Table of Content
Lightweighting has become a common battleground for automotive, aerospace, and wind turbine blades, with carbon fiber composites standing out due to their high specific strength, high specific modulus, and corrosion resistance. However, "expensive, slow, and difficult to mass-produce" has always been a stereotype associated with it in the industry. Traditional autoclave/hot press molding relies on vacuum bags and external pressure to cure prepreg at 120-180 °C, often requiring 4-6 hours for a large fuselage skin. In contrast, the recently emerging high-pressure resin transfer molding (HP-RTM) injects low-viscosity epoxy or PUR resin under high pressure in a closed mold, combined with a fast-curing formula, reducing the molding time to 5-10 minutes, paving the way for mass production of carbon fiber chassis for automobiles. This article will comprehensively compare the two processes from scientific principles, process parameters, cost structure to practical applications, and provide selection and design guidance to help engineers grasp the best cost-effectiveness from the early stages of development.
Scientific principles and process parameters of thermoforming
Hot pressing typically uses prepreg layups, positioned into the mold according to lamination angles (±45°/0°/90°), covered with a polyimide vacuum bag and evacuated to −0.09 MPa to expel air and volatiles; then, pressure is applied and temperature increased at 0.4–0.7 MPa, causing the resin to melt and infiltrate. The pressure ensures a stable fiber volume fraction (Vf) of 55–60 %. Curing kinetics can be described using the Kamal–Sourour model.
dα/dt = (k1+k2αm(1−α)n
Where α is the reaction conversion rate, and k₁ and k₂ increase with increasing temperature according to the Arrhenius equation. Controlling the heating rate to 2-3 °C/min can avoid excessive exothermic reaction that could cause resin microcracks; finally, apply cooling pressure to < 60 °C before demolding to reduce thermal warpage.
High-pressure RTM: The key from resin injection to rapid curing
HP-RTM uses dry fiber reinforcement (DRF) placed in rigid upper and lower molds. After mold closing, epoxy or PUR with a viscosity < 100 mPa·s is injected at a pressure of 50–120 bar, and the temperature is rapidly increased to 120–160 °C to trigger a self-catalytic rapid curing reaction (t₉₀ ≈ 90–180 s). The flow front must cover the entire mold within 10 s; otherwise, gas can easily be trapped in the gaps between fibers, forming pores. The mold is usually equipped with heat pipes or oil temperature control to maintain a temperature uniformity of ±2 °C, ensuring consistent curing shrinkage. Since the cycle time is limited by the three stages of injection + curing + cooling, it can be achieved through:
- Variable viscosity resin: high flow → rapid thickening and curing upon heating.
- Embedded resistance heating mold: direct internal heating, shortening the heating time.
- Dual-mold alternation: One injection molding machine serves two sets of molds, improving OEE.
Performance/Cost and Environment: A Comprehensive Comparison of the Two Processes
| Items | Hot pressing | High-voltage RTM (HP-RTM) |
|---|---|---|
| Typical fiber volume content Vf (%) | 55 – 60 | 50 – 55 |
| Porosity (%) | < 1 (can be optimized down to 0.2) | 1 – 3 |
| Molding pressure (MPa) | 0.4 – 0.7 | 5 – 12 (50–120 bar) |
| Cycle time (min) | 60 – 240 | 5 – 10 |
| Equipment investment (relative) | High-pressure autoclave/hot press | Medium (Injection Molding Machine + Rigid Mold) |
| Material costs (relative) | High (prepreg) | Low (dry fiber + low viscosity resin) |
| Feasibility of annual production capacity ≥ 100,000 units | 難 | 可 |
| Volatile substances/emissions | Almost zero | Volatile monomers need to be recovered |
Selection Guide and Application Cases: From Supercars to Wind Turbine Blades
- Supercar Monocoque
- Requirements: High rigidity, batch size of 5k – 10k/year.
- Solution: HP-RTM + carbon fiber fabric, cycle time 7 min, rigidity-to-weight ratio increased by 20 %.
- Large passenger aircraft wing skin
- Requirements: Extremely low porosity, length > 20 m.
- Solution: Hot pressing + tape laying machine (AFP), porosity < 0.5 %, meets aerospace-grade fatigue life.
- Wind turbine blade root joint
- Requirements: Thickness of 30 mm or more, high fiber content.
- Solution: Dual-station HP-RTM, injection of high-toughness PUR, cycle time 15 min, reducing the total weight of the blade by 8 %.
Decision-making process
- < 5 min cycle & medium production → HP-RTM is the preferred choice.
- High fiber volume content & ultra-low porosity → Hot pressing still has advantages.
- For complex hollow components, consider a combination of RTM + foamed core or hot pressing + soluble core.
Process selection determines competitive advantage
Hot pressing (HP-RTM) and high-pressure RTM are not mutually exclusive; rather, they each have their strengths in terms of batch size, porosity, fiber volume content, cycle time, and capital investment. Facing the pressures of automotive electrification and net-zero carbon emissions, HP-RTM has carved out a new path for mass production with its low material costs and high cycle time. However, for aerospace components with extremely stringent structural safety requirements and a porosity requirement of <0.5 %, the precise control of hot pressing remains an irreplaceable safeguard. If engineers can utilize the comparison tables and case studies provided in this article at the product concept stage to quantitatively assess the cost per square meter (€/m²) and production cycle time (pcs/h), they can gain valuable time in equipment procurement and process verification, truly transforming the high performance of carbon fiber composites into a competitive market advantage.
