< DIRECT & INDIRECT RUNNERS
RISERS (VENTS & AIRS)
Risers allow displaced air volumes and casting gases to escape the investment mould cavity as metal flows in. Air can easily become trapped within an mould, especially if the design is complicated, or has large undercuts over it’s surface. Trapped air often creates a back pressure, forcing the metal charge out of affected areas, which in turn leads to a loss of detail or an incomplete cast.
Gasses are also evolved as the metal charge enters the investment mould. If these gases are not released from the mould cavity they can become trapped within the solidifying charge and cause gas porosity faults in the cooled cast. A series of well placed risers allows the release of air pockets and casting gases, allowing a sound cast to form inside the mould.
As with runners, the complexity of the riser system depends upon a variety of factors including the type of investment system and runner system, the inticacy of the sculptural design, and the type and grade of METAL ALLOY used to form the cast. Simple casts formed in ceramic shell moulds may have no need for risers as such, instead the mould is allowed to vent via the pouring cup and the shell wall (which is especially porous to gases). In practice though, most wax patterns in the art foundry have at least one dedicated riser attached.
Risers are similar in appearance to RUNNERS, though they are often smaller in cross sectional area, this is because they do not have to function as a supply channel for a large volume of molten metal. As a rule, risers are placed on the high points and out-crops of a wax pattern. For example, on an portrait head inverted for casting, the tip of the nose could easily become a potential air trap if not relived by a riser. The riser outlet is usually exited at the head of the mould, adjacent to the pouring cup, but not in a position that would allow fluid metal to run from the cup down into the riser and prevent it venting during the pour. Risers are commonly placed on high points of detail, also on the any edges or upward facing surfaces of the wax pattern, especially near the pouring cup – positioning risers here releases air and gas forced to the top of the mould by the rising charge.
As well as ‘open risers’, which are vented at the head of the mould, the founder may also add in a feature known as a BLIND RISER. If we return to the example given above of an upward facing nose on an inverted portrait head, the founder may choose not to vent this feature directly to the top of the cup, but instead, either loop a short enclosed wax channel between the lips to the nose tip to create a through route, or else place a short blind stub of wax at the end of the nose, thus creating an additional space for metal to flow into. Either of these solutions will assist the filling of isolated detail, (especially fingers, toes etc), looping and blind risers are also useful if the only other alternative involves placing a very lengthy riser on the pattern, which might be prone to damage during the investment process.
SAND VENTING WITH BLIND RISERS >
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