Moulage de précision, also known as precision investment casting, is the highest fidelity casting process on earth. It is a manufacturing process that can produce very thin and most complex features, and it will capture down the fingerprint detail. Whether you are making precision cast parts for the aerospace industry or you are a sculptor creating small works and also a monumental scale, it offers excellent surface finish and excellent dimensional accuracy.
Virtually there is no alloy which cannot be manufactured using investment casting process. That is the unique and greatest feature of the investment casting process. The range of alloys covers virtually the complete spectrum of alloys in engineering use.
The History and Evolution of Precision Casting
In the twentieth century, several developments of the investment casting process occurred. Initially, the solid mould or the block mould investment casting was used where the entire mould was filled with the ceramic material. Later the ceramic material was replaced by the plaster mould to give a better surface finish. Later mercast process was developed where mercury was used as the pattern material, but this is no more in use. Finally ceramic shell investment casting was developed recently and this has gained importance all over the world.
Very little has changed in the 3000 years that bronze has been cast. We now have electric blowers rather than bellows and we have natural gas instead of dried animal dung, otherwise everything else is the same. The mold is made out of earthen materials just like in the old days and we burn out the wax.
Special Features of the Investment Casting Process
What are these special features of investment casting process? It can produce very thin and most complex features. Second feature is it offers excellent surface finish; most of the times no machining is required or very negligible machining is required. Next special feature is it offers excellent dimensional accuracy. Finally it can be used to cast all metals and alloys.
If we see different manufacturing process or the casting process, ductile iron, tool steel, and super alloys cannot be cast using moulage sous pression process. Titanium alloys cannot be manufactured using moulage en sable process. But in the investment casting process, all the alloys can be manufactured. Virtually there is no alloy which cannot be manufactured using investment casting process.
The Ceramic Shell Investment Casting Process Steps
These are the major steps involved in the ceramic shell investment casting process:
- Wax Injection: Wax will be injected into the pattern die and it will be removed.
- Pattern Assembly: Hundreds of such patterns are produced in a single session and several such patterns are joined through a central tree to achieve economy.
- Shell Building: The wax assembly will be dipped inside the ceramic slurry, taken out, and stucco will be sprinkled around the pattern.
- Dewaxing: The wax inside the shell has to be drained out completely.
- Casting : Pouring the molten metal into the ceramic shell.
- Knockout: Breaking that shell and removing the casting outside.
- Cut Off: Separating the individual castings from the assembled tree.
- Finishing and Secondary Operations: Appropriate surface finish is polished on the parts.
Shell Building: Ingredients and Slurry Stages
How do we make the ceramic shell? We need to learn the ingredients of the ceramic slurry. One is the refractory powder or flour, which comprises sixty to eighty percent. The most commonly used refractory powders are zirconium silicate, fused silica, and fused aluminum oxide. Second ingredient is the liquid binder, comprising fifteen to thirty percent, mainly ethyl silicate or colloidal silica. Finally a solid binder will be used from five to ten percent.
The first dip slurry is a fine texture that can easily get down to the details of the grooves and engraving graphics. The next slurry coating is coarser than the first dips as it builds a thick ceramic shell around the wax tree. This cycle will be repeated about five to seven times. The slurry has to dry between successive dippings; temperature and humidity are carefully controlled.
The Science of Dewaxing and the Autoclave Oven
Prior to the pouring of the molten metal into the ceramic shell, wax inside the shell has to be drained out completely and this process is known as dewaxing. Generally an autoclave oven is used. High steam pressure at about 8 kg per square centimeters is injected into the sealed oven. The wax will be melted and it will be removed from the ceramic shell.
We make really sure to burn these molds out clean. If the bronze came in contact with wax it would be a real problem. The bronze would make the wax immediately turn to a gas and a gas is a sudden expansion of volume. The definition of an explosion is a sudden expansion of volume. The bronze could quite well shoot back up out of the mold because of the explosion of a wax in contact with bronze. This wax again it will be refined, it will be purified, and again it will be used for making the pattern.
