Schematic set-up of sand molding / casting process

Transcription

1 UNIT I Casting Processes The casting process involves pouring of liquid metal in to a mold cavity and allowing it to solidify to obtain the final casting. The flow of molten metal into the mold cavity depends on several factors like minimum section thickness of the part, presence of corners, non-uniform cross-section of the cast, and so on. The casting processes can be broadly classified into expendable mold casting and permanent mold casting processes. Sand Casting Sand casting is widely used for centuries because of the simplicity of the process. The sand casting process involves the following basic steps: (a) place a wooden or metallic pattern in sand to create a mold, (b) fit in the pattern and sand in a gating system, (c) remove the pattern, (d) fill the mold cavity with molten metal, (e) allow the metal to cool, and (f) break the sand mold and remove the casting. The sand casting process is usually economical for small batch size production. The quality of the sand casting depends on the quality and uniformity of green sand material that is used for making the mold. schematically shows a two-part sand mold, also referred to as a cope-and-drag sand mold. The molten metal is poured through the pouring cup and it fills the mold cavity after passing through downsprue, runner and gate. Schematic set-up of sand molding / casting process Shell molding Shell molding is similar to sand casting. Normally a machined pattern of grey iron or aluminum is used in this process. The pattern is heated to C to C and the sand resin mixture is poured over its surface. The heated pattern melts the resin creating bonds between the sand grains. After a dwell period the pattern and sand inverted and extra sand is cleaned off. The mold cavity is now formed by a hardened shell of sand. The mold is then heated in an oven for further curing. The shell thus formed constitutes one half of the mold. Two such halves are placed over one another to make the complete mold. The sands used in shell molding process are usually finer than the same used in sand casting. This process is ideal for complex shaped medium sized parts. represents the steps of shell mold casting. This method can be employed for making an integrate shapes, thin and sharp corners small projection which are not possible in green sand mold. Subsequent machining operations are also reduced due to more dimensional accuracy

2 Schematic set-up of shell mold casting process Investment casting Investment casting is also referred to as lost-wax casting since the pattern is made of wax. The wax patterns are first dipped into a slurry of refractory material and subsequently, heated so that the wax melts away keeping a refractory mold. The mold is then further cured to achieve proper strength. Very high melting temperature material can be cast in investment casting process because of the refractory mold. schematically shows an investment casting process. The molten metal is poured into the mold and is taken out after solidification by breaking the mold. Very high dimensional accuracy and surface finish can be achieved in investment casting process. However, the tooling cast is usually high and hence, investment casting process is primarily used for large size batch production or for specific requirements of complex shape or casting of very high melting temperature material. Vacuum Casting In this process, a mixture of fine sand and urethane is molded over metal dies and cured with amino vapor. The molted metal is drawn into the mold cavity through a gating system from the bottom of the mold. The pressure inside the mold is usually one-third of the atmospheric pressure. Because the mold cavity is filled under vacuum, the vacuum casting process is suitable for thin walled, complex shapes with uniform properties

3 Schematic set-up of and sequences involved in investment casting Schematic set-up of vacuum casting process

4 DEFECTS IN CASTING PROCESSES Schematic pictorial presentation of various casting Mould shift It results in a mismatching of the top and the bottom parts of the casting, usually at the parting line. Swell It is an enlargement of the mould cavity by molten metal pressure resulting in localized or general enlargement of the casting. Fins and Flash These are thin projections of the metal not intended as a part of casting. These usually occurs at the parting line of the mould. Sand Wash It usually occurs near the in the gates as rough lumps on the surface of a casting. Shrinkage It is a crack or breakage in the casting on the surface of the work piece, which results from un equal contraction of the metal during solidification. Hot Tear It is an internal or external ragged discontinuously in the metal casting resulting just after the metal has solidified. Sand Blow or Blow Hole It is smooth depression on the outer surface of the casting work piece.

