There currently exists the requirement to improve reproducibility and mechanical properties of SLS Nylon parts for RRapid MManufacturing (RM). In order to achieve this, further fundamental research is needed and this paper addresses this need by investigating effects of potential sources of the lack of reproducibility (i.e. build procedure/parameters and powder blend) and reports effects in relation to crystal structure, microstructure, chemical structure (molecular weight) and mechanical properties.Different and γ p crystal forms were identified and related to the un-molten particle cores and the molten/crystallised regions of the microstructure. The melt point of the γ p form varied depending on processing conditions but the γ v remained principally constant. Observable differences were also present when comparing the microstructure of the parts. Molecular weight of parts was significantly higher than virgin powder but used powder (powder already held at elevated temperature) also showed an increase in molecular weight. This was related to improved elongation at break of parts built from the used powder, consistent with previous studies. Tensile strength showed some increase with parameters selected for improved strength but Young's modulus values were broadly similar.

PurposeSelective laser‐sintered (SLS) parts are known to include un‐melted regions, where insufficient energy has been input into the powder to fully melt all particles. Previous research has shown the presence of two distinct peaks on a differential scanning calorimetry (DSC), and the purpose of this paper is to demonstrate that these peaks relate to the melted and un‐melted regions of the part.Design/methodology/approachSLS specimens were produced under different build parameters, in order to vary the amount of energy input, and DSC traces produced for each. DSC results were also compared with optical microscopy images to confirm the findings.FindingsDSC analysis of SLS Nylon‐12 parts has shown the presence of two distinct melt peaks. It has been shown that these correspond to the melted and un‐melted regions of the part, and that the amount of energy input in the SLS process affects the degree of melting. It has also been identified, via correlation between DSC charts and optical microscopy images, that the un‐melted, or particle core, peak provides the most adequate indication of the proportion of melting. In order to avoid confusion with the commonly used term “degree of sintering”, which provides only a qualitative description, the new term “degree of particle melt (DPM)” has been defined in order to describe the quantitative variations in the completeness of sintering.Research limitations/implicationsFurther work will correlate the DPM, as measured by the core peak height, with the mechanical properties of the parts produced.Practical implicationsResults have shown that it is possible to identify the level of melting in SLS parts via the use of a DSC chart. Owing to the small size of specimen required for DSC, and the relatively automated DSC procedure, this has the potential for use as quality control in SLS.Originality/valueThis is believed to be the first time that DSC has been used to indicate the DPM within SLS parts.

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Selective laser sintering (SLS®, a trademark of 3D systems Inc.) is a manufacturing process which has emerged from numerous other technologies as the leading process considered viable for rapid manufacturing (RM). SLS of polymers has found use in a wide range of industries ranging from aerospace to medicine. The ability to manufacture easily parts that previously have been difficult or impossible to produce, without tooling, has proved invaluable for many applications. A major area of focus within RM is the requirement to produce parts with more repeatable mechanical properties than can currently be achieved. This research has investigated the use of a novel method of interpreting a differential scanning calorimetry curve to indicate the level of melting within semicrystalline selective laser-sintered parts, or the degree of particle melt (DPM). The DPM has been shown firstly to be affected by the amount of energy input into the process whereby, as the energy input increases, the DPM also increases. Results have also shown that, as the DPM increases, the tensile strength and elongation at break also increase, whilst there is no significant effect on the Young's modulus. These findings will enhance the ability to optimize and predict the properties of the SLS process, an area which is critical when producing end-use parts, particularly when considering demanding applications such as in the aerospace and automotive industries.

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