Investment casting is one of the metallurgical processes that are used worldwide which was discovered many years ago. It is also known as lost wax process casting. In the early days they use to produce copper, bronze and gold. This technique re-emerged in the late 19th century when dentists began using the technique to make crowns and inlays. Today, more complex designs around the world are being produced daily.
It is a metal casting process where wax is one of the important tools in producing the desired shape. The wax models are engineered to be slightly larger than the casting due to the volumetric of metal shrinkage after it has solidified. The wax patterns are gated onto a sprue, and then repeatedly dipped into liquid ceramic slurry to form a layered, into which, the seamless mold. After the ceramic has dried, the wax that has been disposed is melted out leaving a hallow cavity into which the metal, melted from existing recipes to achieve the desired alloy, is poured and allowed to solidify. The origin of the term investment comes from the solid mold process where a plaster type material is poured into a container that holds a clustered tree of small patterns that are identical to the casting being produced. It refers to the ceramic materials that are used to build a hallow shell into which the molten metal is poured to make the casting. Investment casting is very helpful processes because we can produce small castings that are very difficult to produce using other processes like die casting, sand cast and other metallurgical processes.
Why is it called investment casting?
The term investment refers to the process of the ceramic slurry application. There are two main techniques to encase wax patterns in the high temperature ceramics which eventually become the mold to hold molten metal. One method is pouring ceramics slurry around the wax tree assembly that is inside a container, and the second is to deep the wax into the slurry to apply the slurry in layers. When any object is dipped into a liquid, and that liquid stays on the surface, that is called investing, and hence this is where the name comes from.
The accuracy of investment casting
Investment casting process is one of the accurate processes in metallurgy. However, a process will not be 100% accurate; we do have a tolerance of 0.010 for the first inch and 0.005 for each inch after that; is standard for investment casting. This varies depending on the complexity of the part that must be produced. The smaller the casting, the more dimensionally the accurate it can be investment cast, and for this reason large investment casting start to lose the dimensional accuracy and because of that, other processes are used for large castings.
The cost of this process
Investment casting can be slightly expensive than other metallurgical processes used today such as sand cast, forged, or some machine parts. In metallurgy we try to use a good process that is also economically. However, this process is still used in many parts of the world as many design elements can be obtained through investment casting, since almost any configuration can be cast. The key to economical use of this process is to fully utilize its flexibility and incorporate as much added value into the cast.
Advantages of investment casting
There are advantages in using this process.
‘ The dimensional accuracy is good.
‘ Investment casting can be used to replace die casting where short runs are involved.
‘ Many intricate forms with undercuts can be cast.
‘ A very smooth surface is obtained with no parting line.
‘ Certain unmachinable parts can be cast to pre-planned shapes.
Disadvantages of investment casting
‘ This process is expensive; it’s usually limited to small casting, and presents some difficulties where cores are involved.
‘ It requires very long production-cycle times versus other casting.
‘ This process is practically infeasible for high volume manufacturing, due to its high cost and long cycle times.
‘ Many of the advantages of the investment casting process can be achieved through other casting techniques if principles of thermal design and control are applied appropriately to existing processes that do not involve the shortcomings of investment casting.
‘ It is a disadvantage that holes cannot be smaller than 1.6mm and should be no deeper than about 1.5 times the diameter.
Examples of investment casting parts