Foundry - Pattern making, moulding and casting PDF

Unit III Foundry- Pattern making, moulding and casting:  Sand casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. Cast...

Unit III Foundry- Pattern making, moulding and casting:  Sand casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. Casting is probably one of the most ancient processes of manufacturing metallic components. The process involves the following basic steps : 1. Melting the metal. 2. Pouring it into a previously made mould or cavity which conforms to the shape of the desired component. 3. Allowing the molten metal to cool and solidify in the mould. 4. Removing the solidified component from the mould, cleaning it and subjecting it to further treatment, if necessary The solidified piece of metal, which is taken out of the mould, is called as ‘‘Casting’’. A plant where the castings are made is called a ‘‘Foundry’’ Metal casting (or simply casting) it based on the property of liquid to take up the shape of the vessel which contains it. The process of metal casting involves pouring of molten metal into a mould, which is a cavity formed in some moulding material such as sand. The mould cavity exactly resembles in shape and size with the product to be made. After pouring, the molten metal is allowed to freeze there, taking up the shape of the mould cavity and the product thus cast, is called a casting. Flask: A moulding flask is one which holds the sand mould intact. Drag: Lower moulding flask. Cope: Upper moulding flask. Parting line: This is the dividing line between the two moulding flasks that makes up the sand mould. Core: It is used for making hollow cavities in castings. Pouring basin :A small funnel shaped cavity at the top of the mould into which the molten metal is poured. Sprue: The passage through which the molten metal from the pouring basin reaches the mould cavity. Runner: The passage ways in the parting plane through which molten metal flow is regulated before they reach the mould cavity. Gate: The actual entry point through which molten metal enters mould cavity. Chaplet: Chaplets are used to support cores inside the mould cavity to take care of its own weight and overcome the metallostatic forces. Chill: Chills are metallic objects which are placed in the moulds to increase the rate of cooling of castings to provide uniform or desired cooling rate. Riser: It is the reservoir of molten metal provided in the casting so that hot metal can flow back into the mould cavity when there is a reduction in volume of metal due to solidification. Core print :An impression in the form of a recess is made in the mould with the help of a projection suitably placed on the pattern, for supporting the cores in the mould cavity. This projection is known as a core print.  Steps in casting (i) Pattern making: The pattern is an replica of the article to be cast. The patterns are designed and prepared as per the drawing. (ii) Moulding and core making :The moulds are prepared in either sand or similar materials with the help of patterns so that a cavity of the designed shape is produced. To obtain hollow portions, cores are prepared separately in core boxes. The moulds and cores are then baked to impart strength and finally assembled for pouring. Moulding can be done either manually or by machines depending on the output required. Provision of gates and risers are also made for flow of molten metal. (iii) Melting & casting: Correct composition of molten metal is melted in a suitable furnace and poured into the moulds. The moulds are then allowed to cool down for the metal to solidify. The castings are finally extracted by breaking the moulds. (iv) Fettling :The castings as obtained after solidification carry unwanted projections. Also sand particles tend to adhere to the surface of castings. The castings are therefore sent to fettling section when the projections are cut off and surface cleaned for further work. The casting may also need heat treatment depending on the specific properties required. (v) Testing & Inspection: Finally, before the casting is despatched from foundry, it is tested and inspected to ensure that it is flawless and confirms to the specifications desired. PATTERNS A pattern is an element used for making cavities in the mould, into which molten metal is poured to produce a casting. It is not an exact replica of the casting desired. There are certain essential differences. It is slightly larger than the desired casting, due to the various allowances (shrinkage allowance, machining allowance etc.) and it may have several projections or bosses called core prints. It may also have extensions to produce runners and gates during the moulding process. Pattern Materials. The requirements of a good pattern are : 1. Secure the desired shape and size of the casting. 2. Cheap and readily repairable. 