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METAL CASTING PROCESSES AND EQUIPMENT
INTRODUCTION TO CASTING In the casting processes material is first melted, heated to proper temperature and treated to modify its chemical combination. The molten material is then pour into a cavity or mold that holds it in the desired shape during solidification. Cast parts range in size from a fraction of a centimeter to over 10 meters and many tons, example huge propellers. Complex shapes or parts having hollow sections or internal cavities, irregular curved surfaces can be produced by using casting processes.
BASIC REQUIREMENT OF CASTING PROCESSESSix basic steps are required in most casting processes:
1. A container must be produced with a mold cavity having the desire shape and size with due allowance for shrinkage of the solidifying material. Any geometrical feature desired in the finish casting must be present in the cavity.
2. A melting process must be capable of providing molten material at the proper temperature in the desired quantity with acceptable quality and at a reasonable cost.
BASIC REQUIREMENT OF CASTING PROCESSES3. A pouring technique must be devised to introduce the molten metal into the mold. Provision should be made for the escape of all air or gases present in the cavity prior to poring to produce high-quality casting that is fully dense and free of defects. 4. The solidification process should be properly designed and controlled. Casting should be designed so that solidification and solidification shrinkage can occur without producing internal porosity or voids. 5. It must be possible to remove the casting from the mold (i.e. mold removal). With single-use molds that are broken apart and destroyed after each casting, mold removal presents no serious difficulty. With multiple-use molds, however, the removal of a complex shaped casting may be a major design problem.
BASIC REQUIREMENT OF CASTING PROCESSES6. Various cleaning, finishing, and inspection operations may be required after the casting is removed from the mold. Extra material is usually attached where the metal entered the cavity, excess material may be present along mold parting lines and mold material may cling to the casting surface. All of these must be removed from the finished casting.
Figure 1: Two part mold cross sectional view
CASTING TERMINOLOGIES1. Pattern: An approximate duplicate of final casting. 2. Flask: The rigid metal or a wood frame that holds the molding material. 3. Cope: The top half of the mold. 4. Drag: The bottom half of the mold. 5. Core: A sand (or metal) shape that is inserted into a mold to produce internal features of a casting such as holes or passages for water cooling. 6. Core print: A feature that is added to a pattern, core or mold and is used to locate and support a core within the mold.
CASTING TERMINOLOGIES7. Riser: An additional void in the mold that also fills with molten metal. Its purpose is to provide a reservoir of additional liquid that can flow into the mold cavity to compensate for any shrinkage that occurs during solidification. 8. Gating system: The network of connected channels used to deliver the molten metal to the mold cavity is known as the gating system. 9. Pouring cup: The pouring cup is the portion of gating system and receives the molten metal from the poring vessel and controls its delivery to the rest of the mold. 10. Sprue: The vertical portion of the gating system. 11. Runner: The horizontal channels of the gating system.
CASTING TERMINOLOGIES12. Mold: A hollow form of a component or frame which is used to give a particular shape to something in molten state. 13. Parting line: The interface that separates the cope and drag halves of mold or flask. 14. Draft: The taper on a pattern or casting that permits it to be withdrawn from the mold. 15. Casting: The process and the product when molten metal is poured and solidified in a mold.
Figure 1: Two part mold cross sectional view
TYPES OF PATTERNSMany types of patterns are used in the foundry industry with selection being based on the number of duplicate castings required and the complexity of the part. One piece or solid patterns: They are simplest and least expensive as shown in Figure 2. They are generally used when the shape is relatively simple and the number of duplicate castings is small.
Split patterns: They are used when moderate quantities of castings are desired. The pattern is divided into two segments. The top and bottom segments of the pattern are positioned in the cope and drag portions of the flask. Figure 3 shows a split pattern.
Figure 2: Single piece pattern for a pinion gear.
TYPES OF PATTERNS
Figure 3: Split pattern showing the two sections together and separated
TYPES OF PATTERNSMatch plate patterns: In the moulding of small castings where large number of parts are required, it is customary to mount several patterns on a plate called a match plate. This plate is of metal and approximately 3/8 inch thick.
