The Complete Buying Guide: Investment Casting

What is investment casting?

Investment casting is also known as lost wax casting or precision casting.

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This is a manufacturing technique in which wax patterns are used to mold disposable ceramic molds.

The wax pattern takes the shape of the desired product (the casting).

The wax pattern is often coated using a refractory material, primarily a ceramic slurry.

Once this material dries to harden, it is heated to remove the wax.

The hard ceramic shell obtained is then filled up with molten metal and left to solidify. After solidification, the casting is broken out from the shell, and it is called an investment casting product. 

Generally, the term investment casting came from the process itself, which is investing or surrounding the wax pattern with a refractory material.

Precision investment casting is often preferred over other casting techniques because it guarantees fine surface finishes on every product manufactured. Investment casting products can be produced with thin walls and complex designs. 

When you compare investment casting vs. sand casting, you realize that investment casting is far superior and less costly because it does not require drafts. 

The investment casting process steps may be a bit complex to follow.

However, the entire process is cost-effective, since it does not require final post-processes, such as machining. Additionally, lost wax investment casting can be used with almost every metal and metal alloy, making it very convenient. 

Investment casting applications are vast. Investment casting is used for the manufacture of medical equipment, firearm parts, golf products, turbine blades, jet engine components, and other parts with complex geometry. 

What is lost wax investment casting?

Investment casting is considered a solution to manufacturing parts with complex geometry and in large quantities.

This is achieved by injecting wax into permanent dies to create the wax pattern. Investment casting manufacturers can also produce investment castings in lost wax patterns.

These are economically viable products when produced in low volumes using manual tooling or high volumes using automatic systems. 

The history of investment casting, also known as the lost wax investment casting process, can be traced back to BC.

During this period, products were mainly hand-sculptured using wax patterns with fine detail.

After the Second World War, the technique evolved and expanded into almost every industry to what it is today. 

The investment casting process

The investment casting process consists of many steps.

Even though every manufacturer may have a different way of producing investment casting products, these steps are often the same throughout. 

Constructing metal die

The wax patterns and ceramic molds are always destroyed to remove the casting in precision investment casting.

Therefore, every production cycle ion investment casting requires the creation of a new wax pattern.

Adie or mold is created to construct the wax pattern.

This may not be the case when the process is used to make products in small volumes.  

The size of the die or mold is predetermined after considering factors such as shrinkage to ensure it fits well. 

Manufacturing wax patterns 

Creating molds

At this phase, a gating system is attached to the mold.

Smaller products may need several wax patterns attached to form a tree assembly. 

The resulting assembly is dipped in a refractory material several times to achieve the required thickness.

The mold is then left to dry and harden. Upon solidification, the ceramic mold is heated to dewax, leaving a hollow ceramic shell. 

Pouring 

Molten metal is poured into the mold cavity through the gating system.

The mold shapes the molten metal to achieve every detail of the desired product. 

Cooling 

The metal in the mold is left to cool and solidify. The time taken to solidify depends on the size and thickness of the mold and the product. 

Shakeout 

After the casting has cooled and solidified, the ceramic mold is broken to remove the product. The mold can be broken manually or using water jets. 

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Finishing 

Post-processes may be applied, though not necessary since precision investment casting produces parts with fine surface finishes. 

Advantages and disadvantages of Investment casting

Advantages of investment casting

• Investment casting can be used to manufacture parts with complex geometry and intricate features. 
• Precision investment casting can guarantee the production of products without cold shut defects when the length is kept short and the depth shallow.  
• The lost wax investment casting process guarantees excellent dimensional precision and tighter tolerance. 
• Compared to other casting techniques, investment casting achieves an excellent surface finish, eliminating the need for post-processes. 
• Lost wax investment casting offers freedom inters of investment casting materials used. Investment casting manufacturers can use both ferrous and non-ferrous metals and metal alloys.
• The wax used in investment casting can be reused, making it cost-effective. 
• The process eliminates the need for additional machining.
•  The manufactured parts do not have flash since the process does not have parting lines.

Disadvantages of investment casting

• The investment casting process is very complex, with many steps that require training. This aspect makes the technique more expensive compared to other casting methods. 
• Dies used in investment casting to make wax patterns are expensive. This aspect limits precision casting to large production volumes.
• The process requires specialized equipment. It also involves a high cost of labor and refractory materials.
• Parts that require a core with holes smaller than 1.6 mm or deeper than 1.5 times the diameter are challenging to manufacture. 

Conclusion

In precision investment casting, wax patterns are dipped in refractory material, mainly ceramic slurry.

Once the pattern is coated and hardened, the process is repeated to achieve the desired thickness and strength.

The resulting coating is dewaxed by heating to make the mold.

Molten metal is poured into the mold and left to solidify. 

The mold is then broken away to reveal the final product. 

Investment casting has been applied in almost every industry due to its vast benefits.

This article is generated to help you understand every aspect of the investment casting process.

You can get similar resources in the forms of investment casting process pdf or investment casting ppt.

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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.

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.

What Is Investment Casting?

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. 

How Investment Casting Differs 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.

To learn more, see our guide on Casting Methods.

Industries That Use Investment Casting

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.

The Basics of Investment Casting

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.

Step-by-Step Process of Investment Casting

Listed and discussed below is the step-by-step process of investment casting:

1. Create and Assemble the Wax Patterns

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.

2. Mount the Wax Patterns

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.

3. Make a Mold Shell

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.

4. Remove the Wax

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.

5. Melt and Cast the Metal

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.

6. Finalize the Castings

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.

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