3. Simple in design for ease of manufacture. 4. Light in mass and convenient to handle. 5. Have high strength and long life in order to make as many moulds as required. 6. Retain its dimensions and rigidity during the definite service life 7. Its surface should be smooth and wear resistant. 8. Able to withstand rough handling.  Pattern Materials Wood Metals Plastics Rubbers Plasters Waxes  Pattern Making Allowances Usually, the pattern is always made larger than the desired size of the casting on account of allowance which should be allowed for machining, shrinkage, distortion and rapping etc. Machining Allowance The extra amount of metal provided on the surfaces of casting to be machined is called as a machining allowance. The amount of this allowance depends upon the method of casting used, metal of casting, method of machining. Size and shape of casting etc. Ferrous types of metals require more allowance comparative to non-ferrous metals. Shrinkage Allowance Metals used for casting usually shrink and contract due to solidification and cooling. It is compensated by providing adequate amount of allowance in the pattern which is called as shrinkage allowance. Distortion Allowance Casting of irregular shape and design tend to distort during cooling period. Distortion of casting will take place due to uneven metal thickness, shrinkage and rate of cooling. To eliminate this defect, distortion in opposite direction is provided in the pattern so that this effect of distortion may be neutralized. Rapping Allowance When a pattern is withdrawn from a mould, rapping is used in the pattern. As a result of this rapping, the cavity in the mould is slightly increased. Therefore, a negative allowance is to be provided in the pattern to compensate the same. Draft Allowance To facilitate easy and early withdrawal of pattern from the mould without injuring the vertical surfaces and edges of mould, patterns are given a slight taper on all vertical surfaces. This slight Distortion Allowance  Core Prints Cores are used to make holes, recesses in the castings. Core prints are used to support the core inside the mould cavity. Core print is the added projection on the pattern and it forms seat in the mould on which sand core will rest during casting. Core print must be adequate in size and shape , so that it can support weight of core during the casting operation. Horizontal Core Print This forms seat for a horizontal core. Horizontal core print is often found on the split or two-piece pattern. Vertical Core Print It forms seat to support a vertical core in the mould. Balancing Core Print It forms seat on one side of the mould and the core is supported at one end only, i.e. the core remains partly in this formed seat and partly in the mould cavity. The print of core in the mould cavity should balance the part which rests in the core seat. Cover or Hanging Core Print It is used when the whole surface of pattern is rammed in the drag and the core is suspended from top of the mould. Wing Core Print At that place, where the cavity to be cored is above or below the parting line in the mould, wing core print is referred Wing core  Core Box Core box is essentially a type of pattern made of wood or metal into which sand is rammed or packed to form a core. Half core box is used when a symmetrical core is prepared in two identical halves which are later on pasted or cemented together to form a complete core Split Core Box is made in two parts like a split pattern. Both the parts are joined together by means of dowel pins to form the complete hollow cavity for making the core Dump Core Box is designed to form a complete core that requires no pasting. For making the slab or rectangular shape of core, dump core box is used. The box is made with open one side and sand is rammed up level with edges of this opening. Loose Piece Core Box is used for the preparation of core with the provisions of boxes or hubs and also when the two halves of a core of which the halves are not identical in shape and size is to be prepared in the same core-box  Strickle Type Core Box For the preparation of unsymmetrical or irregular shapes of cores, strickle type of core-boxes are often used Left and Right Hand Core Box: When two half cores made in same core box cannot be pasted together to form an entire core. A gang core box is employed when a large number of small sized cores are required in a single operation  Essential Characteristics of Core - Sufficiently collapsible i.e they should disintegrate and collapse after the metal solidifies, to facilitate removal of core from the casting during shakeout.  