Selective Laser Sintering Process Research Articles

Computational Framework to Model the Selective Laser Sintering Process.

Selective laser sintering (SLS) is one of the most well-regarded additive manufacturing (AM) sub-processes, whose popularity has been increasing among numerous critical and demanding industries due to its capabilities, mainly manufacturing parts with highly complex geometries and desirable mechanical properties, with potential to replace other, more expensive, conventional processes. However, due to its various underlying multi-physics phenomena, the intrinsic complexity of the SLS process often hampers its industrial implementation. Such limitation has motivated academic interest in obtaining better insights into the process to optimize it and attain the required standards. In that regard, the usual experimental optimization methods are time-consuming and expensive and can fail to provide the optimal configurations, leading researchers to resort to computational modeling to better understand the process. The main objective of the present work is to develop a computational model capable of simulating the SLS process for polymeric applications, within an open-source framework, at a particle-length scale to assess the main process parameters' impact. Following previous developments, virgin and used polymer granules with different viscosities are implemented to better represent the actual process feedstock. The results obtained agree with the available experimental data, leading to a powerful tool to study, in greater detail, the SLS process and its physical parameters and material properties, contributing to its optimization.

A novel process study of biomass-based graphene-enriched body based on selective laser sintering technology

Laser-induced graphene (LIG) on natural wood is a novel method for producing porous graphene composite materials with low cost and high efficiency. However, the three-dimensional (3D) fabrication of biomass-derived LIG remains challenging, restricting its applications in various domains. This paper introduces a “SCG” (sintering, carbonization and grapheneization) preparation strategy, which integrates selective laser sintering and multiple laser processes to achieve rapid fabrication of graphene-enriched structures from natural biomass materials with complex shapes. The experiment demonstrates that the sintering necks are preserved during the carbonization and graphitization process, maintaining the original interlocking state of the upper and lower layers. The laser power and irradiation times for producing porous graphene have an optimal range, beyond which the quality of the synthesized graphene deteriorates. This study validates the effectiveness of this method and investigates the optimal process parameters and application directions. This method not only offers a 3D fabrication technique for biomass-derived LIG for the first time, but also enables 3D printing of biomass carbon electrodes, which have the benefits of wide applicability, no need for activators, low cost, and high efficiency.

Process Study of Selective Laser Sintering of PS/GF/HGM Composites.

To address the issues of insufficient strength and poor precision in polystyrene forming parts during the selective laser sintering process, a ternary composite of polystyrene/glass fiber/hollow glass microbeads was prepared through co-modification by incorporating glass fiber and hollow glass microbeads into polystyrene using a mechanical mixing method. The bending strength and dimensional accuracy of the sintered composites were investigated by conducting an orthogonal test and analysis of variance to study the effects of laser power, scanning speed, scanning spacing, and delamination thickness. The process parameters were optimized and selected to determine the optimal combination. The results demonstrated that when considering bending strength and Z-dimensional accuracy as evaluation criteria for terpolymer sintered parts, the optimum process parameters are as follows: laser power of 24 W, scanning speed of 1600 mm/s, scanning spacing of 0.24 mm, and delamination thickness of 0.22 mm. Under these optimal process parameters, the bending strength of sintered parts reaches 6.12 MPa with a relative error in the Z-dimension of only 0.87%. The bending strength of pure polystyrene sintered parts is enhanced by 15.69% under the same conditions, while the relative error in the Z-dimension is reduced by 63.45%. It improves the forming strength and precision of polystyrene in the selective laser sintering process and achieves the effect of enhancement and modification, which provides a reference and a new direction for exploring polystyrene-based high-performance composites and expands the application scope of selective laser sintering technology.

