Overview of casting technology - Nguyen Ngoc Ha

in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity • The term casting also applies to the part made in the process • Steps in casting seem simple: 1. Melt the metal 2. Pour it into a mold 3. Let it freeze Example – Sand Casting Example – Die Casting Example – Investment Casting Evaporative Pattern Casting (a) Metal is poured into mold for lost-foam casting of a 60-hp. 3-cylinder marine engine; (b) finished engine block. Source: Courtesy of Mercury Marine General characteristics of casting processes 2. CAPABILITIES AND ADVANTAGES OF CASTING • Can create complex part geometries • Can create both external and internal shapes • Some casting processes are net shape; others are near net shape • Can produce very large parts • Some casting methods are suited to mass production 3. DISADVANTAGES OF CASTING • Different disadvantages for different casting processes: – Limitations on mechanical properties – Poor dimensional accuracy and surface finish for some processes; e.g., sand casting – Safety hazards to workers due to hot molten metals – Environmental problems 4. PARTS MADE BY CASTING • Big parts – Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, pump housings • Small parts – Dental crowns, jewelry, small statues, frying pans Typical Cast Parts (a) Typical gray-iron castings used in automobiles, including the transmission valve body (left) and the hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation. (b) A cast transmission housing. (c) The Polaroid PDC-2000 digital camera with a AZ191D die-cast high-purity magnesium case. (d) A two-piece Polaroid camera case made by the hot-chamber die-casting process. Source: Courtesy of Polaroid Corporation and Chicago White Metal Casting, Inc. 5. THE MOLD IN CASTING • Contains cavity whose geometry determines part shape – Actual size and shape of cavity must be slightly enlarged to allow for shrinkage of metal during solidification and cooling – Molds are made of a variety of materials, including sand, plaster, ceramic, and metal Open Molds and Closed Molds • Two forms of mold: (a) open mold and (b) closed mold for more complex mold geometry with gating system leading into the cavity Two Categories of Casting Processes 1. Expendable mold processes – use an expendable mold which must be destroyed to remove casting – Mold materials: sand, plaster, and similar materials, plus binders 2. Permanent mold processes – use a permanent mold which can be used to produce many castings – Made of metal (or, less commonly, a ceramic refractory material Two Categories of Casting Processes 6. CASTING TERMINOLOGY 6. CASTING TERMINOLOGY • Flask The box containing the mold • Cope The top half of any part of a 2-part mold • Drag The bottom half of any part of a 2-part mold • Core A shape inserted into the mold to form internal cavities • Core Print A region used to support the core 6. CASTING TERMINOLOGY • Mold Cavity The hollow mold area in which metal solidifies into the part • Riser An extra cavity to store additional metal to prevent shrinkage • Gating System Channels used to deliver metal into the mold cavity • Pouring Cup The part of the gating system that receives poured metal • Sprue Vertical channel 6. CASTING TERMINOLOGY • Runners Horizontal channels • Parting Line / Parting Surface Interface that separates the cope and drag of a 2-part mold • Draft Taper on a pattern or casting that allows removal from the mold • Core Box Mold or die used to produce cores • Casting The process and product of solidifying metal in a mold 7. STEPS IN CASTING 1. Prepare mold for pour 2. Pour molten metal into sand mold 3. Allow metal to solidify 4. Break up the mold to remove casting 5. Clean and inspect casting 6. Heat treatment of casting is sometimes required to improve metallurgical properties 7. STEPS IN CASTING 8. TRENDS 1. Continuing mechanization & automation of the casting process 2. Increasing demand for high-quality castings 3. Computer-aided design 4. Rapid (free-form) pattern making Computer modeling of casting processes • Rapid advances in computers and modeling analysis led innovations in modeling different aspects of casting including: fluid flow, heat transfer, and microstructures developed during solidification; under various casting- process conditions. • Specifically, software may provide: Modeling fluid flow in molds (Bernoulli’s and continuity). Predict velocity and pressure of the molten metal in the gating system all the way into the mold cavity. Modeling of heat transfer in casting.  Fluid flow and heat transfer (with surface conditions, thermal properties of materials) are coupled. Modeling the development of microstructure in casting. Computer modeling of casting processes • The benefits of such user-friendly software are to increase productivity, improve quality, and easily plan and estimate cost. Also quicker response to design changes. • Several commercial software programs now are available for modeling of casting processes: 1. Magmasoft, 2. ProCast, 3. Solidia, and 4. AFSsolid. 9. ECONOMICS OF CASTING • The cost of the cast part (unit cost) depends on several factors: including materials, tooling, equipment, and labor. • Preparations for casting a product include the production of molds and dies that require raw materials, time, and effort – all of which also influence product cost. Cost Characteristics of Casting Cost Characteristics of Casting • As shown in table 12.6, relatively little cost is involved in molds for sand casting. On the other hand, molds for various processes and die-casting dies require expensive materials and a great deal of preparation. • There are also major costs involved in making patterns for casting. • Costs also are involved in melting and pouring the molten metal into molds and in heat treating, cleaning, and inspecting the casting. Cost Characteristics of Casting • Heat treatment in an important part of the production of many alloys groups (especially ferrous castings) and may be necessary to produce improved mechanical properties. • The equipment cost per casting will decrease as the number of parts cast increase. Sustained high-production rates, therefore, can justify the high cost of dies and machinery. • However, if the demand is relatively small, the cost-per-casting increases rapidly. It then becomes more economical to manufacture the parts by sand casting.