Metal casting process begins by creating a mold, which is the ‘ shape of the part we need. The mold is made from a refractory material, for example, sand. The metal is heated in an oven until it melts, and the molten metal is poured into the mould cavity. The liquid takes the shape of cavity, which is the shape of the part. It is cooled until it solidifies. Finally, the solidified metal part is removed from the mould.
A large number of metal components in designs we use every day are made by casting. The reasons for this include:
(a) Casting can produce very complex geometry parts with internal cavities and hollow sections.
(b) It can be used to make small (few hundred grams) to very large size parts (thousands og kilograms)
(c) It is economical, with very little wastage: the extra metal in each casting is re-melted and re-used.
(d) Cast metal is isotropic- it has the same physical/ mechanical properties along any direction.
There are several types of metal casting method:
Sand casting uses natural or synthetic sand (lake sand) which is mostly a refractory material called silica (SiO2). The sand grains must be small enough so that it can be packed densely; however, the grains must be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized molds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cut-off and re-used also.
Typical sand molds have the following parts:
– The mold is made of two parts, the top half is called the cope, and bottom part is the drag.
– The liquid flows into the gap between the two parts, called the mold cavity. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is (almost) identical to the shape of the part we need to make.
– A funnel shaped cavity: the top of the funnel is the pouring cup; the pipe-shaped neck of the funnel is the sprue- the liquid metal is poured into the pouring cup, and flows down the sprue.
– The runners are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity. The region where any runner joins with the cavity is called the gate.
– Some extra cavities are made connecting to the top surface of the mold. Excess metal poured into the mould flows into these cavities, called risers. They act as reservoirs; as the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows back down to avoid holes in the cast part.
– Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the air in the cavity to escape.
– Cores: Many cast parts have interior holes (hollow parts), or other cavities in their shape that are not directly accessible from either piece of the mold. Such interior surfaces are generated by inserts called cores. Cores are made by baking sand with some binder so that they can retain their shape when handled. The mold is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mold. After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off.
Shell-mold casting yields better surface quality and tolerances. The process is described as follows:
-The 2-piece pattern is made of metal (e. g. aluminum or steel), it is heated to between 175 ºC-370 ºC, and coated with a lubricant, e.g. silicone spray.
-Each heated half-pattern is covered with a mixture of sand and a thermoset resin/epoxy binder. The binder glues a layer of sand to the pattern, forming a shell. The process may be repeated to get a thicker shell.
-The assembly is baked to cure it
Expendable-pattern casting (lost foam process)
The pattern used in this process is made from polystyrene (this is the light, white packaging material which is used to pack electronics inside the boxes). Polystyrene foam is 95% air bubbles, and the material itself evaporates when the liquid metal is poured on it.
The pattern itself is made by molding -the polystyrene beads and pentane are put inside an aluminum mold, and heated; it expands to fill the mold, and takes the shape of the cavity. The pattern is removed, and used for the casting process, as follows:
-The pattern is dipped in a slurry of water and clay (or other refractory grains), it is dried to get a hard shell around the pattern.
-The shell-covered pattern is placed in a container with sand for support, and liquid metal is poured from a hole on top.
-The foam evaporates as the metal fills the shell; upon cooling and solidification, the part is removed by breaking the shell.
-The patterns are removed, and the two half-shells joined together to form the mold; metal is poured into the mold.
-When the metal solidifies, the shell is broken to get the part.
The process is useful since it is very cheap, and yields good surface finish and complex geometry. There are no runners, risers, gating or parting lines- thus the design process is simplified. The process is used to manufacture crank-shafts for engines, aluminum engine blocks, manifolds etc.
The mold is made by mixing plaster of paris (CaSO4) with talc and silica flour; this is a fine white powder, which, when mixed with water gets a clay-like consistency and can be shaped around the pattern (it is the same material used to make casts for people if they fracture a bone). The plaster cast can be finished to yield very good surface finish and dimensional accuracy. However, it is relatively soft and not strong enough at temperature above 1200 ºC, so this method is mainly used to make castings from non-ferrous metals, e.g. zinc, copper, aluminum, and magnesium.
