Investment Casting: Choosing The Right Material For Your Project

Investment casting, also known as precision casting or lost-wax casting, is an ancient manufacturing process that uses a wax pattern to shape a disposable ceramic mould. The wax pattern is a replica of the final product geometry. The wax is then melted out, leaving a cavity in the shape of the final product. Molten metal is poured into the cavity, where it solidifies and cools. The ceramic is then removed, leaving a precise metal component.

The process

Investment casting, also known as lost-wax casting, is an ancient manufacturing process that has been used in various forms for the last 5,000 years. It is a near-net shape process that can produce components with accuracy, repeatability, versatility, and integrity in a variety of metals and alloys.

• Create the master pattern: A master pattern serves as a standard that can be reused for multiple casting cycles. Common materials for manufacturing the master pattern include wax, wood, metal, and silicone rubber. This step may be done using advanced techniques like 3D printing or die casting.
• Assemble the wax pattern: The wax patterns are assembled into a single investment casting mold, known as a 'tree'. A sprue/runner/riser network is attached to allow the metal to flow smoothly to all parts of the mold. This assembly process can be manual or robotic, depending on the volume and design of the casting.
• Apply refractory materials (dipping): The tree is dipped repeatedly into a bath of ceramic slurry and sand stucco to achieve the desired properties. This process creates a ceramic shell around the wax pattern, increasing its thickness and temperature resistance.
• Dewaxing: The wax inside the ceramic shell is removed by melting it in an autoclave or flash fire furnace. This step is crucial as intense temperatures can induce defects in the mold shell.
• Preheating and casting: The mold is preheated to minimize thermal shock when it comes into contact with the liquid metal. The liquid metal is then poured into the mold in a controlled manner to minimize turbulence-related defects.
• Finalize the castings: Once the metal has cooled, the refractory material is broken away from the newly cast metal. This step requires more care if the casting has delicate features or is made of softer metals like gold or silver.
• Post-processing: The cast material is separated from the sprue, vents, and supports, and any cosmetic damage is repaired. The part then undergoes semi-finishing operations such as grinding, blasting, milling, and drilling to achieve the required surface quality.
• Quality inspection: The part is inspected for defects such as cracks and porosity, as well as other quality factors like geometric and dimensional tolerances, hardness, and strength.

Materials

Investment casting can be used with a wide range of materials, including stainless steel alloys, brass, aluminium, carbon steel, glass, copper alloys, and various other metals and alloys.

Materials used

Investment casting is a manufacturing process that uses a wide range of materials to create precise, complex, and intricate shapes with high dimensional accuracy and fine surface finishes.

Investment casting, also known as lost-wax casting, uses a variety of materials to create precise and intricate shapes. The process involves creating a wax pattern, which is then coated with a refractory ceramic material. This ceramic material is often made from silica, zircon, aluminium silicates, or alumina. The specific type of silica used is fused silica, although quartz is sometimes used as a more cost-effective alternative. Aluminium silicates are a mixture of alumina and silica, with alumina content ranging from 42% to 72%. When the alumina content reaches 72%, the compound is known as mullite.

The choice of refractory material depends on the desired properties of the final product. For example, zircon-based refractories are commonly used during the initial coating stages as zirconium is less likely to react with molten metal. The binders used to hold the refractory material together include ethyl silicate, colloidal silica, sodium silicate, or a hybrid of these substances, controlled for pH and viscosity.

Once the ceramic coating is dry and hardened, the wax is melted and drained, leaving a cavity in the shape of the desired product. Molten metal is then poured into this cavity. The metals used in this process include stainless steel alloys, brass, aluminium, carbon steel, and even glass.

The versatility of investment casting allows for the production of small and large castings, ranging from a few ounces to several hundred pounds. The process is particularly suitable for small parts produced in large volumes.

Other Materials

In addition to the materials used for the refractory coating and the molten metal, there are other materials involved in the investment casting process. The wax pattern can be made from wax, wood, metal, or silicone rubber. The choice of material depends on various factors, such as pattern life, design complexity, thermal expansion and contraction, and surface quality.

The mould, or master die, can be made by casting directly from a master pattern made of steel or rubber. Alternatively, the master die can be machined independently without creating a master pattern.

Cores, which are used to form internal features, can be made from soluble wax or ceramic. Soluble wax cores melt out of the investment coating along with the rest of the wax pattern, while ceramic cores are removed after the product has hardened.

Investment casting is a versatile process that can utilise a wide range of materials to create precise and intricate shapes. The specific materials used depend on the desired properties of the final product, with options available to suit various applications and industries.

Industries that use it

Investment casting, also known as lost-wax casting, is a manufacturing process that uses a ceramic shell built over a wax pattern to produce parts with smooth surfaces and high dimensional accuracy. It is used in a wide range of industries, including:

Aerospace

Investment casting is used in the aerospace industry to manufacture complex and intricate components for gas turbines, such as turbine blades and fuel components. The process allows for the production of parts with tight tolerances and fine surface finishes, which are crucial for aerospace applications.

