Casting of Metals: Top 11 Methods | Industries | Metallurgy

The following points highlight the top eleven methods adopted for casting of metals in an industry. The methods are: 1. Semi-Permanent Mould Casting 2. Slush Casting 3. Pressed Casting 4. Squeeze Casting 5. Centrifugal Casting 6. Investment of Lost-Wax Casting 7. Frozen-Mercury Moulding (Mercast Process) 8. Plaster Mould Casting 9. Antioch Casting 10. Continuous Casting 11. Chill Casting.

Method # 1. Semi-Permanent Mould Casting:

It is similar to permanent mould casting except the difference that while permanent moulds use metallic cores, the semi-permanent casting employs sand cores. It is used where cored openings are so irregular in shape, with undercuts, recesses, etc. that solid metal cores would be difficult to withdraw from the solidified castings.

Both the types of moulds are made in two or more pieces, which when fitted and clamped together define the outline of the part to be cast as well as the gates and risers. Stationary or moveable cores are used to form holes of any desired shape in the casting. Mould thickness is usually 25- 50 mm.

Principal factors in the production of sound castings are:

(a) Mould must be designed with parting lines, gates, vents etc. so that molten metal can enter by gravity without turbulence. Vents should be so arranged, that the air in the mould is pushed ahead of the gradually rising level of mol­ten metal.

(b) Gating thickness and external contours of the mould should be such as to make possible progressive solidi­fication of the molten metal in unbroken sequence.

(c) Mould temperature must be controlled within a definite range.

(d) Minimum diameter of cored holes is 6 to 10 mm to permit required strength at elevated temperature.

(e) Inserts can be easily cast into casting.

(f) Minimum section thickness is around 4 mm for magnesium alloys, 3 mm for aluminium and copper alloys.

(g) Usual draft angles on external and internal sur­faces are 3° and 2° respectively.

(h) Generous fillets should be provided in corners es­pecially where heavy and thin sections meet.

Method # 2. Slush Casting:

This method is a special application of permanent mould casting in which hollow castings are produced without the use of cores. Molten metal is poured into the metallic mould and allowed to solidify upto the required thickness. The mould is then turned over so that the remaining liquid metal falls out and castings of desired thickness can be obtained.

Normally small thickness castings of lead, zinc and low melting alloys are obtained by slush casting method. The thickness of casting depends upon the time for which the metal is allowed to solidify into the permanent mould.

Since control is not precise, this method is not adopted to other than ornamental parts and for parts where only external features of the casting are important and uniformity of thickness is not important.

In order to facilitate the removal of casting, the moulds are made in two halves. Ornaments, statues, toys and other novelties are the examples of slush casting.

The casting by this method must be made of a relatively pure metal as most metals do not form a strong solid skin. Owing to obvious drawbacks, this method is used only on a limited scale for low melting point alloys like lead or zinc.

Method # 3. Pressed Casting:

It is another method of producing hollow castings from permanent moulds but differs from gravity die casting and slush casting in operation. In this case a definite amount of molten metal is poured into the mould and then close fitting cores are pushed in the cavities so that the molten metal can be forced into the mould cavities.

When the metal sets into the cavities, the core is removed and hence we get a thin walled hollow casting. Pressed die castings are limited to ornamental articles. This method was developed by Carthias of France and hence it is also popular by the name of Carthias casting.

Method # 4. Squeeze Casting:

It produces non-ferrous castings having mechanical properties comparable with forgings. After the liquid metal has been metered into the open half of the lower die, a closely fitting upper die moves down and compresses the liquid at a high pressure, and this pressure is maintained during solidification. Complex shaped components can be produced by this process.

Method # 5. Centrifugal Casting:

In this process the molten metal is poured in a mould and allowed to solidify while the mould is revolving, i.e. the metal solidifies under the pressure of centrifugal force. The centrifugal forces cause the metal to take up the impression of the mould cavity. The pressure is selective, that is, the greater force is exerted on the denser components.

This is of considerable benefit in eliminating non-metallic and gases during casting. This process is best suited for mass production. Cylindrical parts and pipes are most adaptable to this process. In centrifugal casting, the molten metal is subjected to centrifugal force due to which if flows in mould cavities easily and results in the production of high density castings.

