SINGLE STAGE WAX BURNOUT & FIRING
Before commencing a wax burnout, the founder makes sure wax drain-ways are clear and RISER/VENT outlets and CUP are not fouled with loose refractory. The mould is then carefully positioned in a kiln which has been constructed to encourage air to circulate evenly and allow evacuated wax to drain off. When collected, spent wax may later be recycled, reclaimed wax often used by the founder to make non-critical assembly elements such as pouring cups and wax bars for runners and risers.
The careful placing of multiple refractory moulds inside a kiln permits the even distribution of heat and aids the rapid discharge of evacuated wax. Poor positioning may cause a mould to be over or under ‘cooked’ relative to it’s neighbour as well as making it difficult for wax to clear the mould. Insufficient drainage can also cause wax to a build up on the kiln floor and potentially ignite as a wax fire.
Exceptionally large refractory moulds which do not fit inside a standard kiln may instead have a temporary ‘conical beehive’ kiln constructed around them. This type of kiln is normally made from refractory fire bricks which are fixed in position with a weak refractory mortar – a basic PLASTER & GROG mix can pass as an alternative. The temporary kiln can then be fired using a portable gas cylinder and torch as the heat source, or else a lit wood fire which is fuelled from a slow burning tree species. For the most part though, the burnout and firing of moulds in modern foundries is a controlled by a automated or semi-automated process. Programmed kilns usually require little supervision on the founder’s part once the firing cycle is initiated.
PLASTER & GROG moulds together with HYBRID moulds are subjected to a gradual increase in kiln temperature over a period of some hours, or in the case of the largest refractory moulds, days. This incremental elevation of the temperature inside the kiln allows heat to penetrate through the wall thickness of the mould into to the core mass, minimising the adverse effects of thermal expansion. Too sharp a temperature gradient can lead to thermal shock and the premature expansion of the refractory or contained wax assembly, this can cause the mould to fracture, or in more extreme cases explode.
During the early stages of kiln firing, the wax assembly softens before becoming fluid and evacuating the mould. Softening wax expands significantly and increases in volume to reach a point of maximum pressure on the refractory mould’s walls. The refractory itself also expands slightly, but to a lesser degree and at a much slower pace. It is this difference in expansion characteristics (co-efficients), that can cause a mould to fracture and fail in the kiln, especially if it has been badly constructed or built from inferior refractory material.
The wax assembly contained within the mould usually drains off some considerable time before the optimum firing temperature of the refractory has been reached. A marginally oxygen rich (OXIDISING) atmosphere inside the kiln ensures any residual carbon deposits left by the spent wax are eventually burnt out of the interior. By this stage, it is the core pins (plaster & grog) or continuous ceramic jacket (ceramic shell), which alone maintain the air gap between the outer mould and inner mass of the core/shell wall.
For moulds constructed with water activated gypsum as a binder (plaster and grog), moisture is driven off at a temperature slightly above 100°C. Beyond this temperature, the plaster component starts to convert to an anhydrous calcium sulphate. The mineral silica content within the mould also undergoes a chemical change, though the ultimate product varies according to the exact minerals present in the refractory and temperature levels the refractory is exposed to.
If a refractory material is raised to too high a temperature, vitrification of the refractory could occur. Vitrification converts the silica content into a glass substance, effectively disabling the mould’s porous qualities and neutralising it’s ability to dissipate casting gases through the mould's wall thickness. Highly elevated temperatures also degrade most refractories, seriously weakening the mould by lowering particle cohesion.
Too low a temperature in the kiln will result in an incomplete conversion of the refractory material, this also causes weakness in a refractory mould. In addition, if the moisture content contained within the mould has not been driven off to an acceptable level, there is a risk of steam being generated during contact with the metal charge. Steam reactions would almost certainly lead to casting faults and in more severe cases could result in an explosion.
Traditional PLASTER & GROG moulds are usually reach an ultimate kiln temperature of about 1000 °F (550 °C), according to the refractory content and the founder's experience. Refractory moulds constructed entirely in CERAMIC SHELL are can be fired to a much higher temperature, in the region of 1650-1830°F (900-1000°C) is common. Once the optimum temperature for the process has been attained in the kiln, the mould is held at a constant level or SOAKED for an extended period of time. Soaking ensures the applied heat fully penetrates through the wall thickness of the mould and into the internal mass (esp cores). Some refractory moulding systems, usually those used for producing jewellery and dental casts, are highly sensitive to temperature inaccuracies in the kiln and require precise thermostatic control. An advantage of ceramic shell refractories, is that they are able to tolerate a greater degree of imperfect firing without adverse effect to the mould or cast.
The length of time a refractory mould is temperature soaked depends heavily upon the refractory moulding system used and the volume of material used in the mould. A traditional plaster and grog block mould with a large core mass may require a kiln soak well in excess of forty eight hours. Combined with the initial period of kiln temperature elevation and later, temperature reduction, exceptionally large moulds of this type may take a week or more to complete a burnout/firing/cooling cycle. On average though, mid-sized traditional plaster and grog moulds are of a volume and density that requires a three to four day burnout/firing cycle. This suggests approximately twenty four hours each for temperature elevation/wax removal, temperature soaking and finally the cooling down of the mould prior to casting. Hybrid moulds generally require a slightly shorter firing cycle than equivalent plaster and grog block moulds. OPEN CORE ceramic shell moulds need significantly less kiln time than the moulds of any other system (from a few minutes to a couple hours at most), in this respect, shell moulds are extremely efficient.
Plaster & grog block moulds
placed in a traditional kiln.
Flasked refractory mould
inside a modern electric kiln.
(Photo's cc ANPP)
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