The process of casting metal (also known as liquid forming) is a method of forming metal products of a certain shape and properties after solidification by melting and casting the metal into a pre-made mould (a container that makes the liquid metal a solid casting). Liquid forming was one of the earliest manufacturing processes and although the history of casting over the centuries has made the process less mysterious, it is still a fundamental forming technology.

Advantages of casting: low production costs, great process flexibility, adaptability, suitability for producing castings of different materials, shapes, and weights and for mass production.

Disadvantages: large tolerances (maximum limit size minus minimum limit size), susceptibility to internal defects, and the need for more materials and equipment. At the same time, casting production generates dust, harmful gases, and noise, which pollute the environment.

The casting process usually consists of the following three stages.

1. Casting preparation: Casting according to the materials used can be divided into sand, metal, ceramic, clay, graphite, etc., according to the number of times users can be divided into one-time casting, semi-permanent casting, and permanent casting. One-time casting is generally cast in the process of taking out the casting is destroyed, its cavity is commonly used wax, wood, plastic, or metal made of mould to manufacture. Permanent casts are a more costly but reusable type of casting. Because of the high cost and the extreme expense of on-the-spot modifications, some of which are not even repairable, it requires more design preparation time to prepare the cast.

The quality of the mould preparation is a major factor in the quality of the casting.

2. Melting and pouring of casting metals: casting metals (casting alloys) are mainly cast iron, cast steel, and casting non-ferrous alloys.

3. Casting processing and inspection: casting processing, including the removal of the core and casting surface foreign matter, removal of pouring mouth, shovel grinding burrs and phi seams and other protrusions, as well as heat treatment, shaping, rust treatment, and rough machining.

There are many types of casting, divided into the following according to the modelling method.

1. Ordinary sand casting, including wet and dry sand type, and chemical hardening sand type two categories.

2. Special casting, according to the modelling material can be divided into natural mineral sand and stone as the main modelling material of special casting (such as investment casting, shell casting, solid casting, ceramic casting, etc.) and metal as the main casting material of special casting (such as metal casting, pressure casting, continuous casting, low-pressure casting, centrifugal casting, etc.) two categories.

(a) Ordinary sand casting

1. Wet and dry sand casting

Wet sand type is damp sand compacted into a sandbox with wooden moulds or metal halves, the assembled cast can be with or without a core, molten metal cast into the final formation of the cavity, remove the parts when the cast is destroyed. The sand used for dry sand casting has a slightly higher moisture content in the wet state than the sand used for wet casting. After the sand is made, the surface of the cavity should be coated with refractory paint, and refractory materials to avoid overheating of the sand, and then placed in the oven to dry, after it has cooled down can be combined and poured. It takes a long time to dry the clay sand type, consuming a lot of fuel, and the sand type is easy to produce deformation in the drying process so the accuracy of the casting is subject to typecasting.

2. Chemically Hardened Sand

The chemically hardened sand type used in the sand is called chemically hardened sand. Its binder generally acts as a hardener, enabling molecular polymerization and thus forming a three-dimensional structure of the material, commonly used in various synthetic resins and water glass. The chemically hardened sand casting process is characterised by.

(1) the mould can more accurately reflect the size and contour shape of the pattern, and the resulting casting has high dimensional accuracy.

(2) Lightweight equipment structure, low power consumption and high productivity, and the treatment work parts can be simplified.

(3) When using chemically hardened sand moulding, the mould material can be selected according to production requirements, such as wood, plastic, and metal.

(b) Special Casting

1. Investment Casting

Investment casting, also known as lost wax casting, precision casting, or full mould casting, involves creating a replica of the final product by pressing paraffin-like materials into a precise and clean wax mould. The wax mould is coated with multiple layers of refractory materials, dried and hardened to form a shell. The wax inside the shell is then melted and drained, and the shell is roasted to strengthen and dry it. Finally, molten metal is poured into the shell, cooled and solidified, resulting in a precise and clean casting. It is mainly used for producing blades for steam and gas turbines, impellers for pumps, cutting tools, and small parts for aircraft, automobiles, tractors, wind turbines, and machine tools.

