Key Engineering Materials Vol. 958

Abstract: Additive manufacturing of microfluidic devices is a field of increasing interest due to the great variety of fields where they can be used, especially in physicochemical, biological and medical ones. These devices include internal channels whose manufacture can be challenging as it takes place close to or into the micro geometric scale. On the other hand, the use of low-cost techniques can provide access to certain services in depopulated areas in different fields, so this approach can be of interest in the development of new products under new production and living contexts. In this work, the geometrical ranges of practical application for the manufacture of microfluidic channels by two of the most common additive manufacturing techniques for polymeric materials (Fused Filament Fabrication and the Stereolithography) are analyzed by means of an evaluation of the dimensional accuracy obtained in samples with channels of circular section. The circular channels present diameters that vary from 2 mm to the minimal size feasible which each printer (a Markforged Onyx One, a Ultimaker S5 and a Formlabs Form3). The Ultimaker S5 (FFF) equipment is the one that presents the best results, being the dimensional deviations around 0.2 mm in a wide range 1 < d_nom (mm) < 2; and contrary to the expectations, the SLA system provides the worst results, with a growing trend starting from deviations of 0.6 mm. An obturation effect in the channels has been also detected, being critical in the case of nominal diameters lower than 0.8 mm for the Ultimaker S5 system. In general, it can be concluded that the FFF technology is a more reliable option compared to SLA under the printing parameters considered in this work and for the materials used in this study.

Abstract: 3D anatomical models play an increasing important role in the 3D surgical planning area allowing specialists to have an anatomical representation of the patient before the intervention. The support material to print models is fundamental to ensure optimal finishing, which is why soluble support is a good option for these cases. BVOH (butanediol vinyl alcohol copolymer) is a water-soluble thermoplastic optimized for support generation on FDM (fuse deposition modelling) printing process. In addition, PVA (polyvinyl acetate) is another well-known soluble thermoplastic used for support generation. Compared to PVA, BVOH has some benefits and improvements such as better surface quality, solubility times and ease of printing resulting in a better finish of the model. In this study, we have compared the time and cost of printing the same case combining PLA or ABS with BVOH for the generation of support material and the same printing with supports of the same printing material. The obtained results show an increase in cost and printing time of 33 % with respect to printing with conventional supports. However, this increase in cost and time is offset by the finishes obtained; obtaining much better results compared to the use of non-soluble material to generate the support. It also represents an increase in productivity since for the post-processing of the piece it is only necessary to leave it in water for approximately 22min at a temperature of 50°C or 45min at a water temperature of 22°C. Moreover, compared to PVA, BVOH has shorter solubility times.Furthermore, in the cases where it is necessary to generate internal support, soluble support is the best option since it will be removed with water without leaving visible marks.In conclusion, the use of soluble support presents a clear advantage in terms of finish and increased performance of the 3D printing staff (i.e reduce the time that a person needs to speed removing non-soluble support material), but on the other hand, it presents an increase in cost and printing time.

Abstract: Within the technologies that make up Additive Manufacturing (AM), one of the ones that have taken the greatest prominence in recent times is DED (Direct Energy Deposition), particularly that of wire feedstock. The W-DED/LB technique has some benefits compared to other AM methods, such as the fabrication of relatively larger parts, repair capabilities of the damaged areas of a component, cladding of different materials on existing parts, and reduced material waste.This study describes the optimisation of processing parameters for the manufacturing stainless steel (SS316L) and Inconel 718 alloys (INC718) using W-DED/LB. This is performed by modifying processing aspects like deposition trajectories, laser power, displacement speeds of the DED head, etc, with the aim of obtaining high deposition rates and a density above 99.5%. Once the alloy systems are optimised, a characterisation campaign has been performed, which includes a series of tests as well for defectology analysis using X-ray Computed Tomography (CT). Finally, the influence of different heat treatments on the tensile behaviour is analysed.This work has developed the technology of DED assembling in a Kuka-robot, so the challenge has not only been to control the DED system, but also the communication with the robotic arm to guarantee perfect harmony between all the parts that make up the W-DED/LB system.

Abstract: This work presents a study regarding the mechanical characterization of polymethyl methacrylate (PMMA) patterned samples manufactured via material-extruded additive manufacturing. In recent years, literature about mechanical analysis in additive manufacturing has been growing increasingly, especially for material extrusion-based techniques. However, this trend surpasses the speed of information released by standard councils, existing no clear specifications for polymer characterization apart from conventional techniques. This issue has led to premature breakage as well as fracture not located in the constant cross-section region of samples. The main purpose of this present research is focused on the analysis of diverse modifications of the standard injection geometries to tackle the mentioned problems. Several printing methodologies were compared, changing slicing and geometrical parameters such as number of walls, and fillet radius. Then, the manufacturing of PMMA samples with a material extrusion printer took place to characterize both the material and the effective properties of the structures. With the information post-processed from tensile and compression tests, disparities were found between different geometrical designs for both elastic modulus and ultimate stress. Moreover, diverse location of fractures were observed for the studied geometries. The data obtained from the analysis was valuable to establish a proper protocol for further studies. The experiments suggest that for tensile tests the golden standard is selecting rectangular specimens since they do not induce premature breakage nor fracture outside of gauge length.

