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Chapter 9 – Change of FormMetal Casting: Solidification of molten metal poured into a mold or injected into a die.4 Most Popular Casting Processes:Sand casting, Investment casting, Permanent mold casting, Die castingSAND CASTING (Expendable Mold classification) – mold is produced around a pattern that is withdrawn to leave a cavity. Sand allows gasses & stem to escape. Used only once and must be broken up to free the solidified casting Important factors in casting operations: The flow of the molten metal into the mold cavity Heat transfer during solidification and cooling of the metal in the mold (The metal grain sizes are smaller near the mold than in the center) Influence of the type of mold material Solidification of the metal from its molten stateChill Zone - area near the mold where the metal cools faster and causes smaller grains to form, thus making this area stronger (ex. rubber band). Melting and pouring- Charge - the mixture of alloying elements that will be at the specified composition upon melting- Fluxes - various compounds that can be mixed with the metal to react with the melt- Melt - the molten metal that will be used to produce the casting- Tapping - when melt reaches proper temperature, method used to extract the metal from furnace- Pouring - transferring the tapped material to the casting Casting techniques:- Mulling is the mixing of the sand with the bonding agent. - Riddling is the sifting of the sand into the flask. - Green-sand casting uses water is as the bonding agent for the sand. When the water dries out, the mold falls apart. - Shell molds use a resin or other organic bonding agent and are baked to harden the mold. Since they do not dry out, they last a long time. 1Parts of a Sand Casting:- Pattern is the shape that is to be cast, about which the Mold is made. - Mold is the container into which liquid metal, plastic, or other material is poured in making a casting. - The Cope is the top half of the casting flask. The Drag is the bottom part of the flask. - Pouring Basin - Molten metal is poured through a pouring basin (cup) andflows through the gating system.- Sprue - vertical channel through which the molten metal flows downward into the mold - Runners - channel to carry molten metal from sprue to gate- Gates - portion of the runner through which the molten metal enters the mold cavity, traps contaminate, prevents gas buildup and provides for ease of separation- Riser (Feeder head) – let out trapped air, reservoirs to supply the molten metal necessary to prevent shrinkage during solidification.Casting Alloys Cast steels - readily welded to build up components of unusually large size and complexity- Railroad equipment (wheels, truck, frames, couplers)- Construction and mining equipment- Metal working machineryWhite cast irons - hard and brittle, wear resistant parts (virtually unmachineable except by grinding)- Grinding balls- Liners for ore-crushing mills- Some agricultural machinery partsGray Iron - (low cost) is preferred choice in all fields where ductility and high strength are needed- Weights- Motor gears- Pump and pipe fitting- Machine tool bases (damping capacity)2INVESTMENT CASTING (lost wax, Expendable Mold classification) – Pattern made of wax or polystyrene. Dipped into refractory material, then wax is melted out and metal is poured in. Advantages of investment casting versus sand are greater detailed parts, better surface finish, less machining, less tool wear.PERMANENT MOLD OR GRAVITY DIE CASTING- Made of iron or steel andis mostly used with aluminum alloys.- High cost of manufacturing and maintaining a permanent mold. - Used for high-volume production runs.DIE CASTING ALSO PERMANENT MOLD – Molten aluminum or magnesium is forced into die cavity by hydraulic ram (1000-30,000 psi). Die is cooled by circulating oil or water.- Hot-chamber die casting is used for low-melting temperature casting materials. - Cold-chamber die cast is used for higher melting temperature alloys. Investment casting is a one-shot, one-mold-per-part method in which the temporary mold is poured around the pattern. The investment mold is destroyed after casting.Die casting uses permanent, re-usable molds.


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