CLASSIFICATION OF CASTING PROCESSESCasting processes can be classified into two major categories: 1. Expendable (single use) mold 2. Permanent (multiple use) mold Expendable mold processes are further categorized as: 1. Expendable mold with permanent patterns 2. Expendable mold with expendable patterns
EXPENDABLE MOLD WITH PERMANENT PATTERN CASTING PROCESSESExpendable mold processes include: 1. 2. 3. 4. with permanent pattern casting
Sand casting Shell-mold casting Plaster-mold casting Ceramic-mold casting
SAND CASTINGSand casting: Sand casting process consists of the following steps: 1. 2. 3. 4. 5. 6. Placing a pattern having the shape of the desired casting Incorporating a gating system Filing the molten cavity with molten material Allowing the melt to cool until it solidifies Breaking away the sand mold Removing the casting and finishing (also watch the video clip)
Sands: Product of the disintegration of rocks over a extremely long period of time. It is inexpensive and suitable mold material because of its resistance to high temperature. Two types of sands are used generally: 1. Naturally bonded (bank sand) 2. Synthetic (lake sand) - its composition can be controlled more accurately
SAND CASTING PROCESS ILLUSTRATION
SAND CASTINGTypes of sand molds: There are three basic types of sand molds: 1. Green mold sand: The sand in this mold is moist or damp while the molten metal is being poured into. It is a mixture of clay, sand and water and is the least expensive method of making molds. Skin-dried molds: In this method of making molds, the mold surfaces are dried using torches or is baked. They are stronger than green sand molds and impart better dimensional accuracy and surface finish. No-bake molds: A synthetic liquid resin is mixed with the sand and the mixture hardens at room temperature. Because bonding of the mold takes place without heat no-bake mold processes are also known as cold setting processes. These molds are dimensionally more accurate than green sand molds but are expensive to produce.
SAND CASTINGAdvantages: Can produce large parts, Can form complex shapes Many material options, Low tooling and equipment cost, Scrap can be recycled, Short lead time possible Disadvantages: Poor material strength, High porosity possible, Poor surface finish and tolerance, Secondary machining often required Low production rate, High labor cost Applications: Engine blocks and manifolds, machine bases, gears, pulleys
SHELL-MOLD CASTING- metal, 2-piece pattern, 175C-370C - coated with a lubricant (silicone) - mixture of sand, thermoset resin/epoxy - cure (baking) - remove patterns, join half-shells mold - pour metal - solidify (cooling) - break shell part
SHELL-MOLD CASTINGAdvantages: Can form complex shapes and fine details, Very good surface finish, High production rate, Low labor cost, Low tooling cost, Little scrap generated Disadvantages: High equipment cost Applications: Cylinder heads, connecting rods etc
PLASTER MOLD, CERAMIC MOLD CASTINGPlaster-mold slurry: plaster of paris (CaSO4), talc, silica flour Ceramic-mold slurry: silica, powdered Zircon (ZrSiO4) The slurry forms a shell over the pattern Dried in a low temperature oven Remove pattern baked (burn-off volatiles) cast the metal break mold part
ILLUSTRATION OF CERAMIC-MOLD CASTING PROCESS
SLIP CASTINGPlaster-mold slurry: plaster of paris (CaSO4), talc, silica flourIn slip casting, a suspension of clay in water with sodium silicate or sodium polyphosphate (clay slip) is poured into a plaster mold and allowed to sit. Water is absorbed into the mold and clay is deposited on the interior surface of the mold. When the mold is emptied and opened a slip cast pot is removed. Slip casting is a technique for making multiple, essentially identical, pots inexpensively. Slip casting are the ways most commercial dinnerware is made today.
EXPENDABLE MOLD AND EXPENDABLE PATTERN CASTING PROCESSESExpendable mold processes include: with expendable pattern casting
1. Evaporative or lost-foam casting 2. Investment casting
EVAPORATIVE (LOST-FOAM) CASTING- Polystyrene pattern - dipped in refractory slurry dried - sand (support) - pour liquid metal - foam evaporates, metal fills the shell - cool, solidify - break shell part
INVESTMENT (LOST-WAX) CASTING(a) Wax pattern (injection molding) (b) Multiple patterns assembled to wax sprue
(d) dry ceramic melt out the wax fire ceramic (burn wax)
(c) Shell built immerse into ceramic slurry immerse into fine sand (few layers)
(e) Pour molten metal (gravity) cool, solidify [Hollow casting: pouring excess metal before solidification
(f) Break ceramic shell (vibration or water blasting)
(g) Cut off parts (high-speed friction saw) finishing (polish)
INVESTMENT (LOST-WAX) CASTINGAdvantages: Can form complex shapes and fine details Many material options, High strength parts, Very good surface finish and accuracy, Little need for secondary machining Disadvantages: Time-consuming process, High labor cost High tooling cost, Long lead time possible Applications: Turbine blades, armament parts, pipe fittings, lock parts, handtools, jewelry etc
PARTS/COMPONENTS MADE FROM INVESTMENT CASTING
PERMANENT MOLD CASTING PROCESSES Permanent molds are made of metals that maintain their strength at high temperature. These molds are better heat conductors therefore solidifying of casting takes place at higher rate of cooling. Two-halves of the mold are made from materials such as steel, bronze, refractory metal alloys or graphite. The mold cavity and the gating system are machined into the mold itself and thus become an integral part of it. The cores are made of metals or sand and are placed in the mold prior to casting. The surfaces of the mold cavity are coated with refractory slurry or sprayed with graphite to increase the flow and life of the mold.
