< PREPARING WAXES FOR INVESTMENT
THE POURING CUP
Also sometimes known as a POURING BASIN (especially when sand casting), and referred to in jewellery casting as a BUTTON, the pouring cup has the obvious function of providing an opening into the investment mould for the introduction of molten metal. To think of the pouring cup as nothing more than an entrance in the investment however, is to rather oversimplify what is a very diverse and effective feature.
Positioned at the highest point (the ‘head’), of the wax assembly, and directly attached to the primary RUNNERS, the pouring cup can have a variety of quite different functions during the investment and casting process. Even before any metal is poured into the cup, it can act as a stand for the drying investment whilst the refractory mould is built up. Later on in the process, the cup may support the investment mould when stood in the kiln. As the temperature rises in the kiln, the cup acts as the main drain, enabling molten wax to rapidly run out of ceramic shell investment moulds.
The vast majority of sculptures in the art foundry are cast by a gravity system. This means that no artificial force, such as a vacuum or centrifuge is applied as metal enters the investment mould. Instead, the founder relys upon gravity to force the metal into the far reaches of the investment. The pouring cup is critical in controlling entry of metal into the mould. Both the size, and the design of the cup can be adapted to influence the flow and force of metal as it enters the running system, so it is important that the founder uses an appropriate sized and shaped cup for the job in hand.
Wide brimmed/shallow pouring cups (think of a shallow oil funnel), tend to dissipate the force of an entering metal CHARGE , this style of cup design usually leads to a slow, gentle filling of the mould – though too slow and gentle and the mould may not fill properly, leading to a ‘short run’.
Deeper cups (large polystyrene soda drinks containers for example), encourage a ‘column’ of metal at the head of the mould. This type of cup tends to maintain a higher pressure on the entering charge – filling the mould more forcefully and rapidly than a shallow cup. However, if the cup is too large and deep for the job, there is an increased risk of bursting the investment mould as it fills, the failure usually due to excessive pressure of molten metal on the investment mould’s outer walls.
To summarise, extreme designs of either type of cup invite casting difficulties. Commercial founders often use standard formulae or computer programmes to determine the optimum cup size and design for each job. Art founders on the other hand, nearly always have to rely to experience and judgement, this is because of the unique and individual nature of much of their workload.
As well as providing an entrance and a ‘head pressure’ of entering metal into a mould, the cup has another important function which comes into play once the mould has filled with metal, the control of shrinkage and contraction. In brief, shrinkage and contraction are a common feature of materials transiting from one a liquid state of matter to solid; indeed most of us are aware that metals expand when heated, and contract when cooled. The behaviour of material under changing conditions is a science in itself, suffice to say here, that once a molten charge has filled the investment mould it begins to cool, solidify and contract to a lesser volume.
If you are able to observe a foundry pour, (wearing a full face visor) look down into the pouring cup of a just poured mould. Note the level of metal in the cup. Returning half an hour or so later, look again, and there should be notable reduction in the amount of metal remaining in the cup; there is likely to be a ‘tide mark’ higher up on the inside wall of the cup indicating the earlier level of poured metal. This is a physical demonstration of the shrinkage and contraction of a cooling metal.
As the metal solidifys and contracts in the relatively thin walled cast, the larger mass of metal in the cup remains mobile for a longer period. A proportion of this excess metal is gradually drawn down into the cast, topping up the volume in the mould’s main body. If an inadequate quantity of metal is deposited in the cup and the runner system, the cast may contract to the point where casting faults occur, or in extreme cases, parts of the cast are lost altogether. For this reason it is always a good policy to err on the side of caution and use a decent sized cup, then fill it at least 2/3 full during the pour (the cups of some exceptionally large casts may even need ‘topping up’ some time after the initial pour).
One last, but no less useful function of a pouring cup is as a clamping point for holding the cast during metal finishing. Many of the cast alloys used in the art foundry are relatively soft and easily damaged by tooling. Leaving the cup on a small scale cast allows the work to be clamped securely in a vice, without fear of damage. The cup is cut off at the last moment and the remaining stub made good before finally cleaning and finishing the cast.
THE METAL FEEDING & VENTING SYSTEM >
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