Investment Casting: Art Of Molding Liquid Metals

Investment casting, also known as lost-wax casting, is a manufacturing process that has been used in various forms for the last 5,000 years. It involves creating a wax pattern, layering it with sand and slurry, and then melting out the wax and pouring molten metal into the ceramic shell to create small, precise metal components. This process is used to make complex parts with excellent surface finishes and tight tolerances, and is particularly well-suited for the production of aluminium components. Investment casting is commonly used in industries such as aerospace, military, automotive, jewellery, and medical equipment.

Wax pattern production

Creating the Pattern

The first step is to create a pattern with the same details as the final part, taking into account allowances for thermal contraction (shrinking). Patterns are typically made of wax using a metal injection die. The pattern can also be made using other materials such as clay, wood, or plastic. In recent times, 3D printing technology has been utilised to create patterns with high resolution and accuracy.

Mounting the Wax Patterns and Creating the Tree

Once the wax pattern is produced, it is assembled with other wax components to form the gate and runner system for metal delivery. Multiple wax patterns may be combined and attached to a wax sprue to create a pattern cluster or a "tree". This assembly allows for the efficient casting of multiple parts in a single batch pour.

Creating the Mold Shell

The entire wax pattern assembly is then dipped into a ceramic slurry and covered with sand stucco. This process is repeated in cycles, with each cycle creating a new layer of the shell. The thickness of the shell is determined by the size and configuration of the desired final product. The dipping and stuccoing process is repeated until the desired thickness is achieved, usually around 5 to 15 mm.

Drying and Hardening the Shell

After the desired thickness is reached, the shell is left to dry completely. This step ensures that the shell becomes strong enough to retain the molten metal during casting. The drying process can take anywhere from 16 to 48 hours, and it can be accelerated by applying a vacuum or reducing environmental humidity.

Removing the Wax

Once the shell is dry, it is placed in a furnace or autoclave to melt and remove the wax pattern. This step is crucial as it creates the cavity that will be filled with molten metal. The wax is heated rapidly to minimise stress and prevent shell failures. Any remaining wax is burned out in a furnace, leaving behind a ceramic mould with a cavity in the shape of the desired cast part.

The above steps outline the key stages of wax pattern production in the investment casting process. This process allows for the creation of precise and intricate metal parts while minimising waste and energy consumption.

Ceramic mould creation

Ceramic mould casting is a versatile casting process that can be used for both simple foundry casting and intricate industrial castings. This process uses ceramic materials to form moulds for casting end-use items, mainly metal. The ceramic moulds are expendable and must be recreated each time. However, the use of reusable and cost-effective patterns made from materials such as metals, plastic, wood, or rubber ensures reproducibility.

The process begins with the creation of a pattern, which is an exact replica of the desired final product. A ceramic slurry, a mixture of fine ceramic particles and a liquid binder, is then applied to the pattern's surface to form a thin coating. The slurry solidifies, creating a ceramic shell or mould over the pattern.

The next step is to eliminate volatiles through low-temperature heating. This is typically done by placing the mould in a low-temperature oven or using a flame torch. The heat helps to drive off any remaining moisture and ensures the mould is fully dried and ready for casting.

After the elimination of volatiles, the mould is further processed to achieve greater strength and stability. It is placed in a high-temperature furnace where it undergoes a firing process to harden the ceramic shell. This is typically done at a temperature of around 1000 °C. This step is crucial to ensure that the mould can withstand the high temperatures of the casting process.

Once the ceramic mould is prepared, the two halves, known as the cope and drag sections, are joined together to create a complete mould for the metal casting. To enhance the mould's strength, fireclay material may be used to support both sections. In some cases, the ceramic mould is preheated before the molten metal is poured into it.

With the mould now ready, the desired metal is melted and brought to a suitable pouring temperature. The molten material is then carefully poured into the ceramic mould. It is left to cool and solidify into its final shape within the mould.

The Shaw process and true ceramic moulding are the two types of ceramic mould casting. The Shaw process uses a mixture of refractory aggregate, hydrolyzed ethyl silicate, alcohol, and a gelling agent to create a mould. This slurry mixture is poured into a slightly tapered flask, and a reusable pattern is used. The slurry hardens almost immediately to a rubbery state, and the flask and pattern are then removed. A torch is used to ignite the mould, causing most of the volatiles to burn off and creating ceramic micro-cracks. These cracks are important as they allow gases to escape while preventing the metal from flowing through. After the burn-off, the mould is baked at 1,800 °F (980 °C) to remove any remaining volatiles.

