Investment Casting

1. What Is Investment Casting?

Investment casting is one of the oldest metal casting methods in the industry. For thousands of years, precision investment casting was 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
 

Difference between Investment and Lost Wax Casting

Some people use both terms synonymously, however, there is a slight difference in the casting process, namely whether or not a wax pattern is used. Lost Wax Casting patterns usually are made manually for each piece, such as jewelry 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 a wax pattern ideal for mass manufacture of more intricate components.

2. The Investment Casting Process Explained

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

1. Creating the Pattern

This step involves creating a master pattern that has the same details as the finished part. The patterns are generally made of wax using a metal injection die (master 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. The term investment means to surround the pattern with refractory materials (ceramic) to make the mold/die.

investment casting part1
 

2. Creating the Mold Shell

The wax assembly is called a tree, consisting of sprue and multiple gates that connect the master patterns. The tree is placed in a ceramic slurry and stucco, and left there until it dries. After several 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

In the dewaxing stage a ceramic mold is placed upside down and moved into a heating device (an autoclave or a heating chamber) in order to melt out the wax. Subsequently, the mold is 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 are mounted open-side up into a container that is filled with sand. The molten metal is poured with gravity or force (by positive air pressure or other forces) into the tree and all parts of the mold cavity. Then, the shell releases 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 in-gate 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, they will go through a few quality inspection procedures (non-destructive testing) before being packaged and shipped to the customer.
 

3. Casting Materials

Stainless Steel

Precision casting with stainless steel results in 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 material 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.

Carbon Steel

Carbon Steel contains up to 2% pure carbon. It also generally contains small amounts of silicon, manganese, sulfur, and phosphorus. The higher the carbon content, the higher the strength and the hardness. This kind of steel is very cost-effective, readily available, easy to process while being rigid and offering great wear resistance. All these properties make it a popular choice for investment castings. Carbon steel castings are used in automotive, military, oil and gas, mining, and more.

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 a 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.

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 casting the ductile iron and cast iron lies in the form of graphite that exists in the alloy. Cast iron has graphite flakes, whereas ductile iron has a spherical graphite structure in the metal. Investment casting with ductile iron 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 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 a 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 that permits cracks to pass straight through, grey cast iron has graphite flakes that deflect a passing crack, and this will initiate countless new cracks if the material breaks.

Soft Metals

Soft metals refer to brass, titanium, copper, aluminum, etc. that have higher ductility and malleability. Copper-based alloys (brass and bronze) are ductile and conductive to heat and electricity. They make strong but light casts, and are therefore suitable for jewelry and pipe fittings. It is used in packaging, construction, and transportation applications. Titanium castings are light and strong, typically used in aerospace and defense sectors.


4. Advantages and Disadvantages

Investment casting is the best option if you want to cast complex patterns. You can achieve very thin structures while not compromising on accuracy. As a result, it remains popular today. The many advantages make it the perfect candidate for high-performance alloy materials in the metalworking industry.
 

However, there are also some drawbacks that you need to take into account. The overall production cost can be higher than with other methods. Product size and structure are also major concerns. Let us talk about pros and cons of investment casting.
 

Advantages

 

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.

 

Disadvantages

 

Cost

One disadvantage is that the ceramic cell used during the process is not reusable, slightly increasing the overall cost of this technology. This leads to higher costs for short-run productions. Also, this process involves a lot of manpower. It requires trained and skilled operators to control the machine, assemble the patterns, pour the molten metal, and divest the shells, etc.

Size Limitations

Though the size range precision investment casting can process is wide enough for most applications, it still has an upper limitation less than other methods, such as sand casting.

Structural Limitations

Generally speaking, this process is suitable for making thin-walled products. However, it does not allow for small internal cavities. Creating small interiors requires the use of cores, which is quite challenging to this process. The holes within the part cannot be smaller than 1.6 mm and no deeper than 1.5 times the diameter.

Time

Involving all the steps mentioned above, investment casting is a relatively time-consuming process. Even though it usually does not need additional finishing steps, the time it takes to dry the sprue, cool the metal, and retrieve the cast is still considerably long.

5. Applications

 

Precision investment casting can produce small precision castings and also 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, it can create complex net shapes that may be challenging with other methods. It also provides 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/defense, jewelry, automotive, electronics, agriculture, transportation, construction, energy, and 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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