Investment Casting (or) Lost Wax Method
A wax duplicate of the desired casting is created to be invested into a "Ceramic Slurry".
The slurry covered investment can be dipped into alternating coatings of sand and slurry until a suitable thickness of shell is achieved that can hold the molten metal after the investment is burnt out.
The "Burn-Out" process requires that the investment and coating are inverted in an oven that is fired to 1800F so that the investment can flow out and be recovered. The refractory coating is also cured in this procedure.
This process is beneficial for casting metals with high melting temperatures that cannot be moulded in plaster or metal.
Parts that are typically made by investment casting include those with complex geometry such as turbine blades or fire arm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.
Principle
Method also called as precision investment casting. The method involves the use of expendable Pattern with a shell of refractory material surrounded to form a casting mould. Since the pattern made up of wax is melted out and gets destroyed. That is why the name-"Lost wax method".
Process parameters of Investment casting
Process principle
Refractory slurry is formed around a wax or plastic pattern and allowed to harden. The pattern is then melted cut under mould is baked. The molten metal into the mould and solidifies.
Size limits
As small as (1/10) inch but usually less than 10 lb.
Thickness limits
As thickness as 0.025 inch but less than 3 inch.
Typical tolerance
Approximately 0.005 inch.
For the first inch and 0.002 inch for each additional inch.
Draft allowance
Not required.
Surface finish
50 to 125 micron.
Procedure
1. Produce a master pattern
The pattern is a modified replica of the desired product made from metal, wood, plastic, or some other easily worked material.
2. From the master pattern, produce a master die
This can be made from low-melting-point metal, steel, or possibly even wood. If low-melting-point metal is used.
3. Produce wax patterns
Patterns are made by pouring molten wax into the master die, or injecting it under pressure, and allowing it to harden. Plastic and frozen mercury have also been used as pattern material.
4. Assemble the wax patterns onto common wax sprues
The individual wax patterns are attached to a central sprues and runner system by means of heated tools and melted wax. In some cases, several pattern pieces may first be united to form a complex.
5. Coat the cluster with a thin layer of investment material
This step is usually accomplished by dipping the cluster into a watery slurry of finely ground refractory material.
6. Produce the final investment around the coated cluster
After the initial layer is formed, the cluster can be redipped, but this time the wet ceramic is coated with a layer of sand and allowed to dry. This process can be repeated until the investment coating is the desired thickness (typically 5 to 15 mm).
7. Allow the investment to fully harden
8. Melt or dissolve the wax pattern to remove it from the mould
This is generally accomplished by placing the moulds upside down in an oven, where the wax melts and runs out, and any residue subsequently vaporizes.
9. Preheat the mould in preparation for pouring
Heating to 550 to 1100°C (1000 to 2000°F) ensures complete removal of the mould wax, curves the mould to give added strength, and allows the molten metal to retain its heat and flow more readily into all of the thin sections.
10. Pour the molten metal
Various methods, beyond simple pouring, can be used to ensure complete filling of the mould, especially when complex, thin sections are involved.
11. Remove the casting from the mould
This is accomplished by breaking the mould away from the casting. Techniques include mechanical vibration and high-pressure water.
Advantages
i) Smoother surfaces (1500 to 2250 micro-mm root mean-square).Close tolerance (of +0.003 mm/mm)
ii) High dimensional accuracy
iii) Intricate shape can be cast
iv) Castings do not contain any disfiguring parting line
v) Machining operations can by eliminated
Disadvantages:
i) Process is relatively slow.
ii) Use of cores makes the process more difficult.
iii) The process is relatively expensive than other process.
iv) Pattern is expandable.
v) Size limitation of the component part to be cast. Majority of the castings produce weight less than 0.5 kg.
Applications
The products made by this process are vanes and blades for gas turbines, shuttle eyes for weaving, pawls and claws of movie cameras, wave guides for radars, bolts and triggers for fire arms, stainless steel valve bodies and impellers for turbo chargers.
While investment casting is actually a very old process and has been performed by dentists and jewellers for a number of years, it was not until the end of World War II that it attained any degree of industrial importance.
Developments and demands in the aerospace industry, such as rocket components and jet engine turbine blades, required high-precision complex shapes from high-melting-point metals that are not readily machinable.
Investment casting offers almost unlimited freedom in both the complexity of shapes and types of materials that can be cast.