Abstract: Traditional dehumidification equipment is based on vapour compression units. However, they depend mainly on electrical energy and use polluting gases. An alternative to this equipment is desiccant dehumidification systems, which is based on adsorbent materials. These desiccant systems are an efficient way of removing moisture from the air in buildings with high latent loads. This work presents a new way to manufacture fixed-bed desiccant elements that can remove moisture from an air flow. The desiccant element is obtained by material extrusion-based additive manufacturing (fused filament fabrication or FFF). This technology is cost-effective and provides a precision and finish suitable for the intended use. The filament used is Pine, consisting of an easy printable thermoplastic matrix (polylactic acid, PLA, 80 wt%) and a filler based on pine wood powder (20 wt%). This composite material reached a water absorption capacity of 11.5 %. The experimental results of the desiccant air unit demonstrated high dehumidification capacity, up to 39 mg/s, for a regeneration air temperature of 50 °C. The volumetric adsorption rate was also high, up to 30 g/s·m³, for low pressure drop values, below 522 Pa. The proposed method allows the customised, on-demand and just-in-time manufacturing of air dehumidification systems based on the use of biodegradable desiccant materials of organic origin. Such solutions contribute to the circular economy promoted by The United Nations in the Sustainable Development Goals.

Abstract: Heat exchangers have traditionally been produced on mass using metal alloys and complex manufacturing processes. This work proposes an alternative production via for the additive manufacturing of a cost-effective air-to-air heat exchanger, based on the use of the stereolithography technology. The element has been produced on a FormLabs Form3 printer using standard photosensitive resin. The dimensions of the heat exchanger were 100 × 100 × 100 mm³ and the wall thickness was 0.5 mm. The manufacturing cost of the element was 53.11 €. The heat exchanger was experimentally tested in an air handling laboratory under different climatic conditions. The thermal power of the equipment was 200 W, which is equivalent to a power-volume ratio equal to 200 kW/m³. The experimental energy efficiency was equal to 0.54 (for a number of heat transfer units equal to 1.4) and an overall energy transfer coefficient (U) equal to 1823 W/m²K. In addition, the results showed that the thermal conductivity of the material was less influential the smaller the thickness of the heat exchanger channels. The obtained results show that stereolithography is an economical alternative to obtain customized and high compactness heat exchangers, on demand and just in time.

Abstract: Additive Manufacturing (AM) technologies, also known as 3D printing, are increasingly used to produce usable and appealing end products. Besides allowing greater design freedom, 3D printing can increase material efficiency and drastically reduce production time. However, processes are still lacking in the quality of products. Among dozens of parameters that can influence the printing process, the study analyses the impact of the printing orientation (PO) on the product's geometrical accuracy and the process's overall efficiency. An experimental analysis has been designed and executed. A Fused Deposition Modelling (FDM) machine has been used to print the specimens. In particular, the experimental results showed the influence of PO on the geometrical characteristics of the specimens (bending angle, torsion angle, deviations, flatness tolerance and thickness). Moreover, the variability of production time, cost and resource consumption by varying PO has been investigated. As expected, the worst geometrical performance is shown by upright specimens. Furthermore, the ANOVA analysis underlines that only the orientation of the specimen with respect to the Z-axes (flat, on-edge and up-right) is statistically significant for the geometrical accuracy of the specimens. The method ELECTRE I has been used as Multi Criteria Decision-Making Method to rank the build configurations according to the resulting product's geometrical accuracy, the process performance and decision-maker’s priorities.

Abstract: The excellent properties of ceramic materials make them essential for fields such as aeronautic, biomedical, dental, jewelry or electronic. However, the main drawback when manufacturing parts with these materials is their high brittleness. Additive manufacturing processes for ceramic materials allow this limitation to be overcame. Nevertheless, the required stages for additive manufacturing of ceramic parts make the process extremely difficult and with significant influence on the dimensional and geometrical precision.The stereolithography additive process for ceramic is similar to the conventional polymeric process, excepting that the raw material is a slurry of ceramic particles mixed with photosensitive resins. Consequently, two thermal post-processes are necessary to obtaining the final part: (i) debinding and (ii) sintering. Each of the stages in the process is critical because deformation, cracks or fractures can happen, mainly due to temperature changes during the post-processes. Consequently, a deep study of the stages of printing, debinding and sintering is necessary for optimizing them and to produce high-quality parts.In this work, two different types of supports have been studied to prevent deformations during the printing stage. Despite the great importance of supports in this additive manufacturing process, scarce information is available about their design beyond very general recommendations. This paper contains a preliminary study that reveals the effectiveness of using adequate supports to avoid deformations in the printing process. Parts have been scanned with a 3D structured light scanner and the obtained meshes compared with the nominal CAD. This comparison has revealed deformations during the sintering process; therefore, the need to extend the study of support design to the post-processing stages. Future work is necessary to extend this preliminary analysis of support design to improve the quality of SLA ceramic parts with different shape and size.

Abstract: With technological advances, additive manufacturing processes have been gaining prominence in several industrial areas including maintenance, repair, and overhaul (MRO) processes. A process with great potential for repairing and rebuilding metal parts is laser metal deposition (LMD) technology. Despite the high potential, LMD implementation in the repair industry is not straightforward, due to the geometry variability of parts and damages to be repaired. This paper presents a repairability study that evaluates the remaining volume of the repair of different types of damages in AISI 316L parts by LMD, and determines the most appropriate deposition strategies to adapt to the repair process. This study involves the characterization and classification of common defects in metallic parts and the development of a design of experiments, in which, given the damage geometry, volume, and location, the best repair toolpath to be adopted and the ideal parameterization for the repair process are determined. The ability to correct part damage is assessed from a geometric, mechanical and energetic approach, and explores the possibility of including LMD in an adaptive and intelligent MRO system. The result of this work establishes a new deposition strategy approach based on a modified contour-parallel deposition strategy for repairing metal parts. This study also demonstrates that in surface damage cases, a fixed point strategy is highly effective, especially when using higher laser power values and larger laser spot diameters, enabling an easier process automation. However, in edge and corner damage cases, the best repair approach is using trajectory strategies that constitute material support between deposition tracks and layers. Additionally, it is demonstrated that the corners are the most critical zones that require temperature control throughout the entire repair process.