Investment Casting PDF

The process of investment casting has come to occupy a key position in the range of modern metal casting techniques. Over the half-century dating from 1940,what had been a small and highly specialised sector of casting activity developed into a worldwide and distinctive industry, reflecting the importance of the product in the intensifying search for close accuracy of shape and dimensions in materials forming. The nearnet-shape objective is seen, not only as a means of providing the engineer with a direct, efficient and economical route to the manufacture of a finished component, but also as a contribution to the conservation of costly materials and energy.

The term investment casting derives from the characteristic use of mobile ceramic slurries, or 'investments', to form moulds with extremely smooth surfaces. These are replicated from precise patterns and transmitted in turn to the castings. Although certain variants employ permanent patterns and multi-part moulds analogous to those used in sand casting, investment casting has become closely identified with the expendable pattern principle typified in the long-established lost wax process. In brief, disposable replicas of the required casting are formed by injecting molten wax into a die with the appropriately shaped cavity. The wax patterns are connected, singly or in groups, to a wax sprue and gating system and the whole is clothed in investment slurry. The wax is melted out and the investment consolidated by heating, leaving a hard ceramic mould to receive the molten metal. The mould is finally broken up to extract the solidified product.

A special feature conferred by the use of expendable patterns is the one-piece mould; the absence of the partings normally required for pattern extraction eliminates a major source of errors arising from misalignment of separate mould parts on assembly. Smooth, hard, precise jointless moulds are the key to the product characteristics that have given investment casting its increasing importance in the wider world of metal manufacture.

Edited by: P.R. Beeley, R. F. Smart