CORE SAND It is special kind of molding sand. Considerations while selecting core sand involves : (i) The cores are subjected to a very high temperature and hence the core sand should be highly refractory in nature (ii) The permeability of the core sand must be sufficiently high as compared to that of the molding sands so as to allow the core gases to escape through the limited area of the core recesses generated by core prints (iii) The core sand should not possess such materials which may produce gases while they come in contact with molten metal and (iv) The core sand should be collapsible in nature, i.e. it should disintegrate after the metal solidifies, because this property will ease the cleaning of the casting.  CORE SAND The main constituents of the core sand are pure silica sand and a binder. Silica sand is preferred because of its high refractoriness. For higher values of permeability sands with coarse grain size distribution are used. The main purpose of the core binder is to hold the grains together, impart strength and sufficient degree collapsibility, produces minimum amount of gases when the molten metal is poured in the mould. The common binders which are used in making core sand as follows: Cereal binder, Protein binder, Thermo setting resin, Sulphite binder, Dextrin, Pitch, Molasses, Core oil  Core Making Core Sand Core Core Baking Preparation moulding Finishing  Core Sand Preparation (Mixture must be homogeneous so that the core will be of uniform strength throughout) Roller mills Core mixers ( (Suitable for cereal (Suitable for any type of binder) core binder)  Core Moulding Cores are made manually or with machine Core box is required for preparation of core, whose interiors have desired shape and dimension, into which sand is rammed. Core Making Machines Core Blowing Core Ramming Machine Machine (Core sand is filled into core box by compressed air) (Jolting, Squeezing, slinging) Classification of Moulding Machines  Classification of Moulding Machines Squeezer Jolt-squeezer Slinging Jolt machine machine machine machines  Core baking (to drive away the moisture and harden the binder, thereby giving strength to the core. ) Core ovens Dielectric bakers Continuous type Batch type ovens ovens  Core baking Continuous type ovens Continuous type ovens are preferred basically for mass production. In these types, core carrying conveyors or chain move continuously through the oven. The baking time is controlled by the speed of the conveyor. The continuous type ovens are generally used for baking of small cores. Batch type ovens Batch type ovens are mainly utilized for baking variety of cores in batches. The cores are commonly placed either in drawers or in racks which are finally placed in the ovens. Dielectric bakers These bakers are based on dielectric heating. Material to be heated dielectrically is placed between electrodes(Asbestos) and a high frequency current is passed through it. The main advantage of these ovens is that they are faster in operation and a good temperature control is possible with them.  CORE FINISHING The cores are finally finished after baking and before they are finally set in the mould. The fins, bumps or other sand projections are removed from the surface of the cores by rubbing or filing. The dimensional inspection of the cores is very necessary to achieve sound casting. Cores are also coated with refractory or protective materials using brushing, dipping and spraying means to improve their refractoriness and surface finish. The coating on core prevents the molten metal from entering in to the core.  Types of moulding Sand Green sand Dry sand Loam sand Green sand is also Green sand that has Loam sand is high in known as tempered or been dried or baked in clay (50%) natural sand which is a suitable oven after the Used for loam molding just prepared mixture making mold and cores, of silica sand with 18 to is called dry sand. 30 % clay, having It possesses more moisture content from strength, rigidity and 6 to 8%. thermal stability. The clay and water provide the bond for It is mainly suitable for green sand. larger castings. It is fine, soft, light, and Mold prepared in this porous. sand are known as dry Molds prepared by this sand molds. sand are not requiring backing and hence are known as green sand molds.  Facing sand Backing sand System sand Facing sand forms the face of Backing sand or floor In mechanical the mould. sand is used to back up foundries no facing It is directly next to the the facing sand and is sand is used. surface of the pattern and it used to fill the whole System sand is used to comes into contact molten volume of the molding fill the whole molding metal when the liquid metal flask. flask. is poured.. Used molding sand is The used sand is This sand is subjected mainly employed for cleaned and re- severest conditions and must this purpose. activated by the possess, therefore, high The backing sand is addition of water and strength refractoriness. sometimes called black special additives. This is It is made of silica sand and sand because that old, known as system sand. clay, without the use of used repeatedly used sand. molding sand is black in The layer of facing sand in a color due to addition of mold usually ranges from 22- coal dust and burning 28 mm. From 10 to 15% of on coming in contact the whole amount of with the molten metal. molding sand is the facing sand.  