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Process Know How


The investment casting process (also called lost-wax casting) is widely used to manufacture complex orthopaedic implants because it provides excellent dimensional accuracy, smooth surface finish, and the ability to produce intricate shapes in biocompatible alloys such as cobalt-chromium (Co-Cr) and stainless steel. Titanium implants are more commonly made by precision forging or specialized vacuum casting due to titanium's high reactivity.

Investment Casting Process for Orthopaedic Implant Manufacturing


1. Implant Design

  • A 3D CAD model of the implant (e.g., hip stem, knee component, bone plate) is created.
  • The design includes allowances for shrinkage and machining.

2. Wax Pattern Production

  • Molten wax is injected into a precision metal die.
  • After cooling, the wax pattern replicates the exact geometry of the implant.

3. Pattern Assembly

  • Multiple wax patterns are attached to a central wax sprue to form a "tree."
  • This allows several implants to be cast simultaneously.

4. Ceramic Shell Building

  • The wax assembly is dipped into a ceramic slurry.
  • It is coated with fine refractory sand (stucco).
  • The dipping and coating process is repeated 6–10 times until a strong ceramic shell forms.

5. Dewaxing

  • The ceramic shell is heated in an autoclave or furnace.
  • The wax melts and drains out, leaving a hollow mold.

6. Shell Firing

  • The ceramic mold is fired at approximately 900–1100°C.

  • This:

    • Removes residual wax.
    • Strengthens the shell.
    • Improves thermal stability.

7. Metal Melting

  • Biocompatible alloys are melted under controlled conditions.

  • Common materials include:

    • Cobalt–chromium alloys
    • 316L stainless steel
    • Titanium alloys (with vacuum/inert atmosphere)

8. Pouring

  • Molten metal is poured into the preheated ceramic mold.
  • Vacuum or inert gas may be used to minimize oxidation and porosity.

9. Solidification and Cooling

  • The metal solidifies inside the ceramic shell.
  • Controlled cooling helps achieve the desired microstructure and mechanical properties.

10. Shell Removal

  • The ceramic shell is broken away using vibration, water jet, or mechanical methods.
  • Individual castings are cut from the sprue.

11. Finishing Operations

  • Gates and runners are removed.
  • Grinding and polishing improve the surface finish.
  • Heat treatment may be performed to optimize mechanical properties.
  • Critical dimensions are machined to achieve tight tolerances.

12. Surface Treatment

Depending on the implant type:

  • Mirror polishing (joint surfaces)
  • Sand blasting
  • Hydroxyapatite coating
  • Porous coating for bone ingrowth
  • Passivation to improve corrosion resistance

13. Inspection and Quality Control

The finished implant undergoes:

  • Dimensional inspection (CMM)
  • Radiographic (X-ray) testing
  • Dye penetrant inspection
  • Ultrasonic testing
  • Mechanical testing
  • Metallographic examination
  • Biocompatibility and corrosion testing

  • Advantages of Investment Casting for Orthopaedic Implants

  • High dimensional accuracy (typically ±0.1–0.2 mm depending on size)
  • Excellent surface finish (often 1.6–3.2 µm Ra before polishing)
  • Suitable for complex implant geometries
  • Reduced machining requirements
  • High material utilization with minimal waste
  • Good repeatability for mass production