Features of investment casting

(1) High casting precision and good surface quality. For example, turbine engine blades cast by investment casting can meet no-machining allowance requirements;

(2) Capable of manufacturing complex-shaped castings, combining multiple parts into a single complex component, suitable for single casting of complex shapes.

(3) No limitation on alloy type; especially advantageous for high melting point and difficult-to-machine alloys.

(4) Production batch size is virtually unlimited, suitable for both large-scale and small-scale production, including mass and single-piece manufacturing.

(5) Complex processes, long production cycles, high production costs; castings should not be excessively large or too long.

2. Shell Casting

Shell casting is a casting method that uses a thin shell to produce castings.

The process involves using heat-hardened sand (generally resin sand) coated onto a heated metal mould (heating temperature usually around 300°C) to form a thin shell. The shell thickness is generally 6 to 12 mm, with sufficient strength and rigidity. The upper and lower shell halves are clamped or glued with resin, forming a complete mould without the need for a sand box, into which the molten metal is poured.

Shell casting has the following characteristics.

(1) Significantly reduces the amount of moulding sand used.

(2) The castings produced have a clear outline, a clean surface, and precise dimensions, often requiring little or no mechanical processing. It is particularly suitable for producing large batches of castings with high dimensional accuracy, thin walls, and complex shapes.

(3) The resin used in shell casting is expensive, the template must be precision machined, the cost is high, and there is also an irritating odour when pouring, which to some extent limits the widespread use of this method.

3. Solid casting

Solid casting is the use of polystyrene foam plastic mould samples instead of ordinary mould samples, to make a good model without removing the mould samples into the molten metal, the role of the molten metal, plastic mould samples burning, gasification, disappearance, the molten metal replaces the original plastic mould space occupied by the position, cooling and solidification to obtain the required casting method.

Solid-type casting has the following characteristics.

(1) Due to the use of a foam mould pattern that vapourises when it contacts the molten metal, there is no need to create a mould, no parting surface, no core, and therefore no flying burrs.

(2) Foam moulds can be bonded and moulded as a whole for complex castings of various shapes, reducing processing and assembly time, reducing casting costs by 10% to 30%, and providing full freedom in the structural design of the casting.

(3) Simplifies the production process of castings, shortens the production cycle, and increases the moulding efficiency by 2 to 5 times compared to sand casting.

(4) Solid casting mould samples can only be used once, and foam density is low, with low strength, making mould samples easy to deform, affecting the casting size accuracy, and the gas generated by the mould sample during casting pollutes the environment.

4. Ceramic casting

Ceramic casting is a new process developed on the basis of sand investment casting. The ceramic type uses pure refractory materials with high thermal stability for modelling, with ethyl silicate hydrolysis solution as a binder, through processes such as grouting, gluing, mould starting, baking, and others. The castings produced by this method have high dimensional accuracy and surface finish, so this method is also called ceramic precision casting.

(1) Sand sleeve modelling: the first water glass sand is used to make the sand sleeve. Manufacturing sand jacket mould B and then casting mould A should be larger than the thickness of the ceramic material. The sand jacket is manufactured in the same way as sand casting.

(2) Grouting and gluing: the process involves fixing the casting mould sample metal type, pouring liquid metal into the mould under gravity, brushing on the parting agent, attaching the sand sleeve, and preparing for casting. The casting type made of metal involves filling the ceramic slurry from the pouring mouth into the sand sleeve, which can be reused hundreds to thousands of times. Once the metal mould material has been manufactured, the ceramic paste begins to gel. The ceramic slurry consists of refractory materials (e.g., corundum powder, bauxite, etc.), binder (e.g., ethyl silicate hydrolysate), etc.

