Casting
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Sand casting Sand which can be natural or synthetic is used as the mold materials. Patterns which typically made out of wood or metal are made for bottom and top halves. Core that made of sand mixed with binder is used to provide cavity within casting. One half of the pattern is placed down on a box which form the bottom halve of the mold, this called drag. Casting sand is then poured into the box and is compacted to obtain a good shape. Then, the drag is turned over that revealing the pattern mold face. White powder which acts as the parting agent is dusted on the surface. The top half of the pattern is added. The patterns for the riser and down sprue are positioned on the drug. While the runner, groove which allow molten metal to flow between them is marked in. A top box which called cope is placed top on the drag. The riser and down sprue is replaced. Sand is poured on the half cope and compacted by vibration or by flinging the sand. Next, the cope is carefully removed from the drag, and the pattern is extracted without damaging the sand mould cavity. The molten metal is poured in the pouring cup connected by a vertical sprue to the horizontal runner. This connects to the casting by gates - which need to be cut off later. Vents are also needed to allow air to escape.
description
Simple notes regarding the process of various types of casting like sand, investment,die and lost foam casting.
Transcript of Casting
Sand casting Sand which can be natural or synthetic is used as the mold materials. Patterns which typically made out of wood or metal are made for bottom and top halves. Core that made of sand mixed with binder is used to provide cavity within casting. One half of the pattern is placed down on a box which form the bottom halve of the mold, this called drag. Casting sand is then poured into the box and is compacted to obtain a good shape. Then, the drag is turned over that revealing the pattern mold face. White powder which acts as the parting agent is dusted on the surface. The top half of the pattern is added. The patterns for the riser and down sprue are positioned on the drug. While the runner, groove which allow molten metal to flow between them is marked in. A top box which called cope is placed top on the drag. The riser and down sprue is replaced. Sand is poured on the half cope and compacted by vibration or by flinging the sand. Next, the cope is carefully removed from the drag, and the pattern is extracted without damaging the sand mould cavity. The molten metal is poured in the pouring cup connected by a vertical sprue to the horizontal runner. This connects to the casting by gates - which need to be cut off later. Vents are also needed to allow air to escape.Generally, sand castings have a rough surface with surface impurities and surface variations. A machining (finish) allowance is made for this type of defect.
Investment casting
Investment casting process also called the lost wax process can be used for high temperature metal and hard machined alloy. Complex shape can be formed in high accuracy however it takes a week or months to manufacture it. At first, a wax model is made for the process. Usually, wax is used rather than plastic because wax is easy to melt and can be reused. Wax patterns are made and attach to the wax bar which serves as the central sprue. Then, this wax model is dipped in the wet ceramic slurry whose composition includes extremely fine grained silica, water and binders. A thin ceramic layer can be seen over the surface of wax model. The pattern is repeatedly dipped in the slurry until the layer becomes thicker and thicker. Once the coat is thick enough, it is allowed to dry in air in order to make it harder. After drying, the wax is removed from the ceramic mold. The ceramic mold is then heated to a temperature of around 1000 degree Celsius for 2-3 hours to remove any moisture, residual wax and prevent cracking when it comes to contact the molten metals. The molten metals are poured into the hot ceramic mold. A better and dimensional accuracy is obtained since the mold and casting will shrink together as they cool. After cooling, the ceramic mold is then broken from the investment casting by vibration and individual parts are cutting down. Investment casting is a manufacturing process that allows the casting of extremely complex part with good and smooth surface finish. It allows the tolerances as low as .003in (.076mm) which considered is in a high dimensional accuracy.
Die castingDie casting is a metal casting process that forcing molten metals under high pressure into the mold cavity. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter and tin based alloys. Cold or hot chamber machine is used depending on the type of metal being cast.Cold chamber casting
As the die cast mold is closed, the molten metal is poured into the filing chamber. The molten metal is slowly pushed towards the die cavity by injection piston. The air is first removed from the cavity before molten metal entered it. Within a second, the die cavity is filled with the molten metals. Molten material is cooled in the cavity and solidified under pressure. After that, the die is opened to remove the casting. The die casting machine is cleaned and sprayed with releasing agent before the next cycle is started.Hot chamber casting The pressure chamber which connected to the die cavity is immersed in the molten metal. The plunger is positioned at the top of the hot chamber (unpressurized) which makes the inlet port of the pressurizing cylinder becomes uncovered. When the plunger is forced downward by power cylinder, it will travels past the ports and cutting off the flow of liquid metal to the hot chamber. At this point the plunger travels further downward which forcing the molten metal into the cavity. Usually, the pressure exerted on the molten metal in hot chamber that needed to fill the cavity is around 5MP to 35MP.The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc and lead that tend not to alloy easily with steel at their melt temperatures.Die casting gives close tolerance, as for zinc which has a tolerance of .0015in which is very high in dimensional accuracy. Die casting process can also manufacture a smooth surface finish casting with thin intricate walls. However, the equipment cost of die casting is relatively high compare to others.
Lost foam casting
Lost foam casting is a process that similar to the investment casting except foam is used instead of wax. Metals such as cast irons, aluminium alloys, steels, and nickel alloys are casted by lost foam casting process. For large and low quantity parts block of Expanded Polystyrene (EPS) foam, they will be machined to the shape of final metal part using hot-wire foam cutter.. However, smaller but high in quantity can be made using EPS steam injection. Then, the surface of the machined EPS parts will be sprayed or painted with a refractory coat. The painted EPS part is placed in a mold. Feed and upriser components are added. The mold is filled with casting sand and compressed. Next, molten metal is poured through the feed. This high temperature of metal will burns away all the EPS foam and leaving only the same shape in metal. The casting is removed from the sand mold and the remaining paints or sand is removed by vibration. The feed and upriser components are removed. Cast product is finished.
Lost foam casting is generally more economical than investment casting because it involves fewer steps. Foam is easy to manipulate, carve and glue, due to its unique properties. Lost foam casting gives a high dimensional accuracy and excellent surface finish.
