Use of Insulation to Increase Internal Homogeneity of Castings


In order to achieve high internal homogeneity, it is neces- sary to have in the heat axis of castings at solidification a sufficient temperature gradient, i.e., a sufficient longitudi- nal temperature gradient. X-ray examination of test castings revealed the occurrence of axial porosity, in particular in cylinders of 10 mm (0.393 in.) and 20 mm (0.787 in.) in diameter. Experiments were therefore carried out, the aim of which was to establish the temperature gradient in thermally non-insulated, insulated and partially insulated shell moulds. The Sibral blanket was again used for insulation.


Measurement was conducted on shell moulds with 3 cyl- inders of 50 mm (1.968 in.) in diameter: one cylinder was not insulated, one was insulated along the entire periphery, and the third was insulated in the upper half. Two PtRh-Pt thermocouples were placed in the axis of each cylinder, namely at ¼ and ¾ of the cylinder height (Figure 16). The temperature waveforms can be seen in Figure 17 (“U” = up- per part, “L” = lower part of cylinder). It is obvious that the temperature gradients along the height of non-insulated and insulated cylinders are minimal. Using partial insulation resulted in a high temperature gradient. This high gradient is a precondition for achieving a high internal homogeneity of castings.


(a)


(b)


Figure 13. Thermally insulated mould a) Insulation along the shell mould periphery and b) Insulation of individual castings in the shell mould.


Figure 14. Temperature curves in thermally insulated mould – insulation of periphery.


International Journal of Metalcasting/Spring 10


Figure 15. Symmetry of temperatures on inner and outer sides of shell mould.


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