Manufacturing Technology, ME-202-E. Cores:

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1 Syllabus: Cores: Types of cores, core prints, chaplets, and chills. Gating Systems: Gates and gating systems risers. Melting Furnaces: Cupola, charge calculations. Casting Defects & Cleaning: Fettling, defects in castings and their remedies, methods of testing of castings for their soundness. Cores: These are the materials used for making cavities and hollow projections which cannot normally be produced by the pattern alone. Any complicated contour or cavity can be made by means of cores so that really intricate shapes can be easily obtained. These are generally made of sand and are even used in permanent moulds. In general, cores are surrounded on all sides by the molten metal and are therefore subjected to much more severe thermal and mechanical conditions and as a result, the core sand should be of higher strength than the moulding sand. The normal characteristics desired of a core are: 1) Green strength: A core made of green sand should be strong enough to retain the shape till it goes for baking. 2) Dry strength: It should have adequate dry strength so that when the core is placed in the mould, it should be able to resist the metal pressure acting on it. 3) Refractoriness: Since in most cases, the core is surrounded all around it is desirable that the core material should have higher refractoriness. 4) Permeability: Some of the gases evolving from the molten metal and generated from the mould may have to go through the core to escape out of the mould. Hence cores are required to have higher permeability. 5) Collapsibility: As the casting cools, it shrinks, and unless the core has good collapsibility (ability to decrease in size) it is likely to provide resistance against shrinkage and thus can cause hot tears. 6) Friability: After the casting is completely cooled, the core should be removed from the casting before it is processed further. Hence the friability (the ability to crumble) should also be a very important consideration. 7) Smoothness: The surface of the core should be smooth so as to provide a good finish to the casting. 8) Low gas emission: Because of the high temperatures to which a core is subjected to, it should allow only a minimal amount of gases to be evolved such that voids in the castings can be eliminated. Core Sands: The core sand should contain the sand grains, binders and other additives to provide specific properties: 1) Sand: The silica sand which is completely devoid of clay is generally used for making core sands. Coarse silica, because o f its higher refractoriness is used in steel foundries whereas the finer sands are used for cast irons and non-ferrous alloys. 2) Binders: As explained earlier, core sands need to be stronger than the moulding sand and therefore the clay binder used in moulding sands is not enough but somewhat better binders need to be used. The normal binders are organic in nature, because these would be burnt away by the heat of the molten metal and thus make the core collapsible during the cooling of the casting. The amount of binder required depends to a great extent on the fineness of the sand grains. Also, the amount of clay left in the sand increases the consumption of the binder. The binders generally used are, linseed oil, core oil, resins, dextrin, molasses, etc. Core oils are mixtures of linseed, soy, fish and petroleum oils and coal tar. The general composition of a core sand mixture could be core oil (1%) and water (2.5 to 6%). The actual composition to be used depends on the size and shape of the care and the alloy being cast. Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 1

2 Types of Cores: Various types of cores of different designs and sizes are employed in different ways, in foundry work. A general way of classify them is to do so according to their shapes and positions in the prepared moulds. The main types of these are described below: 1) Horizontal Core. It is the most common and simple type of core. It is assembled in the mould with its axis horizontal. Depending upon the shape of the cross-section of the hole to be made in the casting, It may have any shape of its cross-section but the most commonly used, shape is cylindrical. This core is supported in the mould at its both ends. Unless it has a non-uniform cross-section, it is held in the mould on the parting line such that its one half remains in the cope and the remaining half in the drag. exception that it is supported only on one end. The other end remaining free in the mould cavity. It is, thus a sort of Cantilever- Since it has to support the weight of the overhanging portion. The core print provided on the pattern should be long enough so that sufficient length of the core may be embedded in the sand to balance the weight of the overhung. However, if the overhung is too much, such balancing will not be enough and the overhanging length of the core will have to be supported by means of Chaplets. 