2 Casting Terms Mold consists of two halves: Cope = upper half of moldDrag = bottom halfMold halves are contained in a box, called a flaskThe two halves separate at the parting lineMold cavity is formed by packing sand around a pattern, which has the shape of the part
3 Casting Terms contd.When the pattern is removed, the remaining cavity has desired shape of cast partThe pattern is usually oversized to allow for shrinkage of metal as it solidifies and coolsSand for the mold is moist and contains a binder to maintain shapeThrough downsprue, metal enters a runner leading to the main cavity
4 Casting Terms contd.At top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprueRiser is a reservoir in the mold which is a source of liquid metal to compensate for shrinkage during solidificationThe riser must be designed to freeze after the main casting in order to satisfy its function
5 Types of Patterns (a) Single Piece pattern (b) split or two piece pattern(c) match‑plate pattern(d) cope and drag pattern
6 Single Piece PatternMade from one piece and does not contain loose pieces or joints.Inexpensive.Used for large size simple castings.Pattern is accommodated either in the cope or in the drag.
7 Split PatternThe upper and the lower parts of the split piece patterns are accommodated in the cope and drag portions of the mold respectively.Parting line of the pattern forms the parting line of the mould.Dowel pins are used for keeping the alignment between the two parts of the pattern
8 Loose Piece PatternCertain patterns cannot be withdrawn once they are embedded in the molding sand. Such patterns are usually made with one or more loose pieces for facilitating from the molding box and are known as loose piece patterns.The main body of the pattern is drawn first followed by loose piece.
10 Match Plate PatternIt consists of a match plate, on either side of which each half of split patterns is fastened.The match plate with the help of locator holes can be clamped with the drag.After the cope and drag have been rammed with the molding sand, the match plate pattern is removed from in between the cope and drag.Match plate patterns are normally used in machine molding.By using this we can eliminate mismatch of cope and drag cavities.
11 Cope & Drag patternEach half of the pattern along with gating system is fixed to a separate metal/wood plate.The two moulds of each half of the pattern are finally assembled with the help of alignment pins and the mould is ready for pouring.Cope and drag patterns are used for producing big castings which as a whole cannot be conveniently handled by one molder alone.
12 Sweep PatternIt is used for generating large shapes which are axi-symmetrical.Making a sweep pattern saves a lot of time, money and labour as compared to making a full pattern
14 Follow Board PatternA follow board is a wooden board and is used for supporting a pattern which is very thin and fragile and which may give way and collapse under pressure when the sand above the pattern is being rammed.
18 Pattern AllowancesA pattern is larger in size as compared to the final casting, because it carries certain allowances due to metallurgical and mechanical reasons for example, shrinkage allowance is the result of metallurgical phenomenon where as machining, draft, distortion, shake and other allowances are provided on the patterns because of mechanical reasons.
19 Shrinkage AllowanceLiquid Shrinkage:- reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. Riser is used to accommodateSolid Shrinkage:- Reduction in volume caused when metal loses temperature in solid state. Shrinkage allowance is provided on Pattern.
20 The metal shrinkage depends upon: The cast metal or alloy.Pouring temp. of the metal/alloy.Casted dimensions(size).Casting design aspects.Molding conditions(i.e., mould materials and molding methods employed)
22 Machining Allowance Machining Allowance is provided for Castings get oxidized in the mold and during heat treatment; scales etc., thus formed need to be removed.It is the intended to remove surface roughness and other imperfections from the castings.It is required to achieve exact casting dimensions.Surface finish is required on the casting.
23 Machining allowance depends on Nature of metals.Size and shape of casting.The type of machining operations to be employed for cleaning the casting.Casting conditions.Molding process employed
25 Draft or Taper allowance It is given to all surfaces perpendicular to parting lineIt is given to avoid damage to mould cavity while withdrawing pattern.
26 An illustration of a pattern shown below having proper draft allowance An illustration of a pattern shown below having proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity.
