RISER

RISER:- Most alloys shrink during solidification. As a result of this volumetric shrinkage, voids are formed which are known as hot spots. So a reservoir of molten metal is maintained from which the metal can flow steadily into the casting. These reservoirs are known as risers. Design considerations:- The metal in riser should solidify at the end and the riser volume should be sufficient for compensating the shrinkage in the casting. To solve this problem, the riser should have highervolume. Types:- (a) top riser- This type of riser is open to the atmosphere. It is very conventional & convenient to make. It looses heat to the atmosphere by radiation & convention. To reduce this, insulation is provided on top such as plaster of paris and asbestos sheets. (b) blind riser :- This type of riser is surrounded by the moulding sand and looses heat very slowly.
(c) Internal rise:- It is surrounded on all sides by casting such that heat from casting keeps the metal in the riser hot for a longer time. These are used for cylindrical shapes or hollow cylindrical portions casting. Chill:- Metallic chills are used to provide progressive solidification or to avoid the shrinkage cavities. These are large heat sinks. Use of chill will form a hard spots, which needs further machining.