The amount of accumulated heat depends on the ratio of metal mass to mould mass, and on the initial mould tempera- ture. When pouring into moulds with a high initial tempera- ture (subsequent to annealing) the significance of heat ac- cumulation is reduced and, on the contrary, the significance of heat radiation increases. The heat accumulation capability of shell moulds is significant in particular in thin-wall cast- ings with short solidification times. In thick-wall castings of compact shape the heat portion removed from the mould into the surroundings is of greater significance.
Heat transfer from the mould into the surroundings proceeds via convection and radiation. The bottom part of shell mould is also affected by heat transfer by conduction into the sand bed. When making castings of complex shapes and in the case of moulds made up of several castings the individual walls and cluster elements are mutually irradiated by heat. Heat transfer must therefore be solved as transfer in a completely or partially closed space, where both the shape configuration of the mould itself and the radiation effect of the surroundings Qsurr – 1
have to be taken into consideration (Figure 2).
The overall intensity of heat flow from the mould into the surroundings depends on the difference between the tem- perature of the outer mould surface Tm of the surroundings Tsurr
and the temperature
total effective heat transfer coefficient αtot by the convection αcon
and the radiation components αrad
, on the cooled surface S, and on the , which is formed .
dQm-surr = αtot αtot = (αcon
(Tm + αrad )
– Tsurr
). S . dt
Equation 1 Equation 2
This heat situation can be solved for real shell mould con- figurations by numerical simulation only. For the calcula- tion it is necessary to analyze, with sufficient precision, the boundary conditions and the effect of geometrical layout of the whole system.
Solution of Heat Transfer in the Model Mould
Heat flow was quantitatively analyzed on a model shell mould formed by 8 cylindrical castings (50 mm in diameter and 300 mm in length) placed symmetrically in vertical po- sition on the pitch circle around the central gating system (Figure 3). The shell mould is placed in a bed of dry sand, with a sheet-metal cover around the cluster.
Figure 3. A model of a shell mould formed by 8 cylindrical castings. This is a ground plan without details of the inlet system).
(a) (b)
Figure 1. Portions of accumulated and removed heat in massive moulds and shell moulds during metal solidification a) pouring into a massive mould and b) pouring into a shell mould.
Figure 2. Heat transfer by radiation among mould elements is shown.
72
is shown.
Figure 4. Dependence of mould radiation coefficient of heat transfer αrad
on mould temperature and emissivity International Journal of Metalcasting/Spring 10
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