Processes for Casting of Metals


Processes for Casting of Metals

Casting of metals is a process of manufacturing materials made of metals. It is a forming process for the forming of hot liquid metals. It is the simplest, most direct route to a near net shape product, and often the least expensive.  It is a process, in which liquid metal is poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to cool and solidify. The solidified part is also known as a casting, which is ejected or removed out of the mould to complete the process. Casting is very often used for making complex shapes which are difficult or uneconomical to make by other methods.

The processes for the casting of metals (Fig 1) have two distinct subdivisions namely (i) non-expendable mould casting, and (ii) expendable mould casting. It is further broken down by the mould material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure.

processes-for-casting-of-metals

Fig 1 Processes for the casting of metals

Non-expendable mould casting processes

Non-expendable mould casting is a casting process in which the mould need not be reformed after each production cycle. Non-expendable mould casting is a casting technique which has at least 4 distinct casting processes. These are (i) continuous casting, (ii) centrifugal casting, (iii) die casting, and (iv) permanent mould casting. This form of casting also results in improved repeatability in parts produced and delivers near net shape casting.

Continuous casting process

Continuous casting of metals can be defined as a refined process of casting for continuous production in high volume of metal shapes with constant cross-section. During the process, the pouring of liquid metal takes place into a water-cooled, open-ended copper mould. This allows a skin of solid metal being formed above the ‘still-liquid’ centre. The liquid metal in continuous casting gradually solidifies from outside towards the centre. After initial solidification, the strand, as it is often called, is continuously withdrawn from the mould. Predetermined length of the strand is cut off by either mechanical shears or traveling oxy- fuel torches and transferred to further forming processes, or to the intermediate storage. Cast sizes can range from slab, thin slab, strip, bloom or billet. Continuous casting is normally used where the requirement is a continuous production of a standard product, and also increased quality of the final product. It is widely used due to its cost-effectiveness. The metals which are continuously cast are steel, aluminum, copper, and lead.

Centrifugal casting process

Centrifugal casting was invented by Alfred Krupp, who used it to manufacture cast steel tyres for railway wheels in 1852.  In this process liquid metal is poured in the mould and allowed to solidify while the mould is rotating. Metal is poured into the centre of the mould at its axis of rotation. Due to centrifugal force, the liquid metal is thrown out towards the periphery.

Centrifugal casting uses a permanent mould which is continuously rotated about its axis at high speeds ranging from 300 rpm (revolutions per minute) to 3000 rpm as the liquid metal is poured. Centrifugal forces cause the metal to be thrown out towards the inside of mould walls, where it solidifies after cooling. Parts cast by this method have a fine grain microstructure towards the outer diameter due to the chilling effect at the mould surface. Finer grain microstructure in the outer surface makes the cast part resistant to atmospheric corrosion and 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.

Centrifugal casting machines can be with either horizontal or vertical-axis. Horizontal axis machines are preferred for long, thin cylinders while the vertical machines for rings. In centrifugal casting, the castings are solidified from the outside first. This aspect is used to encourage directional solidification of the casting, and thus to provide useful metallurgical properties to it. Sometimes the inner and outer layers are discarded and only the intermediary columnar portion is used.

Die casting process

Die casting is a metal casting process which is characterized by forcing of liquid metal under high pressure into a mould cavity. The mould cavity is created using two hardened dies made of tool steel. These dies are machined into shape and work similarly to an injection mould during the process.  Majority of die castings are made from specifically zinc, copper, aluminum, magnesium, lead, pewter (alloy of tin, copper and antimony) and tin-based alloys. Die castings of ferrous metal are also possible.

Depending on the type of metal/alloy being cast, either a hot chamber or cold chamber machine is used. In a hot chamber process (used for Zinc alloys, and magnesium) the pressure chamber connected to the die cavity is filled permanently with the liquid metal. The basic cycle of operation includes (i) die is closed and gooseneck cylinder is filled with liquid metal, (ii) plunger pushes liquid metal through gooseneck passage and nozzle and into the die cavity where metal is held under pressure until it solidifies, (iii) die opens and cores, if any, are retracted, casting stays in ejector die, plunger returns, pulling liquid metal back through nozzle and gooseneck and (iv) ejector pins push casting out of ejector die. As plunger uncovers inlet hole, liquid metal refills gooseneck cylinder. The hot chamber process is used for metals which (i) have low melting points and (ii) do not alloy with the die material, steel. Examples are tin, zinc, and lead.

In a cold chamber process, the liquid metal is poured into the cold chamber in each cycle. The operating cycle consists of (i) die is closed and liquid metal is ladled into the cold chamber cylinder, (ii) plunger pushes liquid metal into die cavity where the metal is held under high pressure until it solidifies, (iii) die opens and plunger follows to push the solidified slug from the cylinder, if there are cores, they are retracted away, and (iv) ejector pins push casting off ejector die and plunger returns to original position. This process is particularly useful for high melting point metals such as aluminum, and copper (and its alloys).