2) Vertical core. It is quite similar to a horizontal core except that it is fitted in the mould with its axis vertical, as shown in Fig. It rests on the seat made at the bottom of the mould by the core print and is further supported by being located in a similar seat made in the cope. The top end of the core is provided with more taper in order to have a smooth fitting of the cope on the core. A major portion of the core usually remains in the drag part of the mould. 4) Hanging or Cover Core. A core which hangs vertically in the mould and has no support at its bottom is known as a hanging core. In such a case it is obvious that the entire mould cavity will be contained in the drag only. A good many green sand cores formed in the cope by the pattern are of hanging type. Even otherwise, dry sand cores can also be suspended from copes by being suitably fastened to them by means of wires or rods etc., extending from within the core body to the top of the cope. Unlike this, a hanging core may be supported on a seat made on the parting surface in the drag, as shown in Fig below It is then known as a Cover Core. 3) Balanced core. It is used to produce a blind hole along a horizontal axis in a casting. As a matter of fact it is nothing but a horizontal core with the Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 2 5) Wire core. It is also known as a Stop-off-Core. It is employed when the hole is desired to be produced in the casting at such a position that its axis falls either above or below the parting line. In such a core, its one side remains flush with the inner surface of the mould and the core, thus, acts as a stop-off. The back surface of this core is provided. With enough taper for its easy location. Many other

3 names like Tail Core and Saddle Core etc. are also given to this type of core according to its shape and use. Apart from the above there are a few other types also, but they are not so commonly used. One such rarely used type is a Ram-up Core, which is embedded in the mould. Such a need arises when placement of core is not possible after ramming. Similarly, when such a pattern is to be used which carries no core prints, the core is held between cope and drag simply due to the pressure put by the former. It is known as a Kiss core. Core Prints: The core prints are provided so that the cores are securely and correctly positioned in the mould cavity. The design of core prints is such as to take care of the weight of the core before pouring and the upward metallostatic pressure of the molten metal after pouring. The core prints should also ensure that the core is not shifted during the entry of the metal into the mould cavity. The main force acting on the core when metal is poured into the mould cavity is due to buoyancy. The buoyant force can be calculated as the difference in the weight of the liquid metal to that of the core material of the same volume as that of the exposed core. It can be written as P=V(ρ - d) where P = buoyant force, (N) V = volume of the core in the mould cavity, cm 3 ρ = weight density of the liquid metal, N/cm 3 d = weight density of the core material = 1.65 x 10-2 N/cm 3 Chaptets: Chaplets are metallic supports often kept inside the mould cavity to support the cores. These are of the, same composition as that of the pouring metal so that the molten metal would provide enough heat to completely melt them and thus fuse with it during solidification. Some of the types of chaplets normally used are shown in Fig. below. Though the chaplet is supposed to fuse with the parent metal, in practice it is difficult to achieve and normally it forms a weak joint in the casting. The other likely problem encountered in chaplets is the condensation of moisture which finally ends up as blow holes. Generally the chaplets before they are placed in the mould, should be thoroughly cleaned of any dirt, oil or grease. Because of the problem associated with chaplets, it is desirable to redesign the castings, as far as possible. In order to calculate the required chaplet area, we need to know that what the unsupported load is. Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 3

4 Chills: In a casting, metallic chills are used in order to provide progressive solidification or to avoid the shrinkage cavities. Chills are essentially large heat sinks. Whenever, it is not possible to provide a riser for a part of the casting which is heavy, a chill is placed close to it as shown in Fig., so that more heat is quickly absorbed by the chill from the larger mass making the cooling rate equal to that of the thin sections. Thus, this does not permit the formation of a shrinkage cavity. But use of a chill means essentially providing higher cooling rate which is also likely to form a hard spot at the contact area with the chill and may therefore cause a problem if that area needs further processing by way of machining. Elements of a Gating system: A gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity. The various elements that are connected with a gating system are shown in fig. the mainelements of a gating system are as follows: Pouring basin Sprue Sprue base well Runner Runner extension Ingate Riser Any gating system designed should aim at providing a defect free casting. This can be achieved by making provision for certain requirements while designing the gating system. These are as follows: 1. The mould should be completely filled in the smallest time possible without having to raise metal temperature nor use higher metal heads. 