Investment casting is an excellent manufacturing process for precision net-shape production of metal parts or components.
It is perfect for achieving high accuracy and complex geometric casts to a near-final form, thereby minimizing resource waste.
This article gives an investment casting definition, its step-by-step process, casting materials, and its benefits and disadvantages in simple terms.
What is Investment Casting?
Investment casting is the covering of a wax pattern of a final product in refractory ceramic to form an internal cavity after melting and draining out the wax.
After this, investment casting foundry technicians pour molten metal into the ceramic mold, allowing it to cool to form a net or near-net metal part or component.
‘Investment‘ in this context is derived from its historical meaning, dating back over 5000 years, which means to ‘clothe’ or ‘surround’ a pattern with ceramics.
Why Is Investment Casting Also Called the Lost Wax Casting?
Investment casting is also known as lost wax casting or precision casting. It is easy to understand why it is called precision casting since it offers an exquisite cast surface finish with little to no machining required.
But why is it also known as ‘lost wax casting?’
This is simple. During manufacturing, the pattern maker makes a wax pattern that mirrors the desired final component through injection molding. Then, they attach the pattern to a wax sprue to form a tree.
They thereafter dip the tree into refractory ceramic repeatedly until the desired thickness is achieved. At this point, they allow the ceramic coating, or ‘investment,’ to dry and then heat it to further reinforce the ceramic and cause the wax to melt out of the ceramic mold.
When this is done, the wax is lost from the mold. This is why the process is referred to as lost wax investment casting.
The Investment Casting Process
The investment casting process can be explained in 6 easy steps, and they are as follows:
1. Making of the wax pattern
Making a lost wax casting wax pattern is integral to the entire casting process. The pattern is made in the exact shape of the component to be cast.
Pattern makers craft wax patterns by hand, particularly for singular ornate jewelry or art production. More popularly, they produce it by injecting molten wax into a metal die and leaving it to dry.
In the case of large productions, pattern makers employ rapid prototyping techniques like fused deposition modeling (FDM), selective laser sintering (SLS), stereolithography, and polyjetting using 3D Computer-aided Design (CAD) data.
When making wax for investment casting, they measure dimensions by making allowances for the thermal contraction of the wax, ceramic, and metal.
In place of wax, they also use plastic and frozen mercury patterns.
2. Assembling the wax pattern
After making the precision casting wax pattern, the assembler assembles it into a wax sprue. A wax sprue consists of a central sprue, a runner system, and a feed system to form a tree or pattern cluster.
The wax sprue serves as a structure for the pattern and as a passageway through which the wax can be melted out of the ceramic mold after investment. It also allows the molten metal to be introduced into the ceramic mold.
3. Ceramic investing
After assembling the tree, robots dip or ‘invest’ it in a ceramic slurry containing colloidal silica and alumina in the dipping room. They then dip it into a coarser ceramic called stucco for further coating.
After drying the first coat, the robots repeat this step until they achieve a thick ceramic mold around the wax pattern.
The mold technicians calculate the thickness based on configurations such as the size, weight, and pouring temperature of the metal to be cast.
4. Dewaxing
After investing, the dewaxing operator dewaxes the coated tree by heating it upside down in a steam autoclave. The mold is then placed in a flash-fire dewaxing furnace to melt residual wax.
It also reinforces the ceramic mold, which is in the shape of the desired component to be cast.
5. Casting
The casting technicians preheat the ceramic mold to cure it before pouring molten metal into it. They then leave it to solidify.
6. Ceramic mold removal and finishing
After the metal cools and solidifies, the finishing technicians remove the ceramic coating through chemical cleaning, blasting, high-pressure water jets, or hammering. They then cut the casting from the sprue by sawing, vibration, or liquid nitrogen and clean it.
They conclude the investment casting process steps by shot blasting, sandblasting, grinding, and machining, where necessary.
Additionally, they can put the final part through steps such as vacuum solution annealing, metal hardening, tempering, and precipitation hardening.
Finally, they conduct tests such as magnetic particles, fluorescent penetrants, and radiographic inspections before employing surface treatments like painting.
Different Types of Investment Casting Materials
The type of material you use for precision investment casting depends on factors such as the final product, cost, and design requirements.
The materials suitable for lost wax casting are:
- Aluminum alloy
- Copper alloy
- Cast steel, which includes alloy steel, carbon steel, and stainless steel
- Cast aluminum
- Cast iron
- Steel
- Bronze
- Magnesium
- Beryllium copper
- Super alloy
- Ductile iron
- Cobalt
The Advantages of Investment Casting
Investment casting advantages and disadvantages help determine if investment lost wax casting is the best for your casting project. Here are investment casting advantages:
- It produces high-quality, intricate geometric casts with net or near-net precision.
- With precision investment casting, engineers achieve products with tighter tolerance, improving accuracy.
- It creates complicated designs, which would have been impossible with other casting methods.
- Even though the wax and ceramic mold are destroyed during lost wax casting, they can be recycled and reused.
- It can cast various products, from thin-wall casts with complex passageways to larger models.
- It can produce casts with different materials, such as aluminum, cast iron, magnesium, and steel.
- With its near-net precision production, lost wax casting requires minimal machining, thus reducing material waste.
The Disadvantages of Investment Casting
Investment casting disadvantages are as follows:
- The process is complicated and time-consuming, making it more expensive than sand casting or die casting. However, the cost per part decreases for high-volume productions.
- The difficulty of casting large parts or components has limited the number of engineers who produce bulky components with lost wax casting investment. So, they cast parts over 75 pounds using other casting methods.
- It is labor intensive, leading to high labor costs.
- Precision casting is disadvantageous because casting small holes is challenging, making it impossible to cast cores with it.
What is Investment Casting Used For?
Precision casting creates high-quality, complex patterns with high accuracy and tolerance. It is typically used when products cannot be molded, forged, or pressure cast in sand or plaster.
So far, engineers have employed it in producing machinery and devices for several industries, such as agriculture, aerospace, automotive, medical, military, firearms, energy, mining, energy, electric vehicles, and firearms.
Here are some of the products that lost wax casting is used for:
- Firearms and gunnery components
- Dental implants
- Limited edition jewelry
- Armored vehicle components
- Boat steering wheels, cleats, and anchor
- Firefighting machinery
- Motor mounts and housings
- Automotive engine cylinder head
- Fitness equipment
- Pumps and valves
- Security Hardware
- Snow plow parts
- Turbine blades and engines
Investment Casting Manufacturers
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Conclusion
Investment casting, also called precision and lost wax casting, is an integral method in metal part manufacturing. It involves making a shell around a wax pattern using ceramic and pouring melted metal into the cavity of the ceramic mold after the wax is melted out. Manufacturers use this casting method to make products in different industries, such as medicine, agriculture, aerospace, military, and defense. It is mostly effective for intricate designs and impossible for sand or die casting.
