Heat Transfer Stages


Steam energy transfer across the shell


Shell Properties


Thermal conductivity Heat transfer coefficient Thermal expansion Specific heat capacity


Heating of the wax


Thermal expansion Thermal conductivity Density


Specific heat capacity


Melting of the wax and flow out of mould


Shell and wax interactions and properties relating to failure


Permeability Mechanical properties


Melting point Viscosity


Surface tension


Wax Properties


polymer, the shell offers a stronger, more forgiving system to absorb the pressure build up. The schematic of heat transfer during autoclaving and complete mechanism that occur during dewaxing is illustrated in Figure 2 and Figure 3 respectively. The mechanism will be discussed in two topics – Shell and Wax.


Shell Properties - Prime coat It is important to relieve sufficient pressure through the prime without compensating the surface roughness and integrity of the prime coat. Polymers are sometimes added to the prime slurry which offers flex in the shell and absorb the wax expansion. More advantages of adding polymers into the slurry has been addressed by Dr. Gavin Dooley in his AdBond®


Quikset™ Figure 3: Mechanism that occur simultaneously during the de-waxing process.


article (RemetUK


Insights). For good shell permeability, the prime coat must be readily wetted by the liquid wax. Factors influencing the wetting of a surface includes prime coat surface roughness, heterogeneity of the surface, temperature, rate of wetting and wax liquid characteristics. A simple test was carried out by Jones et al. in Figure 4 to show the wettability of wax on the prime coat at different temperature. The test shows that the contact angle (wettability) of wax on prime coat does varies with temperature.


Shell Properties - Back Up Coat Back up coats require the most attention when it comes to preventing shell cracking. Back up coats offers the extra strength required to prevent shell cracking. Back up coats can vary from as little as 4 coats up to 10 or even 12 coats depending on the slurry system and mechanical strength increases in correlation to the shell thickness. Increasing shell thickness would however increases the heat capacity of shell in the Boilerclave®


and in turn


Figure 4: Plan view of wax contact angle results at different temperature: (a) side view at 21°C : (b) side view at 180°C (3)


®


more steam energy is required to melt the wax. It is therefore important to have a balance of porous shell system to allow adequate amount of steam to penetrate the shell and good mechanical strength. Again, addition of polymers will increase the green strength and therefore increase the survivability rate


November 2018 ❘ 27


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