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CASTING

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PALANIVENDHAN …

PALANIVENDHAN
MANUFACTURING TECHNOLOGY
CASTING
SPECIAL CASTING PROCESS
MOULDING MATERIAL

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  • 1. M.PALANIVENDHAN Department of Automobile Engineering SRM University, kattankulathur campus
  • 2.  The solidified piece of metal, which is taken out of the mould, is called as casting. A plant where the casting are made is called a foundry.
  • 3.  A wooden or metal shape or model of real component.  Pattern is replica (an exact copy) of the object to be made by casting process.
  • 4.  Single piece pattern  Split pattern  Match pattern  Gated pattern  Cope and drag pattern  Loose piece pattern  Sweep pattern  Follow pattern  Skelton pattern
  • 5.  These are inexpensive and the simplest type of patterns.  They are made single piece.  Very small scale production or in prototype development
  • 6. Single piece pattern
  • 7.  When the contour of the casting makes its withdrawal from the mould difficult, or depth of casting is too high, then the pattern is split into two parts so one part is in drag and other in the cope.  The two halves of the pattern should be aligned properly by making use of dowel pins which are fitted to the cope half. These dowel pins match with the precisely made holes in the drag half of the pattern and thus align the two halves properly.
  • 8.  This is an improvement over the simple pattern where the gating and runner system are integral with the pattern.  To eliminated the hand cutting of the runners and gates.
  • 9.  To use this type of pattern in machine moulding.  Here the cope and drag patterns along with the gating and the riser are mounted on single matching metal.
  • 10.  Similar to split patterns  In addition to splitting the pattern, the cope and drag halves of the pattern along with the gating and riser are attached separately to metal or wooden plates along with the alignment pins.
  • 11.  Some castings have certain portions which are structurally weak. If those portion of the pattern is not supported properly they are likely to break under the force of ramming. In this case a special type of pattern called follow board pattern is adopted. A follow board is a wooden board used to support a pattern during moulding. It acts as a seat for the pattern
  • 12.  Sweep pattern can be advantageously used for preparing moulds of large symmetrical castings, particularly of circular cross section. The equipment consists of abase, suitably placed in the sand mass, a vertical spindle and a wooden template called sweep. The outer end of the sweep carries the contour corresponding to the shape of the desired casting. The sweep is rotated about the spindle to form the cavity as shown in fig. Then the sweep and spindle are removed leaving the base in the sand. The hole made by the removal of spindle is patched up by filling the sand.
  • 13.  When the size of the casting is very large, but easy to shape and only a few numbers are to be made, it is not economical to make a large solid pattern of that size. In such cases a pattern consisting of wooden frame and strips is made called skeleton pattern. It is filled with moulding sand and rammed properly.
  • 14.  Certain single piece patterns are made to have loose pieces in order to enable their easy withdrawal from the mould. These pieces from an integral part of the pattern during moulding. After the mould is complete the pattern is withdrawn leaving the pieces in the sand. These pieces are later withdrawn separately through the cavity formed by the pattern.
  • 15.  Number of quantity required  Types of mould  Size of the component to made  Types of sand used  Require accuracy & surface finish
  • 16.  Wood  Metal  Plastic  Plaster  Wax
  • 17.  These are used where the no. of castings to be produced is small and pattern size is large.
  • 18.  Inexpensive  Easily available in large quantities  Easy to fabricate  Light in weight  They can be repaired easily  Easy to obtain good surface finish
  • 19.  Susceptible to shrinkage and swelling  Possess poor wear resistance  Absorb moisture, consequently get wrapped  Cannot withstand rough handling  Life is very short
  • 20.  These are employed where large no. of castings have to be produced from same patterns.
  • 21.  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 machine able cast
  • 22.  Expensive  Require a lot of machining for accuracy  Not easily repaired  Ferrous patterns get rusted  Heavy weight , thus difficult to handle
  • 23. Advantages:  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
  • 24.  It may not work well when subject to conditions of severe shock as in machine moulding (jolting).
  • 25. Plaster may be made out of Plaster of Paris or Gypsum cement. Advantages:  It can be easily worked by using wood working tools.  Intricate shapes can be cast without any difficulty.  It has high compressive strength
  • 26. 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.
