The lowdown on low viscosity powders
A global rheological model has been developed to help predict the combined effect of shear rate, temperature and solid loading on low viscosity feedstocks used in low pressure powder injection moulding (LPIM).
This is designed to help manufacturers identify any potential problems that may occur during injection, and could also reduce the cost and time required to develop new feedstocks.
Producing ceramic and metal alloy parts using LPIM offers a more economical alternative to high pressure powder injection moulding (HPIM) for small series production in industries such as aerospace, medical device and automotives. It enables use of softer tooling and is less energy intensive, helping to produce small complex parts or prototypes, as well as higher mass components with thicker cross sections.
However, ‘LPIM has yet to reach its full potential’, says Francis Lapointe at Ecole Polytechnique de Montreal, Canada, where the research was conducted in partnership with Maetta Sciences, a Canadian manufacturer of LPIM parts. Lapointe explains that, unlike HPIM, ‘there has been no study of the rheology of LPIM feedstock, particularly for metal alloys’.
The team used statistical analysis to determine separately the optimal models for viscosity as a function of each of the three parameters. They made and analysed 18 different feedstocks from each of the two stainless steel 17-4PH powders (one water and one gas atomised) under investigation, at a d50 of around 10µm. Existing models Herschel-Bulkley (shear rate), Arrhenius (temperature) and Maron and Pierce (solid loading) have been found most suitable.
Lapointe explains, ‘A low solid loading, for example, decreases feedstock viscosity, which improves injectability, but a solid loading too low brings various problems, such as bad control over sintered dimensions. Optimal solid loading is strongly influenced by the critical solid loading, which depends on powder characteristics such as atomisation type’.
Multiplying the contributions of these individual sub-models is said to have enabled a global model that can flexibly analyse the feedstocks’ complete rheological behaviour.
The researchers believe this will help manufacturers identify the optimum shear rate, solid loading and temperature for each feedstock.
Bob Blake, a specialist in PIM for the powders sector of the UK’s Materials Knowledge Transfer Network, sees promise in trying to bring all the influencing factors together, aiding product development and helping to create the right tooling for part production. ‘If you have spent money making tooling and you have got it wrong, that is a problem.’
However, he suggests the need to make the model compatible with computational fluid dynamics so that it can be applied by product designers and manufacturers on a practical level. ‘If they are left as academic models, perhaps they will not be used,’ he notes.
Lapointe proposes further work using different metal powder sizes, adding the effect of granulometry as another sub-model. ‘Such an experiment would confirm that the global model can be used for ceramic powders that typically have a lower d50,’ he adds.Materials World Magazine, 01 Apr 2010
This is designed to help manufacturers identify any potential problems that may occur during injection, and could also reduce the cost and time required to develop new feedstocks.
Producing ceramic and metal alloy parts using LPIM offers a more economical alternative to high pressure powder injection moulding (HPIM) for small series production in industries such as aerospace, medical device and automotives. It enables use of softer tooling and is less energy intensive, helping to produce small complex parts or prototypes, as well as higher mass components with thicker cross sections.
However, ‘LPIM has yet to reach its full potential’, says Francis Lapointe at Ecole Polytechnique de Montreal, Canada, where the research was conducted in partnership with Maetta Sciences, a Canadian manufacturer of LPIM parts. Lapointe explains that, unlike HPIM, ‘there has been no study of the rheology of LPIM feedstock, particularly for metal alloys’.
The team used statistical analysis to determine separately the optimal models for viscosity as a function of each of the three parameters. They made and analysed 18 different feedstocks from each of the two stainless steel 17-4PH powders (one water and one gas atomised) under investigation, at a d50 of around 10µm. Existing models Herschel-Bulkley (shear rate), Arrhenius (temperature) and Maron and Pierce (solid loading) have been found most suitable.
Lapointe explains, ‘A low solid loading, for example, decreases feedstock viscosity, which improves injectability, but a solid loading too low brings various problems, such as bad control over sintered dimensions. Optimal solid loading is strongly influenced by the critical solid loading, which depends on powder characteristics such as atomisation type’.
Multiplying the contributions of these individual sub-models is said to have enabled a global model that can flexibly analyse the feedstocks’ complete rheological behaviour.
The researchers believe this will help manufacturers identify the optimum shear rate, solid loading and temperature for each feedstock.
Bob Blake, a specialist in PIM for the powders sector of the UK’s Materials Knowledge Transfer Network, sees promise in trying to bring all the influencing factors together, aiding product development and helping to create the right tooling for part production. ‘If you have spent money making tooling and you have got it wrong, that is a problem.’
However, he suggests the need to make the model compatible with computational fluid dynamics so that it can be applied by product designers and manufacturers on a practical level. ‘If they are left as academic models, perhaps they will not be used,’ he notes.
Lapointe proposes further work using different metal powder sizes, adding the effect of granulometry as another sub-model. ‘Such an experiment would confirm that the global model can be used for ceramic powders that typically have a lower d50,’ he adds.Materials World Magazine, 01 Apr 2010
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