2. The metal should flow-smoothly into the mould without any turbulence. A turbulent metal flow tends to form dross in the mould. 3. Unwanted material such as slag, dross and other mould material should not be allowed to enter the mould cavity. 4. The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented. 5. A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities of distortion. 6. Metal flow should be maintained in such a way that no gating or mould erosion take place. 7. The gating system should ensure that enough molten metal reaches the mould cavity. 8. The gating system design should be economical and easy to implement and remove after casting solidification. 9. Ultimately, the casting yield should be maximized. To have all these requirements together is a tall order, still a mould designer should strive to achieve as many of the above objectives as possible. Pouring Basin: The molten metal is not directly poured into the mould cavity because it may cause mould erosion. Molten metal is poured into a pouring basin which acts as a reservoir from which it moves smoothly into the sprue. The pouring basin is also able to stop the slag from entering the mould cavity by means of a skimmer or skim core as shown in Fig. It holds Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 4

5 back the slag and dirt which floats on the top and only allows the clean metal underneath it into the sprue. The pouring basin may be cut into the cope portion directly or a separated dry sand pouring basin may be prepared and used as shown in Fig. The molten metal in the pouring basin should be full during the pouring operation, otherwise a funnel is likely to form through which atmospheric air and slag may enter the mould cavity. One of the walls of the pouring basin is made inclined at about 45" to the horizontal. The molten metal is poured on this face such that metal momentum is absorbed and vortex formation is avoided. In some special cases the pouring basin may consist of partitions to allow for the trapping of the slag and maintaining constant metal height in the basin. Sprue: Sprue is the channel through which the molten metal is brought into the parting plane where it enters the runners and gates to ultimately reach the mould cavity. The molten metal when moving from top of the cope to the parting plane gains in velocity and as a consequence requires a smaller area of cross section for the same amount of metal to flow at the top. If the sprue were to be straight cylindrical as shown in Fig. then the metal flow would not be full at the bottom, but some low pressure area would be created around the metal in the sprue. Since the sand mould is permeable, atmospheric air would be breathed into this low pressure area which would then he carried to the mould cavity. To eliminate this problem of air aspiration the sprue is tapered to gradually reduce the cross section as it moves away from the top of the cope as shown in Fig. below. Sprue base well: This is a reservoir for metal at the bottom of the sprue to reduce the momentum of the molten metal. The molten metal as it moves down the sprue gains in velocity, some of which is lost in the sprue base well by which the mould erosion is reduced. This molten metal then changes direction and flows into the runners in a more uniform way. Runner: It is generally located in the horizontal plane (parting plane) which connects the sprue to its ingates, thus letting the metal enter the mould cavity. The runners are normally made trapezoidal in cross section. It is a general practice for ferrous metals to cut the runners in the cope and the in-gates in the drag. The main reason for this is to trap the slag and dross which are lighter and thus trapped in the upper portion of the runners. For effective trapping of the slag, runners should flow full as shown in the Fig. When the amount of molten metal coming from the down sprue is more Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 5

6 than the amount flowing through the ingates, the runner would always be full and thus slag trapping would take place. But when the metal flowing through the ingates is more than that flowing through the runners then the runner would be filled only partially as shown in Fig. and the slag would then enter the mould cavity. Runner Extension: The runner is extended a little further after it encounters the ingate. This extension is provided to trap the slag in the molten metal. The metal initially comes along with the slag floating at the top of the ladle and these flows straight, going beyond the ingate and then trapped in the runner extension. Gates: They are also called the ingates, these are the openings through which the molten metal enters the mould cavity. The shape and the cross section of the ingate should be such that it can readily be broken off after casting solidification and also allow the metal to enter quietly into the mould cavity. Depending on the application, various types of gates are used in the casting design. They are: Top gate: This is the type of gating through which the molten metal enters the mould cavity from the top as shown in Fig. Since the first metal entering the gate