Casting Processes
Understanding Modern Casting Processes
Different Methods for Different Needs
Casting isn’t a single technique — it’s a family of processes, each suited to different shapes, materials and production requirements. Every method offers its own balance of precision, strength, finish and cost, allowing manufacturers to create reliable components for a wide range of applications. The sections below outline the key processes and what sets each one apart.
From Concept to Finished Component
Choosing the right casting process is an important early step in any project. Factors such as geometry, material specification, required tolerances and production volume all influence which method will deliver the best outcome. This page introduces each process in clear, practical terms so you can understand how they work — and when they’re the right fit.
Casting processes
A variant of investment casting, and used to produce high integrity steel castings using a ceramic moulding process, the Shaw Process uses ethyl silicate to produce the shells and has a shorter lead time and can be done with lower cost tooling, compared with investment casting.
The Shaw process is a precision casting process capable of the production of accurate moulds with excellent surface finish and metallurgical integrity. Moulds are produced using highly refractory aggregates bonded with silica provided by a liquid ethyl silicate binder. A high temperature firing treatment is a feature of the production sequence and this produces an inert mould into which the majority of commercial ferrous and non-ferrous alloys can be cast with confidence.
The process has been used commercially for many years; it was known before the Second World War that silicon esters could be used as refractory aggregate binders’. As with most processes there has been a continuous development, in particular with respect to the binder system and the methods of mould production.
Using a resin coated sand, thin shells (sometimes termed biscuits) are produced using heated metal dies and then paired to make the mould cavity into which the metal is poured to produce castings with high dimensional accuracy and high productivity rates.
The shell moulding process is a precision sand casting process capable of producing castings with a superior surface finish and better dimensional accuracy than conventional sand castings. These qualities of precision can be obtained in a wider range of alloys and with greater flexibility in design than die-casting and at a lower cost than investment casting. The process was developed and patented by Croning in Germany during World War II and is sometimes referred to as the Croning shell process. The fundamental feature of the process is the use of fine-grained, high purity sand that contributes the attributes of a smooth surface and dimensional accuracy to moulds cores and castings alike. In conventional sand moulding the use of such fine sand is precluded because it would dramatically reduce mould permeability.
A further improvement in casting accuracy can be obtained if zircon sand is used instead of silica sand. That arises because the expansion of zircon sand, caused by the heat of the cast metal, is both lower and more predictable than that of silica sand. Foundry production of castings by the process is comparatively straightforward and the process lends itself readily to close control, with the advantage of consistency in the castings produced.
Typical applications include crankshafts, camshafts, oil & gas parts and general engineering.
Similar to sand casting, the mould is made of Plaster of Paris, rather than sand, and is used when an excellent surface finish and good dimensional accuracy is required. A more unusual process, and only applicable for lower melting point alloys, the process is more expensive.
In the conventional process, plaster of Paris is mixed with water to produce a slurry which is poured over a permanent pattern contained within a moulding box.
Upon setting a rigid mould is produced which, after pattern stripping, is dried at an elevated temperature to remove free and chemically combined water before the metal is cast into the mould. This method produces a strong, dense, but inherently impermeable mould. Hence, metal casting must be conducted using vacuum and/or pressure assistance to ensure complete filling of the mould by the metal. The insulating nature of the plaster improves the fluid life of the metal that aids mould filling and thin section production.
Typical applications include large pumps, valve bodies, ship propellers, large engineering castings.
A tool or die is designed and manufactured to the required shape of the component to be made. Molten metal is pushed into the die under low pressure and good surface finish and thin wall thickness are achievable. Once solidified the casting is removed from the die which can be used many times to produce identical parts.
A mould or die, having a horizontal parting line, is mounted on a holding furnace and is connected to the molten metal by a feed tube or stalk. The furnace is pressurised by the introduction of air above the surface of the molten metal causing it to rise steadily in the stalk and quietly fill the mould. The air in the mould cavity is expelled through suitably positioned vents in the die and when the cavity is filled, solidification commences.
Directional solidification, commencing at the extremities and terminating at the sprue, is effected by correct die design and eliminates the need for conventional feeding systems. When the metal has solidified as far back into the sprue as is required, pressure is released in the furnace and the molten metal left in the stalk returns to the holding furnace.
A further short cooling period is allowed to ensure that all sections of the casting are solid, the mould is opened and the casting removed.
Typical applications
- Aluminium automotive parts: wheels, cylinder heads, blocks, manifolds and housings.
- Critical aerospace castings
- Electric motor housings
- Domestic kitchen ware such as pressure cookers
A variation of the investment casting process, the pattern in lost foam casting is made of expanded polystyrene, dipped (invested) in a ceramic shell. Design flexibility is a key feature of this process, which lends itself to large castings, complex shapes and low part numbers.
