Investment Casting: Definition, Process, Materials & More

Einführung

Dating back more than 5,500 years to the Bronze Age, investment casting, or lost-wax casting, is one of the oldest manufacturing methods. Yet this casting technique is widely used to produce intricate parts with highly detailed patterns, such as jewelry, turbine blades, and other ornaments

As the technology advances, the investment casting process has been upgraded. But the basic working principle still remains the same. Now it supports several technologies, such as CAD/CAM, additive manufacturing, and real-time monitoring. This offers a range of benefits from improved part quality to cost-effectiveness.

In this article, we will discuss the fundamentals of the investment casting process. We will break down the detailed step-by-step method of how the casting works, and more.

Was ist Feinguss?

Feinguss– also referred to as lost wax casting or precision casting- is a manufacturing method that involves coating a wax pattern with a refractory ceramic material. This produces a disposable ceramic shell mold, which is then dried and hardened. The mold is turned upside down and heated until the wax melts and drains from the mold’s internal cavity. This ceramic now serves as an expandable investment mold.

The Molten metal to be cast is carefully transferred into the mold. As the metal has solidified, the ceramic mold is stripped away to acquire the metal casting.

Investment casting is widely preferred as the resulting castings are highly detailed. It delivers smooth surface finishes and tight tolerances with net-to-near-net precision.

Why is it called Investment Casting?

The term “investment” here in this context historically means ‘putting on a covering’ or ‘surrounded’. Literally, investment casting involves the sacrificial covering of a refractory material over the wax pattern.

Investment casting is used in various industries to manufacture precise parts. These include components for aerospace and automotive industries, jewelry items, turbine blades, intricate machinery parts, golf club heads, medical equipment, and much more.

Step-by-Step Process of Investment Casting

Here is the detailed process explained:

1. Engineering review

This is the first and foremost step of the manufacturing process. It begins with a comprehensive DFM (design for manufacturing) review, during which all the details are discussed with the customer. These include:

• Print dimensions
• Design and build considerations, i.e., gate placement
• Material requirements
• Post-processing or finishing requirements
• Tolerances and inspection

This collaborative review ensures a smooth partnership and guarantees that the final part meets the standard requirements.

2. Mold design and build

Now, start the investment-molding process by designing the high-precision wax injection mold. Typically, there are three routes for delivering a wax pattern injection mold:

• For jewelry and decorative items: Here, precision is a low priority. That’s why wax patterns are created using basic shapes, or solid parts are locally melted and shaped to form the desired shape. These wax patterns are also sometimes hand-carved for detailing and complex designs.
• For mass-produced items: For bulk parts or parts that require precision, the mold is usually made of steel. This steel tooling features an internal wax-casting cavity. So the resulting parts will be identical and accurate.
• 3D printing for small parts or prototypes: Manufacturers now use 3D printing, thanks to recent developments. Small changes and adjustments to these rapid prototypes can be made without the high cost of building a metal mold.

This is the master die, and that’s why it has the same pattern as that of the finished casting part. Dimensional calculations must be accurate and account for expected shrinkage.

3. Wax pattern production

Wax patterns are produced according to the number of parts needed to be manufactured. For each casting, a new wax pattern will be required.

To create wax patterns, manufacturers can use different methods:

• First, fill the master die with hot wax and pour it back after it is evenly coated. Then it is allowed to solidify. The desired wall thickness can be achieved by repeating the process. This process will produce a hollow was pattern exactly like the finished part.
• Another method is to completely inject the material into the die cavity. Here, manufacturers can opt for pressure injection, as in die casting or direct filling. Pressure filling is suitable for highly precise parts. This process produces solid wax patterns. For internal features, cores can be added after solidifying. Core material can vary. It may be wax or ceramic.

4. Mounting the wax pattern and creating a tree

After the wax patterns are produced, they are arranged on the central wax gating system. This forms a tree where up to wax patterns are tree’d, depending on the size. The tree cluster is now sent for the dipping process.

