Those portions of the mold close to the mold cavity are heated by the sudden charge of hot metal. So mold surfaces and cores may expand in directions that press against surfaces of the solidifying cast metal. The cavity is surrounded by mold sand. Molten metal is poured into the cavity through a mold sprue.

Considerable effort has been expended to develop insular riser/sprue sleeves that not only perform the traditional function of facilitating the pouring and uniform filling of a sand cast mold cavity with molten metal, but also filters the molten metal to remove foreign particles as the molten metal is being poured without materially hindering the flow of the molten metal into the mold cavity. One or more risers are provided to permit air to escape from the cavity while the cavity is being filled and to provide a reservoir for molten metal while the metal in the cavity is cooling and contracting. There are shapes in aluminum castings, such as those formed by surfaces in the cast body having intersecting faces at angles of about ninety degrees and lower,which may shrink extensively against acute angles (for example), adjacent sand mold surfaces and experience unwanted compressive or tensile stresses.

But aluminum alloys have a high volumetric shrinkage upon solidification, and there is additional shrinkage as solidified cast metal experiences further cooling. Several attempts have been made to construct such sleeves having filter systems mounted within the sleeves themselves. One such sleeve is commercially available under the brand name “Dypur” from Foseco, International Ltd. of Birmingham, England. The sand mold body is initially at ambient temperature and it has relatively low thermal conductivity.

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