In this context, investment refers to the “putting on of clothes or coverings.” Literally, a sacrificial wax object is reproduced in metal by first investing it with a covering or ceramic and then burning out the wax to leave a perfect cavity in refractory material. Molten metal is then poured into the cavity and allowed to cool and solidify, reproducing the fine details of the original wax pattern.
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Investment (or lost wax) casting is an ancient process. The story of lost wax metalworking is the story of Bronze Age technological innovation. In the modern world, investment casting still has a highly active place in jewelry making. More importantly, the process is used for precision net-shape production of otherwise near-impossible objects like complex and hollow turbine blades. The mass production of such parts is achieved by first CNC machining steel molds into which the wax can be rapidly cast. This enables a degree of repeatability that could not be achieved by individually hand-made masters. This article will further discuss investment casting, how it works, its advantages, and give some examples.
Investment casting is a precision casting process used to create complex and intricate metal parts with high dimensional accuracy and fine surface finishes. This casting method is ideally suited to producing relatively high-quality components with intricate details, delicate features, and tight tolerances.
Investment casting is commonly used in industries such as: aerospace, automotive, jewelry, and medical devices, in which high precision and intricate designs are essential. It also produces castings with very tight tolerances and high dimensional accuracy. Investment castings typically have smooth and fine surface finishes. Investment casting can be used with a variety of materials, including various metals and alloys. It can also produce parts with complex shapes, internal cavities, and thin walls that are challenging for other casting methods.
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.
To learn more, see our guide on Casting Methods.
Investment casting is most used in industries in which precision, complexity, and fine details are crucial, and volumes are insufficient to justify the cost of die casting. The aerospace industry uses investment casting for the manufacture of high-complexity gas turbine components for gas flow such as turbine blades and fuel components. In automotive applications, it is used to manufacture components for low-volume and race vehicles, heavy goods/construction equipment, tractors, and agricultural machinery. In the medical sector, it is used to produce parts for MRI machines, wheelchairs, hospital beds, operating room equipment, and surgical tools.
Investment casting works by pre-applying coatings to the pattern—before investment casting mold creation, or to the mold—once it is fired. This serves several important purposes. Refractory coatings are applied to the interior surface of the mold to provide a barrier between the mold material and the molten metal. This reduces oxidation, contamination, and marking. An internal stucco layer in the mold can improve the release and texture of the surface to improve cosmetics by concealing slight flaws. Parting compounds applied to the mold surfaces ease the release of the casting at breakout.
Thermal barrier coatings applied selectively to the pattern can control the cooling rate and prevent rapid solidification. Anti-penetration coatings applied to the pattern can reduce the penetration of melt into the refractory material, to improve the quality of the surface finish.
Listed and discussed below is the step-by-step process of investment casting:
There are three basic routes to delivering the precision wax patterns that are the starting point for investment casting. For decorative items and one-off products such as jewelry or technical items for which precision is a lower priority, it is common to hand carve the wax master to the desired dimensions/shape. This starts with either assembling preformed wax shapes to form the part or extracting the part from a solid block by cutting or melting locally. For mass-produced items that will be investment cast in relatively large volumes, it is generally preferred to build a steel tool that forms a wax casting cavity. A more recent development has been the ability to 3D print (rapid prototype) relatively small wax parts, to allow volume production without the cost of a wax casting tool. This is particularly useful in iterative development, allowing multiple generations of small changes to be trailed without huge cost.
Attach a feeder structure consisting of a sprue and channel to the part to allow the fill to be successful. Attach the part and feeder structure to a casting board that forms the structure to allow the flask (the completed cast tool) to stand securely. Any areas that are liable to vent poorly require air passage features to be added. Where multiple parts are to be cast in one pour, these structures are best established as individual cavities/galleries. Feed them from a single sprue/pour point. Care and experience are required to enable effective fill when the gallery structure or part geometry is more complex.
