< WAX REMOVAL & MOULD FIRING
SINGLE STAGE BURNOUT & FIRING
Before starting a wax burnout, the founder makes sure any protruding wax drain-ways into the mould are clear, and that the riser vent outlets and cup are not fouled with loose investment. The mould is then carefully positioned in a kiln which is constructed in a fashion that encourages air to circulate evenly, and allows evacuated wax to drain away from the kiln body to an outer collection point. Collected wax may later be recycled and reclaimed wax is often used by the founder to form non-critical assembly elements such as pouring cups and wax bars for runners and risers.
The careful placing of multiple investment moulds inside a kiln facilitates an even distribution of applied heat and eases the rapid drainage of evacuated wax. Poor positioning may cause a mould to be over or under ‘cooked’ relative to it’s neighbour, and can make it difficult for wax to rapidly clear the mould. Insufficient drainage to an outside collection point can cause wax to a build up on the kiln floor and potentially ignite as a kiln fire.
Exceptionally large investment moulds which cannot be fitted inside a standard kiln may have a temporary ‘conical beehive’ kiln structure constructed around them. This type of kiln is normally made from fire bricks which are fixed in position with a weak refractory mortar – a basic plaster and grog mix will suffice as an alternative. The temporary kiln can then be fired using a heat source provided either by a portable gas cylinder and torch, or else a wood fire fuelled by a slow burning tree species. For the most part though, the burnout and firing of moulds in modern foundries is a controlled by a more or less 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 investment moulds, are subjected to a gradual increase in kiln temperature over a period of some hours, or in the case of the largest investments, a number of days. The incremental elevation of the temperature inside the kiln allows heat to penetrate through the wall thickness of the mould into to the core mass, this minimises the 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 lead to a mould fracturing, or in extreme cases exploding.
During the early stages of kiln firing, the wax assembly softens before becoming fluid and evacuating the mould. Softening wax expands and increases in volume to reach a point of maximum pressure on the investment’s walls. The investment also expands slightly, but to a lesser degree and at a much slower rate. It is this difference in expansion characteristics that can cause a mould to fracture and fail in the kiln, especially if it has been badly constructed, or formed using inferior refractory materials.
The wax assembly contained within the mould usually drains off some considerable time before the optimum firing temperature of investment 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 or continuous ceramic jacket which alone maintain the air gap between the outer investment mould and inner mass of the core/shell wall.
In the case of moulds constructed using a water activated gypsum binder (plaster and grog), moisture is driven off at a temperature slightly above 100°C. Beyond this temperature point, 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 result varies according to the type of minerals present in the investment and the maximum temperature to which the mould is exposed.
If an investment is raised to too high a temperature, vitrification of the refractory could occur. Vitrification converts silica into a glass substance, effectively ‘sealing’ the mould’s porous fabric and neutralising it’s ability to dissipate casting gases through it’s wall thickness. Highly elevated temperatures also degrade most investments, seriously weakening the mould by lowering particle cohesion.
Too low a temperature in the kiln will result in an incomplete conversion of the investment material, this also causes weakness in a refractory mould. In addition, if the moisture content contained within the mould has not been driven off, there is a risk of steam being generated as the metal charge enters. Steam reactions would almost certainly lead to casting faults, and in more severe cases this could lead to an explosion.
Traditional art foundry investments are usually taken to a kiln temperature of between 1000-2200°F (550-1200°C) according to the refratory content (most are fired toward the lower end of this scale). Investments constructed entirely in ceramic shell are normally fired to a temperature in the region of 1650-1830°F (900-1000°C). Once an optimum temperature 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, and into the internal mass. Selected investment systems, usually those used for producing jewellery and dental casts, are highly sensitive to temperature inaccuracies in the kiln and therefore require precise thermostatic control. An advantage of ceramic shell investments, 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 an investment mould is temperature soaked depends heavily upon the investment system used, and the overall volume of material contained in the mould. A traditional plaster and grog block mould containing a large core mass may require a kiln soak in excess of forty eight hours. With an initial period of kiln temperature elevation, and later temperature reduction, exceptionally large investments may take nearly a week to satisfactorily complete a firing cycle. On average though, mid-sized traditional plaster and grog investment moulds are of a volume and density that requires a three day burnout/firing cycle. This allows approximately twenty four hours each for temperature elevation/wax removal, temperature soaking, and finally, a cooling of the mould prior to casting. Hybrid moulds generally require a slightly shorter firing cycle than equivalent plaster and grog block investments. 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), and in this respect, these moulds are extremely cost and time efficient.
FLASH DE-WAXING FOR SHELL MOULDS >
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