On casting surfaces better details of numerals and numbers can be obtained and thin parts of high strength can be easily produced. The castings are produced with promoted directional solidification as the colder metal is thrown to outer side of the castings and the hotter metal nearer the axis of rotation which further acts as a feeder during solidification of metal.

No core is needed to form the hole in the middle. Centrifugal casting finds its best use in mass production operation. The use of the machinery and equipment for centrifugal casting can be justified only when a large quantity of identical castings are required.

Typical Liner Casting Machine:

This is horizontal type of centrifugal machine. There are two rollers at bottom and two at the top. The mould is arranged between the rollers so as to revolve freely. At the end of the mould is fitted a gear which meshes with a gear on a motor driven shaft. Thus the mould can be driven at a constant, predetermined speed.

The ends of the hollow mould are partially closed by covers which can be easily detached when casting is to be pushed out of the mould. At both end covers a central hole is provided. From one side, the molten metal is poured from a ladle and from the other the hot gases escape out.

For casting pipes, it would be preferable to have inclined machine to facilitate the flow of metal from one end of mould to the other. In order to avoid the chilling effect, the moulds have to be generally preheated to the requisite temperature. After centrifugal casting, the castings are immediately subjected to proper heat treatment in order to have the desired qualities and properties in the casting.

Selection of Proper Speed in Centrifugal Casting:

Selection of proper speed is very important factor as the centrifugal force exerted on the molten metal is directly proportional to the square of the speed. In centrifugal casting, it is a common practice to produce a centrifugal force about 60 to 75 times of gravity for sand moulds with horizontal axis, 40—60 times for metal moulds and about 100 times for moulds revolving about a vertical axis.

If centrifugal force is more, then longitudinal hot tears in the outer surface of the casting are produced, whereas with low centrifugal force, slipping or raining of the molten metal during casting is experienced.

Exact spinning speed is dependent on several factors like application and shape of casting. Thus it is very difficult to determine precisely the exact value of rotation.

Material Considerations:

Grey iron is centrifugally cast in large tonnages because of its relatively low pouring temperature and fluidity. Centrifugal casting of steel has replaced the forging methods. The heavy non-ferrous alloys, especially the copper base alloys, are readily formed by centrifugal casting.

Light metals have been centrifugally cast to some degree, although in some cases, where the oxides are as dense as the metal, centrifugal force is of little or no value in eliminating the non-metallic. Centrifugal casting has proved to be an efficient and economical method for producing annular components in special composition and heavy walled tubing of the common alloys, particularly for those alloys which are very difficult to forge or roll.

The bonding between two metals by this process is said to be complete and continuous and thus it is best suited for producing parts having soft lining on hard metals or vice versa.

Advantages of Centrifugal Casting:

(i) The castings produced are sounder with dense structure, cleaner and the foreign inclusions are eliminated completely (these being segregated at the inner surface). This calls for simplified inspection techniques.

(ii) Mass production is possible with less rejections.

(iii) Use of runners and risers and cores is eliminated.

(iv) Mechanical and physical properties of castings are improved.

(v) Parts are produced closer to finished dimensions with consequent saving in machining.

(vi) Thinner sections can be cast because of the pres­sure exerted on the metal.

(vii) Any metal can be cast by this process.

Limitations of Centrifugal Casting:

(a) The process is limited to only cylindrical and annular parts with a limited range of sizes.

(b) It involves high initial cost and requires skilled labour for its maintenance.

(c) Too high speed may result in surface cracks caused by high stresses set up in the mould.

Method # 6. Investment of Lost-Wax Casting:

This process is called the lost-wax process or precision casting. This process uses wax pattern which is subsequently melted from the mould, leaving a cavity having all the details of the original pattern.

Castings obtained by this process have very close tolerances or the order of ± 0.005 mm. Generally this process is used for producing light and intricate parts. This process does not need a parting line or any form of split mould.

The process of investment casting consists to two stages. First of all a master pattern is made of steel or brass and it is replica of the part to be cast. Around it, a split mould is formed from gelatin or an alloy of low melting point.

This alloy is poured over the master pattern. After solidification master mould is obtained. This master mould is used for making the wax or lost-pattern.

The following are the materials used for preparing master mould:

(i) Plaster of Paris or gypsum products for non-ferrous castings.

(ii) Ethyl silicate, sodium silicate and phosphoric acid for steel castings.

(iii) Sometimes fine-grain silica sand is also used for preparing master mould.