Architected flexible syntactic foams: Additive manufacturing and reinforcing particle driven matrix segregation

Polymer syntactic foams are transforming materials that will shape the future of next-generation aerospace and marine structures. When manufactured using traditional processes, like compression molding, syntactic foams consist of a solid continuous polymer matrix reinforced with stiff hollow particles. However, polymer matrix segregation can be achieved during the selective laser sintering process with thermoplastic polyurethane (TPU). It is uncertain what role hollow particles play in forming this matrix segregation and its impact on the corresponding mechanical properties of syntactic foams. We show that the size of the hollow particles controls the internal microscale morphology of matrix segregation, leading to counter-intuitive macroscale mechanical responses. Particles with diameters greater than the gaps between the cell walls of the segregated matrix get lodged between and in the walls, bridging the gaps in the segregated matrix and increasing the stiffness of syntactic foams. In contrast, particles with smaller diameters with higher particle crushing strength get lodged only inside the cell walls of the segregated matrix, resulting in higher densification stresses (energy absorption). We show that stiffness and densification can be tuned while enabling lightweight syntactic foams. These novel discoveries will aid in facilitating functional and lightweight syntactic foams for cores in sandwich structures.

Thermo-Mechanical Simulation of the Powder Bed of Polyamide 12 during the Selective Laser Sintering Process

The selective laser sintering process is considered to be a new process among the most innovative techniques in the field of additive manufacturing, to facilitate the development and the technical improvement of this process, Thermo-mechanical investigation is necessary to control the SLS process, in this paper a thermo-mechanical simulation of selective laser sintering of polyamide 12 will be presented., this thermo-mechanical simulation was made under the COMSOL Multiphysics software, using Hook's law with the equilibrium and accounting equations for the calculation of the residual stresses appearing in the sintered layer of polyamide 12, this simulation was made for the prevention of deformations appeared in the final printed parts. The calculation of von Mises stress and the calculation of the magnitude of displacement within a sintered layer of polyamide 12 was done, with the analysis of the volumetric strain and the calculation of the volume: stress tensors, principal strain directions.

An Update on 3D-printed Orthodontic Aligners

Clear Aligner Treatment (CAT) is an orthodontic technique used to align teeth with removable and scarcely visible appliances. Conventionally, these are produced through the process of thermoforming. The inherent disadvantages of thermoforming include increased surface roughness leading to plaque accumulation, compromised biomechanics due to a reduction in force delivered and flexure of the aligner, and altered mechanical properties, such as increased opacity, water sorption, and hardness. Direct Three-dimensional (3D)-printed aligners, or Direct Printed Aligners (DPA), introduce a new frontier to aligner technology and are a recent addition to the ever-improving field of orthodontics. Through 3D printing, the various disadvantages of thermoformed aligners like surface roughness, extent and definition of aligner borders, undercuts, and differential thickness of the aligner can be controlled to enhance the accuracy of aligner fit with lesser reliance on attachments. 3D printing of aligners is more environmentally friendly since there is no subtractive process for thermoforming or post-processing of the TA. Various methods of 3D printing, such as selective laser melting, selective laser sintering, Stereolithography (SLA), and Digital Light Processing (DLP), can be applied to the printing of clear aligners. Challenges in printing primarily involve maintaining transparency and designing support during the printing process. The present review aimed to include a detailed description of all aspects of direct 3D-printed aligners.

Investigation of the ageing conditions of PEEK powder for the selective laser sintering process

ABSTRACT Ageing mechanism of PEEK powders at optimal powder bed temperature (T b) and factors affecting their reusability remain unclear. This study investigated effects of gas type (air, nitrogen, and argon) and holding time (0–8 h) on physico-chemical properties of PEEK powders at optimal T b. The hardness of sintered single-layers was measured to evaluate the quality of the parts. Results revealed that the flowability, morphology, and crystallinity of PEEK powders changed with holding time in different gases. Further, the ageing mechanism involved chain crosslinking within molecular chains in all gases but chain breakage in air. Further, laser power required for SLS decreased with the increase in holding time. However, the hardness of single-layers was stable for powders aged in argon for up to 8 h, but decreased by 7% for powders aged in nitrogen for 4 h. Severe degradation and blackening were observed for single-layers sintered in air using both virgin and aged powders.