Pre-forms of the parts to be cast are moulded in expanded polystyrene (or other expandable polymers) using aluminium tooling. Gluing EPS mouldings together can form complex shapes. The pre-forms are assembled into a cluster around a sprue then coated with a refractory paint. The cluster is invested in dry sand in a simple moulding box and the sand compacted by vibration. Metal is poured, vaporising the EPS pre-form and replacing it to form the casting.
Typical applications include large pumps, valve bodies, ship propellers, large engineering castings.
Used to produce large numbers of high quality castings for aerospace and medical applications, investment casting uses a wax copy of the component, dipped (invested) in a ceramic slurry to produce a ceramic mould. The wax is then removed to leave the ceramic shell, into which molten metal is poured.
Also known as the lost wax process, Patterns of the castings to be made are moulded by injection of a special wax into a metal die. Cores of pre-formed ceramic may be incorporated into the wax patterns as they are moulded. The patterns are assembled into a cluster (often comprising tens or even hundreds of patterns) around a wax runner system. The ‘tree’ of patterns is then coated with 8-10 layers of a refractory material, each layer being dried or chemically cured before the next layer is applied.
The assembly is heated to remove the wax, then fired at high temperature to bond the refractory mould strongly. The hot mould is cast and when cool the mould material is removed by impact, vibration, grit blasting, high pressure water blasting or chemical dissolution leaving the castings, which are then removed from the runner system.
Used for special purpose parts and with specialised alloys, typical applications include Aircraft engine turbine blades, jewellery, medical implants, statues
A volume casting process used to produce castings with very good surface quality, at high production rates and low wall thicknesses are readily achieved. The dies and equipment used in the process are expensive but the dies may be re-used to produce hundreds of thousands of identical components.
High pressure diecasting (HPDC) takes place suing steel dies which are loaded into hydraulically operated machines which exert high pressure onto the dies. Metal, mostly aluminium but also zinc and magnesium is forced into the die under high pressure to rapidly fill the cavity within. The metal chills off and once solid the casting is removed from the die. This can be done by hand but in large volume facilities tends to be done using automated equipment such as robots. Complex castings weighing in the region of 15kg (30lb), for the automotive industry are produced.
Typical markets are for car engine, body panels, steering and suspension
Medical – Monitor housings and control joy sticks for MRI scanners
Technology – cases and frames for computers, mobile communication devices
A highly flexible process, greensand casting can be used for high volume, fully automated casting or for very low number of parts. Sand, bound with clay and water is used to make moulds around a pattern made of metal, wood or resin – metal is poured into the mould and once solidified, the sand can be re-used over and over again making greensand casting one of the most cost effective and environmentally friendly of all the casting processes.
Clay-bonded sands have provided the principal medium from which moulds for castings have been produced for centuries. In essence the mould material consists of sand, usually silica in the quartz form, clay and water. The water develops the bonding characteristics of the clay, which binds the sand grains together. Under the application of pressure, the mould material can be compacted around a pattern to produce a mould having sufficient rigidity to enable metal to be poured into it to produce a casting. When the mould is used in its moist condition it is referred to as green and the method of producing the moulds as the green sand moulding process. If the mould is dried at a temperature just above 100°C (212°F) the majority of the free moisture will be removed. This is the principal of the dry sand moulding process. Removal of the free moisture is accompanied by a significant increase in the strength and rigidity of the mould. This enables the mould to withstand much greater pressures and so, traditionally, the dry sand process has been used in the manufacture of large, heavy castings.
Typical applications are machine tools, sculptures, architectural, general engineering
A tool or die is designed and manufactured to the required shape of the component to be made. Molten metal is introduced into the die by under gravity. Once solidified the casting is removed from the die which can be used many times to produce identical parts with good surface finish.
The production of castings from a reusable mould or die having two or more parts each located in relation to other parts. The metal is poured in by hand or automated ladles and uses gravity to fill the die. The die contains an impression of the casting together with its running, feeding and venting systems. Provision is made for the removal of the casting by some means. The die can readily be cleared of debris such as hot metal splashes and sand so that the casting’s accuracy is maintained.
The process is capable of producing castings in cast iron, copper base alloys, aluminium alloys and other metals including zinc. It is used for medium to long runs of castings with weight ranges typically from ½ kg to 50kg. It can be automated to a certain extent on the closing and pouring side of the process.
Used in most markets and applications where light alloys are needed as most of the production is of aluminium parts.
Sand is bound together around a pattern, which is designed in the shape of the component to be made – the pattern is removed and molten metal is poured into the cavity. The pattern may be re-used and the sand can generally be recycled.
Chemically Bonded sand moulds are created using a wood, metal, or plastic pattern. Sand is mixed with a binder and hardener in a high-speed mixer. This sand is deposited into a box containing the pattern and all essential gating, risers and chills for pouring. This sand mixture sets hard in a few minutes and the mould is removed from the pattern. Cores for forming internal passages in the castings may be made using the same process. The moulds are then closed and are ready for pouring.
Used for most market sectors for producing large castings due to the stability of the moulds.
