Lecture 3 4 Casting.pdf

Workshop Practice Dr. Anuj Sharma Dr. Amit Kumar Assistant Professor of Mechanical Engineering  LECTURE 3 & 4 CASTING PROCESSES Sand casting is a manufacturing process in which molten metal is poured into a cavity (of the desired shape) in a disposable sand mould,...

Workshop Practice Dr. Anuj Sharma Dr. Amit Kumar Assistant Professor of Mechanical Engineering  LECTURE 3 & 4 CASTING PROCESSES Sand casting is a manufacturing process in which molten metal is poured into a cavity (of the desired shape) in a disposable sand mould, where it solidifies to create the cavity-shaped component. Relatable to key duplication  Outline Necessity of casting The sand casting process Pattern making Moulding sand Moulding Cores Defects in casting Advantages and limitations 3 When casting becomes inevitable ? – Very large parts can be produced in single piece – For complicated shapes having internal cavities or hollow sections – Casting can utilize materials that are difficult to process by other means. – Economically competitive with other manufacturing processes.  Casting Fig. 1: Cast steel mill housing (280 tonnes) (Peter Beeley, 2001) Outline of a typical sand-casting operation Mould for sand casting Gate 7 SAND CASTING 8  Pattern Making A pattern is a model or the replica of the object (to be casted). It is used to form impressions (cavity) in sand (which is known as mould) Some modifications (allowances, position for core print etc.) may be required. When the mould/cavity is filled with molten metal, the molten metal solidifies and produces a casting (product). So the pattern is the replica of the casting.  Wood, metal and plastic are used for pattern materials 9  WOOD Easily available Low weight Low cost It absorbs moisture and hence dimensions will change Lower life Suitable for small quantity production and very large size castings. 10  METAL Used for mass production High wear resistance For maintaining closer dimensional tolerances on casting. More life when compared to wooden patterns Cu, Al, steel, Brass, etc are used as metal patterns. Al is widely used. Metals like Cu, and steel are heavy. Some metals may get corroded over time. 11 PLASTIC Low weight Easier formability Do not absorb moisture Good corrosion resistance 12 Selection of Pattern Material Size of casting (W>P>M) Shape of casting (M>P>W) Dimensional accuracy of casting (M>P>W) Machinability of the pattern (W>P>M) Quantity of castings required (P>M>W) Economy (W>P>M) 13  Types of patterns Single piece pattern (flat-back pattern/Solid pattern) Split pattern Loose piece pattern Gated pattern Match-plate pattern Disposable pattern 14  Single Piece Pattern Simple shape castings are produced by this type of patterns. Solid pattern is made of single piece without joints, partings lines or loose pieces. It is the simplest form of the pattern. Single Piece Pattern Single Piece Pattern in Sand Mould 15  Split patterns Used when patterns cannot be made as a single piece.  When solid pattern is difficult to withdraw from the mold cavity, then solid pattern is split in two parts.  Split pattern is made in two pieces which are joined at the parting line by means of dowel pins.  The splitting at the parting line is done to facilitate the withdrawal of the pattern. 16  Loose piece patterns Used when withdrawal of pattern from mould is not possible or castings is having projections, undercuts, etc After ramming is over, first main pattern is removed and then the loose pieces and then loose pieces are withdrawn through the gap generated by the main pattern.  Gated pattern USED FOR PRODUCING SMALL SIZED CAVITIES IN ONE MOULD. mass production of castings, multi cavity moulds are used. Such moulds are formed by joining a number of patterns and gates and providing a common runner for the molten metal, 18  Match-plate patterns Split patterns attached on either side is known as Match plate pattern. It increases production and helps in maintaining uniformity in the size and shape of the castings. The gates and runners are also attached to the plate. 19  Disposable pattern The disposable patterns are made from polystyrene (thermocol). The pattern is left in the mould instead of being removed from the sand. The pattern material vaporizes when the molten metal is poured in to the mould. The method is also known as cavity less method. Polystyrene Polystyrene Sand Sand pattern Hot Hotmetal metal pattern Sand Sand (a) Disposable pattern (b) Hot metal replacing (a) Disposable pattern in sand mould (b)disposable Hot metalpattern replacing in sand mould disposable pattern 20 Is pattern same as the casting? The pattern is slightly larger in every dimension – shrinkage allowance Sufficient extra dimensions to get required surface finish – machining allowance Extra dimension for pattern removal– draft Other allowances 21  Pattern allowances Intentional deviations made in the dimensions of a pattern from the final desired dimensions of a finished product. Shrinkage allowance Machining allowance Draft or Taper allowance Distortion allowance Rapping or Shake allowance Allowance is basically the difference in dimensions between pattern and casting. 