Parting sand Core sand Parting sand is used to keep used for making cores and it the green sand not to stick to is sometimes also known as the pattern and also to allow oil sand the sand on the parting This is rich silica sand mixed surface the cope and drag to with binders such as core oil separate without clinging which composed of linseed oil, resin, light mineral oil and other bind materials.  Properties of Moulding sand Permeability Flowability Collapsibility porosity behave like a fluid. After the molten to allow the escape It will flow metal in the mould of any air, gases or uniformly to all gets solidified, the moisture present portions of pattern sand mould must or generated in the when rammed and be collapsible so mould when the distribute the that free molten metal is ramming pressure contraction of the poured into it. evenly all around in metal occurs and All these gases all directions. this would generated during naturally avoid the pouring and tearing or cracking solidification of the contracting process must metal. escape otherwise the casting becomes defective  Adhesiveness Cohesiveness Green strength It is property of It is property of The green sand after molding sand to get molding sand by water has been stick or adhere with virtue which the sand mixed into it, must foreign material such grain particles have sufficient sticking of molding interact and attract strength and sand with inner wall each other within the toughness to permit of molding box molding sand. the making and handling of the mould. By virtue of this property, the pattern can be taken out from the mould without breaking the mould and also the erosion of mould wall surfaces does not occur during the flow of molten metal  Dry strength Refractoriness As soon as the molten Refractoriness is defined metal is poured into the as the ability of molding mould, the moisture in sand to withstand high the sand layer adjacent to temperatures without the hot metal gets breaking down or fusing evaporated and this dry thus facilitating to get sand layer must have sound casting. sufficient strength to its Molding sand with poor shape in order to avoid refractoriness may burn erosion of mould wall on to the casting surface during the flow of molten and no smooth casting metal surface can be obtained.  SAND TESTING Moisture Clay Content Content Test Test Grain Fineness Permeability Test Test Green Mould Compression Hardness Test Strength  Moisture Content Test By drying a weighed amount of 20 to 50 grams of molding sand to a constant temperature up to 100°C in a oven for about one hour. cooled to a room temperature reweighing the molding sand. The moisture content in molding sand is thus evaporated. This loss in weight gives percentage moisture content in given sand. Speedy moisture teller Principle: When water and calcium carbide react, they form acetylene gas which can be measured and this will be directly proportional to the moisture content. Pressure gauge calibrated is provided to read directly the percentage of moisture present in the molding sand. Moisture testing instruments are based on principle that the electrical conductivity of sand varies with moisture content in it.  Clay Content Test A standard sample of 50gm moulding sand (weight W1) is taken. The sample is mixed & stirred in sodium hydroxide, water, clay and sand to separate them Larger sand grains will settle down. Content on top i.e Clay which fails to settle is removed. This process is repeated until water is clear after 5 minutes of settling period. Sample is kept in oven for drying out. Percentage of clay is determined by difference in initial and final weights of sample. Grain Fineness Test Place the sample of dry sand (clay removed sand) in upper sieve. Vibrate the sieve shaker for a definite period. Weigh the amount of sand retained on each sieve Compute the grain fineness number Grain Fineness Number= Sum of (Sand retained* Multiplier)/ total % of sand Permeability Test The quantity of air that will pass through a standard specimen of sand at a particular pressure condition is called the permeability of sand Major parts of permeability test equipment's : 1.An inverted bell jar, which floats in a water 2. Specimen tube(For holding sand specimen) 3. Manometer (for measuring the air pressure) Steps involved : 1. The air (2000cc volume) held in the bell jar is forced to pass through the sand specimen 2. At this time , air entering the specimen is equal to the air escaped through the specimen. 3. Take the pressure reading in the manometer. 