The influence of the energy density on dimensional, geometric, mechanical and morphological properties of SLS parts produced with single and multiple exposure types

Selective Laser Sintering (SLS) is a Powder Bed Fusion technology that embraces a large number of variables influencing the properties of the parts produced. The well-known dependence and complex interaction established between the main process parameters demands continuous empirical research for effective SLS monitoring. The assessment of the energy density supplied by the laser beam to the powder bed during the process, that depends on the combination of the laser power, hatch distance, scan speed and layer thickness, is frequently considered for that purpose. Therefore, this research intends to evaluate the influence of the energy density on the dimensional, geometric, mechanical and morphological properties of SLS parts produced with conventional Polyamide 12 material. In this study, we considered different hatching and contour parameters in the energy range between 0.158 J/mm3 and 0.398 J/mm3 through single and multiple exposure types defining individual and combined parameterization sets, respectively. Results from X-ray computed tomography, tensile tests and scanning electron microscopy show that the implementation of a skin/core configuration allows the production of SLS parts with a valuable set of properties, minimizing the trade-off between mechanical strength and overall accuracy.This work was co-funded by the European Regional Development Fund through the Operational Competitiveness and Internationalization Programme (COMPETE 2020) [Project No. 47108, “SIFA”; Funding Reference: POCI-01-0247-FEDER-047108] and by the Foundation for Science and Technology (FCT) through the PhD scholarship 2020.04520.BD

Analytical characterization of polyamide 11 and related polymers used for additive manufacturing

In order to meet the quality requirements for polymers used in additive manufacturing, especially in the laser sintering and the multi jet fusion process, as well as to guarantee high quality of printed parts, a comprehensive characterization of the intrinsic properties of the applied materials is necessary. Therefore, methods were developed and optimized to analytically investigate polyamide 11 and 12 samples both at the macromolecular and the low molecular weight range

Determination of Optimal Processing Conditions for the Production of Polyamide 11 Parts using the Laser Sintering Process

Due to the advancements during the last decade, the laser sintering process has achieved a high technical level, allowing for Rapid Manufacturing of parts in some applications. However, only few polymers are commercially available for the process. Polyamide 12 dominates the market with share of nearly 90 %. Other laser sintering materials differ in part properties from PA 12. Therefore, they are more suitable for some specific applications. Within these, Polyamide 11 has the highest distribution on the market. PA 11 offers some advantages like significant higher part ductility but also some disadvantages like more warpage or higher processing temperatures. However, literature provides in general only little information on the processing of PA 11 and how to achieve optimal processing conditions. A DOE approach using the response surface methodology was utilized to study the correlations between process parameters and part properties. Laser power, scan speed, hatch distance, layer thickness and outline energy density were varied in order to improve the part quality considering mechanical properties, surface roughness and part density. Additionally, results for process influences and part properties were compared with those found for Polyamide 12 in order to derive general correlations. Based on the performed study, optimized process parameter sets are established for PA 11 resulting in improved part properties

Suppression of Orange Peel and Acceleration of Partcake Internal Cooling in Polymer Laser Sintering Process

東京都立大学Tokyo Metropolitan University博士（工学）doctoral thesi

Investigations into glass additive manufacturing by selective laser melting and directed energy deposition

Glass is a desirable material for many industrial applications, offering unique properties such as transparency, chemical durability, and high thermal resistance. Current production of complex glass shapes is typically achieved through the use of moulds. Customisation of glass geometries therefore often increases the production cost of bespoke glass pieces. Additive Manufacturing methods are capable of fabricating complex geometries at efficient cost for small production volumes, making customisation cost-effective. The opportunities that AM presents for glass manufacturing could be exploited for a number of applications, including fabrication of customised microfluidic devices, and bespoke décor for high value glass packaging. The potential applications for additively manufactured glass parts have driven research and industry to face the associated challenges, such as thermal stresses from high temperature gradients causing parts to crack or fracture, high transmittance in the near infrared (NIR) range reducing laser absorption at certain wavelengths, and porosities and cracking compromising transparency. In this thesis, research into glass processing by two AM techniques is presented: Selective Laser Melting (SLM), and powder-fed Directed Energy Deposition (DED). Investigations were carried out to define suitable processing parameters for SLM and DED of a common glass composition, soda lime silica. Investigations evaluated the effect of laser parameters and processing set ups on glass processing. For SLM, soda lime silica was processed onto two substrates: soda lime silica glass and alumina. Different geometries were fabricated, including single walls, cubes, hollow cylinders, lattices, and text structures. Channel structures were fabricated to demonstrate the potential for customised glass continuous flow reactor (CFR) production by SLM. For on-glass processing, adhesion of parts to substrates was inspected, highlighting the impact of SLM processing on crack formation in glass substrates for the first time. The effect of substrate heating on glass SLM was also investigated, showing promising results on transparency and porosity in glass SLM. For applications requiring removal of parts from substrates, alumina discs provided suitable adhesion to glass powders during processing, and easy removal of parts post process. Energy densities between 80-110 J/mm3 are recommended for processing 3D structures on alumina substrates, and for on-glass processing energy densities above 28 J/mm2 were found necessary to achieve glass consolidation. Novel glass processing by a powder-fed DED method is presented in this work, demonstrating customisation of glass bottle packaging. Process maps are presented for powder-fed DED of soda lime silica glass onto glass substrates for the first time, evaluating the effect of laser power and scan speed on glass powder consolidation and substrate cracking. Suitable processing parameters were identified, with cracking found to associate with laser power, and consolidation of glass correlating with energy density. Parameters of laser power below 115 W and energy density above 11 J/mm2 are recommended. Challenges including the transmission of laser energy through transparent feedstocks and substrates and delivery of glass powder through nozzle systems were evaluated and overcome. Darkened base plates are recommended below transparent substrates to reduce laser reflection, and a single layer of cellophane tape was used to improve glass melting and adhesion to substrates by acting as a heat source during processing. Also highlighted in this research was the flowability of glass powder feedstocks for AM methods, and the effect of flowability on forming homogenous powder beds for SLM and achieving consistent powder delivery for DED. A case study on glass powder spheroidisation is presented, comparing methods of altering angular glass powder morphologies for improved flowability. Flame spheroidisation and plasma spheroidisation are presented as promising techniques for improving flowability of glass materials for AM, and their limitations are evaluated. The work done during the course of this PhD contributes to understanding of glass processing by SLM and through powder-fed DED, demonstrating the potential for glass processing by these AM methods. Recommendations are made for future work to further develop these methods of glass processing, with the hopes of establishing AM as a valid technique for forming customised, complex geometries for high value applications

