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The metal injection moulding technique (MIM) is already established for serial production of complex, mostly stainless steel parts. However for other materials, especially for titanium parts, there is still the need for superior purity and enhanced surface quality. Facing the challenge of obtaining suitable medical titanium MIM parts, advances have been made at Fraunhofer IFAM over the last few years. Investigations show the effect of very fine powders and different surface finishes of the mould: with polished moulds a very low surface roughness of less than 0.8 pm of the sintered part was obtainable. Chemical and mechanical properties of the sintered parts show the complexity of interactions between sintering conditions, purity, particle size and binder components. Additionally tests concerning feasibility of moulding high aspect ratios, wall-thicknesses of less than 200 pm and special features like a fine internal thread and hexagon head set structures were performed as well.
Introduction
Titanium exhibits very good biocompatibility and is therefore seen as the optimal material for medical components. But the demands made by manufacturers and distributers of medical parts concerning precision and purity of components are high. Hence, until now, moulded titanium parts are treated with reservation because of insufficient quality and high cost [1,2].The problem is that titanium and its alloys are very reactive and pick up light elements rapidly, which is a challenging problem. In particular, high levels of oxygen are often found in titanium powder sources. Also carbon can be readily introduced due to insufficient binder removal prior to sintering or to damaging reactions between the decomposing binder, the debinding atmosphere, and the metal phase. As a result the mechanical properties often don't match the materials standards. Additionally moulded parts often exhibit rough surfaces which is undesirable for most medical applications because of e.g. resulting poor fatigue strength [3].
This study deals with the MIM process for precise, thin-walled titanium medical parts with low surface roughness in the dental field. Consequently developments were necessary in the areas of tooling, powder development, injection moulding and sintering technique.
Materials and Methods
For the experiments, three titanium alloy powders with mean particle size of less than 25 pm were obtained by TLS Special powder GmbH & Co. KG. Subsequent to a gas atomisation process, one powder...