**7. Benefits of countergravity casting**

Some of the advantages highlighted for the countergravity casting technique may be achievable in other, more conventional processes. However, the countergravity technique provides a more complete solution. The process easily lends itself to automation for large scale production; while at the same time can be scaled down for small-scale and jobbing applications.

point of the sprue on the mould. These non-ideal, but very real instances may require a more interactive vacuum system, wherein pressure feedback is used to constantly adjust the

An effort to bring down overall system costs have led to the use of re-usable sprues. These are usually in the form of metallic pipes. Re-usable sprues must however be used with caution because of the tendency of accumulated impurities in the sprue channel to

Fig. 12. Premature solidification of melt inside the sprue due to inadequate pre-heat

Some of the advantages highlighted for the countergravity casting technique may be achievable in other, more conventional processes. However, the countergravity technique provides a more complete solution. The process easily lends itself to automation for large scale production; while at the same time can be scaled down for small-scale and jobbing

flask vacuum pressure.

contaminate the melt.

**6.5 Melt contamination by reusable sprue** 

**7. Benefits of countergravity casting** 

applications.

The possibility of more economical use of the melt is good for the bottom line of foundry operation and was actually the original goal of the countergravity technique. This has motivated a growing list of companies and industrial sector to adopt the technology.

The combination of precision near net shape and strength has resulted in countergravity die casting being used to produce parts formerly made of steel that required a significant amount of secondary machining (Aurora Metals LLC, 2009).

Net shape casting, particularly for thin sections is easily achievable in countergravity casting. Countergravity cast part may have walls as thin as 0.5 mm (National Institute of Industrial Research, 2005).

In order to make the benefits of this casting technique more accessible, low-cost countergravity equipment have been developed. A low-cost design developed by the authors is presented in figure 13.

The design utilizes a simplified vacuum control system and manual positioning of mould and moulding flask. Such low cost alternatives would be invaluable for small scale operations.

Fig. 13. A low-cost machine for countergravity casting

Size restrictions have been tackled by many recent designs. Jie *et al* (2009) reported a system using compressed air to assist the up-flow of melt for large-sized castings. The Check Valve (CV) process is has been developed Hitchiner for larger sized casting. This allows for

Aluminium Countergravity Casting – Potentials and Challenges 17

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portions of the melt in the down sprue to be returned to the furnace whilst keeping the portion delimited by the valve in the moulding flask.

Vacuum control in countergravity casting has benefited significantly from advances in control technology and instrumentation. Li et al (2008) demonstrated a pressure control system based on fuzzy-PID control and a digital valve system and achieved pressure error of less than 0.3 KPa. Other workers such as Khader et al (2008) have carried out extensive system modelling of the countergravity casting machine with the goal of developing an automatic controller for control of machine operation.
