Investment Casting lost wax
What is lost-wax investment casting?
Section titled “What is lost-wax investment casting?”Lost-wax investment casting is a precision casting process in which a wax, plastic, or similar material pattern is coated with a refractory ceramic mold. The mold is heated to remove the wax (lost wax) and preheated to approximately 1000 °C / 1832 °F to eliminate residues and harden the ceramic binder. Molten metal is then poured into the mold cavity, faithfully replicating the original shape. The process allows complex geometries, tight tolerances, and good surface finishes without the need for draft angles. Casting can be done by gravity, pressure, or vacuum, depending on the mold configuration and material.
Operating Parameters
Section titled “Operating Parameters”Process parameters vary with the material being cast, but general typical values are summarized below. The ceramic mold is always preheated prior to casting to ensure complete filling and gas removal.
| Parameter | Typical Value |
|---|---|
| Mold preheating temperature | 1000 °C / 1832 °F |
| Minimum wall thickness (easily cast alloys) | 0.5 – 1.0 mm / 0.020 – 0.040 in |
| Typical part weight range | 200 g to 8 kg / 7 oz to 15 lb |
| Maximum achievable weight | 35 kg / 80 lb |
| Typical part lengths | From a few millimeters up to approximately 500 mm / 20 in (depending on configuration) |
| Casting method | Gravity, pressure, or vacuum depending on mold permeability |
Pouring Parameters by Material
Section titled “Pouring Parameters by Material”The metal pouring temperature and minimum thickness are adjusted according to the alloy. The following values are representative of normal production conditions.
| Material | Pouring Temperature | Typical Minimum Wall Thickness | Remarks |
|---|---|---|---|
| Aluminum alloys | 700 – 760 °C / 1290 – 1400 °F | 0.5 – 1.0 mm / 0.020 – 0.040 in | Excellent castability; light parts and thin walls are obtained. |
| Bronzes and brasses | 950 – 1150 °C / 1740 – 2100 °F | 0.5 – 1.0 mm / 0.020 – 0.040 in | Good detail reproduction; used in artistic and industrial applications. |
| Stainless steels | 1450 – 1600 °C / 2640 – 2910 °F | 1.0 – 1.5 mm / 0.040 – 0.060 in | Require controlled atmosphere or partial vacuum to avoid oxidation. |
| Tool steels | 1400 – 1500 °C / 2550 – 2730 °F | 1.0 – 1.5 mm / 0.040 – 0.060 in | High wear resistance; high-refractoriness ceramic molds are used. |
| Nickel/cobalt-based superalloys (Hastelloy, Stellite) | 1350 – 1500 °C / 2460 – 2730 °F | 1.0 – 2.0 mm / 0.040 – 0.080 in | Suitable for turbine blades and high-temperature components; often vacuum cast. |
| Precious metals (gold, silver) | 1000 – 1200 °C / 1830 – 2190 °F | 0.3 – 0.5 mm / 0.012 – 0.020 in | Allow very fine geometries; the process enables wax recovery. |
In addition to the materials listed, it is possible to cast glass, although this case goes beyond the typical scope of metal casting.
Dimensional Tolerances
Section titled “Dimensional Tolerances”The usual tolerance in investment casting is expressed as a percentage of the nominal dimension. For medium-sized parts, ±0.5 % of the length is achieved, which can be reduced to ±0.15 % on small dimensions with rigorous process control. These values approximately correspond to tolerance grades IT9-IT12 per ISO 286, although the process can reach IT7-IT8 under optimized conditions.
The following table illustrates the linear tolerance for various typical nominal lengths.
| Nominal Dimension | Typical Tolerance (±0.5%) | Adjusted Tolerance (±0.15%) |
|---|---|---|
| 25 mm / 1 in | ±0.125 mm / ±0.005 in | ±0.038 mm / ±0.0015 in |
| 50 mm / 2 in | ±0.25 mm / ±0.010 in | ±0.075 mm / ±0.003 in |
| 100 mm / 4 in | ±0.5 mm / ±0.020 in | ±0.15 mm / ±0.006 in |
| 200 mm / 8 in | ±1.0 mm / ±0.040 in | ±0.3 mm / ±0.012 in |
Tolerances may be wider in areas near the metal entry or in very fine details. It is recommended to consult with the foundry regarding the specific capability for the target geometry.
Surface Finishes
Section titled “Surface Finishes”Surface finish directly depends on the binder method used in the ceramic coating. Two main variants are distinguished: the sodium silicate (water glass) process and the silica sol process. The latter produces smoother surfaces but has a higher cost.
| Binder Method | Surface Roughness (Ra) |
|---|---|
| Sodium silicate (water glass) | 3.2 – 6.3 µm / 125 – 250 µin |
| Silica sol (silica sol) | 1.6 – 3.2 µm / 63 – 125 µin |
The indicated values correspond to the “as-cast” condition, without additional polishing or blasting operations. For finishes finer than Ra 1.6 µm, subsequent mechanical polishing or electropolishing is necessary.
