Technical Paper: PIM International, Vol.1 No. 3 September 2007, pages 59-62, 2374 words

Authors: José M. Contreras, Antonia Jiménez-Morales and José M. Torralba

Universidad Carlos III de Madrid, Departamento de Ciencia e Ingeniería de Materiales e Ingeniería Química, Av. Universidad 30, 28911 Leganés, Spain

Abstract

Metal injection moulding (MIM) is an alternative fabrication route for metal parts made to net-shape. With the aim of diminishing the raw material costs, the possibility of changing the powder usually used in MIM (spherical shape and particle size lt; 20 μm) by other types of more cost-effective powder was researched. In order to do this nine bronze 90/10 feedstocks were prepared with the same binder (HDPE-paraffin wax), mixing three different kinds of powder: spherical gas atomised (size lt;22 μm) and two irregular water atomised powders (lt;45 μm and lt;100 μm). With the help of a capillary rheometer, the rheological properties from the feedstocks were evaluated to perform a comparative study of the characteristics and processability of them calculating different rheological parameters as flow index, yield stress and activation energy.

Introduction

In the last 15 years, processing by MIM has undergone rapid development becoming a profitable technology with the ability to produce small parts with complex geometries to the final desired shape. The process can be divided in four main steps: in the first one it is necessary to obtain a mixture with the metal powder and binder (usually a thermoplastic polymer) called feedstock which must have specific rheological characteristics that allow it to flow and to be injected in the second stage of the process using a plastic injection moulding machine. The parts obtained in this way, composed of metallic powder and polymers, are called green parts. The next stage is the binder removal, known as debinding. This removal can be done by a chemical treatment, thermal treatment or a combination of both. These parts are called brown parts and they are formed by the metallic powder and a small portion of binder that confers it enough strength to maintain the shape until, in the last stage, they are submitted to a heat treatment of sintering in which the links between powder particles takes place obtaining a material with densities and mechanical properties in the same range that is obtained by casting and even as wrought material [1].

Further sections of this article include:

- Experimental procedure
- Results and discussion
- Viscosity measurements
- Flow index calculation
- Yield stress estimation
- Activation energy calculation
- Conclusions
- Acknowledgements
- References

Figures and Tables:

Fig. 1 Images obtained by SEM of the three types of powder used in this study; a) OSP at 650X, b) W50 at 650X y c) W100 at 150X.

Fig. 2 Viscosity of the nine mixtures powder-binder with a solids loading of 60% in volume of powder, measured at 170ºC and shear rates of 100 and 1000 s-1

Table 1 Main features and chemical composition of the bronze 90/10 powders used in this study

Table 2 Composition (% in vol.) and apparent density of the blends used in the study

Table 3 Flow index values (FI) and yield stress values (YS) [kPa] from mixtures fabricated with a 55%, 60%, 63% and 65% in volume of powder calculated between 160ºC y 190ºC

Table 4 Activation energy values [kJ/mol] of mixtures fabricated with several solids loading calculated at shear rate of 1000 s-1.