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In accordance with the high demands of the market for products with complicated geometry that are difficult or impossible to manufacture using classical methods, additive manufacturing has great advantages because it enables production of complicated geometry in a short time without the need for moulds and/or tools. Additive manufacturing has been present on the market for more than 37 years and every day technologies are improved, new materials and new 3D printers are developed. Selective laser sintering (SLS) is a procedure in additive manufacturing in which material must be exclusively in the form of powder, and by passing the laser beam only towards the cross-section of the 3D CAD model, the lighted particles soften, and their mutual sintering/fusion occurs. The rest of the unfused powder serves as a support structure and after production it is sifted and can be reused, and as such constitutes a recycled material. This possibility to use recycled materials is a great advantage of the SLS process since huge extraction of raw materials from nature (the global extraction of raw materials is expected to increase by 60% by 2060) raises environmental concerns and increases the price of raw materials.
Therefore, recycling of material is becoming an increasingly important factor in the SLS technology, as well as the mixing ratio of the original and the already used material. When using polymer materials, especially polyamide (PA), from the very beginning of the SLS process, a mixture of 50% recycled and 50% original material was used. But the question arises whether the production of a product with 100% recycled material that has undergone several processing cycles can satisfy the dimensional stability and roughness in comparison with the original material itself, given that unfused material was also exposed to indirect temperature caused by the laser beam. The orientation in the machine working space also affects the dimensional stability and roughness of the final product. An analysis of the impact of three different types of product orientation in the machine working area (with a total height of 4 mm, 10 mm, and 150 mm) has been performed. From the analysis, carried out with the same production parameters (laser power 21 W, laser beam speed 2500 mm/s and hatch distance 0.25 mm), it was concluded that 100% recycled material has a higher roughness (mean arithmetic deviation of the profile Ra) by 35% from original material and mixed in a ratio of 50:50%. Furthermore, products with the lowest height had the lowest roughness, i.e. when the orientation is such that the total height is 4 mm. The difference in dimensions in the application of recycled material is manifested exclusively in the z direction, i.e. the deviations in the height of the product are higher when using recycled material, while in other directions no significant difference is noticed, which leads to the conclusion that the material can be maximally used after several cycles of processing where it previously served as a support structure.
additive manufacturing; dimensional precision; polyamide; recycling; roughness; selective laser sintering
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