Investigations into glass additive manufacturing by selective laser melting and directed energy deposition

Glass is a desirable material for many industrial applications, offering unique properties such as transparency, chemical durability, and high thermal resistance. Current production of complex glass shapes is typically achieved through the use of moulds. Customisation of glass geometries therefore often increases the production cost of bespoke glass pieces. Additive Manufacturing methods are capable of fabricating complex geometries at efficient cost for small production volumes, making customisation cost-effective. The opportunities that AM presents for glass manufacturing could be exploited for a number of applications, including fabrication of customised microfluidic devices, and bespoke décor for high value glass packaging. The potential applications for additively manufactured glass parts have driven research and industry to face the associated challenges, such as thermal stresses from high temperature gradients causing parts to crack or fracture, high transmittance in the near infrared (NIR) range reducing laser absorption at certain wavelengths, and porosities and cracking compromising transparency. In this thesis, research into glass processing by two AM techniques is presented: Selective Laser Melting (SLM), and powder-fed Directed Energy Deposition (DED). Investigations were carried out to define suitable processing parameters for SLM and DED of a common glass composition, soda lime silica. Investigations evaluated the effect of laser parameters and processing set ups on glass processing. For SLM, soda lime silica was processed onto two substrates: soda lime silica glass and alumina. Different geometries were fabricated, including single walls, cubes, hollow cylinders, lattices, and text structures. Channel structures were fabricated to demonstrate the potential for customised glass continuous flow reactor (CFR) production by SLM. For on-glass processing, adhesion of parts to substrates was inspected, highlighting the impact of SLM processing on crack formation in glass substrates for the first time. The effect of substrate heating on glass SLM was also investigated, showing promising results on transparency and porosity in glass SLM. For applications requiring removal of parts from substrates, alumina discs provided suitable adhesion to glass powders during processing, and easy removal of parts post process. Energy densities between 80-110 J/mm3 are recommended for processing 3D structures on alumina substrates, and for on-glass processing energy densities above 28 J/mm2 were found necessary to achieve glass consolidation. Novel glass processing by a powder-fed DED method is presented in this work, demonstrating customisation of glass bottle packaging. Process maps are presented for powder-fed DED of soda lime silica glass onto glass substrates for the first time, evaluating the effect of laser power and scan speed on glass powder consolidation and substrate cracking. Suitable processing parameters were identified, with cracking found to associate with laser power, and consolidation of glass correlating with energy density. Parameters of laser power below 115 W and energy density above 11 J/mm2 are recommended. Challenges including the transmission of laser energy through transparent feedstocks and substrates and delivery of glass powder through nozzle systems were evaluated and overcome. Darkened base plates are recommended below transparent substrates to reduce laser reflection, and a single layer of cellophane tape was used to improve glass melting and adhesion to substrates by acting as a heat source during processing. Also highlighted in this research was the flowability of glass powder feedstocks for AM methods, and the effect of flowability on forming homogenous powder beds for SLM and achieving consistent powder delivery for DED. A case study on glass powder spheroidisation is presented, comparing methods of altering angular glass powder morphologies for improved flowability. Flame spheroidisation and plasma spheroidisation are presented as promising techniques for improving flowability of glass materials for AM, and their limitations are evaluated. The work done during the course of this PhD contributes to understanding of glass processing by SLM and through powder-fed DED, demonstrating the potential for glass processing by these AM methods. Recommendations are made for future work to further develop these methods of glass processing, with the hopes of establishing AM as a valid technique for forming customised, complex geometries for high value applications