Investment casting is one of the most diverse casting methods used in manufacturing. It is the preferred choice of engineers as it offers high dimensional accuracy, smooth surface finish, design complexity, fast production, and more benefits.
In this article, we explore the ins and outs of what is investment casting. We will briefly touch upon its history, discuss its process step-by-step in detail, and go over its applications, material capabilities, and advantages/disadvantages.
Overview of Investment Casting
Investment casting, also known as a lost wax casting, utilizes expendable patterns made of wax-like materials to manufacture complex parts with high precision. It is a near-net shape process with incredible surface quality since it uses ceramic molds with elite thermal qualities.
Its numerous benefits and versatility give tough competition to alternative casting methods like sand casting and die casting, some of which we will also compare later on.
Without further ado, let us begin with a historic overview of this casting technique.
History of Investment Casting
Similar manufacturing methods have been around for more than 5,000 years. In ancient times, people used pure beeswax patterns to cast softer metals like gold and copper. Some interesting items discovered by archeologists from those times include jewelry, idols, and artworks.
As mankind’s knowledge of dealing with metals increased, people experimented with new metal casting methods. In the late 19th century, its use for manufacturing dental crowns gave it a boost.
The event that brought this technique mainstream attention, however, was World War II. Military applications required a casting method to productively manufacture strong, repeatable, and accurate parts. Consequently, efforts in those years refined the technique now known as investment casting.
How Does Investment Casting Work?
The investment casting process is a step-by-step procedure that engineers have perfected for optimal casts over the years. In this section, we follow this 6-step procedure in detail.
Create the Master Pattern
The wax pattern is a replica of the final part geometry. a ‘master pattern’ serves as a standard that is reusable for numerous casting cycles. Common materials for manufacturing the master pattern dies are wax, wood, metal, and silicone rubber. The choice of material depends on factors like pattern life, design complexity, thermal expansion/contraction, surface quality, etc. Using this master pattern, operators produce wax patterns for each casting cycle.
Nowadays, engineers use advanced techniques like 3D printing or die casting to manufacture patterns. Conventionally, wax patterns were made from die casting. However, making patterns with 3D printing is an innovation that is quickly getting popular. Engineers prefer FDM 3D printing due to its decent accuracy and low cost.
Assembly of Wax Pattern
The next step is to assemble the multiple patterns produced in the previous step into a single investment casting mold. This combined pattern is known as a ‘tree’ as its shape resembles that of a tree.
Operators then assemble the patterns with a sprue/runner/riser network. This gating network provides a connected pathway for the metal to flow to all parts of the mold smoothly.
This assembly process can be manual or robotic. A manufacturer may choose to opt for manual assembly for low-volume production or when each casting has a different design. On the other hand, if the casting process is very repetitive, it is wise to invest in a robotic assembly unit.
Apply Refractory Materials (Dipping)
Next, operators apply a coat of refractory materials to the pattern to achieve desired properties. The coating process takes place in multiple dipping cycles. They dip the tree repeatedly into a bath of ceramic slurry and sand stucco with a gradual, rotary motion. The ceramic helps in making the mold temperature-resistant and the stucco enhances its strength.
With each dipping cycle, the thickness of the ceramic shell around the wax pattern increases. It is important to take care when applying the initial layer, also known as the primary core, as it has a strong effect on the part quality and geometry.
This process repeats until it achieves the required shell thickness. Typically, the shell thickness ranges from 3/8” – 1/2” and depends upon the size and weight of the cast, as well as the material to cast. If the part is heavy and the metal has a high melting temperature, the shell should be thick enough to bear the temperature and stresses.
In the dewaxing process, the wax inside the ceramic shell is removed by melting. Industrial setups commonly include steam autoclaves or flash fire furnaces to heat and melt the wax.
Operators will place the mold down so molten wax can flow out easily. In an autoclave, the steam heats up the mold for up 12 hours at 90oC–175oC and a majority of the wax flows out. After this, bake the mold at higher temperatures of up to 1000oC for a few hours to burn off the remaining wax and evaporate moisture content.