Since plaster has lower thermal conductivity, the casting cools slowly, and therefore has more uniform grain structure (i.e. less warpage, less residual stresses).
Ceramic mold casting
Similar to plaster-mold casting, except that ceramic material is used (e. g. silica or powdered Zircon ZrSiO4). Ceramics are refractory (e. g. the clay hotpot used in Chinese restaurants to cook some dishes), and also have higher strength that plaster.
-The ceramic slurry forms a shell over the pattern;
-It is dried in a low temperature oven, and the pattern is removed 8
-Then it is backed by clay for strength, and baked in a high temperature oven to burn off any volatile substances.
-The metal is cast same as in plaster casting.
This process can be used to make very good quality castings of steel or even stainless steel; it is used for parts such as impellor blades (for turbines, pumps, or rotors for motorboats).
Investment casting (lost wax process)
This is an old process, and has been used since ancient times to make jewellery-therefore it is of great importance to HK. It is also used to make other small (few grams, though it can be used for parts up to a few kilograms). The steps of this process are shown in the figure 10 below.
An advantage of this process is that the wax can carry very fine details- so the process not only gives good dimensional tolerances, but also excellent surface finish; in fact, almost any surface texture as well as logos etc. can be reproduced with very high level of detail.
This process is also called counter-gravity casting. It is basically the same process as investment casting, except for the step of filling the mold (step (e) above). In this case, the material is sucked upwards into the mould by a vacuum pump. The figure 9 below shows the basic idea – notice how the mold appears in an inverted position from the usual casting process, and is lowered into the flask with the molten metal.
One advantage of vacuum casting is that by releasing the pressure a short time after the mold is filled, we can release the un-solidified metal back into the flask. This allows us to create hollow castings. Since most of the heat is conducted away from the surface between the mold and the metal, therefore the portion of the metal closest to the mold surface always solidifies first; the solid front travels inwards into the cavity. Thus, if the liquid is drained a very short time after the filling, then we get a very thin walled hollow object, etc.
Permanent mold casting
Here, the two halves of the mold are made of metal, usually cast iron, steel, or refractory alloys. The cavity, including the runners and gating system are machined into the mold halves. For hollow parts, either permanent cores (made of metal) or sand-bonded ones may be used, depending on whether the core can be extracted from the part without damage after casting. The surface of the mold is coated with clay or other hard refractory material – this improves the life of the mold. Before molding, the surface is covered with a spray of graphite or silica, which acts as a lubricant. This has two purposes -it improves the flow of the liquid metal, and it allows the cast part to be withdrawn from the mold more easily. The process can be automated, and therefore yields high throughput rates. Also, it produces very good tolerance and surface finish. It is commonly used for producing pistons used in car engines, gear blanks, cylinder heads, and other parts made of low melting point metals, e.g. copper, bronze, aluminum, magnesium, etc.
Die casting is a very commonly used type of permanent mold casting process. It is used for producing many components of home appliances (e.g. rice cookers, stoves, fans, washing
and drying machines, fridges), motors, toys and hand-tools- since Pearl river delta is a largest manufacturer of such products in the world, this technology is used by many HK-based companies. Surface finish and tolerance of die cast parts is so good that there is almost no post-processing required. Die casting molds are expensive, and require significant lead time to fabricate; they are commonly called dies. There are two common types of die casting: hot
and cold-chamber die casting.
Centrifugal casting uses a permanent mold that is rotated about its axis at a speed between 300 to 3000 rpm as the molten metal is poured. Centrifugal forces cause the metal to be pushed out towards the mold walls, where it solidifies after cooling. Parts cast in this method have a fine grain microstructure, which is resistant to atmospheric corrosion; hence this method has been used to manufacture pipes. Since metal is heavier than impurities, most of the impurities and inclusions are closer to the inner diameter and can be machined away surface finish along the inner diameter is also much worse than along the outer surface.