< METAL FEEDING & VENTING
RUNNERS
Runners are connected to the pouring cup at the head of the wax pattern and terminated at lower, more remote areas. The runners are eventually burnt out in the kiln, together with other attachments and the wax pattern itself to leave a negative ‘air gap’ impression of the entire wax running system and pattern design inside the investment mould. During the pour, metal enters the cup and is then distributed to the various parts of the mould cavity, ensuring the mould fills completely before the charge freezes.
Because the runners are burnt off in the kiln, the founder must construct this feature from a material that easily breaks down when heated. Almost all art founders use wax runners, though alternative materials such as balsa wood, straw, bamboo, flexible polystyrene and plastics can also be used. The Greeks were known to have used hollow ceramic tubes to flow metal into their moulds [ref], and some modern sand founders use exothermic feeder sleeves to direct metal into sand moulds. Few art founders though, find reason to use non-wax runners for feeding lost wax castings.
Runners are usually formed within plaster or other rigid moulds as this allows large quantities of suitable tubing to be produced quickly and easily. Many founders use recycled and filtered wax to make runners and similar attachments, this wax is reclaimed from earlier kiln burn-outs. Others buy in proprietory wax runners from specialist foundry suppliers. Commercially produced runners are often hollow and specially formulated to collapse at low kiln temperatures, this helps minimise the pressure on the investment mould’s wall from expanding wax, reducing the chance of a mould stress fracture. Steel bars can also be inserted into hollow runners to support an entire wax assembly, this is especially common when investing large scale ceramic shell moulds.
Runners of various sizes are usually made round or square in cross section, though commercially produced runners may be available in a variety of other sections, including triangular. The advantage of using square or triangular sectioned runners is that these shapes tend to reduce turbulence and eddying in the metal charge as it enters the mould cavity, thus smoothing flow and minimising the potential for casting faults.
The exact layout of a runner system depends upon a large number of factors including the form and scale of the sculpture, the investment material used, and the founder’s own personal ‘style’ and preferences. Judgement in the art foundry is usually determined by long experience rather than by rigid formulae, so the information given in the following paragraphs can only be offered as an overview.
The main aim of the founder is to fill the investment mould as quickly and smoothly as possible, using the minimum number of runners possible. Excessive numbers of runners can interfere with the surface quality of the cast, they also use up quantities of metal and generally add to the overall workload. By the same token, too few runners (or runners placed incorrectly), can result in mis-runs and incomplete casts.
As well as designing a runner system to fill the investment cavity, CERAMIC SHELL founders may construct the runner system in a way that also supports the wax assembly during the investment cycle. Small scale assemblies can be stood on their pouring cups, with the runners used as a structural device to ‘suspend’ the wax pattern clear of any surface that might damage newly deposited investment layers. Larger waxes may need additional support, this can be provided by passing steel bars through the main wax runners. The exposed ends of the steel support are formed into a loop or stand that can be used to hang or support the drying assembly. The exposed steelwork is usually given a light coating of wax, this prevents the slurry from being contaminated with oxides from the steel, and also helps ease the removal of the steelwork from the investment after firing in the kiln.
In all cases, the main runners to the wax pattern are collected into a ‘bunch’ at the ‘head’ of the mould. The pouring cup is then directly attached to the bunch, effectively completing the runner system. Main or PRIMARY RUNNERS may follow the entire length of the wax pattern and terminate close to the base of the design. The founder will also want to distribute metal to intermediate parts of the mould, so shorter ‘branches’ or SECONDARY RUNNERS are taken off a main runner at appropriate points along it’s length and fixed to the wax pattern (this gives the average wax assembly it’s typically distinctive ‘arterial’ appearance).
We have already examined how the founder can use the pouring cup design to control the initial flow of metal into the investment mould. The founder can further regulate the flow of metal by adjusting the cross sectional area of the runners where they connect with the mould cavity. Tapering down a wax runner where it connects with the wax pattern (the INGATE), will create a ‘choke’ effect, increasing the pressure and flow rate of metal entering the mould. If on the other hand the founder flares the runner, the pressure is decreased and the flow rate of metal into the mould is reduced. By carefully balancing the size and shape of the pouring cup, and the distribution and layout of the runner system, the art founder can except a great deal of control over the way in which an investment mould fills.
DIRECT & INDIRECT RUNNERS >
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