Technical Article Wax on Wax off – De-Waxing the Shell by Kevin Lee and Gavin Dooley, REMET®; Howard Pickard, LBBC Technologies T
he de-waxing process is one of the most important steps in the investment casting process. Kevin
Lee has completed immense work on this area for his PhD and has researched many aspects of the process. For this, we have also included LBBC Technologies’ Howard Pickard to give us his extensive insight into the best working practices of Boilerclave®
technology.
Introduction The process of removing the wax is thought to be one of the most important processes to produce a sound investment cast mould. Any microcracks built in during shell build are then evident when the dewax occurs. We know the green or de-waxed shells before firing are vulnerable and fragile. The shell’s components are basically held together by colloidal networks and occasionally polymers and fibres. Most foundries use autoclaving
method to remove wax patterns. We are aware that some foundries are using flash firing to de-wax but I will leave that out for now. The main advantage of using steam as opposed to air (in an oven) is to transfer ‘energy’ to the wax interface layer as quickly as possible. Other advantage of using this method is that the melted wax can be reclaimed and also reduces emission of polluting gas compared to flash firing method. The amount of wax left over after autoclave de-waxing is relatively low (see Figure 1) and therefore less ash produced during the firing process. During the autoclaving process,
the steam instant heat energy source is introduced to the surface of the shell, the solid wax in the shell then starts to change phase and transform into liquid phase. The phase transformation and liquid volume expansion starts to exert stress on the inside surface of shell. The pressure developed due to
26 ❘ November 2018 ® Figure 2: Schematic of the heat transfer in the autoclave (2)
the expansion must be relieved quick enough before the shell cracks. The mechanism for the pressure relieve is that when the steam penetrates the shell and melted the surface of the wax, the
wax will penetrate the shell by passing into the prime coat. The volume of wax lost after penetrating the shell helps to relieve the pressure build up in the shell. At the same time, with the presence of
Figure 1: Morphology of wax on inner surface of shell after Boilerclave® filler particles remain on the surface (1)
dewaxing with
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