Manufacturing Processes Lecture 8 PDF

Manufacturing Processes (MENG 222) Lecture (8) Dr. Sabry Said Youssef Assistant Professor Mechanical Engineering Dept. Faculty of Engineering, Fayoum University  METAL CASTING PROCESSES 1. Sand Casting 2. Other Expendable Mold Casting Processes 3....

Manufacturing Processes (MENG 222) Lecture (8) Dr. Sabry Said Youssef Assistant Professor Mechanical Engineering Dept. Faculty of Engineering, Fayoum University  METAL CASTING PROCESSES 1. Sand Casting 2. Other Expendable Mold Casting Processes 3. Permanent Mold Casting Processes 4. Furnaces for Casting Processes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Two Categories of Casting Processes 1. Expendable mold processes - mold is sacrificed to remove part  Advantage: more complex shapes possible  Disadvantage: production rates often limited by the time to make mold rather than casting itself 2. Permanent mold processes - mold is made of metal and can be used to make many castings  Advantage: higher production rates  Disadvantage: geometries limited by need to open mold ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Overview of Sand Casting  Most widely used casting process, accounting for a significant majority of total tonnage cast  Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium  Castings range in size from small to very large  Production quantities from one to millions ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Sand casting weighing over 680 kg (1500 lb) for an air compressor frame (photo courtesy of Elkhart Foundry). ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Sand Casting 1. Making the sand mold 2. Pour the molten metal into sand mold 3. Allow time for metal to solidify 4. Break up the mold to remove casting 5. Clean and inspect casting  Separate gating and riser system 6. Heat treatment of casting is sometimes required to improve metallurgical properties ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Making the Sand Mold  The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern  The mold must also contain gating and riser system  If casting is to have internal surfaces, a core must be included in mold  A new sand mold must be made for each part produced ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Sand Casting Production Sequence  Production sequence in sand casting, including pattern‑making and mold‑making ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  The Pattern Full‑sized model of part, slightly enlarged to account for shrinkage and machining allowances in the casting Pattern materials:  Wood - common material because it is easy to work, but it warps  Metal - more expensive to fabricate, but lasts longer  Plastic - compromise between wood and metal ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Types of Patterns  Types of patterns used in sand casting: (a) solid pattern, (b) split pattern, (c) match‑plate pattern, (d) cope and drag pattern ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Core Full‑scale model of interior surfaces of part  Inserted into mold cavity prior to pouring  The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces  May require supports to hold it in position in the mold cavity during pouring, called chaplets ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Core in Mold  (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Desirable Mold Properties  Strength ‑ to maintain shape and resist erosion  Permeability ‑ to allow hot air and gases to pass through voids in sand  Thermal stability ‑ to resist cracking on contact with molten metal  Collapsibility ‑ ability to give way and allow casting to shrink without cracking the casting  Reusability ‑ can sand from broken mold be reused to make other molds? ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Types of Sand Mold  Green‑sand molds - mixture of sand, clay, and water  “Green" means mold contains moisture at time of pouring  Dry‑sand mold - organic binders rather than clay  Mold is baked to improve strength  Skin‑dried mold - drying mold cavity surface of a green‑sand mold to a depth of 10 to 25 mm, using torches or heating lamps ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Other Expendable Mold Processes  Shell Molding  Investment Casting (Lost wax casting)  Ceramic Mold Casting ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Shell Molding Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder Steps: (1) A metal pattern is heated and placed over a box containing sand mixed with thermosetting resin ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Shell Molding  (2) Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Shell Molding  (3) Box is repositioned so loose uncured particles drop away ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Shell Molding  (4) Sand shell is heated in oven for several minutes to complete curing  (5) shell mold is stripped from pattern ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Shell Molding  (6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished  (7) Finished casting with sprue removed ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Shell Molding: Advantages and Disadvantages  Advantages:  Smoother cavity surface permits easier flow of molten metal and better surface finish  Good dimensional accuracy  Mold collapsibility minimizes cracks in casting  Can be mechanized for mass production  Disadvantages:  More expensive metal pattern  Difficult to justify for small quantities ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Investment Casting (Lost Wax Process) A pattern made of wax is coated with a refractory material to make the 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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Investment Casting  (1) Wax patterns are produced  (2) Several patterns are attached to a sprue to form a pattern tree ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Investment Casting  (3) Pattern tree is coated with a thin layer of refractory material  (4) Full mold is formed by covering the coated tree with sufficient refractory material to make it rigid ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Investment Casting  (5) Mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity  (6) Mold is preheated to a high temperature, the molten metal is poured, and it solidifies ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Investment Casting  (7) Mold is broken away from the finished casting and the parts are separated from the sprue ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Investment Casting: Advantages and Disadvantages  Advantages:  Parts of great complexity and intricacy can be cast  Close dimensional control and good surface finish  Wax can usually be recovered for reuse  This is a net shape process  Additional machining is not normally required  Disadvantages:  Many processing steps are required  Relatively expensive process ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Permanent Mold Casting Processes  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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  The Basic Permanent Mold Process Uses a metal mold constructed of two sections designed for easy, precise opening and closing  Molds used for casting lower melting point alloys are commonly made of steel or cast iron  Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Permanent Mold Casting  (1) Mold is preheated and coated for lubrication and heat dissipation ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Permanent Mold Casting  (2) Cores (if any are used) are inserted and mold is closed ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Steps in Permanent Mold Casting  (3) Molten metal is poured into the mold, where it solidifies ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Permanent Mold Casting: Advantages and Limitations  Advantages of permanent mold casting:  Good dimensional control and surface finish  Rapid solidification caused by 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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Applications and Metals for 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  Unsuited to steels because of very high pouring temperatures ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Hot-Chamber Die Casting  Hot‑chamber die casting cycle: (1) with die closed and plunger withdrawn, molten metal flows into the chamber ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Hot-Chamber Die Casting  (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification. ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Hot-Chamber Die Casting  (3) Plunger is withdrawn, die is opened, and casting is ejected ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Cold‑Chamber Die Casting Cycle  (1) With die closed and ram withdrawn, molten metal is poured into the chamber ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Cold‑Chamber Die Casting Cycle  (2) Ram forces metal to flow into die, maintaining pressure during cooling and solidification ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Cold‑Chamber Die Casting Cycle  (3) Ram is withdrawn, die is opened, and part is ejected ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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 onto cavity surfaces to prevent sticking ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Die Casting: Advantages and Limitations  Advantages:  Economical for large production quantities  Good accuracy and surface finish  Thin sections possible  Rapid cooling means small grain size and good strength in casting  Disadvantages:  Generally limited to metals with low metal points  Part geometry must allow removal from die ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  True Centrifugal Casting  Setup for true centrifugal casting ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Semicentrifugal Casting Centrifugal force is used to produce solid castings rather than tubular parts Molds use 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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Semicentrifugal Casting ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Centrifuge Casting Mold is designed with part cavities located away from axis of rotation, so 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 ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Centrifuge Casting ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Furnaces for Casting Processes  Furnaces most commonly used in foundries:  Cupolas  Direct fuel‑fired furnaces  Crucible furnaces  Electric‑arc furnaces  Induction furnaces ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Cupolas Vertical cylindrical furnace equipped with tapping spout near base Used only for cast irons  Although other furnaces are also used, the largest tonnage of cast iron is melted in cupolas The "charge," consisting of iron, coke, flux, and any alloying elements, is loaded through a charging door located less than halfway up height of cupola ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Cupola for melting cast iron ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Direct Fuel‑Fired Furnaces Small open‑hearth in which charge is heated by natural gas fuel burners located on side of furnace Furnace roof assists heating action by reflecting flame down against charge At bottom of hearth is a tap hole to release molten metal Generally used for nonferrous metals such as copper‑base alloys and aluminum ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Crucible Furnaces Metal is melted without direct contact with burning fuel mixture Sometimes called indirect fuel‑fired furnaces Container (crucible) is made of refractory material or high‑temperature steel alloy Used for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum Three types used in foundries: (a) lift‑out type, (b) stationary, (c) tilting ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Three Types of Crucible Furnaces  (a) Lift‑out crucible, (b) stationary pot - molten metal must be ladled, and (c) tilting-pot furnace ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Electric‑Arc Furnaces Charge is melted by heat generated from an electric arc  High power consumption  But electric‑arc furnaces can be designed for high melting capacity  Used primarily for melting steel ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Induction Furnaces  Uses alternating current passing through a coil to develop magnetic field in metal  Induced current causes rapid heating and melting  Electromagnetic force field also causes mixing action  Since metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purity  Common alloys: steel, cast iron, and aluminum ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Induction Furnace ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  Ladles  Two common types of ladles to transfer molten metals to molds: (a) crane ladle, and (b) two‑man ladle ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e