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FLASH DE-WAXING FOR SHELL MOULDS
Founders using the OPEN CORE ceramic shell system, can opt to burnout the wax assembly under an immediate and intense heat. This technique dramatically reduces the time needed to reach optimum temperature levels in the kiln compared to traditional investments, which are instead fired on a far more gradual temperature gradient model.
An open core CERAMIC MOULD can be rapidly fired and attain it’s full working properties after a relatively short period of kiln soaking. This method of rapid wax evacuation and mould firing is often referred to as FLASH DE-WAXING, so called because the initially high kiln temperature has the effect of ‘flashing’ wax out of the investment mould very quickly. Unlike the comparatively fragile PLASTER & GROG mould, a ceramic shell investment survives flash de-waxing intact due to the refractory’s excellent strength, thermal resilience and expansion characteristics.
The absence of a solid core mass and an even wall thickness in the mould, allows the entire shell mould to expand and contract at a more or less uniform rate, thus reducing the potential for damage to the fabric of the investment. Similarly, the design and construction of the mould, in particular the relatively thin investment wall, allows heat to penetrate into the inner volumes of the mould over a comparatively short period of time. The thermal efficiency of the ceramic investment is such that it is possible to completely burnout and fire small moulds without a kiln, simply by applying direct heat from a propane torch.
The kilns specifically built for for flash de-waxing differ from traditional investment kilns in a number of ways. Flash de-wax kilns are designed and fuelled to attain a pre-heat temperature of at least 2200°F (1000°C), before to any investment mould is inserted into the kiln body. To enable a rapid temperature build up and instant control, these kilns are nearly always gas/air fuelled. Because the mould is going to be introduced into a pre-heated kiln body, an access system (typically a vertical lift mechanism directly attached to a removable kiln base), enables the mould to be safely set up on a base remote of the kiln interior, then elevated up into the raised kiln body once conditions are suitable. A collection tray for evacuated wax is positioned under the kiln base, which in turn has built in drain ways to direct wax away from the kiln body and into the tray.
The ceramic shell mould is normally placed with the pouring cup face down to the kiln base, this enables wax to easily drain from the mould cavity via the cup, then out from the kiln body. Although the pouring cup is usually the main route of wax evacuation, temporary drains may also be pre-drilled through the investment wall and into the underlying wax pattern. Wax can easily escape the upper reaches of the inverted mould through drilled holes, helping to minimise the internal pressure exerted by expanding wax. Once the kiln base carrying the moulds has been raised up to the kiln body, heat rapidly penetrates through the thin walls of the ceramic investment causing an evacuation of the contained wax assembly. When the wax assembly has been flushed form the mould (usually within a few minutes, even for the largest investments), the temperature of the kiln is lowered to some 1470°F-1650°F (800-900°C) to allow for a controlled firing of the refractory. The amount of time that the mould remains within the kiln for firing is determined by the size and wall thickness of the mould. However, kiln soak times for open core ceramic shell moulds are normally a fraction of those for an equivalent sized work invested in a plaster and grog block mould.
Once the investment is fired, the kiln burner is shut down and the platform retaining the kiln base lowered away from the kiln body to allow for a visual inspection of the mould. Any released metal hooks (inserted into the wax assembly for suspending the drying investment during mould construction), can now be removed and discarded. Any minor mould faults and temporary drain holes in the investment wall can be plugged and repaired using refractory cements formulated for this purpose. Ceramic shell moulds repaired with refractory cements can then be returned to the kiln and briefly fired again to ensure a sound repair.
Upon completion of the burnout/firing cycle, the founder is able to remove the still hot ceramic shell mould directly from the kiln to the foundry floor, where a prepared metal charge can be poured immediately into this pre-heated investment. This technique of hot mould pouring allows the founder to produce casts of a much thinner metal wall thickness than possible when pouring cooler or room temperature moulds (the molten charge chills rapidly on contact with investment, arresting it’s flow). As well as improving the flow of metal through the mould, pouring metal into a pre-heated mould also minimises the thermal shock and rapid expansion to which the investment is subjected during casting, thus minimising the opportunity for damage or loss of the mould at this critical stage of the process.
The speed of the ‘flash’ burnout and firing sequence is such, that in combination with modern rubber moulding compounds, and open core ceramic shell investment techniques (especially the alcohol based ethyl silicate system), it is theoretically possible for a founder to produce a small, hollow metal cast from an artist’s master pattern, within a twenty four hour time period – a feat which would normally be considered impossible using traditional investments.
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