Maximilian Drexler

Characterization of Polymer Materials and Powders for Selective Laser Melting

Concerning individualization, the requirements to products have increased. The trend towards individualized serial products faces manufacturing techniques with demands of increasing flexibility. Additive manufacturing techniques generate components directly out of a CAD data set while requiring no specific tool or form. Due to this additive manufacturing processes comply, in opposite to conventional techniques, with these increased demands on processing technology. With a variety of available additive manufacturing techniques, some of them have a high potential to generate series products with reproducible properties. Selective laser melting (SLM) of powder materials shows the highest potential for this application. If components made by SLM are desired to be applied in technical series products, their achievable properties play a major part. These properties are mainly determined by the processed materials. The range of present commercially available materials for SLM of polymer powders is limited. This paper shows interrelations of various material properties to create a basic understanding of sintering processes and additional qualifying new materials. Main properties of polymer materials, with regard to their consolidation are viscosity and surface energy. On the one hand the difference of the surface energy between powder and melt influences, the wetting behavior, and thus the penetration depth. On the other hand, a high surface tension is fundamental for good coalescence of bordering particles. To fulfill these requirements limits of the surface tension will be determined on the basis of a reference material. For these reason methods for determining surface tension of solids, powders and melts are analyzed, to carry out a possible process-related material characterization. Not only an insight into observed SLM phenomena is provided but also hints concerning suitable material selection.

Fundamental investigation of laser beam melting of polymers for additive manufacture

By selective laser sintering (SLS), polymer powders are molten layer by layer to build conventional prototypes or parts in small series with geometrical freedom that cannot be achieved by other manufacturing technologies. The SLS process is mainly defined by the beam–matter interaction between powder material, laser radiation and different material characteristics by itself. However the determination of these different material characteristics is problematic because powder material imposes certain requirements that cannot sufficiently be provided by conventional measurement methods. Hence new fundamental investigation methods to determine the optical and thermal material characteristics like the thermal diffusivity, thermal conductivity, or the influence of different heating rates on the melting behavior are presented in this paper. The different analysis methods altogether improve the process of understanding to allow recommendations for the future process controlling.

Derivation of heating rate dependent exposure strategies for the selective laser melting of thermoplastic polymers

The selective laser melting of polymer powder is for rapid prototyping applications an established technology, although a lack in basic process knowledge appears. Considering demands of series production the selective laser melting technique is faced with varies challenges concerning processable material systems, process strategies and part properties. Consequently basic research is necessary to shift from rapid prototyping to rapid manufacturing of small lot sized series. Based on basic research the high potential of selective laser melting for the production of complex parts without any tools can be opened up. For the derivation of part quality increasing process strategies knowledge about interactions between sub-processes of selective laser melting and resulting part properties is necessary. The selective laser melting consists of three major sub-processes: Geometry exposure, tempering and powder feeding. According to the interaction of sub-processes resulting temperature fields during the selective laser melting process determine the part properties by changing micro structural pore number and distribution. Beneath absolute temperatures also the time-dependency of the thermal fields influences the porosity of molten parts. Present process strategies tend to decrease building time by increasing scanning speed and laser power. Although the absolute energy input into the material is constant for increasing scanning speed and laser power in the same ratio, time dependent material effects are neglected. The heating rate is a combined parameter derived from absolute temperature and time. Within the paper the authors analyze the basic interactions between different heating rates and part properties (e.g. porosity, mechanical strengths). Therefore with different heating rates produced specimens are analyzed with imaging technologies as well as mechanical tests. Based on the done basic investigations new heating rate dependent process strategies can be established considering time dependent material behavior.

Modelling of the aging behavior of polyamide 12 powder during laser melting process

Concerning individualization, the requirements to products have increased. Additive manufacturing technologies, such as selective laser melting allow manufacturing of complex parts without tools and forms. Due to this additive manufacturing processes comply, in opposite to conventional techniques, with these increased demands on processing technology. Due to the high temperature during processing, a degradation of the used plastic powder occurs. The non-molten material in the building chamber, the so-called partcake, can be removed after building from the finished component and reused for another process. To realize reproducible part properties refreshing of partcake powder with 30 up to 50 % virgin powder is necessary. However, these refreshing strategies lead to varying component properties due to an undefined aging state. Previous investigations on oven aged powder for selective laser melting showed for short periods of storage near the melting point thermally induced post condensation is the predomina...

Fundamental investigation of part properties at accelerated beam speeds in the selective laser sintering process

Purpose Despite the recent progress in basic process understanding considering the selective laser sintering (SLS) of thermoplastics, several aspects of the mechanisms of the beam and powder interaction are not fully understood yet. Recent studies covered the correlation of mechanical properties and part density with the heating rate. The surface roughness of the test specimens was also considered but showed no distinct relation to the part mechanics. The purpose of this paper is to provide a new fundamental model for describing the decreasing mechanical properties with increasing beam speed. Design/methodology/approach While the dependence of mechanical properties with total energy input during exposure is well published, the correlation of the exposure speed with the degree of particle melt (DPM) is the subject of the present study. The DPM is accessible through differential scanning calorimetry measurements. Supporting the previously introduced method of the core-peak height, the interpretation via the core-peak area is proposed as a means to ascertain the melting behaviour for different processing conditions. Further support of the observations is given by x-ray computed tomography and microscopy which allows for a correlation with the respective porosity and inner structure of the parts. Findings The authors show a novel way of describing the decreasing mechanical properties with increasing speed of energy input by showing the dependence of the DPM on the heating rate during exposure. Practical implications The results offer an addition to the understanding considering the reliability and reproducibility of the SLS process. Originality/value The paper extends the existing models of the time-dependent material behaviour, which allows for the derivation of new efficient and stable process strategies.

A novel process for production of spherical PBT powders and their processing behavior during laser beam melting

Additive manufacturing processes like laser beam melting of polymers are established for production of prototypes and individualized parts. The transfer to other areas of application and to serial production is currently hindered by the limited availability of polymer powders with good processability. Within this contribution a novel process route for the production of spherical polymer micron-sized particles of good flowability has been established and applied to produce polybutylene terephthalate (PBT) powders. Moreover, the applicability of the PBT powders in selective laser beam melting and the dependencies of process parameters on device properties will be outlined. First, polymer micro particles are produced by a novel wet grinding method. To improve the flowability the produced particles the particle shape is optimized by rounding in a heated downer reactor. A further improvement of flowability of the cohesive spherical PBT particles is realized by dry coating. An improvement of flowability by a fa...

Selective laser melting of polymers: influence of powder coating on mechanical part properties

Abstract For the derivation of part quality increasing process strategies, knowledge about interactions between sub-processes of selective laser melting (SLM) and resulting part properties is necessary. The SLM process consists of three major sub-processes: powder coating, exposure, and material consolidation. According to the interaction of sub-processes, resulting processing conditions during SLM determine the part properties by changing micro structural pore number and distribution. In addition to absolute temperatures, the time-dependency of the thermal fields also influences the porosity of molten parts. Present process strategies tend to decrease building time by acceleration of the subprocesses. Apart from prior investigated acceleration of the exposure, the powder coating step is focused. Within the paper, the authors analyze the basic interactions between different coating parameters and part properties. The authors estimate an interaction between coating speed and resulting part properties due to a force impact caused by the moved coating mechanism. Therefore, specimens produced with different coating speeds are analyzed with imaging technologies as well as mechanical tests. Based on the investigations, new processing strategies can be established considering the forces applied to the powder bed during the coating process, as well as the unique compaction behavior of current and future used powders.