4. Note the time required for 2000cc of air to pass the specimen  Mould Hardness Test How hard the mould has been rammed. It is based on the principle of Brinell hardness testing machine. In standard hardness tester a half inch diameter steel hemi-spherical ball is loaded with a spring. Ball is made to penetrate into the mold sand The penetration of the ball point into the mould surface is indicated on a dial The dial is calibrated to read the hardness directly. Classification of Moulding Machines Cupola Furnace Parts of Cupola: 1. Shell 2. Foundation 3. Drop-bottom door 4. Working bottom 5. Tapping hole 6. Slag hole 7. Air Blower, Wind Box and Tuyeres 8. Charging door 9. Spark arrester Zones in Cupola: 1. Crucible Zone /Well/ Hearth 2. Combustion or Oxidizing Zone 3. Reducing Zone 4. Melting Zone 5. Pre-heating Zone 6. Stack Zone Cupola Operation: 1. Preparation of Cupola 2. Firing the cupola 3. Charging the cupola 4. Soaking of iron 5. Air blast 6. Tapping & Slagging 7. Closing the cupola Electric Arc Furnace Advantages of Electric Arc Furnace: 1. Faster cooling rate can be achieved (do not have layers like cupola ) 2. Hence process time will be minimised 3. Close temp control at any point can be achieved by varying power supply 4. Hence metal can melt in very short duration 5. Molten metal is not in contact with coke like cupola. 6. Hence good quality molten metal is achieved. 7. Addition of expensive alloying elements such as chromium, nickel, tungsten etc can be done without loss by oxidation. Induction Furnace Advantages of Induction Furnace: 1. Do not require electrode. 2. No carburization of metal 3. Simplified control of process 4. Continuous stirring action is produced by electromagnetic force in crucible. This enables homogeneous metal to be obtained. 5. Small amount precious metal can be melted efficiently.  Defects in Casting Shift 1. Misalignment of two halves of 1.Ensure proper alignment of pattern, pattern. moulding boxes. 2. Improper location of core. 2.Checking of locating pins before use 3. Faulty core boxes. Warpage 1. Unintentional & undesirable 1.Add sufficient rib like shape to provide deformation in casting occurs during equal cooling rates in all directions. solidification. Swells 1. It is enlargement of mould 1. Sand should be cavity by metal pressure, rammed properly. resulting in enlarged casting. 2. Cause: Improper ramming of mould Drop 1. Occurs when upper 1. Sand should be surface of mould cracks rammed properly. and pieces of sand fall into the molten metal. 2. Caused by low strength and soft ramming of sand  1. Smooth round holes , due to (i) The moisture content in the sand Blow holes entrapped bubbles of gases must be controlled and kept at Causes: desired level. 1. Excessive moisture in the sand. (ii) High permeability sand should 2. Low Permeability of the sand. be used. 3. Sand grains are too fine (iii) Sand of appropriate grain size 4. Too hard rammed sand. should be used. 5. Insufficient venting is provided. (iv) Sufficient ramming should be done. (v) Adequate venting facility should be provided. Metal 1. Occurs when the molten This defect can be Penetration metal flows between eliminated by using high sand particles in the strength, small grain size, mould and we get rough moderate permeability or uneven casting and proper ramming of surface. sand. 2. It is caused due to low strength, large grain size, high permeability and soft ramming of sand. Pinholes 1. They are numerous small i) By reducing the holes of about 2 mm in moisture content of the size, caused by sand with molding sand. high moisture content, absorption of hydrogen (ii) Good fluxing and gas melting practices should be used. (iii)Increasing permeability of the sand. Shrinkage 1. When metals solidify, there is a This defect can be prevented by Cavity volumetric shrinkage, and if adequate adequate feeding of molten metal feeding does not compensate for the and designing a gating system to shrinkage, voids will occur inside the enable directional solidification. casting. Misrun 1. When metal is unable to fill (i) Increasing the pouring mold cavity completely & temperature of the thus leaves unfilled cavities molten metal increases , it is called misrun defect the fluidity. 2. (i) Low fluidity of the (ii)Proper gating system molten metal. (iii) Too thin section is (ii) Low temperature of the avoided. molten metal which decreases its fluidity. (iii) Too thin section and improper gating system. Cold Shut 1. When two metal streams (i) Increasing the pouring meeting in the mould cavity temperature of the , do not fuse together molten metal increases properly, causing the fluidity. discontinuity or weak spot (ii)Proper gating system inside casting , it is called as (iii) Too thin section is cold shuts. avoided. 