True Ceramic Moulding involves the initial bonding of refractory grain with ammonium or calcium phosphates. Ceramic moulds are usually created using the dry pressing method, where a clay mixture with a specified moisture percentage is pressed into dies using a pressure of around 1-10 tons per square inch. After being removed from these dies, the moulds are ready to be baked in furnaces at temperatures ranging from 899°C to 1316 °C.

Pouring

The pouring stage of the investment casting process involves placing the investment mould open-side up into a tub filled with sand. Molten metal is then poured into the cavity where the wax pattern was. The metal solidifies within the ceramic cavity, cools, and the ceramic is removed from the metal casting. This results in a net to near-net precision metal component which can be used for a broad range of applications in various industries.

The metal may be gravity poured or forced by applying positive air pressure or other forces. Vacuum casting, tilt casting, pressure-assisted pouring, and centrifugal casting are methods that use additional forces and are especially useful when moulds contain thin sections that would be otherwise difficult to fill.

Vacuum casting, also known as counter-gravity casting, is a variation on the gravity pouring technique. It involves filling the mould using a vacuum. A common form of this technique is the Hitchiner process, which uses a downward fill pipe that is lowered into the melt. A vacuum draws the melt into the cavity; once the important parts have solidified, the vacuum is released, and the unused material leaves the mould. This technique can use substantially less material than gravity pouring as less material solidifies in the gating system.

Vacuum pressure casting (VPC) uses gas pressure and a vacuum to improve the quality of the casting and minimise porosity. Typically, VPC machines consist of an upper and lower chamber—the upper chamber, or melting chamber, houses the crucible, and the lower casting chamber houses the investment mould. Both chambers are connected via a small hole containing a stopper. A vacuum is pulled in the lower chamber, while pressure is applied in the upper chamber, and then the stopper is removed. This creates the greatest pressure differential to fill the moulds.

Solidification

The solidification process can be divided into two types: directional solidification and unidirectional solidification. Directional solidification is a faster process where cooling starts from the bottom of the mould and moves upwards. This rapid cooling results in a less smooth surface and is often used for making turbine blades. On the other hand, unidirectional solidification is a slower process, with cooling starting from the top of the mould and moving downwards. This method yields a smoother surface and is employed in the manufacturing of turbine vanes.

During solidification, the molten metal loses energy, and crystals begin to form near the mould walls, eventually growing into grains. The rate of cooling determines the length of these grains. If the metal cools quickly, shorter grains form, while slower cooling results in longer grains. As the metal continues to solidify, it shrinks, and risers are used to feed this shrinkage to prevent voids and defects in the final casting.

The solidification process is a critical step in the casting procedure, and skilled foundry workers are required to control it effectively to avoid casting defects that can arise from improper handling.

Shakeout and cleanup

Shakeout is the process of separating the casting from its mould. It is usually done manually in small foundries or with small castings, where a worker will dig into the mould with tongs to grab a sprue or runner. The casting is then lifted out while the mould crumbles, and is tapped with another tool to cause excess debris to fall away. Alternatively, this can be done on an agitation/vibration table that shakes as unflasked moulds are dropped onto it. The shaking dislodges the sand, which drops from the casting and sifts through holes in the table. The sand is then recaptured and used again for moulding.

For investment castings, the hard mould of ceramic sand needs to be cracked open manually or removed with powerful water jets. Sand cores, which are made with resin or another substance to provide greater structure, may need more than a vibrating table to be removed. Their removal depends on the type of material used for the core. Some sand cores are constructed to be highly collapsible after sitting in the heat of the mould. Others maintain their structure throughout the pouring but can be baked out using lower temperatures for longer times. Some are built around a wire frame that will need to be mechanically extracted.

After shakeout, the casting will be in its final shape, but it will still need to be cleaned. Castings are traditionally cleaned through a process called fettling, which involves removing the sprue, gate, risers, runners and any small imperfections, as well as the final remnants of burnt-on sand. Cleaning can be done with shot blasters, air-blast machines, tumblers, water jets, pneumatic hammers or drills, and hydroblast systems.

Frequently asked questions