fasting Methods S CASTING METHODS A Sere eLnaaiee ee ean mace ein ans Ue 3.1 INTRODUCTION : 31 NS Most commercial metals can be cast in sand moulds. Roughly 80% of the total output of castings are made in sand moulds. Sand moulds are cheap, but contribute rejections of castings due to defects such as blow holes, dimensional deviations and surface irregularities etc. Also, sand casting is not suitable for mass production. To overcome these limitations, a variety of special casting processes have been developed. The following special casting methods along with sand moulding are described in this chapter. ee 1. Die casting 2. Centrifugal casting 3. CO,-process, and 4. Investment casting. .2_ MOULDING PROCEDURE : The procedure for making a simple mould by using solid pattern is as follows : The drag is first placed upside down on moulding board and then the pattern is kept centrally on the board inside the flask. The parting powder is dusted on pattern surface. Next, the facing sand is riddled on to the face of the pattern to a depth of 25 mm.Sprue Base Fig. 3.1 MAKING A SIMPLE MOULD. The drag now filled with backing sand, which is then rammed suitably. After ramming, the excess of sand is cut off with a strike off bar to a level of the box. The mould is vented. The drag is then turned over and the cope is now placed on drag. The parting sand sprinkled over the pattern and entire surface of drag to stop the bottom and top layers of sand from binding together. Sprue and riser pins are placed in position and sand is filled in the cope and rammed. Any excess sand is removed. The sprue and riser pins are then withdrawn and at this stage cope vented. The cope is now lifted off and set aside. The side of the pattern are moistured with swab and the pattern is rapped. It is then withdrawn carefully by using draw spikes. Channel (ingate) is made from mould and its surface is coated with graphite to give smooth surface finish. Finally, the mould is closed by Positioning the cope on drag. The cope is loaded or clamped with drag to prevent separation of boxes when the metal is poured in, as there is a tendency for the cope to float and cause misalignment between the top and bottom halves an noe The molten metal is now poured into the mould through sprue aaa seen flowing up the riser, When cooled, the mould is broken totsand moulds. Moulds are desi, Ved Most commercial metals can be cast in Casting ig after solidification of castings and hence can not be reused. Sang not suitable for mass production and some special casting methods are for large production of castings with close dimensional tolerances and q ee surface finish. The principle of special casting methods are given below. 3.6.1 Die Casting: Die casting involves use of permanent metal moulds or dies. The die is made in two halves and are closed to form the cavity. The principle of pressure die casting involves the forcing of molten metal into die cavity under pressure ang maintains the pressure until it solidities. A ram is used for opening and closing the die halves and ejection pins automatically eject the casting as the die is opened. If the object to be made is small, the die may be constructed ona multi-cavity plan (multiple-cavity die) by which it is possible to produce several parts in a single cycle. In pressure die casting no riser is needed in the mould, but very small vents are provided at the parting line of the die halves in order to allow the escape of ait. The speed of transfer of the molten metal into the die cavity results little loss of heat during injection and liquid metal can be forced into the recesses ofa complex-shaped mould. Rapid solidification under pressure largely eliminates the effect of srinkage, and there is little wastage of metal. Applications : The die casting process was developed in the early 1900s. It is employed for producing small and medium size castings of low-melting point non-ferrous alloys. ina dtan elwate asting Meth Advantages 1, Large number of identical Large nu Parts can be quickl Y produced, resulting in low 2. Parts with extremely fine finis hes and i obtained eliminating toatiink close dimensional tolerances can be INg operations, fa ‘apes may be produced with comparative ease, 4, Mechanical properties of die castings are superior to sand castings. 3. Very complex sh 5. Thin sections may be cast, and holes may be cored accurately. Disadvantages : 1. Tooling cost is high. Only economical for mass production. 2. The range of metal suitable for die casting is limited 3. Size of castings produced is limited. 4. No flexibility of processing conditions. Die casting machines: The following types of die-casting machines are in use : 1. Hot chamber (gooseneck) machine (a) Hot chamber machine operated by plunger (b) Hot chamber machine operated by compressed air 2. Cold chamber machine 3. Vacuum die-casting machine. 