What is investment casting?
Investment casting is a foundry process that makes metal parts from a single-use pattern and a single-use ceramic mould. A wax replica of the part is produced for every casting, coated ("invested") with layers of fine refractory material, and then removed by melting. The hollow ceramic shell that remains is fired and filled with molten metal. After solidification the shell is broken away and the castings are cut from their feeding system.
Because the mould is built around the pattern in one piece, there is no parting line and no need to draw a pattern out of the mould. This is the origin of the design freedom that distinguishes the process from sand and die casting.
Operating principle
Three properties of the process explain most of its characteristics:
- The pattern is expendable. Undercuts, internal passages and re-entrant shapes can be formed with multi-part dies, soluble cores or ceramic cores, because the pattern never has to be withdrawn from the mould.
- The mould is a thin, hot, inert ceramic. The fine first coat reproduces surface detail; pouring into a preheated shell lets metal run into thin sections before it freezes.
- Nearly every castable alloy can be poured. The shell withstands the pouring temperatures of steels, nickel and cobalt alloys as well as aluminium and copper alloys.
The complete sequence is described in the manufacturing process section.
Historical development
Lost-wax casting is one of the oldest metal-forming techniques; cast copper-alloy objects made this way are known from more than five thousand years ago, and the method was used for sculpture, jewellery and ritual objects across many cultures. Industrial use began in the early twentieth century with dental castings and expanded rapidly in the 1940s, when aircraft engines required complex parts in alloys that were hard to machine. From the 1950s the ceramic shell replaced the earlier solid block mould and made mechanised, larger-scale production possible. Since then, vacuum melting, ceramic cores, solidification simulation and printed patterns have extended the range of parts that can be made.
Manufacturing characteristics
| Characteristic | Typical situation |
|---|---|
| Part mass | From a few grams to several tens of kilograms; larger parts in specialised foundries |
| Geometry | High complexity, undercuts, cast-in holes, lettering, thin walls |
| Alloys | Carbon, low-alloy and stainless steels, nickel and cobalt alloys, aluminium, copper alloys, titanium in specialised foundries |
| Dimensional accuracy | Tighter than sand casting; see tolerances |
| Surface | Fine as-cast texture; see surface finish |
| Quantities | Prototypes without hard tooling up to series of many thousands |
These are general characteristics of the process. What a particular foundry can achieve depends on its equipment, alloys and experience, and must be confirmed for each part.
Production volume and economics
The cost of an investment casting is made up of tooling (the wax injection die), labour- and energy-intensive process steps, the alloy, and finishing and testing. Tooling is usually cheaper than die-casting or forging dies but is not negligible, so very small quantities are often made from printed or machined patterns. The process becomes attractive when it removes machining, welding or assembly: the comparison has to be made on the total cost of the finished component, not on the price per kilogram of casting. The process comparison page sets investment casting against the main alternatives.
Terminology
How the terms investment casting, lost-wax casting, precision casting and ceramic shell casting relate, where they overlap and where their use differs.
02Advantages and limitations
A balanced engineering view of what investment casting does well, where its limits lie and which part characteristics make it the economic choice.