PurposeThe purpose of this paper is to provide macromechanical insight into the fatigue behaviour of laser sintered parts and to understand the influence of the laser sintered surface structure on this behaviour.Design/methodology/approachA background on the technological maturity of manufacturing processes and the demand for structural and aesthetic properties of laser sintered plastic products is given. As the contribution of surface structure on part quality was the focus, laser sintered specimens with and without surface finishes, as well as injection moulded specimens were used. The latter simply served as a comparison and was not intended to qualify injection moulding. The study comprises the determination of short‐term tensile properties, the load increase method for investigating fracture and deformation behaviours, and fatigue crack propagation analysis.FindingsAccording to the test results, the contribution of laser sintered surface structures to relevant mechanical properties can be neglected. Under dynamic loading conditions, laser sintered specimens achieved a longer lifetime but showed less deformation capabilities in contrast to injection moulded specimens. In general, laser sintered specimens presented considerable resistance to crack initiation and propagation.Research limitations/implicationsBecause of the long‐term approach of the research, the number of tests conducted per lot was limited. Thus, the effects of different process settings and the reproducibility could not be fully analysed.Practical implicationsThe studied fatigue behaviour of laser sintered specimens has implications for the functional testing of parts or components, for the product and process design as well as for the general compatibility of laser sintering as a manufacturing technology of end‐customer products.Originality/valueThe value of this paper lies in the better understanding of deformation and fracture behaviours of laser sintered polymers.

Material aging of Polyamide 12 (Laurinlactam) is a very common problem in laser sintering (LS). For stable process conditions, recycled material used in previous processes should be refreshed with 30–50% virgin powder material. However, even by following these refreshing strategies, material quality drops to an insufficient level after several process cycles which leads to poor part quality showing orange peel or poor mechanical properties when processed. In order to avoid this, a quality assurance system has been established to provide recommendations for robust process conditions and material qualities. A detailed study on aging processes in LS comparing two different machines was performed in order to analyze correlations between material quality, process parameters and part properties. Energy input allowing for robust processing conditions should be in a range between 0.325 and 0.42 J/mm3 showing almost identical values for both machines. Optimal material quality ranges was found to be machine specific, while the lower limit lies between 20 and 25 cm3/ 10 min for both machines used. Additionally, material aging characteristics in an oven and a LS machine were compared, in order to simulate material aging in the LS process by simple experiments in an oven. POLYM. ENG. SCI., 54:1540–1554, 2014. © 2013 Society of Plastics Engineers

Air rings, used today in film blowing systems, have been developed down the years through trial and error experiments. They represent the limiting factor concerning the efficiency of film blowing lines with respect to throughput and take-off speed. Numerical simulation of the cooling process in film blowing provides a tool for the design and optimization of future cooling systems. Individual process and geometric parameters can be changed interactively with the effects on flow and heat transfer processes being analysed and applied to similar situations. The knowledge acquired regarding the physical interactions in the cooling of blown film makes a targeted optimization possible. The purpose of this paper is to present the foundamentals for calculating turbulent air flows in blown film extrusion and, additionally, a model for complete FE-simulation of the flow and heat transfer processes. Furthermore, the achieved simulation results will be discussed.

In laser sintering (LS) un-molten Polyamide 12 (PA12) powder is usually re-used (recycled) in further processes. However, LS processing time at powder bed temperature leads to material property changes. As a consequence, un-molten PA12 powder that is re-used or recycled in further processes leads to process and part properties deviations. In this context, powder particle size, shape and distribution is assumed to affect surface roughness and porosity of LS parts. In order to investigate this process effect on changes in powder size, shape and distribution, PA12 powder was systematically aged in a vacuum oven at conditions close to the LS process. According to this procedure, polymeric powder was obtained with aging times up to 120 hours and analyzed by dynamic image analysis. At first, fresh powder was investigated as a reference. The effect of LS processing time and temperature, i.e. powder bed temperature of approx. 174°C was measured with respect to changes in size distribution and shape whereas particles were considered of size up to 500μm. The influence of LS processing time at powder bed temperature was found to be neither significant on changes in particle size nor distribution. With respect to particle shape, a higher deviation to the reference was observed for particle size bigger than 100 μm and longer aging times. Consequently, influences on particle shape changes on surface roughness are assumed to be more likely than influences on part porosity due to LS processing conditions.

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