22 SHRINKAGE ALLOWANCE Provided to compensate for shrinkage of material Pattern is made slightly bigger Amount of allowance depends upon type of material, its composition, etc. Required Actual Casting Pattern size size 23  Three types of Shrinkage Liquid shrinkage: The contraction that occurs when liquid metal or alloy temperature falls from the pouring temperature to the liquid point temperature. Solidification shrinkage: Contraction as a result of cooling from the liquid to the solidus temperature (below melting point), also known as solidification contraction. Solid shrinkage: Contraction that follows until the temperature reaches room temperature. This pattern allowance is referred to as solid contraction. 24  MACHINING ALLOWANCE – Provided to compensate for machining on casting. – Pattern is made slightly bigger is size. – Amount of allowance depends upon size and shape of casting, type of material, machining processes to be used, accuracy and surface finish required etc. 25  DRAFT OR TAPER ALLOWANCE  Provided to facilitate easy withdrawal of the pattern.  Typically taper ranges from 1 degree to 3 degree for wooden patterns. h(a)zero draftzero (no) draft (b) Pattern(b)withPattern (no)with Pattern draftwith draft (Not to scale) (Not to scale) Sand mould 26  DISTORTION ALLOWANCE (a)(a)(a) (b) (b)(b) (c) (c) (c) equired Required quired shape ofofof shape shape Casting Casting Casting produced produced produced when whennono when no Patternwith Pattern Pattern withdistortion with distortion distortion casting casting casting distortion distortion distortion allowance allowance allowance isisis provided provided provided allowance allowance allowance Provided on patterns whose castings tend to distort on cooling 27 DISTORTION ALLOWANCE 28  RAPPING OR SHAKE ALLOWANCE – Provided to permit easy withdrawal of the pattern. – When a pattern is rapped for easy withdrawals, the mould cavity is enlarged – To account for this increase in size of cavity, the pattern size is reduced, – It is a negative allowance. 29  MOULDING SANDS INGREDIENTS Sand grains (Silica), Clay, Moisture & special additives like coal dust, fuel oil. TYPES OF MOULDING SAND Natural foundry sand Synthetic (or Artificial) foundry sand Moulding sand possesses the necessary properties— high fusion temperature and good thermal stability—for making moulds. special additives like coal dust—to improve surface finish, fuel oil—to improve mouldability 30  COMMON MOULDING SAND Green Sand:  18-30% clay, 4-8% water.  Collected from natural resources.  Maintains moisture content for long time. Dry sand:  It is green sand dried and baked.  Yields porosity absent castings, as there is no moisture.  Suitable for very large size castings. 31  TYPES OF MOULDING SAND Loam sand:  Clay and silica are mixed in equal proportions. Parting sand:  Used to permit easy withdrawal of the pattern from mould cavity. Core sand:  Sand used for making cores.  It should be stronger than moulding sand. 32 PROPERTIES OF MOULDING SAND Cohesiveness or Strength - compressive strength Chemical resistivity Permeability - ability of sand to allow gases to escape through Adhesiveness (with the flask) Refractoriness - ability of sand to remain solid at high temperatures Flowability - ability of sand to flow around and over the pattern during ramming Collapsibility 33  Numerical Problem 1 The casting shown in Figure is to be made in plain carbon steel using a wooden pattern. Assuming only shrinkage allowance, calculate the dimensions of the pattern. Consider the shrinkage allowance is 21.0 mm/m. For dimension 100 mm, allowance is 100 × 21.0/1000 = 2.10 mm 100 mm 102.1 mm Dimensions taking shrinkage into account 34  Numerical Problem 2 The casting shown in Figure is to be made in plain carbon steel using a wooden pattern. Assuming only machining allowance, calculate the dimensions of the pattern. Consider the machining allowance for all surfaces = 3 mm Final dimension, 100 + 3 + 3 = 106 mm 100 mm 106 mm Dimensions taking machining allowance into account 35  Numerical Problem 3 Provide draft allowance to the pattern shown in Figure. Take the draft angle is 0.75° for external details Taper on one side = 109 × tan(0.75) ≈ 1.40 mm Final dimension at the top is: 211 + 2 × 1.40 = 213.80 mm 36  Core Cores are used to obtain holes or other internal cavities in castings. For the core, there is no provision has been made in the pattern. Generally produced separately from the sand mould. Baked to make it strong and facilitate handling during setting into the mould. It is placed in specially created cavities called core prints. It is made from sand, metal, or ceramics. 