UNIT3-Special casting processmechanica.ppt
- 1. UNIT-3 Special Casting Process
- 5. Centrifugal Casting • In this casting process, molten metal is poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force. • This process makes hollow product. • There are three main types of centrifugal casting process: 1. True centrifugal casting process 2. Semi centrifugal casting process 3. Centrifuging process.
- 6. Centrifugal Casting 1. True Centrifugal casting process: • This is normally used for the making of hollow pipes, tubes, hallow bushes, etc. Which are axisymmetric with a concentric hole. • The axis of rotation can be either horizontal, vertical or any angle between. • Very long pipes are normally cast with horizontal axis, whereas short pieces are more conveniently cast with a vertical axis.
- 7. Process • In centrifugal casting, a permanent mold is rotated continuously about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured. • The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. • The casting is usually a fine-grained casting with a very fine-grained outer diameter, owing to chilling against the mould surface. • Impurities and inclusions are thrown to the surface of the inside diameter, which can be machined away. • Casting machines may be either horizontal or vertical-axis. Horizontal axis machines are preferred for long, thin cylinders, vertical machines for rings. • Most castings are solidified from the outside first. This may be used to encourage directional solidification of the casting, and thus give useful metallurgical properties to it. Often the inner and outer layers are discarded and only the intermediary columnar zone is used.
- 8. • Features of centrifugal casting • Castings can be made in almost any length, thickness and diameter. • Different wall thicknesses can be produced from the same size mold. • Eliminates the need for cores. • Resistant to atmospheric corrosion, a typical situation with pipes. • Mechanical properties of centrifugal castings are excellent. • Only cylindrical shapes can be produced with this process. • Size limits are up to 3 m (10 feet) diameter and 15 m (50 feet) length. • Wall thickness range from 2.5 mm to 125 mm (0.1 - 5.0 in). • Tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15 in). • Surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.
- 9. 9 Advantages Good mechanical properties can be achieved No cores are required for making concentric holes in the case of true centrifugal casting. There is no need for gates and runners, Which increases the casting yield , reaches to almost 100%. Limitations Castings which are axi-symmetric and having concentric holes are suitable. Equipment is expensive.
- 10. Materials • Typical materials that can be cast with this process are iron, • steel, • stainless steels, • glass, and • alloys of aluminum, • copper and nickel.
- 11. • Typical parts made by this process are • pipes, • boilers, • pressure vessels , • flywheels, • cylinder liners and • other parts that are axi-symmetric. • It is notably used to cast cylinder liners and sleeve valves for piston engines, parts which could not be reliably manufactured otherwise.
- 14. Semi - Centrifugal Casting • Semi –centrifugal casting is used for jobs which are more complicated than those possible in true centrifigal casting, but are axisymmetric in nature. • The casting like symmetrical shape, pulley, wheel, disk, gear like big shape products manufactured. • More than one casting achieved. • Here vertical axis machine is used. In center the hub is provided to create hollow core.
- 16. Centrifugal Casting 3. Centrifuging: • This process is used for non-symmetrical castings having intricate details and also for precision castings. • The centrifugal force provides high-fluid pressure to force the molten metal into mould cavity. • A number of similar components can be cast simultaneously.
- 18. • WHAT IS A DIE? • A die is a specialized tool used in manufacturing industries to cut or shape material mostly using a press tool , mould & die casting. Like molds, dies are generally customized to the item they are used to create. Products made with dies range from simple paper clips to complex pieces used in advanced technology • PRESS TOOL MOULD DIE CASTING •
- 19. DEFINETION OF DIE CASTING • Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. • Most die castings are made from non-ferrous metals, specifically 1.) zinc 2.) copper 3.) aluminium 4.) magnesium 5.) lead 6.) tin based alloys o Depending on the type of metal being cast, a hot- or cold- chamber machine is used.
- 20. Gravity die casting, also typically known as permanent mold casting, uses reusable molds made of metal, like steel, graphite etc. to fabricate metal and metal alloys. This type of metal casting can manufacture various parts like gears, gear housing, pipe fittings, wheels, engine pistons, etc. 1. Gravity Die Casting:
- 21. In this process, the direct pouring of molten metal into the mold cavity takes place under the effect of gravity. For better coverage, the die can be tilted to control the filling. The molten metal is then allowed to cool and solidifies within the mold to form products. As a result, this process makes casting of materials like lead, zinc, aluminum, and magnesium alloys, certain bronzes, and cast iron more common.
