Investment Casting

What is Investment Casting?

Investment casting is a manufacturing process in which a wax pattern is coated with a refractory ceramic material. Once the ceramic coating material is dry and hardened, the wax is melted out and leaves an internal cavity the shape of the final product’s geometry. Molten metal is poured into the cavity where the wax pattern was. The metal solidifies within the ceramic cavity, cools, and the ceramic is removed from the metal casting. The result of this process is a net to near-net precision metal component which can be used for a broad range of applications in various industries.

Why Is Investment Casting Also Called the Lost Wax Process?

Lost wax is a very literal alternative name for the process. The investment in refractory material is a covering that is formed wet, over a wax pattern. This covering is then allowed to dry, before baking at high temperature. This serves two purposes.

First, the refractory investment is solidified and chemically fused to form a strong material.

Secondly, the wax is burned away, making the investment a tough and hollow body that perfectly and precisely represents the now-absent wax, as a space to be filled with metal. Thus, the wax is lost, hence the naming of the process.

How Does Investment Casting Differ From Other Types of Casting?

All casting methods use a heat-resistant cavity that is the shape of the desired part as a volume to be filled with liquid metal, which is removed once solidified. The means by which the cavity is formed is the primary differentiator of casting methodologies. Die casting uses a two-part (or more) hardened-steel cavity tool that is expensively and precisely machined from billet material. Sand casting forms a cavity by packing sand with a binding agent around a reusable pattern or master of the part. Investment casting forms the cavity by drying and then kiln baking a skin of refractory material over a pattern or master that is sacrificial.

Precision and extent of detail differ greatly between casting methods. Die casting is high precision, although gravity (poured) die casting is less precise than the various forms of pressure die casting. Pressure die casting facilitates compensation for shrinkage, which retains/reflects the shape and dimensions of the cavity more faithfully in the cast part. Pressure die-cast parts can maintain very high levels of detail, so long as that detail can be extracted from the tool features. Sand casting is of moderate precision because the process does not lend itself to forming very accurate and repeatable cavities. It can maintain only relatively thick sections and coarse details. Investment casting is renowned for its combination of precision and fine detail while avoiding the major up-front costs of die casting.

Investment Casting Process Steps (Lost Wax)

How Long Does It Take Investment Casting To Finish?

The time it takes to complete investment casting depends on several factors including: the material cast, the design, and the machines used. For small casting, below 1 kg, for example, it can commonly take 5 to 7 days to fully process a flask ready to cast.

How Accurate Is Investment Casting?

The investment casting process is very accurate in creating precise reproductions of the pattern when the flask has been prepared with skill and care. Pattern accuracy is highly dependent on the manufacturing process used to make it. A range of techniques can be used to improve the precision of the wax pattern. For example, if the pattern is molded in a tool, the injection pressure of the wax can be adjusted to moderate dimensional errors. Parts of the pattern requiring high precision can also be post-machined or “coined” (press molded with a precise steel cavity in two parts.

Investment Casting Process Steps at Soject

The following steps are what we follow to produce parts with complex geometries and intricate details.

A pattern that replicates the finished part geometry is made using one of two primary methods:
a. Build a wax injection die
b. 3D print pattern
● If a wax injection die will be used, the first step involves designing and building a metal die from Aluminum or Steel. This die creates a wax replica of the desired part by injecting melted wax under high pressure into the cavity. The die can be made as a simple one cavity manual tool or a complex multi-cavity automatic tool depending on volume requirements.
● If a 3D printed pattern will be used, a CAD model which contains the part geometry is sent to a printer and the part is printed.

Next, the wax patterns are assembled onto runners and into a finished tree which is ready to be dipped.

The assembly is then dipped into a high-grade ceramic slurry to build a ceramic shell around the wax tree.

After the slurry coating is done, particles of sand are dropped onto the surface of the wet tree assembly. This helps to thicken and strengthen the layer of coating on the wax assembly surface.

Repeat steps 2 and 3 until the assembly achieves the desired shell thickness. The assembly is then allowed to set and dry.

The wax inside the newly built shell is now removed. Dewaxing is done using a steam-dewaxing autoclave or flash fire furnace.

Now the desired molten metal is poured into the pre-heated mold cavity.

The mold then sits to allow the molten metal to cool and solidify which then becomes the final casting.

The shell material is then removed through processes hammer knockout, vibration, and steel grit blasting.

The finished parts are then cut free from the gating and runner system.

Various finishing techniques are then employed including grinding, sand blasting and coating to achieve the final surface needed.

Once the finishing operations are done, the parts are inspected for surface and sub-surface defects. Visual and fluorescent penetrant inspection is done for surfaces and X-ray is employed for sub-surface defect identification.

Gallery of Investment Casting

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