Investment Casting: Process, Benefits & Applications

Investment casting is one of the most widely used casting processes in the industry. Manufacturers often use investment casting because it offers many advantages. These include high dimensional accuracy, smooth surfaces, the ability to handle complex designs, and fast production.

In this article, we will tell you everything you need to know about investment casting. We will briefly describe the history of investment casting and explain the process in detail, discussing its uses, material properties, and advantages and disadvantages.

What is Investment Casting?

Investment casting is also known as “lost wax casting”. It involves using a wax-like material to produce a consumable model and then creating an exact copy of the model. The process uses ceramic molds with excellent thermal properties. As a result, it is possible to produce parts in almost net shape with a very high surface quality.

Its many advantages and adaptability make it a strong competitor to other casting methods such as sand casting and die casting. For now, let’s continue to learn about investment casting.

History of Investment Casting

People have been using similar methods to make objects for over 5,000 years. Models made from pure beeswax were used to cast soft metals such as gold and copper. Archaeologists have found jewelry, idols, and works of art from that era, which are fascinating to look at.

As knowledge of metal processing grew, people began experimenting with new metal casting techniques. The technique was further developed at the end of the 19th century when it was used to make dental braces.

The Second World War made the process well-known to most people. The military needed a way to produce strong, precise, and reusable parts. As a result, the process that is now known as investment casting began to be improved.

How Does Investment Casting Work?

Over the years, manufacturers have worked hard to perfect the investment casting process to produce the best castings. In this section, we will take a closer look at the six steps.

Creating the Master Model

A wax model is an exact replica of the finished product geometry. The “master model” is the standard that can be used repeatedly for casting. Wax, wood, metal, and silicone rubber are the most common materials used to create the master model mold. Which material is used depends on the model’s lifespan and the complexity of the design. In addition, the degree of expansion or contraction of the material when heated or cooled, and the surface quality are also factors to consider. In each casting cycle, the operator creates a wax model based on the master design.

Today, engineers use cutting-edge methods such as 3D printing or the investment casting process to create models. In the past, the die casting process was often used to create wax models. Nowadays, 3D printing is a new technology that is rapidly gaining popularity. Engineers like FDM 3D printing because it is cheap and fairly accurate.

Assembling the Wax Model

The next step is to assemble all the models created in the previous step into a lost wax casting mold. The assembled model looks like a tree, hence the name “tree”.

The operator then assembles the model together using a network of gates/runners/risers. This network of gates allows the liquid metal to flow easily to all parts of the mold.

This assembly process can be carried out manually or automatically. Manufacturers may opt for manual assembly when the production quantities are low or when each casting has a unique shape. On the other hand, if the casting process is to be repeated, it makes sense to purchase an automated production cell.

Applying the Refractory Material (impregnation)

Next, the operator applies a layer of refractory material to the mold to achieve the desired properties. The coating process involves several impregnation steps. They slowly rotate the tree and dip it again and again into a mixture of ceramic slurry and sand plaster. The clay helps the mold withstand high temperatures, while the plaster makes it stronger.

As the dipping process continues, the ceramic shell around the wax model becomes thicker and thicker. Take extra care when you finish the first initial layer (also called the master core). This has a significant impact on the quality and shape of the part.

This process is repeated until the shell has reached the right thickness. In most cases, the shell thickness is between 3/8 inch and 1/2 inch. This depends on the casting material, size, and weight. If the part is heavy and the metal has a high melting point, the shell should be thick enough to withstand the high temperatures and pressures.

Dewaxing

In the dewaxing process, melting removes the wax from the clay shell. To heat and melt the wax, factories usually use a steam autoclave or a flash furnace.

The operator then lowers the mold so that the molten wax can flow out easily. The mold is heated in the autoclave for 12 hours at temperatures of up to 90°C to 175°C, and most of the wax flows out. The mold is then baked at higher temperatures (up to 1000°C) for several hours to remove any remaining wax and water.

This is a very important step in the casting process. This is because the mold shell may develop defects at high temperatures. Engineers generally try to avoid cracks. Therefore, in investment casting plants, the mold is usually dewaxed using methods that do not damage the mold.

