International Journal of Engineering Research & Science (IJOER) ISSN: [2395-6992] [Vol-2, Issue-4, April- 2016] Page | 184 Duplex Stainless Steel Self-ligating Orthodontic Brackets by Micro-powder Injection Moulding María E. Sotomayor 1 , Alberto Cervera 2 , Alejandro Várez 3* , Belén Levenfeld 4 1,3,4 Departamento de Ciencia e Ingeniería de Materiales, Universidad Carlos III de Madrid, Madrid, SPAIN 2 Euroortodoncia SA, Madrid, SPAIN Abstract— This work deals with the production of duplex stainless steel dental brackets by micro-powder injection moulding employing a thermoplastic binder based on high density polyethylene and paraffin wax. The starting powder was a mixture of ferritic and austenitic gas-atomized powders in a volume ratio of 50/50. Feedstocks with powder loadings of 50, 60, 65 and 68 vol.% were prepared. All feedstock compositions presented a pseudoplastic flow behaviour. The critical powder volume concentration experimentally determined through oil absorption method and applying a rheological model was 71 vol.%. A mixture with a 65 vol.% of solid loading was chosen to perform the complete route. Injection moulding stage was successfully carried out and green parts were obtained with an accurate dimensional precision. Green brackets were thermally debound under a cycle developed on the basis of thermogravimetric analysis. The cycle took place under argon atmosphere to prevent the oxidation of metallic powder and it lasted only 4 hours. Samples were successfully sintered in low vacuum at 1250ºC, final parts reached densities close to 98%. Scanning electron micrographs revealed a biphasic microstructure distinctive of duplex stainless steels. Keywords— duplex stainless steel, thermoplastic binder, micro-powder injection moulding. I. INTRODUCTION A few years ago micro-powder injection moulding (μ-PIM) became a manufacturing process of growing interest gaining importance in several fields as automotive, electronics and medical technology. This process is one of the key technologies for manufacturing 3D near net shape micro-components. Adapted from PIM, μ-PIM allows processing of stainless steel powders through a profitable and advantageous method for the fabrication of micro-components with a high shape complexity [1]. This process consists of similar steps than PIM: mixing, injection moulding, debinding and sintering [2]. It begins with a suitable selection of powder and binder components. In the case of μ-PIM, the starting powder must be small enough to replicate the finest details, and multicomponent binder systems are normally used to meet the requirements for the process success. Usually, multicomponent binders constituted of polymers and other additives improve the debinding process: the binder removal occurs in a wider temperature range and in a more gradual way. The powder and binder are mixed together to obtain a mixture called feedstock. The feedstock should have low viscosity for filling the micro-cavities and enough strength for avoiding the breaking of the pieces during ejection process. After moulding, the binder is removed to obtain the so-called “brown part” that retains the original shape, and finally it is sintered to shrink in an isotropic way till near full density. A specific application of μ-PIM in medical technology is dental brackets [3]. Orthodontic brackets are precision parts where the arch slot tolerances must be maintained. On the other hand, each tooth demands its own bracket. In this sense, PIM is a powerful tool to obtain this kind of pieces through an advantageous method as it was demonstrated for alumina brackets [4]. The choice of materials available in orthodontics depends on factors such as physical, mechanical and biological properties. Plastic brackets are esthetically satisfying but have several undesirable effects such as high wear and failure to deliver sufficient torque because of their low modulus. Aesthetic ceramic brackets provide significantly better mechanical properties, increased transparency [5], and decreased reactivity with the oral environment than plastic brackets. Finally, stainless steel has been proved to be a very good material for orthodontic appliances due to its low cost, greater strength, higher modulus of elasticity, good formability, and high corrosion resistance in the mouth. The austenitic stainless steel alloy 316L, which is used primarily for bracket manufacturing, contains nickel in the range of 10-14 wt.%. Metal brackets have in general good strength but possess some problems associated with the allergenic potential of nickel, whose allergic effects are well described nowadays. On the other hand, pitting corrosion of orthodontic appliances is common due to the aggressive action of Cl - ions in saliva, or from food and drink [6]. Furthermore, when the bracket and wire are combined with a ligature wire, crevice and galvanic corrosion can occur, and further types of corrosion may develop when the bracket is soldered to the metal band. The release of Ni ions, as a consequence of corrosion process, could cause adverse effects. Their predominant