37  Core Properties It must be strong to retain the shape while handling, It must resist erosion by molten metal, It must be permeable to gases, It must have high refractoriness, and It must have good surface finish to replicate it on to the casting. 38  CHAPLETS If the cores are very large in size, then it is not possible to provide the sufficient support for a core in the mould. The cores are supported by rigid metal pieces called chaplets, placed between the core and the mould face. The material of chaplets must conform to the molten metal for proper fusion. 39 Figure  GATING SYSTEM In order to avoid the erosion of the bottom of mould cavity the molten metal is introduced into the mould cavity through a gating system. Gating system has following components: Pouring basin Sprue Sprue well Runner Gates Riser 40  Gating System Pouring cup Riser Sprue Casting Sprue base Runner Gate It refers to the passage through which molten metal passes to enter mould cavity. 41  Pouring Cup Pouring Pouring cup cup Riser Liquid Liquid metal metal Sprue Casting Sprue base Sprue Runner Gate Gate Acts as a reservoir from which molten material flows smoothly in the sprue. (b) Tapered sprue It reduces the momentum of the molten metal flowing into the mould by settling first into it. It avoids turbulence 42  SPRUE Metal pouring cup Metal pouring cup Liquid metal Liquid Metal Metalpulling pulling down down Low Sprue Sprue Lowpressure pressure Corners zone zone Corners Gate Gate Gate (a)Straight (a) Straight sprue sprue (b) Tapered sprue sprue (b) Tapered v = velocity (cm/s); Volume flow rate, Q = A.v A = cross-sectional area of the liquid, cm2  Sprue Base Pouring cup Riser Sprue Casting Sprue base Runner Gate This is a reservoir for the metal at the bottom of the sprue to reduce the momentum of the falling molten metal. It prevents mould erosion 44  Runner Pouring cup Riser Sprue Casting Sprue base Runner Gate Runner is used to take the molten metal from the sprue base and distribute it to several gates (passageways) 45  GATES Pouring cup Strainer core Cope Mould Cope cavity Mould Drag Drag cavity Mould Cope cavity Drag (a) Top gate (b) Bottom gate (c) Parting gate Passage through which molten metal flows to fill the mould cavity 46  RISER Allows the air to escape from the mould as the Pouring molten metal is poured cup Riser It creates pressure; hydrostatic head to Sprue Sprue ensure that the cavity is Casting completely filled Ensures that there willSprue Sprue base base not be any shrinkage. Runner Gate Workers can see through the riser that cavity is full 47  Two Types of Risers Blind riser Open riser 48  Requirement of a riser A riser must be the last part to be solidified. Volume of the riser must be sufficient to compensate for metal shrinkage. It must cover the casting section completely that requires feeding. Fluidity of the metal inside the riser must be maintained. 49 How to avoid impurities (i) Pouring basin This reduces the eroding force of the liquid metal stream coming directly from the furnace (ii) Strainer A ceramic strainer in the sprue removes dross. (iii) Splash core A ceramic splash core placed at the end of the sprue reduces the eroding force of the liquid metal stream. Sprue base can also be used. (iv) Skim bob It is a trap placed in a horizontal gate to prevent heavier and lighter impurities from entering the mould. 50  ENGINEERING ANALYSIS OF POURING Bernoulli’s theorem states that the sum of the energies (head, pressure, kinetic) at any two points in a flowing liquid are equal (ignoring friction losses). This can be written in the following form: Bernoulli’s Expression h Where, h = head (cm), p = pressure on the liquid; ρ = density; v = flow velocity; g = gravity constant; Subscripts 1 and 2 indicate two locations in the liquid flow. 51  Engineering analysis of pouring Assuming the system remains at atmospheric pressure throughout: Pouring Metal pouring cup cup Liquid metal 1 Metal pulling down Low pressure h Sprue If point 2 is used as the reference plane, Corners zone Gate then the head at that point is zero (h2 2 Gate = 0) and h1 is the height (length) of the (a) Straight sprue Sprue (b) Tapered sprue sprue. The initial velocity at the top is zero (v1 = 0). 