  • 27.  Shrinkage allowance  Draft allowance  Machining allowance  Shake allowance
  • 28.  Since most metals shrink in volume, when solidifying from liquid state and again on cooling, it is obvious, that the pattern should be made slightly larger than the size of finished casting. This difference in size of the pattern is called shrinkage allowance.  The metal shrinkage is of two types: ◦ Liquid Shrinkage ◦ Solid shrinkage
  • 29.  On many occasions, castings produced in the foundry shop are machined subsequently. The object of machining is to get exact sizes and better surface finish on the component. If such is the case, a layer of 1.5–2.5 mm thick material has to be provided all round the casting. This is done by making the pattern suitably bigger than the casting. This increase in size of pattern is called “machining allowance”.
  • 30.  It facilitates withdrawal of pattern from the mould. It is provided on vertical surfaces. The idea is to give an inclination of 2–3 degrees to vertical surfaces, so that while lifting the pattern, the upper surface is wider and withdrawing the pattern with draft provided will not damage the sand mould. On inner vertical surfaces, draft is provided in such a way that top surface is narrower and bottom portion of pattern is wider.
  • 31.  A patter is shaken or rapped by striking the same with a wooden piece from side to side. This is done so that the pattern a little is loosened in the mold cavity and can be easily removed.  In turn, therefore, rapping enlarges the mould cavity which results in a bigger sized casting.  Hence, a –ve allowance is provided on the pattern i.e., the pattern dimensions are kept smaller in order to compensate the enlargement of mould cavity due to rapping.  The magnitude of shake allowance can be reduced by increasing the tapper
  • 32. Special Casting Process  Expandable Mold Casting Processes ◦ Shell mold Casting ◦ Investment casting ◦ Vacuum mold Casting ◦ Expanded polystyrene mold ◦ Plaster mold and ceramic mold  Permanent Mold Casting Processes ◦ Die casting ◦ Centrifugal casting ◦ Basic permanent mold ◦ Variations of permanent mold
  • 33. Special Casting Process  Expandable Mold Casting ◦ In expandable mold casting, the mold is destroyed to remove the casting and a new mold is required for each new casting.
  • 34. Expandable Mold Casting Processes  The pattern used in this process is made from polystyrene (this is the light, white packaging material which is used to pack electronics inside the boxes). Polystyrene foam is 95% air bubbles, and the material itself evaporates when the liquid metal is poured on it.  The pattern itself is made by molding – the polystyrene beads and pentane are put inside an aluminum mold, and heated; it expands to fill the mold, and takes the shape of the cavity.
  • 35.  The pattern is removed, and used for the casting process, as follows: ◦ The pattern is dipped in a slurry of water and clay (or other refractory grains); it is dried to get a hard shell around the pattern. ◦ The shell-covered pattern is placed in a container with sand for support, and liquid metal is poured from a hole on top. ◦ The foam evaporates as the metal fills the shell; upon cooling and solidification, the part is removed by breaking the shell.
  • 36. Expandable Mold Casting Processes Expandable mold casting
  • 37. Expandable Mold Casting Characteristics  The process is useful since it is very cheap, and yields good surface finish and complex geometry.  There are no runners, risers, gating or parting lines – thus the design process is simplified.  The process is used to manufacture crank-shafts for engines, aluminum engine blocks, manifolds etc.
  • 38. Shell Mold Casting  Shell moulding is a process for producing simple or complex near net shape castings, maintaining tight tolerances and a high degree of dimensional stability.  The process was developed and patented by Croning in Germany during World War II and is sometimes referred to as the Croning shell process.
  • 39. Shell Mold Casting Process  Initially preparing a metal-matched plate  Mixing resin and sand  Heating pattern, usually between 505-550 K  Inverting the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill)  Curing the shell and baking it  Removing investment
  • 40.  Inserting cores  Repeating for the other half  Assembling the mould  Pouring the metal  Removing casting  Cleaning and trimming
  • 41. Process Detail  Example 1
  • 42.  Example 2 The shell-molding process, also called dump-box technique.