(3) Mould starting and spray firing: slurry pouring lasts for 5 to 15 minutes, during which the slurry still retains some elasticity, allowing the mould sample to be removed. To accelerate curing and improve mould strength, the entire cavity must be sprayed with an open flame.

(4) Roasting and moulding: the mould is heated to 350-550°C and roasted for 2-5 hours to burn off residual moisture and enhance the mould's strength.

(5) Pouring: the pouring temperature can be slightly higher to achieve a clear outline of the casting.

Characteristics of ceramic type casting.

(1) The ceramic surface layer begins the mould in an elastic state, while the ceramic surface layer is resistant to high temperatures and exhibits minimal deformation, so the dimensional accuracy and surface roughness of the castings are similar to those of investment mould casting.

(2) The weight of ceramic castings is almost unlimited and can range from a few kilograms to several tonnes.

(3) Suitable for single-piece, small batch production with low investment, short production cycle, generally feasible for foundries.

(4) Not suitable for large batch production, lightweight or complex-shaped castings, as the production process is difficult to mechanise and automate.

5. Metal casting

Metal moulding is a method of pouring liquid metal into a metal mould under gravity to obtain a casting. The castings are made of metal and can be used hundreds to thousands of times repeatedly. The melting point of the material used to make the metal type should generally be higher than the melting point of the alloy poured, such as pouring tin, zinc, magnesium, and other low melting point alloys, available grey cast iron for manufacturing metal types; for pouring aluminium, copper, and other alloys, it is necessary to use alloy cast iron or steel metal types.

Characteristics of metal type casting.

(1) High dimensional accuracy, small surface roughness, and minimal machining allowances.

(2) Fine grain of the casting, good mechanical properties.

(3) Can achieve multi-casting of a type, improve labour productivity, and save moulding materials.

(4) High manufacturing cost of metal types, not suitable for the production of large, complex-shaped, and thin-walled castings.

(5) Due to the rapid cooling rate, the surface of cast iron parts is prone to white mouth, making cutting processing difficult.

(6) Restricted by the melting point of the metal type material, alloys with high melting points are not suitable for casting with metal types. It is mainly used for mass production of copper alloys, aluminium alloys, and other castings, such as pistons, connecting rods, cylinder heads, etc. Metal type castings of cast iron parts have also been developed, but their size is limited to 300mm, and the weight does not exceed 8kg, such as the bottom plate of an electric iron.

6. Pressure casting

Pressure casting is a casting method in which liquid or semi-liquid metal is filled at high speed under high pressure and solidified into a casting under pressure. The pressure used is 4~500MPa, and the metal filling speed is 0.5~120m/s. The die casting machine used for pressure casting is divided into two types: hot chamber die casting machine and cold chamber die casting machine. The hot chamber die-casting machine immerses the pressure chamber in liquid metal; when the piston is at its highest position, the metal enters the pressure chamber, then the piston moves down, pressing the metal through the gate into the die cavity, where it quickly solidifies into shape. The cold chamber die-casting machine has a separate chamber and holding furnace. During casting, the liquid metal is first poured into the chamber and then pressed into the cavity by the injection piston. The hot chamber die-casting machine is suitable for die casting lead and zinc alloys with low melting points and can also be used for magnesium alloys. Cold chamber die-casting machines are suitable for die casting aluminium, copper, or magnesium alloys. Castings with more pores are not easy to heat treat or weld, affecting their performance, so most production involves several special pressure casting methods, commonly vacuum die-casting, inflatable die-casting, and fine speed dense die-casting.

(1) Vacuum die-casting: die-casting with the air in the cavity of the mould evacuated in advance.

(2) Inflatable die-casting: die-casting with oxygen-filled casting cavity, causing the liquid metal and oxygen to form a solid oxide, which diffuses within the casting;

(3) Fine speed dense die casting: liquid metal fills the casting cavity at low speed, and after the cavity is filled, small pistons are used to pressurise further.

