Wax Mould Burnout Furnaces

The investment casting process is a technique for producing castings of precise dimensions and clean surface finish. It is widely used to make aerospace components, complex shapes used in cars and trucks, jewellery, 3D artwork and statues, golf club heads, replacement hip joints and many other metal pieces.

In its simplest form investment casting involves making a wax precursor in the shape of the required product, forming a ceramic mould from the precursor and then pouring liquid metal into the ceramic mould. The metal solidifies and the product can then be broken out of the mould.

The ceramic mould is produced from one or more precursors and runner bars. Typically a tree of precursors is dipped into ceramic powders and binders in a controlled sequence to build up a ceramic shell. The coated tree is then heated in a steam autoclave which melts out the wax leaving the ceramic layer. This mould is still 'green' but is sufficiently strong to be transferred to the burnout furnace.

The mould burnout operation has two purposes: to fire the ceramic shell, creating sufficient strength for the metal casting; and to remove all the wax that remains after autoclaving. It is this wax removal function that can produce a lot of smoke.

There is always some wax remaining in the mould after autoclaving. The non-draining areas of the mould trap pools of wax; also some wax just sticks to the internal surfaces of the mould.

The mould burnout furnace is a lot hotter that the autoclave and almost immediately the mould is inserted the wax melts and then vapourises. As long as there is sufficient oxygen the wax vapour just burns to CO2 and water. However, as soon as the oxygen starts to become depleted it reacts only with the hydrogen part of the wax molecule leaving long carbon chains of soot. This soot is emitted to atmosphere and appears as a dense black smoke. In cases of more serious oxygen shortage there is also a smell of wax.

The soot particles can be removed from the furnace flue gas by filtering it with a ceramic filter. This is an effective technique which in nearly all cases yields a smoke-free emission. The upper photo shows the soot collected by a ceramic filter in 2-3 days operation.

Caldo has considerable experience supplying ceramic filters for this duty. The lower photo shows a typical Caldo ceramic filter suitable for a 10 therm gas-fired mould burnout furnace.