3.6.1.1 Hot-Chamber Die-Casting Machine : In the hot chamber machine the metal holding furnace is incorporated with the injection system unit. This type is also known as the ‘gooseneck’ machine because of the shape of the injector. The two types of hot chamber machines are shown in Fig. 3.7.1. In plunger type machine Fig. 3.7.1 (b), molten metal is forced into die cavity at a pressure of 14 MPa. Plunger moving down first closes the port through which liquid metal enters into the cylinder. Further movement of the plunger increases the pressure and molten metal is forced through the godseneck into the die cavity. After solidification the castings are ejected at a rapid rate. At the same time the plunger moves up, uncovers the port and as a result the molten metal enters into the cylinder. Again when the plunger moves down it closes the port and the cycle is repeated. All these operations are automatic while the machine is in operation. AE (Radiant) rssHydrautig Yylinder (HE = s 2 3.7.1 HOT CHAMBER DIE CASTING MACHINES In air operated die-casting machine Fig. 3.7.1 (a), the molten metal ig forced into die cavity at a pressure of 4.2 MPa. This machine is Provided with a Suitable mechanism to raise or lower the gooseneck which is submerged in the pot carrying molten metal. In operation the gooseneck is lowered to receive the molten metal from the pot. Then it is raised up and held in Position against the nozzle. The compressed air blown into the gooseneck pot forces the molten metal into the die cavity. After solidification the casting is ejected. The gooseneck again lowered, die closed and whole cycle is repeated. the hot The cold chamber process has a longer operating cycle than that of chamber machine,UW ‘tasting Methods’ (FE Mould Moving Cavity Fixed Platen Platen Liguia Metal —— ZA _ J Injection iston Fig. 3.7.2 COLD CHAMBER DIE CASTING MACHINE 6.1.3 Vacuum Die-Casting Machine : Vacuum die-casting machine is similar to hot chamber and cold chamber die casting machines; but air is completely evactuated from the die cavity. This prevents the formation of blow holes and other similar defects. Vacuum Fig. 3.8 VACUUM CHAMBER DIE CASTING MACHINE 6.2 Permanent Mould (Gravity die) Casting : Permanent mould casting refers to the process in which molten metal is transferred into the cavity by gravity. Therefore, it is also known as gravity die casting. Permanent moulds (made of cast iron or other metal) are made in two halves. The mou Ids are coated with refractory materials and are closed. The molten metal is poured into the dies and after solidification of casting the dies are opened. T he casting is removed from dies with the help of ejectors. If necessary cores may be inserted in the mould and the gating system is built in the mould halves it sell. Fig, 3.9 illustrate the principle of permanent mould casting for casting aluminium bearing.3.16 Vertical Parting line ey Fig. 3.9 PERMANENT MOULD CASTING Permanent mould castings are superior than the sand castings. Their surfaces are smooth: and close dimension tolerances can be maintained. Also they Possess uniform and good mechanical properties, and the rate of production high. This process is mainly used for aluminium, magnesium, copper alloys and gray cast iron because of their low melting point. Steels can also be cast using graphite or heat - resistant metal moulds... Permanent mould casting is usually referred to as gravity die casting. The term die casting is used for the pressure die casting. 3.6.3 Centrifugal Casting : Centrifugal casting is the method of producing castings in a rotating mould. The molten metal is poured into mould (while it is in rotation at a speed of 1500 r.p.m.) and centrifugal force (produced as a result of rotation) spreads the molten metal uniformly along the entire length of the mould and holds it there until solidification is complete. Solidification progresses from the outer surface and any impurities Present are also pushed towards the centre which is often removed by machining. Applications : The centrifugal casting process is particularly suitable for cylindrical hollow products e.g. large pipes for carrying water, gas or Caan Products upto about 5 m long with uniform thickness can be cast. The metho can also be used for casting cylindrical shell bearings of copper-base alloys and lead-tin alloys. sam (Radiant) ES_ Ado 1 2. 3. 4. lowing advantages * for all casting metals and alloys. suitable ptable for the large castings having the mass of several tonne: S. Mechanical properties of the products are relatively good. Castings of good quality, dimensional accuracy with good surface finish + Gan be obtained. pisadvantages : 1. 2. Centrifugal casting was first sugs Not suitable for intricate castings Manufacturing cost is high. ested in the early 1800s. There are three types of centrifugal castings. (2) True centrifugal casting (b) Semi-centrifugal casting, and (c) Centrifugins. (a) True centrifugal casting: itis the process of producing hollow casting without use of ore The molten metal is fed into the revolving mould. The axis of rotation is usually horizontal but can be seatieal for short pieces. Moulds are made of steel or graphite and may be coated with a refractory lining to increase mould life. The metal is forced against the mould surface by the centrifugal force until it solidifies. After solidification mould rotation is stopped and the casting pulled out. This is the most effective method of making cylindrical parts Piich as pipes, gun barrels and streetlamp posts.Fo strtete tee seen waneu steer Castings, 3.6.5 Investment Casting : Investment casting is also known as lost wax or precision casting This method is used for making intricate castings from metals which cannot be shaped by other methods. In this method of casting the expendable pattern are prepared from wax. To prepare the pattern, the wax is melted and injected into a master die at a pressure of 3.5 to 7 N/mm?. When the wax pattern is solidified, it is removed from the die and the ends of wax gates are suitably trimmed. The patterns are attached to centre sprue to form an assembly or tree. Next the entire assembly is oe by dropping in a refractory slurry and is then dusted with refractory sand.