5 Honeycombing or Slag holes These are smooth depression on the upper surface of the casting. They usually occur near the ingates. Scabs These are patches of sand on the upper surface of the casting component. Cold Shut and Misruns These happens when the mould cavity is not completely filled by the molten and insufficient material or metal. Run-outs and Bust-outs These permit drainage of the metal from the cavity and result in incomplete casting. Porosity Porosity is a phenomenon that occurs in materials, especially castings, as they change state from liquid to solid during the manufacturing process. Blister This is a scar covered by the thin layers of the metal. Dross The lighter impurities are appearing on the top of the cast surface is called the dross. It can be taken care of at the pouring stage by using items such as a strainer and a skim bob. Wash It is a low projection on the drag surface of a casting commencing near the gate. It is caused by the erosion of sand due to high velocity liquid metal. Buckle It refers to a long fairly shallow broad depression at the surface of a casting of a high temperature metal. Due to very high temperature of the molten metal, expansion of the thin layered of the sand at the mold face takes place. Rat tail It is a long shallow angular depression found in a thin casting. The cause is similar to buckle. CENTRIFUGAL CASTING In centrifugal casting process, molten metal is poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force. It is employed for mass production of circular casting as the castings produced by this process are free from impurities. Due to centrifugal force, the castings produced will be of high density type and of good strength. The castings produced promote directional solidification as the colder metal (less temperaturemolten metal) is thrown to outside of casting and molten metal near the axis or rotation. Thecylindrical parts and pipes for handling gases are most adoptable to this process.

6 PATTERN & PATTERN MATERIAL: Pattern is an mirror image of the casting, when it is used with suitable moulding material it forms a cavity called as mould. When this cavity is filled by molten metal and after solidification we get the desired casting. Some allowance are provide in the pattern. They are as: Allowance for shrinkage of metal. Allowance for machining. Draft allowance for easy removal of sand. Core prints in the form of extra projections to produce seats for cores. Tools used for making the pattern includes the tools already discussed in carpentry shop i.e. planes such as rib bet plane, block plane, router plane, plough plane, spoke shave, draw knife etc. the saw such as coping saw, bow saw, meter saw etc. the measuring and marking tools such as carpenter s scale or folding rule, dividers, trammels, calipers and miscellaneous tools suchas gauges, files etc. Pattern material: The most common material for making a pattern is wood due to some advantages: It is economical and easily available. Easy to convert in required shape and sizes. Light in weight and good finish is obtained only by planning. Can be preserved for a long time. There are some short comings also in case of wood for example wear and tear for wooden item is more. It is effected by the atmospheric variations, due to this defect such as bending/warpage. Crooks etc. may develop in the patterns Plaster:

7 Plaster of paris can be casted very easily to any shape. It has a very high compressive strength and can be used to make patterns of smaller sizes with close dimension control. It has the property that it expands on solidification. In case proper plaster is selected the effect of shrinkage is automatically neutralized. Metals: It is used when pattern are made in mass production with more accuracy. It overcomes almost all the shortcomings of wood. There are some limitations also in the use of metals. They are as: In comparison to wood it is costlier. In this case machining is required which will increase the cost of pattern. It is heavier as compared to wooden pattern. It is affected by atmospheric corrosion due to which treatment is a must which increase the cost. The metals used for making the patterns are: CAST IRON: It is economical. It can be casted to any shape having good machinability, resistance to abrasion, gives better surface finish, it is very heavy.

8 BRASS: Used for making patterns of smaller sizes. It has more strength, more resistance to corrosion, can be machined very easily, suitable for good surface finish, can be casted into any shape. It is heavier than cast iron. ALUMINIUM: Pattern of bigger sizes are made by this metal. Since the weight is less and it is economical. It can be casted and machined easily for better finish. It is not as stronger as other metals. PLASTICS: Plastic is used for making a pattern due to the properties lighter in weight, more strength with lesser wear, gives better finish and low shrinkage during melting also mot much costlier. Plastic that are used for pattern making are called thermosetting resins and phenolic resins. Initially the moulds are made from plaster of paris then the resin is poured in moulds at a particular temperature, the resin solidifies for getting plastic patterns. WAX: In this the mould is made into two halves and the wax is poured in this. The cooling of the die takes place by circulation of water. After setting, the die is separated and the pattern is taken out. FACTORS AFFECTING THE SELECTION OF PATTERN MATERIAL These are the points that must be considered while selecting materials for the pattern. a. Rate of production of casting.