  • 43.  Process Steps in Detail
  • 44.  Process Steps in Detail
  • 45. 1. Heated metal pattern ready for shell formation from resin-bonded sand. 2. Box inversion for shell formation. 3. Partial cured shell layer hardened; box re-inverted to allow the loose sand particles to be separated. 4. Sand shell is heated to complete the curing. 5. Sand shell removed from the pattern 6. Two halves of the sand shell are assembled and ready for pouring. 7. Finished shell casting with sprue removed.
  • 46. Shell Mold Casting Process  Advantages ◦ Good surface finish (up to 2.5 mm) ◦ Good dimensional accuracy (±0.25 mm) ◦ Suitable for mass production  Disadvantages ◦ Expensive metal pattern
  • 47.  Area of application ◦ Mass production of steel casting of less than 10 kg ◦ Crankshaft fabrication ◦ Steel casting parts, fittings ◦ Molded tubing fabrication ◦ Hydraulic control housing fabrication ◦ Automotive castings (cylinder head and ribbed cylinder fabrication).
  • 48. Process Characteristics  Is superior to other sand casting processes in the accurate duplication of intricate shapes and dimensional accuracy  Process can be completely mechanized  Uses a thin-walled non-reusable shell composed of a sand-resin mixture  Requires a heated metal pattern for producing the shell molds  The process was optimized to get a better shell by varying the temperature of the metal pattern, holding time of sand – resin mixture and final curing time of shell and pattern.
  • 49. Investment casting  Also Named as lost wax casting, because of the use of wax patterns which are coated with a refractory (i.e. the patterns are invested in alternate layers of slurry and stucco), with the wax patterns subsequently melted out to leave a hollow shell into which the metal is cast.  It is an extremely slow process and the production rate is governed by the time to make the mould.
  • 50. Investment casting Process  This process uses wax patterns assembled in tree forms on a runner. The completed assembly is coated with a ceramic slurry, allowed to dry and then heated to melt out the wax leaving a ceramic mould into which the molten alloy is poured.  Parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved.
  • 51. Processing Steps  Example 1
  • 52. Process Steps in Detail 1. Wax Pattern made. 2. Patterns attached to wax sprue. 3. Pattern tree coated with thin layer of refractory material. 4. Pattern tree coated with sufficient refractory material. 5. Invert tree and melt wax by heat. 6. Preheat mold to high temperature to induce flow and remove contaminants. 7. Mold broken and parts remove.
  • 53.  Example - 2
  • 54.  Process Steps in Detail ◦ Produce a master pattern ◦ Produce a master die ◦ Produce wax patterns ◦ Assemble the wax patterns onto a common wax sprue ◦ Coat the tree with a thin layer of investment material ◦ Form additional investment around the coated cluster ◦ Allow the investment to harden
  • 55.  Remove the wax pattern from the mold by melting or dissolving  Heat the mold  Pour the molten metal  Remove the solidified casting from the mold
  • 56.  Example - 3
  • 57.  Example - 4
  • 58.  Advantages ◦ Excellent accuracy and flexibility of design. ◦ Useful for casting alloys that are difficult to machine. ◦ Exceptionally fine finish. ◦ Suitable for large or small quantities of parts. ◦ Suitable for most ferrous / non-ferrous metals. ◦ Complex shapes can be cast ◦ Thin sections can be cast ◦ Machining can be eliminated or reduced  Disadvantages ◦ Limitations on size of casting. ◦ Higher casting costs make it important to take full advantage of the process to eliminate all machining operations.
  • 59.  Area of application ◦ One of the oldest casting methods ◦ complex parts such as art pieces, jewelry, dental fixtures from all types of metals. ◦ Products such as rocket components, and jet engine turbine blades
  • 60. Permanent Mold Casting  In Permanent Molding Process, instead of using sand as the mold material, a metal is used as a mold (Die).  In contrary to sand casting, in permanent mold casting the mold is used to produce not a single but many castings. In all the above processes, a mold need to be prepared for each of the casting produced.  For large-scale production, making a mold, for every casting to be produced, may be difficult and expensive. Therefore, a permanent mold, called the die may be made from iron or steel, although graphite, copper and aluminum have been used as mold materials.
