< CORE VENT
CORE PINS
When poured into a wax pattern, the core material is initally held in place by the surrounding wax wall. This retaining wax wall will later be removed from the investment mould during kiln firing. Unless independently secured, the core mass would most likely move out of position as soon the wax pattern is evacuated from the mould. Loose cores can be surprisingly buoyant and typically float to the top of the mould cavity as a metal charge enters. A floating core alters the uniform wall thickness of the cast, causing it to be too heavy in some areas, and too thin or non-existent in others. To prevent this lopsided effect, the founder inserts a number of metal pins through the wax pattern and into the core mass. A portion of the pin is left extended outside of the wax pattern, and this head is later encased within the surrounding investment mould, effectively holding and suspending the core in it’s correct position.
Core pins are always inserted before the wax pattern is invested with a refractory coating (be it ceramic shell or plaster and grog). This allows the pins to be positioned accurately, and with minimal disruption to the important surface detail on the wax pattern. The size of the inserted core pin can range from the width and length of a small tack, to a bar of some inches in diameter and several feet long – variable according to the mass of the suspended core and the overall scale of the job in hand. Metal alloys suitable for use as core pins vary, with stainless steel being popular for use with CERAMIC SHELL systems (stainless steel is less likely to unbalance the pH level of the CERAMIC SLURRY). Cut mild steel nails and brass tacks are usually preferred for use with traditional investments.
Care is taken to use the minimum number of pins necessary to support the core and to cause as little disruption as possible to wax pattern’s surfaces when placing the pins. Once the design has been cast, the core pins are removed and the residual holes in the metal’s fabric made good by welding or pinning during the FINISHING stages.
CORE PRINTS >
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