RTM Machine Product

Overview

Resin Transfer Molding (RTM) is a closed-mold composite manufacturing process combining the design flexibility of manual layup with the production efficiency of injection molding. Dry fiber performs (mats, rovings, woven fabrics) are arranged inside a closed mold cavity; then, catalyzed resin (polyester, vinyl-ester, or epoxy) is injected under low pressure (2–8 bar), filling the cavity and impregnating the fiber. The sealed mold provides:

1. No fiber exposure: Closed mold eliminates volatile emissions (styrene) and occupational exposure.
2. Excellent surface finish: Both mold surfaces (upper and lower) create Class A aesthetics without secondary sanding.
3. High production rate: Cycle times 10–45 minutes, supporting 20–100 parts/day with multiple molds in production.
4. Fiber volume control: Preform architecture and injection pressure dictate final fiber content (45–60%), reproducible batch-to-batch.

RTM is widely used for automotive (body panels, door skins, floor pans), marine (deck structures), sports (bicycle frames, helmet shells), and industrial products (electrical enclosures). Unlike prepreg (expensive, short tack life, requires freezer storage), RTM uses low-cost dry fiber and room-temperature resin, reducing material and logistics costs by 50–70%.

Mold & Clamping System

The Mold Clamping System system is a large hydraulic press clamping Upper Mold and Lower Mold together at forces 500–5000 kN. Molds are typically aluminum or composite (to reduce weight and thermal inertia).

Clamp force calculation: The mold must resist injection pressure. At a 2 m² cavity filled at 5 bar:

Clamping force = Cavity area × Injection pressure = 2 m² × 5 bar = 10 m² × 0.5 bar/cm² ≈ 100 kN

(In practice, mold contact area is smaller and pressure peaks higher, so 200–300 kN typical for this part.)

The Clamping Hydraulics proportional cylinder applies force smoothly, with Alignment Pins ensuring ±0.1 mm mold alignment (critical for thin walls, parting line flash). Mold Release (PTFE film or silicone wax) coats cavity surfaces, reducing demolding effort and part scratching.

A dry fiber preform is manually positioned inside the lower mold cavity before mold closure; this is the only labor-intensive step in RTM.

Resin Metering System

The Resin Metering System system dispenses precise resin volume (±2% accuracy) to ensure correct fiber volume fraction and repeatable properties. Components:

• Resin Tank: Pressurized stainless vessel (50–500 L) holding catalyzed resin, pressurized to 2–5 bar nitrogen to assist pump displacement.
• Resin Pump: Gear pump (0.1–2.0 cc/rev) delivering 0.5–5 L/min, driven by Pump Motor (0.5–2 kW with VFD control).
• Flow Meter: Turbine or Coriolis meter (±2% accuracy) measuring total volume dispensed per cycle.

The PLC monitors Flow Meter feedback; when the target volume is reached, the pump stops and a solenoid shutoff valve closes, isolating the mold. This ensures each part receives identical fiber content.

Catalyst Metering & Mixing

The Catalyst Metering is a secondary pump system synchronized with the resin pump, maintaining stoichiometric catalyst ratio (e.g., 100:30 resin:hardener by volume). This separate system is critical because:

1. Instant gelation prevention: If catalyst is pre-mixed with resin, the entire Resin Tank cure and gel over hours, ruining the resin.
2. Adjustable gelation time: By varying the catalyst concentration, operators control pot life (10–60 min) independently of resin batch.

The Catalyst Pump is variable-displacement (0.1–0.5 cc/rev), driven by a Catalyst Motor servo motor that adjusts pump displacement to maintain a fixed flow ratio (e.g., 0.3× catalyst flow for every resin unit). A Catalyst Meter monitors catalyst volume dispensed.

Mixing ratio accuracy: ±5% tolerance typical (±1–2% variations in hardener concentration cause 10–20% variation in gelation time, acceptable for most applications).

In-Situ Mixing & Reaction

The Static Mixhead Assembly is a simple static mixer where resin and catalyst combine just before injection. The Mixhead Housing (aluminum billet) has two inlet ports (resin and catalyst) and one outlet feeding the Injection Tube.

Inside the mixhead, a Mixing Element (disposable polypropylene helical cartridge) turbulently blends the two liquids. The element is discarded after each shot (5–10 minutes) to prevent cross-contamination and exothermic temperature rise. A Mixhead Cooler water cooler maintains mixhead temperature at 20–30°C, slowing the exothermic reaction and extending pot life to 30–60 minutes.

Cavity Injection & Venting

Mixed resin enters the Injection & Venting at the Injection Gate (typically 5–20 mm diameter, located at the thickest section of the part to maintain low injection pressure). As resin fills the cavity, it displaces air, which escapes via vent holes (typically 1–2 mm diameter, located at thin sections).

Vent design is critical: If vents are blocked or too small:

• Air pockets trap in the cavity, creating voids (reduces strength 10–30%).
• Injection pressure rises dramatically (5–10 bar), risking mold damage or fiber waviness.