PERMANENT MOLD CASTING PROCESSESPermanent mold casting processes include: 1. 2. 3. 4. 5. 6. Slush casting Pressure casting Vacuum casting Die casting Centrifugal casting Squeeze casting
SLUSH CASTING The molten metal is poured into the mold and begins to solidify at the cavity surface. When the amount of solidified material is equal to the desired wall thickness, the remaining slush (material that has yet to completely solidify) is poured out i.e the mold is inverted and the remaining liquid metal is poured out. The mould halves are removed and the casting is removed. It is generally used for making ornamental and decorative objects and toys.
PRESSURE CASTING In pressure casting process, instead of being poured, the molten metal is forced into the mold by gas pressure. The molten metal is forced upward into the graphite or metal mold by gas pressure which is maintained until the metal has completely solidified in the mold. It is also known as pressure pouring or low pressure casting.
The pressure casting process utilizing graphite molds for the production of steel railroad wheels.
VACUUM CASTING It is similar to low pressure casting, but vacuum pressure is used to fill the mold. This process is used when air entrapment is a problem. Finer details and thin walls can be molded and the mechanical properties of the castings are improved. The main disadvantage to this process is the high price for the equipment.
DIE CASTINGCommon uses: components for rice cookers, stoves, fans, drying machines, fridges, motors, toys, hand-tools, car wheels etc Hot Chamber: (low mp e.g. Zn, Pb; non-alloying) (i) die is closed, gooseneck cylinder is filled with molten metal (ii) plunger pushes molten metal through gooseneck into cavity (iii) metal is held under pressure until it solidifies (iv) die opens, cores retracted; plunger returns (v) ejector pins push casting out of ejector die
DIE CASTINGCold Chamber: (high mp e.g. Cu, Al) (i) die closed, molten metal is ladled into cylinder (ii) plunger pushes molten metal into die cavity (iii) metal is held under high pressure until it solidifies (iv) die opens, cores retracted (v) ejector pins push casting out of ejector die
ADVANTAGES OF DIE CASTINGThe die casting manufacturing process offers many advantages and benefits: provides complex shapes within closer tolerances than many other mass production processes. high rates of production with little or no machining produced with thinner walls than those obtainable by other casting methods. durable, dimensionally stable, and appearance of quality.
DISADVANTAGES OF DIE CASTING Casting weight must be between 30 grams and 10 kg. High initial cost. A certain amount of porosity is common.
APPLICATIONSEngine components, pump components, appliance housing etc
CENTRIFUGAL CASTING Molten is rotated about cylindrical mold at 300 ~ 3000 rpm Inner surface of the casting remains cylindrical because molten metal is uniformly distributed by centrifugal forces. Cylindrical parts ranging from 13mm to 3m in diameter and 16 m long can be centrifugally cast.
Schematic illustration of the centrifugal casting process
CENTRIFUGAL CASTINGAdvantages: Can form very large parts, Good mechanical properties, Good surface finish and accuracy, Low equipment cost, Low labor cost, Little scrap generated Disadvantages: Limited to cylindrical parts, Secondary machining is often required for inner diameter, Long lead time possible Applications: Pipes, wheels, pulleys, nozzles, gun barrels, lampposts etc
SQUEEZE CASTING It involves solidification of molten metal under high pressure. The pressure applied by the die / punch keeps the entrapped gases in solution. The high pressure contact at the die-metal interface promotes heat transfer. The higher cooling rate results in fine microstructure with good mechanical properties.
Schematic illustration of the squeeze casting process