The factors affecting the dimensional accuracy and surface quality in the investment casting process

Investment casting is one of the important modern manufacturing technique and its prior applications are dating back to ancient times. Investment casting technique is used for fabrications of complex shaped components or hard metals and alloys which are difficult to produce with machining. The precision of the technique comes from disposable pattern. Generally patterns are made from wax or similar polymer based materials which have low melting points and can be easily shaped. Patterns are removed from moulds with melting and there is no parting line on the cast parts. Depending on the pattern and mould materials, dimensional accuracy and surface quality of the investment castings are significantly higher than conventional sand cast products. Investment casting is a net or near net shape process so it minimizes the further processes.There are two types of investment castings process; investment shell mould casting and investment flask mould casting (block mould). In the shell mould process, pattern trees are covered with ceramic layer form a solid shell around the patterns. In the flask mould process, cylindrical hollow metal flasks are used for mould making. The flask is placed around the pattern tree and the ceramic slurry is poured into it. The main uses of the investment flask moulds are jewellery and oral prosthesis castings.In this study, determination of moulding procedure and development of plaster bonded and high alumina cement bonded investments to produce castings with high dimensional accuracy and surface quality were aimed. Laboratory made and commercial investments were used for casting experiments and effects of several parameters on the dimensional accuracy and surface finish were investigated by using various characterization tests.Plaster bonded investments are used for especially jewellery castings made of relatively low melting points (< 1200 °C) metals and alloys. In the laboratory conditions plaster bonded investments were developed with similar characteristics to the commercial ones and successfully used. The investments which are appropriate to the high melting point (> 1200 °C) castings are generally phosphate bonded ones. These mould materials are also used for jewellery and dental castings for many years. High alumina cement bonded investments are an alternative to the phosphate bonded investments. High alumina cement was produced in the laboratory conditions and this bonding component is used for developing new investments and their characteristics were compared with the commercial phosphate bonded investments.In the results of the experimental studies first, development of the plaster bonded and the high alumina cement bonded investments that is the main aim of the thesis was successfully realized. The factors affecting the surface quality and dimensional accuracy in the flask mould investment castings were largely determined. As a further work, research studies can be carried to modify and develop more suitable morphologies of the bonding components.