In addition, heat treatment of the refractory material is a side benefit of this process. The shell will become stronger and therefore less likely to react with the molten metal.

Preheating and Pouring

From this point on, the investment casting process begins in earnest. The mold is preheated so that it does not react too quickly when it comes into contact with the hot liquid metal.

The liquid metal is poured into the mold in a controlled manner, either by an operator or by a machine. It is important to maintain a constant filling rate. This prevents casting defects caused by turbulence. In addition, if the molten metal is prone to reacting with air, it is necessary to ensure that there is no air around the mold. Once the filling is complete, the metal solidifies and cools inside the mold.

Post-processing

Once the metal has solidified, the mold is broken using a hammer blow or water jet. After the part is removed from the mold, it is subjected to a series of semi-finishing steps.

One task is to close the pouring system, as it is not part of the product. Rough surfaces, sharp edges, and other defects can also be removed using grinding or sandblasting. CNC milling and drilling are two machining processes that can also be used to machine holes and surfaces. In addition, parts can be subjected to a range of surface treatment processes to achieve the desired surface finish.

After the parts have been post-processed, they are subjected to a quality inspection. The inspection team uses methods such as penetrant testing to check the parts for defects such as cracks and holes. They also check other quality factors such as strength, hardness, geometry, and dimensional limits.

Different Types of Investment Casting Materials

One of the great advantages of investment casting is that it can be used with a wide range of materials, including ferrous and non-ferrous metals. This means that the final parts can have different mechanical properties.

• Steel: Mild steel, high-carbon steel, and other special alloy steels are well suited for investment casting. Steel parts made by investment casting are used in many areas, such as shipbuilding, aircraft, and the military.
• Stainless Steel: Stainless steel is strong, rust-resistant, and has a smooth, shiny surface. Pump housings, valve parts, gears, and bushings are common parts.
• Aluminum Alloys: These metals are light, strong, and rust-resistant. Aircraft and car parts are the main uses for aluminum alloy parts.
• Nickel Alloys: Nickel alloys are very strong, do not rust, and remain stable at high temperatures. Castings made from these metals are used in high-temperature, oxidizing environments and can withstand high levels of stress.
• Cobalt Alloys: These alloys are very strong and do not wear, rust, chemically react overheat or cool down. This high-performance alloy is used in heavy machinery, medicine, and food processing, for example.
• Cast Iron: Cast iron is extremely durable and very strong. It is used to make electrical parts, pipe fittings, tool gears, etc.
• Copper Alloys: Copper alloys such as bronze and brass are attractive in appearance, have moderate tensile strength, and are good conductors of heat. These metals are therefore used for plumbing parts, radiators, heat exchangers, and other components.

How to Finish Investment Casting Parts

After dewaxing, the surface of an investment casting is somewhat rough. This is due to the marks left on it by the main core of the clay mold. The surface roughness of these parts is usually 125 Ra, but there may be some differences depending on the material and quality of the mold.

There are therefore many surface treatments available to achieve the desired smooth surface. Some processes achieve a smooth surface by removing material. Other methods involve painting the surface to smooth it and hide the rough surface beneath a layer of paint.

The type of surface treatment is chosen based on factors such as the desired appearance, dimensional tolerances, and mechanical qualities. The following are different methods of surface treatment:

• Anodizing
• Polishing
• Painting
• Powder Coating
• Passivation

Uses of Investment Casting

The investment casting process can be used in many areas. Here are some common areas of application:

• Energy Industry: This manufacturing process is very useful in the energy industry. The blades of wind turbines are very large and heavy and need to be very strong and have a smooth surface. One advantage of the investment casting process is that it has all these properties.
• Shipbuilding: In the shipbuilding business, parts made of metal are often produced using investment casting. Special alloys are used to make strong metal plates. These plates do not rust and can be used in ships and submarines. This process is suitable for this industry because it is compatible with many metal alloys.
• Aerospace: Because it can handle complex shapes and high strength. Therefore, this process can be used to manufacture important parts of aircraft engines and fuselages.
• Medical Industry: This method is useful in the medical field. Because it can manufacture small parts with a better surface finish. For example, some common uses are prosthetics, implants, and surgical tools.
• Automotive industry: Investment casting can manufacture high-performance parts for the automotive industry. Such as transmission system parts, rocker arms, and powertrain components.

