**3. Conventional techniques and casting defects**

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 rejects in a batch of cast products.

Aluminium Countergravity Casting – Potentials and Challenges 7

Fig. 5. A Blow hole defect in an aluminum casting at 100× magnification

Fig. 6. Gas porosity in aluminium casting at 1000× magnification

Fig. 4. The vacuum is maintained until the cavity is completely filled. Vacuum pressure is released causing un-solidified melt to flow back into the furnace
