Feature article: PIM International, Vol.1 No. 1 March 2007, pages 34-39, 4005 words

Author: Professor Randall M. German, San Diego State University, USA

College of Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1326, USA

Introduction

Professor Randall German reviews the development of feedstock for powder injection moulding and considers what the future holds in terms of new materials and applications.

The feedstock for powder injection moulding (PIM) has been the subject of much study. Indeed, feedstock formulations are the area of greatest patent coverage in PIM. Yet, in spite of much effort, feedstock design can be one of the largest frustrations for the PIM practitioner. The ideal feedstock is easy to mould, easy to debind, yet has a high solids loading.

This introduction to PIM feedstock shows the basis for its design using historical trends. The article subdivides the criteria for feedstock formulation into its constituents, for example discussing what makes for a good powder. The integration of powder and polymer is reflected by the feedstock formulation and the development of an effective mixing technology. Although a sophisticated aspect of PIM, it seems the era of discovery is ending. Current research is focused on low-cost titanium feedstock and formulations customized for moulding microminiature components. A few comments are offered on how difficult it is to enter this business with a new idea.

Powder injection moulding (PIM) feedstocks are formulated from mixtures of polymers and powders. At the moulding temperature they are viscous mixtures, since the proportion of particles is high, even when the polymer is inherently low in viscosity. Viewed from a distance, PIM feedstock is similar to paint, ice cream, toothpaste, and chocolate, since all of these are mixtures of polymers and powders. In PIM feedstock the proportion of solid particles is very high, almost near the limit for flow.

A mental experiment helps understand PIM feedstock formulation. Start with a jar of water and add a little sand to the water. The viscosity of that mixture is low. Einstein treated the prediction of the viscosity of such mixtures for dilute concentrations of spheres, showing the mixture viscosity increased in proportion to the amount of solid particles. However, add more sand to the water and eventually the slurry becomes thick; then viscosity increases dramatically with small increases in the solids loading. As this process of adding sand to the water continues, then at the critical solids loading the mixture is resistant to flow. Indeed, at the critical solids loading the mixture is solid-like in character. Thus, the critical solids loading correspond to the composition where the particles are touching one another to give sufficient friction between the particles to inhibit flow. At this composition all of the voids between the particles are filled with water, which is very low in viscosity, but the mixture is thick. Similar conceptual approaches of sequential powder additions to molten binder are ...

Further sections of this article include:

- History
- Powder requirements
- Polymer requirements
- Feedstock formulations
- System integration
- Prospects for the future

Figures and Tables:

Fig. 1 A scanning electron micrograph of the high pressure water atomized stainless steel powder. It is a lower cost option designed for PIM applications.

Fig. 2 An example of a poor packing powder that is used in PIM because of its low cost, but this titanium powder is difficult to mould because of the particle shape.

Fig. 3 A polished cross-section through a high pressure water atomised stainless steel powder showing how the particles are dense.

Fig. 4 A picture of a model high packing density particle with an elongated shape which would be ideal for moulding

Fig. 5 A typical hard shell chocolate candy that has the desirable aspect ratio sought for PIM powders.

Fig. 6 Cracks in sintered PIM components resulting from improper feedstock design, showing a cell phone vibrator weight and an alumina jetting plate.

Fig. 7 Birdshot is one of the highest production items by PIM. The feedstock is self-mixed for this application.

Fig. 8 A high volume cell phone component which relies on self-mixed feedstock.

Fig. 9 Small orthodontic components showing the size range that appears to be the growth direction for PIM.

Fig. 10 An example of a small medical biopsy component fabricated by PIM with features in the 100 micrometer size range (this device is 8 mm in length and 0.1 mm thick).

Table 1 Weight Percent Binder for 60 vol. % Solids Loading with Different Powders

Table 2 Examples of PIM Stainless Steel Powders

Table 3 Attributes Associated with a PIM Powder

Table 4 Key attributes for PIM binders