  • 61.  The process in which we use a die to make the castings is called permanent mold casting or gravity die casting, since the metal enters the mold under gravity.  Some time in die – casting, we inject the molten metal with a high pressure. When we apply pressure in injecting the metal it is called pressure die casting process.  Permanent mold casting is typically used for high- volume production of small, simple metal parts with uniform wall thickness.
  • 62.  Processing Steps
  • 63. Process Steps in Detail 1. Mold is preheated and coated with lubricant for easier separation of the casting; 2. Cores (if used) are inserted and mold is closed; 3. Molten metal is poured into the mold; 4. Mold is open and finished part removed. 5. Processing Finished part
  • 64.  Advantages ◦ Good dimensional accuracy ◦ Good surface finish ◦ Finer grain structure (stronger casting) ◦ Possibility for automation  Disadvantages ◦ Only for metals with low melting point ◦ Castings with simple geometry  Area of application ◦ Mass production of non-ferrous alloys and cast iron
  • 65. Die casting  Die casting is a very commonly used type of permanent mold casting process.  It is used for producing many components of home appliances (e.g rice cookers, stoves, fans, washing and drying machines, fridges), motors, toys and hand-tools.  Surface finish and tolerance of die cast parts is so good that there is almost no post-processing required. Die casting molds are expensive, and require significant lead time to fabricate.
  • 66. Types of Die Casting  Hot-chamber die-casting ◦ In hot chamber die-casting, the metal is melted in a container attached to the machine, and a piston is used to inject the liquid metal under high pressure into the die.  Cold chamber die casting ◦ In cold-chamber die-casting, molten metal is poured into the chamber from an external melting container, and a piston is used to inject the metal under high pressure into the die cavity.
  • 67. Hot-chamber die-casting Schematics of Hot-chamber die-casting
  • 68. Hot-chamber die-casting Process Details  In a hot chamber process (used for Zinc alloys, magnesium) the pressure chamber connected to the die cavity is filled permanently in the molten metal  Die is closed and gooseneck cylinder is filled with molten metal;  Plunger pushes molten metal through gooseneck passage and nozzle and into the die cavity; metal is held under pressure until it solidifies;  Die opens and cores, if any, are retracted; casting stays in ejector die; plunger returns, pulling molten metal back through nozzle and gooseneck;  Ejector pins push casting out of ejector die. As plunger uncovers inlet hole, molten metal refills gooseneck cylinder
  • 69. Advantages ◦ High productivity (up to 500 parts per hour) ◦ Close tolerances ◦ Good surface finish Disadvantages ◦ The injection system is submerged in the molten metal ◦ Only simple shapes Area of application ◦ Mass production of non-ferrous alloys with very low melting point (zinc, tin, lead)
  • 70. Cold chamber die casting Schematics of Cold-chamber die-casting
  • 71. Cold chamber die casting Process details  In a cold chamber process, the molten metal is poured into the cold chamber in each cycle  Die is closed and molten metal is ladled into the cold chamber cylinder;  Plunger pushes molten metal into die cavity; the metal is held under high pressure until it solidifies;  Die opens and plunger follows to push the solidified slug from the cylinder, if there are cores, they are retracted away;  Ejector pins push casting off ejector die and plunger returns to original position.
  • 72. Advantages ◦ Same as in hot chamber die-casting, but less productivity. Disadvantages ◦ Only simple shapes Area of application ◦ Mass production of aluminium and magnesium alloys, and brass
  • 73. Centrifugal casting  Centrifugal casting uses a permanent mold that is rotated about its axis at a speed between 300 to 3000 rpm as the molten metal is poured. Centrifugal forces cause the metal to be pushed out towards the mold walls, where it solidifies after cooling
  • 74.  This process is normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are axi symmetric with a concentric hole.  Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole.  The mold can be rotated about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously.  The length and outside diameter are fixed by the mold cavity dimensions while the inside diameter is determined by the amount of molten metal poured into the mold.