This is a crucial stage in the casting process as such intense temperatures can induce defects in the mold shell. Cracking is a common concern that engineers try to avoid. As a result, it is a good practice at an investment casting foundry to dewax the mold in a way that the wax melts but no damage is done to the mold.
A positive side effect of this process is the heat treatment of the refractory material. This improves the shell’s strength and also decreases its reactivity with molten metal.
Preheating & Casting
In this step, the actual investment casting process takes place. Operators preheat the mold to minimize its thermal shock upon contact with high-temperature liquid metal.
Operators or machinery will pour the liquid metal into the mold in a controlled manner. Maintaining the pouring rate is very important at this point to minimize turbulence-related casting defects. What’s more, if the molten metal tends to react with air, it’s also suggested to create a vacuum environment around the mold. Once the pouring process is complete, the metal solidifies and cools down inside the mold.
Operators break apart the mold by impact knockoff via hammer or water blasting in this stage. Once out of the mold, the part goes through several semi-finishing operations.
These operations include cutting off the gating system as it is not part of the product. Similarly, rough surfaces, jagged edges, and other defects are flattened with grinding or blasting. Moreover, machining operations like milling and drilling are able to finish holes and surfaces. The parts can also proceed to a range of surface finish treatments to achieve the required surface quality.
Then, the part goes through quality inspection after post-processing operations. The inspection team checks the part for defects like cracks and porosity through tests like die-penetrant testing. Furthermore, they also check other quality factors like geometric and dimensional tolerances, hardness, and strength.
[Further Reading: Investment Casting vs Die Casting: Which Is Suitable for your Application]
Different Types of Investment Casting Materials
One of the main advantages of lost wax casting is that it works well with a diverse catalog of materials, both ferrous and non-ferrous materials. As a result, a variety of combinations of mechanical properties can be achieved in the final part.
- Steels: Investment casting performs exceptionally with mild steel, high carbon steels, and other special steel alloys. Investment casted steel parts have a wide range of applications in aerospace, military industries, and shipbuilding.
- Stainless Steel: Stainless steel has good strength, corrosion resistance, and a fine shiny surface. Common products include pump housings, valve components, gears, and bushings.
- Aluminum Alloys: These are lightweight, corrosion-resistant, and have a good strength-to-weight ratio. The main aluminum products are aircraft and automotive parts.
- Nickel Alloys: Nickel alloys have superior strength, corrosion resistance, and high thermal stability. Parts cast from these alloys are used for high-intensity applications in hot, oxidizing environments.
- Cobalt-Alloys: Cobalt alloys have strong resistance against wear, corrosion, chemical reactions, and extreme temperatures. These high-performance alloys have several applications in medical, food processing, and heavy machinery industries.
- Cast Iron: Cast iron has good strength and durability. It is useful in manufacturing electrical components, pipe fittings, tool hardware, etc.
- Copper Alloys: Copper alloys such as brass and bronze have medium-level tensile strength, ductility, thermal conductivity, and a great appearance. Owing to this, these alloys have applications in plumbing components, heat sinks, heat exchangers, etc.
Surface Finishing for Investment Cast Parts
Investment castings are near-net shape but have a slightly rough surface after the dewaxing stage. This is due to the patterns left on it by the primary core of the ceramic mold. Typically, the surface finish is 125 Ra, varying slightly with the material and quality of the mold.
As a result, for a smooth surface, a number of surface finishing treatments are available to achieve the desired surface quality. Some are based on material removal and others on coating the surface to neutralize and bury the rough surface beneath a layer of external coating.
The choice of surface finish technique depends on factors like dimensional tolerance, preferred appearance, and mechanical properties. Enlisted below are the surface finish operations one can choose from:
Applications of Investment Casting
Investment casting has innumerable applications in a range of industries. We summarize some of them below.
- Energy: The energy sector has a very specific use for this manufacturing technique. Wind turbine blades are huge, weigh tons, and require superior strength and surface finish. This combination of qualities is an advantage of this method.
