Investment casting, also known as precision casting or lost-wax casting, is a manufacturing process in which a wax pattern shapes a disposable ceramic mold. A wax pattern is fabricated in the exact shape of the item to be cast. This pattern is coated with a refractory ceramic material. Once the ceramic material is hardened, it is turned upside-down and heated until the wax melts and drains out. The hardened ceramic shell becomes an expendable investment mold. Molten metal is poured into the mold and is left to cool. The metal casting is then broken from the spent mold.
History of machines investment casting
Ancient Egyptians first used it between 4000 and 3000 B.C. After World War II, its industrial importance and popularity grew due to the growing demand for parts in the aircraft engine and airframe sector.
Machined investment castings had changed from the days when clay was packed around the bee’s wax pattern to the use of ceramics shells and the introduction of special dewaxing ovens.
The high manufacturing cost of master dies traditionally limited investment casting to large production quantities. Still, the introduction of additive manufacturing or 3D printing of wax patterns in recent years has eliminated the manufacturing cost of dies and enabled shorter runs.
The investment casting process
It has several steps:
- metal die construction
- wax pattern production
- ceramic mold creation
1. Metal die construction
The wax pattern and ceramic mold are destroyed during the investment casting process, so each casting requires a new wax pattern. Unless investment casting is being used to produce a very small volume (as is common for artistic work or original jewelry), a mold or die is needed to manufacture the wax patterns.
The size of the master die must be carefully calculated; it must consider the expected shrinkage of the wax pattern, the expected shrinkage of the ceramic material invested over the wax pattern, and the expected shrinkage of the metal casting itself.
2. Wax pattern production
The number of wax patterns always equals the number of castings produced; each casting requires a new wax pattern.
Hot wax is injected into the mold or die and allowed to solidify. Cores may be needed to form any internal features. The resulting wax pattern is a replica of the part to be produced. The method is similar to die-casting, but with wax used instead of molten metal.
3. Mold creation
A gating system (sprue, runner bars, and risers) is attached to the wax mold. Several wax patterns are attached to a central wax gating system for smaller castings to form a tree-like assembly. A pouring cup, typically attached to the end of the runner bars, introduces molten metal into the mold.
The assembled “pattern tree” is dipped into a slurry of fine-grained silica. It is dipped repeatedly, being coated with progressively more refractory slurry with each dip. Once the refractory coating reaches the desired thickness, it is allowed to dry and harden; the dried coating forms a ceramic shell around the patterns and gating system.
The thickness of the ceramic shell depends on the size and weight of the part being cast and the pouring temperature of the metal being cast. The average wall thickness is approximately 0.375 in. (9.525 mm). The hardened ceramic mold is turned upside down, placed in an oven, and heated until the wax melts and drains away. The result is a hollow ceramic shell.
The ceramic mold is heated to around 1000 – 2000°F (550 – 1100°C). The heating process further strengthens the mold eliminates any leftover wax or contaminants, and evaporates water from the mold material.
Molten metal is poured into the mold while it is still hot – liquid metal flows into the pouring cup, through the central gating system, and into each mold cavity on the tree. The pre-heated mold allows the metal to flow easily through thin, detailed sections. It also creates a casting with improved dimensional accuracy, as the mold and casting will cool and shrink together.
After the mold has been poured, the metal cools and solidifies. The time it takes for a mold to cool into a solid state depends on the cast material and the thickness of the casting being made.
Once the casting solidifies, the ceramic molds break down, and the casting can be removed. The ceramic mold is typically broken up manually or by water jets. Once removed, the individual castings are separated from their gating system tree using manual impact, sawing, cutting, burning, or cold breaking with liquid nitrogen.
Finishing operations such as grinding or sandblasting are commonly employed to smooth the gate’s part and remove imperfections. Depending on the metal that the casting was poured from, heat-treating may be employed to harden the final part.
When to use investment casting
Due to its complexity and labor requirements, investment casting is a relatively expensive process – however, the benefits often outweigh the cost. Practically any metal can be an investment cast. Parts manufactured by investment casting are normally small, but the process can be used effectively for 75 lbs or more parts.
Investment casting is capable of producing complex parts with excellent as-cast surface finishes. Investment castings do not need to have taper built in to remove the components from their molds because the ceramic shells break away from the part upon cooling. This production feature allows castings with 90-degree angles to be designed with no shrinkage allowance built-in and with no additional machining required to obtain those angles.
The investment casting process creates parts with superior dimensional accuracy; net-shape parts are easily achievable, and finished forms are often produced without secondary machining. Each unique casting run requires a new die to produce wax patterns. Tooling for investment casting can be quite expensive; depending on the complexity, tooling costs can run anywhere between $1000 and $10,000.
For high-volume orders, the time and labor saved by eliminating or decreasing secondary machining easily make up for the cost of new tooling. Small casting runs are less likely to make up for the investment. Generally, investment casting is a logical choice for a run of 25 parts or more.
It usually takes seven days to go from a fresh wax pattern to a complete casting; most of that time is taken up by creating and drying the ceramic shell mold. Some foundries have quick-dry capabilities to produce castings more quickly. The time and labor-intensive nature of investment casting don’t only affect cost. Foundries have limited equipment and production capacity, so longer lead times for investment casting are common.
Investment casting has been used in various forms for the last 5,000 years. In its earliest forms, beeswax was used to form patterns necessary for the casting process. Today, more advanced waxes, refractory materials, and specialist alloys are typically used for making patterns. Machines investment casting is valued for its ability to produce components with accuracy, repeatability, versatility, and integrity in various metals and high-performance alloys.