- 22. Permanent Mold Casting Typical parts include gears, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine
- 23. Die Casting 2. Pressure die casting: • In pressure die casting metal flows under high pressure • Also as the die is metallic, the casting rate is high and thus mass production is possible. The following are the types of pressure die casting (A) Hot chamber die-casting (B) Cold chamber die casting.
- 24. HOT CHAMBER DIE CASTING In hot chamber die casting manufacture, the supply of molten metal is attached to the die casting machine and is an integral part of the casting apparatus for this manufacturing operation. The metal for casting is maintained at an appropriate temperature in a holding furnace adjacent to, if not part of, the machine. The injection mechanism is located within the holding furnace and a substantial part of it is therefore in constant contact with the molten metal. Pressure is transmitted to the metal by the injection piston, which forces it through the gooseneck and into the die. On the return stroke metal is drawn into the gooseneck for the next shot. In this process there is minimum contact between air and the metal to be injected, thus minimizing the tendency for turbulent entrainment of air in the metal during injection. Due to the prolonged contact between the metal and parts of the injection system hot chamber is restricted to zinc-base alloys. The Zinc alloys are the most widely used in the die casting process. They have very desirable physical, mechanical and casting properties. They also have the ability to be readily finished with commercial electroplated or organic coatings.
- 26. Some applications of Zinc Die Castings: • Automotive Industry • Fuel Pumps • Carburetor Parts • Valve Covers • Handles hot-chamber machines are primarily used with zinc, tin, and lead based alloys.
- 27. • The essential feature of this process is the independent holding and injection units. In the cold chamber process metal is transferred by ladle, manually or automatically, to the shot sleeve. • Actuation of the injection piston forces the metal into the die. This is a single-shot operation. • This procedure minimizes the contact time between the hot metal and the injector components, thus extending their operating life. • However, the turbulence associated with high-speed injection is likely to entrain air in the metal, which can cause gas porosity in COLD CHAMBER DIE CASTING
- 28. • Next to zinc aluminum is the most widely used die-casting alloy. The primary advantage is it light weight and its high resistance to corrosion. Magnesium alloy die-castings are also produced and are used where a high strength–to–weight ratio is desirable. • The mold has sections, which include the “cover” or hot side and the “movable” or ejector side. The die may also have additional moveable segments called slides or pulls, which are used to create features such as undercuts or holes which are parallel to the parting line. The machines run at required temperatures and pressures to produce a quality part to near net- shape.
- 29. Die Casting • Cold Chamber Cold-chamber machines are used with a large composition of aluminium, magnesium and copper. Some application for Aluminum Die Castings: Automotive industry Home Appliances Communication Equipment Sports & Leisur
- 30. Advantages of die casting • Advantages: – Thin section (0.5 mm thickness) can be easily made. – Impression or complicated design can be achieved on component walls. – The production rate is high (300 per hour approx). – Die set can be used many times. – All non-ferrous products are produced with this method. – Better surface finish is achieved.
- 31. Disadvantages of die casting • It is not economic for small quantity of production. • It is used for only small casting (10 kg weight approx). • Initial cost is high for die and other equipment. • Only non-ferrous products are casted. • If proper care is not taken then the defects like blow hole can be possible.
- 32. Investment Casting • Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mold. • Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mold after it has been formed. • The mold is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mold. • However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be
- 33. Investment Casting • This process is called the lost-wax process or precision casting. The following are the different stages (1) Die making (2) Making wax pattern (3) Precoating the wax pattern assembly (4) Investment the wax pattern in mould box (5) Removal of wax pattern (6) Pouring molten metal (7) Cleaning of casting
- 34. • Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel • This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. • Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components.
- 35. • Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding. The process steps include the following:
- 36. Process • Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. • Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter.
- 38. • The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting. • Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity.. • Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.
- 39. • Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. The ceramic mold is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen). • Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part.
- 41. Advantages Investment Casting (1)Very smooth surface of casting without parting lines are achieved. (2) Machining of intricate shape can be manufactured. (3) Die casting can be replaced when short runs are involved. (4) Castings are sound and have large grains as the rate of cooling is slow. (5) It represents the only method suitable for manufacture of precision shaped castings of high melting point metals which would cause too rapid die failures is normal die casting process.
- 42. Disadvantages Investment Casting (1) It is expensive process and hence is adopted only where small number of intricate and highly accurate parts particularly high melting point alloys are to be manufactured. (2) This process is suitable for small size parts. (3) This presents some difficulties when cores are to be used.