There is large capital costs involved because of the casting equipment and the metal dies and this tends to limit the process to high volume production. Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small to medium sized castings. Because of this, die casting produces more castings than any other casting process.

Die castings have two variants namely (i) pore free die casting which is used to eliminate gas porosity defects, and (ii) direct injection die casting which is used with zinc castings to reduce scrap and increase yield.

Die casting method is used where finer parts are needed.  It is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.

Semi solid metal (SSM) casting is a modified die casting process which reduces or eliminates the residual porosity present normally in many die castings. Rather than using liquid metal as the feed material, SSM casting uses a higher viscosity feed material which is partially solid and partially liquid. A modified die casting machine is used to inject the semi-solid slurry into re-usable hardened steel dies. The high viscosity of the semi-solid metal, along with the use of controlled die filling conditions, ensures that the semi-solid metal fills the die in a non-turbulent manner so that harmful porosity can be essentially eliminated. SSM castings are used commercially mainly for aluminum and magnesium alloys. These castings can be heat treated to the T4, T5 or T6 tempers. The combination of heat treatment, fast cooling rates (from using un-coated steel dies) and minimal porosity provides excellent combinations of strength and ductility. Other advantages of SSM casting include the ability to produce complex shaped parts, net shape, pressure tightness, tight dimensional tolerances and the ability to cast thin walls.[

Permanent mould casting process

Permanent mould casting is a metal casting process which employs reusable moulds (permanent moulds), usually made from matal. The most common process uses gravity to fill the mould. However, gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys. Iron and steel are also cast in graphite moulds. Permanent moulds, while lasting more than one casting still have a limited life before wearing out.

Here, the two halves of the mould are made of metal, usually cast iron, steel, or refractory alloys. The cavity, including the runners and gating system are machined into the mould halves. For hollow parts, either permanent cores (made of metal) or sand-bonded ones are normally used, depending on whether the core can be extracted from the part without damage after casting. The surface of the mould is coated with clay or other hard refractory material for the improvement in the life of the mould. Before moulding, the surface is covered with a spray of graphite or silica, which acts as a lubricant. This has two purposes namely (i) it improves the flow of the liquid metal, and (ii) it allows the cast part to be withdrawn from the mould more easily. The process can be automated, and therefore yields high throughput rates. Also, it produces very good tolerance and surface finish.

Moulds for the casting process consist of two halves. Casting moulds are generally formed from gray cast iron because it has about the best thermal fatigue resistance, but other materials include steel, bronze, and graphite. These metals are chosen because of their resistance to erosion and thermal fatigue. They are usually not very complex because the mould offers no collapsibility to compensate for shrinkage. Instead the mould is opened as soon as the casting is solidified, which prevents hot tears. Cores can be used and are usually made from sand or metal. Mould is heated prior to the first casting cycle and then used continuously in order to maintain as uniform a temperature as possible during the cycles. This decreases thermal fatigue, facilitates metal flow, and helps control the cooling rate of the casting metal. Venting usually occurs through the slight crack between the two mould halves, but if this is not enough then very small vent holes are used. They are small enough to let the air escape but not the liquid metal. A riser is also used to compensate for shrinkage. This usually limits the yield to less than 60 %. Mechanical ejectors in the form of pins are used when coatings are not enough to remove casts from the moulds. These pins are placed throughout the mould and usually leave small round impressions on the casting.

The four main types of permanent mould casting are (i) gravity, (i) slush, (iii) low-pressure, and (iv) vacuum.

The gravity process begins by preheating the mould to 150 deg C to 200 deg C to ease the flow and reduce thermal damage to the casting. The mould cavity is then coated with a refractory material or a mould wash, which prevents the casting from sticking to the mould and prolongs the mould life. Any sand or metal cores are then installed and the mould is clamped shut. Liquid metal is then poured into the mould. Soon after solidification the mould is opened and the casting is removed to reduce chances of hot tears. The process is then started all over again, but preheating is not required because the heat from the previous casting is adequate and the refractory coating usually lasts several castings.  The metal is poured at the lowest practical temperature in order to minimize cracks and porosity.

Slush casting process is a variant of permanent mould casting to create a hollow casting. In the process the liquid metal is poured into the mould and allowed to cool until a shell of material forms in the mould. The remaining liquid metal is then poured out to leave a hollow shell. The resulting casting has good surface detail but the wall thickness can vary. The process is usually used for low melting point metals. It uses less material than solid casting, and results in a lighter and less expensive product. Hollow cast figures generally have a small hole where the excess liquid was poured out. Similarly, a process called ‘slush moulding’ is used in automotive thermoplastic dashboard manufacture, where a liquid resin is poured into a hot, hollow mould and a viscous skin forms and then excess slush is drained off, the mould is cooled, and the moulded product is stripped out.

