Development of the 2-component-injection moulding for metal powders

2-Component-Metal Injection Moulding (2-C-MIM) is a technique derived from plastics industry which has been adapted to metal powders. By using this technology, the production of titanium parts combining dense and porous parts becomes possible. Such a structure with a gradient in porosity is attractive for biomedical implants, as the pores promote a mechanical interlock between bone tissue and implant material. The dense part of the structure is responsible for the mechanical stability of the implant. For the fabrication of metal parts with a gradient in porosity, feedstocks with and without space holder particles are employed. A 2-C-MIM machine and a specifically designed tool are used for the combination of these feedstocks in the same green part. After removal of the binder and space holder material, the parts are sintered and a structure with a gradient in porosity is obtained. First 2-C-MIM experiments were conducted with a standard binder system previously developed at the institute IEK-1 at Forschungszentrum Jülich. Results achieved with this binder system indicated binder-powder phase separation during injection, which occurred due to the low viscosity of the binder system used (0.15 Pa·s). The need for the development of a new binder system with a more suitable flowing behaviour became clear. Moreover, the partial debinding method employed so far (wicking) needed to be substituted by another method with a more industrial approach. New binder systems comprising a wide range of viscosity were developed, where the partial debinding method employed was solvent extraction in n-hexane. A binder system with a viscosity of 12.4 Pa·s was chosen for further investigations, due to the suitability of flowing behaviour of its feedstock. The viscosity of feedstocks was measured, where the use of space holder particles was found to decrease viscosity. After addition of stearic acid and optimisation of the solids content, feedstocks with 72 and 64 Vol.% solids loading (with and without space holder, respectively) were used in injection moulding trials. After optimisation of the injection temperature of feedstock, prototypes of titanium spinal implants with a gradient in porosity were successfully produced and characterised. The availability of the net-shape production of such implants by 2-C-MIM means a reduction of costs in case of large scale production, as compared to the fabrication method current employed (pressing and green machining).