5. Creation of a shell mold

Wax patterns cannot withstand high temperatures. This is the reason ceramic coating is done. So, the tree cluster is now immersed in a ceramic refractory slurry of fine-grained silica. The refractory coating material completely contacts the wax pattern, forming a layer. The process is repeated until the specific thickness is achieved. It is allowed to dry in the drying oven for 24 hours, forming a solid, hard layer. This results in a hardened ceramic shell surrounding the wax pattern and gating system. It is known as “investment.”

The thickness of the ceramic mold is the key here. While the standard thickness is 0.375 inches, it can vary depending on factors such as the material and the application. These include the pouring temperature of the molten metal, the weight, and the size of the final part.

6. Removing the wax

Now, the ceramic shell is placed upside down in the furnace, where the wax melts and hardens the shell. The temperature here is 1000-2000 °F. The melted wax drains out and is saved for reuse. Now, only the ceramic mold remains for the casting process.

7. Pouring of molten metal and cooling

Before the actual casting process begins, the ceramic molds are tested for cracks or other defects. If defects are found, they are rectified using ceramic slurry.

However, once testing is successful, the molten metal is poured into the heated molds under external pressure and gravity. The heat keeps the metal liquid and completely fills all the ceramic molds on the tree cluster. It also ensures the molten metal flows easily.

Manufacturers may also opt for vacuum filling for filling molten metal. While it’s an efficient option with a material yield of upto 95%, it may be a bit costly.

8. Removal of Casting from the Mould

Before removing the cast part, the molten metal is allowed to cool and solidify. The rate of solidification depends on several factors. These include:

• Material used
• Thickness of ceramic casting

Now, once the part is solidified, the brittle ceramic shells are broken using various techniques. This includes water jetting, vibration, chemical dissolution, and media blasting. This ensures safe removal of the final part.

Other components, such as the runner, gating system, and sprues, are also separated and reused.

9. Post-processing operations

Final post-processing operations involve grinding, machining, trimming, heat treatment, and polishing. This is to remove imperfections on the final cast part. Although investment casting delivers clean, smooth finishes. They do not need subsequent finishing. But if needed, it is done before shipping to the customer.

Common Materials Used in Investment Casting

1. Ferrous Metals (Iron-Based)

These are popular for their high strength and durability.

Stainless Steel: The most common choice for precision parts. It is highly corrosion-resistant and is frequently used for medical tools, food processing equipment, and marine hardware. Common grades suitable include 316, 304, etc.

Carbon Steel: An economical, low-cost option for general-purpose parts. It has high strength but can rust easily, so a protective coating is applied. It’s commonly used in agricultural machinery and automotive parts.

Alloy Steel: Created by adding elements like chromium or nickel to carbon steel to improve toughness and wear resistance. These are used for heavy-duty industrial components.

2. Non-Ferrous Metals

These are typically lighter or more corrosion-resistant than steel.

Aluminum Alloys: Prized for being lightweight and having an excellent strength-to-weight ratio. They are the top pick for aerospace, electronics, and automotive components, where reducing weight is critical.

Copper Alloys (Bronze and Brass): Known for excellent electrical and thermal conductivity. They are also naturally resistant to wear and are often used for plumbing fixtures, bearings, and decorative items.

Titanium: Offers the strength of steel at a fraction of the weight. It is highly corrosion-resistant and biocompatible, making it a standard choice for aerospace parts and medical implants.

3. Superalloys

These are specialized “exotic” materials designed for the most extreme environments.

Nickel-Based (e.g., Inconel): Maintains exceptional strength and resists oxidation even at temperatures exceeding 1000 degrees. Essential for jet engine parts and gas turbines.

Cobalt-Based (e.g., Stellite): Offers superior resistance to wear, galling, and chemical aggression. Frequently used in medical implants and high-wear valve components.