A tier-2 auto parts supplier in Dongguan runs two carbon fiber hood production lines side by side: one prepreg-autoclave, one RTM. Same geometry (a BMW M3-style hood), same part weight target (<4.5 kg), same customer requirements. After three years of production data, the differences between the two processes are sharper than the marketing brochures suggest.
What’s surprising isn’t which process wins — it’s how the choice of process decides everything else downstream: tooling budget, scrap rate, surface finish quality, and whether the part is even profitable at volume. Here’s the comparison, based on real production numbers, not spec sheets.
Prepreg Autoclave: The Gold Standard With a Price Tag
Prepreg — carbon fiber fabric pre-impregnated with epoxy resin — goes into the mold, gets vacuum-bagged, and cures under heat and pressure in an autoclave. The process has been the default for high-end automotive carbon fiber since the 1990s. It produces the lightest, strongest parts. It also produces the most expensive ones.
Material cost is the big line item. Automotive-grade 3K twill prepreg runs $25–45/m² depending on tow grade and resin system. A hood uses roughly 4–5 m² of prepreg in a 2–3 ply layup. That’s $100–225 in raw carbon per hood before you account for the 15–25% material waste from trimming.
Cycle time is 2–4 hours per part. Most of that is autoclave time — ramp to 120–150°C, hold for 60–90 minutes, cool-down before demolding. A single autoclave running one shift produces 2–4 hoods per day. Adding a second shift doubles output but not proportionally — you lose time to re-loading and vacuum checks between cycles.
Surface finish is excellent straight from the mold. This is prepreg’s biggest advantage for visible carbon parts. The resin is precisely controlled (typically 35–42% by weight), flow is minimal, and with a good mold surface, you get a Class A clear-coat-ready finish. Wet sand and clear, done. The Dongguan supplier’s prepreg hoods average 0.8 hours of finishing labor per part.
Tooling is moderate in cost but high in precision. Prepreg tools don’t need the massive clamping force of SMC tools, but they do need to hold vacuum (no leaks) and survive repeated autoclave cycles. Aluminum or steel tools cost $8,000–15,000 for a hood-sized tool. Heated tooling adds cost but cuts cycle time by 30–40%.
Mechanical properties hit the top of the range. Fiber volume fraction typically reaches 55–60% in a well-processed prepreg part. Tensile strength runs 600–900 MPa per ASTM D3039. Flexural modulus is in the 50–70 GPa range. For a hood that needs to pass pedestrian impact standards while staying under weight, prepreg gives the most margin.
RTM: Faster, Cheaper Tooling, More Process Control Required
Resin Transfer Molding flips the material equation. Dry fiber preforms go into a closed mold. Low-viscosity epoxy is injected under pressure. The part cures in the mold. No autoclave. No freezer storage for prepreg rolls. No hand layup of tacky fabric.
Material cost is lower, but preforming adds a step. Dry carbon fabric costs $12–25/m² — roughly half the price of prepreg. A hood uses 4–5 m². But you need to build a preform first: cut the dry fabric plies, stack them, and bind them with a thermoplastic binder so they hold shape during mold loading. The Dongguan supplier preforms by hand (30 minutes per hood) but is evaluating automated preforming for volume.
Cycle time is 15–45 minutes per part. Injection takes 2–5 minutes. Cure at 80–120°C takes 8–25 minutes depending on resin chemistry. The mold stays closed the whole time. One RTM press produces 12–20 hoods per day — roughly 5× the output of one autoclave. For production volumes above 500 units/year, RTM starts to pull ahead purely on throughput.
Surface finish is the weak point. RTM parts come out of the mold with resin-rich surface layers. But pinholes, dry spots, and fiber print-through happen more often than with prepreg. The Dongguan supplier’s RTM hoods average 2.5 hours of finishing labor per part — three times the prepreg line. Most of that is filling pinholes, sanding, and primer before clear coat. If the hood is being painted (not visible carbon), this gap narrows.
Tooling is more expensive but longer-lasting. RTM molds see injection pressures of 5–15 bar — not huge, but enough that the tool needs to be rigid and precisely sealed. Steel tools for an RTM hood run $20,000–40,000. But they last 5,000–10,000 cycles versus 500–1,000 for an aluminum prepreg tool, so the per-part tooling amortization is actually lower at volume.