Anisotropic and elastoplastic mode‐I fracture toughnesses of three additively manufactured polymers fabricated via material extrusion and powder bed fusion

AbstractTwo types of additive manufacturing (AM) techniques, fused filament fabrication (FFF) and selective laser sintering (SLS), were employed to fabricate specimens from three widely used AM polymers: polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyamide (PA): the first two using FFF and PA using SLS. The single‐edge notch bending (SENB) test was employed to measure the mode‐I fracture toughnesses of bulk and AM specimens in three directions relative to the printing direction. Unlike their bulk counterparts, AM polymers exhibited anisotropic fracture toughness. The degree of anisotropy in FFF‐PLA was much larger than FFF‐ABS and SLS‐PA. The smallest fracture toughness value for each AM polymer was achieved along the transverse printing direction. Different AM techniques affect fracture toughness differently. For instance, FFF is shown to increase PLA's plastic deformation, while SLS reduces PA's ductility. The decrease in fracture toughness is consistently observed when transitioning from bulk polymers to AM ones, reaching as much as 75%. This study provides insights into the fracture toughness of AM parts made via popular plastics, which are crucial for designing structural parts using FFF and SLS processes.

Dry Friction and Wear Behavior of Laser-Sintered Graphite/Carbon Fiber/Polyamide 12 Composite.

Carbon fiber-reinforced polymers (CFRPs) are being used extensively in modern industries that require a high strength-to-weight ratio, such as aerospace, automotive, motorsport, and sports equipment. However, although reinforcement with carbon fibers improves the mechanical properties of polymers, this comes at the expense of abrasive wear resistance. Therefore, to efficiently utilize CFRPs in dry sliding contacts, solid lubricant is used as a filler. Further, to facilitate the fabrication of objects with complex geometries, selective laser sintering (SLS) can be employed. Accordingly, in the present work, graphite-filled carbon fiber-reinforced polyamide 12 (CFR-PA12) specimens were prepared using the SLS process to explore the dry sliding friction and wear characteristics of the composite. The test specimens were aligned along four different orientations in the build chamber of the SLS machine to determine the orientation-dependent tribological properties. The experiments were conducted using a pin-on-disc tribometer to measure the coefficient of friction (COF), interface temperature, friction-induced noise, and specific wear rate. In addition, scanning electron microscopy (SEM) of tribo-surfaces was conducted to specify the dominant wear pattern. The results indicated that the steady-state COF, contact temperature, and wear pattern of graphite-filled CFR-PA12 are orientation-independent and that the contact temperature is likely to approach an asymptote far below the glass transition temperature of amorphous PA12 zones, thus eliminating the possibility of matrix softening. Additionally, the results showed that the Z-oriented specimen exhibits the lowest level of friction-induced noise along with the highest wear resistance. Moreover, SEM of tribo-surfaces determined that abrasive wear is the dominant wear pattern.

Effect of heat treatment on microstructure and mechanical properties for laser powder bed fusion of nickel-based superalloy: A review

Laser powder bed fusion (LPBF) is most widely used method for presenting an innovative manufacturing approach that operates layer by layer. This technique relies on a geometrical model and offers a viable substitute for material processing. The utilization of LPBF for producing nickel alloys has garnered significant interest across various domains due to their exceptional attributes. These attributes include excellent corrosion resistance, favorable mechanical properties, and the quality of the manufactured alloys. The particular solidification conditions arising from the metal fusion in the selective laser fusion process, combined with layer-by-layer deposition, give rise to unique microstructural characteristics. These include the development of supersaturated solid solutions, significant microstructural refinement, and the introduction of residual stresses. As a result, adjustments might be necessary for heat treatment and hot isostatic pressing, practices commonly employed with conventionally manufactured nickel alloys. These adaptations would accommodate the metallurgical nuances specific to Ni alloys produced through direct metal laser deposition, effectively addressing issues arising from the inherent characteristics of the process. This review paper compiles the most recent scientific discoveries concerning thermal treatment applied to additively manufactured nickel alloys using the selective laser sintering process.

3D Printing of Personalised Carvedilol Tablets Using Selective Laser Sintering.

Selective laser sintering (SLS) has drawn attention for the fabrication of three-dimensional oral dosage forms due to the plurality of drug formulations that can be processed. The aim of this work was to employ SLS with a CO2 laser for the manufacturing of carvedilol personalised dosage forms of various strengths. Carvedilol (CVD) and vinylpyrrolidone-vinyl acetate copolymer (Kollidon VA64) blends of various ratios were sintered to produce CVD tablets of 3.125, 6.25, and 12.5 mg. The tuning of the SLS processing laser intensity parameter improved printability and impacted the tablet hardness, friability, CVD dissolution rate, and the total amount of drug released. Physicochemical characterization showed the presence of CVD in the amorphous state. X-ray micro-CT analysis demonstrated that the applied CO2 intensity affected the total tablet porosity, which was reduced with increased laser intensity. The study demonstrated that SLS is a suitable technology for the development of personalised medicines that meet the required specifications and patient needs.

