(( Manufacturing )) Expendable Mold Casting Processes: Types of expendable mold casting are: 1 ) Sand casting. 2 ) Shell molding. 3 ) Vacuum molding. 4 ) Investment casting. 5 ) Expanded polystyrene process. 6 ) Plaster & Ceramic mold casting. 1. Overview of Sand Casting: Most widely used casting process, it can be considered as 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 and for complicated shapes [see fig. (1) below]. Production quantities from one to millions. Fig. (1): Some casting with large or complicated shape manufactured by sand casting. Sand Casting Production Sequence: The steps include not only the casting operation but also pattern-making and mold-making. The sand casting sequences can be described by the below flow chart, in which the steps of sand casting take place [see fig. (2) below]. The chart shows that pattern shape making and sand preparation are first applied, then the mold is made by pressing sand around pattern with fingers. When the mold complete, the following steps is done: 1. Pour the molten metal into sand mold. 2. Allow time for metal to cool and solidify 3. Break up the mold to remove casting. 4. Clean and inspect casting (Separate gating and riser system). 5. Heat treatment of casting is sometimes required to improve metallurgical properties. 1
Desirable Mold Properties: Strength - the mold s ability to maintain its shape and resist erosion caused by the flow of molten metal; it depends on grain shape, adhesive qualities of the binder. Permeability - capacity of the mold to allow hot air and gases from the casting operation to pass through the voids in the sand. Thermal stability - ability of the sand at the surface of the mold cavity to resist cracking and buckling upon contact with the molten metal. Collapsibility - ability of the mold to allow the casting to shrink without cracking the casting; it also refers to the ability to remove the sand from the casting during cleaning. Reusability can the sand from broken mold be reused to make other molds? Binders Used with Foundry Sands: Sand is held together by a mixture of water and bonding clay: Typical mix: 90% sand, 3% water, and 7% clay Other bonding agents also used in sand molds: Organic resins (eg, phenolic resins) [acidic compound C 6 H 5 OH ]. Inorganic binders (eg, sodium silicate and phosphate) Additives are sometimes combined with the mixture to increase strength and / or permeability. 2. Shell Molding: Casting process in which the mold is a thin shell of sand (typically 9 mm thickness) held together by thermosetting resin binder. The process is described and illustrated in the following figure (3): Fig. (3): Steps in shell-molding. Steps in Shell molding: (1) A match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin. (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. (3) Box is repositioned so that loose uncured particles drop away. (4) Sand shell is heated in oven for several minutes to complete curing. (5) Shell mold is stripped from the pattern. 2
(6) Two halves of the shell mold is assembled, supported by sand or metal shot in a box, and pouring is accomplished. (7) The finished casting with sprue removed. Advantages of shell molding: Smoother cavity surface permits easier flow of molten metal and better surface finish Good dimensional accuracy - machining often not required Mold collapsibility minimizes cracks in casting Can be mechanized for mass production More expensive metal pattern Difficult to justify for small quantities 3. Investment Casting (Lost Wax Process): A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal [see fig. (4) below]. "Investment" comes from a 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 [see fig. (4) photo no. (8)]. Fig. (4): Steps in investment-molding. Steps in Investment molding: (1) Wax patterns are produced. (2) Several patterns are attached to a sprue to form a pattern tree. (3) The pattern tree is coated with a thin layer of refractory material. (4) The full mold is formed by covering the coated tree with sufficient refractory material to make it rigid. (5) The mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity. (6) The mold is preheated to a high temperature, the molten metal is poured, and it solidifies. (7) The mold is broken away from the finished casting and the parts are separated from the sprue. (8) A one-piece compressor stator with 108 separate airfoils made by investment. 3
Advantages of investment casting: Parts of great complexity and intricacy can be cast Close dimensional control and good surface finish Wax can usually be recovered for reuse Additional machining is not normally required - this is a net shape process Disadvantages Many processing steps are required Relatively expensive process 4. Vacuum Mold ing: Uses sand mold held together by vacuum pressure rather than by a chemical binder [see fig. (5) below]. The term "vacuum" refers to mold making rather than casting operation itself. It is developed in Japan around 1970. Fig. (5): Steps in Vacuum - molding. Steps in Vacuum molding: (1) A thin sheet of preheated plastic is drawn over a match-plate or copedrag pattern by vacuum; the pattern has small vent holes to facilitate vacuum forming. (2) A specially designed flask is placed over the pattern plate and filled with sand, and a sprue with pouring cup is formed in the sand. (3) Another thin plastic sheet is placed over the flask, and a vacuum is drawn that causes the sand grain to be held together, forming a rigid mold. (4) The vacuum on the mold pattern is released to permit the pattern to be stripped from the mold. (5) The drag is made in the same way (without the sprue and pouring cup). This mold is assembled with its matching half to form the cope and drag, and with vacuum maintained on both halves, the molten metal is poured. When the metal has solidified the vacuum is turned off. Advantages of vacuum molding: Easy recovery of the sand, since no binders Sand does not require mechanical reconditioning done when binders are used Since no water is mixed with sand, moisture-related defects are absent Slow process Not readily adaptable to mechanization. 4
5. Expanded Polystyrene Process: Fig. (6): Steps in Expanded Polystyrene process. Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold [see fig. (6) above]. Other names: lost-foam process, lost pattern process, evaporative-foam process, and full-mold process. Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed). Mold does not have to be opened into cope and drag sections. Steps in Expanded Polystyrene process : (1) Pattern of polystyrene is coated with refractory compound. (2) Foam pattern is placed in mold box, and sand is compacted around the pattern. (3) Molten metal is poured into the portion of the pattern that forms the pouring cup and sprue. As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus the resulting mold cavity is filled. Advantages of expanded polystyrene process: Pattern need not be removed from the mold. Simplifies and speeds mold-making, because two mold halves are not required as in a conventional green-sand mold. Mass production of castings for automobile engines. A new pattern is needed for every casting Economic justification of the process is highly dependent on cost of producing patterns 6. Plaster Mold Casting: Similar to sand casting except mold is made of plaster (gypsum - CaSO 4-2H 2 O). In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set [see fig. (7) below]. Wood patterns not generally used due to extended contact with water Plaster mixture readily flows around pattern, capturing its fine details and good surface finish. Advantages of plaster mold casting: Good accuracy and surface finish Capability to make thin cross-sections 5
Mold must be baked to remove moisture, which can cause problems in casting. Mold strength is lost if over-baked. Plaster molds cannot stand high temperatures, so limited to lower melting point alloys. 7. Ceramic Mold Casting: Fig. (7): Steps in Plaster mold casting process. Fig. (8): Steps in Ceramic Casting process. Similar to plaster mold casting except that mold is made of refractory ceramic material that can withstand higher temperatures than plaster [see fig. (8) above]. Can be used to cast steels, cast irons, and other high-temperature alloys. Applications similar to those of plaster mold casting except for the metals cast Advantages (good accuracy and finish) also similar. Steps in Ceramic casting process: 1. Preparing the pattern and setting it in flask. 2. A mixture of fine grain Zircon (ZrSiO 4 ), aluminum oxide, bonding agents and water creates a ceramic slurry. The slurry is poured over the pattern and let set. 3. The pattern is removed and the mold is left to dry and then the mold is fired. The firing will burn off any unwanted material and make the mold harder & rigid 4. Pouring the molten metal into the ceramic mold and let the metal to cool and solidified. 5. The finished casting. 6