A first immersion in the correctly formulated slurry will coat the surfaces of the pattern/sprue/support structure. Once the assembly is coated, it can be withdrawn and allowed to drip. Once drained, apply a layer of stucco—dry powder sprinkled all over to stabilize the first layer. Repeat the dip/drain/stucco steps until sufficient thickness is developed all over. The rest/drying time between layers can be several hours, to allow sufficient drying and stabilization for each layer. Set aside to completely dry, giving this enough time for the coating to become firm and rigid.
Bake the completed and dried flask to remove the wax and fully integrate the refractory coating. The supplier-recommended temperatures and durations for this stage are dependent on the type of material such as: water glass, clay, and silica. At the end of this baking process, the mold or flask will be fully fused into a single, strong body. The wax will have been fully volatilized or burned off. The cavity or cavities, sprue, and galleries will be clear and open. The support structure of investment and braces will be able to support the structure, ready to fill.
Lift the crucible with tongs and pour the contents into the upper opening to the sprue. Fill/charging should continue until the entire structure is full. This is usually confirmed by the visible presence of molten metal in the upper vents, showing good flow through.
When the cast has cooled, break away the refractory material without distorting the newly cast metal. In some cases, such as for simple and robust castings in stainless steel and bronze, this can be a vigorous and rapid process. Where more delicate features and softer metals such as gold and silver are present, much greater care must be taken.
The removal of cast material from sprue, vents, and supports will damage the desired casting. Any cosmetic damage can be repaired using die grinding, sawing, and clipping followed by surface restoration by sanding and polishing as required.
The Investment Casting Institute recently completed its 71st Annual Technical Conference and Exposition in Milwaukee – the highlight of which may well have been the results of its 23rd Investment Casting Design Contest. Investment castings is a high-precision process that demands skill and precision from the operators, and frequently reveals those capabilities in the way they duplicate the fine details and extensive geometric variety in a single form.
The annual contest presents awards to the best examples of product design, development, and execution using investment casting in six product areas – automotive, aerospace, defense, oil and gas, power generation, and fine art casting.
This mesh trimmed bracket produced by Aristo-Cast, Almont, MI, is a critical element in a chassis node used to support a plastic automotive chassis column. Cast in 17-4 PH stainless steel, the 3.247 x 2.236 x 2.134-in. part replaces a 3D metal printed component.
This investment casting must withstand the rigors of impact crash testing, while staying rigid inside the plastic column without cracking near the node – and it must be lightweight. These requirements were achieved by adopting the investment cast thin-walled mesh structure.
Aristo-Cast used printed investment casting shells (PICS) in the production of this mesh bracket, which provides improved material integrity and can withstand more impact force and has improved dimensional stability when compared with its 3D-printed metal predecessor. Taking this approach, the customer realized a 35% pricing reduction.
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It was Aristo-Cast’s first application where the metal part was designed to be over-molded into a plastic chassis column to form the column’s node.
Turbo Cast’s customer developed a new product, the E-Fan, to be used as a cooling solution for Urban Air Mobility. The customer used 3D-printed metal components to validate the concept, after which they turned to Turbo Cast for a production solution.
Turbo Cast was contacted to evaluate the feasibility of part development. Using the investment casting process, they produced the E-Fan’s Housing, successfully reducing manufacturing cost. This component is characterized by an intricate geometry, tight dimensional tolerances and 0.8mm wall thickness.
In consideration of this challenge, investment casting proved to be the only option to make this part. Turbo Cast worked closely with the customer’s design team to understand the application and part requirements. After more than one year of concurrent engineering, the design was finalized, and Turbo Cast successfully developed a robust investment casting process.
A Virginia Tech professor presented a challenge to students – and student Lillian Coussens found that making the dragon was an exciting opportunity to synthesize everything that she had learned about investment casting.
