Investment Casting – Working Principle, Process & More

Like sand casting, investment casting is one of the oldest casting processes practiced already in ancient civilisations. Dating back to 3700 B.C. in the Levant region for making jewelry, ornaments, and idols, this casting technique was also used by Harappa, Mesopotamia, Aztec, Mayan, and Benin civilisations, and found in the tombs of Egyptian rulers.

The founding principles of investment casting have remained the same over thousands of years. However, adding supporting technologies such as CAD/CAM, additive manufacturing, and real-time monitoring capabilities has significantly improved the final part quality, consistency, and affordability of this industrial process.

In this article, we shall explore the workings of modern investment casting processes, along with their advantages, disadvantages and use cases.

What Is Investment Casting?

Investment casting, also sometimes known as precision casting or lost wax casting, is one of the most popular metal forming techniques. The term ‘investment’ originates from the archaic meaning of ‘invest,’ which refers to clothing or surrounding an object with a layer of material.

In this process, a wax pattern is encased in refractory material to form a ceramic shell mould. The pattern is slightly larger in dimensions than the final product to accommodate shrinkage when the material cools.

Since the shell mould has a hard surface, investment casting delivers smoother surface finishes than sand casting. The process can work with most metals. Some popular metals that are highly compatible with investment casting are bronze, magnesium, stainless steel alloys, glass, carbon steel, brass, aluminium and aluminium alloys.

Investment Casting Process

Investment Casting Process Walkthrough

The investment casting process is more intricate than the widely used sand casting process, thus involving more stages. Investment casting involves 8 stages and these are as follows:

1. Creation of the master pattern

2. Creation of the master die

3. Creation of the wax pattern

4. Creation of the shell mould

5. Removal of wax

6. Pouring of molten metal and subsequent cooling

7. Removal of casting from the shell mould

8. Post-processing operations

Let’s look closer at what is happening in each of the stages:

1. Creation of the Master Pattern

The master pattern is an exact duplicate of the product required. It may be made from wood, metal, plastic, clay, wax or other specialist alloys. Over the years, manufacturers have shifted to 3D printing master patterns due to their compatibility with CAD/CAM and improved dimensional accuracy.

2. Creation of the Master Die

The master pattern is used to create a master die. The master die’s creation technique depends on the master pattern’s material. For instance, if the master pattern is from steel, it can be placed in molten aluminium, since aluminium melts at 660°C (1,220°F), whereas steel melts at around 1500°C (2,732°F). Thus, the molten aluminium will not dissolve the steel. Upon solidification, it will create an aluminum-based master die.

3D-printed master dies are increasingly being used because they offer a cost-effective alternative to metal dies when only a few castings are needed.

3. Creation of the Wax Pattern

There are several ways to produce wax patterns from master dies. One method involves filling the die with a small amount of wax and shaking it until it evenly coats the inner surface of the die and solidifies. This process is repeated until the desired thickness is achieved, resulting in a hollow wax pattern.

The second method involves filling the master die completely with wax. The wax may be fed directly or through high-pressure injection into the die. The high pressures enable the wax to fill all of the cavity’s features in the case of complex components. The wax patterns thus produced are solid.

During this stage, cores can be added to create internal cavities in the wax pattern. Common materials for cores include soluble wax or ceramic. The soluble wax is removed in stage 5, while ceramic cores are removed after the final product has hardened.

4. Creation of the Shell Mould

Wax patterns cannot withstand the high temperatures of molten metal. Therefore, ceramic moulds are created from wax patterns to endure the heat of molten metal. The ceramic mould is formed by immersing the wax pattern into a slurry of refractory material and allowing it to solidify.

The process is essentially the same as coating vanilla ice cream with a layer of chocolate. When the cold vanilla ice cream comes into contact with hot melted chocolate, the chocolate sticks to the ice cream and solidifies almost immediately, creating a thin shell of chocolate covering the ice cream.

Similarly, when the ceramic slurry comes into contact with the wax pattern, it adheres to it and solidifies. Several passes of the wax pattern are made, starting with a finer slurry and then a coarser one to achieve the desired thickness of the ceramic mould. The mould is then left to cool down.

