Investment Casting

1. The Basics

Investment casting is one of the oldest metal casting methods in the industry. For thousands of years, investment casting and precision investment casting were used to form precision molds for various industrial and craft applications. The history indicated that the first group of archeologists found beeswax for the formation of patterns, and the pattern formed was later used for the purpose of casting processes. It is an old method but very accurate and convenient. The image below shows exactly how sophisticated the resulting products can be:

metal parts
Fig.1 Metal Parts Produced by Investment Casting
 

Is lost wax casting the same thing ?

Some people use both terms synonymously, however there is a slight difference in the casting process, namely whether or not a wax pattern mold is used. Lost Wax Casting patterns usually are made manually for each piece, such as jewellery casting or even fashion accessories. While Investment Casting metal forms create high quality, critical casting components. Investment casting also injects wax into permanent dies and molds to form wax patterns, ideal for mass manufacture of complex parts.

2. The Process Explained

The investment casting process can be broken down into a few steps. Let us take a look:

1. Creating the Pattern

This step involves creating a pattern that has the same details as the finished part. The patterns are generally made of wax using a metal injection die. Depending on the attributes of the finished parts, multiple wax patterns may be utilized and processed before they are assembled with other wax components.

casting part1
 

2. Creating the Moldshell

The wax assembly is then placed in a ceramic slurry and waited to dry. After cycles of wet dipping, a shell of desired thickness is finally created. And once the ceramic shell has dried, it is now strong enough to undergo casting.

casting part2
 
 

3. De-waxing

The mold shell will be placed upside down and moved into a heating device in order to melt out the wax. Subsequently, the mold will be subjected to burnout at a high temperature up to 1095 °C, eliminating any moisture and wax, sintering the model.

casting part3
 

4.Pouring and Divesting

The investments will be mounted open-side up into a container that is filled with sand. The molten metal may either be poured with gravity or be forced by adding positive air pressure or applying other generated forces in the system. Then, the shell will release the casting through different methods such as hammer, media blast, vibration, water-jet, or chemically dissolve. The casting will be finished up to remove any marks of the casting process, generally by grinding.

casting part4
 

5. Finishing and Inspection

In addition to ingate grinding, the steel cast will also be further treated to eliminate any impurities and defects through hand tooling or welding, and many more. After the parts are finished a few final quality inspection procedures before being packaged and shipped to the customer.



3. Advantages and Drawbacks

 

Investment casting is the best option if you want to cast complex patterns. You can achieve very thin structures while not compromising on accuracy. One minor disadvantage is the ceramic cell used during the process is not reusable, slightly increasing the overall cost of this technology.

Also most steps even in today’s world of automation, need to be performed manually. But now let us take a look at all the advantages of this casting procedure. Investment casting remains popular today. It can produce parts repeatedly and accurately. Its many advantages make it the perfect candidate for high performance alloy materials in the metalworking industry. Here are some of its key benefits:

Fast Production

Reduced total production time as a lot of machining steps after the casting process are not necessary anymore.

Perfect Finishes of Complex Parts

Investment castings can meet very tight tolerances, which also reduces the amount of finishing required in post-production. For instance, the surface finish of a professional investment casting foundry can reach roughly 125 RA surface finish accuracy.

Tooling Variety

The equipment needed is usually rather cheap and it is a safe process overall, hardly ever causing additional expenses.

Part Size & Material Range

It is suitable for casting both large and small parts weighing from under one ounce up to hundreds of pounds. Read on to learn more about the most common materials used for investment casting.

Another wonderful advantage of investment casting is the ability to work multiple parts simultaneously. It can also cast objects with great details, such as holes, bosses, splines, littering, threads, etc. All of these conveniences allow business owners to save a lot of money and energy, and explain why the method is widely used even today.

4. What materials can be cast?

Stainless Steel

Investment casting with stainless steel produces durable products with superb corrosion resistance. The built-in chromium oxide layer of the stainless steel can offer the ideal anti-corrosive ability, making it a popular alloy material for investment casting across industries.

This casting method is widely used in the aerospace and power generation fields in order to produce turbine blades with complex shapes or cooling systems. It is also widely used by firearms manufacturers to produce firearm triggers, receivers, and hammers at minimum cost. Industries including military, medicine, commerce, and automotive also utilize stainless steel investment casting vastly.

Cast Iron

Investment casting with cast iron is also one of the oldest casting methods over the course of its development history. It is a metalworking process that produces a group of iron based workpieces. Iron-carbon alloys with carbon content greater than 2% are conventional iron casting products.

The usefulness of iron casting lies in its relatively low melting temperature. The alloy constituents affect its color when fractured. The colors of fractures are indications of the state of the casting process. All operators should be able to identify these colors in case of mutations.

Ductile Iron

The key difference between the casting of ductile and cast iron lies in the form of graphite that exists in the alloy. Cast iron has graphite flakes, whereas ductile iron has spherical graphite structure in the metal. Investment casting with ductile irone has a great many benefits, including high resistance to wear, impact, strength and fatigue. While it is used extensively in many applications in the industry, casting of ductile iron does tend to be more expensive. So cast iron is still more popular on a general level.

Minerals

Mineral investment casting is a type of polymer-bonded mineral casting. Different from iron casting, this is a metalworking process that produces a group of iron-carbon alloys with carbon content greater than a certain level of percentage. The alloy constituents affect its color when fractured, that is, white cast iron has a carbide impurity which permits cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack, and this will initiate countless new cracks if the material breaks.

5. Applications

Investment casting can be used for producing both small and precision castings and also some large components weighing over a hundred pounds. Although the process may be more costly than die casting or sand casting, it is undoubtedly the best solution for high volume applications.

Furthermore, investment casting can create complex patterns that may be challenging with other metal forming methods. No matter the complexity of the patter, the casting products have better surface qualities and tight tolerances with less or no surface finishing tasks in most cases.

part examples
Fig.2 Part Examples (Courtesy of Proval)
 

With these benefits, investment casting is widely utilized in power generation, aerospace, medical, military, jewelry, automotive, sports industries to produce high-quality and complex components like blades, or cooling systems and equipment. The development of 3D printing technology can be utilized to create much larger molds that can be used in the steel investment casting to extend the usage.


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