METAL CASTING LECTURE 12

CASTING Basically involves i. Pouring molten metal into a mould patterned after the part to be made WITHOUT TURBULANCE , SERIES OF EVENTS TAKES PLACE INFLUENCE SIZE, SHAPE, UNIFORMITY OF THE GRAINS FORMED, AND THUS THE OVERALL PROPERTIES. ii. Allow it to cool HEAT TRANSFER DURING SOLIDIFICATION iii. Remove from the mold INFLUENCE OF THE TYPE OF MOULD MATERIAL SIMILARITY WITH POURING CAKE MIX INTO A PAN

POURING CAKE MIX INTO A PAN (MOULD) & BAKING IT POURING CAKE MIX INTO A PAN (MOULD) & BAKING IT   *SELECT THE KIND AND SIZE OF PAN, *CONTROL THE COMPOSITION OF THE MIX, * CAREFULLY POUR THE MIX, * SET THE PROPER BAKING TEMPERATURE, * SET THE TIMER FOR PROPER BAKING TIME, * LEAVE THE CAKE IN THE MOULD FOR A CERTAIN AMOUNT OF TIME BEFORE REMOVING.   (CASTING OF PLASTICS & CERAMICS - DIFFERENT)

Knowledge of certain fundamental relationships is essential to produce good quality economic castings This knowledge helps in establishing proper techniques for mould design and casting practice.   Castings must be free from defects, must meet the required strength, dimensional accuracy, surface finish

ADVANTAGES OF CASTING PROCESS Process is cheap More suitable for mass production Most suitable for manufacturing complex/complicated/intricate shaped products. Large parts weighing several tonnes and also small components weighing a few grams can be cast. No limitation on the size of component. Directional properties absent in castings. Components with uniform properties as well as with varying properties at different locations can be cast. By use of cores, saving in machining of holes achieved. Internal stresses are relieved during solidification in many types of castings. Even some materials which cannot be made by other processes made by casting: eg. Phosphor-Bronze.

Outline of production steps in a typical sand casting operation - pattern making - Core making - Gating system Moulding Mould Sand Melting Pouring casting Heat Treat Clean Inspect Furnaces Solidification Shakeout Addl. Heat Treatment Defects, pressure tightness, dimensions Outline of production steps in a typical sand casting operation

PATTERN Model of a casting constructed such that it forms an impression in moulding sand

PATTERN 1st step- Prepare model (pattern) Differs from the casting Differences Pattern Allowances. To compensate for metal shrinkage, Provide sufficient metal for machining Easiness in moulding As Shrinkage allowance, Draft allowance, Finishing allowance, Distortion or camber allowance, Shaking or rapping allowance

MATERIAL Types- many WOOD. METAL Al, CI, Brass, 3. For special casting processes, Polystyrene which leaves mould as gas when heated also used. Types- many Simple-Identical patterns; Complex, intricate- with number of pieces. Single or loose piece; Split; gated; Match Plate; Sweep; Segmental; Skeleton(frame, ribbed), skell; Boxed Up; Odd shaped etc. Sketches

Material 1. WOOD. (+) Cheap, easily available, light, easiness in surfacing, preserving (by shellac coating), workable, ease in joining, fabrication (-) Moisture effects, wear by sand abrasion, warp during forming, not for rough use. Must be properly dried/ seasoned, free from knots, straight grained Egs. Burma teak, pine wood, mahogany, Sal, Deodar, Shisham, Walnut, Apple tree

2. METAL: For durability, strength Egs: Al alloys, Brass, Mg alloys, Steel, cast Iron for mass production (first, wooden pattern is made, then cast in the metal) Type of material depends on shape, size, number of castings required, method of moulding etc.