27 Taper amount depends on Shape and size of pattern in the depth direction in contact with the mould cavity.Moulding methods.Mould materials.Draft allowance is imparted on internal as well as external surfaces; of course it is more on internal surfaces.
29 Distortion or cambered allowance: A Casting will distort ifIt is of irregular shape,All it parts do not shrink uniformly i.e., some parts shrinks while others are restricted from during so,It is u or v-shape,The arms possess unequal thickness,It has long, rangy arms as those of propeller strut for the ship,It is a long flat casting,One portion of the casting cools at a faster rate as compared to the other.
31 Shake AllowanceA Pattern is shaked or wrapped to take it out of mould. This in turn enlarges the mould cavity. Hence a –ve allowance is given to pattern.If the draft angle is provided, shake allowance reduces.
32 Pattern Material No. of castings to be produced. Metal to be cast. Factors affecting selection of material:-No. of castings to be produced.Metal to be cast.Dimensional accuracy & surface finish.Shape, complexity and size of casting.Casting design parameters.Type of molding materials.The chance of repeat orders.Nature of molding process.Position of core print.
33 Desirable Properties of Pattern Material Easily worked, shaped and joined.Light in weight.Strong, hard and durable.Resistant to wear and abrasion.Resistant to corrosion, and to chemical reactions.Dimensionally stable and unaffected by variations in temperature and humidity.Available at low cost.
34 Material for pattern Making WoodMetalPlasticPlasterWax.
35 WoodThese are used where the no. of castings to be produced is small and pattern size is large.Advantages:InexpensiveEasily available in large quantitiesEasy to fabricateLight in weightThey can be repaired easilyEasy to obtain good surface finish
36 Wood cont. Limitations: Susceptible to shrinkage and swelling. Possess poor wear resistance.Abraded easily by sand action.Absorb moisture.Cannot withstand rough handling.Life is very short.
37 Metal Patterns Advantages: These are employed where large no. of castings have to be produced from same patterns.Advantages:Do not absorb moistureMore strongerPossess much longer lifeDo not wrap, retain their shapeGreater resistance to abrasionAccurate and smooth surface finishGood machinability
38 Metal Patterns cont. Limitations: Expensive Require a lot of machining for accuracyNot easily repairedFerrous patterns get rustedHeavy weight , thus difficult to handle
39 Plastic Pattern Advantages: Durable Provides a smooth surface Moisture resistantDoes not involve any appreciable change in size or shapeLight weightGood strengthWear and corrosion resistanceEasy to makeAbrasion resistanceGood resistance to chemical attack
40 Plastic Cont. Limitations: Plastic patterns are Fragile These may not work well when subject to conditions of severe shock
41 Plaster Pattern Advantages: It can be easily worked by using wood working tools.Intricate shapes can be cast without any difficulty.It has high compressive strength
42 Wax PatternWax patterns find applications in Investment casting process.Advantages:Provide very good surface finish.Impart high accuracy to castings.After being molded, the wax pattern is not taken out of the mould like other patterns;rather the mould is inverted and heated; the molten wax comes out and/or is evaporated.Thus there is no chance of the mould cavity getting damaged while removing the pattern.
43 Pattern Color codeThe patterns are normally painted with contrasting colorsThe color code used is,Red or orange on surface not to be finished and left as castYellow on surfaces to be machinedBlack on core prints for unmachined openingsYellow stripes or black on core prints for machined openings
44 Moulding MaterialHot strength:- when sand reaches to high temp. its strength to retain shape of cavity.Permeability:-ability of sand to allow gases to escape from the mould
45 Properties of Moulding Material Refractoriness:-Ability to withstand high temperature of the molten metal and avoid fusion.Green Strength:-Sand with moisture is green sand and its property to retain shape of mould is green strength.Dry strength:- Sand without moisture is dry sand. Its ability to retain shape of cavity and withstand metallostatic force is dry strength.
