< AIR SET, RESIN & OIL BONDED SANDS
SHELL (CRONING OR C) PROCESS
The shell or CRONING sand casting process (after Johannes Croning), is notable, not for any wide use in the production of art and design work, but because it can occasionally be confused with CERAMIC SHELL investment casting. Shell moulding here refers to a resin bonded sand process with mould hardening achieved by the application of heat. Sand is combined with a dry PHENOLIC RESIN and a HEXAMINE CATALYST. These components allow mixed sand to keep it’s free flowing quality, distinct from most of the other bonded sands which coagulate into a plastic mass when the binder is added. The master pattern for this moulding process takes the form of one or more metal plates, which are usually constructed in iron. The plates contains an impression both of the design and the running/venting channels, registration points are included for locating multiple mould sections.
The iron pattern is pre-heated to some 480°F (250°C) or so before the dry sand/binder/catalyst mix is spread over the plate. The heat retained within the plate causes the sand mix to set over the pattern’s indentations and raised profiles, where it cools and hardens. When the plate is inverted, unused sand falls away from the pattern, leaving a refractory shell of about 1 inch (25mm) or so in thickness. Most moulds are made up of two such plates (BI-VALVE MOULDS). The sections may have a preformed core inserted and fixed in place to make a hollow cast, before the mould parts are glued together for casting. The shell process is one of the more expensive sand casting procedures, though it is capable of producing a very high standard of ‘as cast’ finish [ref].
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