Low-pressure permanent mould casting uses a gas at low pressure, usually between 20 kPa to 100 kPa to push the liquid metal into the mould cavity. The pressure is applied to the top of the pool of liquid, which forces the liquid metal up a refractory pouring tube and finally into the bottom of the mould. The pouring tube extends to the bottom of the ladle so that the liquid metal being pushed into the mould is very clean. No risers are required because the applied pressure forces liquid metal in to compensate for shrinkage. Yields are normally more than 85 % because there is no riser and any metal in the pouring tube just falls back into the ladle for reuse.  The vast majority of low pressure permanent mould castings are from aluminum and magnesium, but some are copper alloys. Advantages include very little turbulence when filling the mould because of the constant pressure, which minimizes gas porosity and dross formation. Mechanical properties are around 5 % better than gravity permanent mould castings. The disadvantage is that cycles times are longer than gravity permanent mould castings.

Vacuum permanent mould casting retains all of the advantages of low pressure permanent mould casting, plus the dissolved gases in the liquid metal are minimized and the cleanliness of the liquid metal is even better. The process can handle thin-walled profiles and gives very good surface finish. Mechanical properties are normally 10 % to 15 % better than gravity permanent mould castings.

Expendable mould casting processes

Expendable mould casting is a generic classification which includes sand, plastic, shell, plaster, and investment (lost wax technique) mould castings. This method of mould casting involves the use of temporary, non-reusable moulds.

Sand casting process

Sand casting process also known as sand moulded casting process, is one of the most popular and simple type of casting. The process uses sand as the mould material. It has been used for centuries. Sand casting allows for smaller batches than permanent mould casting and at a low cost. Sand casting allows most metals to be cast depending on the type of sand used for the moulds. Sand castings are produced in specialized workshops known as foundries. Over 70 % of all metal castings are produced via sand casting process.

Sand for casting is relatively cheap and sufficiently refractory even for the production of steel casting. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop the strength and plasticity of the clay and to make the aggregate suitable for moulding. The sand is typically contained in a system of frames or mould boxes. The mould cavities and gate system are created by compacting of the sand around patterns which are carved directly into the sand.

Sand casting requires a lead time of days, or even weeks sometimes, for production at high output rates (1–20 pieces/hr-mould) and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit ranging from 2,300  kg to 2,700 kg. Minimum part weight ranges from 0.075 kg to 0.1 kg. The sand is bonded together using clays, chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little maintenance.

Sand casting uses natural or synthetic sand (lake sand) which is mostly refractory material called silica (SiO2). The sand grains are to be small enough so that it can be packed densely and large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized moulds use green sand (mixture of sand, clay and some water). Sand can be re-used, and excess metal poured is cut­off and re-used also.  The typical sand mould 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 mould cavity. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the pattern is (almost) identical to the shape of the part to be cast. There is a funnel shaped cavity. The top of the funnel is the pouring cup while 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 mould. Excess liquid 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 are inserted to create interior surfaces of the cast parts. This is necessary since many cast parts have interior holes (hollow parts), or other cavities in their shape which are not directly accessible from either piece of the mould. Cores are made by baking sand with some binder so that they can retain their shape when handled. The mould is assembled by placing the core into the cavity of the drag, and then placing the cope on top. The mould after assembly is locked. After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off.

Plaster mould casting process

Plaster mould casting process is similar to sand casting except that plaster of paris (gypsum, CaSO4) is substituted for sand as a mould material. The plaster is not pure plaster of paris, but rather has additives to improve green strength, dry strength, permeability, and castability. For example, talc or magnesium oxide is added to prevent cracking and reduce setting time. Addition of lime and cement limit expansion during baking while addition of glass fibers increases strength. Sand can be used as a filler material. The ratio of ingredients is 70 % to 80 % gypsum and 20 % to 30 % additives.

Plaster of paris is a fine white powder, which, when mixed with water gets a clay-like consistency and can be shaped around the pattern. 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 deg C, so this method is mainly used to make castings from non-ferrous metals such as zinc, copper, aluminum, and magnesium.

During the plaster mould casting, first, the plaster is mixed and the pattern is sprayed with a thin film of parting compound to prevent the plaster from sticking to the pattern. The plaster is then poured over the pattern and the unit is shaken so that the plaster fills any small features. The plaster sets generally in about 15 minutes and the pattern is then removed. The mould is then baked, between 120 deg C and 260 deg C, to remove any excess water. The dried mould is then assembled, preheated, and the metal poured. Finally, after the metal has solidified, the plaster is broken from the cast part. The used plaster cannot be reused.

