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Casting and types
- 1. ME8352: MANUFACTURING TECHNOLOGY - I CASTINGS Prepared by S.Kannan Assistant Professor Department of Mechanical Engineering Vel Tech Engineering College Avadi-Chennai-62
- 2. INTRODUCTION Casting Pouring molten metal into a mold shaped after the part to be produced, allowing it to harden, and removing it from the mold
- 3. TYPES OF CASTING 1. Investment Casting (Lost Wax Process) A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal • "Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern • It is a precision casting process - capable of producing castings of high accuracy and intricate detail
- 4. Investment Casting Figure 2 Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree
- 5. Investment Casting Figure 3 Steps in investment casting: (3) the pattern tree is coated with a thin layer of refractory material, (4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid
- 6. Investment Casting Figure 4 Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies
- 7. Investment Casting Figure 5 Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue
- 8. Investment Casting Figure 6 A one piece compressor stator with 108 separate‑ airfoils made by investment casting (photo courtesy of Howmet Corp.).
- 9. Advantages and Disadvantages • Advantages of investment casting: – Parts of great complexity and intricacy can be cast – Close dimensional control and good surface finish – Wax can usually be recovered for reuse – Additional machining is not normally required this is a‑ net shape process • Disadvantages – Many processing steps are required – Relatively expensive process
- 10. 2. Plaster Mold Casting Similar to sand casting except mold is made of plaster of Paris (gypsum CaSO‑ 4 2H‑ 2O) • In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set – Wood patterns not generally used due to extended contact with water • Plaster mixture readily flows around pattern, capturing its fine details and good surface finish
- 11. Advantages and Disadvantages • Advantages of plaster mold casting: – Good accuracy and surface finish – Capability to make thin cross sections‑ • Disadvantages: – Mold must be baked to remove moisture, which can cause problems in casting – Mold strength is lost if over-baked – Plaster molds cannot stand high temperatures, so limited to lower melting point alloys
- 12. 3. Ceramic Mold Casting Similar to plaster mold casting except that mold is made of refractory ceramic material that can withstand higher temperatures than plaster • Can be used to cast steels, cast irons, and other high temperature alloys‑ • Applications similar to those of plaster mold casting except for the metals cast • Advantages (good accuracy and finish) also similar
- 13. 4. Permanent Mold Casting • Economic disadvantage of expendable mold casting: a new mold is required for every casting • In permanent mold casting, the mold is reused many times • The processes include: – Basic permanent mold casting – Die casting – Centrifugal casting
- 14. Permanent Mold Casting Figure 7 Steps in permanent mold casting: (1) mold is preheated and coated
- 15. Permanent Mold Casting Figure 8 Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.
- 16. Advantages and Limitations • Advantages of permanent mold casting: – Good dimensional control and surface finish – More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger • Limitations: – Generally limited to metals of lower melting point – Simpler part geometries compared to sand casting because of need to open the mold – High cost of mold
- 17. Applications of Permanent Mold Casting • Due to high mold cost, process is best suited to high volume production and can be automated accordingly • Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles • Metals commonly cast: aluminum, magnesium, copper base alloys, and cast iron‑
- 18. 5. Die Casting A permanent mold casting process in which molten metal is injected into mold cavity under high pressure • Pressure is maintained during solidification, then mold is opened and part is removed • Molds in this casting operation are called dies; hence the name die casting • Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes
- 19. Die Casting Machines • Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity • Two main types: 1. Hot chamber machine‑ 2. Cold chamber machine‑
- 20. Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the die • High production rates - 500 parts per hour not uncommon • Applications limited to low melting point‑ metals that do not chemically attack plunger and other mechanical components • Casting metals: zinc, tin, lead, and magnesium
- 21. Hot-Chamber Die Casting Figure 9 Cycle in hot chamber casting: (1) with die closed and‑ plunger withdrawn, molten metal flows into the chamber (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.
- 22. Cold Chamber Die Casting Machine‑ Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity • High production but not usually as fast as hot chamber machines because of pouring‑ step • Casting metals: aluminum, brass, and magnesium alloys • Advantages of hot chamber process favor its‑ use on low melting point alloys (zinc, tin, lead)‑
- 23. Figure 10 Cycle in cold chamber casting : (1)‑ with die closed and ram withdrawn, molten metal is poured
- 24. Cold Chamber Die Casting‑ Figure 11 Cycle in cold chamber casting: (2) ram forces metal to flow into die,‑ maintaining pressure during cooling and solidification.
- 25. Molds for Die Casting • Usually made of tool steel, mold steel, or maraging steel • Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron • Ejector pins required to remove part from die when it opens • Lubricants must be sprayed into cavities to prevent sticking
- 26. Advantages and Limitations • Advantages of die casting: – Economical for large production quantities – Good accuracy and surface finish – Thin sections are possible – Rapid cooling provides small grain size and good strength to casting • Disadvantages: – Generally limited to metals with low metal points – Part geometry must allow removal from die
- 27. 6. Centrifugal Casting A family of casting processes in which the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity • The group includes: – True centrifugal casting – Semicentrifugal casting – Centrifuge casting
- 28. True Centrifugal Casting Molten metal is poured into rotating mold to produce a tubular part • In some operations, mold rotation commences after pouring rather than before • Parts: pipes, tubes, bushings, and rings • Outside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces
- 29. True Centrifugal Casting Figure 11.15 Setup for true centrifugal casting.
- 30. Semicentrifugal Casting Centrifugal force is used to produce solid castings rather than tubular parts • Molds are designed with risers at center to supply feed metal • Density of metal in final casting is greater in outer sections than at center of rotation • Often used on parts in which center of casting is machined away, thus eliminating the portion where quality is lowest • Examples: wheels and pulleys
- 31. Centrifuge Casting Mold is designed with part cavities located away from axis of rotation, so that molten metal poured into mold is distributed to these cavities by centrifugal force • Used for smaller parts • Radial symmetry of part is not required as in other centrifugal casting methods
- 32. A casting that has solidified before completely filling mold cavity Figure 13 Some common defects in castings: (a) misrun General Defects: Misrun
- 33. Two portions of metal flow together but there is a lack of fusion due to premature freezing Figure 14 Some common defects in castings: (b) cold shut General Defects: Cold Shut
- 34. Metal splatters during pouring and solid globules form and become entrapped in casting Figure 15 Some common defects in castings: (c) cold shot General Defects: Cold Shot
- 35. Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze Figure 16 Some common defects in castings: (d) shrinkage cavity General Defects: Shrinkage Cavity
- 36. Balloon shaped gas cavity caused by‑ release of mold gases during pouring Figure 17 Common defects in sand castings: (a) sand blow Sand Casting Defects: Sand Blow
- 37. Formation of many small gas cavities at or slightly below surface of casting Figure 18 Common defects in sand castings: (b) pin holes Sand Casting Defects: Pin Holes
- 38. When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metal Figure 19 Common defects in sand castings: (e) penetration Sand Casting Defects: Penetration
- 39. A step in cast product at parting line caused by sidewise relative displacement of cope and drag Figure 20 Common defects in sand castings: (f) mold shift Sand Casting Defects: Mold Shift