- What is it?
- Design Notes
- Technical Notes
- The Economics
- Typical Products
- The Environment
- Competing Processes
What is it?
If you have gold fillings in your mouth, be thankful for this process – it was used to make them. The lost wax process – the old name for investment casting – has been practice for at least 3000 years; sophisticated jewelry, ornaments, and icons were being made in Egypt and Greece well before 1500 bc. In investment casting, wax patterns are made and assembled (if small) into a tree with feeding and gating systems. The assembled pattern is dipped into refractory slurry, then covered in refractory stucco and allowed to dry. The procedure is repeated a number of times until about eight layers have built up, creating a ceramic investment shell. The wax is then melted out and the ceramic shell fired before the molten metal is cast. Gravity casting is adequate for simple shapes, but air pressure, vacuum, or centrifugal pressure is needed to give complete filling of the mold when complex, thin sections are involved. The mold is broken up to remove the castings. The process is suitable for most metals of melting temperatures below 2200 c. Because the wax pattern is melted out, the shape can be very complex, with contours, undercuts, bosses, and recesses. Hollow shapes are possible: bronze statues are hollow, and they are made by an elaboration of this process.
As complex as you like.
The process is extremely versatile, allowing great freedom of form. It offers excellent reproduction of detail in small three-dimensional components.
Investment casting is one of the few processes that can be used to cast metals with high melting temperatures to give complex shapes; it can also be used for low melting metals. The traditional uses of investment casting were for the shaping of silver, copper, gold, bronze, pewter, and lead. Today the most significant engineering applications are those for nickel, cobalt, and iron-based alloys to make turbine blades.
Investment casting lends itself both to small and large batch sizes. For small batch sizes the process is manual, with low capital and tooling costs, but significant labor costs. When automated, the capital costs are high, but quality control and speed are greater. The production rate is increased by the use of multiple molds.
Jewelry, dental implants, statuary, metal sculpture and decorative objects, high temperature gas turbines and similar equipment, monumental statues.
There are the usual hazards associated with casting molten metal, but procedures for dealing with these are routine. The mold materials cannot, at present, be recycled.