2. (i) Low fluidity of the molten metal. (ii) Low temperature of the molten metal which Hot tears 1. Hot tears are internal or external cracks i) Proper mold design can having ragged edges occuring immediately easily eliminate these types after the metal has solidified. of casting defects. 2. If casting is poorly designed and abrupt sectional changes, no proper fillets and corner radii are provided. Special type of Casting: SHELL MOLD CASTING Preparation of the metal match plate cope and drag type patterns. Heat the pattern. Mix the investment material (Silica + Phenol formaldehyde). Invest the pattern using dump-box. Curing the Shell. Remove the Shell from pattern plate. Repeat the steps 2 to 6 for the other half of the metal match plate pattern to produce the other half of the shell Assemble the Shells. Pour the molten metal. Remove the Casting after cooling and solidification.  Special type of Casting: Investment Casting OR Lost - wax method.  Permanent Mould Casting In sand casting moulds are destroyed after solidification of castings, where as the moulds are used repeatedly in the permanent mould castings. This requires mould material that has i) Sufficiently high melting point to withstand erosion by liquid metal at pouring temperature ii) High enough strength not to deform in repeated use, iii) high thermal fatigue resistance to resists premature crack that would leave marks on casting iv) Low adhesion Materials used for making moulds(dies): Cast iron, Alloy steel, Molybdenum alloy(refractory metal alloy), Graphite moulds (for simple shapes) The resistance of mould to melting can be increased by using refractory coatings Metals that can be cast by permanent Mould Casting: Zinc, Copper, Aluminium, Lead, Magnesium, Tin alloys Casting produced by permanent mould method should be relatively simple having uniform wall thickness, without any undercuts. Advantages of Permanent Mould Casting Over Sand Mould Casting Closer dimensional tolerances Better surface finish Greater mechanical strength Lower percentage of rejection More economical production in larger quantities Disadvantages of Permanent Mould Casting Over Sand Mould Casting Lack of permeability High cost of moulds Difficulty in removing the casting from the mould since the mould cannot be broken up. PERMANENT MOLD OR GRAVITY DIE CASTING It makes use of a mold or metallic die which is permanent. Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity. The metallic mold can be reused many times before it is discarded or rebuilt. The mold is made in two halves in order to facilitate the removal of casting from the mold  PRESSURE DIE CASTING Unlike permanent mold or gravity die casting, molten metal is forced into metallic mold or die under pressure in Pressure Die Casting. The pressure is generally created by compressed air or hydraulic means. Two Types of Pressure Die Casting: (i) Hot chamber type (a) Gooseneck or air injection management (ii) Cold chamber type  Hot chamber die-casting A cast iron gooseneck is so pivoted in the setup that it can be dipped in the surface of the molten metal to receive the same when needed. The molten metal fills the cylindrical portion and the curved passageways of the gooseneck. Gooseneck is then raised and connected to an airline which supplies pressure to force the molten metal into the closed die. The two mould halves are securely clamped together before pouring. On solidification of the die cast part, the gooseneck is again dipped beneath the molten metal to receive the molten metal again for the next cycle. The die halves are opened out and the die cast part is ejected and die closes in order to receive a molten metal for producing the next casting. The cycle repeats again and again.  Cold Chamber die-casting Melting unit is generally not an integral part of the cold chamber die casting machine. Molten metal is brought and poured into die casting machine with help of ladles. Die made in two halves : Fixed and Movable which are aligned in position by means of ejector pins Measured quantity of molten metal is poured into the cold chamber by means of ladle. Plunger of the piston is activated and forces the molten metal rapidly into the die cavity. The pressure is maintained during solidification process. After metal cools and solidifies, plunger moves backwards and movable die half can be opened by means of ejector pins and the casting can be removed.  CENTRIFUGAL CASTING Molten metal is poured into a rotating mold The mold is rotated at high speed so that the molten metal is distributed by the centrifugal force to the outer regions of the die cavity. This cause the metal to take the shape of the mold cavity. Impurities being lighter in weight are pushed towards the centre, which can be machined out.(eg. Boring) Solidification progresses from outer surface to inward.Thus the area of weakness is at the centre of wall. The use of gates, feeders and cores is eliminated.Hence making the method less expensive and complicated.  