Process
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.
Investment casting has been used in various forms for the last 5,000 years. In its earliest forms, beeswax was used to form patterns necessary for the casting process. Today, more advanced waxes, refractory materials and specialist alloys are typically used for making patterns. Investment casting is valued for its ability to produce components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.
The fragile wax patterns must withstand forces encountered during the mold making. Much of the wax used in investment casting can be reclaimed and reused.
Investment casting derives its name from the pattern being invested (surrounded) with a refractory material. Many materials are suitable for investment casting; examples are stainless steel alloys, brass, aluminium, carbon steel and glass. The material is poured into a cavity in a refractory material that is an exact negative replica of the desired part. Due to the hardness of refractory materials used, investment casting can produce products with exceptional surface qualities, which can reduce the need for secondary machine processes.
The process can be used for both small castings of a few ounces and large castings weighing several hundred pounds. It can be more expensive than die casting or sand casting, but per-unit costs decrease with large volumes. Investment casting can produce complicated shapes that would be difficult or impossible with other casting methods. It can also produce products with exceptional surface qualities and low tolerances with minimal surface finishing or machining required.
Basic steps
Tool Design / Build
Designing the tool as per the component’s geometry and quantity requirements considering various factors like shrinkage properties of wax, metal and it’s design.
Wax injection
Firstly, the wax replica of the required casting is produced with the help of injection molding. Such wax replicas are known as patterns.
Assembly
Secondly, the wax pattern is attached to a sprue (central wax stick) for forming an assembly or casting cluster.
Shell building
Thirdly, a shell is made by dipping the assembly inside a container comprising of liquid ceramic slurry followed by putting it into a bed comprising of very fine sand approximately up to at least 8 layers can be applied in such a manner.
Dewax
Fourthly, the wax is immediately melted out the moment the ceramic is dried out, forming a negative impact of the assembly inside the shell.
Chemical Analysis
Small sample is taken from the molten metal and it is quickly solidified by the help of quenching it in the water. It is then tested in Spectrometer to know it’s chemical properties.
Liquid Metal Pouring
In case of the conventional process, molten metal is filled in the shell via gravity pouring. When the metal cools down, the pouring cup, sprue, gates and parts turn into one solid casting.
Knockout
When the metal gets cooled and solidified, next the ceramic shells are broken with the help of water blasting or vibration.
Cut-off
The parts are traditionally cut away and separated from the main sprue with the help of a friction saw of high speed.
Heat Treatment
Heat treatment is a controlled process used to alter the microstructure of metals such as stainless steel, carbon steel and alloy steels to impart properties which benefit the working life of a component, for example increased surface hardness, temperature resistance, ductility and strength.
Mechanical Analysis
Tensile Strength, Yield Stress, Elongation and Reduction of area are important parameters that can be achieved by testing the Tensile Test bars in Universal Testing Machine.
Finished Castings
The metal castings that are identical to that of the original one is finally complete post minor finishing operations.
Various Tests
Various Destructive and Non-Destructive Tests like Bend Test, Impact Test, IGC test, Radiography Test, Liquid Penetrant Test, Magnetic Particle Examination, Ultrasonic Tests, Eddy Current Tests, Microscopic Examination.