  • 75. Types ◦ Horizontal Centrifugal Casting ◦ Vertical Centrifugal Casting  Horizontal Centrifugal Casting ◦ True centrifugal casting ◦ In this casting, molten metal is poured into a rotating mold to produce tubular parts such as pipes, tubes, and rings
  • 76. Horizontal Centrifugal Casting
  • 77. Vertical Centrifugal Casting  In this method, centrifugal force is used to produce solid castings rather than tubular parts.  Density of the metal in the final casting is greater in the outer sections than at the center of rotation.  The process is used on parts in which the center of the casting is machined away, such as wheels and pulleys.
  • 78. Vertical Centrifugal Casting
  • 79. Centrifugal Casting Vertical Centrifugal CastingHorizontal Centrifugal Casting
  • 80. Advantages  Rapid production rate.  Suitable for Ferrous / Non-ferrous parts.  Good soundness and cleanliness of castings.  Ability to produce extremely large cylindrical parts.  Formation of hollow interiors in cylinders without cores  Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity
  • 81. Disadvantages  More segregation of alloy component during pouring under the forces of rotation  Contamination of internal surface of castings with non - metallic inclusions  Inaccurate internal diameter  Only cylindrical shapes can be produced with this process. Application  Pipe and Tube Manufacturing
  • 82. Major part of Molding material in sand casting are 1. 70-85% silica sand (SiO2) 2. 10-12% bonding material e.g., clay cereal etc. 3. 3-6% water
  • 83.  Green Sand – Sand in its natural more or less moist state  Dry sand – For making large castings mold is dried  Loam sand – high clay content(50%)  Facing sand – is used directly next to the pattern  Backing sand – Used to back up the facing sand  Parting sand – Clean clay free silica sand. It is used to prevent green sand sticking to the pattern
  • 84.  Refractory sand grains + Binder + Water + Additives  REFRACTORY SAND GRAINS – Mixture of silica sand, Alumina and Magnesium oxide.  It provides Refractoriness, Chemical resistivity and permeability
  • 85.  BINDERS – clay, Kaolinite or fire clay , Bentonite.  It provides necessary binding strength  WATER – 2 to 8%  It develops necessary plasticity
  • 86.  ADDITIVES – to enhance the properties of molding sand. ◦ Coal dust – for protecting mold surface against action of molten metal, surface finish ◦ Dextrin – for increasing toughness & collapsibility ◦ Iron oxide powder – for hot strength & Anti –metal penetration property. ◦ Molasses – for dry strength and collapsibility
  • 87.  Refractoriness – with stand high temperature of molten metal  Strength – sufficient strength to retain shape of the mold  Flowability – Aquire the required shape  Permeability – to allow the gases picked up by the molten metal or for escape or trapped gases in the mold
  • 88.  Cohesiveness – ability of molding sand to adhere to each other.  Adhesive ness – to adhere to the walls of molding sand  Collapsibility – readiness of molding sand to get collapsed after solidification  Chemical resitivty  Durability
  • 89. Dimension of pattern is always greater than that of the casting.  Shrinkage allowance(10-20 mm/m)  Draft allowance(1 – 3 deg)  Machining allowance(2mm)  Distortion allowance  Rapping or shake allowance
  • 90. Riddle Shovel Rammer Trowel Strike –off bar Vent wire
  • 91. Lifter Slick Swab Draw Spike Gate Cutter
  • 92. Sand, mixed with clay binder & water (so it will hold its shape) plus coal dust to improve surface finish Pattern (a copy of the shape you want to produce, made of wood, plastic or metal) Basic materials & equipment for green sand-casting Container of molten metal (filled from furnace) Top and bottom mold forms (made of metal, open at top and bottom) Rammer (tool to compact the sand; often a pressing machine is used) Midland Metals CASE STUDY
  • 93. MOLDING: Sand placed into bottom mold form & compacted MOLDING: Pattern placed into mold A very basic summary of the sand casting process. . . First of all, mix the sand. 1 2 Midland Metals CASE STUDY THEN
  • 94. MOLDING: Add the top mold form 3 4 MOLDING: Fill top form with compacted sand. A tube or pipe provides a path to pour the metal in Pattern is still inside! Midland Metals CASE STUDY
  • 95. MOLDING: Take the top mold off and remove pattern & pipe or post 5 6 MOLDING: Replace the top mold and fasten securely! Pouring hole In the middle of the sand is a cavity shaped like the pattern! Midland Metals CASE STUDY
