Investment Casting

For investment casting in art, see lost-wax casting.

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Investment casting, also called lost-wax casting, is one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.

The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive than die casting or sand casting, but can produce complicated shapes that require little rework.

Investment casting offers high production rates, particularly for small or highly complex components, and extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) with very little machining. The drawbacks include the specialized equipment, costly refractories and binders, many operations to make a mould, and occasional minute defects.

Investment casting is used in the aerospace and power generation industries to produce single-crystal turbine blades, which have more creep resistance than equiaxed castings. Other industries that use standard investment-cast parts include military, medical, commercial and automotive.

History

Investment casting dates back thousands of years. Its earliest use was for idols, ornaments and jewellery, using natural beeswax for patterns, clay for the moulds and manually operated bellows for stoking furnaces. Examples have been found in Egypt's tombs of Tutankhamun (1333 – 1324 BC), in Mesopotamia, Mexico, and the Benin civilization in Africa where the process produced detailed artwork of copper, bronze and gold.

The earliest known text that describes the investment casting process (Schedula Diversarum Artium) was written around 1100 A.D. by Theophilus Presbyter, a monk who described various manufacturing processes, including the recipe for parchment. This book was used by sculptor and goldsmith Benvenuto Cellini (1500 - 1571), who detailed in his autobiography the investment casting process he used for the Perseus and the Head of Medusa sculpture that stands in the Loggia dei Lanzi in Florence, Italy.

Investment casting came into use as a modern industrial process in late 19th century, when dentists began using it to make crowns and inlays, as described by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. Its use was accelerated by Dr. William H. Taggart of Chicago, whose 1907 paper described his development of a technique. He also formulated a wax pattern compound of excellent properties, developed an investment material, and invented an air-pressure casting machine.

In the 1940s, World War II increased the demand for precision net shape manufacturing and specialized alloys that could not be shaped by traditional methods, or that required too much machining. Industry turned to investment casting. After the war, its use spread to many commercial and industrial applications that used complex metal parts.

Modern investment casting techniques stem from the development in the United Kingdom of a shell process using wax patterns known as the Investment X Process. This method resolved the problem of wax removal by enveloping a completed and dried shell in a vapor degreaser. The vapor permeated the shell to dissolve and melt the wax. This process has been evolved over years into the current process of melting out the virgin wax in an autoclave.

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.   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.   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 off or by dipping them in liquid nitrogen and breaking them off with a hammer and chisel. The parts are then finished. Many cast parts need grind the gate and runner bar attachments. Because the molten metal cools slowly, it does not finish as hard as some forging and machining processes. Cast parts may be 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.