Mechanical properties are close but not equal to prepreg. Fiber volume fraction in RTM typically reaches 45–55% — a bit below prepreg but well above wet layup. Tensile strength runs 500–750 MPa. The gap comes from fiber alignment — RTM resin flow can shift fibers during injection, especially in complex geometries with ribs or mounting bosses. A well-designed preform and controlled injection parameters minimize this, but it’s an extra variable that prepreg doesn’t have.
Head-to-Head: Prepreg vs RTM for Carbon Fiber Hoods
| Factor | Prepreg Autoclave | RTM |
|---|---|---|
| Raw material cost (per hood) | $100–225 | $50–125 |
| Cycle time | 2–4 hours | 15–45 minutes |
| Daily output (per station) | 2–4 hoods | 12–20 hoods |
| Tooling cost | $8,000–15,000 | $20,000–40,000 |
| Tool life | 500–1,000 cycles | 5,000–10,000 cycles |
| Finishing labor (per part) | 0.5–1.5 hours | 2–4 hours |
| Fiber volume fraction | 55–60% | 45–55% |
| Tensile strength (ASTM D3039) | 600–900 MPa | 500–750 MPa |
| Best for volume | <500 units/year | >500 units/year |
| Visible carbon quality | Excellent | Good (needs more finishing) |
Which Process Fits Your Production?
The Dongguan supplier came to a practical division that most hood manufacturers end up with, whether they plan it or not.
Go prepreg if: you’re making under 500 hoods per year, the customer wants visible carbon weave with a show-quality finish, and your labor cost per finishing hour is below $15. The higher material cost is offset by lower tooling investment and less finishing complexity. Prepreg also makes sense for development and low-volume validation runs — the tooling is cheaper to modify.
Go RTM if: you’re making over 500 hoods per year, the part will be painted (not visible carbon), and you have a dedicated preforming station. RTM’s cycle time advantage becomes real above this volume. The tooling investment amortizes to less per part than prepreg tooling. But you need to invest in process control — injection pressure, temperature ramp, and resin flow front monitoring — or your scrap rate will eat the material savings.
One thing nobody tells you about switching: the labor pool is different. Prepreg layup needs a person who can handle tacky fabric carefully, position plies precisely, and understand fiber orientation. RTM needs someone who understands fluid dynamics and can troubleshoot a resin injection that’s not filling the mold right. The Dongguan supplier learned this the hard way — their best prepreg technician couldn’t run the RTM line, and their best RTM operator found prepreg work frustratingly slow.
FAQ
Why not just 3D print or forge carbon fiber hoods?
Forged carbon (chopped fiber in a press) is fast but has 30–50% lower mechanical properties than continuous-fiber prepreg or RTM. For non-structural trim pieces, forged carbon works. For a hood that contributes to chassis stiffness or needs to pass crash standards, continuous fiber is the requirement. 3D-printed carbon is still in the prototyping phase for automotive body panels — the surface finish isn’t there yet.
Can you mix processes — RTM base with prepreg surface layer?
Yes and some suppliers do this: an RTM structural core with a prepreg cosmetic layer on the A-surface. It gets you the cycle time advantage of RTM with the surface finish of prepreg. The trick is getting a good bond between the RTM resin and the prepreg layer — they need to be chemically compatible and gelled at the right point in the cycle. It adds complexity but for visible carbon at volume, it’s an option worth looking at.
What’s the actual weight difference between prepreg and RTM for the same hood design?
With the same geometry and ply schedule, an RTM hood typically comes out 8–15% heavier than a prepreg hood. The extra weight is mostly resin — RTM parts have slightly higher resin content by volume. On a 4.5 kg prepreg hood, the RTM version would be 4.9–5.2 kg. Most customers don’t care about 400 grams. If they do, you can optimize the RTM preform to close the gap.
What resin systems work best for each process?
Prepreg typically uses 120–150°C cure epoxies with a 30–60 minute cure cycle. These are optimized for autoclave dwell time, not speed. RTM resins are formulated for lower viscosity (50–200 cps at injection temperature) and faster cure — some snap-cure epoxies hit full properties in 5–8 minutes at 120°C. The resin chemistry drives cycle time more than the process itself.
