UAV Wing Spar Manufacturing: Autoclave vs. Out-of-Autoclave CFRP Cure Cycle Comparison for 600g Target Weight

For UAV manufacturers targeting a 600g wing spar, selecting the right cure cycle is critical to balancing weight, strength, and production cost. Autoclave curing has been the gold standard for aerospace-grade composites, but out-of-autoclave (OOA) prepregs offer reduced capital investment and cycle times. This article provides a technical comparison of both processes using Toray T700S carbon fiber / Hexcel 8552 epoxy prepreg, with a worked example demonstrating how to achieve the 600g target while meeting structural requirements per ASTM D3039.

Design Requirements for a 600g UAV Wing Spar

The target UAV wing spar is a 1.2 m long, constant cross-section C-channel with a mass of 600 g. Assuming a quasi-isotropic laminate layup of [0/90/±45]s, the required laminate thickness can be derived from the target mass and material density.

Worked Example:

• Target mass: 600 g = 0.6 kg
• Spar length: 1.2 m
• Cross-section area (C-channel): approximate web area = 40 mm × 4 mm, flange area = 30 mm × 4 mm each → total cross-section area = (40×4) + 2×(30×4) = 160 + 240 = 400 mm² = 4×10⁻⁴ m²
• Volume = cross-section area × length = 4×10⁻⁴ m² × 1.2 m = 4.8×10⁻⁴ m³
• Required density: ρ = m/V = 0.6 kg / 4.8×10⁻⁴ m³ = 1250 kg/m³

Typical cured CFRP density for Toray T700S / Hexcel 8552 with Vf ≈ 62% is 1580 kg/m³. The target density of 1250 kg/m³ is lower, indicating the need for a lightweight core or reduced fiber volume. A practical approach is to use a foam core (e.g., Rohacell 51 IG-F, density 52 kg/m³) in the web and flanges, with CFRP skins. Assuming a 1 mm CFRP skin on each side and a 3 mm foam core, the effective density becomes: (2×1×1580 + 3×52) / 5 = (3160 + 156)/5 = 663.2 kg/m³, which is too low. Adjusting to 1.5 mm CFRP skins and 2 mm core: (2×1.5×1580 + 2×52)/5 = (4740 + 104)/5 = 968.8 kg/m³. To reach 1250 kg/m³, use 2 mm CFRP skins and 1 mm core: (2×2×1580 + 1×52)/5 = (6320 + 52)/5 = 1274.4 kg/m³, close to target. Thus, a laminate with 2 mm CFRP skins and a 1 mm foam core meets the 600 g mass target.

Autoclave Cure Cycle for CFRP Wing Spars

Autoclave curing applies heat (typically 135°C for Hexcel 8552) and pressure (3–7 bar) to consolidate the laminate. The standard cure cycle for Hexcel 8552 per manufacturer data sheet is:

• Ramp at 1–3°C/min to 120°C, hold for 30 min
• Ramp to 180°C at 1–3°C/min, hold for 120 min
• Cool down at ≤3°C/min
• Apply 6.9 bar (100 psi) pressure throughout

For the 600g spar, autoclave processing yields void content <1% (per ASTM D2734) and fiber volume fraction (Vf) of 62–65%. Mechanical properties per ASTM D3039 for Toray T700S/8552: tensile strength 2.55 GPa, tensile modulus 135 GPa, and compressive strength 1.5 GPa. The high pressure ensures excellent consolidation, especially at corners and radii, critical for C-channel geometry.

However, autoclave capital cost is significant (≥$500k for a 1.2 m diameter unit), and cycle time (including ramp and cool) exceeds 6 hours. For low-to-medium volume UAV production (100–500 spars/month), autoclave amortization can add $50–100 per spar.

Out-of-Autoclave (OOA) Cure Cycle for CFRP Wing Spars

Out-of-autoclave prepregs are designed to cure under vacuum bag pressure only (0.8–1.0 bar). OOA cure cycles typically use a lower temperature (e.g., 120°C for 120 min) with a slower ramp rate to allow air evacuation. For Hexcel 8552, an OOA cure cycle (modified) involves:

• Dwell at 60°C for 60 min under vacuum
• Ramp at 0.5–1°C/min to 120°C, hold for 120 min
• Cool down at ≤2°C/min
• Vacuum only (0.8 bar)

OOA processing achieves void content typically <2% (ASTM D2734) and Vf of 58–62%. Mechanical properties are slightly lower: tensile strength 2.30 GPa, tensile modulus 130 GPa, compressive strength 1.3 GPa (per manufacturer data for OOA cure). For the 600g spar, the lower Vf means the laminate must be slightly thicker (e.g., 2.2 mm CFRP skins) to achieve equivalent stiffness, increasing mass. However, the 600g target can still be met by adjusting core thickness.

OOA eliminates autoclave capital cost, reduces cycle time to ~4 hours, and allows lower tooling costs (aluminum or composite molds). For small UAV manufacturers, OOA is often more cost-effective at volumes <1000 spars/year.

Comparison Table: Autoclave vs. OOA for 600g UAV Wing Spar

Note: Costs are estimates based on industry averages and may vary by supplier and location.

Selecting the Right Process for Your UAV Spar

The choice between autoclave and OOA depends on performance requirements and production volume. For high-performance UAVs demanding maximum strength-to-weight ratio (e.g., military drones), autoclave is preferred due to higher Vf and lower void content. For commercial UAVs where cost is a primary driver, OOA offers a viable alternative with adequate mechanical properties.

At Dongguan Flex Precision Composites, we have experience with both processes using Toray T700S and T800H prepregs. Our autoclave (1.2 m diameter × 2.5 m length) and OOA oven (1.5 m × 1.5 m × 3 m) allow us to optimize cure cycles for each project. We consistently achieve ±0.05 mm tolerances on C-channel spars, verified by Zeiss Contura CMM inspection.