9 b. Moulding is done manually or by machine. c. What are the casting methods. d. Accuracy and surface finish required in castings. Master patterns: It is used for making moulds for castings. It is a wooden pattern having both the shrinkage allowance and the machining allowance. Suppose aluminium is used to make moulds for brass castings. So aluminium pattern should have dimensions greater than brass casting equal to shrinkage that takes place while casting and machining allowance for processing the casting. PATTERN ALLOWANCE Shrinkage allowance: Most of the metal have a tendency to contract during solidification of the metal. The amount of shrinkage also differs from metal to metal, the factors that affect the shrinkage are temperature while pouring the metals, material of the mould specifications of the casting, method of moulding, casting material. Machining allowance:

10 Machining may be required by the casting, may be partially or fully. In drawing the portion to be machined is identified and in those portions machining allowance is also provided, apart from shrinkage allowance. Machining allowance also depends upon casting metal, machining methods, specification of casting and the finish required. Draft allowance: Pattern are given slight taper on all vertical surfaces, this taper is draft allowance. This is either in degrees or linear measurements and it is provided in internal and external surfaces It is provided for easy withdrawal of the pattern. Draft allowance depends upon its vertical height and moulding method. Shake allowance:

11 Before withdrawal of the pattern first of all it is shaken so that it is free from adjoining walls, due to this size of mould cavity increases so a negative allowance is given to pattern. Distortion allowance: There are certain casting in which the cooling of the metal is not uniform throughout the casting due to very complicated shape. Due to this there is distortion in the castings. To minimize its effect distortion in opposite direction is given in the pattern. Movement of mould wall: Due to excessive heat the walls of the mould have a tendency to move, this movement of the wall affects the final size of the castings. In order to minimize the effect, allowance is added

12 in the pattern and it is also controlled by controlling three parameters moulding sand composition, density and temperature of molten metal. USE OF A PATTERN a. To form a cavity of proper shape and size in the moulding material so that required casting is obtained by molten metal. b. Providing seating surface for the cores that are used for making the cavity. This resting surfaces are called core prints. c. To form several locating points that are used as reference points for checking dimensions of the casting and the measurement during machining. d. It reduces casting defects. e. It minimizes the cost of casting. MOULDING SAND PROPERTIES AND ITS CLASSIFICATION: The moulding is a process of making a cavity or mould out of sand by means of a pattern. The molten metal is poured into the moulds to produce casting. Properties of moulding sand 1: porosity or permeability It is the property of sand which permits the steam and other gases to pass through the sand mould. The porosity of sand depends upon its grain size, grain shape, moisture and clay components are the moulding sand. If the sand is too fine, the porosity will be low. 2: Plasticity It is that property of sand due to which it flows to all portions of the moulding box or flask. The sand must have sufficient plasticity to produce a good mould. 3: Adhesiveness It is that properties of sand due to it adheres or cling to the sides of the moulding box. 4: Cohesiveness It is the property of sand due to which the sand grains stick together during ramming. It is defined as the strength of the moulding sand. 5: Refractoriness The property which enables it to resist high temperature of the molten metal without breaking down or fusing. Classification of Moulding sand according to their use: 1: Green sand The sand in its natural or moist state is called green sand. It is also called tempered sand. It is a mixture of sand with 20 to 30 percent clay, having total amount of water from 6 to 10 percent. The mould prepared with this sand is called green sand mould, which is used for small size casting of ferrous and non-ferrous metals.

13 2: Dry Sand The green sand moulds when baked or dried before pouring the molten metal are called dry sand moulds. The sand of this condition is called dry sand. The dry sand moulds have greater strength, rigidity and thermal stability. These moulds used for large and heavy casting. 3: Loam Sand A mixture of 50 percent sand grains and 50 percent clay is called loam sand. It is used for loam moulds of large grey iron casting. 4: Facing Sand A sand which is used before pouring the molten metal, on the surface is called facing sand. It is specially prepared sand from silica sand and clay. 5: Backing or Floor Sand A sand used to back up the facing sand and not used next to the pattern is called backing sand. The sand which have been repeatedly used may be employed for this purpose. It is also known as black sand due to its colour. 6: System Sand A sand employed in mechanical sand preparation and handling system is called system sand. This sand has high strength, permeability and refractoriness. 7: Parting Sand A sand employed on the faces of the pattern before the moulding is called parting sand. The parting sand consists of dried silica sand, sea sand or burnt sand. 8: Core Sand The cores are defined as sand bodies used to form the hollow portions or cavities of desired shape and size in the casting. Thus the sand used for making these cores is called core sand. It is sometimes called oil sand. It is the silica sand mixed with linseed oil or any other oil as binder CORES A core can be defined as a body of sand which is used to form a cavity of desired shape and size in a casting. Core are prepared separately in core boxes. Types of cores: Horizontal core:

14 It is most common and simple type of core. It is assembled in the mold with its axis horizontal. It is supported in the mold at its both ends. Vertical core: It is quite similar to a horizontal core except that it is fitted in the mold with its axis vertical. 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 exception that it is supported only on one end, the other end remaining free in the mold cavity. Hanging or cover core: A core which hangs vertically in the mold and has no support at its bottom is known as hanging core. In such case it is obvious that the entire mold cavity will be contained in drag only.

15 MELTING FURNACES A melting furnace is a very necessary equipment in foundry shop. It is used to melt the metal to be casted. TYPES OF FURNACES The main types of furnaces used in foundries for melting of various varieties of ferrous and non-ferrous metals and alloys are described as: Crucible furnaces: These are the simplest of all the furnaces used in foundries. They are used in most of the small foundries where melting is not continuous and a large variety of metal is to be melted in small quantities. In these furnaces the entire melting of metal takes place inside a melting pot called crucible, which is made of clay and graphite. These furnaces can be classified as: Coke fired furnaces: These furnaces are generally installed in a formed pit and are used for melting small quantities of ferrous metals for producing iron casting and also non-ferrous metals and alloys. They are provided with refractory lining inside and a chimney at the top. Coke is used as fuel. Broken pieces of metal are placed in the crucible. Bed coke is fired in the furnace and the crucible placed into it. Afterwards more coke is placed all around the crucible. Oil and gas fired furnaces: These furnaces utilize oil or gas as a fuel. In fact a mixture of gas and air or oil and air is fed into the furnace which burns inside to produce the desired temperature. The furnace essentially consists of cylindrical steel shell, provided with refractory lining inside and proper passage for entry of the fuel mixture. The crucible is seated on a pad formed at the bottom.

16 Cupola furnace: For melting of cast iron in foundry the cupola furnace is used. It has a construction in the form of a hollow vertical cylinder made of strong mild steel plates. The kindling material, generally soft and dry piece of wood, is first placed over the sand bed followed by a small amount of coke charge known as bed charge. The coke for this charge is put in the furnace through the charging door. The kindling material is ignited through the hole. This fire spreads slowly into the coke around the kindling material. Additional coke is fired until the bed charge acquires the required height. Cover plated opposite the tuyeres are opened to allow the free entry of air to aid combustion and they are left open till the entire bed charge is fully ignited. They are repeated in alternate layers, of course a predetermined quantity of each, until the cupola is full to the charging door. If the cupola, on account of its fixed capacity, is unable to take up the entire material to be melted at a time, the remainder is fed into it after the initial charge has been melted.

17 ADVANTAGES OF USING CUPOLA 1. The initial cost is comparatively lower than other types of furnaces of same capacity. 2. Operation and maintenance of the furnace does not involve too many complications. 3. Cost of operation and maintenance are comparatively lower. 4. The floor area required is hardly a fraction of that required for other furnaces of similar capacity. 5. It can be operated for a number of hours at a stretch. 6. It does not involve very complicated problems in its design which is comparatively simpler. PRECAUTIONS IN OPERATING THE CUPOLA 1. A superior refractory lining should be used to withstand high temperature produce inside the furnace. 2. The man who fires the coke and charge should place the metal charge in the center. 3. The molten metal should be tapped out well before its level rises too high in the well. 4. The tap hole should be properly closed by means of well prepared clay bott or plug. 5. In closing the tap hole care should be taken to press the plug downwards in the hole so that the splash of the molten metal does not fall on the hands. 6. The amount of air supply should be properly controlled. An excess amount of air will result in lowering of temperature inside.