**2. Description of the countergravity casting process**

The basic process steps for the vacuum casting process are presented as follows. In the diagram in figure 1, a preheated investment mould with an integrated down-sprue (fill pipe) is positioned in the moulding flask.

The sprue, with a conical-shaped intersection point with the rest of the mould, pokes through and sits in the conical depression of the lock-nut. The "square" fit of the two, depicted in figure 2, ensures a sealing of the flask interior from the external environment.

Aluminium Countergravity Casting – Potentials and Challenges 5

The otherwise solid investment mould is made permeable by a single opening at its apex. This opening effectively connects the mould cavity with the interior space of the moulding flask, making it an extension of the moulding flask and enabling its evacuation along with the rest of the flask. The flask lid hosts the casting valve, a connecting hose to the vacuum system and lid locking mechanism. The electrical resistance furnace melts the aluminium charge, usually by a superheat of about 40 °C above the melting temperature (660 °C) of aluminium to reduce melt viscosity and ease melt up-flow into the mould. During countergravity casting, the moulding flask with the mould assembly inside, is placed on the

The vacuum system evacuates the moulding flask and the ensuing low pressure thus created causes ambient atmospheric pressure on the melt to push up the molten metal, up

Fig. 3. The evacuation of the moulding flask (a) also evacuates the investment mould cavity.

Apart from investment material, the mould could be a metal mould or a ceramic mould. The vacuum system is calibrated so that just the right volume of melt flows inside the mould for a period long enough for the melt to solidify. The vacuum is released after allowing enough time for melt solidification in the mould cavity. This allows un-solidified melt along the sprue length to be flow back into the furnace. The illustration in figure 4 shows the vacuum being maintained until the cavity is completely filled. Vacuum pressure is then released

Conventional gravity- or pressure-assisted aluminium metal casting techniques like sand casting, investment casting and die casting are fraught with problems. These include gas defects, melt oxidation, shrinkage defects and pouring defects. Defects are naturally undesirable because they can result in low strength, poor surface finish and high number of

This causes molten aluminium to rise up into the mould cavity (b)

causing un-solidified melt in the sprue to flow back into the furnace

**3. Conventional techniques and casting defects** 

rejects in a batch of cast products.

furnace lid with the down-sprue poking through a hole in the furnace lid.

inside the mould. See figure 3.

Fig. 1. Typical setup of the countergravity casting process

Fig. 2. Down sprue, with conical base (a) is integrated with the rest of the investment mould "tree" (c). The assemble rests inside the conical depression of the lock-nut (b)

Fig. 2. Down sprue, with conical base (a) is integrated with the rest of the investment mould

"tree" (c). The assemble rests inside the conical depression of the lock-nut (b)

Fig. 1. Typical setup of the countergravity casting process

The otherwise solid investment mould is made permeable by a single opening at its apex. This opening effectively connects the mould cavity with the interior space of the moulding flask, making it an extension of the moulding flask and enabling its evacuation along with the rest of the flask. The flask lid hosts the casting valve, a connecting hose to the vacuum system and lid locking mechanism. The electrical resistance furnace melts the aluminium charge, usually by a superheat of about 40 °C above the melting temperature (660 °C) of aluminium to reduce melt viscosity and ease melt up-flow into the mould. During countergravity casting, the moulding flask with the mould assembly inside, is placed on the furnace lid with the down-sprue poking through a hole in the furnace lid.

The vacuum system evacuates the moulding flask and the ensuing low pressure thus created causes ambient atmospheric pressure on the melt to push up the molten metal, up inside the mould. See figure 3.

Fig. 3. The evacuation of the moulding flask (a) also evacuates the investment mould cavity. This causes molten aluminium to rise up into the mould cavity (b)

Apart from investment material, the mould could be a metal mould or a ceramic mould. The vacuum system is calibrated so that just the right volume of melt flows inside the mould for a period long enough for the melt to solidify. The vacuum is released after allowing enough time for melt solidification in the mould cavity. This allows un-solidified melt along the sprue length to be flow back into the furnace. The illustration in figure 4 shows the vacuum being maintained until the cavity is completely filled. Vacuum pressure is then released causing un-solidified melt in the sprue to flow back into the furnace