Advantages and Disadvantages of Investment Casting

Like other casting processes, investment casting has advantages and disadvantages. In order to make an informed choice of casting method, it is important to be fully aware of both. Let’s discuss these in more depth next.

Advantages

The investment casting process has many advantages, which is why these cast parts are so popular.

• Compatibility with Complex Geometries: It can handle complex geometries. Such as internal features, sharp curves, thin walls, and shallow channels. All of these can be manufactured as a single part. So you don’t have to assemble parts that have been manufactured separately. Which is something you would have to do with other casting methods.
• Fine Surface Finish: This casting process can provide you with a surface finish of up to 3 microns. Compared to sand casting, investment casting produces even better results. In addition, investment cast parts do not have parting lines, which makes them look more aesthetically pleasing.
• Large Parts: This method is suitable for parts of different sizes. It can handle parts weighing anything from 100 grams to one ton. In addition, it can manufacture huge turbine blades as well as tiny medical tools.
• Excellent Dimensional Accuracy: Investment casting is almost a near-net-shape casting process, with extremely small tolerances for finishing. Parts have an amazing dimensional tolerance of +/- 0.075 mm after finishing.
• Wide Range of Materials: The process is very flexible and can be used with a wide range of materials. It is suitable for and precise with materials such as steel, nickel alloys, and aluminum alloys.

Disadvantages

On the other hand, the investment casting process also has some disadvantages. Anyone who is involved in casting must be aware of these.

• High Tool and Equipment Costs: Initial molds, masters, and machines (furnaces, ceramic coating machines) are very expensive. This process is therefore not suitable for small-batch production. However, if cost is not your main concern, then it may be suitable for some small-batch production. For example, the manufacture of wind turbine blades.
• A Highly Technical Process: As already discussed, the process has many steps. Each step in the process requires different skills and knowledge. Any part of the process is prone to error. This can be very costly for a company. If you want to use this casting process, finding the right people and paying them enough can be a problem.
• Time-consuming: The casting process can be very slow because it has many steps. Some of these steps take a long time, such as cooling and finishing. Therefore, companies that need parts quickly should also consider other casting methods when looking for production services.

Yonglihao Machinery Offers Custom Casting Services

It is clear that investment casting is a very complex process. It needs to be done by people with a lot of skill and experience. Do you need some parts cast? Yonglihao Machinery can provide you with unrivaled casting services. For example, investment casting, hot chamber die casting, and high-pressure die casting processes.

Yonglihao Machinery has its own manufacturing plant. We are equipped with advanced casting equipment, as well as a skilled technical team and an experienced team of engineers. We are able to produce parts quickly and to a high standard, providing fast and attentive service to our customers.

FAQ

What is the Difference Between 3D Printing and Investment Casting?

In investment casting, molten metal is poured into a preformed mold and allowed to solidify. 3D printing is an additive manufacturing technique. 3D printing technologies such as FDM and SLS build parts layer by layer.

It takes a long time to become proficient in investment casting. However, this casting process can handle a wide range of metals and achieve perfect surface finishes. 3D printing is still in its infancy and is not as good as casting when it comes to manufacturing metal parts. Surface roughness, porosity, and unevenness are some of the problems with 3D printed parts compared to cast parts.

The Difference Between Investment Casting and Sand Casting

Investment casting can produce parts with very complex geometries and extremely high surface finishes. On the other hand, sand casting is superior. This is because the sand casting process can produce large quantities of parts, thereby reducing costs. In addition, this process can also produce very large parts.

Is Investment Casting Environmentally Friendly?

Yes. In investment casting, wax and metal are regularly recovered and used in other casting processes, thereby avoiding material waste. Although investment casting consumes energy and produces environmentally harmful fumes. However, these problems can be minimized with good fume control and waste disposal.