Carbon Fiber Prototypes Without a Production Mold

A matched metal mold is the right tool for making a few thousand carbon fiber parts. It's the wrong tool for making three. Tooling for composite prototypes usually steps up as the quantities grow, and moving up a step too early adds cost.

One to five parts: one good face is usually enough

Most early prototypes only need one cosmetic side. That points to an open mold: a single tool you lay the carbon into, with the back side bagged under vacuum.

The mold can be machined tooling board. Polyurethane and epoxy boards cut quickly on a CNC router and seal up to a smooth surface. Check the board's temperature rating against your cure cycle before you cut it, because ratings vary a lot from one board to the next.

It can also be a 3D print, sealed and coated. That works for room-temperature or low-temperature cures. A standard PLA print starts to soften around 60 °C, well below most prepreg cure temperatures, so use a high-temperature print material or a resin system that cures cool.

Wet layup under vacuum is the cheapest way to make the part itself. Out-of-autoclave prepreg in an oven gives more consistent resin content, if you have the oven. Either way, plan to trim the edges afterwards.

What you get is a part that looks right and is close on weight. What you don't get is tight thickness control or a finished back face.

Five to fifty parts: a tool that holds up

Board and printed molds wear, chip and drift after a few parts. The usual next step is a composite tool: carbon fiber tooling prepreg laid up on a master pattern and post-cured. It survives repeated oven or autoclave cycles if it's looked after, and it grows with heat at nearly the same rate as the part.

That last point catches people out. Aluminum expands about 23 µm per meter for every degree Celsius. A quasi-isotropic carbon laminate expands only around 2 to 4. On a part 1 m long, cured 100 °C above room temperature, an aluminum mold grows about 2.3 mm while the part grows a few tenths of a millimeter. On long, precise parts that mismatch shows up as locked-in stress or parts that come out the wrong size. A carbon tool mostly avoids it.

Fifty parts and up, or both faces matter

When both surfaces need to be finished, the wall thickness has to hold, or something has to fit inside, a matched mold in a heated press is the answer. That's compression molding. The mold is a real investment, but cycle times drop and every part comes out the same. It's how we make the shell of our SSD Enclosure 1, because a circuit board and a fan have to fit inside every one of them. (The full process is here.)

Details that cost time if you skip them

• Draft. Carbon parts need draft to release, just like molded plastic. A degree or two on vertical walls saves a lot of grief at demold.
• Weave direction. On a cosmetic part the weave has to run straight across the visible face. Plan seams and darts before anyone cuts fabric.
• Inserts and bonding. Threaded inserts, bushings and bonded brackets belong in the tooling plan, not in a rescue job after the first part.
• Trimming. Every part needs its edges trimmed and holes drilled. A simple trim fixture pays for itself within a handful of parts.

Send a drawing or a sketch, the quantity and the finish you need, and we'll suggest the lowest-cost tooling route that still gives you a useful prototype. See carbon fiber and prototyping.