- 43. Defects in Casting • Casting defects, their causes & remedies: (1) Blow holes Holes on casting surface is blow holes. This defect occur because of air comes out from surface. Causes: Moisture level is high in mould sand, improper baking of core. unnecessaryy carbonic binder, unwanted ramming, small vent hole, fine sand etc. Remedies: Proper moisture level, proper baking of core, proper use of binder, proper ramming, proper vent hole with vent rod, selection of sand particle.
- 44. Blow holes Casting Defects
- 45. Casting defects, their causes & remedies (2) Shrinkage When metal transfer from liquid to sold its volume will decrease. During this process if it will not get more molten metal then in the internal surface of casting voids are developed, that is known as shrinkage. Causes: Defective runner, gate and riser, molten metal's pouring temperature. Remedies : Proper arrangement of runner, riser and gate for proper directional solidification. If required, the design can be changed, maintaining proper molten metal temperature.
- 47. Casting defects, their causes & remedies • (3) Crack : • Due to solidification metal shrinks in mould. The improper shrinkage develops stress on surface, due to that cracks propagates on surface also known as pull. • Causes: Non availability of collapsibility property of mould and core, improper design, hard ramming of mould. • Remedies: Collapsibility property can be improved, design can improved. properly
- 49. Casting defects, their causes & remedies • 4) Inclusions : • Unwanted ingredients such as metal oxide, slag, sand particles give defects known as inclusions in metal castings. • Causes: Improper gating system, improper pouring, low quality mould and core sand, improper ramming, impurity in metal charge. • Remedies: By modifying gating system, turbulence free pouring, using good quality mould and core, proper ramming of mould sand, proper and pure metal charge should be used and by using oxide free molten metal crucible.
- 51. Casting defects, their causes & remedies • (5) Lift and shift : • Some part of casting gets distortion known as lift and shift. • Causes: Improper alignment of pattern parts, improper support of core, improper clamping of mould box, improper strength of mould sand. • Remedies: By aligning the mould box with help of dowel pin, properly supporting core in mould, properly clamping of mould box, providing proper strength mould and core sand. • (6) Swell • Due to molten metal the some part of mould cavity become large so the casting becomes larger than required which known as swell. • Causes : Pressure of molten metal on surface, improper ramming of sand, low strength of core sand. • Remedies: By properly ramming of sand, by increasing core strength so molten metal can easily flow in mould. • (7) Fins • Thin edge on casting known as fins. Generally it is on parting line. • Causes: Improper clamping of mould b0x, improper arrangement of mould and core. • Remedies: Properly clamping of cope and drag, proper assembly of mould and core.
- 52. Casting defects, their causes & remedies • (8) Misrun and cold shut :Molten metal cannot reach in all the parts of mould, so this improper filled casting known as misrun. Molten comes from different sides, sometimes cannot mix each other. This defect known as cold shut. • Causes: Defective design of gating system, low fluidity of molten metal, thin wall of casting, non-continues pouring of metal. • Remedies: Modification of gating system, increasing temperature of molten metal, continuously pouring of molten metal, increasing porosity of sand. • (9) Metal Penetration: Surface of casting becomes rough due to metal penetration. • Causes: Bigger size of sand, less ramming, low strength of moulding sand and core, higher permeability. • Remedies : Fine grain sand, proper ramming of mould sand, proper mixture should be used to increase moulding and core porosity. • (10) Hard spot: Some part of casting solidifies very fast and that surface becomes tough, that known as hard spot. • Causes: Bad casting design, improper métal composition, improper use of chills. • Remedies: Proper design of castings so metal solidifies in same time, proper metal Composition, proper use of chills in design.
- 53. Casting defects, their causes & remedies • (11) Run out: While pouring, molten metal comes out (leaks out) from casting known as run out. • Causes: Defective mould box, defectively moulding process. • Remedies Moulding box should be changed, modification in moulding process. • (12) Drop: A drop occurs when cope surface cracks and breaks, thus the pieces of sand fall into the molten metal. • Causes Due to either low green strength or improper ramming of the cope flask, improper reinforcement. • Remedies: Proper mixing of binder for strength improvement, proper arrangement of steel rod for reinforcement in core and mould, proper ramming. • (13) Warpage: After or before solidification casting may twist or change shape known as warpage. • Causes: Improper design, lack of directional solidification, internal stress. • Remedies: Proper design of casting to get directional solidification. By proper heat treatment the stress can be removed.
Editor's Notes
- #37: Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity. Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.
- #40: Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity. Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.
- #41: Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity. Mold creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.