The investment casting process involves the production of engineering castings using an expendable pattern. The principles can be traced back to 5000 BC when Early Man employed the method to produce rudimentary tools. This was followed by centuries of use OF jewellery and artistic products before the advent of the 2nd World War saw the development of aerospace and subsequently engineering components.
The term investment casting derives from the characteristic use of mobile ceramic slurry, or ‘investments’, to form a mould with an extremely smooth surface. These are replicated from precise patterns and transmitted in turn to the casting. Investment casting allows dimensionally accurate components to be produced and is a cheaper alternative than forging or machining, since waste material is kept to a minimum. Production of the investment casting ceramic shell mould is a crucial part of the whole process. There are some basic steps in the production of an investment cast component using a ceramic shell mould. First, multicomponent slurries are prepared composed of a fine mesh refractory filler system and a colloidal binder system. A pattern wax is then dipped into the slurry, sprinkled with coarse refractory stucco and dried. The purpose of the stucco is to minimise drying stresses in the coatings by presenting a number of stress concentration centres which distribute and hence reduce the magnitude of the local drying stress. The second main purpose of the stucco is to present a rough surface, thus facilitating a mechanical bond between the primary coating and the back-up or secondary investment.
When the primary coat has set (air-dried until the binder gels), the assembly is systematically dipped into a secondary slurry and stuccoed until the required thickness of shell is built up. The particle size of the stucco is increased as more coats are added to maintain maximum mould permeability and to provide bulk to the mould. Each coating is thoroughly hardened between dippings. Thus, an investment casting mould consists of individual layers of fine refractory material and granular refractory material held together by a binder that has been set to a rigid gel. Flexibility exists in changing the composition of each layer. Different methods can be used to remove the wax pattern, normally steam autoclave, leaving a hollow shell. Shells are fired and filled with molten metal that solidifies inside the shell. After casting, the ceramic shell is removed through mechanical or chemical methods to obtain the parts.
The investment casting process has increasingly been used to produce components for the aerospace industry and it has been particularly successful for the production of single crystal turbine blades.
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