With the accelerating interest in micro powder injection moulding for high volume production of metal and ceramic components, the comprehensive review of µPIM technology just published in the Journal of Micromechanical and Microengineering (Vol.21, No.4, 2011) puts trends and future challenges in this area into perspective. The authors of the review, Usama M Attia and Jeffrey R. Alcock at Cranfield University, UK, state that according to definitions in the literature, µPIM can be divided into the following three classes:
- Micro-parts: parts with a maximum size below 10 mm and features in the micron range.
- Microstructured parts: parts with dimensions between several millimetres and several centimetres with 3D microstructures located on one or more surface area.
- Micro-precision parts: parts of unlimited size, but with tolerances in the micron range or smaller.
In their review the authors have covered the design of µPIM components including mouldable shape and geometries, and the influence of dimensional changes – particularly during the sintering process. They state that, “The changes in dimensions during the µPIM process chain affects the overall volume of the produced part,” and that “very little is available in the literature about achievable dimensional tolerances by μPIM.” However, typical reported tolerances are said to be between ±0.2% and ±0.5% of nominal dimensions. They also cite a recent study which suggested that with thorough process optimisation, accuracies of ±0.1% of the nominal dimension of some ceramic components could be reached in certain directions of the final part.
The authors state that with regard to mouldable shapes and geometries very little is available in the literature about the relation between component geometry and filling quality in μPIM. Similarly, very little information is available in databases or standards for materials suitable for µPIM, especially for relevant properties such as particle size, impurity, and agglomeration. They state that ceramic powders are reported to be easier to handle in μPIM compared to metallic materials, as the latter are often pyrophoric in the nanopowder size range. In addition, because of their relatively lower thermal conductivity, ceramic-based feedstocks tend to less prematurely freeze during injection moulding due to lower cooling rate. Optimum powder loadings, binder systems and feedstock characterisation are covered by the review, with recommendations for future developments in these areas.
The various approaches and challenges to micro injection moulding of PIM feedstock are also included in the review as are debinding, sintering and the resulting dimensional accuracies and surface roughness of µPIM components. Finally, given the small size of the micro components or micro features, the authors state that assessing the quality of the produced µPIM parts depends on the ability to measure relevant properties, such as dimensions, weights, roughness values or mechanical properties. Different techniques and instruments are said to have been implemented to inspect micro-structures produced by μPIM, but the need for specialised equipment for micro-components is still said to persist.
The authors have addressed the challenges and research gaps throughout the µPIM process. They also state that variant processes, already showing success in μIM, are likely to expand the range of μPIM capabilities. Two-component moulding, micro-overmoulding, lost-core technology and similar variants are examples of potential research areas that would enable combining structural complexity with the high-volume capabilities of μPIM. The review includes 145 references.