< DRAINS
CORE VENT
When a solid core is used to form a hollow cast (as is usually the case with plaster and grog investments), it is essential to provide a means of venting this internal refractory mass to atmosphere. Venting is necessary because the molten charge reacts on contact, evolving gases that could otherwise blow holes in the cast, contaminate the metal and cause POROSITY faults.
Where practical, cores with an open surface are positioned ‘core face up’ for investment and casting, this helps gas to rise harmlessly through the outer investment mould. Unfortunately it is not always possible or adequate to vent a core in so simple a way, so a suitable venting tube (or series of tubes), is inserted into a core to provide a more efficient gas release route.
Core vents which can be routed in a way that avoids contact with either the wax pattern or running system, can be formed from a thin wax bar like a riser. If the core vent passes through the wall of the wax pattern it must be fashioned from a metal or ceramic tube, with the open ends temporarily blocked off with wax to prevent accidental filling of the tube with investment and other debris. Were a wax core vent to directly contact the wax pattern, the eventual channel would provide a route for molten metal to enter the core mass, blocking escaping gases and in some extreme cases causing a break up of the core. Once correctly placed through a wax pattern, the free end of a metal core vent can then be connected to a wax tube which like a riser, exits at the head of the mould, usually in close proximity to the pouring cup’s lip.
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