The Vent One-Way Valves are one-way pneumatic check valves (3–5 bar cracking pressure), allowing air to escape but closing if resin approaches the vent (preventing resin loss).

Fill pattern: Injection typically occurs in 1–5 minutes for automotive parts; slow fill rates (<1 m/min resin front velocity) minimize fiber floating and ensure good impregnation. The Perimeter Seal Dam (clay or plastisol bead) around the mold perimeter traps resin leakage.

Vacuum Pre-Dwell

Before injection, a Vacuum & Air Removal may apply −0.5 bar to the cavity for 5–10 minutes, evacuating air from fiber interstices. This vacuum-assisted RTM (VARTM) improves:

• Void content: Reduced from 3–5% to <1% by pre-evacuating air.
• Fiber wetting: Lower pressure differential across fiber bundles reduces trapped gas.

The Vacuum Pump (rotary vane, 5–10 m³/h) pulls vacuum; the Vacuum Valve solenoid shutoff isolates the cavity once vacuum dwell is complete (preventing vacuum loss when resin injection starts).

Gelation & Cure

Once resin is injected, the exothermic polymerization reaction raises temperature. The Mold Heating & Cure accelerates gelation by maintaining mold temperature at 60–120°C via Heater Cartridges (2–5 kW total, embedded in mold blocks). The Heater Control PID loop maintains ±2°C accuracy.

Gelation kinetics: Unsaturated polyester at 60°C gels in ~10 minutes; at 80°C, ~3 minutes. The PLC coordinates:

1. Resin injection phase (1–5 min): Pump resin, mold stays at room temperature or low heat (slow polymerization, better flow).
2. Gelation phase (5–15 min): Heater activates; temperature ramps to 80°C (accelerates cure, solidifies the part).
3. Post-cure (optional, 10–30 min): Higher temperature (100–120°C) for complete cross-linking.

The entire cycle (fill + cure + demolding) is typically 15–45 minutes, allowing 5–10 parts per hour per mold tool.

Demolding & Part Removal

Once the resin gels (typically gel time + 5 minutes to allow handling strength), the mold opens. The Cooling Fan accelerates cooling post-cure, reducing residual stresses (thermal gradients cause warping). A demolding robot or manual press ejects the part via pins or vacuum.

Part quality: Excellent; both surfaces are molded (no hand-layup waviness on one side). Flash (excess resin at parting line) is typically 1–3 mm, requiring light edge trimming (manual or CNC router) post-cure.

Typical RTM Cycle Example

Scenario: Automotive door inner panel, 1.5 m × 0.6 m × 3 mm average thickness, polyester resin.

1. Preparation (2 min): Mold cleaned, release agent applied, dry fiber preform (mat/woven) positioned. Resin tank pressurized to 3 bar.
2. Mold closure (1 min): Press applies 500 kN clamping force.
3. Vacuum pre-dwell (optional, 5 min): Cavity evacuated to 50 mbar if VARTM process.
4. Resin injection (2 min): Pumps dispense 2.5 L resin + 0.75 L catalyst (mixed in Static Mixhead Assembly). Injection pressure peaks at 5 bar.
5. Gelation (10 min): Heater ramps to 80°C; Heater Control monitors Mold Thermocouples. Resin gels in ~8 minutes.
6. Cooling (5 min): Cooling Fan cools part to <50°C.
7. Demolding (1 min): Mold opens, part ejected by air pressure pins or robot.

Total cycle: 26 minutes. Production rate: ~2.3 parts/hour/mold (5–10 parts/day with multiple mold sets rotating).

Material & Cost Advantages

Compared to prepreg layup:

• Resin cost: $5–8/kg (vs. $20–40/kg prepreg), saving $200–800 per m² of part.
• Fiber cost: Dry mat/roving ~$8–12/kg (vs. prepreg integration, reducing material waste and logistics).
• No freezer storage: Resin and fiber stored at room temperature, no cooling infrastructure.
• Volatile emissions: Closed mold contains styrene vapor (polyester resin), requiring ductwork but eliminating operator exposure (vs. open-mold hand layup).

Typical RTM automotive part:

• Material cost: $10–20/kg.
• Labor (manual preform placement + machine monitoring): $5–10/part.
• Finished part selling price: $50–100/kg (depending on size and complexity).

Quality Metrics

• Void content: 1–3% typical (better than hand layup, worse than prepreg/autoclave).
• Surface finish: Excellent (Class A both sides, no lay-up wrinkles).
• Fiber volume fraction: 45–60% (repeatable, controlled by preform areal weight).
• Dimensional accuracy: ±0.5–1 mm (comparable to injection-molded plastics).

Defects:

• Dry spots: Fiber not fully impregnated; caused by vent blockage, slow injection, or fiber bridging. Reduces strength 20–40%.
• Fiber waviness: High injection pressure (>8 bar) can shift fibers. Minimized by slow fill (<1 m/min resin velocity).
• Flash: Excess resin at parting line, requiring trimming (labor).

RTM is production-ready for series volumes 1000–100,000+ parts/year, dominating automotive OEM supply for Class B/C structural panels.