Since plaster has lower thermal conductivity, the casting cools slowly, and therefore has more uniform grain structure (i.e. less warpage, less residual stresses).

Generally, the form takes less than a week to prepare, after which a production rate of 1 unit to 10 units /hr.mould is achieved, with items as massive as 45 kg and as small as 30 grams with very good surface finish and close tolerances. Plaster mould casting is an inexpensive alternative to other moulding processes for complex parts due to the low cost of the plaster and its ability to produce near net shape castings.

Shell moulding process

Shell moulding process, also known as shell mould casting, is an expendable mould casting process which uses a resin covered sand to form the mould. Shell moulding is similar to sand casting, but the moulding cavity is formed by a hardened ‘shell’ of sand instead of a flask filled with sand. The sand used is finer than sand casting sand and is mixed with a resin so that it can be heated by the pattern and hardened into a shell around the pattern. Because of the resin and finer sand, it gives a much finer surface finish. The process is easily automated and more precise than sand casting. This process is ideal for complex items which are small to medium-sized and which need higher precision. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labour requirements.

In shell moulding process, like sand casting, the liquid metal is poured into an expendable mould. The mould is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell moulds. A reusable pattern allows for higher production rates, while the disposable moulds enable complex geometries to be cast. Shell moulding process requires the use of a metal pattern, oven, sand-resin mixture, dump box, and liquid metal.

Shell moulding process allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.

Shell moulding process yields better surface quality and tolerances. The process consists of making of the 2-piece pattern of metal (e.g. aluminum or steel) which is then heated to between 175 deg C to 370 deg C, and then coated with a lubricant (e.g. silicone spray). Each of the 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 can be repeated to get a thicker shell. After this, the assembly is baked to cure it. The patterns are then removed, and the two half-shells joined together to form the mould. The metal is then poured into the mould. When the metal solidifies, the shell is broken to get the part.

Investment casting process

Investment casting process is also known as lost-wax casting process. It is a process which has been practiced for thousands of years, with the lost-wax process being one of the oldest known metal forming techniques. From 5000 years ago, when beeswax 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.

Investment casting derives its name from the fact that the pattern is invested, or surrounded, with a refractory material. The wax patterns require extreme care for they are not strong enough to withstand forces encountered during the mould making. One advantage of investment casting is that the wax can be reused.

The steps in the investment casting process are (i) wax patterns are produced by injection moulding, (ii) multiple patterns are assembled to a central wax sprue, (iii) a shell is built by immersing the assembly in a liquid ceramic slurry and then into a bed of extremely fine sand and there can be need for several layers,  (iv) The ceramic is dried, the wax is melted out and the ceramic is fired to burn all wax, (v) the shell is filled with liquid metal by gravity pouring. On solidification, the parts, gates, sprue and pouring cup become one solid casting. Hollow casting can be made by pouring out excess metal before it solidifies, (vi) after metal solidifies, the ceramic shell is broken off by vibration or water blasting, and (vii) the parts are cut away from the sprue using a high speed friction saw and minor finishing is given to the final part.

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 aluminum casting 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 require little or no rework once cast.

Vacuum casting process

This process is also called counter-gravity casting. The process is used when air entrapment is a problem, there are intricate details or undercuts, or if the material is  fiber or wire reinforced. It is basically the same process as investment casting, except for the step of filling the mould which is the step (v) above. In this case, the material is sucked upwards into the mould by a vacuum pump. The mould appears in an inverted position from the usual casting process, and is lowered into the flask with the liquid metal. One advantage of vacuum casting is that by releasing the pressure for a short time after the mould is filled, the un-solidified metal can be released back into the flask. This allows creation of a hollow casting. Since most of the heat is conducted away from the surface between the mould and the metal, therefore the portion of the metal closest to the mould 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 it is possible to get a very thin walled hollow object.

Evaporative pattern casting process

Evaporative pattern casting process is a type of casting process which uses a pattern made from a material which will evaporate when the liquid metal is poured into the mould cavity. This means that there is no need to remove the pattern material from the mould before casting. The most common evaporative-pattern material used is polystyrene foam. The two main processes are lost foam casting and full mould casting.

Lost foam casting is a type of evaporative-pattern casting process which is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mould.

Full mould casting is an evaporative-pattern casting process which is a combination of sand casting and lost foam casting. It uses an expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mould, which vaporizes the foam upon contact.

The main difference is that lost-foam casting uses an-unbonded sand and full-mould casting uses a bonded sand (or green sand). Because this difference is quite small there is much overlap in the terminology. There are many non-proprietary terms which have been used to describe these processes. These include cavity less casting, evaporative foam casting, foam vaporization casting, lost pattern casting, the castral process, and expanded polystyrene moulding. Proprietary terms used include Styro-cast, Foam cast, Replicast, and Policast.