  • 96. 7 CASTING: Pour the metal (container is filled from furnace immediately before you are ready to pour) 8 Wait for the metal to cool (minutes to days, depending on the size of the casting) Midland Metals CASE STUDY
  • 97. 8 SHAKE OUT: Break apart the two halves of the mold & take out the part—usually requires vibrating or striking the mold to break apart the sand CLEANING. Sand is cleaned off the part, the “tab” where metal flowed in must be removed. 9 A copy of the pattern has now been made in metal 10 Mold forms are reused 11 Sand is broken up, screened to remove debris and clumps, and sent for remixing Midland Metals CASE STUDY
  • 98. Sand-casting is simple in concept, but demanding in execution. It is a process essential to making basic and advanced products. . . it is also hazardous and energy-intensive! Midland Metals CASE STUDY
  • 99. 1. Casting is taken out of the mould by shaking and the Moulding sand is recycled often with suitable additions. 2. The remaining sand, some of which may be embedded in the casting, is removed by means of Shot blasting. 3. The excess material in the form of sprue, runners, gates etc., along with the flashes formed due to flow of molten metal into the gaps is broken manually in case of brittle casting or removed by sawing and grinding in case of ductile grinding. 4. The entire casting is then cleaned by either shot blasting or chemical pickling. 5. Sometimes castings are heat treated to achieve better mechanical properties.
  • 100. Hollow castings are obtained by using sand cores which are made in boxes known as core boxes Cores are separate shape of sand that are required to form hollow interior of the castings or a hole in casting Core Box is essentially a type of pattern made of wood or metal into which sand is rammed or packed to form core For supporting the core in mold an impression in the form of recess is made in the mold with the help of projection in the pattern. This Projection on the pattern is called as core print. Core Print
  • 101.  It must be strong to retain its shape  Permeable to allow the gases to escape  Highly refractory to withstand high temperature of the molten metal  Stable – i.e with minimum of contraction and expansion  Collapsible after the metal solidifies  Smooth surface  Minimum generation of gases when heated by the pour.
  • 102. Core Sand ( Sand + Binder) ( sand containing more than 5 % of clay are not used for cores) Sand Binders Zircon Olivine Carbon Chamotte oils, cereals, dextrine, resins, molasses and protein
  • 103. Core sand preparation Core molding Baking Core finishing
  • 104.  Mix and prepare the sand properly  Mixing should be homogeneous so that core is having uniform strength  Core sands are mixed  Roller mills – the action of mullers and plough give uniform mixing – specially for cereal binders  Core mixers – all types of binders
  • 105.  Green sand cores are made by ramming the sand mixtures into the boxes, the interiors of which have desired shapes and dimensions.  Methods to ram the core are by machines  Core blowing machines  Core ramming machines [ jolting , squeezing, slinging]  Cores are often reinforced with steel wires and rods to have sufficient stregth.
  • 106.  After cores are prepared they are placed on metal plates or core carriers and they are baked to remove the moisture [ 150 – 400 deg c]  Core plates called driers are usually perforated to permit the circulation of gases and to lessen the sticking
  • 107.  After baking cores are given finishing operations  The rough places and unwanted fins are removed by filing  Cores are also coated with refractory or protective materials by brushing, dipping or spraying.  This coating prevents the metal from penetrating and gives smooth surface to the casting  Graphite ,Silica, Mica, Zircon, Four and rubber base chemical spray
  • 108. Chaplets are used to support the cores which tend to sag or sink. These are made of same materials being cast.
  • 109. 1. Minimize turbulent flow so that absorption of gases, oxidation of metal and erosion of mould surfaces are less 2. Regulate the entry of molten metal into the mould cavity 3. Ensure complete filling of mould cavity, and 4. Promote a temperature gradient within the casting so that all sections irrespective of size and shape could solidify properly 5. Avoiding erosion 6. Removing inclusions 7. Minimizing scrap and secondary operations
  • 110.  Runner is commonly a horizontal channel which connects the sprue with the gates thus allowing the molten metal to enter into the cavity.
