< RISERS
SAND VENTING WITH BLIND RISERS
An alternative means of venting investment moulds is SAND or SOFT VENTING. Where this method is used the wax pattern is almost always fitted with a DIRECT RUNNER system (see preceeding pages). Traditional RISERS that vent at the cup head are almost totally dispensed with, instead the founder positions a series of short blind wax stubs on any potential air traps on the wax pattern’s surface. Wax stubs are also placed around the top and bottom edges of the wax pattern, where they can also function as wax drains during the kiln burn out. As the outer investment mould wall is built up, the free end of the riser stubs is kept clean and exposed, so that after burnout there is a series of small open holes into the mould – these holes directly connect to the mould’s internal cavity.
Before the fired investment mould is filled with metal, the exposed vent holes are plugged with flexible nylon tubing (the flexible nylon core vent as used in sand casting is best). The free end of each tube is collected into a bunch and taped to the outside of the pouring cup. Once all the holes into the investment have been plugged, both the mould and the nylon ‘risers’ are encased in a hard set sand to a level just below the pouring cup lip (see SAND CASTING for a description of hard set sands). Encasing the investment mould in a sand ‘jacket’ permits the metal charge to flow through the investment mould cavity and out through the flexible risers, venting displaced air and gases as the mould fills. Molten metal exiting the tubes freezes rapidly on contact with the outer sand jacket, preventing an excessive loss of metal from the body of cast.
Sand venting is a very fast and efficient method of venting investment moulds especially effective for use on large scale sections which can benefit from the added support of a sand jacket. The main disadvantage of this method is the additional equipment and materials that have to be kept in stock – mixers, flasks, sand and so forth. For this reason sand venting is usually carried out in foundries that already have a sand casting facility, and convenient access to the necessary equipment and materials (see also INVESTMENT).
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