Q. Why is it desirable to design a casting to have directional solidification sweeping from the extremities of the mould to riser based on Chvorinov's rule? What would be an ideal shape for a riser? Discuss a desirable shape from a practical perspective.
Ans. The optimum riser diameter for a given casting can be opened by a Chvorinov's Rule :
Chvorinov's Rule : It states that the freezing time of a casting.
t = q2 (A)
q = solidification constant, depending on the composition of cast metal and the positioning of the mould cavity, like, along a vertical or a horizontal axis.
V = ratio of volume of the casting to its surface area.
Values of (V)2 and freezing time have been found out by actual
experiments for different metals and alloys and for different cylinder diameters.
Best riser is one whose (V)2 is 10% to 15 larger than that of the casting.
Riser Design : If no riser is provided during casting, the solidification will start from walls and liquid metal in the centre will be surrounded by a solidified shell and the contracting liquid will produced voids towards the centre of the casting. Further cooling of the solid in centre sets up undersirable stresses in the casting.
Provision of risers overcomes these problems as these supply molten metal for a solidifying casting. For this purpose, the risers must be large enough to remain liquid after the casting has solidified and must contain sufficient metal to provide for the contraction losses. Further these should be so positioned that they continue to supply metal throughout the solidification period.