- Marine: Investment casting of metal parts is common in the marine industry. Strong, corrosion-resistant metallic panels for ships and submarines are manufactured with special alloys. Since this method is compatible with numerous metal alloys, it is well suitable for this application.
- Aerospace: This technique can manufacture crucial parts in aircraft engines and bodies owing to the requirements of complex geometries and strength.
- Medical: The ability to cast small parts with a superior surface finish puts this method at an advantage in the medical industry. Prosthetics, implants, and surgical equipment are some standard applications.
- Automotive: In the automotive industry, high-performance parts like drive train components, rocker arms, and power train elements are made.
Advantages and Disadvantages of Investment Casting
Investment casting has numerous benefits and a few drawbacks. One must be fully aware of both of these to make an informed decision when choosing a casting method. Let us discuss these aspects in detail.
This casting technique has many strong points that make investment cast parts in high demand.
- Complex Geometry Compatible: It can handle intricate geometries such as internal features, sharp curvatures, thin walls, and shallow passages. All of these can be manufactured as a single part, eliminating the need for joining separately manufactured components afterward via welding, as with other casting processes.
- Fine Surface Finish: A surface finish of up to 3 microns can be achieved with this casting method. This is at par or much better than other popular methods like sand casting. Moreover, investment casting parts also have no parting lines, further enhancing part aesthetics.
- Large Parts: This method is capable of dealing with different part sizes. It is suitable for parts as lightweight as 100g and as heavy as a ton. It produces huge turbine blades of the same quality as minute medical equipment.
- Great Dimensional Accuracy: It is a near-net shape casting process with minimal finishing tolerances. After finishing, parts have an impressive dimensional tolerance of +/- 0.075 mm.
- Wide Material Selection: It is a versatile process that is compatible with numerous materials. As discussed above, it can handle steels, nickel alloys, aluminum alloys, etc., all with the same quality and precision.
There are some cons to this process as well. These points are important to know for any industrialist dealing with casting.
- High Tooling and Equipment Costs: The initial tooling, master pattern, and machinery (furnace, ceramic coater) are quite expensive and are not justified for low-volume production. But It may be chosen for specific low-volume manufacturing, like wind turbine blades, if the cost is not the most important consideration for you.
- Highly Technical Process: The process has many steps, as discussed above. Each step in the procedure requires specific expertise and skilled labor. So, mishandling the process at any point is easy and can prove to be very costly for the manufacturer. Thus, having the correct manpower and its cost can be a predicament in adopting this casting process.
- Time Consuming: Due to the several steps involved and time-taking steps like cooling and coating, the casting process can be very slow. Companies seeking manufacturing services that can make parts in a very fast lead time should also look into alternative casting techniques.
RapidDirect Provides Custom Casting Services
It is clear that investment casting is a highly technical process that requires skill and experience for proper execution. Do you need some casted parts? RapidDirect is your best partner in providing unbeatable casting services including investment casting and die casting services.
RapidDirect boasts a self-owned manufacturing complex with state-of-the-art casting equipment, and an experienced team of engineers, complemented by a professional technical team. We also pride ourselves on high-quality part manufacturing with fast lead times.
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Investment casting is a method where molten raw material is poured into a pre-shaped mold and allowed to solidify. 3D printing is an additive manufacturing process where the part is created layer by layer by a 3D printing technology such as FDM, SLS, etc.
Investment casting has been perfected over a long time. It can handle a variety of metals with high precision and a surface finish. In contrast, 3D printing is still in its heyday and is not at par with casting when it comes to metals. Issues such as poor surface finish, porosity, and non-homogeneity render 3D printed parts inferior to cast parts.
Investment casting is capable of creating parts with high tolerance, and surface finishes, and dealing with very complex geometries. On the other hand, sand casting has the advantage of being more economical for high-volume production and is capable of manufacturing very huge parts.
Yes. Wax and metal in investment casting are regularly recycled and reused in subsequent casting operations, saving wastage of material. While it does consume energy and produce fumes that are potentially harmful to the environment, proper fume management and waste disposal can minimize these harmful effects.