Sand mold casting - Nguyen Ngoc Ha

of sand: • Naturally bonded (bank sand) • Synthetic (lake sand) –Preferred by foundries b/c its composition can be controlled 1.1. Advantages • 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 • Parts ranging in size from small to very large • Production quantities from one to millions 1.2. Disadvantages • Porosity • Poor dimensional control for some processes • Poor surface finish for some processes • Limitation on mechanical properties • Safety hazard • Environmental hazard 1.3. Sand-Cast Parts A large sand casting weighing 680 kg for an air compressor frame 1.4. Sand Casting Mold Characteristics • Tolerances: Non-Ferrous  1/32 to 6 Add  .003 to 3,  3/64 from 3 to 6. Across parting line add  .020 to  .090 depending on size. (Assumes metal patterns) • Surface Finish: Non-Ferrous: 150-350 RMS Ferrous: 300-700RMS • Minimum Draft Requirements: 1° to 5° Cores: 1° to 1 1/2° • Normal Minimum Section Thickness: Non-Ferrous: 1/8 - 1/4 Ferrous: 1/4 - 3/8 • Ordering Quantities: All quantities 1.5. Steps in Sand Casting 1.5. Steps in Sand Casting 2. TYPES OF SAND MOLDS a) Green Sand Molds: The most common type consisting of forming the mold from damp molding sand (silica, clay and moisture) b) Skin-dried Molds: It is done in two ways; (1) The sand around the pattern to a depth of about 1/2 in(10 mm). is mixed with a binder so that when it is dried it will leave a hard surface on the mold. (2) Entire mold is made from green sand, but a spray or wash, which hardens when heat is applied, is used. 2. TYPES OF SAND MOLDS c) Dry Sand Molds: These molds are made entirely from fairly coarse molding sand mixed with binders (linseed oil: bezir yağı or gelatinised starch: nişasta). They baked before being used. A dry sand mold holds its shape when poured and is free from gas troubles due to moisture. Green Sand • Contains bonding agents and water • Typical “green sand” is: – 88% silica – 9% clay – 3% water 3. SAND CASTING PATTERN • Patterns may be made from a variety of materials: – Wood – Metal – Hard Polymers – EPS (Styrofoam) 3.1. Types of Sand Casting Patterns • Variety of patters are used in casting and the choice depends on the configuration of casting and number of casting required – Single-piece pattern – Split pattern – Follow board pattern – Cope and drag pattern – Match plate pattern – Loose-piece pattern – Sweep pattern – Skeleton pattern Characteristics of pattern materials Characteristic Cast iron Plastic Steel Aluminum Wood Machine-ability G G F G E Wear resistance E F E G P Strength G G E G F Weight P G P G E Repair-ability P G G P E Resistance to: Corrosion Swelling P E E E P E E E E P E, excellent; G, good; F, fair; P, poor (a) Split pattern (b) Follow-board (c) Match Plate (d) Loose-piece (e) Sweep (f) Skeleton pattern 3.2. One-Piece Pattern One-Piece (Using a Follow Board) 3.3. Split Pattern • Split pattern, showing the two sections together and separeted. (The light –colored portions are core prints) 3.4. Match-Plate 3.5. Cope-and-Drag Split Patterns Cope-and-drag pattern for producing two heavy parts. (Left) Cope section; (right) drag section. (Note: These are two separete pttern boards) 3.5. Cope-and-Drag Split Patterns 3.6. Loose Piece Pattern 4. MOLDING PROCEDURE (a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be built into the drawing. (b-c) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. 4. MOLDING PROCEDURE (d-e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). 4. MOLDING PROCEDURE (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the sprue and risers. Continued on next slide. (g) The flask is rammed with sand and rthe plate and inserts are removed. 4. MOLDING PROCEDURE (h) The drag half is produced in a similar manner with the pattern inserted. A bottom board is placed below the drag and aligned with pins. (i) The pattern , flask, and bottom board are inverted; and the pattern is withdrawn, leaving the appropriate imprint. 4. MOLDING PROCEDURE (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. The flasks the are subjected to pressure to counteract buoyant forces in the liquid, which might lift the cope 4. MOLDING PROCEDURE (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, inspected, and heat treated (when necessary). Sand-molding machines • The oldest known method of molding, which still used for simple castings, is to compact the sand by hand hammering or ramming it around the pattern. For most operations, however, the sand mixture is compacted around the pattern by molding machines, Fig.6. These machines have the following advantages: • Eliminate labor cost, • Offer high quality casting by improving the application and distribution of forces, • Manipulate the mold in a carefully controlled fashion, • Increase the rate of production Squeeze Heads Various designs of squeeze heads for mold making: (a) conventional flat head; (b) profile head; (c) equalizing squeeze pistons; and (d) flexible diaphragm. Source: © Institute of British Foundrymen. Used with permission. Automatic molding methods Jolting the assembly • Jolting the assembly can further assist mechanization of the molding