MODULE 2 COURSE OUTCOMES Fundamentals of Casting process Solidification of metals Fluid flow of molten metal Various casting process Casting defects & quality
: Casting / FoundryCasting processes basically involve the introduction of a moltenmetal into a mold cavity, where upon solidification, the metal.takes on the shape of the mold cavity :ApplicationsCylinder blocks, liners, machine tool beds, pistons, piston rings, mill rolls, wheels, housings, water supply pipes, bells
Examples of Cast PartsCrank handle formed by casting; some areaswere machined and assembled after casting
Examples of Cast PartsC-clamps formed by casting (left) and machining)(right
Examples of Cast Parts Complex part formed by casting
Aluminum piston for an internal: combustion engine.a) as-cast (b) after machining (
Flask : A molding flask is one which holds the sand mould intactDrag : Lower molding flaskCope : Upper molding flaskPattern : It is a replica of final object to be made with somemodifications. The mold cavity is made with the help of pattern materialMolding sand : It is the freshly prepared refractory material used for.making the mold cavity.Core : It is used for making hollow cavities in the castingsCore Print : A region used to support the corePouring basin : A small funnel shaped cavity at the top of the mold.into molten metal is poured
Parting Line / Parting Surface : Interface that separates thecope and dragRunner: The channel through which the molten metal is carried.from the sprue to the gateGate: A channel through which the molten metal enters the moldcavityChaplets: Chaplets are used to support the cores inside the mold.cavity. Riser: It is a reservoir of the molten metal provided in the castingVent: Small opening in the mold to facilitate escape of air and.gasesSprue : The passage through which the molten material from the.pouring basin reaches the mold cavity
Pattern MaterialWood, metals & alloys, plastic, plaster of Paris, plasticand rubbers, wax, and resins.Material selection depends on size and shape of casting.To be suitable for use, the pattern material should be:1. Easily worked, shaped and joined2. Light in weight3. Strong, hard and durable4. Resistant to wear and abrasion5. Resistant to corrosion, and to chemical reactions6. Dimensionally stable and unaffected by variations in temperature and humidity7. Available at low cost
Pattern AllowancesPattern allowance is a vital feature as it affects thedimensional characteristics of the castingThe selection of correct allowances greatly helps toreduce machining costs and avoid rejections.1. Shrinkage or contraction allowance2. Draft or taper allowance3. Machining or finish allowance4. Distortion or camber allowance5. Rapping allowance or shake allowances
Shrinkage or Contraction AllowanceAll most all cast metals shrink or contract volumetrically on cooling.The metal shrinkage is of two types: (1) Liquid Shrinkage: It refers to the reduction in volume when the metal changes from liquid state to solid state at the solidustemperature.To account for this shrinkage; riser, which feed the liquid metal to thecasting, are provided in the mold. (2) Solid Shrinkage: It refers to the reduction in volume during thecooling of the cast metal to room temperature.To account for this, shrinkage allowance is provided on the patterns.The rate of contraction with temperature is dependent on thematerial.For example steel contracts to a higher degree compared toaluminum.
Shrinkage Metal Percent Contraction (-) Expansion(+) Aluminum -7.1% Zinc -6.5% Gold -5.5% Copper -4.9% Brass -4.5%Carbon Steel -2.5-4% Lead -3.2%Gray Cast Iron +2.5%
Draft or Taper Allowance Taper is provided on all vertical surfaces of the pattern so that itcan be removed from the sand without tearing away the sides of thesand mold. Draft allowance varies with the complexity of the sand job. Inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side ofthe pattern to be extracted; the intricacy of the pattern; the method ofmolding; and pattern material.
Pattern Height of the Draft angle Draft angle material given surface (External (Internal (inch) surface) surface) 1 3.00 3.00 1 to 2 1.50 2.50Wood 2 to 4 1.00 1.50 4 to 8 0.75 1.00 8 to 32 0.50 1.00 1 1.50 3.00 1 to 2 1.00 2.00Metal and 2 to 4 0.75 1.00plastic 4 to 8 0.50 1.00 8 to 32 0.50 0.75 Draft Allowances of Various Metals
Taper on patterns for ease of removal from the sand mold
Machining or Finish Allowance Machining or finish allowances are added in the pattern dimension to have good surface finish or dimensionally accurate The amount of machining allowance to be provided is affected bythe method of molding and casting used viz. hand molding or machinemolding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size andshape of the casting; the casting orientation; the metal; and the degreeof accuracy and finish required.