reaches the bottom and hotter metal is at the top, a favourable temperature gradient towards the gate is achieved. Also, the mould is filled very quickly. But as the metal falls directly into the mould cavity through a height, it is likely to cause mould erosion. Also, because it causes turbulence in the mould cavity, it is prone to form dross and as such top gate is not advisable for those materials which are likely to form excessive dross. It is not suggested for non-ferrous materials and is suggested only for ferrous alloys. It is suitable only for simple casting shapes which are essentially shallow in nature. To reduce the mould erosion pencil gates are provided in the pouring cup. This type of gate requires minimum of additional runners to lead the liquid metal into the Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 6 cavity, and as such provides higher casting yield. Bottom gate: When molten metal enters the mould cavity slowly as shown in Fig., it would not cause any mould erosion. Bottom gate is generally used for very deep moulds. It takes somewhat higher time for filling of the mould and also generates a very unfavourable temperature gradient. The preparation of the gating also requires special sprue as shown or special cores for locating the sprue in the drag. These gates may cause unfavourable temperature gradients compared to the top gating. Thus the system may have to use additional padding of sections towards risers and large riser sizes to compensate for the unfavourable temperature distribution. Bottom gating may sometimes be preferable in conjunctionwith the use of side risers since the metal enters the riser directly without going through the mould cavity.

7 Parting gate: This is the most widely used gate in sand castings. A s the name implies, the metal enters the mould at the parting plane when port of the casting is in the cope and part in the drag as in Fig. For the mould cavity in the drag, it is a top gate and for the cavity in cope it is a bottom gate. Thus this type of gating tries to derive the best of both the types of gates, i.e. top and bottom gates. Of all the gates this is also the easiest and most economical in preparation. However, if the drag portion of the mould cavity is deep, it is likely to cause mould erosion and aggravate dross formation and air entrapment in the case of nonferrous alloys. This can be somewhat reduced by making the gate area large such that the liquid metal velocity is minimised and it flows slowly along the walls into the mould cavity. Step gate: Such gates are used for heavy and large castings. The molten metal enters mould cavity through a number of ingates which are arranged in vertical steps. The size of ingates are normally increased from top to bottom such that metal enters the mould cavity from the bottommost gate and then progressively moves to the higher gates. This ensures a gradual filling of the mould without any mould erosion and produces a sound casting. In designing a casting, it is essential to choose a suitable gate, considering the casting material, casting shape and size so as to produce a sound casting. Riser: Most of the foundry alloys shrink during solidification. Table 10.1 shows the various volumetric shrinkages for typical materials. As a result of this volumetric shrinkage during solidification, voids are likely to form in the castings as shown in Fig. unless additional molten metal is fed into these places which are termed as hot spots since they remain hot till the end. Hence a reservoir of molten metal is to be maintained from which the metal can flow readily into the casting when the need arises. These reservoirs are called risers. As shown in Table, different materials have different shrinkages and hence the risering requirements vary for the materials. In grey cast iron, because of graphitization during solidification, there may be an increase in volume sometimes. This of course, depends on the degree of graphitization in grey cast iron which is controlled by the silicon content. In order to make them effective, the risers should be designed keeping the following in mind. The metal in the riser should solidify in the end. The riser volume should be sufficient for compensating the shrinkage in the casting. In order to satisfy the above requirements, risers of large diameters are generally used. But it proves to be a very expensive solution since the solidified metal in the riser is to be cut off from the main casting and is to be melted for reuse. Higher the riser volume, lower is the casting yield and as such it is very uneconomical. The risers are normally of the following types, top risers which are open to the atmosphere; blind risers which are completely concealed inside the mould cavity itself and internal risers which are enclosed on all sides by the casting. Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 7

