The effect of aging and degradation on the re-use of polyamide 12 within powder bed fusion processes

Sanders, Benjamin Luke ORCID: 0000-0003-3818-598X (2025). The effect of aging and degradation on the re-use of polyamide 12 within powder bed fusion processes. University of Birmingham. Ph.D.

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Abstract

Powder bed fusion (PBF) of thermoplastic materials, such as polyamide-12 (PA-12), can provide complex, individualised components with a high strength-to-weight ratio. However, during every build, 80-90% of powder remains un-sintered. The remaining powder is recoverable and can be re-used, but it has been exposed to physical aging and degradation processes that hinder the recyclability of the material. As a result, PBF processes are capable of generating significant amounts of waste. This work aims to improve the sustainability of PBF, by determining optimal strategies for quantifying the effect of aging on the properties of recycled PA-12 powder, and how to maximise its re-use across future builds, without detriment to the quality of final components.
To understand its complex aging behaviour, PA-12 powder was conditioned at 170 °C, within an oxygenated environment, for up to 336 hours. This time-temperature profile aims to replicate the harsh thermal conditions that PA-12 powder could be exposed to if repeatedly re-used across multiple PBF builds. Differential scanning calorimetry revealed an initial increase in melting temperature and crystallinity, representative of secondary crystallisation. However, for extended storage times beyond 100 hours, all measured variables indicate that thermo-oxidative degradation becomes the dominant aging mechanism. In an attempt to counteract the effect of aging, used material is typically blended with virgin powder before re-use; a 50:50 refresh ratio is most common. This study quantified the effectiveness of employing a more resource efficient 70:30 refresh rate. Successive powder re-use affected various material properties, such as reduced material coalescence and melt flowability, deteriorated particle morphology, and a 4.5 °C increase in melting temperature. Nonetheless, a 20% reduction in particle flowability had the most significant effect on the quality of final parts. Despite identifying a direct correlation between specific powder and part properties, an 11% reduction in strength across 7 build cycles is relatively modest. Therefore, a 70:30 refresh ratio offers a good compromise between maintaining part performance, particularly for non-critical applications, without having to add an unnecessary amount of virgin powder.
The functionality of PBF parts is also dependent on the crystallisation process. With increased powder re-use, polycondensation and cross-linking cause structural changes that reduce the rate and extent of primary crystallisation. As a result, the common Avrami model is unsuitable for describing the crystallisation kinetics of aged PA-12 powder. Alternatively, the Hay theory accounts for both primary and secondary crystallisation, so can model the full phase transformation of re-used PA-12 powder more accurately. Despite limited reports in the literature, secondary crystallisation is a crucial aging process in the context of PBF. Flash scanning calorimetry (FSC) verified that secondary crystallisation, via lamellar thickening, can occur when PA-12 is exposed to elevated temperatures. FSC quantified that the mechanism and rate of lamellar thickening is dependent on temperature, time, and the polymorphic nature of PA-12. This insight into the crystallisation behaviour of PA-12 could be utilised to improve the consistency of PBF parts.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):