Machining Allowances of Various Metals Metal Dimension (inch) Allowance (inch) Up to 12 0.12Cast iron 12 to 20 0.20 20 to 40 0.25 Up to 6 0.12Cast steel 6 to 20 0.25 20 to 40 0.30 Up to 8 0.09Non ferrous 8 to 12 0.12 12 to 40 0.16
Distortion or Camber Allowance.Castings get distorted, during solidification, due to their typical shapeFor example, if the casting has the form of the letter U, V, T, or L etc. itwill tend to contract at the closed end causing the vertical legs to look. slightly inclinedThis can be prevented by making the legs of the U, V, T, or L shapedpattern converge slightly (inward) so that the casting after distortionwill have its sides vertical. The distortion in casting may occur due to internal stressesThese internal stresses are caused on account of unequal cooling of. different section of the casting and hindered contraction:To prevent the distortion in castings includei. Modification of casting designii. Providing sufficient machining allowance to cover the distortionaffectiii. Providing suitable allowance on the pattern, called camber or )distortion allowance (inverse reflection
Rapping Allowance Before the withdrawal from the sand mold, the pattern is rapped allaround the vertical faces to enlarge the mold cavity slightly, whichfacilitate its removal. Since it enlarges the final casting made, it is desirable that theoriginal pattern dimension should be reduced to account for thisincrease. There is no sure way of quantifying this allowance, since it is highlydependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to thosedimensions that are parallel to the parting plane.
Fluid flow• 2 principles of fluid flow are relevant to gating design: Bernoulli’s theorem and the law of mass continuity.
Fluid flowBernoulli’s theorem• Based on - principle of conservation of energy - frictional losses in a fluid system 2 h = elevation p v p = pressure at elevation h+ + = Constant ρg 2 g v = velocity of the liquid ρ = density of the fluid• Conservation of energy requires that, p1 v12 2 p 2 v2 h+ + = h2 + + +f ρg 2 g ρg 2 g
Fluid flow Mass continuity • States that for an incompressible liquid the rate of flow is constant.Q = A1v1 = A2 v2 Q = volumetric rate of flow A = cross-sectional area of the liquid stream v = velocity of the liquid • Subscripts 1 and 2 pertain to two different locations in the system.
Fluid flowSprue profile• Relationship between height and cross- sectional area at any point in the sprue is given by A1 = h2 A2 h1• Velocity of the molten metal leaving the gate is v = c 2 gh• When liquid level reached height x, gate velocity is v = c 2g h − x
Fluid flowFlow characteristics• Reynolds number, Re, is used to characterize aspect of fluid flow.• It represents the ratio of the inertia to the viscous forces in fluid flow and is defined as vDρ v = velocity of the liquid Re = D = diameter of the channel η ρ = density n = viscosity of the liquid.
Flow Characteristics 0 < Re < 2000 => laminar flow 2000 < Re < 20 000 =>mixture of laminar and turbulent flow , generally regarded as harmless in gating systems. Re > 20 000 => severe turbulence In gating systems, Re typically ranges from 2000 to 20,000 Techniques for minimizing turbulence • Dross or slag can be eliminated by vacuum casting • Use of filters eliminates turbulent flow in the runner system
Turbulence can be reduced by the design of a gating system that promotes a more laminar flow of the liquid metal. Sharp corners and abrupt changes in sections within thecasting can be a leading cause of turbulence. Their affectcan be mitigated by the employment of radii.
Fluidity of molten metalFluidity of Molten Metal : The capability of molten metal to fill mold cavities is called fluidity.The following influence fluidity Characteristics of molten metal – Viscosity (How runny is it when hot) – Surface tension (Development of film ) – Inclusions – Solidification pattern of the alloy Casting parameters – Mold design (Risers, runners, gates, etc.) – Mold material and its surface characteristics – Degree of superheat – Rate of pouring – Heat transfer
Heat TransferImportant consideration in casting – Heat flow in the system • Complex • Depends of flow characteristicsSolidification Time – A function of the volume of a casting and its surface area • Solidification time = C volume 2 surface area – Effects on solidification time • Mold Geometry • Skin thickness
Heat transfer• Temperature distribution in the mold- liquid metal interface is shown below.
Solidification of Metals Involves liquid metal turning back in to solid metal The process is different for Pure metals and alloys Can be divided into two steps: Formation of stable nuclei Growth of crystals Pure Metals• Have a clearly defined melting point• Temperature remains constant during freezing • Solidifies from the walls of the mold toward the center of the part
Grain Structure for Pure Metals• Two types of grains are formed for a pure metal – Fine equiaxed grains – Columnar• Rapid cooling at the walls produces fine equiaxed grains• Columnar grains grow opposite of the heat transfer throughout the mold following the chill zoneEquiaxed Grains• If crystals can grow approximately equally in all directions – equiaxed grains will grow.• Large amounts of under cooling is needed near the wall of the mold.