The master mould is then filled with either liquid wax or thermoplastic polystyrene resin which when solidified forms a wax-pattern. This wax pattern is used for making the final casting. Then the process of investment of the pattern is followed which consists of casting the wax-pattern with slurry consisting of silica sand or graphite mixed with water.

Coarse sand is sprinkled over the wet slurry to form the quartz shell. This wax-pattern is used for making the final mould in the same fashion as the conventional moulding process. This mould is then dried in air for 2 to 3 hours and then baked in an oven so that the wax may melt out. When the temperature reaches 100 to 120°C, the wax melts out and is collected through a hole in the bottom plate.

To improve the resistivity, the mould is further heated up to 1000°C called the sintering of the mould and finally cooled to 100°C for obtaining the castings. The castings can be obtained by gravity, pressure, vacuum or centrifugal operation.

After the metal is cooled, the plaster is broken away and gates and feeders are cut out. The castings so obtained are finally cleaned by sand blasting, grinding or other finishing processes.

The castings so obtained have good surface finish and are exact reproduction of master pattern. This process is used for making jewellery parts, dental castings, castings of satellite tools (which are difficult to be produced either by forging or machining), turbine blades, parts of motor cars and sewing machines, type writers, calculating machines and various other intricate parts.

Advantages of Lost Wax Casting:

(i) High dimensional accuracy of the order of ± 0.08 mm can be attained.

(ii) A very smooth surface of the casting (of the order of 0.015 to 0.025 mm r.m.s. value) without parting line, can be easily obtained.

(iii) It is suitable for both high and low melting point alloys since the ceramic material can be selected to have the appropriate refractory properties and bonded with any desired agent to give the required strength and permeability.

(iv) It is a flexible process and can reproduce surface details and dimensions with precision, especially for high melting point alloys.

(v) Undercuts and other shapes, which would not allow the withdrawal of a normal pattern are easily provided. No cores or loose pieces are required.

(vi) Machining of intricate parts can be eliminated.

(vii) Very thin sections of the order of 3/4 mm can be cast easily.

(viii) Die casting can be replaced when short runs are involved.

(ix) Castings are sound and have large grains as the rate of cooling is slow.

(x) It represents the only method suitable for manufacture of precision shaped castings of high melting point metals which would cause too rapid die failures in normal die casting process.

Limitations of Lost Wax Casting:

(a) It is an expensive process and hence is adopted only where small number of intricate and highly accurate parts particularly high melting point alloys is to be manufactured.

(b) This process is suitable for small size parts.

(c) This presents some difficulty where cores are to be used.

Method # 7. Frozen-Mercury Moulding (Mercast Process):

In this process frozen mercury is used for the production of precision castings. In this case, the metal mould is prepared to the necessary shape with gates and sprue- holes. It is then placed in cold bath and filled with acetone (which acts as a lubricant). Mercury is poured into it and freezing of mercury takes place at 20°C after about 10 minutes of pouring.

The patterns are then removed and are given dipping’s in a cold ceramic slurry bath, until a shell of about 3 mm is built up. Mercury is then melted and removed at room temperature. The shell is dried and heated at high temperature to form a hard permeable shape. The shell is then placed in a flask, surrounded by sand, preheated and filled with metal. After solidification of metal, the castings can be removed.

Both the ferrous and non-ferrous metals can be cast by this process, but its application is limited for commercial use due to high cost of casting process.

Castings obtained by this process have the following characteristics:

(i) Very accurate details can be obtained even in intricate shapes.

(ii) The surface finish is excellent and machining or cleaning cost is minimum.

(iii) The accuracy obtained by this process is of the order of 0.002 mm per mm.

(iv) Both ferrous and non-ferrous metals can be cast (maximum pouring temperature being around 1650°C). How­ever the cost of castings is high.

Method # 8. Plaster Mould Casting:

For casting silver, gold, aluminium, magnesium, copper and alloys of those metals (particularly brass and bronze), plaster of Paris or gypsum (CaS0₄.nH₂0) is extensively used. Gypsum is particularly used for investment casting or for cope and drag moulding. For preparing the mould a slurry is used which consists of 100 parts of metal casting plaster and 160 parts of water.

It is important to note that plaster is to be added to water and not water to plaster of Paris. They are then stirred slowly to creamy consistency. This slurry is poured over a carefully made match plate type pattern, usually of metal. The mould is vibrated slightly to ensure plaster’s filling all small cavities. The initial setting takes place at room temperature after few minutes of pouring of slurry and then the pattern can be removed.

