Investment Casting
Investment Casting (The Lost Wax Process) has been practised for thousands of years, with lost wax process being one of the oldest known metal forming techniques. From 5000 years ago, when bees wax formed the pattern, to today’s high technology waxes, refractory materials and specialist alloys, the castings ensure high quality components are produced with the key benefits of accuracy, repeatability, versatility and integrity.
The process is suitable for repeatable production of net shape components, from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 300 kg and aluminium castings of up to 30 kg. Compared to other casting processes such as die casting or sand casting it can be an expensive process, however the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so requiring little or no rework once cast.
[edit] History of the Process
Investment Casting is one of the earliest forms of casting, dating back in use over 4000 years when production of idols, ornaments and jewellery using natural beeswax for patterns, clay for the moulds and manually operated bellows for stoking furnaces was carried out. Examples of investment cast production have been found in Egypt from the tombs of Tut-Ankh-Amun (1333 BC – 1324 BC). Other examples of early investment cast articles have also been found in Mesopotamia, Mexico, and the Benin civilization in Africa where the process was used to produce intricately detailed artwork of copper, bronze and gold.
The earliest known foundry text describing the investment casting process (Schedula Diversarum Artium) was written by a monk, Theophilus Presbyter around 1100AD who is known for recording differing processes from the recipe for making parchment to the lost wax process. This book was used by the sculptor and goldsmith, Benvenuto Cellini (1500AD - 1571AD) who describes in his autobiography in great detail the investment casting process used to produce his famous statue Perseus and the Head of Medusa, which stands to this day in the Loggia dei Lanzi in Florence, Italy.
The investment casting process was largely ignored as a modern industrial process until the late 19th century when it started to be used by the dental profession for producing crowns and inlays. An authenticated record of the use of investment castings in dentistry appears in a paper written by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. However, the significance of this process was not realised until research published by Dr. William H. Taggart of Chicago in 1907, in which Dr. Taggart not only developed and described a technique, but also formulated a wax pattern compound of excellent properties, developed an investment material, and even invented an air pressure casting machine.
Investment casting really started to become a modern industrial process in the 1940's as the onset of World War II increased the demand for precision net shape manufacturing processes and the use of specialized alloys that could not be shaped by traditional methods. Investment casting could produce these near net shape components more quickly and accurately than traditional machining methods that were becoming overwhelmed by the demand.
Investment Casting Process was found practical for many wartime needs--and during the postwar period it expanded into many commercial and industrial applications where complex metal parts were needed.
Modern investment casting techniques stem from the development in the United Kingdom of a shell process utilizing 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 Investment Casting industry, is one of the principal suppliers of precision net shape components to several markets, including:
- Aerospace
- Automotive
- Defense
- Medical
- Commercial
[edit] The Investment Casting Process
A pattern of the component to be cast is produced by injection moulding specialist waxes into a metal die, pre-formed ceramic cores can be included in the wax pattern as it is moulded enabling intricate hollows within the finished casting. The patterns are assembled into a tree around a wax runner system (riser & sprue) which may contain hundreds of patterns. Once a tree has been assembled a pour cup is attached to the tree.
The completed tree is dipped (invested – hence the name Investment Casting) either by hand or via a robotic control into a ceramic slurry of either ethyl silicate (Alcohol based and chemically set), colloidal silica (Water based, also known as silica sol, set by drying) or a hybrid of these dependant upon the exact process used by each foundry which is controlled for pH and viscosity. A fine sand is then applied to the invested tree in a fluidised bed or rain tower sander or if being hand made the sand is applied by hand. During the primary coat(s) the sand will typically be a zircon based sand, 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 either mullite, molochite, chamotte or fused silica refractory material. This process is repeated until sufficient shell thickness has been built up to withstand the mechanical shock of pouring the molten metal into the shell. The production time for a single tree can be as little as 48 hours or as great as 1 week dependent upon the drying system being used. There will normally be a final dry time on the tree of between 24 and 48 hours to allow the refractories to set.
Turbo charger shell
After the shell (Fig 1.) has been constructed to the desired specifications, the wax must be removed, this is achieved using either an autoclave or in a de-wax furnace, this is where the name “lost wax process” comes from. This de-waxing process is the cause of most shell failures during the whole process, 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 1100 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.
View of the ceramic impression in a Turbo charger shell
Most foundries remove the shells from the furnace whilst still hot and pour the desired 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. The molten metal is then allowed to cool within the shell.
Once the molten metal has cooled the shell is removed. This is generally done with water jets, vibration, grit blasting or chemical dissolution. The foundry is left with the cooled parts although still attached to the tree, the parts are removed from the tree at the gate. Some foundries remove the parts by cold-break (dipping in liquid nitrogen and breaking the parts off with a hammer and chisel) or with cut-off saws.
The parts are then finished, for a lot of investment cast parts the only finishing work required is to grind the gate and any runner bar attachments. Due to the slow cooling of the molten metal the hardness is not as high as some forging and machining processes. Investment cast parts that require further hardening processes are subjected to various post casting operations such as heat treatment, surface hardening, HIP (Hot Isostatic Pressing) hardening (Known as HIPping).
Other post casting processes include inspection which will be carried out visually, and in the case of special application parts X-ray examination will be carried out either at the foundry or by third party suppliers.
Special characteristics of the Investment Casting process:
- High production rates, particularly for small components
- High dimensional accuracy and consistency
- High integrity castings
- Extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness)
- Highly complex shapes can be cast
- Long or short runs can be accommodated
- Machining is virtually eliminated
- Almost any alloy can be cast
The negatives of Investment Casting are:
- Specialized equipment needed
- The process can be expensive because costly refractories and binders are used and many operations are required to make a mould.
- Because of the exceptional surface finish possible and expected, minute defects can cause rejection of castings and scrap rates can be high.
Investment casting is often used in the aerospace and power generation industries to produce single-crystal turbine blades, which exhibit superior creep resistance compared to those produced by equiaxed castings. A combination of slow cooling rates, seed crystals, and an elaborate sprue and runner system referred to as a “pigtail” are used to produce single-crystal castings. Standard Investment cast parts are used in aerospace, military, medical, commercial and automotive industries.