Alloys• Solidification in alloys begins when the temperature drops below the liquidus TL and is complete when it reaches the solidus, TS.
Alloys• Within the TL and TS Temperature range, the alloy is like a slushy with columnar dendrites
Effects of Cooling Rates• Slow cool rates results in course grain structures (102 K/s)• Faster cooling rates produce finer grain structures (104 K/s)• For even faster cooling rates, the structures are amorphous (106 – 108 K/s)• Grain size influences strength of a material• Smaller grains have higher ductility and strength• Smaller grains help prevent hot tearing and/or cracks in the casting
Shell molding– a mounted pattern, made of a ferrous metal or aluminum, is heated to 175-370 0 C, coated with a parting agent such as silicone, and clamped to a box or chamber containing a fine sand coated with a 2.5 - 4.0% thermosetting resin binder– the sand mixture is blown over the heated pattern, coating it evenly– the assembly is placed in an oven to complete the curing of the resin– the shell is formed by removing the pattern– two half shells are made and are clamped together in preparation for pouring
Shell moldingAdvantages• Better surface finish• Better dimensional tolerances.• Reduced Machining.• Less foundry space required.• Semi skilled operators can handle the process.• The process can be mechanized.
Shell molding Disadvantages The raw materials are relatively expensive.The process generates noxious fumes which must be removed.The size and weight range of castings is limited. (Size limits of 30 g to 12 kg )
Shell MoldingApplications• -Crankshaft fabrication• -Steel casting parts, fittings• -Molded tubing fabrication• -Hydraulic control housing fabrication• -Automotive castings (cylinder head and ribbed cylinder fabrication).
Expendable Mold Uses a polystyrene foam pattern which evaporates with molten metal to form a cavity for the casting. Polystyrene foam pattern includes sprue, risers, gating system and internal cores (if needed) Polystyrene inexpensive and easily processed into patterns
Advantages of expanded polystyrene process:1.Pattern need not be removed from the mold2.Simplifies and speeds mold-making, because twomold halves are not required as in a conventionalgreen-sand moldDisadvantages:1.A new pattern is needed for every casting2.Economic justification of the process is highlydependent on cost of producing patterns
Evaporative Pattern Casting of an Engine Block (a ) (b )a) Metal is poured into mold for lost-foam casting of a 60-hp. ( .3-cylinder marine engine; (b) finished engine block
Investment Casting – Characteristics • Advantages: Complex shapes possible Thin wall sections possible High production rates High dimensional accuracy • Disadvantages: Limited weight range Expensive Machinery & Dies Expensive Unit Cost, Labor Intensive Mold is not reusable
. Typical parts produced by investment castingProducts such as rocket components, and jetengine turbine blades
Die casting The molten metal is injected into die cavity under high pressure Pressure maintained during solidification Die casting typically makes use of non-ferrous alloys. The four most common alloys that are die cast areAluminum alloys, Copper alloys, Magnesium alloys,Zinc alloys
Advantages of die casting Excellent dimensional accuracy Smooth cast surfaces Thinner walls can be cast Inserts can be cast-in (such as threaded inserts,heating elements, and high strength bearingsurfaces). Reduces or eliminates secondary m/c ing operations. Rapid production rates.
Disadvantages of die casting The main disadvantage - very high capital cost. To make die casting an economic process a largeproduction volume is needed. Die casting is limited to high fluidity metals (Zinc,Aluminum, Magnesium, Copper, Lead and Tin) (Notapplicable for high melting point metals and alloys(eg. steels) Casting weights must be between 30 grams and 10 kg Limited die life
Centrifugal CastingIn this process, the mold is rotated rapidly about itscentral axis as the metal is poured into it.Because of the centrifugal force, a continuous pressurewill be acting on the metal as it solidifies.The slag, oxides and other inclusions being lighter, getseparated from the metal and segregate towards thecenter.This process is normally used for the making of hollowpipes, tubes, hollow bushes, etc., which are ax- symmetricwith a concentric hole.
Centrifugal CastingSince the metal is always pushed outward because of thecentrifugal force, no core needs to be used for making theconcentric hole.The mold can be rotated about a vertical, horizontal or aninclined axis or about its horizontal and vertical axessimultaneously.The length and outside diameter are fixed by the moldcavity dimensions while the inside diameter isdetermined by the amount of molten metal poured.