What Is the Difference Between Investment Casting and Injection Molding

The main difference between investment casting and injection molding is that investment casting creates metal parts using a wax pattern and molten metal, while injection molding forms plastic parts by injecting molten plastic into a mold. Investment casting suits complex metal components; injection molding excels in high-volume plastic production.

Investment Casting

Investment casting is a metal forming process that creates precise metal parts by pouring molten metal into ceramic molds. The process uses a wax pattern covered in ceramic material to form the mold. Manufacturers use this method to produce complex shapes with excellent surface finishes and tight tolerances.

Step-by-Step Investment Casting Process

1. Create the wax pattern: Workers inject wax into a metal die to form an exact replica of the desired part. Multiple patterns attach to a central wax sprue to create a tree-like assembly.
2. Build the ceramic shell: The wax assembly gets dipped into ceramic slurry, then coated with sand particles. This coating process repeats 5-10 times until the shell reaches the required thickness.
3. Remove the wax: The ceramic shell goes into an autoclave or furnace at 200-375°F. The heat melts out the wax, leaving a hollow ceramic mold.
4. Fire the mold: The empty ceramic shell heats to 1,800-2,000°F. This step removes any remaining wax residue and strengthens the ceramic.
5. Pour the metal: Workers melt the chosen metal and pour it into the hot ceramic mold. Gravity or vacuum assistance helps the metal fill all cavity details.
6. Break out the casting: After cooling, workers break away the ceramic shell using hammers or water jets. The metal parts separate from the sprue system.
7. Finish the part: Final operations include grinding, machining, and heat treating to meet specifications.

Injection Molding

Injection molding forces molten material into a steel mold under high pressure. The process creates identical parts at speeds of 15-120 seconds per cycle. Two main types exist: plastic injection molding and metal injection molding (MIM).

• Plastic injection molding produces parts from thermoplastic and thermoset polymers.
• Metal injection molding combines fine metal powders with plastic binders to create a feedstock.

Step-by-Step Injection Molding Process

Plastic Injection Molding

1. Load plastic pellets – Fill the hopper with plastic resin pellets. Automated systems feed 50-500 pounds per hour into the machine.
2. Melt the plastic – The screw rotates and pushes pellets through a heated barrel at 300-600°F. Complete melting takes 10-30 seconds.
3. Inject into mold – Ram the molten plastic into the closed steel mold at 10,000-20,000 psi. Injection fills the mold in 1-5 seconds.
4. Cool the part – Circulate coolant through mold channels at 50-150°F. Cooling takes 10-60 seconds depending on wall thickness.
5. Open and eject – Separate the mold halves and push out the finished part with ejector pins. The cycle then repeats.

Metal Injection Molding

1. Mix feedstock – Blend metal powder (2-20 microns) with polymer binders at a 60:40 ratio. Mix for 2-4 hours at 300-400°F.
2. Inject the feedstock – Load feedstock pellets and inject into the mold at 200-400°F. Pressure reaches 15,000-25,000 psi.
3. Remove from mold – Cool for 20-40 seconds and eject the “green” part containing metal and binder.
4. Debind the part – Dissolve or burn out 90% of the binder using solvents or heat. This creates a porous “brown” part over 12-48 hours.
5. Sinter the metal – Heat to 2,000-2,500°F in a controlled atmosphere furnace. Metal particles fuse together, shrinking the part by 15-20% over 8-24 hours.
6. Finish as needed – Perform secondary operations like machining, plating, or heat treating to meet specifications.

Investment Casting Vs Injection Molding

Characteristic	Investment Casting	Plastic Injection Molding	Metal Injection Molding (MIM)
Core Principle	Expendable (single-use) ceramic mold created from a wax pattern.	Reusable, high-precision steel or aluminum mold.	Reusable steel mold with post-molding debinding and sintering steps.
Primary Materials	Ferrous and non-ferrous alloys: stainless steel, aluminum, carbon steel, superalloys.	Thermoplastics (ABS, PC, PP, PE, Nylon), thermosets, elastomers.	Powdered metal feedstock: stainless steel, tool steel, titanium, cobalt alloys.
Typical Production Volume	Low to Medium: 10s to 10,000s of parts.	High to Very High: 10,000s to millions of parts.	Medium to High: 10,000s to millions of parts.
Tooling Cost	Low to Medium: Master die for wax patterns is less expensive.	Very High: Hardened steel molds can cost over $100,000.	High: Precision steel molds required, but smaller than many plastic molds.
Per-Unit Cost	High to Medium: Driven by labor and long cycle times.	Very Low: Benefits from automation, fast cycles, and cheap materials.	Low: Higher material cost than casting, but automated for high volume.
Geometric Complexity	Very High: Can produce intricate internal cavities with cores; no parting lines.	High: Undercuts and complex features require expensive mold actions (sliders, lifters).	Very High: Excels at producing thin walls, sharp features, and intricate details.
Typical Tolerances	Good: Typically ±0.5 mm, can be improved with secondary operations.	Excellent: Highly repeatable; specific tolerance depends on material and part design.	Excellent: Often ±0.05 mm or tighter, superior to investment casting.
Surface Finish (Ra)	Very Good: Typically around 3.2 µm (125 Ra microinches).	Excellent: Can be cosmetic/optical grade directly from a polished mold.	Excellent: Typically around 1 µm, often requires no secondary finishing.
Key Advantage	Unmatched material integrity and design freedom for complex metal parts.	Unbeatable scalability and low unit cost for polymer parts.	High-precision, high-volume production of small, complex metal parts.