Sometimes, the initial setting time is decreased by heating or by adding a small quantity of terra-alba. Copes and drags are made simultaneously on separate lines and dried in ovens at 200—425°C, until all free and combined moisture is removed. Normally 20 hours is the time for drying purposes.

In order to prevent the cracking of moulds, 20 to 30% talc is added to the plaster while mixing. In addition other compounds such as terra-alba or magnesium oxide are added to reduce the initial setting time.

Sometimes lime or cement is also added to control the expansion of plaster during caking. Mould sections obtained by this process are very fragile and require care in assembling.

Characteristics and Advantages of Plaster Mould Casting:

(i) The dimensional accuracy obtained by this process is of the order of 0.008 to 0.01 mm per mm.

(ii) Because of no sand or other inclusions, excellent surface finish, which neither requires machining nor grinding, can be obtained.

(iii) Advantage of this process is that non-ferrous castings having intricate and thin sections can be obtained with good dimensional accuracy and excellent surface finish. Because of low thermal conductivity of plaster the metal does not chill rapidly and thus very thin sections can be cast.

Limitations of Plaster Mould Casting:

(a) Its application is limited to non-ferrous castings as sulphur of gypsum reacts chemically with ferrous metal at high temperature, giving very bad casting surface.

(b) Since the metal moulds are used, the plaster casting possesses low permeability because the combined water and moisture cannot be fully taken out. The moulds are not permanent. They are destroyed when the castings are removed.

Method # 9. Antioch Casting:

This process is a further application of plaster casting. It was first development by Monis Beam for making special engineering parts of complex shape requiring minute details and thin sections. For preparing the mould a creamy slurry is obtained by adding water to a dry mixture of gypsum, sand, asbestos, talc and sodium silicate.

This slurry is piped by hose into metal core-boxes or cope and drag flasks fitted to special match plates. After the initial setting which is somewhat faster than plaster moulding, the patterns are drawn and then the mould is assembled in a green condition. After keeping them at room temperature for nearly 6 hours, they are autoclaved in steam at about 0.7 kg/cm² pressure, cured in air and finally dried in an oven at temperature of 230°C for 10 hours.

This autoclaving develops a special permeable structure in the mould but greatly reduced dry strength. Drying in oven removes the fee and combined water and hence good permeability is obtained.

Advantages of the Antioch Process:

(a) The major advantage of this process is that it develops a high degree of permeability in plaster mould and hence making it easier to obtain fine-details by allowing any moisture and other gases present to escape.

(b) Good finish and dimensional accuracy can be obtained even in large castings.

(c) This process lends itself to incorporate chills in the moulds, which can be used to control the metallurgical properties.

Method # 10. Continuous Casting:

Continuous casting has proved itself to be a most economical way of casting wherever feasible and several methods have been devised and successfully used. In this process the molten metal is continuously poured into a mould around which there are facilities for rapidly chilling the metal to the point of solidification. The solidified metal is then continuously removed from the mould at the calculated rate.

Method # 11. Chill Casting:

It is used, where very hard outer surfaces and wear resistant castings are required. This process is nearly similar to sand casting. Moulds are made of sand or cast iron and for the purpose of chilling the cast iron, steel blocks are used as shown in Fig. 3.76. The figure shows an example of composite mould of sand and metallic chills for casting a cast iron wheel.

Metallic chills are used at outer surfaces so that the rate of cooling increases and hence hardness increases. Where hardness is of extreme importance, metallic moulds are used as in the case of railway brake shoe. In order to reduce the excessive chilling effect, moulds are preheated.

In case of bushes and bearings, the inner surfaces of the holes should be hard and wear resistant and in order to fulfil the above requirement core chills are used in the moulds. The core chills help in increasing the cooling rate of the bored surfaces by coming in contact with the chills. Extensive chills are used to reduce the possibility of the defect called hot-tear.

The rate of cooling has a considerable effect upon the hardness of the surface; greater the rate of cooling the lesser amount of carbon will come out in graphite state. In other words, carbon will be in the combined form and hence casting will be hard.

The examples of chill castings are wheel tread, railway brake shoe, tram-car wheels, crusher jaw, chilled rolls used in rolling mills and sideways of machine tools.