Pressure casting allows the manufacture of complex castings, saving material, energy, and machining time. The weight of a die casting can range from a few grams to several tens of kilograms. The pressure casting method is suitable for mass production of castings, with high efficiency, and the process is easy to mechanise and automate, making it widely used in the automotive, instrumentation, agricultural machinery, electrical appliances, medical equipment, and manufacturing industries.

7. Continuous casting

Continuous casting is a casting method that uses a continuous crystalliser to pour liquid metal at one end and continuously draw out the moulded material from the other end. The crystalliser is generally made of materials with good thermal conductivity and certain strength, such as copper, cast iron, graphite, etc. The moulded material is cast into sections of various shapes, including square, rectangular, round, flat, tubular, or others.

A schematic diagram of a horizontal continuous ingot casting, with the crystalliser at the lower part of the ladle, through which steel is continuously drawn into ingots, with the surface of the ingots being fully solidified by secondary cooling with jet water below the crystalliser. When the ingot material reaches a certain length, it is cut into segments by a cutter for further processing.

The characteristics of continuous casting are:.

(1) Suitable for the production of castings with constant cross-sectional shapes and large lengths of alloys such as iron, steel, copper, lead, and magnesium.

(2) Continuous casting using equipment and processes are very simple, high production efficiency and metal utilisation, used and rolling mill to form a production line, but also substantial energy savings.

8. Low-pressure casting

In low-pressure casting, the metal liquid is pressed from the heated crucible into the mould through the riser tube under certain air pressure, filling the bottom without the turbulence that typically occurs in gravity-filled casting. After the casting has solidified, the pressure is reduced, allowing the remaining metal to flow back into the crucible.

The process of low-pressure casting: dry compressed air is slowly introduced into the crucible furnace; the metal liquid is pressurised by the gas, filling the cavity from the bottom up along the riser and pouring system; the mould is opened and the casting is removed.

Characteristics of low-pressure casting.

(1) The pressure and speed of pouring can be adjusted, so it can be applied to different casting types (e.g. metal type, sand type, etc.), casting various alloys and various sizes of castings.

(2) The metal liquid fills the mould smoothly and without splashing, which can avoid the involvement of gas and the scouring of the mould wall and core, and improves the qualified rate of the casting.

(3) Castings crystallised under pressure, castings with dense organisation, clear contours, clean surfaces, and high mechanical properties, particularly beneficial for the casting of large thin-walled parts.

(4) Increased metal utilisation rate from 90% to 98%.

(5) Low labour intensity, good labour conditions, simple equipment, easy-to-achieve mechanisation, and automation.

9. Centrifugal casting

Centrifugal casting is a method of casting in which molten metal is poured into a rotating mould so that the liquid metal fills the mould and solidifies under the action of centrifugal force. The liquid metal is injected into the high-speed rotating drum-shaped mould and the dense metal is pressed against the outer wall, resulting in a pipe with a uniform density of grain tissue. The process is mainly used for the production of cast iron tubes, cylinder liners, copper liners, bimetallic bearings, seamless billets of special steels, paper machine drums, and other castings.

Features of centrifugal casting.

(1) The liquid metal can form a hollow free surface in the casting pattern, and hollow castings can be cast without a core, simplifying the production process of sleeves and tube castings.

(2) Due to the role of centrifugal force generated by the liquid metal when rotating, centrifugal casting can improve the ability of the metal to fill the casting, so some poor mobility of the alloy and thin-walled castings can be produced by centrifugal casting.

(3) Due to the role of centrifugal force, gas, and non-metallic inclusions are also easy to discharge from the metal liquid, producing shrinkage, shrinkage, and porosity; and other defects are less likely.

(4) Gas, slag, and other inclusions in the metal, because of the lighter density and concentration on the inner surface of the casting, so the size of the inner hole is not accurate, the quality is also poor, the casting is easy to produce composition and density segregation.