Investment Casting Process1. 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 from which to manufacture the wax patterns is needed.
The size of the master die must be carefully calculated; it must take into consideration 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 to be produced; each individual 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 an exact 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. For smaller castings, several wax patterns are attached to a central wax gating system to form a tree-like assembly. A pouring cup, typically attached to the end of the runner bars, serves to introduce 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 of 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 strengths 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 material that was cast 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 by cold breaking with liquid nitrogen.
Finishing operations such as grinding or sandblasting are commonly employed to smooth the part at the gates and remove imperfections. Depending on the metal that the casting was poured from, heat treating may be employed harden the final part.