46 Moulding Sand Silica Sand (SiO2) Clays are binding agents to provide strengthMoisture
47 Types of sandFacing Sand:-This sand is used next to pattern to obtain cleaner and smoother casting surfacesMould Wash:-Carbonaceous material applied on the inner cavity after the pattern is withdrawn. This is done to prevent metal penetration and prevent sand fusion.
49 Core & core printsCore is a Body made of refractory material which is set into prepared mould before closing and pouring it.Core should be able to collapse after the metal is solidifyChaplets are used to support cores if the core are very big in size. They are made with same metal or alloys being casted.
50 Type of CoresGreen Sand core:-it is formed by the pattern itself using moulding sand.Dry Sand Core:- They are made separately and positioned in the mould after the pattern is taken out.
51 Backing sand:-Foundry sand which is used for ramming after facing sand is applied to pattern. Parting sand:-Sand which is sprinkled on the parting surfaces of mould before they are prepared.
52 Classification of casting Processes Casting processes can be classified into following FOUR categories:1. Conventional Molding Processesa. Green Sand Moldingb. Dry Sand Moldingc. Flask less Molding2. Chemical Sand Molding Processesa. Shell Moldingb. Sodium Silicate Moldingc. No-Bake Molding
53 3. Permanent Mold Processes a. Gravity Die castingb. Low and High Pressure Die Casting4. Special Casting Processesa. Lost Waxb. Ceramics Shell Moldingc. Evaporative Pattern Castingd. Vacuum Sealed Moldinge. Centrifugal Casting
55 ProcedureSand and additives are mixed in a muller for 1min and then resin is added and heated for another 3 min.Only metal patterns can be used as it is subjected to C.Coated sand is allowed to be in contact with metallic pattern and is heated.Pattern is removed and clamped together and ready for pouring.
56 Cont.Advantages:Dimensionally accurate as well as tolerance of 0.03 – 0.13mm can be achieved.Smoother surface can be achieved.Lower draft angle required.Thin sections can also be made.Small amount of sand is used.
57 Limitations:Patterns are expensive, so only used for large scale productionSize of casting is limitedHighly complicated shapes can not be preparedSophisticated equipments are required for handling
58 Precision Investment casting A wax pattern is prepared by injecting molten wax at pressure of 2.5MPa in metallic diePattern is ejected and cluster of wax patterns are attached to gating system by application of heat.Prepared pattern is dipped into a slurry made by suspending fine ceramic materials
60 Cont.When sufficient thickness is attained, the mould is reverted and is heated to take wax out of it (Removal of pattern).Now, the pattern is preheated and molten metal is poured in it
61 AdvantagesSince the pattern is withdrawn by melting , so shapes which are difficult to produce by any other method are possible to fabricate.Very fine details can be incorporated.Dimensional accuracy and good surface finish can be achieved.Casting is ready for use as very little or no machining is required.
62 Cont. Controlled mechanical properties can be achieved. Limitations Size is limited.Expensive due to larger manual labour involved.
63 Die CastingDie casting involves injecting molten metal at high pressure into a metallic die
64 It involves one stationary half and a moving half. Moving half is used to withdraw casting.Lubricant is sprayed on inner surface of metallic die in order to avoid fusion.Casting is solidify under pressure.Hot chamber die casting: Furnace is integral with die.
65 Cold Chamber die casting:- Furnace is not integral with casting
67 Casting defects Gas defects Shrinkage cavities Molding material defectsPouring metal defectsMetallurgical defects
68 Gas DefectsThese defects are mainly caused by lower gas passing tendency of the mould.Blow holes and open blows:-Spherical, flattened or elongated cavities caused by moisture left in the core.On the surface they are called open blows and inside they are called blow holes
69 Air Inclusion:-the atmospheric and other gases absorbed by molten metal in furnace, laddle or during flow in mould, if not allowed to escape will weaken it.Pin hole Porosity:-when the molten metal solidify, its solubility with gases decreases. If hydrogen is present, it will expell from molten metal and will cause a long pin hole shape cavity. High pouring temperature is main reason for that defect.
