Investment casting is an industrial process of metal casting, with the use of alloys. It also known as lost wax casting due to the use of wax to make molds. This wax is then heated and melted out of the casts which are then filled with the alloy, with which, the metal part is to be made. The lost wax process is one of the oldest manufacturing processes.
The Egyptians are known to have made jewelry for their pharaohs through this process some 5000 years ago. Though initially beeswax was used, nowadays high technology wax are used in the industries. The advantage of this process is that it makes metal parts with great structural precision.
Wax Pattern Production
The investment casting process begins with the production of a single piece of wax pattern, that has the exact same shape, as that of the metal part to be manufactured. This pattern is made by injecting wax into a metal wax injection die.
These patterns have the exact geometrical shape of the finished product. However, they are a little larger in size in order to make room for shrinkage in volume of the alloy, during the process.
Making the Pattern Assembly
A pattern assembly is made by attaching individual patterns to one or more runners. Each runner is then attached to the pouring cup. The patterns, runners, and the pouring cup form the tree or cluster, which is required for the next step.
The number of patterns on each runner cup and their arrangement, depends upon the size and configuration of the casting as well as the alloy type.
Making the Ceramic Mold
This process involves dipping the cluster into a ceramic slurry. The excess ceramic slurry is drained. Then the cluster is coated with fine ceramic sand and then the cluster is left to dry.
After the first coating of ceramic material has dried, the process is repeated with ceramic material of increasing coarseness. This results in a hard, self supporting ceramic mold in the shape of the cluster.
After the ceramic mold is made, it is placed in a high temperature furnace. Here the molten wax patterns run out of the runners and the pouring cup, leaving behind a ceramic mold with cavities in the shape of the wax patterns.
The ceramic molds are further heated to a temperature of 1600 to 2000 degrees Fahrenheit, to prepare them for the casting process. This heating also removes any last traces of wax, that might have escaped the previous step.
Hot molten metal or alloy is then poured into the ceramic mold that, enters the cavities and takes up the desired shape. While pouring the hot molten material into the ceramic molds, vacuum, pressure, or centrifugal force is used. This ensures the precise replication of the shape.
Removing the Ceramic Mold
After the previous process is done, the ceramic mold is removed by chemically treating the entire structure. Mechanical vibration or abrasive blasting is also employed in some cases.
Removing the Casting
The individual castings are removed from the cluster with the help of cut-off wheels. With the help of the process of belt grinding, any protrusions that might be left by the runners are removed.
The castings thus produced are ready for further treatment like straightening, heat treatment, and inspection, before they are delivered to customers.
This process is widely used in industries, to make high precision tools with metals, that are too hard to be processed by normal manufacturing techniques. This technique is used to make airplane parts and objects that have complex shapes like turbine blades.