What is investment casting?

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What is investment casting?

Investment casting (ie. lost wax casting or precision casting) is used for producing complex-shaped components which require tighter tolerances, thinner walls and better surface finish that can be obtained through sand casting.


The distinguishing feature of investment casting is the way the mold is made. A pattern of the part is made with wax. The wax pattern is dipped into a fine ceramic slurry that contains colloidal silica and alumina. Dried and then heated in an oven, the wax melts out leaving behind a ceramic shell mold for casting.


The investment casting method, also called the lost wax process, is used for precision casting applications, for example, aerospace components such as gas turbine blades.


The Investment Casting Process

1. Pattern Creation

Wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form internal features on the pattern. Several of these patterns may attached to a central wax gating system, to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity.


2. Mold Creation

This “pattern tree” is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around both the patterns and gating system.


This process is repeated until the ceramic shell is thick enough to withstand the molten metal it will encounter. The shell is then heated in an oven, draining the molten wax and leaving a hollow ceramic shell that acts as a one-piece mold, hence the name “lost wax” casting.


3. Pouring

The mold is preheated in a furnace to approximately 1000°C and the molten metal is poured into the gating system of the mold, filling the mold cavity. Pouring is typically gravity fed, but other methods such as vacuum or pressure are sometimes used.


4. Cooling

After the mold has been filled, it is allowed to cool and solidify into the shape of the final investment casting parts. The cooling time allowed depends on the thickness of the part, the thickness of the mold, and the casting material used.


5. Casting Removal

After the molten metal has cooled, the mold can be broken, and the casting removed. The ceramic mold is typically broken away using water jets (Several other methods exist). Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).


6. Finishing

Finishing operations such as grinding, or sandblasting are used to smooth the part around the gates. Heat treatment may also be used to harden the final part.




Advantages and Disadvantages of Investment Casting

Advantages

Excellent surface finish.

Tight dimensional tolerances.

Complex and intricate shapes may be produced.

Capability to cast thin walls.

Wide variety of metals and alloys (ferrous and non-ferrous) may be cast.

Draft is not required in the molds design.

Low material waste.

Disadvantages

Individual pattern is required for each casting.

Limited casting dimensions.

Relatively high cost (tooling cost, labor cost).

Z&Z casting have been working with a number of highly experienced investment casting foundries to offer you more choice and flexibility. We offer different core types (such as waxless cores, soluble wax cores and urea wax cores) and specialized waxes (including low-temperature wax, medium temperature wax and low-temperature wax with polymer additives).