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A review of rapid prototyping integrated investment casting processes

Investment casting has been used to manufacture weapons, jewellery and investment casting during the ancient civilization. Today, its applications include jewellery/art castings, turbine blades and many more industrial/scientific components. The present paper reviews various investigations made by researchers in different stages of investment casting and highlights their importance. The paper initially highlights the investigations made on pattern wax properties, effects of blending, additives and fillers. Different ways through which pattern properties (like surface finish, dimensional accuracy, etc.) could be enhanced by properly controlling the injection processing parameters are thoroughly discussed. The paper also looks into the investigations made to enhance the strength, surface finish, etc. of ceramic shell for ferrous alloys/non-ferrous alloys as well as superalloys in investment casting. Investigations made on incorporation of nylon fibers and polymer additions confirm that a ceramic shell reinforced with nylon fibers attains additional permeability compared to the one with polymer additions.

Different investigations carried out on autoclave dewaxing and microwave dewaxing conclude that the wax properties are less altered with microwave dewaxing when compared to an autoclave dewaxing. Some recent investigations carried out on pouring and post-treatment operations are also discussed in the paper. The advent and emergence of rapid prototyping in shell mold casting are broadly exposed in the subsequent sections of the paper. Various aspects of rapid prototyping like rapid investment casting, rapid freeze prototyping, etc., along with their advantages are projected. The emerging areas of applications of rapid prototyping like dentistry, etc., are duly discussed.

The casting of titanium based alloys presents considerable problems, including the extensive interactions that occur between the metal and refractory. In this work, CaO stabilised zirconia was used as a primary coat material on the investment casting mould. The reaction between the zirconia face-coat and a Ti-46Al-8Nb-1B alloy was evaluated at three mould pre-heating temperatures: 500°C, 1000°C and 1200 °C. The effect of casting dimensions on interaction was also included in this work and the computer simulation of metal cooling profiles was carried out to assist the analysis. Higher mould pre-heat temperature and larger casting dimensions enhanced the interaction between the shell and the TiAl alloy associated with longer metal solidification time. During the high temperature casting process, not only were O and Zr observed penetrating into the metal from the decomposition of the face-coat materials, but also Si which had penetrated from the backup coat was found to have interacted with the metal.

Investment casting is competitive with all other casting processes where the size of the product is within a mutually castable range. Though investment casting is used to produce metal parts of any intricate shapes with excellent surface finish, it suffers from long lead time and high tooling costs, which makes it uneconomical for the production of either single casting, or small and medium production units. These problems could be overcome by the applications of rapid prototyping and rapid tooling technologies for low-volume investment casting production runs. The present article analyzes different classifications of rapid prototyping techniques and it reviews various investigations made on the usability of rapid prototyping- and rapid tooling-integrated investment casting process, with their advantages and limitations. The emerging areas of applications of rapid prototyping like dentistry, jewelry, surgical implants, turbine blades, etc., are accordingly discussed. Further, an elaborate discussion is made on the application of newer technologies for directly developing ceramic shells. This article also emphasizes on various future scopes possible in rapid prototyping-integrated investment casting process.

Investment casting process is known to its capability of producing clear net shape, high-dimensional accuracy and intricate design. Consistent research effort has been made by various researchers with an objective to explore the world of investment casting. Literature review revealed the effect of processing parameters on output parameters of cast specimen. This article highlights the advancements made and proposed at each step of investment casting and its hybridization with other process. Besides, investment casting has always been known to manufacture parts such as weapons, jewellery item, idols and statues of god and goddess since 3000 BC; this article reviews the present applications and trends in combination of rapid prototyping technique as integrated investment casting to serve in medical science. Advancements in shell moulding with incorporation of fibre and polymer, development of alternative feedstock filament to fused deposition modelling are duly discussed. The aim of this review article is to present state of art review of investment casting since 3200 BC. This article is organized as follows: in section ‘Introduction’, introduction to investment casting steps is given along with researches undertaken at each step; in section ‘Rapid prototyping technique’, background is given on the concept of rapid prototyping technique by examining the various approaches taken in the literature for defining rapid prototyping technique; section ‘Biomedical applications of RPT’ presents the medicine or biomedical applications of investment casting and rapid prototyping technique; section ‘Future trends’ provides some perspectives on future research and section ‘Conclusion’ closes the article by offering conclusions.

In order to improve the properties of silicon sol shell for shell mold casting process, natural plant fibers combined with aluminum silicate fibers at natural-to-aluminum silicate fibers mass ratio of 1:1 were mixed into the slurries preparing for fiber-reinforced shell. The flexural strength of specimens of green shells, fired shells at different temperatures and the self-loaded deformation of the latter at elevated temperature were investigated. The fracture surface of shell specimens was observed by SEM. The results show that the green strength of shell specimens increases firstly and then decreases with variation of content of fiber from 0.2% to 1.0%. However, the self-loaded deformation at elevated temperature firstly decreases and then increases. The green strength of shell specimens reinforced with 0.6% fibers reaches the maximum of 2.94 MPa. The bending strength of shell specimens reinforced with 0.6% fibers fired at 900℃ reaches 4.04 MPa, approaching that of the non-reinforced shell specimens. It is found by SEM that the failure of the fiber-reinforced shell specimens at the applied load is resulted in breakdown of silicon sol films and pulling-out, fracturing and debonding of fibers in the shell.

The development of manufacturing processes for high-performance investment casting components in turbomachinery is an iterative process, which takes a lot of development time for engineers and foundry occupation. The reduction of these expensive preliminary tests is possible by combining probabilistic methods with modern simulation tools for the numerical description of the what is investment casting and solidification processes. Starting from the deterministic simulation of the casting process, the casting and solidification parameters including their production tolerances are taken into account in the probabilistic simulation. Through a multi-dimensional statistical analysis of the numerous parameters of the casting process and the achieved virtual casting results, the correlations between the process parameters and component quality can be worked out. Furthermore, a design of experiment (DoE) was performed with real castings to confirm the influence of the main parameters on the result quantities. Mechanical and microstructural characterizations of appropriate cast specimens allow a validation of the simulation results and the formulation of casting parameter–microstructure–property relations. The mechanical properties are studied by uniaxial hot tensile tests using standard and small-scale specimens. Furthermore, the uniaxial fatigue behavior and the life times at elevated temperatures are investigated.

In order to reduce the interaction between the Ti alloys and ceramic shell during the casting, materials with high thermal and chemical inertness were used in investment casting. An investigation was undertaken to analyze the influence of the change of binder systems on the slurries, facecoats and the thermo-chemical properties of the facecoat systems using an Y2O3–ZrO2 filler material. The results showed that, using alumina-sol as the binder in the slurry gave the longest life of around three days followed by that using the commercially available zirconia-sol at around 6 h, and the yttria sol based slurry giving a shortest life of around 1.5 h. Meanwhile using the alumina-sol can also enhance the facecoat sintering properties. There was no obvious evidence observed that the change of the binder system influenced the facecoat chemical inertness.
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