70 Shrinkage cavityThese are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required.
71 Moulding material defects These defects are caused by characteristics of molding materialCuts & Washes:-Appears as rough spots and area of excess metal due to erosion of moulding sand by the flowing molten metal.Caused by poor strength of moulding sand or molten metal flowing at high velocity.
72 Metal penetration:-When the molten metal enters the gap between sand grains resulting in rough casting surface. This could be due to coarse grain size or no mould wash is applied to mould cavity.Fusion:-fusion of sand grains with molten metal resulting in brittle, glassy appearance. Reason may be lower refractoriness of clay in moulding sand or higher pouring temperature.
73 Runout:-A runout is caused when the molten metal leaks out of the mould. Caused by faulty mould making or faulty flask.Rat tails and buckles:-caused by compression failure of the skin of mould cavity. Due to excessive heat, sand expands resulting in a small line on casting surface. Buckles are rat tails that are severe.Reasons may be poor expansion properties of sand or high pouring temperature.
74 Swell:-Under the metallostatic force mould wall my move back causing a swell in the dimension. This may result in increased feeding requirement.Drop:-The dropping of loose moulding sand or lumps from the cope surface into the mould cavity. Caused by improper ramming of the cope flask.
76 Pouring metal defect Misrun & Coldshut:- Misrun is caused when the metal is unable to fill the mould cavity completely.Coldshut is caused when two metal streams while meeting in the mould cavity do not fuse together properly.These are caused due to lower fluidity of molten metal or section thickness is too small.
77 Metallurgical defects Hot tears:-Since metal has low strength at higher temperature, any unwanted cooling stress may cause rupture of the casting.Hot spots:-These are caused by chilling of the casting.
78 CASTING SYSTEM DESIGNAim of an effective gating system are as follows:-Mould should be completely filled in smallest possible time.Metal should flow smoothly in cavity without turbulence.Unwanted material should not be allow to enter mould cavity.Entry of metal should be properly controlled so that atmospheric air can be released.
79 Cont……..A proper thermal gradient should exist in casting so that it can be cooled without any shrinkage cavity or distortion.Metal flow should not cause gating or mould erosion.Adequate supply of molten metal is ensured.Gating design should be economical, simple and easy to remove after solidification.
80 Pouring BasinMain function of PB is to reduce the momentum of flowing liquid.Molten metal if poured directly in cavity can cause erosion. Molten metal is poured in pouring basin and act as a reservoir.It also separates dirt and slag which float on the surface, with the help of a skim core.
81 SprueSprue is the channel through which molten metal is brought into parting line.While moving downward, metal velocity increases, hence requires smaller cross section area.A1V1 =A2V2So a straight sprue will result in air inclusion in mould. Hence a tapered sprue is always used.
82 RunnerLocated in horizontal plane, runner connects the sprue to ingates.For ferrous metal, runner is in the cope and the in-gate is in drag in order to trap slag and dross.Runner should always be full in order to trap slag.
83 Gates or In-gatesOpening through which molten metal enters the mould cavity.Top gate:-molten metal enters the cavity from the top.Advantages:Mould cavity is filled very quicklyFavorable temperature gradient exist
84 Disadvantages:-As metal directly falls into the mould through height, it is likely to cause mould erosion.It may cause turbulence in mould cavity.Not suitable for nonferrous alloys.Application:-Suitable for ferrous alloys and simple casting shapes
85 Bottom gate Metal enters the mould cavity from bottom. Advantages:- It would not cause mould erosionVery useful for deep moulds.Limitations:Cause unfavorable temperature gradient.Side risers are to be used with these.
86 Parting gateMost widely used gates, metal enters the cavity at the parting plane.For cope it is bottom gate and for drag it is top gate.Advantages:-Easiest and most economical to prepare.Incorporate advantages of both top and bottom gate.
87 Limitations:Not useful if drag part is too deep.
88 Step gateMolten metal enters the cavity through number of vertical gates. Such gates are used for heavy and large casting.