Investment Casting: Definition, Process, Pros and Cons

When it comes to investment casting of metal parts manufacturing factors such as design requirements, cost, and feasibility to manufacture dictate which casting process is most suitable to manufacture a product.

This article describing investment casting is intended to help you make an informed casting decision.

Investment casting produces precise components while minimizing material waste, energy, and subsequent machining. It can also ensure the production of very intricate parts. This makes the investment casting process quite useful to design engineers.

The goal is to understand what is meant by investment casting. So, What exactly is the investment in “investment” casting? The term “invested” historically carries the meaning of “clothed” or “surrounded.”

Investment casting employs a shell made of ceramic, plaster, or plastic that is formed around a wax pattern. The wax pattern is melted and removed in a furnace and metal is poured into the shell to create the casting.

What is investment casting used for? Let’s break down the investment casting production process for a greater understanding:

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 it Called Investment Casting?

The ancient art and science of investment casting is also known as the lost wax process.

Investment casting was developed over 5500 years ago and can trace its roots back to both ancient Egypt and China.

Parts manufactured in the industry by this process include dental fixtures, gears, cams, ratchets, jewelry, turbine blades, machinery components, and other parts of complex geometry.

How 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.

The Investment Casting Process

Castings can be made from an original wax model (the direct method) or from wax replicas of an original pattern that need not be made from wax (the indirect method).

The following steps describe the indirect process, which can take two to seven days to complete.

#1. Produce A Master Pattern.

An artist or mould-maker creates an original pattern from wax, clay, wood, plastic, or another material.

In recent years, the production of patterns using 3D printing of models created by computer-aided design software has become popular.

This is done mainly using resin-based Stereolithography (SLA) or DLP 3D printers for high-resolution patterns, or standard PLA filament when high levels of accuracy are not required. If you are using a 3D printed pattern, proceed directly to step 5.

#2. Create A Mould.

A mould, known as the master die, is made to fit the master pattern.

If the master pattern was made from steel, the master die can be cast directly from the pattern using metal with a lower melting point. Rubber moulds can also be cast directly from the master pattern.

Alternatively, a master die can be machined independently—without creating a master pattern.

#3. Produce Wax Patterns.

Although called wax patterns, pattern materials may also include plastic and frozen mercury. Wax patterns can be produced in one of two ways.

In one process, the wax is poured into the mould and swished around until an even coating, usually about 3 mm (0.12 in) thick, covers the inner surface of the mould.

This is repeated until the desired pattern thickness is reached. Another method involves filling the entire mould with molten wax and letting it cool as a solid object.

If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern; ceramic cores are removed after the product has hardened.

#4. Assemble Wax Patterns.

Multiple wax patterns can be created and assembled into one large pattern to be cast in one batch pour. In this situation, patterns are attached to a wax sprue to create a pattern cluster, or tree.

To attach patterns, a heating tool is used to slightly melt designated wax surfaces, which are then pressed against each other and left to cool and harden. As many as several hundred patterns can be assembled into a tree.

Wax patterns can also be chased, which means parting lines or flashings are rubbed out using the heated metal tool. Finally, patterns are dressed by removing imperfections to look like finished pieces.

#5. Apply Investment Materials

The ceramic mould, known as the investment, is produced by repeating a series of steps—coating, stuccoing, and hardening—until a desired thickness is achieved.

• Coating involves dipping a pattern cluster into a slurry of fine refractory material and then draining to create a uniform surface coating. Fine materials are used in this first step, also called a prime coat, to preserve fine details from the mould.
• Stuccoing applies coarse ceramic particles by dipping patterns into a fluidised bed, placing it in a rainfall-sander, or by applying materials by hand.
• Hardening allows coatings to cure. These steps are repeated until the investment reaches its required thickness—usually 5 to 15 mm (0.2 to 0.6 in). Investment moulds are left to dry completely, which can take 16 to 48 hours. Drying can be accelerated by applying a vacuum or minimizing environmental humidity. Investment moulds can also be created by placing the pattern clusters into a flask and then pouring liquid investment material from above. The flask is then vibrated to allow entrapped air to escape and help the investment material fill any small voids.

Materials: common refractory materials used to create the investments are: silica, zircon, various aluminum silicates, and alumina.

Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive.

Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have an alumina content from 42 to 72%; at 72% alumina the compound is known as mullite. During the primary coat(s), zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal.

Prior to silica, a mixture of plaster and ground up old moulds (chamotte) was used. The binders used to hold the refractory material in place include: ethyl silicate, colloidal silica , sodium silicate, and a hybrid of these controlled for pH and viscosity.

#6. Dewax.

Once ceramic moulds have fully cured, they are turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax.

Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it—as the wax is heated it expands and introduces stress.

To minimize these stresses the wax is heated as rapidly as possible so that outer wax surfaces can melt and drain quickly, making space for the rest of the wax to expand. In certain situations, holes may be drilled into the mould before heating to help reduce these stresses.

Any wax that runs out of the mould is usually recovered and reused.

#7. Burnout Preheating.

The mould is then subjected to a burnout, which heats the mould to between 870 °C and 1095 °C to remove any moisture and residual wax, and to sinter the mould.

Sometimes this heating is also used to preheat the mould before pouring, but other times the mould is allowed to cool so that it can be tested.