Semi-Centrifugal Casting It is similar to true centrifugal casting but only with a difference that a central core is used to form the inner surface. This casting process is generally used for articles which are more complicated than those possible in true centrifugal casting, but are axi-symmetric in nature. A particular shape of the casting is produced by mold and core and not by centrifugal force. The centrifugal force aids proper feeding and helps in producing the castings free from porosity. Since the speeds are low, high pouring pressure are not produced and the impurities are not rejected towards the centre as effectively as in the true centrifugal casting.  Centrifuging This casting process is generally used for producing non-symmetrical small castings having intricate details. A number of such small jobs are joined together by means of a common radial runner with a central sprue on a table which is possible in a vertical direction of mold rotation. The sample article produced by this process is depicted in Fig.  Slush Casting This is a special form of permanent mould casting in which hollow castings are produced without use of cores. The mould is filled with molten metal and waiting for some time, during which an outer metal shell is sufficiently solidified, the mould is turned over to drain off most of the melt. This leaves behind in the mould a hollow thin walled casting with a good outer surface but very rough inner surface. The method is used mainly to produce non structural, decorative parts such as hollow lamp bases, candle sticks, statuettes, ornamental objects, toys and other novelties. The metals used for casting the parts in this method are: Lead, Zinc and other low melting alloys. Die costs are relatively low and that is an advantage for small quantity production. Low - Pressure Permanent - Mould Casting. The permanent mould is mounted directly above the melting or holding furnace. The molten metal is forced by air or inert gas pressure (approximately 1 atm) up a riser tube, into the mould cavity. The Air/gas pressure is released as soon as the cavity is filled with solidified metal. The metal in the riser tube drops back into the sealed crucible. The casting is ejected from the cavity. The cavity is then recoated with a refractory and the process is repeated. The process finds wider applications to aluminium alloys.The weight of the casting can be upto 300N Continuous Casting  Continuous Casting Main feature: This process replaces the casting of ingots, the removal of moulds from ingots, the reheating of ingots and their primary rolling. Any shape of uniform cross section round, rectangular, square can be produced by this process. Process is used for copper, brass, bronze, aluminium, cast iron, steel Melt is transferred from ladle via tundish into mold. Molds are made up of graphite or copper. Molten metal is poured into a mould at a controlled rate, which is open at both the ends. Cooling the melt at a rate consistent with that of pouring. Dummy bar or slab is placed at one end of molud, upon which first liquid metal will fall. Solidified metal is pulled by rolls along with dummy bar. As the casting passes out the rolls, it is cut to a desired length by moving cutter blades.  Heat Treatment Necessity of HT 1. Improve machinability 2. Relieve internal stresses 3. Improve mechanical properties such as ductility, strength, hardness, toughness 4. Change the grain size 5. Increase resistance to heat and corrosion 6. Change the chemical composition 7. Remove gases  Classification of steel depending on % of carbon: Steels are alloys of iron & carbon between 0.008 to 2% carbon. Iron Carbon diagram  Annealing Normalizing Purpose: Purpose: 1. Soften the metal 1. To eliminate coarse grain structure 2. Improve machinability 2. To remove internal stresses caused due to 3. Increase ductility and toughness working 4. Relieve internal stresses 3. To improve mechanical properties 5. Refine grain size 4. Higher yield point and tensile strength than of Process annealing, but ductility and machinability is 1. Heating the steel above 20-of lowered A3(Annealing range) Process 2. Holding at this temp for 3-4 min/mm 1. Heating the metal (Normalizing range 30-40 3. Slow cooling in furnace above A3) 2. Holding at this temp for 15 min 3. Cooling in air Hardening Purpose: 1. To develop high hardness to resist wear 2. To enable to cut other metals 3. To improve strength ,elasticity, ductility, toughness Process 1. Heating above 20-30 of A3 2. Holding 3. Quenching (Rapid cooling) in water, oil, salt bath  Tempering Purpose: 1. To stabilize structure of the metal 2. To reduce internal stresses produced during previous heating 3. To reduce some of the hardness produced during previous hardening 4. To increase ductility of metal Process 1. Reheating the steel after hardening to