process. The flask, molding sand, and pattern are placed on a pattern plate mounted on an anvil, and jolted upward by air pressure at rapid intervals, as shown in Fig.7. The inertial forces compact the sand around the pattern. Jolting produces the highest compaction at the horizontal parting line, whereas in squeezing, compaction is highest at the squeeze head, Fig.6. Thus more uniform compaction can be obtained by combining them, as shown in F . Jolting the assembly (a) Schematic illustration of a jolt-type mold-making machine. (b) Schematic illustration of a mold-making machine combines jolting and squeeze Vertical Flaskless Molding • In this method, the halves of the pattern form a vertical chamber wall against which sand is blown and compacted, Fig.8. Then the mold halves are packed horizontally, with the parting line oriented vertically and moved along a pouring conveyor. Vertical Flaskless Molding Vertical flaskless molding. (a) Sand is squeezed between two halves of the pattern. (b) Assembled molds pass along an assembly line for pouring. 5. CORES AND CORE MAKING 5.1. Introduction • Casting processes are unique in their ability to incorporate internal cavities -or reentrant sections with relative ease. • To produce these features, however, it is often necessary to use cores as part of the mold. • Figure shows an example of a product that could not be made by any process other than casting with cores. • While these cores constitute an added cost, they do much to expand the capabilities of the process, and good design practice can often facilitate and simplify their use. 5.1. Introduction V-8 engine block (bottom center) and the five dry-sand cores that are used in the construction of its mold. (Courtesy of General Motors Corporation, Detroit, MI.) 5.1. Introduction • Consider the simple belt pulley shown schematically in Figure 14-21. Various methods of fabrication are suggested in the four sketches, beginning with the casting of a solid form and the subsequent machining of the through-hole for the drive shaft. • A large volume of metal would have to be removed through a substantial amount of costly machining. • A more economical approach would be to make the pulley with a cast-in hole of the approximate final size. 5.1. Introduction 5.1. Introduction • Figure 14-21b depicts an approach where each half of the pattern includes a tapered hole, Which receives the same green sand that is used for the remainder of mold. • While these protruding sections are an integral part of the mold, they are also known as green-sand cores. • Unfortunately, green-sand cores have a relatively low level of strength. • If the protrusions are narrow or long, it might be difficult to withdraw the pattern without breaking them, or they may not have enough strength to even support their own weight. 5.1. Introduction • Dry-sand cores can be used to overcome some of the cited difficulties. • These cores are made independent from the remainder of the mold and are then inserted into core prints to hold them in position. • The remaining sketches in Figure 14-21 show dry-sand cores in the vertical and horizontal positions 5.1. Introduction • To function properly, cores must have the following characteristics; 1. Sufficient hardness and strength (after baking or hardening) to withstand handling and the forces of the molten metal. Compressive strength should be between 100 and 500 psi. 2. Sufficient strength before hardening to permit handling in that condition. 3. Adequate permeability to permit the escape of gases. Since cores are largely surrounded by molten metal, they should possess exceptionally good permeability. 5.1. Introduction 4. Collapsibility, After pouring, the cores must be weak enough to permit shrinkage of the solidified casting as it cools, thereby preventing cracking. In addition, they must be easily removable from the interior of the finished product via shakeout. 5. Adequate refractoriness, Since the cores are largely surrounded by hot metal, they can become quite a bit hotter than the adjacent mold material. 6. A smooth surface. 7. Minimum generation of gases when heated during the pour. 5.1. Introduction • Various techniques have been developed to enhance the natural properties of cores and core materials. • Internal wires or rods can be used to 'impart additional strength. Collapsibility can be enhanced by making the cores hollow or by placing a material such as straw in the center. • Enhanced collapsibility is particularly important in steel castings, where a large amount of shrinkage is observed. • All but the smallest of cores must be vented to permit the escape of trapped and evolved gases. • Vent holes can be made by pushing small wires into the core. 5.2. Types of Cores Green sand core Dry sand core Most commonly used binder is Linseed oil. The oil forms a film around the sand grain and hardens when baked at 180-2200C for 2 hours. Other binders are wheat flour, dextrin, starch and several types of thermosetting plastics. Chaplets • Cores serve to produce internal surfaces in castings In some cases, they have to be supported by chaplets for more stable positioning: (a) Core held in place in the mold cavity by chaplets, (b) chaplet design, (c) casting with internal cavity Chaplets (Left) Typical chaplets. (Right) Method of supporting a core by use of chaplets (relative size of the chaplets is exaggerated). Examples of sand cores showing core prints and chaplets to support cores 6. MELTING AND POURING 6.1. Introduction • The quality of casting depends on the method of melting. The melting technique should provide molten metal at required temperature, but should also provide the material of good quality and in the required quantity 6.1. Introduction • Molten metal is prevented from oxidation by covering the molten metal with fluxes or by carrying out melting and pouring in vacuum • Ladles which pour the molten metal from beneath the surface are used • The two main consideration during pouring are the temperature and pouring rate • Fluidity of molten metal is more at higher temperature but it results into more amount of dissolved gases and high temperature also damage the mould walls and results into poor surface quality of the casting 6.1. Introduction • To control the amount of dissolved gases low, the temperature should not be in superheated range • In ferrous metals, the dissolved hydrogen and nitrogen are removed by passing CO. In non- ferrous metals, Cl, He, or Ar gases are used. • Therefore, fluidity and gas solubility are two conflicting requirements. The optimum pouring temp. is therefore decided on the basis of fluidity requirements.The temp. should be able to fill the whole cavity at the same time it should enter inside the voids between the sand particles. 6.2. Furnaces for Casting Processes • Furnaces most commonly used in foundries: – Cupolas – Direct fuel-fired furnaces – Crucible furnaces – Electric-arc furnaces – Induction furnaces Cupolas • Vertical cylindrical furnace equipped with tapping spout near base • Used only for cast irons, and although other furnaces are also used, largest tonnage of cast iron is melted in cupolas • The "charge," consisting of iron, coke, flux, and possible alloying elements, is loaded through a charging door located less than halfway up height of cupola Cupola 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 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 Crucible Furnaces Three types of crucible furnaces: (a) lift-out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting-pot furnace 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 Electric arc furnace for steelmaking 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 in liquid metal • Since metal does not contact heating elements, the environment can be closely controlled, which results in molten metals of high quality and purity • Melting steel, cast iron, and aluminum alloys are common applications in foundry work Induction furnace 6.3. Ladles • Moving molten metal from melting furnace to mold is sometimes done using crucibles • More often, transfer is accomplished by ladles Two common types of ladles: (a) crane ladle, and (b) two-man ladle 7. CLEANING AND FINISHING 1. Casting is taken out of the mould by shaking and the Moulding sand is recycled often with suitable additions. 2. The remaining sand, some of which may be embedded in the casting, is removed by means of Shot blasting. 3. The excess material in the form of sprue, runners, gates etc., along with the flashes formed due to flow of molten metal into the gaps is broken manuaaly in case of brittle casting or removed by sawing and grinding in case of ductile grinding. 7. CLEANING AND FINISHING 6. The entire casting is then cleaned by either shot blasting or chemical pickling. 7. Sometimes castings are heat treated to achieve better mechanical properties. 7.1. Trimming • Removal of sprues, runners, risers, parting-line flash, fins, chaplets, and any other excess metal from the cast part • For brittle casting alloys and when cross-sections are relatively small, appendages can be broken off • Otherwise, hammering, shearing, hack-sawing, band-sawing, abrasive wheel cutting, or various torch cutting methods are used 7.2. Removing the Core • If cores have been used, they must be removed • Most cores are bonded, and they often fall out of casting as the binder deteriorates • In some cases, they are removed by shaking casting, either manually or mechanically • In rare cases, cores are removed by chemically dissolving bonding agent • Solid cores must be hammered or pressed out 7.3. Surface Cleaning • Removal of sand from casting surface and otherwise enhancing appearance of surface • Cleaning methods: tumbling, air-blasting with coarse sand grit or metal shot, wire brushing, buffing, and chemical pickling • Surface cleaning is most important for sand casting, whereas in many permanent mold processes, this step can be avoided • Defects are possible in casting, and inspection is needed to detect their presence 7.4. Heat Treatment • Castings are often heat treated to enhance properties • Reasons for heat treating a casting: – For subsequent processing operations such as machining – To bring out the desired properties for the application of the part in service