8 The top riser is the most conventional and convenient to make. But the position where it can be placed is limited. The top being open loses heat to the atmosphere by radiation and convection. To reduce this, often insulation it provided on the top such as plaster of Paris, asbestos sheet. The blind riser since it is surrounded by the moulding sand would lose heat slowly and thus would be more effective. Also it can be located more conveniently than an open riser. The best is the internal riser which is surrounded on all sides by the casting such that heat from the casting keeps the metal in the riser hot for a longer time. These are normally used for castings which are cylindrically shaped or have a hollow cylindrical portion. Melting Practice: After moulding, melting is the major factor which controls the quality of the casting. There are a number of methods available for melting foundry alloys such as pit furnace, open hearth furnace, rotary furnace, cupola furnace, etc. The choice-of the furnace depends, on the amount and the type of alloy being melted. For melting cast iron, cupola in its various forms is extensively used basically because of its lower initial and melting cost. CUPULA: For melting of cast iron in foundry the Cupola Furnace is used. A diagramatic sketch of this furnace is given in Fig. A Fig. B below illustrates a crosssectional view of Cupola. It has a construction in the form of a hollow vertical cylinder made of strong mild steel plates and riveted or welded at the seams. Welded joints are more common in modern practice. In large Cupolas the lower portion is made of comparatively thicker plates so as to make it strong enough to hold the upper structure and fire brick lining. Thus, the stress in the whole structure is distributed uniformly. Also, such Cupolas are further strengthened by providing the Brick Retaining Rings at suitable heights. The Bottom Door of the shell can be in one piece, hinged to a supporting leg, or in two pieces: each piece hinged separately to the two opposite legs. When the cupola is in operation, the Bottom door is supported by a Prop so that it may not collapse due to the Fig. A. COPULA FURNACE Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 8

9 large weight of the charge and coke, etc., it carries. When we do not need the A Wind Chamber or Wind Belt, as COPULA for further operation, the charge feeding is stopped, air supply cut it is more commonly known, off and the Prop is removed. As soon as the Prop is removed the Door drops encircles the cupola shell at a down providing a clear space for the coke fire, residue of the molten metal place little above the bottom of with slag and the sand bed to fall down and thus the fire inside ceases the shell. This belt is connected to gradually. the furnace blower by means of a Blast Pipe. The amount of air required is forced into the chamber by the blower, which enters the furnace through openings called Tuyeres. These Tuyeres are provided all around the shell and have a definite number and size depending upon the amount of air required. Charging Door is located at a suitable height above the charging platform. This platform is of robust mild steel construction, supported on four strong steel legs, and is provided with a Ladder. Weighed quantities of Metal, Coke, Scrap and Flux are collected on this platform, which is charged into the COPULA as and when required. The top of the COPULA is provided with a Mesh Screen and a Spark Arrester. It is a cone shaped construction, as shown in the diagram. This attachment facilitates a free escape of the waste gases at the same time deflects the spark and the dust back into the furnace. In some COPULAS the upper portion is made tapered with the top diameter as about half of the inside diameter of the cupola at the smelting zone. Small Cupola say from 500 kg to 1000 kg capacity, are better known as Cupolettes. They are quite self-sufficient in operation and have almost all the accessories which a large cupola possesses except the Spark Arrester and Charging Door. Since the height of these Cupolettes is Fig. B Cross-sectional view of COPULA Furnace very small, say 2.5 meters to 4 Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 9

10 meters, charging is done from the top of the Cupolette. They are fixed on two reunions inside the bearings, mounted on the supporting legs, so that they can be tilted to become horizontal for providing the fire Brick lining. This lining is provided in all cupolas, irrespective of the size to withstand the high temperature produced inside the furnace. Other Furnaces: Reverberatory Furnace: In these furnaces the fuel burners fire within a refractory hood above the metal bath. These are generally used to melt large amounts of metal for example, aluminium to supply to holding furnaces such as those used with pressure die casting machines. These use gas fired burners located generally high in the furnace transferring the heat by radiation to the walls and roof. As the walls and roof become incandescent they radiate the heat to the metal bath. These furnaces are simple and have relatively low capital cost. Thus these are generally used for melting large volumes of metal. Crucible Furnace: Smaller foundries generally prefer the crucible furnace. The crucible is generally heated by electric resistance or gas flame. In these the metal is placed in a crucible of refractory metal and the heating is done to the crucible thus there is no direct contact between the flame and the metal charge. This type of melting is very flexible since it suits a