Casting Terms Casting Terms • Mold consists of two halves: – Cope = upper half of mold – Drag = bottom half • Mold halves are contained in a box, called a flask • The two halves separate at the parting line • Mold cavity is formed by packing sand around a pattern, which has the shape of the part Casting Terms contd. • When the pattern is removed, the remaining cavity has desired shape of cast part • The pattern is usually oversized to allow for shrinkage of metal as it solidifies and cools • Sand for the mold is moist and contains a binder to maintain shape • Through downsprue, metal enters a runner leading to the main cavity Casting Terms contd. • At top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue • Riser is a reservoir in the mold which is a source of liquid metal to compensate for shrinkage during solidification • The riser must be designed to freeze after the main casting in order to satisfy its function Types of Patterns (a) Single Piece pattern (b) split or two piece pattern (c) match-plate pattern (d) cope and drag pattern Single Piece Pattern • Made 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. Split Pattern • The 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used for keeping the alignment between the two parts of the pattern Loose Piece Pattern • Certain patterns cannot be withdrawn once they are embedded in the molding sand.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. Loose piece pattern Match Plate Pattern • It 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. Cope & Drag pattern • Each 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. Sweep Pattern • It 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 Sweep Pattern Follow Board Pattern • A 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. Follow Board Pattern Skeleton Pattern • A skeleton of pattern is made with the help of wooden strip. It is filled by packing sand. Skeleton Pattern Pattern Allowances • A 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. Shrinkage Allowance • Liquid Shrinkage:- reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. Riser is used to accommodate • Solid Shrinkage:- Reduction in volume caused when metal loses temperature in solid state. Shrinkage allowance is provided on Pattern. The metal shrinkage depends upon: 1. 2. 3. 4. 5. 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) Contraction Allowance Machining Allowance Machining Allowance is provided for i. Castings get oxidized in the mold and during heat treatment; scales etc., thus formed need to be removed. ii. It is the intended to remove surface roughness and other imperfections from the castings. iii. It is required to achieve exact casting dimensions. iv. Surface finish is required on the casting. Machining allowance depends on i. Nature of metals. ii. Size and shape of casting. iii. The type of machining operations to be employed for cleaning the casting. iv. Casting conditions. v. Molding process employed Machining allowance for various metals Draft or Taper allowance • It is given to all surfaces perpendicular to parting line • It is given to avoid damage to mould cavity while withdrawing pattern. 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. Taper amount depends on i. Shape and size of pattern in the depth direction in contact with the mould cavity. ii. Moulding methods. iii. Mould materials. iv. Draft allowance is imparted on internal as well as external surfaces; of course it is more on internal surfaces. Draft values for patterns Distortion or cambered allowance: A Casting will distort if It 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. Shake Allowance • A 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. Pattern Material • 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. Desirable Properties of Pattern Material 1. 2. 3. 4. 5. Easily worked, shaped and joined. Light in weight. Strong, hard and durable. Resistant to wear and abrasion. Resistant to corrosion, and to chemical reactions. 6. Dimensionally stable and unaffected by variations in temperature and humidity. 7. Available at low cost. Material for pattern Making Wood Metal Plastic Plaster Wax. Wood These are used where the no. of castings to be produced is small and pattern size is large. Advantages: Inexpensive Easily available in large quantities Easy to fabricate Light in weight They can be repaired easily Easy to obtain good surface finish 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. Metal Patterns These are employed where large no. of castings have to be produced from same patterns. Advantages: Do not absorb moisture More stronger Possess much longer life Do not wrap, retain their shape Greater resistance to abrasion Accurate and smooth surface finish Good machinability Metal Patterns cont. Limitations: Expensive Require a lot of machining for accuracy Not easily repaired Ferrous patterns get rusted Heavy weight , thus difficult to handle Plastic Pattern Advantages: Durable Provides a smooth surface Moisture resistant Does not involve any appreciable change in size or shape Light weight Good strength Wear and corrosion resistance Easy to make Abrasion resistance Good resistance to chemical attack Plastic Cont. Limitations: Plastic patterns are Fragile These may not work well when subject to conditions of severe shock 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 Wax Pattern Wax 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. Pattern Color code The patterns are