What Is Investment Casting - Its Introduction, Production Process, Applications, Advantages 

Introduction :-

In investment casting, a pattern made of wax is coated with a refractory material to make the mold, after which the wax is melted away prior to pouring the molten metal. The process dates back to ancient Egypt and is also known as the lost-wax process, because the wax pattern is lost from the mold prior to casting. The investment-casting process was first used during the period from 4000 to 3000 B.C. The term investment comes from one of the less familiar definitions of the word invest, which is “to cover completely,” this referring to the coating of the refractory material around the wax pattern. Wax patterns require careful handling because they are not strong enough to withstand the forces encountered during mold making; however, unlike plastic patterns, wax can be recovered and reused. Although the mold materials and labor involved make the lost-wax process costly, it is suitable for casting high-melting-point alloys with good surface finish and close dimensional tolerances; few or no finishing operations, which otherwise would add significantly to the total cost of the casting, are required.

The process is capable of producing intricate shapes, with parts weighing from 1 g to 35 kg, from a wide variety of ferrous and nonferrous metals and alloys. Parts up to 1.5 m in diameter and weighing as much as 1140 kg have been cast successfully by this process. All types of metals, including steels, stainless steels, and other high temperature alloys, can be investment cast. Examples of parts include complex machinery parts, blades, and other components for turbine engines, jewelry, and dental fixtures. Typical parts made are components for office equipment, as well as mechanical components such as gears, cams, valves, and ratchets. Recent advances include the casting of titanium aircraft-engine and structural airframe components with wall thicknesses on the order of 1.5 mm, thus competing with previously used sheet-metal structures.

Advantages of investment casting include:-

(1) parts of great complexity and intricacy can be cast;
(2) close dimensional control—tolerances of 0.075 mm are possible; (3) good surface fi nish is possible;
(4) the wax can usually be recovered for reuse; and
(5) additional machining is not normally required this is a net shape process. Because many steps are involved in this casting operation, it is a relatively expensive process.

Investment Casting Process :-

The pattern is made of wax, or of a plastic such as polystyrene, by molding or rapid prototyping techniques. The pattern is then dipped into a slurry of refractory material such as very fine silica and binders, including water, ethyl silicate, and acids. After this initial coating has dried, the pattern is coated repeatedly to increase its thickness for better strength. Note that the initial coating can use smaller particles to develop a better surface finish in the casting; subsequent layers use larger particles and are intended to build coating thickness quickly.
The one-piece mold is dried in air and heated to a temperature of 90° to 175 °C. It is held in an inverted position for a few hours to melt out the wax. The mold is then fired to 650° to 105 0°C for about four hours to drive off the water of crystallization and to burn off any residual wax. After the metal has been poured and has solidified, the mold is broken up and the casting is removed. A number of patterns can be joined to make one mold, called a tree, significantly increasing the production rate. For small parts, the tree can be inserted into a permeable flask and filled with a liquid slurry investment. The investment then is placed into a chamber and evacuated until the mold solidifies. The flask usually is placed in a vacuum-casting machine, so that molten metal is drawn into the permeable mold and onto the part, producing fine detail.

The Investment Casting Process in detail :

Creating a Wax Pattern :-

• In today’s manufacturing world, wax patterns are typically made by injecting wax into a metal tool or “die”
• With the evolution of Additive Manufacturing, patterns can be printed
• In the art community, one of a kind pieces are carved by the artist from wax blocks
• For multiple castings, a silicon tool is usually made from the artist’s sculpture and wax is injected or poured into the resulting cavity

Wax Tree Assembly :-

• It is typically uneconomical to make small parts one at a time, so wax patterns are typically attached to a wax “sprue”
• The sprue serves two purposes
1. Provides a mounting surface to assemble multiple patterns into a single mold, which will be later filled with alloy
2. Provides a flow path for the molten alloy into the void created by the wax pattern(s)
• The wax between the pattern(s) and the sprue are called “Gates”, because they throttle the direction and flow of the alloy into the void made by the pattern

Shell Building :-

• The next step in the process is to build a ceramic shell around the wax tree
• This shell will eventually become the mold that metal is poured into
• To build the shell, the tree is dipped into a ceramic bath or “slurry”
• After dipping, fine sand or “stucco” is applied to the wet surface
• The mold is allowed to dry, and the process is repeated a number of times until a layered (or laminated) ceramic mold, capable to undergo the stresses of the casting process, has been built

Dewax / Burnout :-

• Before pouring metal into the mold, the wax is removed
• This is typically done using a steam-dewax autoclave, which is like a large, industrial pressure cooker
• Another method is the use of a flash fire oven, which melts and burns off the wax
• Many foundries use both methods in concert
• Autoclave removes the majority of the wax, which can be reconditioned and reused
• Flash fire burns off residual wax and cures the shell, readying it for casting

Metal Pouring :-

• Before the metal is poured into the ceramic mold or “shell”, the mold is preheated to a specific temperature to prevent the molten alloy from solidifying or “freezing off” before the entire mold is filled
• Alloy is melted in a ceramic cup (called a crucible) using a process known as induction melting
• A high frequency electric current creates a magnetic field around the alloy, generating electric fields inside the metal (eddy currents)
• The eddy currents heat the alloy due to the material’s electrical resistance
• When the alloy reaches its specified temperature, it is poured into the mold, and the mold is allowed to cool

Shell Knock Off :-

• Once cool, the shell material is removed from the metal
• This is typically done via mechanical means
• Hammer
• High Pressure Water Blast
• Vibratory Table
• Shell removal can also be accomplished chemically, using a heated caustic solution of either potassium hydroxide or sodium hydroxide, but this approach is being phased out due to environmental and health concerns

Cut Off :-

• Once the shell material has been removed, the parts are cut off the sprue and the gates are ground off
• Part cut off can be done manually
• Chop saw
• Torch
• Laser (limited applications)
• Parts can also be cut off using automation, that is, the mold can be secured using a fixture on a programmable cut off saw

Individual Castings :-

• Once the parts are removed from the sprue, and the gates removed, the surface can be finished via a number of means
• Vibratory/Media finishing
• Belting or hand grinding
• Polishing
• Finishing can be done by hand, but in many cases it is automated
• Parts are then inspected, marked (if required), packaged and shipped
• Depending on the application, the parts can be used in their “net shape” or undergo machining for precision mating surfaces