variety of casting alloys. Degassing and any metal treatment can be completed in the crucible before it is removed for pouring. Melt quality and temperature can also be controlled reasonably well. Induction Furnace: The induction furnaces are used for all types of materials, the chief advantage being the heat source is isolated from the charge and the slag and flux would be getting the necessary heat directly from the charge instead of the heat source. The stirring effect of the electric current would cause fluxes to be entrained in the melt if they are mixed along with the charge. So flux is generally added after switching off the current to the furnace. Then sufficient time must be allowed for the oxides to be removed by the flux as slag before transferring the metal for pouring. High frequencies help in stirring the molten metal and thus help in using the metal dwarf (chips). Low cost raw materials could, therefore, be used and at the same time better control of temperature and composition can be achieved. Ladles: The molten metal from the furnace is tapped into the ladles at requisite intervals and then poured into the moulds. Depending on the amount of metal to be handled, there are different sizes of ladles. They may range between 50 kg to 30 tones depending upon the casting size. For grey cast iron, since the slag can be easily separated, top pouring ladles would be enough. But for steels; to separate the slag effectively, the metal is to be poured from the bottom with the help of the bottom pour ladle. The bottom pour ladle has an opening in the bottom that is fitted with a refractory nozzle. A stopper rod, suspended inside the ladle, pulls the stopper head up from its position thus allowing the molten alloy to flow from the ladle. As the metal in the ladle loses a large amount of heat to the surrounding atmosphere by radiation it is necessary to account for this drop in the temperature of the casting metal. Fettling: The complete process of the cleaning of castings, called 'fettling', involves the removal of the cores, gates and risers, cleaning of the casting surface and chipping of any of the unnecessary projections on surfaces. The dry sand cores can be removed simply by knocking off with an iron bar, by means of a core vibrator, or by means of hydro blasting. The method depends on the size, complexity and the core material used. The gates and risers can be removed by hammering, chipping, hack sawing, abrasive cutoff or by flame or arc cutting. Removal of gates and risers can be simplified by providing a reduced metal section at the casting joints. For brittle materials such as grey cast iron, the gates can easily be broken by hitting with a hammer. For steel and other similar materials, sawing with any metal cutting saw like hack saw or band saw would be more convenient. For large size gates and risers, it may be necessary to use flame or arc cutting to remove them. Similarly, abrasive cut off may also be used Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 10

11 for removal of gates. Most of the abrasive cut off can be carried out by portable grinding machines with an angled grinding head. Typical wheel speeds used are in the range of 45 to 80 m/s. The casting surface after removal of the gates may still contain some rough surfaces left at the time of removal of gates, or sand that is fused with the surface, or some fins and other projections on the surface near the parting line. These need to be cleaned thoroughly before the casting is put to use. The fins and other small projections may easily be chipped off with the help of either hand tools or pneumatic tools. For smoothening the rough cut gate edges either the pedestal or swing frame grinder is used depending on the size of the casting. For cleaning the sand particles sticking to the casting surface, sand blasting is normally used. The casting is kept in a closed box and a jet of compressed air with a blast of sand grains or steel grit is directed against the casting surface, which thoroughly cleans the casting surface. The typical shot speeds reached of the order of 80 m/s. The shots used are either chilled cast iron grit or steel grit. Chilled iron is less expensive but is likely to be lost quickly by fragmentation. In this operation, the operators mould be properly protected. Another useful method for cleaning the casting surface is the tumbling. Here the castings are kept in a barrel which is completely closed and then slowly rotated on a horizontal axis at 30 to 40 rpm. The barrel is reasonably packed, with enough room for castings to move so that they will be able to remove the sand and unwanted fins and projections. However one precaution to be taken for tumbling is that, the castings should all be rigid with no frail or overhung segments which may get knocked off during the tumbling operation. Defects in Castings: Any irregularity in the moulding process causes defects in castings which may sometimes be tolerated, sometimes eliminated with proper moulding practice or repaired using methods such as welding and metallization. The following are the major defects which are likely to occur in sand castings: Gas defects. Shrinkage cavities. Moulding material defects. Pouring metal