**5. Potentials and applications of the countergravity casting technique**

The countergravity technique has numerous potentials, derivable from its advanatges over the conventional metal casting techniques. As such it is gradually making in-roads into traditional investment casting applications and also in novel materials production.

Aluminium Countergravity Casting – Potentials and Challenges 11

Fig. 10. Ceramic mould at 400× magnification shows heavy segregation of impurities at the

Fig. 11. Countergravity cast specimen at 400× magnification. Significant reduction of impurities at the gain boundaries indicates lesser intake of impurities from the melt

grain boundaries

Fig. 8. Cross-section of a countergravity cast rod showing the absence of volumetric shrinkage defect as evident from the convex meniscus at the top of the rod

Fig. 9. An investment mould "tree", simplified structure is characteristic of the countergravity technique

Fig. 8. Cross-section of a countergravity cast rod showing the absence of volumetric

Fig. 9. An investment mould "tree", simplified structure is characteristic of the countergravity

technique

shrinkage defect as evident from the convex meniscus at the top of the rod

Fig. 10. Ceramic mould at 400× magnification shows heavy segregation of impurities at the grain boundaries

Fig. 11. Countergravity cast specimen at 400× magnification. Significant reduction of impurities at the gain boundaries indicates lesser intake of impurities from the melt

Aluminium Countergravity Casting – Potentials and Challenges 13

Composites (MMC) which can be cast into complex, intricate geometries. These materials have found applications in diverse fields, from high quality reflective mirrors to optical and laser equipment (O'Fallon Casting, 2009). There has been increased interest in the use of cast aluminium/silicon carbide MMC for optoelectronics packaging due to its compatible coefficient of thermal expansion, high thermal conductivity, and potential to produce parts at low cost (Berenberg, 2003). In ring laser gyros, these MMCs are displacing traditional favourites like beryllium and stainless steel in the production of dimensionally stable mirrors that can withstand extreme thermal cycling (Mohn and

The afore-mentioned advantages notwithstanding, the process has some challenges militating

Spada (1998) reported the cost of countergravity mould and handling equipment to be typically between \$50,000 to \$1.25 million depending on complexity. Present day prices would naturally be much higher. This is so because the proper utilisation of a countergravity casting equipment requires an ecosystem of support facilities. These include high-temperature mould pre-heating ovens, mould and moulding flask positioning units, and sophisticated vacuum control systems. These added facilities add to the cost of setting up and operation of the technique. In some instances, licensing fees may also apply, further

Countergravity casting is typically restricted to smaller sized components, usually less than 50 kg. This is because the moulding flask tend to be small, to allow for proper operation of the vacuum system. Larger flasks are more difficult to evacuate and maintain at desired

It is essential for the mould and the sprue to be adequately heated prior to carrying out countergravity casting. The pre-heat prevents chilling of the melt as it flows up from the crucible. Improperly pre-heated sprue and mould will cause increased melt viscosity and a tendency for the melt to get stuck in the sprue or incomplete mould filling. Figure 12 shows premature solidification of melt inside the sprue due to inadequate pre-heat of the mould

Proper control of vacuum pressure is paramount in countergravity casting. Too much vacuum will result in splatter of melt inside the moulding flask due to over-filling of the mould cavity. Loss of vacuum during casting is also a real problem for countergravity technique. This may be caused by improperly closed lid, damage to or cracks in the moulding flask, or a poor seal between the recess of the lock-nut and the conical connection

**6. Challenges and limitations of countergravity casting** 

Vukobratovich, 1988).

**6.1 Equipment cost** 

raising up the cost.

partial vacuum.

and sprue assembly.

**6.4 Vacuum control** 

**6.2 Size restriction of products** 

**6.3 Mould and sprue pre-heat temperature** 

against its wide-spread deployment.
