Technical Paper: PIM International, Vol.3 No. 3 September 2009, pages 64-70, 2863 words

Authors: Sachin Laddha [1], Carl Wu [1], Srikar Vallury [1], Gopi Lingam [1], Shiwoo Lee [1], Kevin Simmons [2], Patricia Thomas [3], Belén. Levenfeld [3], Alejandro Várez [3], Seong-Jin Park [4], Seokyoung Ahn [5], Randall M. German [6] and Sundar V. Atre [1]

[1] Oregon Nanoscience and Microtechnologies Institute, Oregon State University
[2] Pacific Northwest National Laboratory, Richland, WA
[3] Universidad Carlos III de Madrid, Leganes, Spain
[4] Pohang University of Science & Technology, Pohang, South Korea
[5] University of Texas-Pan American, Edinburg, TX
[6] San Diego State University, CA

Abstract

Powder injection moulding (PIM) is a cost-effective technique for producing small, complex, precision parts in high volumes from powders. To have a good understanding of the PIM process and to provide the necessary data for simulation studies, detailed characterisation of the powder-polymer mixture (feedstock) is essential. In this paper, the characterisation of feedstocks consisting of alumina powder (median particle size of ~ 400 nm) with polymer/paraffin wax (Standard Mix) and polyacetal (Catamold AO-F, BASF) binder systems for micro powder injection moulding (µPIM) is reported. It was found that the Standard Mix had lower viscosity and heat capacity as well as greater pseudo-plasticity compared to the Catamold AO-F. However, the results from Moldflow simulations and scanning electron microscopy inferred that the Catamold AO-F filled the microcavities (50µm) more efficiently than the Standard Mix. In addition, the micro powder injection moulding of an alumina dental bracket was analysed using the Moldflow package following measurement of key feedstock material properties. A possible correlation was observed between the mould filling behaviour and the dimensional scatter of the sintered parts..

Introduction

The role of powder technologies for the net shape production of complex engineering components from metal and ceramic materials continues to grow [1-3]. One way of net-shaping such components is the use of powder injection moulding (PIM) which is advantageous as far as shape complexity, materials utilisation, energy efficiency, low-cost production, and mass manufacturing are concerned [4-10]. The present paper focuses on the extension of PIM to the fabrication of parts with miniaturised features and is referred to as micro powder injection moulding (µPIM).

Material homogeneity is a critical issue in µPIM because it results in various moulding defects in metal or ceramic mircroparts as shown in Fig. 1. These defects can be avoided by carefully studying the effect of the part geometry, feedstock properties and studying the effect of change in processing parameters on the fill of microscale features.

The main objective of this research is to investigate the material heterogeneity issue in microcavities through the development of the µPIM process for alumina microchannel arrays (MCAs). MCAs have been chosen for this study since they have been widely used as the major component and design feature for many microsystems in a large variety of applications, such as microfluidics, micro optics, and micro heat exchangers. In this paper, the effect of binder composition on the feedstock’s property was studied. In addition, an experimental platform and modelling approaches have been used to study the mould filling behaviour of the feedstocks during the fabrication MCAs by µPIM. Finally, the results were compared with the µPIM of similar microfeatures in dental brackets by a wax-polymer alumina feedstock.....

Further sections of this paper include:

- Experimental Methods
    Part design 
    Materials: Binders 
    Materials: Powder 
    Processing 
    Characterisation
- Results and discussion 
    Rheological studies 
    Pressure-volume-temperature behaviour 
    Specific heat 
    Thermal conductivity 
    Progressive filling of microchannels 
    Comparison of feedstocks in microchannels 
    Fabrication of MCAs 
    Comparison of MCA results with the fabrication of dental brackets
- Conclusions
- Acknowledgement
- References

Figures and Tables:

Fig. 1 Defects in µPIM: (a) incomplete mould fill, (b) ejection crack, (c) porosity distribution

Fig. 2 Alumina MCAs fabricated by µPIM in this study: schematic (top) and molded parts (bottom)

Fig. 3 Scanning Electron Micrograph (SEM) of the alumina

Fig. 4 (a) Catamold AO-F; (b) Standard Mix; Relationship between viscosity (Pa·s) and shear rate (s-1) for Catamold AO-F and Standard Mix

Fig. 5 (a) Catamold AO-F; (b) Standard Mix; PVT relationships for Catamold AO-F and Standard Mix

Fig. 6 Specific heat (J/kg.K) as a function of temperature for Catamold AO-F and Standard Mix.

Fig. 7 Thermal conductivity of Catamold AO-F and Standard Mix as a function of temperature.

Fig. 8 Progressive filling of microchannels using Catamold AO-F at a melt temperature of 190^oC from Moldflow simulations

Fig. 9 Comparison of Catamold AO-F and Standard Mix in microchannels and bulk at a melt temperature of 190^oC

Fig. 10 A and B are the cross-sections of the MCA molded using Catamold AO-F and Standard Mix respectively

Fig. 11 A and B are the cross-sections of the microchannels ribs after ion milling for Catamold AO-F and Standard Mix respectively

Fig. 12 Specimen geometry: (a) sintered injection moulded bracket and (b) CAD drawing

Fig. 13 Dimensional variations in dental bracket: (a) bottom face and (b) top face

Fig. 14 Mould filling behaviour of dental bracket: (a) early stage and (b) final stage

Table 1 Properties of the alumina powder

Table 2 Value and definition of different constants for Catamold AO-F and Standard Mix

Table 3 PVT coefficients for Catamold AO-F and Standard Mix