processes for general shapes

Shell Molding - Resin-bonded sand systems are also used in the shell molding process, in which prepared sand is contacted with a heated metal pattern to form a thin, rigid shell. As in sand casting, two mating halves of the mold are made to form the mold cavity. Common shell molding binders include phenol formaldehyde resins, furan or phenolic resins and baking oils similar to those used in cores. Non-baking resins (furans, phenolics, urethanes) are also available; these can claim lower energy costs because they do not require heated partems.

The shell molding process is capable of producing quite precise castings and nearly rivals metal-mold and investment casting in its ability to reproduce fine details and maintain dimensional consistency. Surface finish, at about 125 gin (3.2 gm) rms, is considerably better than that from green sand casting.

Shell molding is best suited to small-to-intermediate size castings. Relatively high pattern costs (pattern halves must be made from metal) favor long production runs. On the other hand, the fine surface finishes and good dimensional reproducibility can, in many instances, reduce the need for costly machining. While still practiced extensively, shell molding has declined somewhat in popularity, mostly because of its high energy costs compared with no-bake sand methods; however, shell molded cores are still very widely used.