Evaluation of the mechanical properties of polyamide 12 regarding different percentages of reused material in the selective laser sintering process

PurposeAdditive manufacturing (AM) has been one of the most highlighted processes of the last few years. AM prints complex parts and items from 3D files regarding different materials, such as polymers. Moreover, there are different AM techniques available for polymers, such as selective laser sintering. In the SLS technology, polyamides 11 and 12 lead 88% of the market. These materials are high-cost and use an average of 50% of virgin material at each printing. It is possible to use lower rates of virgin material, but at least 30% is recommended. Low rates of virgin material decrease mechanical properties.Design/methodology/approachThis study aims to evaluate the influence on the mechanical properties of the percentage of reused PA12 in parts manufactured by the SLS process. The specimens of PA12 were manufactured with a percentage of virgin/reused polymer of 50/50, 40/60, 30/70, 20/80 and 10/90. We considered three distinct printing directions to compare the mechanical properties of the specimens: horizontal, perpendicular and vertical.FindingsThe results showed that when the percentage of reused material increases, the tensile strength limit (TSL), flexural strength limit and Shore D hardness decrease. Another aspect visualized was the fragile behavior presented in the vertical specimens. In addition, DSC analysis indicated a 2% reduction of crystallinity. Scanning electron microscopy images revealed spherical voids and unfused particles of PA12 at the fracture of tensile test specimens. The material thermal history and unfused particles could decrease the material properties.Originality/valueWe observed that the mechanical properties, such as the TSL, flexural strength limit and hardness, decrease as the percentage of reused material increases. In addition, the process presented a printing-direction dependence, where the vertical direction presented as the more brittle between the ones used.

Preparation and 3D printability study of bio-based PBAT powder for selective laser sintering additive manufacturing

Additive Manufacturing (AM) has been recognized as a highly versatile manufacturing method with high potentiality. Polymers are materials of central importance for AM specifically for fabrication of original and customized parts. However, most of the polymers currently in use derive from fossil sources, which extensive use represents a critical issue for the environment. This scenario encourages the use of sustainable and biodegradable biopolymers. The present paper focuses on the study of the printability of a bio-based powder, starting from an aromatic‐aliphatic biodegradable polyester, poly (butylene adipate-co-terephthalate) (PBAT), as sustainable alternative for Selective Laser Sintering (SLS) conventional materials. Bio-based polymers represent environmentally friendly materials that can be suitable for AM, although several problems still need to be solved for SLS applications. In fact, it is challenging to produce polymeric powders with specific particle size and size distribution, spherical shape, good flowability and processability. From this point of view, PBAT powder was prepared by means of an emulsion solvent evaporation method, characterized from morphological and thermal points of view, and then successfully used for SLS process, with the aim to obtain novel 3D printed objects using for the first time this biodegradable polymer.

Optimization of Selective Laser Sintering Three-Dimensional Printing of Thermoplastic Polyurethane Elastomer: A Statistical Approach

This research addresses the challenge of determining the optimal parameters for the selective laser sintering (SLS) process using thermoplastic polyurethane elastomer (TPU) flexa black powder to achieve high-quality SLS parts. This study focuses on two key printing process parameters, namely layer thickness and the laser power ratio, and evaluates their impact on four output responses: density, hardness, modulus of elasticity, and time required to produce the parts. The primary impacts and correlations of the input factors on the output responses are evaluated using response surface methodology (RSM). A particular response optimizer is used to find the optimal settings of input variables. Additionally, the rationality of the model is verified through an analysis of variance (ANOVA). The research identifies the optimal combination of process parameters as follows: a 0.11 mm layer thickness and a 1.00 laser power ratio. The corresponding predicted values of the four responses are 152.63 min, 96.96 Shore-A, 2.09 MPa, and 1.12 g/cm3 for printing time, hardness, modulus of elasticity, and density, respectively. These responses demonstrate a compatibility of 66.70% with the objective function. An experimental validation of the predicted values was conducted and the actual values obtained for printing time, hardness, modulus of elasticity, and density at the predicted input process parameters are 159.837 min, 100 Shore-A, 2.17 MPa, and 1.153 g/cm3, respectively. The errors between the predicted and experimental values for each response (time, hardness, modulus of elasticity, and density) were found to be 4.51%, 3.04%, 3.69%, and 2.69%, respectively. These errors are all below 5%, indicating the adequacy of the model. This study also comprehensively describes the influence of process parameters on the responses, which can be helpful for researchers and industry practitioners in setting process parameters of similar SLS operations.