5. Wax Removal

Once the slurry has hardened, it is placed in a furnace to remove the wax and for further sintering of the ceramic mould. Sintering is the process through which items are hardened through pressure and heat but without liquefaction. As the temperature rises, sintering hardens the mould while the molten wax is reclaimed for future use. This stage gives us the solid ceramic mould for the actual casting process.

6. Pouring of Molten Metal and Subsequent Cooling

After the wax removal stage, the mould is cooled for testing. If any cracks or other imperfections are detected, they can be rectified using ceramic slurry or special cement.

Upon successful test results, the mould is heated again before pouring liquid metal into it. This preheating ensures that the molten metal remains in the liquid state for a longer period until it uniformly fills the mould.

To pour the metal, the ceramic mould is inverted and placed in a sand-filled container. The molten metal is poured into the mould under gravity or external pressure.

Vacuum-assisted filling may also be used. It comes at a higher price point but offers significantly better resource efficiency compared to gravity and pressure-based filling.

The vacuum pulls the molten metal into the mould, and once all the crucial parts solidify, the vacuum is released, allowing the unused melt to drain out. This process minimises material solidification in the sprue and gates, resulting in up to a 95% material yield, compared to 15-50% in gravity pouring.

The material is then allowed to cool and solidify until it is hard enough to withstand separation from the ceramic shell mould.

7. Removal of Casting From the Mould

The divesting is typically carried out by hammering the mould to release the casting. Other methods to separate the metal casting from the mould include waterjetting, media blasting, vibration and chemical dissolution of the mould. The sprue, runners and other gating system components are then separated and recycled.

8. Post-Processing Operations

The investment casting process generally does not require subsequent machining. However, if the results are not as expected, the casting may need surface finishing operations before use. Typically, surface grinding with machine tools is adequate for polishing and trimming any surface defects. Nevertheless, depending on the situation, other finishing processes such as hand tooling, welding, and hydraulic straightening may be necessary.

Advantages of Investment Casting

The investment casting process offers the following advantages:

• Excellent dimensional accuracy even for complex components

• Smoother surface finish

• Works with a wide range of metals

• Parts are created as a single casting with no parting lines

• Compatible with low- and high-volume manufacturing

• Minimal wastage of resources

• Parts can be combined, eliminating the need for assembly operations downstream

• 90-degree angles can be cast without worrying about shrinkage allowance

• More environmentally friendly than other metal fabrication processes

Limitations of Investment Casting

The investment casting process also has certain limitations:

• A very high number of variables that can affect part quality. A high level of process control is essential for producing good-quality parts

• There is an upper limit on the size of investment castings, which is lower than that of some other casting processes such as sand casting

• Casting objects with internal cavities can be more challenging than other casting processes

• Longer production cycles and lead times

• Investment casting techniques are more expensive than other processes, especially when the quantity is low and the manufacturing process involves the use of permanent tools

Applications of Investment Casting

The industry uses investment casting to produce the following products:

• Turbocharger rotors and turbine blades

• Firearm components such as triggers, hammers and receivers

• Power generation and cooling system parts such as pumps, valves and combustor components

• Intricate jewellery with delicate components and tight tolerances

• Automotive parts including engine components, transmission components, brakes, door handles, gears, housings, brackets, rods and more

When to Choose Investment Casting

After learning about the investment casting process, let’s summarise our knowledge by noting the five scenarios where investment casting provides the best ROI:

Medium size range – Investment casting gives better results when the weight of the product ranges from a few grams up to 1,000 kg. Parts weighing more than 1,000 kg are better suited for the sand casting process.

Extremely complex geometry – Investment casting can create super complicated shapes such as turbine blades, medical equipment, dental crowns and inlays, power generation equipment, aerospace components and more.

High number of parts – Investment casting is not cost-effective when a small number of items need to be manufactured, as the process becomes more expensive. However, as the quantity of items increases beyond a certain threshold, the cost per piece can decrease below that of other methods, including sand casting.

Need for a high surface finish – Investment casting uses a hard mould with a very fine surface, allowing the process to achieve an exceptional surface finish.

Need for high dimensional accuracy – It is possible to cast net or near-net shapes through investment casting. The need for post-processing is also minimal with this process.