Investment Casting (Lost-Wax Process): Precision Manufacturing for Aerospace Turbine Blades, Medical Implants & High-Temperature Alloys

When aerospace engineers need to produce turbine blades with a thickness of just 0.1mm and surface precision up to ±0.05mm, the 5,000-year-old investment casting (lost-wax process) becomes the critical technology. This technique, which underpins 90% of modern aerospace engine components, continues to push precision boundaries in medical devices, luxury goods manufacturing, and other high-value industries.

As a professional foundry enterprise, Tiegu will next provide you with a detailed introduction to investment casting (also known as lost-wax casting), a precision manufacturing process that is widely applied in industries such as aerospace, automotive, and medical devices.

How investment casting works？

The investment casting (lost-wax process) achieves precision metal forming through a soluble pattern replication technique. The core workflow involves: creating wax patterns by injecting low-temperature paraffin into molds, assembling patterns into tree-like structures, and applying ceramic slurry layers to form a robust shell. Steam dewaxing and high-temperature sintering remove the wax and densify the shell, followed by vacuum casting of molten alloys. After cooling, the shell is removed to yield high-precision castings requiring no post-machining.

What is investment casting process？

How Investment Casting Differs From Other Types of Casting

Investment casting excels in precision, material compatibility, and structural complexity:

1. Precision Control: Achieves CT4 tolerance (±0.15mm/25mm), double the accuracy of sand casting, enabling 0.2mm micropores and 30° negative draft angles

2. Material Adaptability: The only process capable of directly casting superalloys (>1300℃ melting point like Inconel 718) and titanium alloys

3. Complexity: Forms intricate internal structures like turbine blade cooling channels in one step, reducing post-assembly needs

Comparison Data:

• Minimum wall thickness: 0.5mm (vs. 3mm for sand casting, 0.8mm for die casting)

• Material utilization: 90% (vs. 40% for traditional machining)

• Unit cost: 5-8x sand casting, but more cost-effective in high-value applications

Why is it Called Investment Casting and Other Names?

The term "investment casting" originates from the Latin word "investire," meaning "to clothe" or "to cover." This refers to the process of encasing the wax pattern with ceramic investment material, which forms the protective shell during casting. While "investment casting" is the standardized term in engineering contexts, the process is also widely known as lost-wax casting due to its core step of melting away the wax pattern before metal pouring.

Alternative Names in Practice（other name）

Depending on regional or industrial contexts, you may encounter these synonyms:

Precision Casting – Highlights its capability to achieve CT4 dimensional accuracy

Expendable Pattern Casting – Focuses on the sacrificial nature of the wax model

Ceramic Mold Casting)– Describes the refractory shell material

Investment Casting Process Explained

A Step-by-Step Guide for Precision Metal Parts

1. Wax Pattern Creation

The process begins with designing 3D models using CAD software, where 0.3-0.6% shrinkage compensation is factored in to account for metal solidification. Low-temperature paraffin wax (48-52℃ melting point) mixed with 15% stearic acid is then injected into precision-machined aluminum molds at 5-8MPa. This results in wax patterns with wall thickness ranging from 0.5-3mm and dimensional accuracy of ±0.05mm.

Technical Edge: Aluminum molds with Ra≤0.8μm surface finish ensure smooth pattern release.

2. Cluster Assembly

Individual wax patterns are welded to runners using heated air guns, forming tree-like clusters. These assemblies undergo X-ray inspection to detect internal bubbles smaller than 0.2mm in diameter, ensuring structural integrity before casting.

Quality Control: Cluster weight variation is maintained within ≤5g to balance metal flow during pouring.

3. Ceramic Shell Formation

The clusters are repeatedly dipped in silica sol-zircon slurry (30-35% SiO₂ content) and coated with refractory sands. After 7-9 layers and 4-6 hours of drying between coats, the shells are sintered at 980℃. This process creates ceramic shells with ≥30MPa bending strength capable of withstanding 1500℃ molten metal.

Material Science: Zircon flour provides high thermal stability, while mullite sand enhances structural rigidity.

4. Dewaxing & Cleaning

Steam at 160℃ removes the wax patterns, leaving less than 0.05% residue. High-pressure air blasting then clears any remaining debris from the shell cavities. Approximately 95% of the wax is recycled for reuse.

Environmental Advantage: Advanced wax recovery systems minimize material waste.

5. Vacuum Casting

Nickel-based superalloys like Inconel 718 are melted at 1550±20℃ in vacuum induction furnaces. The molten metal is poured into the ceramic shells under ≤10Pa vacuum to eliminate porosity, ensuring dense, defect-free castings.

Process Innovation: Vacuum environment reduces oxidation and improves material purity.

6. Cooling & Demolding

Casted parts are cooled gradually—first in the furnace to 600℃, then air-cooled at 5-10℃ per minute. Vibration shaking removes over 99.9% of the ceramic shell, leaving near-net-shape metal components.

Stress Control: Slow cooling prevents thermal distortion in complex geometries.

7. Final Processing & Inspection

Surface finishing via aluminum oxide blasting achieves Ra1.6-6.3μm roughness. Coordinate measuring machines verify CT4 dimensional tolerance (±0.15mm/25mm), while CT scans with 5μm resolution detect internal defects.

Certifications: Processes comply with ISO 9001, AS9100D (aerospace), and ISO 13485 (medical) requirements.