Precision Casting: Melting and Solidifying the Metal
The ceramic moulds must be heated up before molten metal can be poured into them. We preheat the mold until it is orange hot so that it offers the least amount of chill resistance to the bronze as the bronze floods the chamber. Bronze literally solidifies at 1850 degrees; we melt it to about 2000 degrees and pour soon thereafter.
During the melting of the bronze ingots, imperfections such as sand or mold material float to the surface. We skimmed off the dross on the surface. After the shell is filled with the liquid metal, it must be set aside to cool down. Freshly poured ceramic shells generate a lot of ambient heat. Inside the mold, the thinnest areas solidify first and then the thicker areas.
Post Investment Casting: Knockout, Cut Off, and Finishing
Knockout means breaking that shell and removing the casting outside. A motorized chisel is used to break the ceramic shell away from the cast part. The metal tree is sand blasted to remove any remaining ceramic shell traces. Traces are to be removed from the casting; that’s why we are using a sand blasting system.
The castings are cut at the gates, which leaves the excess material on the gate areas; this is the cut off. For that, most of the times a grinding wheel is used. The castings are heat treated to normalize the metal. Appropriate surface finish is polished on the parts; mirror and satin finishes are the most common these days. The final product will be free from flaws and visually appealing.
Investment Cast Alloys and Their Industrial Applications
Common investment cast alloys include:
- Ductile Iron: Manufactured by an alloying process in which the graphite flakes are converted into spheroidal nodules. It is used for defense components, automotive components, and pump and valve components.
- Carbon Steels: Commonly used in a broad spectrum of industries like internal lock mechanisms, mining applications, military and firearms, and transportation parts.
- Tool Steels: Particularly used for making different tools like the milling cutter.
- Stainless Steels: Produced to a mirror finish for the ultimate in a corrosion resistance.
- Aluminum and Magnesium Alloys: Used for aerospace components where there are several thin fins.
- Titanium Alloys: Jet engine diffuser and Formula One race car suspension parts.
- Super Alloys: Like the turbine impeller with very thin blades and complex features.
Innovation: Rapid Prototyping in Investment Casting
The most tough face of the investment casting is the producing the pattern. Producing the pattern is very difficult, so that’s why rapid prototyping has emerged to produce the patterns. Important processes include:
- Stereo Lithography (SLA): Laser beam falls on a liquid resin photopolymer and it becomes solid. It is cured layer by layer.
- Selective Laser Sintering (SLS): Similar to stereo lithography but here powder is used.
- 3D Printing: Laser system is replaced by the liquid adhesive.
- Fused Deposition Modeling (FDM): Liquifier head moves in x and y direction making a thin sheet at a time.
For technical standards regarding these materials, refer to the American Society for Testing and Materials (ASTM).
The Future of the Investment Casting Industry: Ice Patterns
Ice patterns are found to be very effective in the investment casting process. Advantages of ice patterns include better surface finish and elimination of cracks in the shell during the pattern removal. When we raise the temperature, because of the abnormal expansion of the water, the volume will be coming down till four degree centigrade. Because the volume is coming down, the shell won’t be cracking. That is the greatest advantage. Pattern material water or ice is not expensive.
Bronze is a very archival media. When making a bronze, we are not just thinking about an art show this year, but how it will be viewed by future generations. It is a language based on a human story. Art is for the doggedly determined; it is for the ones that refuse to give up.
Deepen Your Precision Casting Manufacturing Knowledge
Explore our detailed guides to see which process fits your specific project needs:
- Zinc Die Casting vs. Aluminum Die Casting: A head-to-head comparison of costs and capabilities.
- Alloy Selection Guide: Understanding which metals work best for complex geometries.
- Secondary Finishing Operations: How we achieve mirror and satin finishes on cast parts.
- Rapid Prototyping Services: Accelerate your production with SLA and FDM patterns.
For more technical background, you can visit the Investment Casting Institute or explore the latest standards at the American Foundry Society.
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