Hot micro-embossing: effect of pressure on 316L metal parts Elsa W. Sequeiros* 1 , V. C. Neto 2 , M. T. Vieira 3 and M. F. Vieira 1 The use of replicative processes has become strategic and critical in industry to produce precise, microscopically detailed metallic parts and devices via low cost manufacturing routes. Metal powder hot embossing is an emerging process that brings some advantages associated with the reduction of production costs relative to powder injection moulding (PIM). The technology involves four distinct steps: preparation of the selected feedstock material (powder and binder); hot embossing; debinding; and sintering. The effect of continuous pressure during the hot embossing step as a means of replicating microdetails in 316L stainless steel parts is examined. Dimensional accuracy, microstructure and mechanical properties of the parts produced were evaluated. For the configuration tested, the most promising results were achieved when processing at 180uC for 30 min at a pressure of 14 MPa. Hot embossing is a replication process for microparts in which forming is achieved by applying temperature and pressure. 1,2 The production of metallic parts and devices characterised by high dimensional precision and microscale detailing at low cost requires new processing technologies capable of meeting the challenges imposed by micro-manufacturing. 3,4 The importance of micro- and nano- manufacturing has been identified in several reports for a range of materials. 4 Hot embossing and micro- plastic injection moulding are replicative technologies that are well established for mass production of polymer components with low associated costs. 1,4–6 However, there are many specifications that polymers cannot meet, such as improved mechanical properties and thermal stability. 7,8 To date, micro-PIM has been almost exclusively used as a shaping technology for the production of small metallic parts. 6 Hot embossing of metal powder, as opposed to polymers or bulk metallic alloys, is an alternative micro-manufacturing powder metallurgy process with potential to produce easily a high level of complex geometries. 9–12 The study reported below was developed within the Tooling EDGE project, a partnership between the Portuguese Scientific and Techno- logical System and industry. The project’s main goal is to develop scientific and technological knowledge, working methods and innovation adapted to the tooling sector that, through a process of demonstration and dissemination, has potential to increase the overall performance of industry and add value to its processes and products. 13 The need to maintain high levels of competitiveness for the Portuguese Engineering & Tooling sector has as a target the optimisation of manufacturing processes to obtain smaller products with tight dimensional tolerances and to facilitate production of complex geometries at acceptable cost, without loss of quality assurance. The final goal of a particular strand of the Tooling EDGE project is to obtain a metallic insert mould for a light guide produced by hot embossing. To achieve this goal it is necessary to optimise this emerging technology, to allow reproduction of the final product with quality assurance. In this particular study, the feasibility to replicate metallic parts in the shape of a light guide, using a 60:40 (by volume) mixture of 316L stainless steel powder and a commercial binder, was studied using a silicone rubber die at different pressures. This is interdisciplinary research, bringing together know-how related to the production and selection of powder materials and binders, optimisation of feedstock, shape forming parameters and good practice for debinding and sintering. The metal powder embossing technology requires four distinct steps: N preparation of the selected feedstocks (powder and binder) N hot embossing (shape forming with temperature and pressure) N debinding N sintering These steps are interrelated and the variables of each interact and affect the properties of the final product/part. Experimental procedures The metal powder used was composed of 316L stainless steel (Sandvick Osprey Powder Group) with a shape factor, typical of spherical particles, close to 1. This powder had a bimodal particle size distribution, which decreases the relative feedstock viscosity (Table 1). However, according to the literature, 10,14,15 this powder has ideal characteristics for PIM. As shown in a previous study, 9 the powder has a two-phase microstructure consisting of austenite (major phase) and delta ferrite. The binder was a commercial system based on mixture of polyolefin waxes and polyethylene, with a density of 1000 kg m 23 . The first component to melt has a melting temperature close to 60uC, which promotes pseudo- plastic behaviour. The binder is almost totally removed at 500uC. 9 The binder constituents were also characterised in the preliminary study. 9 Feedstock samples were produced by torque rheometry (Brabender Plastograph) by mixing at 140uC for 45 min at a blade speed of 30 rev min 21 . The samples had a fixed powder load of 60 vol.-% and the final torque average value was 2 . 2 Nm. Previous studies showed that these mixtures were homogeneous 9,11 and that the 60% powder loading ensures good densification. 14 The feedstock sample were milled and sieved before hot embossing. The embossing step was done using a mould with a silicon rubber die of hardness 50¡2 Sh A (Fig. 1), coupled to a tensile test machine equipped with an 1 CEMUC, Department of Metallurgical and Materials Engineering, University of Porto, Portugal *Corresponding author, email elsa.sequeiros@fe.up.pt 2 Pedro Nunes Institute, Coimbra, Portugal 3 CEMUC, Department of Mechanical Engineering, University of Coimbra, Portugal ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute DOI 10.1179/0032589914Z.000000000193 Powder Metallurgy 2014 VOL 57 NO 4 241