For the SLM process, the 3D CAD model is dissected by the process software into a series of cross-sectional slices. The slices are then printed layer-by-layer by fusion of the metal powder (Fig. 5). Selective laser melting (SLM) is currently one of the most powerful 3D printing technologies.6 An increasing number of metal and alloys can be used, including aluminum, copper, stainless steel, hastelloy, tantalum and tungsten.6These materials provide good chemical stability in addition to excellent temperature and pressure resistance. Furthermore, the materials deliver the thermal conductivity necessary for adequate heat transfer. The reactor present herein was printed with a SLM system from EOS (Fig. S5 in the ESI†).20 As metal powder, a 316 L stainless steel powder with a median particle size of 43.5 μm was utilized (Fig. S6 in the ESI†). The SLM instrument uses an Ytterbium fiber laser with 400 Watt maximum power to fuse the stainless steel powder as it scans across the build area. After the designated areas of the layer have been exposed, the build plate is lowered by 50 μm and another layer of powder is dispensed over the object. The machine then proceeds to scan the next slice cross-section (Fig. 5). The thermal energy of the laser is sufficient to melt to a depth exceeding the layer thickness. Thus, a portion of the previously solidified structure is re-melted to generate a well-bonded, gas-tight, high-density build.6 The printing process takes place inside an enclosed chamber filled with nitrogen to avoid oxidation and degradation of the material.
Fig. 5 Slice cross-section. The lines are showing the exposed route. Different settings were used for routes of different colors (arc energy, laser spot size, scan speed). The inner areas (hatching structures, green) and the outline contours (blue) were drawn with laser spot sizes of 0.5 mm and 0.2 mm, respectively.
At the end of the printing process, the freshly printed reactor is retrieved from the bed and the powder is dusted off with compressed air. In contrast to extrusion based printing technologies, SLM does not need secondary support structures for overhangs and undercuts as the printed structure is supported by the surrounding powder bed. This feature enables greater design freedom compared to other printing technologies. However, after the fabrication process the remaining powder needs to be drained from hollow spaces. To facilitate clearance of the reaction channel, shortcut holes were incorporated at every turn (Fig. 6a and b). With the help of these shortcut holes, the remaining metal powder could be removed without any difficulties from the channels with compressed air. After the channels were cleared and the reactor was cleaned with ultrasound, the holes were closed by laser beam welding and 1/16′′ stainless steel tubings were attached onto the inlets and outlets (Fig. 6e). The tubings were finally equipped with standard 1/16′′ fittings to allow simple integration with peripheral flow equipment (pumps, backpressure regulator and mass flow controller). Unused powder remains unaffected in the SLM process and is recovered for the next 3D print. In total, the printing process took about 14 hours to complete.
Fig. 6 3D printed reactor. (a) Top. (b) Shortcut holes were incorporated at every turn. (c) Front. (d) Back. (e) After the channels were cleared, the shortcut holes were closed, stainless steel tubings were attached and part of the support structure was removed.
To determine the residence time distribution (RTD) in the reactor, a step experiment was performed (see Fig. S7 and S8 in the ESI†). The build resolution and surface roughness of SLM is still relatively poor compared to conventional manufacturing techniques such as computer-numerically controlled machining (CNC). The surface roughness could have a detrimental effect on the RTD. To measure the RTD, inlet A was connected to a syringe pump (Asia Syrris), while the other inlets were closed. A 10−3 M aqueous solution of Rose Bengal was pumped through the reactor with a flow rate of 1 mL min−1. A fast switch from the Rose Bengal solution to water was carried out by means of a three-way valve, while maintaining a flowrate of 1 mL min−1. The concentration of Rose Bengal at the outlet was measured in-line with an in-house developed photometer. From the experimental data, the Bodenstein number for the open-open vessel condition was found to be 31.44. The Bodenstein number and the narrow exit age distribution suggest moderate axial dispersion throughout the reactor, without retention of material, channeling or unexpected dead volumes (see Fig. S7 and S8 in the ESI†). In addition, the residence volume was determined to be 1.85 mL and, thus, corresponds very well with the design volume of 1.89 mL, indicating good clearance of the reaction channel and high dimensional accuracy.