Resin Transfer Moulding (RTM)

Resin Transfer Moulding (RTM) is a closed-mould manufacturing process designed for high-performance composite materials. Its core principle involves pre-placing reinforcement materials such as carbon fibre or glass fibre inside a sealed mould, injecting liquid resin under pressure to fully wet out the fibres, and subsequently applying heat to cure and form the component.

Think of it as a "precision infusion" process for crafting complex-shaped composite components, delivering finished parts renowned for high dimensional accuracy, superior strength, and a smooth, dual-sided surface finish.

⚙️ Working Principle and Basic Process

The RTM process is straightforward and broadly comprises the following steps:

1. Preform Preparation: Tailored to the part geometry, reinforcement materials like carbon fibre or glass fibre (in the form of woven or 3D fabrics) are cut and pre-arranged within the mould cavity.

2. Mould Clamping and Injection: The mould is closed and clamped shut before low-viscosity liquid resin (such as epoxy resin) is injected under pressure. As the resin flows, it progressively purges entrapped air and thoroughly impregnates the reinforcement fibres.

3. Curing and Demoulding: Heat is applied to the mould to initiate cross-linking, curing the resin. Once fully cured, the mould opens to release the finished composite component.

🚗 Why Is It So Crucial in Automotive Lightweighting?

RTM stands as one of the pivotal technologies for manufacturing high-performance composite parts like carbon fibre, striking the ultimate balance between performance, cost, and manufacturing efficiency.

• Delivering Substantial Weight Savings: This is the primary driver behind adopting RTM in the automotive industry. Composites are inherently far lighter than steel, allowing RTM-moulded components to achieve remarkable weight reductions. Research indicates that using composite materials can reduce body weight by 25% to 50%—a feat virtually unattainable with lightweight metals alone. For instance, the new BMW 7 Series shed 130 kg by integrating carbon-fibre-reinforced plastics (partially fabricated via RTM).

• Balancing High Performance with Production Efficiency: RTM produces high-grade components characterised by high fibre volume fractions and low porosity. Concurrently, it eliminates the need for costly autoclaves and lengthy cycle times, thereby cutting production costs. To cater to high-volume automotive production and short takt times, High-Pressure Resin Transfer Moulding (HP-RTM) was pioneered, slashing single-part cycle times down to just 3 to 5 minutes. The carbon-fibre structural body parts on models like the BMW i3 and BMW 7 Series stand as proven success stories of HP-RTM mass production.

🔧 Key Applications and Derivative Technologies

The RTM process (particularly HP-RTM) is widely employed to manufacture high-demand automotive components:

• Body Structural Components: Core structural parts such as the Life Module of the BMW i3 and the side frame/sill structures of the BMW 7 Series.

• Other Components: Structural parts found in models like the Volvo XC90 and BMW 7 Series, as well as battery enclosure covers for new energy vehicles (NEVs).

Furthermore, to accommodate diverse production volumes and cost structures, RTM has branched into several specialised methods, including Vacuum-Assisted RTM (VA-RTM) and Light RTM (L-RTM).

⚠️ Key Challenges and Limitations

Despite its standout merits, the RTM process faces several practical hurdles:

• Costs and Lead Times: High tooling costs demand substantial initial capital investment, alongside stringent control over resin formulations and processing parameters.

• Technical Complexities: Ensuring uniform resin flow and complete fibre wet-out across large, intricate parts remains an engineering challenge.

The Bottom Line: Resin Transfer Moulding is a cornerstone manufacturing process in automotive engineering, governing whether advanced lightweight materials like carbon fibre can be deployed efficiently and reliably in mass-market vehicles. For drivers, composite vehicle architectures engineered through RTM are the unsung heroes delivering lower kerb weight, greater efficiency, and noticeably sharper handling dynamics.