This research focuses on the possible causes of variation in the tensile strength and stiffness of witness specimens that are used in the manufacturing of aerospace grade selective laser sintered parts. Using specimens and data obtained from Materialise of the PA2241FR samples, the crystallinity and porosity of the samples was determined. The effect of the flame-retardant additive was also investigated. A relationship was found between the degree of crystallinity and the tensile strength, and between the porosity and the tensile strength. Unfortunately, no relationship could be found between the crystallinity and stiffness. The effect of the FR-additive could not be tested using NMR and as a result no data on the influence was available. When analysing the other factors, they seemed to indicate the significance of an unknown parameter on the mechanical properties and especially on the stiffness. A variation of the halogenated flame-retardant additive seems to fit this effect.