Compression Molds for Carbon Fiber Parts
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Compression molding is how carbon fiber parts get to production volumes. Prepreg or a chopped fiber charge goes between two matched mold halves in a heated press, which supplies the pressure an autoclave would. Cure times drop from hours to minutes, and both faces of the part come out finished. The mold is what makes it work.
Mold materials
- Steel, such as P20 or H13, for production. It holds a polished surface, survives many thousands of heat cycles and doesn't dent.
- Aluminum for short runs and prototypes. Cheaper and quicker to machine and it heats faster, but it is softer and expands more with heat.
Shear edges and flash
The two halves usually meet at a vertical shear edge: a tight sliding fit around the part outline that traps the material and pinches off the excess. The gap is small, often a few hundredths of a millimeter, so the halves have to be guided precisely, with guide pins or heel blocks taking the side loads.
Heating
Molds are heated by cartridge heaters, oil channels or the press platens. What matters is an even temperature across the surface, because hot spots cure faster and can show on the part. Thermocouples at several points let you check it.
Release and ejection
Cured epoxy sticks to bare steel, so molds get a semi-permanent release coating that is renewed on a schedule. Draft angles of a few degrees help parts come out, and ejector pins or air ejection push them free without cracking them.
Thermal expansion
Steel grows about 12 µm per meter for each degree Celsius, aluminum about 23, and a carbon laminate only 2 to 4. A part cured in a steel mold takes the mold's size at cure temperature, then shrinks much less than the steel as it cools, so it comes out slightly larger than the cold mold: about 0.1 percent for a 120 °C rise. On long parts with tight tolerances, the mold is cut undersize to compensate. Aerospace tooling sometimes uses Invar or carbon fiber molds to avoid the mismatch.
When it pays off
Matched metal molds cost far more than a composite layup tool, but they make the cheapest parts at volume, with consistent thickness and two finished faces. For a handful of parts, an open mold and a vacuum bag is usually the better deal; see carbon fiber prototypes without a production mold. Our SSD Enclosure 1 is compression molded this way.