investment casting The process

A pattern of the component to be cast is produced by injection-moulding special waxes into a metal die. Pre-formed ceramic cores can be included in the wax pattern as it is moulded, which can create intricate hollows within the finished casting. As many as several hundred patterns may be assembled into a tree around a wax runner system (riser & sprue). Once a tree has been assembled, a pour cup is attached.
Fig 2. View of the ceramic impression in a turbocharger shell
Fig 2. View of the ceramic impression in a turbocharger shell

The completed tree is dipped, or invested, by hand or via robotic control into a ceramic slurry of ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying) or a hybrid of these controlled for pH and viscosity. A fine sand is applied to the invested tree in a fluidised bed, rain tower sander, or by hand. During the primary coat(s), the sand will typically be a zircon-based, as zirconium is less likely to react with the molten metal when poured into the shell. The stuccoed tree is then allowed to dry before re-dipping in slurry and applying secondary coats of mullite, Molochite, chamotte or fused silica refractory material. This process is repeated until the shell is thick enough to withstand the mechanical shock of receiving the molten metal. Dry times generally range from 24 to 48 hours, and total production from two days to one week.
Completed turbocharger
Completed turbocharger

After the shell (Fig 1.) has been constructed, the wax is removed in an autoclave or furnace (hence, the lost-wax process). Most shell failures occur at this point, as the fragile stuccoed shell is subjected to extremes of temperature and, in an autoclave, pressure. The shell is then fired at temperatures of around 1,100 degrees Celsius to induce chemical and physical changes in the set refractory materials forming a ceramic shell. This leaves a ceramic impression (Fig 2.) of the part to be cast. Most foundries remove the shells from the furnace while still hot and pour the molten metal into the ceramic shell. Various methods of pouring the molten metal include vacuum casting, anti-gravity casting, tilt casting, gravity pouring, pressure assisted pouring, centrifugal casting. After the molten metal cools, the shell is removed. This is generally done with waterjets, vibration, grit blasting or chemical dissolution. The cooled parts are removed from the tree by sawing them free or by dipping them in liquid nitrogen and breaking them off with a hammer and chisel. The parts are then finished. Many cast parts require grinding of the gate and runner bar attachments. Because molten metal cools slowly, it does not finish as hard as some forging and machining processes. Cast parts often are subsequently hardened by heat treatment, surface hardening, or HIP (Hot Isostatic Pressing) hardening (Known as HIPping). The parts are inspected by eye or in special cases by X-ray at the foundry or by specialty firms.