Applicable Materials
Section titled “Applicable Materials”Investment casting accepts a wide range of metals and alloys, including those difficult to machine.
- Light alloys: aluminum and its alloys (Al-Si, Al-Cu), magnesium (less common).
- Copper alloys: bronzes, brasses, nickel silver.
- Steels: carbon steels, stainless steels (austenitic, ferritic, duplex), air- or oil-hardening tool steels.
- Superalloys: nickel-based (Hastelloy, Inconel), cobalt-based (Stellite), resistant to high temperatures and corrosion.
- Precious metals: gold, silver, platinum, used in jewelry and dentistry.
- Special alloys: titanium (with controlled atmosphere), cobalt-chromium alloys for medical implants.
Thanks to the exact replication of the pattern, details such as logos, knurling, or gears can be incorporated directly into the cast part.
Advantages and Limitations
Section titled “Advantages and Limitations”| Advantages | Limitations |
|---|---|
| Excellent dimensional accuracy (±0.5% of dimension) and repeatability. | High tooling and ceramic mold cost, especially for short runs. |
| Good surface finish, no parting line. | Long manufacturing cycle (several days) due to layer-by-layer mold construction. |
| Capability for complex internal shapes and undercuts without removable cores. | Size limitation: parts rarely exceed 35 kg / 80 lb. |
| No draft angles required. | The need to preheat the mold to 1000 °C can affect the choice of refractory. |
| Wide variety of materials, including those difficult to machine. | Subsequent machining may be required to remove sprue marks and achieve tighter tolerances. |
| Allows combining several components into a single part (reduction of assemblies). | Labor-intensive process with consumables (wax, refractory slurry). |
Selection Guide
Section titled “Selection Guide”Lost-wax investment casting is the preferred choice when the following are required:
- Complex three-dimensional geometry with internal cavities, blades, or shapes that cannot be drafted.
- Excellent surface finish and tight tolerances without secondary operations.
- Difficult-to-machine materials (superalloys, hardenable steels).
- Medium or large production runs where tooling cost is amortized; for single parts, alternatives such as direct metal 3D printing exist, though with different capabilities.
If the required surface roughness is less than Ra 1.6 µm / 63 µin, the silica sol process should be chosen or a finishing step added. If parts are large (>35 kg) or walls require thicknesses greater than 10 mm, other processes (sand casting, forging) may be more economical.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What dimensional tolerances can I expect in an investment cast part?
Section titled “What dimensional tolerances can I expect in an investment cast part?”Typical tolerance is ±0.5% of the dimension, equivalent to ±0.125 mm on a 25 mm dimension, and can reach ±0.15% (≈±0.038 mm on 25 mm) for small parts with rigorous process control.
What is the minimum achievable wall thickness?
Section titled “What is the minimum achievable wall thickness?”Wall thicknesses of 0.5 mm / 0.020 in to 1.0 mm / 0.040 in can be achieved in easily cast alloys such as aluminum and bronzes; for steels and superalloys, the practical minimum wall is 1.0 mm / 0.040 in.
What surface roughness is obtained as-cast?
Section titled “What surface roughness is obtained as-cast?”Average roughness Ra ranges from 1.6 µm / 63 µin (silica sol) to 6.3 µm / 250 µin (sodium silicate), depending on the binder system used.
Why is the mold preheated to 1000 °C?
Section titled “Why is the mold preheated to 1000 °C?”The mold is preheated to approximately 1000 °C / 1832 °F to remove any wax residue, fully harden the ceramic binder, and ensure that molten metal fills thin sections without premature solidification.
What is the maximum weight of an investment cast part?
Section titled “What is the maximum weight of an investment cast part?”Parts can weigh from a few grams up to 35 kg / 80 lb, although the typical production range is between 200 g and 8 kg / 7 oz and 15 lb.
What non-ferrous materials are commonly investment cast?
Section titled “What non-ferrous materials are commonly investment cast?”In addition to aluminum and bronzes, nickel-based superalloys such as Inconel and Hastelloy, cobalt alloys such as Stellite, and precious metals (gold, silver, platinum) are investment cast with excellent detail fidelity.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/machine-processes-tolerance-grades-d_1367.html
- efunda.com: https://www.efunda.com/processes/metal_processing/invest_casting.cfm
- manufacturingguide.com: https://www.manufacturingguide.com/en/lost-wax-casting