Prepreg vs Wet Layup vs Vacuum Infusion
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Most carbon fiber parts are made one of three ways. The fiber can be identical in all three. What changes is how the resin gets in, how much of it ends up in the part, and what the process costs to run.
Wet layup
Dry fabric is laid into an open mold and wetted out with resin by brush or roller, usually a room temperature epoxy. It's the cheapest way to start: the mold can be fiberglass or even a 3D printed pattern, and there's no oven or press. The trade-off is resin content. Hand-wetted laminates usually end up around 30 to 40 percent fiber by volume. Putting the layup under a vacuum bag pushes that up and pulls out some of the air, but expect more voids and more variation from part to part than the other two methods.
Good for one-offs, large parts, prototypes where weight isn't critical, and repairs.
Vacuum infusion
Dry fabric goes into the mold, everything is sealed under a vacuum bag, and the vacuum pulls low-viscosity resin through the stack. Because the fabric is compacted before the resin arrives, infused parts typically reach 50 to 55 percent fiber by volume with low voids, and the shop stays cleaner. The hard part is planning the flow: where resin enters, where it exits, and whether it wets everything before it starts to gel. Dry spots are the classic failure.
Good for large parts like hulls, panels and covers at low to medium volumes.
Prepreg
Prepreg is fabric that comes with the resin already in it, at a controlled ratio, and it's cured with heat: in an oven under a vacuum bag, in an autoclave, or between matched molds in a heated press. Autoclave-cured laminates commonly reach 55 to 65 percent fiber with void contents around 1 percent or less, which is why aerospace and racing parts are made this way. Prepreg has to be stored frozen, has a limited working life at room temperature, and needs molds that can take the cure temperature.
Good for structural parts, cosmetic parts that need a consistent finish, and anything where weight matters most. Pressing prepreg between heated matched molds, or compression molding, brings cure times down to minutes for production. It's how our SSD Enclosure 1 is made.
Side by side
| Wet layup | Vacuum infusion | Prepreg | |
|---|---|---|---|
| Fiber by volume | about 30 to 40% | about 50 to 55% | about 55 to 65% |
| Voids | highest, varies | low | lowest in an autoclave |
| Mold | open mold, cheapest | open mold, sealed bag | must take cure heat |
| Material cost | lowest | low | highest |
| Typical use | prototypes, one-offs, repairs | large parts, low volume | structural, cosmetic, production |
How to choose
- If weight and consistency matter most, use prepreg.
- If the part is big and the budget is tight, infuse it.
- If you need one part next week to check fit, wet layup in a simple mold is often enough.
The process also decides the tooling. We cover how molds step up from prototype to production in carbon fiber prototypes without a production mold. Every method, including RTM, autoclave and compression molding, is compared in composite manufacturing processes. For help choosing for your part, see carbon fiber.