  • 111. RISER - Feeder Head Riser – is used to provide a passage in molding sand to compensate for shrinkage and eliminate cavity. Purpose ◦ Compensate for shrinkage cavities ◦ Permit the escape of air and gases as the cavity is filled with molten metal ◦ Promote directional solidification ◦ Sound castings
  • 112. Progressive solidification Intermediate rate Slow rate Fast rate Riser Temperature gradient rising toward riser Directional solidification Solidification of castings proceed in two ways 1.Progressive solidification 2.Directional solidification
  • 113. Solidification time α (V/A)2 To promote directional solidification, Solidification time of riser is equal to or greater than that of casting.
  • 114.  Chills are metal inserts which are placed in the mold to induce directional solidification.  If a casting consists of section of uneven thickness, the thin section tends to solidify quicker than thick.  This difficulty is removed by providing chill at thicker section and thus promoting uniform solidification. (i.e to speed up the process of cooling of thick sections )
  • 115.  These serve to promote directional solidification by the generation of heat  Which are a mixture of the oxides of the metal to be cast and Aluminum metal in powder form which produce large amount of heat when come in contact with molten metal  They may be added to the surface of molten metal in the riser / mixed with molding sand in the riser walls.
  • 116. Draft in Pattern Patterns Mold
  • 117.  Defects may occur due to one or more of the following reasons: ◦ Fault in design of casting pattern ◦ Fault in design on mold and core ◦ Fault in design of gating system and riser ◦ Improper choice of moulding sand ◦ Improper metal composition ◦ Inadequate melting temperature and rate of pouring
  • 118. Casting defects Surface Defect Internal Defect Visible defects Blow Scar Blister Drop Scab Penetration Buckle Blow holes Porosity Pin holes Inclusions Dross Wash Rat tail Swell Misrun Cold shut Hot tear Shrinkage/Shift
  • 119.  Blow is relatively large cavity produced by gases which displace molten metal from convex surface.
  • 120.  Scar is shallow blow generally occurring on a flat casting surface.
  • 121.  A scar covered with a thin layer of metal is called blister.
  • 122.  Drop is an irregularly-shaped projection on the cope surface caused by dropping of sand.  Caused by Low strength and soft ramming
  • 123.  A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold.  Caused by too fine sand, low permeability, moisture content, uneven ramming
  • 124.  Penetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it.
  • 125.  Buckle is a vee-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face. A proper amount of volatile additives in moulding material could eliminate this defect by providing room for expansion.
  • 126.  Blow holes are large spherical shaped gas bubbles  Caused by excessive moisture, low permeability, too fine grains, too hard ramming, proper venting is insufficient
  • 127.  porosity indicates a large number of uniformly distributed tiny holes.
  • 128.  Pin holes are tiny blow holes appearing just below the casting surface.  Caused by absorption of hydrogen or carbon monoxide gasses, or high moisture content
  • 129.  Inclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are dross.  Caused by slag particles.
  • 130.  Wash is a low projection near the gate caused by erosion of sand by the flowing metal
  • 131.  Rat tail is a long, shallow, angular depression caused by expansion of the sand.
  • 132.  Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand.
  • 133.  Misrun is a casting that has solidified before fillingthe mold cavity  causes - Fluidity is insufficient, pouring temp too low, pouring slowly, too thin sections
  • 134.  Cold shut – occurs when two portions of metal flow together but there is a lack of fusion between them due to premature freezing.
  • 135.  Hot tear is the crack in the casting caused by high residual stresses.
  • 136.  Shrinkage is essentially solidification contraction and occurs due to improper use of Riser.
  • 137.  Shift is due to misalignment of two parts of the mould or incorrect core location.
  • 138.  Sand Casting  Molding sand – Ingredients, Types, Properties  Pattern  Pattern materials  Types of Pattern  Pattern allowances  Molding tools  Green sand molding procedure
  • 139.  Molding Machine  Core  Requirement of core  Core making  Chaplet  Gating system  Runner  Riser  Chill  Exothermic material  Casting Defects