## 2 Materials and Methods  
Two polyamide 12 (PA12) powders mainly used in industrial LS-applications were selected for this study (As mentioned, there is a third main LS-powder on the market from the company EOS (Krailling, Germany), named PA 2200. This material is based on Vestosint powder, and behaves in a similar way to Duraform(r) PA. For this reason, it is not included in the present study):  
* Duraform(r) PA (abbreviated: DF-PA); Company 3D-Systems, Rock Hill, SC, USA;
* (DF-PA is based on Vestosint(r) powder from the company Evonik, Marl, Germany)
* Orgasol(r) Invent Smooth (abbreviated: Orgasol-IS); Company Arkema, Colombes, France. It is important to realize that powders from the \"Orgasol(r)-family\" [14] and \"Vestosint(r)-family\" [15] have been used as PA12 powders in different coating technologies and other applications in huge quantities for decades. Both materials are fine-tuned by the suppliers for their use in LS to a certain extent [16; 17]; however, the basic powder production includes the same principle process for the LS-powders:  
* Vestosint(r) powders are generated by precipitation [18]. * Orgasol(r) powders are produced from direct polymerization [19]. The different production techniques have drastic consequences for the morphology of the particles, as can be revealed with microscopy analysis. Figure 1 depicts the differences by means of ScanningElectron Microscopy (SEM) pictures. The differences in the shape and surface structure of single particles can be clearly recognized. Whilst DF-PA exhibits a shape similar to potatoes (Figure 1, left), Orgasol(r) particles are much more spherical (Figure 1), with a smooth and slightly wavy surface structure (like cauliflower). Comparing both SEM pictures also gives the impression that the particle size distribution is much narrower in the case of Orgasol(r)-IS, which could be acknowledged during the investigation (see Section 3.1). For this study, both materials were processed on a commercial laser sintering machine manufactured by DTM (DTM was purchased in 2001 by 3D-Systems and no longer exists). The exact machine type used was a DTM 2500 Plus HiQ, retro-fitted with a digital scan-head and a six-zone multi- heating system for improved thermal stability. The machine is under active maintenance, used in industrial part production, and the parts present the highest possible quality to date. For the investigation, two processing parameters were set to fixed values:  
* Scan speed: 10 m/s;
* Layer thickness: 0.1 mm. Other process settings like the part bed temperature (\({}^{\circ}\)C), laser power (W), and scan spacing (mm) were adjusted and optimized so that a (self-designed) process-control-part could be produced with maximum resolution details. This means, for the benchmark part depicted in Figure 2, that all dimensions after build are correct; all slits are open; all walls are built and stand upright without breaking (even the thinnest); the corners are edged and not rounded; the space between the walls is open; and no, or at least as few as possible, undesired (partially) melted powder sticks in-between (for further details, see Figure 2). The evaluation of such a benchmark structure cannot be explained in detail here, but these parts have been successfully used in our production facility to monitor machine and process stability for years as a periodical control instrument. In this way, the experience of the LS-machine operators results in great confidence in the results, even if the qualification is partly based on empirical facts. A main purpose of this work is to identify the processing parameters for high-resolution parts for each material as a reference, as these targets are the most relevant for industrial applications. During the procedure, test specimens for mechanical examinations were built with the identified machine settings. It is important to recall that the mechanical properties of LS-parts can be influenced by LS-processing parameters to a great extent, as laser sintering is a primary shaping process.