Rheological modeling of strontium ferrite feedstock for magnetic powder injection molding
Graphical abstract
Introduction
Powder injection molding (PIM) is a productive and widely used technique for the net-shaping of metallic and ceramic components. This technique combines two conventional processes of injection molding process and powder metallurgy, producing high performance and high precision components with complex shape at a low cost [1]. PIM process consists of i) feedstock (powder–binder mixture) preparation; ii) injection molding; iii) debinding; and iv) sintering. Due to many advantages, PIM has been applied to produce various mechanical and biomedical components [2], [3], [4], [5]. For example, mechanical components having three-dimensional complex shapes and/or microstructures such as precision actuator components and microsystem components can be produced by PIM process. Also PIM process has been applied to produce biomedical components such as medical tools for special surgical operations and dental components like orthodontic brackets.
PIM process can also be a promising process for a mass production of magnetic parts [6], [7], [8], [9]. The magnetic PIM process enables a high precision replication of a magnetic part having geometrical complexity or micro-scale structures [7]. This technology can be directly applied to relevant manufacturing industries for production and commercialization of micro magnetic parts.
The magnetic performance of final product from magnetic PIM will be significantly affected by the orientation of the magnetic particles [10]. In this regard, magnetic field is commonly applied in the mold cavity during the injection process in order to align the magnetic particles with a designed direction [8]. The orientation behavior of the magnetic particles is determined by flow-particle interaction, particle–particle interaction and magnetic field–particle interaction. In order to characterize this complex behavior of the magnetic particles during injection process, one needs to understand the rheological behavior of the magnetic powder mixture under various conditions. Also a rheological modeling of the magnetic powder mixture is of particular importance for design and optimization of the process.
There have been several studies regarding the rheology of feedstock and its effect on PIM process. For example, homogeneity effects on feedstock viscosity for PIM were studied by German [11], and it was found that homogeneous mixing of the powder and binder results in the lowest viscosity of the feedstock. Zauner et al. [12] investigated the effect of feedstock viscosity on the dimensional accuracy of the injection molded green part in PIM process. It was noted that the temperature condition of the PIM process affects the dimensional accuracy of the molded part since the feedstock viscosity is significantly affected by the temperature. Wang et al. conducted numerical simulation for filling stage to understand the effects of mold dimensions on rheology of feedstock in micro powder injection molding [13]. He used a power law model and the Arrhenius type viscosity model to relate shear rate and temperature in the FEM simulation and found that PIM process parameters can be affected by the size of mold.
Although there are several studies on the rheology as mentioned above, only few studies have been carried out on the rheology of magnetic powder mixture with particular emphasis on the effects of magnetic field, shear rate, temperature and powder volume fraction with regard to the PIM process. For example, Song [14] studied the rheological behaviors of Neodymium (Nd–Fe–B) powder mixtures for fabrication of bonded magnet, and rheological behavior was characterized by a simple rheological equation of state by nonlinear regression. Kim et al. [6] investigated the rheological behavior of strontium ferrite (Sr–Fe) feedstock for development of magnetic PIM process. In these studies, however, appropriate rheological modeling has not been carried out under magnetic field effect, which limits the application of their results to practical analysis of magnetic PIM process.
In this paper, we present a rheological modeling for Sr–Fe powder mixture based on experimental characterization the shear rheology of the mixture. The effects of magnetic field, shear rate, temperature and powder volume fraction are all investigated comprehensively by using rotational rheometer of plate–plate type. The shear viscosity of the mixture is modeled by employing several rheological models in order to describe the effects of all parameters mentioned above.
Section snippets
Rheological modeling
In general, the shear viscosity of the magnetic powder–binder mixture depends on the shear rate, particle shape and size, temperature, magnetic field flux density and powder volume fraction [6]. In this study of St–Fe feedstock, however, the effect of particle size and shape is found to be almost negligible thus it is not presented. The shear viscosity η can be represented as followswhere T is the absolute temperature, is the shear rate, ϕ is the powder volume fraction and B
Materials
The magnetic feedstock consists of Sr–Fe powder and paraffin wax. There are wet and dry types of Sr–Fe powder depending on the manufacturing method. In this work, the dry type is selected since it is widely used in various applications due to its efficient performance and low cost. Fig. 1 shows microscopic image of the Sr–Fe powder. The magnetic properties are summarized in Table 1. We used coarse-particle wax as binder. It has a density of 0.9 g/cm3. The melting temperature ranges from 42 °C to
Results
First we present the torque rheometer result in Fig. 3. The rotational speed of the screw is fixed at 150 rpm and the temperature is 150 °C. In this experiment, we increase the powder volume fraction from 61% in a step-wise manner in time until the maximum powder volume fraction (ϕm) can be clearly identified. Fig. 3 shows that the increasing slope of the torque changes drastically at ϕ = 70%, which is due to significant friction between particles, indicating that the mixture has reached its
Conclusions
The rheological behavior of strontium ferrite feedstock is investigated for various conditions of external magnetic fields, shear rate, temperatures, and volume fraction of powder. Several rheological models are employed to describe the shear rheology of the feedstock. Three different models by Roscoe, Mooney and Krieger and Dougherty are compared for wide range of powder volume fraction, and it was found that Krieger and Dougherty model could describe the increasing trend of the shear
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2010-0026242/2011-0030075).
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