This is the most traditional and simplest variant of gravity casting. Metal is melted and superheated over the pouring temperature in a melting furnace. Molten metal is poured from the furnace to a foundry ladle used as a transfer vessel, from which the molten metal is poured right into the mould under the effect of gravity force only. The goal is to maintain a smooth and uninterrupted flow of metal to fill the mould with the minimum turbulence to reduce oxidation and foaming, and to prevent slag from entering the mould. It is a critical step in casting production and it requires to:
- Position the pouring lip or nozzle accurately over the mould entry,
- Lower the pouring ladle as low as possible above the top of pouring basin,
- Deliver the correct amount of metal at the required rate,
- Keep the gate and runner system completely full at all times during the pour.
The higher the pouring height, the higher the velocity of the melt when it hits the pouring basin. A higher velocity pour has potential for a more turbulent pour, while the amount of air entrainment as the melt passes into the mould cavity also increases. It results into the increased casting defects. Castings are often hand-poured, although there is a tendency to use fully automated pouring to reduce the variability inherent in manually controlled pouring. However, manual pouring is still the commonest method of filling the moulds. The conventional gravity casting process therefore requires the skill of the highly experienced foundry staff.
Scope and benefits
- Minimal investment required;
- Versatility – used for any castings size and any volume production.
Limitations and drawbacks
- Ladle pours are currently performed by an operator – no guarantee for the optimal pour rate;
- Turbulent flow of the molten metal during the pour causes casting defects;
- Air entrainment (the capture of air in the melt) – oxidation forms inclusions.
Usage
- It is the most traditional and simplest method of casting.