Sprue… Sprue size and shape are important for low porosity casting. When casting simple designs with rounded shanks and only a few prongs almost any sprue, round or flared, light or heavy will function properly. A proper sprue should be round and slightly smaller than the width of the shank.

      A sprue is designed to supply the molten metal into the pattern cavity. Assuming proper metal and flask temperatures, the sprue is critical in properly filling the cavity and allowing a proper solidification. When a sprue is too small, it will solidify before the part cavity. This may cause porosity at the sprue part interface. This is usually seen as a roughness or an indentation at the interface. In some circumstances cracking may also take place, in a similar manner of a hot tear. When oxides are present, as when old metal is used, trapping of the oxides may take place. This appears as a gritty or sandy texture inside the cast part. A flared sprue, hammered to expand the sprue width is very popular. Flaring a sprue makes it easy to engrave style number as well as increase the metal flow rate into the part.

      When solidification occurs in angle, the outer edges solidify more rapidly causing a greater shrinkage to take place. The narrower the sprue-part interface, the higher the velocity of the supplied metal. Round sprues solidify in a more uniform manner than angled sprues. Errors in sprue placement often result in shrinkage porosity in the heavy areas of the castings.

      The ideal sprue placement is at the heaviest cross section area of the casting. This will allow progressive solidification from the thinnest area to the thickest area and provide a supply of fresh molten metal to the thick areas as the metal solidifies.