52  Engineering analysis of pouring Flow velocity can be obtained: h=height of sprue V= velocity at the sprue base Continuity during Pouring: It states that the volume rate of flow remains constant throughout the liquid. The continuity law can be expressed: Q= vA Q = v1 A1 = v2A2 where Q = volumetric flow rate, cm3/s; v = velocity; A = cross- sectional area of the liquid, cm2; Thus, an increase in area results in a decrease in velocity, and vice versa. 53  Mould filling time Time required to fill a mould cavity of volume V as Where, TMF= mold filling time, (s); V=volume of mold cavity (cm3); and Q= volume flow rate (cm3/sec) 54  Numerical problem A mold sprue is 20 cm long, and the cross-sectional area at its base is 2.5 cm2. The sprue feeds a horizontal runner leading into a mold cavity whose volume is 1560 cm3. Determine: (a) velocity of the molten metal at the base of the sprue, (b) volume rate of flow, and (c) time to fill the mold cavity. (a) The velocity of the flowing metal at the base of the sprue is given by (b) The volumetric flow rate is (c) Time required to fill a mold cavity of 1560 cm3 at this flow rate is 55  SOLIDIFICATION TIME Chvorinov’s rule for solidification time: 2  volume  t  c    surface area  where, c is a mould constant that depends upon the mould material, metal properties and temperature t (of riser) > t (of casting) 56  Numerical Problem A sand casting process casts two cubes of the same material and the same size. The top face of one of the cubes is completely insulated. What will be the ratio of the solidification time for the cube with the top face insulated to that of the other cube? 57 CASTING DEFECTS Casting Defects are those characteristics which generate imperfections in a casting exceeding the quality limits imposed by the design or service conditions of the casting. 58  BLOW Appears as small round voids opened to the casting surface. Blowholes Possible Causes – Excess moisture in the moulding sand – Permeability of moulding sand too low because of - hard ramming or excess binder or too fine grain size – Poor venting 59  Pinholes Pinholes are caused by release of gases during pouring, consist of many small gas cavities formed at or slightly below the surface of the casting. 60  Sand wash Sand wash is an irregularity in the surface of the casting that results from erosion of the sand mould during pouring, and the contour of the erosion is formed on the surface of the final cast part. 61  Mold shift and Core shift Mold shift refers to a defect caused by a sidewise displacement of the mold cope relative to the drag, the result of which is a step in the cast product at the parting line. Core shift is similar to mold shift, but it is the core that is displaced, and the displacement is usually vertical. Core shift is caused by the buoyancy of the molten metal 62  Penetration defect Penetration refers to a surface defect that occurs when the fluidity of the liquid metal is high and it penetrates into the sand mould or sand core. Harder packing of the sand mould helps to alleviate this condition. 63  Drop Drop: An irregularly-shaped projection on the cope surface of a casting is called a drop. A drop defect in casting occurs when sand pieces fall into molten metal (while the casting material is still liquid), causing an irregular deformation on the casting's surface 64  SHRINKAGE DEFECT Shrinkage is a casting defect that occurs when the molten metal solidifies unevenly or uncontrollably, resulting in cavities or depressions in the casting. It also when there is not enough metal to fill the space as the casting cools and shrinks. 65  HOT TEARS Appears as external cracks or discontinuities on the casting surface. A crack that develops in a casting due to high residual stresses. Possible Causes – too much of shrinkage of molten metal – poor design of casting 66  MISRUNS Mould cavity remaining unfilled (casting is too thin or temperature is less) Possible causes – section thickness which is too thin – Low fluidity of metal/alloys – Faulty gating system – Pouring temperature too low 67  COLD SHUT Imperfect fusion of molten metal in the mould cavity. Occurs when two portions of the metal flow together but there is a lack of fusion between them due to premature freezing Possible causes – Similar to misrun defect 68  INCLUSIONS Impurities present in the casting and occur due to the presence of oxides, sand, nitrides or non-metallic particles in casting Possible causes – Faulty gating system – Faulty pouring methods 69 THANK YOU 70