temp below A1 2. Holding 3. Slow cooling Case Hardening : Carburising and Nitriding Purpose: 1. To give extreme hardness at the surface Process 1. Heating with carbonaceous substance and ammonia gas respectively  Solidification of molten metal in casting When the molten metal is poured in a cold mould, the solidification of liquid metal will be very rapid along the mould walls. This is the chill zone and a layer of polycrystalline, fine, equiaxed grains will be produced along the mould walls, As the solidification progresses, the grain growth will be predominantly inwards towards the centre of the mould These elongated (columner) solid grains (dendrites), Fig. 3.36, protrude into the unaffected liquid - Solid (L-S) interface, Fig. 3.37 and form what is known as ‘‘Mushy Zone’’. As the heat extraction continues throughout the mass, the simultaneous freezing of the metal at the centre of the mould will take place. Equi-axed B coarse grains will form at the centre of the casting, Solidification of molten metal in casting  Gating System Function Provide continuous and uniform feed of molten metal with less turbulence to mould cavity. To fill mould cavity with molten metal in shortest possible time To supply casting with molten metal at best location to achieve directional solidification. To prevent erosion of mould walls To prevent slag, sand from entering into mould Parts of Gating System Pouring basin: Funnel shaped opening Made at top of mould or at top of sprue in cope Purpose: To direct flow of molten metal from ladle to sprue To maintain rate of flow To reduce turbulence & vortexing at sprue entrance Requirements: Large to fill mould Deep enough to reduce vortex formation Kept full to compensate contraction Sprue: Connects the pouring basin with runners and gates. It may be of circular, square and reactangular cross section Tapered downwards to avoid aspirsation of air & to avoid metal damage Runners: Channel which connects the sprue with gate It is located in drag It should be streamlined to avoid aspiration of air & turbulence Gates: Passage through which molten metal flow from runner to mould cavity They must feed melt to casting at a rate equal to rate of solidification. Should not have sharp edges Located in such a way that can be easily removed without damaging castings.` Types of Gates: Depending upon the level at which the molten metal enters the mould cavity relative to the level of parting line. Top gate Parting line gate Bottom gate Top Gate.: In this type of gating, the metal enters the mould cavity above the level of the parting line. Molten metal enters from the top into mould cavity. The different designs of top gates: 1. Wedge shaped gates: light casting 2. Pencil gate: Sprue is made up of series of slits from pouring cup 3. Finger gate: metal is allowed to entre number of streams 4. Ring gate: Core is used to break fall of molten metal Advantages 1. Proper directional solidification towards the riser located on top of the casting 2. Gates may serve as riser Disadvantage: 1. Erosion of mould by falling of metal. Parting Line Gate.: Here, the molten metal enters the mould cavity at the level of parting line. Adv: 1. Simple to construct 2. Hottest metal reaches riser & promotes directional solidification 3. Cleaning cost reduced as no additional gate is required to connect mould cavity with riser. Disadvantage: 1. Turbulence may occur as liquid metal falls in mould cavity. Skimming gate : Foreign matter lighter than parent metal rises up through the vertical passage and trapped Parting line gate with skimbob & choke: Trap foreign matter and slag & control rate of flow of metal Whirlpool gate: Slag due to whirlpool action comes at centre & rises up in the whirlpool gate Gate with shrink bob: as a metal reservoir to feed casting on shrinkage & slag collector Bottom Gate. For deep cavities, bottom gating may be employed. Here, the metal enters the mould cavity at or near the bottom of the mould cavity Horn Gate: Produce fountain effect in mould cavity. Adv: 1. Mould erosion is prevented while pouring 2. Turbulence of metal is minimum 3. Metal is allowed to rise gently in mould & around the core Disadv: 1. Metal loses heat as it rises in mould cavity 2. Hence directional solidification is difficult to achieve Riser. : Made in cope Permit molten metal to rise above highest point in casting after mould cavity is filled up. Functions: 1. Compensate for solidification shrinkage 2. Freeze last & promotes directional solidification 3. Enable pourer to see metal(indication of complete filling of cavity) 4. Escape of steam, air, gas Types of Risers. : `Top riser' and `Side riser'. If the riser is located between runners and casting , it is known as ‘side riser’ or blind riser. If a riser must be placed at the top of the casting, (Fig. (b)), then it is called as ‘Top riser’ or open riser.