( Wax Pattern Base Plate CI — 5 A Gi Tamed (i) Severat Pattorns Mounted on. Wox Pattern Central Wax Runner Investment Material (¥) Melting Out the ‘Wax Pattern (iv) Manufacture of the Mould (vi) The Finished Mould Fig. 3.12 INVESTMENT CASTING After refractory, the tree is fixed on to a flat bottom plate by wax. A stainless steel (heat resisting steel) flask open at each end is placed over the assembly and the investment material (fine refractory sand) then poured into the flask. The flask is vibrated to remove the air bubbles. After investment has taken an initial set, the base plate is removed and the flask is placed in an oven at about’ 100°-150° C. So that the wax melts and runs out leaving ‘a’mould cavity in the investment material. The mould is heated at about 650° to 1000° C to remove the traces of wax and to obtain fine details in the mould. The molten metal is then cast into the hot mould. After solidification, the castings are stripped from the investment material. Finally individual castings are removed from the assembly, and are cleaned and inspected. * i The stages of investment casting are illustrated in Fig. 3.12. i AT (Radiant) xagp nn t casting is suitable fo _qvestment cast T mass productio: i: i Me ical applications are the blades for gas ane aie Cee es products of investment casting includes; Ceres ; ; special alloy parts used in chemical industries. Tools and dies parts of sewing machines and washing machines advantages : _ produces complicated shapes with extreme dimensional accuracy. _ Parts produced by investment casting does not require machining. _ Thin sections of 0.75 mm thickness can be cast. _ Absent of parting line on casting gives pleasing appearance. Disadvantages : _ The cost of production is high - The process is limited to produce small castings of less than 0.5 kg. 3.7. DEFECTS IN CASTING : Various defects in casting results due to improper sand preparation and process techniques. The defects that commonly occur in castings are explained below. Blow holes are spherical voides having a clean and smooth surface. They appear near to the surface. These defects are due to poor venting and lack of permeability in a moulding sand. Proper venting, not too hard ramming and adjusting the moisture content in the sand may eliminate these defects. Porosity (uniformly distributed tiny cavities in casting) and pin holes (small blow holes occur below the surface of casting) are also caused due to similar * sreasons which are responsible for blow holes. Cold shuts and misruns are discontinuities in the casting as a result of poor fluidity of the molten metal. Misruns are formed when the entire section is not filled during pouring before solidification. Cold shuts formed when two streams of metal do not fuse together. These defects can be minimised by proper gating system and increasing in pouring temperature (i.e. increasing the fluidity of metal). DLAM\ Advanced Won n the casting as a result of contraction provement in the casting design, prope, ram, °° Alt bility in the core and mould may help elim ng ni ion of [3.22 ZA Hot tears av : ification. An im soli P increasing the collapsil tears. a ; hift of the individual parts of a casting With tog Pect to en ismatch is a sh ; , mn 1 The defect results from mismatching of cope and drag. To elim; Need id be properly closed and secured. inate the defects, the flask shoul Shrinkage cavities are voids in the casting. They appear ee of insufficient feeding, poor casting design, incorrect arrangement of pan risers and high temperature of pouring metal. The defect can be eliminates locating the riser at correct place and promoting the directional soliditicn by using chills (pieces of metals kept in mould to extract heat in certain location, Fins or flash are thin projections of metal, not intended as a part of the casting, They commonly appear along the mould joint because of much wear of flask halves or improper clamping of flasks. Slag inclusions (or slag holes) are cavities filled with slag, and produced when the slag gets into the casting when pouring metal into the mould. These defects are due to a poor skimming of metal in the ladle and incorrect gating system. Swell is an expansion of the mould cavity by metal pressure. It is due to insufficient ramming and too rapid pouring of molten metal. Scabs are lumps of excess metal (i.e. irregular projections containing embedded