What is Investment Casting and The Investment Casting Process introduction?

In this article, we will discuss what is investment casting and the investment casting process introduction.

Investment casting, also known as investment casting or lost wax casting, is a manufacturing process that uses wax patterns to form disposable ceramic molds. Wax molds are made in the exact shape of the item to be cast. This pattern is covered with refractory ceramic material.

After the ceramic material hardens, turn it upside down and heat it until the wax melts and drains. The hardened ceramic shell becomes a one-time investment. The mold is filled with molten metal and let to cool. The used mold is then taken out and the metal casting is separated.

The term investment casting comes from the process of “melting” (surrounding) a model with refractory material. Investment casting is often chosen over other forming methods because the resulting castings have fine detail and excellent surface finish. They can also be cast with thin walls and complex internal channels. Investment casting doesn’t need a draft, in contrast to sand casting.

These manufacturing features can provide castings that are final or near final, resulting in significant savings for customers in materials, labor and machining.

Works with most common metals including aluminum, bronze, magnesium, carbon steel, and stainless steel. Parts made using investment casting include turbine blades, medical devices, gun parts, gears, jewelry, golf club heads, and many other machine parts with complex geometries.
Investment casting process
The investment casting process includes several steps: metal mold making, wax mold making, ceramic mold making, pouring, solidification, sandblasting, and cleaning.

1. Metal mold structure
Wax patterns and ceramic molds are destroyed during the investment casting process, so a new wax pattern is required for each casting. Unless investment casting is used to produce very small volumes (which is common in original works of art or jewelry), a mold or mold is needed to make wax casts.

The size of the main mold must be carefully calculated; It should take into account the expected shrinkage of the wax mold, the expected shrinkage of the ceramic material invested in the wax mold, and the expected shrinkage of the metal casting itself.

2. Making a wax mold
The number of wax patterns is always equal to the number of castings to be produced; A new wax pattern is required for each casting.

Hot wax is poured into the mold or mold and allowed to dry. Nuclei may be required to form any internal feature. The resulting wax model is an exact copy of the part to be produced. This method is like die casting, but wax is used instead of molten metal.
3. Molding
The gate system (gates, runners, and risers) is attached to the wax mold. For smaller castings, connect multiple wax molds to a central wax gate system to form a tree-like assembly. A casting cup, usually attached to the end of the runner rod, is used to feed the molten metal into the mold.

Immerse the assembled “pattern tree” in a fine-grained silica slurry. It is repeatedly dipped, and gradually more refractory slurry is applied with each dip. When the refractory coating reaches the desired thickness, it is allowed to dry and harden; The dry coating forms a ceramic shell around the pattern and gate system.

The thickness of the ceramic shell depends on the size and weight of the cast piece as well as the metal pouring temperature. The average wall thickness is approximately 0.375 inches (9.525 mm). Turn the hardened ceramic mold upside down and place it in the oven to heat until the wax melts and the water drains out. The result is a hollow ceramic case.
4. Pouring
Ceramic molds are heated to 1000 to 2000 degrees Fahrenheit (550 to 1100 degrees Celsius). The heating process further strengthens the mold, removes any remaining wax or contamination, and evaporates moisture from the mold material.

The molten metal is poured into the mold while it is still hot – the liquid metal flows into the pouring cup, through the central pouring system, and into each hole in the tree. The preheated mold allows the metal to easily flow into thin, precise parts. Also, due to the cooling and gathering of the mold and the casting together, it makes the casting parts with more dimensional accuracy.

5. Cooling
After pouring the mold, the metal cools and solidifies. The time required for the mold to cool to a solid state depends on the casting material and the thickness of the mold.
6. Shakeout
After the mold solidifies, the ceramic mold breaks and the mold can be removed. Ceramic molds are usually broken by hand or with a water jet. After removal, the individual castings are separated from the tree of the gate system by hand tapping, sawing, cutting, burning, or cold crushing by liquid nitrogen.
7. Finishing
Finishing operations such as grinding, or sandblasting are usually used to smooth the part in the gate and remove defects. Depending on the metal from which the mold is cast, heat treatment may be used to harden the final part.

When to use Investment Casting

Investment casting is a relatively expensive process due to its complexity and labor requirements – but the benefits often outweigh the costs. Almost any metal can be invested. Parts made by investment casting are usually small, but the process can be used effectively for parts weighing 75 pounds or more.

Investment casting can produce complex parts with excellent surface finish. Investment castings do not require a built-in taper to remove the part from the mold, as the ceramic shell falls off the part as it cools. This manufacturing feature allows castings to be designed with 90-degree angles without internal shrinkage allowances and without the need for additional machining to achieve these angles.

The investment casting process produces parts with excellent dimensional accuracy; Lattice parts are readily available and usually do not require secondary machining to produce finished products.

Each unique casting run requires a new mold to produce the wax pattern. Investment casting tools can be very expensive. Depending on the complexity, this tool can cost anywhere from $1,000 to $10,000.

For high-volume orders, the time and labor saved by eliminating or reducing secondary machining can easily offset the cost of new tooling. Small batch casting is unlikely to recoup the investment. In general, investment casting is a reasonable choice for runs of 25 or more pieces.

It usually takes 7 days to go from fresh wax mold to full casting; Most of the time is spent on making and drying the ceramic shell mold. Some castings have quick drying capabilities for faster casting production. The time-consuming and labor-intensive nature of investment casting affects more than just cost. Foundries have limited equipment and production capacity, so long investment times for foundries are common.
Custom casting services
Reliance Foundry works with customers to design patterns and forming methods for each custom casting. Request a quote for more information on how our casting services can meet your project needs.


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