normally painted with contrasting colors The color code used is, 1. Red or orange on surface not to be finished and left as cast 2. Yellow on surfaces to be machined 3. Black on core prints for unmachined openings 4. Yellow stripes or black on core prints for machined openings Moulding Material • Hot 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 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. Moulding Sand Silica Sand (SiO2) Clays are binding agents to provide strength Moisture Types of sand Facing Sand:-This sand is used next to pattern to obtain cleaner and smoother casting surfaces Mould Wash:-Carbonaceous material applied on the inner cavity after the pattern is withdrawn. This is done to prevent metal penetration and prevent sand fusion. Core & Core Prints Core & core prints Core 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 solidify Chaplets are used to support cores if the core are very big in size. They are made with same metal or alloys being casted. Type of Cores Green 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. 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. Classification of casting Processes Casting processes can be classified into following FOUR categories: 1. Conventional Molding Processes a. Green Sand Molding b. Dry Sand Molding c. Flask less Molding 2. Chemical Sand Molding Processes a. Shell Molding b. Sodium Silicate Molding c. No-Bake Molding 3. a. b. Permanent Mold Processes Gravity Die casting Low and High Pressure Die Casting 4. a. b. c. d. e. Special Casting Processes Lost Wax Ceramics Shell Molding Evaporative Pattern Casting Vacuum Sealed Molding Centrifugal Casting Shell Moulding process Procedure • Sand 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 200-350 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. Cont. Advantages: a. Dimensionally accurate as well as tolerance of 0.03 – 0.13mm can be achieved. b. Smoother surface can be achieved. c. Lower draft angle required. d. Thin sections can also be made. e. Small amount of sand is used. Limitations: a. Patterns are expensive, so only used for large scale production b. Size of casting is limited c. Highly complicated shapes can not be prepared d. Sophisticated equipments are required for handling Precision Investment casting • A wax pattern is prepared by injecting molten wax at pressure of 2.5MPa in metallic die • Pattern 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 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 Advantages a. Since the pattern is withdrawn by melting , so shapes which are difficult to produce by any other method are possible to fabricate. b. Very fine details can be incorporated. c. Dimensional accuracy and good surface finish can be achieved. d. Casting is ready for use as very little or no machining is required. Cont. • Controlled mechanical properties can be achieved. Limitations a. Size is limited. b. Expensive due to larger manual labour involved. Die Casting • Die casting involves injecting molten metal at high pressure into a metallic die • 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. • Cold Chamber die casting:Furnace is not integral with casting Die Casting Casting defects • • • • • Gas defects Shrinkage cavities Molding material defects Pouring metal defects Metallurgical defects Gas Defects These 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 • 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. Shrinkage cavity • These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. Moulding material defects These defects are caused by characteristics of molding material • Cuts & 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. • 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. • 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. • 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. Pouring metal defects 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. 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. CASTING SYSTEM DESIGN Aim 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. 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. Pouring Basin • Main 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. Sprue • Sprue 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 =A2V2 So a straight sprue will result in air inclusion in mould. Hence a tapered sprue is always used. Runner • Located 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. Gates or In-gates Opening through which molten metal enters the mould cavity. • Top gate:molten metal enters the cavity from the top. Advantages: a. Mould cavity is filled very quickly b. Favorable temperature gradient exist Disadvantages:a. As metal directly falls into the mould through height, it is likely to cause mould erosion. b. It may cause turbulence in mould cavity. c. Not suitable for nonferrous alloys. Application:Suitable for ferrous alloys and simple casting shapes Bottom gate Metal enters the mould cavity from bottom. Advantages:a. It would not cause mould erosion b. Very useful for deep moulds. Limitations: a. Cause unfavorable temperature gradient. b. Side risers are to be used with these. Parting gate Most 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:a. Easiest and most economical to prepare. b. Incorporate advantages of both top and bottom gate. Limitations: Not useful if drag part is too deep. Step gate • Molten metal enters the cavity through number of vertical gates. Such gates are used for heavy and large casting.