Investment Casting is also known as the lost wax process. This process is one of the oldest manufacturing processes. Intricate shapes can be made with high accuracy. In addition, metals that are hard to machine or fabricate are good candidates for this process. It can be used to make parts that cannot be produced by normal manufacturing techniques, such as turbine blades that have complex shapes.
The process begins with production of a one-piece heat-disposable pattern. This pattern is made by injecting wax into a metal die. A pattern is required for each casting. These disposable patterns have the exact geometry of the required finished part, but they are made slightly larger, to compensate for volumetric shrinkage in the pattern production state and during solidification of metal in the ceramic mould.
Patterns are fastened on to one or more runners and the runners are attached to the pouring cup. Patterns, runners and pouring cups comprise the cluster or tree, which is needed to produce the ceramic mould. The number of runners per section and their arrangement on the pouring cup can vary considerably,depending on alloy type, size, and configuration of the casting.
The ceramic shell mould technique involves dipping the entire cluster into a ceramic slurry, draining it, then coating it with fine ceramic sand. After drying, this process is repeated again and again, using progressively coarser grades of ceramic material, until a self-supporting shell has been formed
The coated cluster is placed in a high temperature furnace where the pattern melts and runs out through the gates, runners and pouring cup. This leaves a ceramic shell containing cavities of the casting shape desired with passages leading to them.
The ceramic shell moulds must be fired to burn out the last traces of pattern material and to preheat the mold in preparation for casting usually in the range of 1600 to 2000 degrees Fahrenheit. The hot moulds may be poured with the assistance of vacuum, pressure and/or centrifugal force. This enables reproduction of the most intricate details and extremely thin walls of an original wax pattern.
After the poured molds have cooled, the mold material is removed from the casting cluster. This is done by mechanical vibration, abrasive blasting, and chemical cleaning.
Individual castings are then removed from the cluster by means of cut-off wheels and any remaining protrusions left by gates or runners are removed by belt-grinding.
The castings are then ready for secondary operations such as: heat-treating, straightening, machining, finishing, inspection, non-destructive testing, and then shipment to the customer.