sand) on the casting as a result of erosion of mould by the stream of molten metal. The defect can be eliminated by proper ramming, using fine facing sand and controlling the flow of metal. Warping i. i ion (i i adn oe distortion (i,e., change in shape) of casting due to internal stres ng cooling proper design of casting elimi i ig eliminates warping.y 7 aoe of rosi ry BY yy VY ce L7] 2a) (Blow Holes, Porosity and Pin Hol g = IL hs lisrun Cold. Shue3.3.7 Shell Moulding : This process was invented (during second world war) by Johannes Cron, and is sometimes called Croning or C-process. It is basically a sand moujg in which the clay is replaced by resin bonding agent (phenol-formaldehyig urea formaldehyde). The process is illustrated in Fig. 3.3 and consists ot le following steps : (a) Preparation of thin shell made of a mixture of sand and thermo-settin resin around a heated metal pattern 7 (b) Separating the shell from the metal pattern (c) Clamping two-halves of the shells to form a mould Preparation of thin shell : Fine silica sand (free from clay) is thoroughly mixed with about 5% thermo. setting resin binder such as phenol-formaldehyde and placed into a container (dump box). The metal pattern plate is heated to about 250° C in an oven and is clamped to the top of the dump box as in 'Fig. 3.4 (a). The metal patterns incorporates ejector pins to facilitate stripping of the completed mould. The dump box is inverted so that the sand resin mixture covers the pattern. After about 30 seconds, the resin cures causing the bonding of sand grains to forma shell around a pattern. Separating the shell from pattern : sition and the surplus sand mixture The dump plate is returned to its original po: ased by falls back into the box. The pattern plate is removed and the shell is rele the ejector pins. The shells are light and thin, usually 5-10 mm thick. A (Radiant) ——o(b) Separation of Shell (c) Clamping of Shells to form a Mould ANT] Pattern Plate Elector AY. Sand with Bonding Agent Fig. 3.4 SHELL MOULDING PROCESS Mould formation : The shell is further hardened by final curing for a few minutes at about 320°C. The two halves of shell are joined together by adhesive to form the mould. It is placed in suitable box and is supported by coarse sand or steel shots held in a box. The mould is ready. The molten metal is poured into the completed mould. After solidification the castings are removed from the sand. Advantages of shell moulding : iL 2. 3. 4. 5. Produce accurate castings with very good surface finish. In most cases, machining operation is not required. Because sizes are very close enough to acceptable. Thin wall sections can be produced. Small core holes can be produced. Shells can be stored for long time and can be reused. Disadvantages : 1. Cost of metal patterns is high SRT (Radiant) Ramesses ESES Advanced Workshop 2. Cost of resin is high 3. Many equipment and control facilities are needed. B 4. Casting size and weight are limited. Applications : Shell moulding is used for making fine castings of ferrous and non-ferrous Metal i.e, process is suitable for all cast metals. It is particularly adapted for the castings that range from few grams to 25kg. Shell moulding applications inl small machine parts requiring high precision as gear housing, cylinder heads and connecting rods. The process is also used in producing high Precision moulding cores.3.6.4 CO, -Process : In Carbon dioxide sand moulding (CO,-process or silicate sand moulding), fine silica sand is mixed with a small amount (3 to 5%) of sodium silicate and mixture is placed in the moulding box and rammed in the same Way as green sand. Sodium silicate is used as the binder for sand. In this case more vent holes are necessary. After moulding, CO, gas at regulated pressures (0.5 N/ mm*) and amounts is allowed to flow through the sand moulds for 10 to 30 seconds. The chemical reaction between the CO, and sodium silicate results the formation of sodium carbonate and silica gel, which bonds the sand grains together. This gives a harder sand moulds with less wall movement. Immediately after gassing (the process of blowing the gas) the moulds are ready for pouring- aN (Radiant) Jaxmiissinnnnmannamnenns a.Casting Methods 7 ents * EER gg 5 | ‘or advantages of —— he male COnProcesses are | 1. The process time is only fey seconds 2 4 g. The process can also be useq to produce cores. Cores made by this process reduce the tendency to tear. 3, Moulds and cores can be used immediately after processing. 4, Moulds are strong and abrasion resistant so that the process is suited to mechanised production. Disadvantages : 1. Difficulty in reclaiming the used sand 2. Poor collapsibility of hardened sand requires special additives (wood flour or coal) to improve the same. 3. Cost of sand mixture is high due to increased cost of binders 4. Use of CO, gas is expensive. -process was first used in 1950s and has been developed further by using other chemicals as binder. CO,-process can be used for both ferrous and non-ferrous castings. The process is employed for making harder moulds and cores. It is particularly suitable for thick walled steel castings. Applications : CO,