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A Case for Low Pressure Sand Casting of Aluminum


Less common than gravity sand casting and low pressure permanent mold casting, low pressure sand molding holds a distinct combination of advantages for large aluminum castings. FRANCO CHIESA, CENTRE DE MÉTALLURGIE DU QUÉBEC, TROIS-RIVIÈRES, QUÉBEC, CANADA, AND JOCELYN BARIL, TECHNOLOGY MAGNESIUM & ALUMINIUM, TROIS-RIVIÈRES, QUÉBEC, CANADA


A


majority of aluminum castings are produced via sand or permanent


mold casting, but for large precision components, another viable option for metalcasters to consider is low pressure sand casting, which uses principles from both low pressure permanent mold (LPPM) and gravity pour sand casting. Low pressure sand casting marries the use of bottom pouring for tranquil filling of the mold (which avoids metal


oxidation) with the flexibility to make larger parts. Te capable process can be ideal when producing large “top qual- ity” aluminum castings. Te process also can be considered when walls are too thin (such as 0.1 in. [2.5 mm]) to be obtained by gravity casting. LPPM produces high quality


castings due to tranquil filling of the mold and the application of pressure to fill the mold efficiently and cleanly. Te two main characteristics of the


LPPM process are:


1. The filling from the bottom of the mold is perfectly controlled compared to the turbulent flow associated with gravity casting. Also, the liquid metal is drawn from under the melt surface, preventing dross entrainment into the mold cavity. 2. Efficient feeding from the bot-


tom injection pipe occurs through pressure applied to the melt during solidification, eliminating the need for risers. Te resulting yield is high: typi- cally 80-90% versus 50-60% for gravity permanent mold casting. However, not all casting geometries are amenable to the LPPM process. In low pressure sand


Fig. 1. The principle of the low pressure process is illustrated. 40 | MODERN CASTING February 2016 Fig. 2. Shown is a low pressure poured sand mold.


casting, the sand mold rests on top of a pressur- ized enclosure as shown in Figures 1 and 2. Te similarity between LPPM and low pressure sand casting is in the controlled tranquil filling of the mold with a dross-free melt. Both processes also share the ability to produce thinner walls than gravity pouring would. However, in contrast with the LPPM process, in low pressure sand casting,


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