Automotive

In the automotive industry, investment casting is used to manufacture components for low-volume and race vehicles, heavy goods/construction equipment, tractors, and agricultural machinery. It enables the production of intricate parts with complex shapes and thin walls, which are challenging to create using other casting methods.

Medical

The medical sector also utilizes investment casting to produce parts for MRI machines, wheelchairs, hospital beds, operating room equipment, and surgical tools. The precision and accuracy of the process ensure that the resulting parts meet the stringent requirements of the medical field.

Jewelry

Investment casting is commonly used in jewelry-making, especially for one-off and limited-edition pieces. The process allows for the creation of intricate and delicate designs with fine details, making it ideal for jewelry applications.

Military

The military industry also employs investment casting for various applications, including firearms fabrication. The ability to produce complex and accurate parts makes investment casting valuable for military equipment and weapons manufacturing.

Other Industries

In addition to the industries mentioned above, investment casting is used in power generation, electronics, semiconductors, and textile industries. It is a versatile process that can be adapted to meet the specific needs of different sectors.

Benefits

Investment casting, also known as precision casting or lost-wax casting, is a manufacturing process that offers several benefits. Here are some key advantages:

Versatility and Intricate Shapes

Investment casting provides the ability to produce complex and intricate shapes with high precision. It allows for thin walls, internal cavities, and delicate features that would be challenging or impossible with other casting methods. This versatility enables the creation of parts with exceptional surface finishes and tight tolerances.

Dimensional Accuracy and Repeatability

The process delivers superior dimensional accuracy, with net-shape or near-net-shape castings. The use of wax patterns ensures that the final product is an exact replica of the desired part. This accuracy and repeatability reduce the need for secondary machining, saving time and costs.

Wide Range of Materials

Investment casting can be used with a broad range of materials, including most metals and alloys. Common materials include stainless steel, aluminium, brass, carbon steel, and even glass. This versatility allows for the production of parts with various desired properties.

Reduced Material Waste and Costs

The investment casting process minimises material waste, energy consumption, and subsequent machining. It also reduces the need for complex assemblies, as multiple components can be consolidated into a single investment cast, reducing potential failure points. This simplification can lead to significant cost savings in material, labour, and machining.

Smooth Surfaces and Fine Details

The use of ceramic moulds results in castings with smooth and fine surfaces. Investment castings have no parting lines, as only one mould is used, enhancing the overall aesthetics and quality of the final product.

Time and Cost Savings

While investment casting may have higher initial costs due to labour and equipment, it can offer time and cost savings in the long run. The precision and accuracy of the process reduce the need for secondary machining and finishing operations, streamlining production and lowering overall costs, especially for high-volume orders.

Limitations

Investment casting, also known as lost-wax casting, is a manufacturing process that has been used in various forms for the last 5,000 years. It is a versatile process that can be used with a wide range of materials, including various metals and alloys, resins, ceramics, cement, and glass. It is commonly used in industries such as aerospace, automotive, jewellery, and medical devices, where it is valued for its ability to produce complex and intricate parts with high accuracy and fine surface finishes.

However, there are some limitations to the investment casting process:

• Cost: Investment casting is a relatively expensive process due to its complexity and labour requirements. The cost is particularly high for short-run productions, as the per-component cost is higher than other casting methods such as sand casting or die casting. However, the benefits of investment casting often outweigh the cost, especially for high-volume orders where the time and labour saved by eliminating or decreasing secondary machining can make up for the initial investment.
• Size and weight limitations: Investment casting is typically used for small parts, and the size and weight of one-piece castings are more limited compared to other casting methods like sand casting. While it can be used for parts weighing 75 lbs or more, there is an upper limit to the size and weight of castings produced by investment casting.
• Longer production cycles: Investment casting requires longer production cycles compared to other casting processes, which can result in longer lead times. This is due to the time- and labour-intensive nature of the process, which includes multiple steps such as wax pattern production, ceramic mould creation, pouring, solidification, and cleanup.
• Difficulties with cores: It can be challenging to cast objects that require cores, and there are limitations on the size of holes that can be created. Holes cannot be smaller than 1/16" (1.6 mm) and should not be deeper than about 1.5 times the diameter.
• Susceptibility to defects: The many steps in the investment casting process leave room for errors, and occasional minute defects can occur. One of the critical stages is the dewaxing process, where the wax is removed from the ceramic shell, as intense temperatures can induce defects in the mould shell. Cracking is a common concern that engineers try to avoid during this stage.
• Quality management challenges: There are many process factors that can affect the quality of the mould and casting, making quality management a challenging aspect of investment casting.

Frequently asked questions