sealed and prepared for part production.

EHT tooling incorporates a constant cross-section, resistively heated foam heating element (identical to the CFOAM used in the tool) just beneath the tool face. A graphite electrode is connected to the element to provide electricity, and the element itself is isolated from the rest of the tool mass, explains Blacker. “In this way, resistive heating occurs only beneath the tool face, making the tool more energy efficient, and the underlying foam substrate that’s not heated acts to thermally insulate the face and minimize heat losses.