Airlift Technologies

Description

Investment Casting The process is suitable for series production of high quality castings particularly of aluminum, steel and special high performance alloys. The process is generally used for small castings, often less than 500gm, although steel castings up to 300kg and aluminum castings of 30kg are being produced. Compared to other casting processes such as sand or die casting, investment casting is an expensive process, but it has versatility approached by few other metals forming processes. Intricate or re-entrant contours can be incorporated so that great freedom of design is possible. In many cases components are cast near to net shape so that little or no machining is necessary. Brief description Patterns of the castings to be made are molded by injection of a special wax into a metal die. Cores of pre-formed ceramic may be incorporated into the wax patterns as they are molded. The patterns are assembled into a cluster (often comprising tens or even hundreds of patterns) around a wax runner system. The 'tree' of patterns is then coated with 8-10 layers of a refractory material, each layer being dried or chemically cured before the next layer is applied. The assembly is heated to remove the wax, then fired at high temperature to bond the refractory mold strongly. The hot mold is cast and when cool the mold material is removed by impact, vibration, grit blasting, high pressure water blasting or chemical dissolution leaving the castings, which are then removed from the runner system. Various post-casting operations may be carried out to meet customer requirements. Special characteristics -High dimensional accuracy and consistency -High integrity castings -Extremely good surface finish obtained -Complex shapes can be cast -Long or short runs can be accommodated -Machining can be eliminated -Minimum shot blast and grinding needed -Almost any alloy can be cast -Environmentally good -Specialized equipment needed -New technology must be learned and great attention to detail is required -The process is expensive because costly refractories and binders are used and many operations are needed to make a mold -Because of the exceptional surface finish possible and expected, minute defects can cause rejection of castings and scrap rates can be high. Principles of the investment casting process The investment casting process is distinguished by the use of an expendable pattern. A metal die is normally used to produce the pattern, now almost universally of wax. The injection dies are made from materials such as duralumin or brass. Pre-formed ceramic or water-soluble cores are used to give precision internal cavities in the casting and these cores are located in the wax die prior to injection or into a pre-formed cavity in a wax pattern. Patterns are mounted onto a runner system to give an assembly ready for subsequent coating with refractory. The covering of the wax assembly with a refractory is known as investing - hence the modern name of the process. Mold production originally used the block mould process in which the wax pattern assembly is firstly coated with a refractory material, typically zircon, and allowed to dry. Around this is poured a ceramic slurry based on a low refractory 'grog' and a binder of sodium silicate or hydrolysed ethyl silicate. The refractory mass, contained in a steel can or tube, is allowed to set (typically for 48 hrs) and is then de-waxed using hot air. On removal from the container, the block is slowly fired to about 950

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