Coussens’ art piece was modeled from an existing figure, having an intricate shape with both thick (>25 mm) and thin (<1.7 mm) section sizes. Many metalcasting principles were demonstrated by the casting, including creating multiple silicone rubber molds from the original; creating multiple wax patterns; assembling the wax patterns into a complete dragon; creating an investment shell mold on the wax pattern assembly, melting out the expendable wax pattern; and then melting and pouring silicon bronze (Everdur, UNS C).
The complex shape of the dragon posed many design challenges; the silicone rubber mold must allow the fragile wax patterns to be removed without breaking and the final investment shell mold must allow the flow of metal into challenging areas, such as the thin, high surface area wings. Problem-solving was required to repair any broken expendable pattern pieces, and to develop strategies to fill thin areas of the silicone rubber mold with wax.
This housing, formerly made from a three-piece sand-cast assembly, is a critical component for the Abrams Tank turbine engine lubrication pump. Atlantic C&E’s large, single-piece A356 investment casting is a complicated, cored component that is credited with a significant cost saving to the customer by reducing machining and assembly requirements. This component has also demonstrated improved reliability and performance.
By switching from sand casting to the investment casting process the customer was able to obtain components with a superior surface finish, which is particularly advantageous in the winding interior passages, providing a more efficient part at a lower price.
Atlantic C&E was also able to hold tighter overall tolerances when compared to sand castings, giving the customer a vastly superior product. Current production runs at over 95% yield for an uninterrupted supply of castings to the customer.
The casting has 26 intricate cores throughout the part, ranging in diameter from 0.25 to 2.5 in., with a dimensional tolerance of ±0.010 in. To lighten the amount of investment material in and around the part, Styrofoam is placed in the flask prior to investment, which allows the material to form around it. This uses less investment material in the mold, which reduces cost.
The part has both thick and thin areas ranging from 0.160 to 1.06 in. thick.
To produce reliable molds the part is invested under vacuum, which helps to create sound investment in the tight, winding cores that must be free of defects and held to tight tolerances.
This casting is poured under vacuum with 100% ingot to avoid defects. As a result, the optional Hot Isostatic Pressing (HIP) and Impregnation formerly used are not needed.
A stabbing guide is used in oil rig operations: It clamps around drill pipe to provide personnel with a secure way to hold pipe while being moved by an overhead crane. The traditional manufacturing method involved the customer procuring raw castings that demanded subsequent machining and welding.
Upon receiving the initial RFQ and accompanying photos of the finished product, TPM identified several opportunities for enhancement through investment casting, allowing the company to optimize its design and manufacturing process. These enhancements afforded the customer multiple advantages over their original approach.
The original assembly was comprised of a single part, used in pairs to make up both sides of the stabbing guide. This approach resulted in large blocks at each end that required extensive machining to fit the latch and hinge assemblies. With the recommended modifications, the component now includes near-net-shape features that necessitate only minimal machining.
Additionally, the handle, which was formerly attached by welding, required the customer to purchase steel rods and perform shaping and welding of the handle to the assembly. The new approach integrally casts the handle directly onto the component. TPM was able to accomplish these design changes with tooling modifications that use interchangeable inserts to make left and right-side parts, as well as adding the handle section. This change not only enhances the handle’s strength but also provides a more professional finish.
As a result, the customer achieved approximately 42% cost savings by reducing machining and eliminating welding operations while also offering a much more aesthetically pleasing part.
This diffuser casting used in nuclear power generation was originally fabricated by welding outsourced plates of T6 material, which required machining. The resulting product was expensive, with poor aesthetics and excess production lead time.
Due to the excessive lead time and manufacturing cost, the customer tried producing this component by sand casting in LM 25 material. This approach resulted in poor radiographic quality. After multiple failures, the customer turned to Turbo Cast.
Turbo Cast produced this component as a single-piece investment casting, improving dimensional conformity and aesthetic quality while reducing manufacturing cost and lead time.
By converting this part from a sand casting to an investment casting, Turbo Cast successfully met the customers cost and lead time requirements while providing an aesthetically pleasing component.