defects. Metallurgical defects. Gas defects: The defects in this category can be classified into blow holes and open blows, air inclusion and pin-hole porosity. All these defects are caused to a great extent by the lower gas passing tendency of the mould which may be due to lower venting, lower permeability of the mould and/or improper design of the casting. The lower permeability of the mould is, in turn, caused by finer grain size of the sand, finer clay, higher moisture, or by excessive ramming of the moulds Blow holes and open blows: These are the. Spherical, flattened, or elongated cavities present inside the casting or on the surface as shown in the Fig. On the surface they are called as the open blows and inside, they are called blow holes. These are caused by the moisture left in the mould and the core. Because of the heat in the molten metal, the moisture is converted into steam; part of this steam is entrapped in the casting ends up as blow hole or ends up as open holes when it reaches the surface. Apart from the presence of moisture they occur due to the lower venting and lower permeability of the mould. Thus in green sand moulds it is very difficult to get rid of the blow holes, unless venting is provided. Air inclusions: The atmospheric and other gases absorbed by the molten metal in the furnace, in the ladle, and during the flow in the mould, when not allowed to escape, would be trapped inside the casting and weaken it. The main reasons for this defect are the higher pouring temperatures which increase the amount of gas absorbed; poor gating design such as straight sprues in unpressurised gating, abrupt bends and other turbulence causing practices in the gating, which increases the air aspiration and finally the low permeability of the mould itself. Remedies would be to choose the appropriate pouring temperature and improve gating practices by reducing the turbulence. Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 11

12 Pin Hole Porosity: This is caused by hydrogen in the molten metal. This could have been picked up in the furnace or by the dissociation of water inside the mould cavity. As the molten metal gets solidified it loses the temperature which decreases the solubility of gases and thereby expelling the dissolved gases. The hydrogen while leaving the solidifying metal would cause very small diameter and long pin holes showing the path of escape. These series of pin holes cause the leakage of fluids under high operating pressures. The main, reason for this is the high pouring temperature which increases the gas pick-up. Shrinkage Cavities: These are caused by the liquid shrinkage occurring during the solidification of the castings, to compensate this proper feeding of liquid metal is required as also proper casting design also has a very large impact towards these sorts of defects. Moulding Material defects: Under this category are those defects which are caused because of the characteristics of the moulding materials. The defects that can be put in this category are: cuts and washes, metal penetration, fusion, run out, rat tails and buckets, swells and drop. These defects occur essentially because the moulding materials are not of requisite properties or due to improper ramming. Cuts and washes: These appear as rough spots and areas of excess metal, and are caused by the erosion of moulding sand by the blowing molten metal. This may be caused by the moulding sand not having enough strength or the molten metal flowing at high velocity' The former can be remedied by the proper change of moulding sand and using appropriate moulding method. The latter can be taken care of by altering the gating design to reduce the turbulence in the metal, by increasing the size of gates or by using multiple ingates. Metal penetration: When the molten metal enters the gaps between the sand grains, the result would be a rough casting surface. The main reason for this is that, either the grain size of the sand is too coarse, or no mould wash has been applied to the mould cavity. This can also be caused by higher pouring temperatures. Choosing appropriate grain size, together with a proper mould wash should be able to eliminate this defect. Fusion: This is caused by the fusion of sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this defect is that the clay in the moulding sand is of lower refractoriness or that the pouring temperature is too high, The choice of an appropriate type and amount of Bentonite would cure this defect. Run out: A run out is caused when the molten metal leaks out of the mould. This may be caused either due to faulty mould making or because of the faulty moulding flask. Shift: A shift is a misalignment between two mating surfaces leaving a small clearance between them and changing their relative location. This may occur at the parting surface between the two parts of the mould called a Mould shift, or at the core prints, providing a gap between the core and core seat. This is known as a Core shift. They result in a mismatch of the casting at the parting line and an incorrect dimension or incorrect location of hole. The clearance left between the mating surfaces is filled up by the molten metal and is found to be attached to the