Preheating allows the metal to stay liquid longer so that it can better fill all mould details and increase dimensional accuracy. If the mould is left to cool, any cracks found can be repaired with ceramic slurry or special cements.

#8. Pouring.

The investment mould is then placed open-side up into a tub filled with sand. The metal may be gravity-poured or forced by applying positive air pressure or other forces.

Vacuum casting, tilt casting, pressure-assisted pouring and centrifugal casting are methods that use additional forces and are especially useful when moulds contain thin sections that would be otherwise be difficult to fill.

#9. Divesting.

The shell is hammered, media blasted, vibrated, waterjeted, or chemically dissolved (sometimes with liquid nitrogen) to release the casting.

The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.

#10. Finishing.

After grinding, the completed casting is then subject to finishing. This usually goes further than grinding, with impurities and negatives being removed via hand tooling and welding.

In the case that the part needs additional straightening, this process is usually carried out by hydraulic straightening presses, which bring the product in line with its tolerances.

What are the Common Defects in Investment Casting?

Investment casting is a complicated process with a lot of steps. Moreover, many factories do most of the process manually. So, it is common to have some defects in the parts if not handled carefully which can offset the investment casting advantages.

We have discussed some common defects of investment casting below,

• Shrinkage Crack. it is possible to get shrinkage cracks when ceramic shell mold will restrict molten metal from contracting during the cooling or solidification process. Irregular shapes, rough holes may form due to this.
• Porosity. Porosity can severely impact the quality of the parts.  Sometimes air entrapment occurs during the pouring or casting of molten metal. Without the proper precautions, It can also form during the cooling process.
• Inclusions. It is the inclusion of other materials into the casting. A very common form of inclusion defect is refractory material getting entering the wax model through unintended cracks.
• Misrun. Misrun is a very common type of defect. It occurs if the investment casting mold isn’t filled by the molten metal. It usually happens if the metal starts solidifying during pouring, and the mold or metal is too cold.
• Deformation. Sometimes deformation in the casting may occur during the solidification process. This type of defect may happen due to shrinkage stress.
• Cold Shuts. When molten metal entering two streams inside a mold fails to merge properly, it is known as a cold shut. It usually happens due to the cooling of the metal streams before they meet each other.

Advantages of Investment Casting

Investment casting has various advantages that make it a critically important manufacturing process. These include:

• Complex geometries, thin walls, and fine features are relatively easy to achieve reliably, providing remarkable design freedom.
• The dimensional accuracy and repeatability of castings are very good, assuming the patterns are regular and identical.
• Surface finishes are generally good and the process can be tuned to deliver exceptional surface quality and precisely defined textures.
• Few limitations exist in material selection for casting—including: resins, ceramics, cement, most metals/alloys, and glass.
• General properties in the cast part faithfully reproduce the properties of the raw material.
• Material wastage is very low, as all feed/sprue parts can be re-used (except thermoset resins).
• Complex assemblies of multiple components can be consolidated into a single investment cast, reducing assembly time and potential failure points.
• Tooling costs are low (in comparison with die casting).
• Less post-processing is required than with typical sand casting.

Disadvantages of Investment Casting

Despite the beneficial nature of lost-wax casting, it does have its drawbacks. The most significant disadvantage is the size limitation.

Because so few design engineers can produce large parts using this technique, it may not be the best option for a client who needs to fabricate a bulky component.

This process is best for casting small intricate components. Parts exceeding 75 pounds are better suited for other casting methods.

Investment casting shells also have limitations on their size and depth. They cannot be smaller than 1.6mm or deeper than 1.5 times the diameter. If your casting design requires cores and cannot fit these requirements, you may need to seek another process.

This technique is more complicated than other casting processes, and it requires a substantial amount of preparation and specialized equipment.

As a result, the upfront cost of investment casting can be more expensive than sand casting or die casting, but the production cost per unit decreases with larger orders.

When to use investment casting?

Due to its complexity and labor requirements, investment casting is a relatively expensive process – however the benefits often outweigh the cost.

Practically any metal can be investment cast. Parts manufactured by investment casting are normally small, but the process can be used effectively for parts weighing 75 lbs or more.

Investment casting is capable of producing complex parts with excellent as-cast surface finishes. Investment castings do not need to have taper built in to remove the components from their molds because the ceramic shells break away from the part upon cooling.

This production feature allows castings with 90-degree angles to be designed with no shrinkage allowance built-in, and with no additional machining required to obtain those angles.

The investment casting process creates parts with superior dimensional accuracy; net-shape parts are easily achievable, and finished forms are often produced without secondary machining. Each unique casting run requires a new die to produce wax patterns.

Tooling for investment casting can be quite expensive; depending on the complexity, tooling costs can run anywhere between $1000 and $10,000.

For high volume orders, the time and labor saved by eliminating or decreasing secondary machining easily makes up for the cost of new tooling. Small casting runs are less likely to make up for the investment. Generally, investment casting is a logical choice for a run of 25 parts or more.

It usually takes 7 days to go from a fresh wax pattern to a complete casting; the majority of that time is taken up by creating and drying the ceramic shell mold. Some foundries have quick-dry capabilities to produce castings more quickly.

The time and labor-intensive nature of investment casting doesn’t only effect cost. Foundries have limited equipment and production capacity, so longer lead times for investment casting are common.

FAQs.