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Sand Casting
Sand casting
Sand casting is utilized to make large components of iron, Bronze, Brass and Aluminium. Melted metal is poured into a mould cavity made out of sand.
Patterns:
The cavity in the sand is made by utilizing a pattern (a rough duplicate of original) which are generally made of wood and rarely of metal. The cavity is contained in an aggregate housed in a box called the flask. Core is a sand shape put into the mould to produce the inner features of the component like holes or internal passages. Core is put in cavity to form holes of the desired shapes. Core print is the portion added to pattern core or mould which is utilized to find and support the core within the mould. A riser is an extra cavity made in mould to hold excess molten material. The reason of this is to feed the molten metal to the mould cavity and as metal solidifies and shrinks and therefore, prevents holes in the main casting.
In a two part mould, the upper half is called cope and the lower half is called drag. The parting line separates cope and drag. The drag is filled with sand in part and the core print, the cores and the gating process are put close to the parting line. The cope is then joined to drag and sand is poured covering the pattern, core and the gating system. The sand is made compact by vibration and mechanical method. After that the cope is separated from drag and the pattern is removed carefully. The purpose is to remove the pattern without breaking the mould cavity. This is facilitated by designing a draft, a slight angular offset from the vertical to vertical surface of the pattern. This is generally of 1° or 1.5 mm (0.060 in) whichever is more. The rougher the surface of design, the more the draft to be given.
Sprues and runners:
The melted material is poured in the pouring cup, it is part of gating process that provides the melted metal to the mould cavity. The vertical part of gating process connected with pouring cup is called sprue and the horizontal part is called the runners and lastly to the multiple points where it connects to the mould cavity is called gates. Alongside there are extensions to the gating process called vents that give the way for built-up gases and the displaced air to vent to the atmosphere.
The cavity is generally made oversize to allow for metal contraction as it cools down to normal temperature. These are linear factors and apply in each direction. These shrinkages for allowance are approximate because the exact allowance is decided by shape and size of casting. Besides different components of casting need separate shrinking allowances.
Sand casting is a way of making rough metal castings utilizing a mould generally made of sand made around a duplicate of the part to be cast that is removed once the sand becomes compact. Castings made this way may be refined by hammer preening, shot preening, polishing forging, plating, rough grinding machining sand castings not refined further are easily recognizable by sand like texture given by the mould. Therefore, the accuracy of part has to be improved by polishing or grinding. And the mould is broken after every casting to take out the part, a fresh mould is to be made for every casting.
Paths for metal pourin during pouring into mould casting are called the runner system and include sprue, various feeders which maintain a good metal ‘feed’ and ‘runners’ and ingates which attach the runner system to cavity. Gas and steam made during casting exit via the permeable sand and riser are added either in the pattern itself or a separate pieces.
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