The total pyrolysis rate of the binder,ρ sum over the actions of the individual rates, Òi
, is then given as the :
Equation 4 where ρb is the initial binder density in the core which can
also be written as a product of the initial binder fraction and the density of the sand.
The first rate identified in3 can be shown to be active already
at room temperature. Specifically the exponential decay time for ∆ω1
is 570 seconds in a room temperature holding ex-
periment. This indicates a fast outgassing reaction such as possibly volatilization of the solvent right after the core is hardened.
The rest of the rates in Table 2 will only produce an appre- ciable amount of gas products under casting conditions. The temperature range in which each rate would be active in a 20C/s (68F/s) steady heating experiment is given in columns four and five of Table 2. A multi-rate model is generally required to accurately represent the wide temperature range of this pyrolysis process.
To further highlight the features of the model, the pyroly- sis speed and binder fraction are computed for a simple flat wall case. In this numerical experiment, aluminum at 720C (1328F) is at the sand core wall at t=0. The heat transfer coefficient to sand is 500 W/m2 ously reported values.8
/C, in the range of previ-
the heat capacity of the bonded core sand should be well ap- proximated by values for dry silica sand.9
Because the binder fraction is low, The thermal con-
ductivity was measured directly in the course of this work with the aid of a transient plane source thermal conductivity system TPS 2500 S.10
It was found to track thermal conduc-
tivity values of the green molding sand to 300C (572F). The PUCB bonded core sand had lower thermal conductivity above 300C (572F), with the largest difference of 30% at 500C (932F).9,10
The results of the numerical pyrolysis experiment are plotted in Figure 1. On the right is the binder fraction in the wall at 7, 15 and 30 seconds. It can be seen that the residue fraction is 40% and the middle of the pyrolysis zone moves into the core wall at approximately 0.1 mm/s. The computed pyroly- sis speed is plotted on the left of Figure1 and shows a peak of 0.13 mm/s at 1.6 seconds and a decay to a value of 0.075 mm/s at 30 seconds. As the pyrolysis zone moves away from
Table 1. Volatile Chemical Compounds Identified by Lytle et al.1 PUCB Isocure®
in a GC/MS Analysis of Pyrolysis of
Performed at 900C (1652F). 92.3% (by Molar Content) of These are Still Volatile at Room Temperature and the Gas Constant for this Mixture is 230 J/kg/C
*See Reference 7. **These are assumed values, for no tabulated values for these gases could be found. International Journal of Metalcasting/Summer 2011 59
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