casting in the form of a thin projection of metal called Fin. The Mould shift is a result of misalignment of the moulding boxes due to worn out or bent clamping pins or due to improper alignment of the two halves of the pattern. The core shift is due to improper supporting of core improper location of core and faulty core boxes producing oversized or undersized cores. The above shifts may also occur due to improper clamping of boxes and placement of inadequate weights over the cope, resulting in the lifting of cope. Rat tails and buckles: Rat tail is caused by the compression failure of the skin of the mould cavity because of the excessive heat in the molten metal. Under the influence of the heat, the sand expands, thereby moving the mould wall Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 12

13 backwards and in the process when the wall gives away, the casting surface may have this marked as a small line, as shown in Fig. With a number of such failures, the casting surface may have a number of crisscrossing small lines. Buckles are the rat tails which are severe. The main causes for these defects are: the moulding sand has got poor expansion properties and hot strength or the heat in the pouring metal is too high. Also, the facing sand applied does not have enough carbonaceous material to provide the necessary cushioning effect. Proper choice of facing sand ingredients and the pouring temperature are the measures to reduce the incidence of these defects. Swell: Under the influence of the metallostatic forces, the mould wall may move back causing a swell in the dimensions of the casting. As a result of the swell, the feeding requirements of castings increase which should be taken care of by the proper choice of risering. The main cause of this is the faulty mould making procedure adopted. A proper ramming of the mould should correct this defect. Drop: The dropping of loose moulding sand or lumps normally from the cope surface into the mould cavity is responsible for this defect. This is essentially, due to improper ramming of the cope flask. Warpage: It is an undesirable deformation in the casting which may occur during or after solidification. The deformation takes place due to the internal stresses developed in the casting due to differential solidification in different sections. Such stresses are also developed and differential solidification occurs in case of castings having very large and wide flat surfaces. Both the causes can be attributed to faulty design of the casting which needs modification to ensure proper directional solidification. Pouring metal defects: The likely defects in this category are mis-runs and cold shuts and slag inclusions. Mis-runs and cold shuts: Mis-run is caused when the metal is unable to fill the mould cavity completely and thus leaves unfilled cavities. A cold shut is caused when two metal streams while meeting in the mould cavity, do not fuse together properly, thus causing a discontinuity or weak spot in the casting. Sometimes a condition leading to cold shuts can be observed when no sharp corners exist in a casting. These defects are caused essentially, by the lower fluidity of the molten metal or when the section thickness of the casting is too small. The latter can be rectified by proper casting design. The remedy available is to increase the fluidity of the metal by changing the composition or rising the pouring temperature. This defect can also be caused when the heat removal capacity is increased such as in the case of green sand moulds. The castings with large surface area to volume ratio are more likely to be prone to these defects. Further cause of this defect-is the back pressure due to gases in the mould which is not properly vented. The remedies are basically improving the mould design. Slag inclusions: During the melting process, flux is added to remove the undesirable oxides and impurities present in the metal. At the time of tapping, the slag should be properly removed from the ladle before the metal is poured into the mould. Otherwise any slag entering the mould cavity will be weakening the casting and also spoil the surface of the casting. This can be eliminated by some of the slag trapping methods such as pouring basin screens or runner extensions. Metallurgical defects: The defects that can be grouped under this category are hot tears and hot spots. Hot Tears: Since metal has low strength at higher temperatures, any unwanted cooling stress may cause the rupture of the casting. The mainly cause for this is the poor casting design. Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 13

14 Hot Spots: These are caused by the chilling of the casting. For example, with grey cast iron having small amounts of silicon, very hard cast iron may result at the chilled surface. This hot spot will interfere with the subsequent machining of this region. Proper metallurgical control and chilling practices are essential for eliminating the hot spots. Assignment for the students: Write about the various Casting cleaning processes? What are the various ways of inspecting a casting? Last date for the submission of assignment: 23 rd February 2010 Mohammad Amir, Lecturer, Department of Mechanical Engineering., BHCET 14