What is investment casting products advantages and disadvantages？

Advantages of Investment Casting

1. Extreme Precision

   
   Achieves CT4 dimensional tolerance (±0.15mm/25mm) and surface roughness Ra1.6-6.3μm, eliminating post-machining for most applications.

2. Complex Geometry Capability

   
   Forms intricate internal structures like turbine blade cooling channels and 30° negative draft angles impossible with sand casting or forging.

3. High-Temperature Material Compatibility

   
   Casts nickel-based superalloys (e.g., Inconel 718, melting point 1330℃) and titanium alloys directly, which are difficult to machine conventionally.

4. Material Efficiency

   
   90% material utilization compared to 40% for CNC machining, reducing waste and costs for expensive alloys.

5. Design Flexibility

   
   Allows rapid design iterations using 3D-printed wax patterns, cutting prototyping time from 30 days to 5 days.

Technical Data

• Minimum feature size: 0.2mm

• Wall thickness range: 0.5-3mm

• Tensile strength retention: 98% of wrought material

Limitations of Investment Casting

1. High Production Costs

   
   Unit costs are 5-8 times higher than sand casting due to labor-intensive ceramic shell processes and material investment.

2. Long Lead Times

   
   Typical production cycle ranges from 15-20 days for small batches, longer than die casting's 3-5 days.

3. Size Restrictions

   
   Maximum casting weight is generally ≤50kg, though advanced composite shells can cast up to 300kg components.

4. Environmental Impact

   
   Traditional processes generate VOC emissions from organic binders, though water-based coatings now reduce this by 90%.

5. Porosity Risks

   
   Improper vacuum levels during casting can lead to porosity exceeding 0.5%, requiring post-casting HIP treatment for critical applications.

Risk Mitigation Strategies

• Simulation Software: ProCAST modeling predicts shrinkage and porosity

• Process Control: Vacuum levels maintained at ≤10Pa during pouring

• Post-Processing: Hot isostatic pressing (HIP) eliminates internal voids

Decision Matrix

What is the application of investment casting?

What Are the Examples of Investment Casting Products?Investment casting is used to produce high-precision components across critical industries, including:

• Aerospace: Turbine blades (used in jet engines to withstand 1200℃ heat), combustion chamber liners

• Medical: Cobalt-chromium hip implants (used for biocompatible bone integration), titanium spinal spacers

• Industrial: High-temperature valves (used in petrochemical plants), marine propellers

• Luxury Goods: 18K gold jewelry settings (used for intricate diamond prongs), stainless steel watch cases

• Energy: Nuclear reactor components (used in radiation environments), wind turbine bearings

Why These Products Use Investment Casting
✅ Complex geometries (e.g., internal cooling channels in turbine blades)
✅ High-temperature materials (e.g., nickel-based superalloys melting at 1330℃)
✅ Surface finishes meeting medical/consumer standards (Ra1.6-6.3μm)

Why Is Investment Casting Expensive?

Investment casting incurs higher costs due to its precision-driven process and material requirements, including:

1. Labor-Intensive Steps: Creating ceramic shells involves 7-9 slurry coatings and sintering, requiring skilled technicians and time (5-7 days for shell preparation alone).

2. High-Temperature Alloys: Materials like nickel-based superalloys (e.g., Inconel 718) cost 3-5 times more than conventional metals, though their 90% utilization rate offsets some expense.

3. Tooling Costs: Aluminum molds for wax patterns can cost $10,000-$50,000 per design, though 3D printing reduces this by 80% for prototypes.

4. Quality Assurance: CT scans (5μm resolution) and tensile testing add $200-$500 per part for aerospace/medical applications.

Cost Mitigation Strategies

• 3D-Printed Wax Patterns: Reduce tooling costs for small batches

• Automated Shelling: Shorten production time by 30%

• Material Optimization: Near-net shape reduces machining waste by 50%

Tiegu's Approach
Leverages automated ceramic coating robots and in-house alloy melting capabilities to lower costs by 25% compared to industry averages.

What Materials Are Utilized in Investment Casting?

Investment casting is a versatile process that can work with a wide range of materials to meet different industrial needs. Here are the main materials employed:

1. Metallic Materials

• Nickel - based Superalloys: These are the go - to choice for aerospace and energy applications. Alloys such as Inconel 718 (which meets ASTM B637) and CMSX - 4 are known for their ability to withstand high temperatures, with melting points exceeding 1300°C.

• Cobalt - based Alloys: Stellite 6, in particular, is valued for its high hardness, reaching up to HRC52, making it suitable for wear - resistant parts.

• Titanium Alloys: Ti - 6Al - 4V (complying with ASTM F136) is widely used in the medical field due to its biocompatibility and high strength - to - weight ratio.

• Stainless Steels: 316L (with a low carbon content of ≤0.03%) and 17 - 4PH (which can achieve HRC42 after heat treatment) are commonly selected for their corrosion resistance.

2. Wax Materials

• Paraffin Wax: It serves as the primary component, typically accounting for 85% of the wax mixture. It has a melting point ranging from 48 - 52°C.

• Stearic Acid: Making up 15% of the mixture, it helps in controlling the viscosity of the wax.

• Additives:  microcrystalline wax is added to reduce the shrinkage rate to less than 0.6%, ensuring dimensional stability of the wax pattern.

3. Ceramic Materials

• Zircon Flour: With a particle size of 270 mesh, it is used in the surface layer of the ceramic shell to provide thermal stability.

• Mullite Sand: Having an 80 - mesh particle size, it is used in the backing layers to enhance the strength of the shell.