The metal injection moulding (MIM) process combines the well-known thermoplastic injection and powder metallurgy technologies to manufacture small and intricate parts to nearly net shape. The design of injection moulds is a very important stage in such a technology as the mixture of metallic powder and thermoplastic binder is injected in the mould with a viscous behaviour strongly different from the pure polymer injection. To improve the mould design by an efficient numerical tool, an explicit 3D software has been developed to perform efficiently the injection simulation. Taking into account the special behaviour of feedstock in MIM injection, a bi-phasic model is used to describe the flows of metallic powder and plastic binder so as to predict accurately the powder segregation in injection. The volume fractions of metallic powder and plastic binder give directly the evolution of segregation during the injection course. The numerical results are compared with experiments, in which a multi-cavity mould specially designed and realised in our laboratory is used. The segregation zones are well predicted. This specially developed software permits to optimise the mould design and processing parameters to get required components.

Improving mould design and injection parameters in metal injection moulding by accurate 3D finite element simulation

Elaboration of PIM feedstocks with 316L fine stainless steel powders for the processing of micro-components

Micropowder injection molding (μ-PIM) is one of the most promising processes of mass production for the fabrication of small complex shaped ceramic or metallic parts with high sintered density. However, dimensional accuracy of finished parts is difficult to achieve because of extremely high shear rates during the injection molding process. This promotes the separation of powder and binder even in highly homogeneous feedstocks leading to a particle density variation in the green part causing anisotropic shrinkage during sintering. The main objective of this study is to investigate the effect of the powder particle distribution in injection molded green metallic microparts with respect to the molding parameters using synchrotron microtomography (S-μCT) and three-dimensional (3D) image evaluation. Image analysis has been performed using the MAVI software package. To get information about the allocation of the metal particles along the sample the 3D CT-scans have been segmented and statistically analyzed via spatially resolved size distributions. Furthermore, the spatial arrangement of the particles has been investigated using the so-called summary statistics from the area of point process statistics. The results show that variations in the size distribution of the metal powder particles can be detected and give consistent evidence for a monotonic increase in particle size with distance to the injection point. In order to give recommendations for the choice of parameters as well as tool construction, knowledge about the causes for separation effects is essential. This study shows that S-μCT is a well-adapted analytical tool to investigate the powder-particle distribution in μ-PIM.

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Micropowder injection molding: investigation of powder-binder separation using synchrotron-based microtomography and 3D image analysis

Experimental and numerical analysis of the particle size effect on the densification behaviour of metal injection moulded tungsten parts during sintering

Segregation measurement of powder injection molding feedstock using thermogravimetric analysis, pycnometer density and differential scanning calorimetry techniques

The paper is concerned with occurrence of processing defects and resulting mechanical properties associated with material processing by metal injection molding (MIM). MIM process is a multistep one that consists first in the injection of metallic powders mixed with a thermoplastic binder, followed by a debinding stage that permits to evacuate the polymeric binder, and then followed by a sintering stage by solid state diffusion that normally leads to a nearly dense component. The main defects arising during MIM processing are associated with powder segregation during injection molding, and uncompleted or heterogeneous mechanical properties resulting from solid state diffusion. The paper first describes a biphasic fluid flow approach that can accurately predict powder volume fraction after injection molding and consequently the associated segregation defects. This analysis is followed and continued by a proper sintering model based on an elastic-viscous analogy that predicts the resulting local densities after sintering and also associated defects. So, from the two subsequent models, it becomes possible to get the final powder densities after processing and to localize the possible resulting defects. This analysis is completed by an analysis using a porous material model to get the final resultant mechanical properties after processing.

Processing Defects and Resulting Mechanical Properties After Metal Injection Molding

The sintering behaviours of injection moulded components in 316L stainless steel powder are investigated by means of the bending and dilatometric tests in the furnace. Various thermal cycles are realised for different tests. The experimental results are used to determine the parameters of uniaxial viscosity and sintering stress in the viscoplastic constitutive law, which is important for description of the sintering behaviours. The sintering models are implemented into the finite element solver, incorporated with the identified parameters, to simulate the size change and shape distortion of powder injection moulded components during the sintering process. The sintering simulation takes into account the green inhomogeneity caused by powder binder segregation in the previous injection process. The simulation results for the sintering process of a three-dimensional wheel component are compared with the experimental ones. It proved validity of the sintering model and the approaches for parameter deter...

J. Song

Papers

Processing Defects and Resulting Mechanical Properties After Metal Injection Molding

Experimental and numerical analysis on sintering behaviours of injection moulded components in 316L stainless steel powder