Shear viscosity measurements on Polyamide-12 polymers for laser sintering

Purpose – Laser sintering kinetics and part reliability are critically dependent on the melt viscosity of materials, including polyamide 12 (PA-12). The purpose of this paper is to characterise the viscosity of PA-12 powders using alternative scientific methods: constrained boundary flows (capillary rheometry) and rotational rheometry. Design/methodology/approach – Various PA-12 powders were selected and characterised by both techniques. Measurement of molecular weight was also carried out to interpret the viscosity data. Findings – Results demonstrate conventional pseudoplastic flow in all PA-12 materials. Zero-shear viscosity has been quantified by rotational rheometry; a notable observation is the striking difference between virgin/used PA-12. This is interpreted in terms of molecular weight and chain structure modifications, arising from polycondensation of PA-12 held at the bed temperature during laser sintering. Research limitations/implications – Accurate zero-shear viscosity data provide scope for use in predictive computational models for laser sintering processes. Careful sample preparation and equipment operation are critical prerequisites for accurate rheological characterisation of PA-12 powders. Practical implications – Differences in flow behaviour and molecular structure allow prediction and deeper understanding of process-property relationships in laser sintering, giving potential for further optimisation of material specification and in-process machine parameter control. Originality/value – This is believed to be the first time that techniques other than melt flow rate (MFR) have been reported to measure the viscosity of PA-12 in a laser sintering context, noting the effects of pre-drying and molecular weight, then predicting differences between virgin/used powders in practical sintering behaviour.