Overview of Composite Molding Processes And Applications

1. Hand Lay-up

Principle: Fibrous fabrics (e.g., fiberglass, carbon fiber) are manually laid onto a mold, coated with resin (e.g., epoxy, polyester), and rolled to remove air bubbles before curing at room temperature or under heat.
Advantages: Simple equipment, low cost, suitable for small-batch production and complex shapes (e.g., boat hulls, sculptures).
Disadvantages: Highly dependent on operator skills, inconsistent product quality, high porosity, and lower mechanical properties.
Applications: Yachts, storage tanks, architectural decorative components.

2. Spray-up

Principle: A spray gun simultaneously dispenses chopped fibers and resin onto a mold, followed by compaction and curing.
Advantages: Higher efficiency than hand lay-up, suitable for hollow or curved parts.
Disadvantages: Lower fiber content, reduced strength, high VOC (volatile organic compounds) emissions.
Applications: Automotive body panels, bathtubs, simple shells.

3. Vacuum Bag Molding

Principle: After laying fibers and resin, a vacuum bag covers the part, and air is evacuated to compact the material and enhance resin flow, ensuring fewer voids.
Advantages: Higher material density than hand lay-up, lower porosity, improved mechanical properties.
Disadvantages: Requires vacuum equipment and involves a more complex process.
Applications: Small aerospace components, localized reinforcement in wind turbine blades.

4. Autoclave Molding

Principle: Prepregs (fibers pre-impregnated with resin) are layered inside an autoclave and cured under high temperature and pressure.
Advantages: High material density, excellent mechanical properties, ideal for high-performance applications.
Disadvantages: Expensive equipment, high energy consumption, long production cycle.
Applications: Aircraft wings, satellite structures, race car components.

5. Resin Transfer Molding (RTM)

Principle: Dry fiber preforms are placed inside a closed mold, and resin is injected to impregnate the fibers before curing.
Advantages: High surface finish, controllable fiber content, suitable for complex structures.
Disadvantages: High mold costs, requires precise resin flow control.
Variants: High-pressure RTM (HP-RTM), Vacuum-Assisted RTM (VARTM).
Applications: Automotive structural parts, UAV fuselages.

6. Compression Molding

Principle: Prepregs or sheet molding compounds (SMC) are placed in a heated mold and compressed into shape.
Advantages: Suitable for mass production, high efficiency, consistent product quality.
Disadvantages: High mold costs, limited fiber orientation control.
Applications: Automotive bumpers, electrical insulation components.

7. Filament Winding

Principle: Continuous fibers impregnated with resin are wound around a mandrel at specific angles before curing.
Advantages: Controlled fiber orientation, excellent strength, suitable for axisymmetric structures.
Disadvantages: Complex equipment, limited to rotationally symmetric shapes.
Applications: Pressure vessels, pipelines, rocket motor casings.

8. Pultrusion

Principle: Continuous fibers pass through a resin bath and are pulled through a heated mold for shaping and curing.
Advantages: Continuous production, high efficiency, ideal for constant cross-section profiles (e.g., rods, beams).
Disadvantages: Limited to straight-line profiles, lower transverse strength.
Applications: Bridge trusses, cable trays, ladder frames.

9. Automated Fiber Placement (AFP)

Principle: A robotic system precisely places narrow strips of prepreg onto a mold following programmed paths, then heat and pressure cure the structure.
Advantages: High precision, high efficiency, suitable for large and complex curved surfaces.
Disadvantages: Extremely high equipment and material costs.
Applications: Aircraft fuselage skins, wind turbine main beams.

10. 3D Printing (Additive Manufacturing)

Principle: Layer-by-layer deposition using fused deposition modeling (FDM) or continuous fiber co-extrusion (e.g., Markforged technology).
Advantages: High design freedom, no need for molds, ideal for prototypes or low-volume complex parts.
Disadvantages: Lower strength, weak interlayer bonding, slower process.
Applications: Customized brackets, lightweight structural prototypes.

Other Molding Processes

• Reaction Injection Molding (RIM): Fast-curing reactive resins are injected into a mold, mainly for polyurethane-based composites.

• Centrifugal Molding: Uses centrifugal force to distribute resin within fibers, ideal for pipe manufacturing.

• Bulk Molding Compound (BMC) / Dough Molding Compound (DMC): Suitable for electrical components using a putty-like composite material.

Key Factors for Process Selection

• Production Volume: Small batches favor hand lay-up or spray-up; large-scale production prefers compression molding or pultrusion.

• Performance Requirements: High-performance parts use autoclave molding or AFP; cost-effective solutions use hand lay-up.

• Shape Complexity: Complex curved surfaces benefit from RTM or AFP, while constant cross-sections suit pultrusion.

• Material Type: Thermoset composites are typically made via RTM or autoclave molding, while thermoplastics can be processed via 3D printing or compression molding.

By selecting the optimal process, manufacturers can balance cost, efficiency, and performance to meet diverse industry demands in aerospace, automotive, energy, and more.