A thermodynamically consistent non-isothermal phase-field model for selective laser sintering

Selective laser sintering (SLS) additive manufacturing possesses the characteristics of extremely non-uniform temperature distribution and high temperature gradient, which are unmanageable by the traditional isothermal models. In this work, we propose a thermodynamically consistent non-isothermal phase-field model (PFM) to investigate the microstructure evolution during SLS process with the full consideration of temperature gradient effect. The model is derived from a thermodynamic framework which invokes the microforce theory and Coleman–Noll procedure. The temperature-dependent parameters in the free energy formulation are determined by the experimentally measurable surface energy and grain boundary energy with a given interface width. Benchmark simulations of sintering two particles under temperature gradient indicate that the grain boundary has the trend to migrate to the higher temperature region, which is further verified by molecular dynamics simulations. This phenomenon cannot be captured by the traditional isothermal PFM. Grain boundary migrations can also be driven by curvature for two unequally-sized particles. Simulations on SLS of powder bed show the dependence of microstructure evolution and final porosity on the laser power and scanning speed. The developed non-isothermal PFM could be a practicable toolkit for the microstructure simulation of SLS process.

Research on selective laser sintering process of ternary composite materials (PP/MCMB/CNT)

The composite material composed of polypropylene and conductive filler has good corrosion resistance, light weight and good conductivity, and is one of the preferred materials for fuel cell bipolar plates. The formation of ternary composite materials composed of polypropylene, mesocarbon microbeads and carbon nanotubes is realized by selective laser sintering. The influences of laser power, scanning speed, scanning spacing and defocus amount on forming quality are studied. The formability of mixed powders with different composition proportions as well as the conductivity, bending strength and hydrophobicity of the formed samples are investigated. The results show that when the input energy density is insufficient, it is easy to form defects such as spheroidized particles and worm-like structures. The parameters with good forming quality are in the following range: defocus amount is + 3 mm; line energy density is 2.94–3.85 J/mm2; peak power density is 1.13 × 104-1.48 × 104 W/mm2 and scanning spacing is 0.1–0.14 mm. The conductive filler in composite material is beneficial to improving the conductivity, but it will reduce the formability of the mixed powders and reduce the bending strength and hydrophobicity of the sample. The effect of mesocarbon microbeads on improving conductivity or on reducing hydrophobicity is smaller than that of carbon nanotubes.

Additive manufactured parts produced by selective laser sintering technology: porosity formation mechanisms

Abstract Additive manufacturing represents a powerful tool for the fabrication of parts with complex shapes by the deposition and the consolidation of materials as opposed to subtractive manufacturing methodology. Selective laser sintering (SLS), one of the most popular powder bed fusion (PBF) technologies for thermoplastic part production, has demonstrated extensive applications in various industrial sectors. The process involves the deposition of homogeneous powder layers and employs a laser source to selectively melt a powder bed according to a CAD model. Due to its layer-by-layer nature, voids and pores are inevitably introduced in the fabricated thermoplastic parts. Porosity represents one of the major limitations of this technology being one of the main causes of the variation of the mechanical properties. With the intention of providing support for reducing the porosity and thus increasing the quality and performance of the final product, in this paper, a brief review was carried out focusing on the SLS process parameters and their interaction with the porosity of the product. In addition, an in-depth look was given to the mechanisms of formation and consolidation of pores within parts made of polymeric material.

Effect of multidimensional filler hybridization on the mechanical properties of thermoplastic polyurethane composites prepared by selective laser sintering

AbstractThis paper aims to improve the tensile strength and thermal stability performance of selective laser sintered thermoplastic polyurethane elastomer/spherical alumina/graphene oxide/silicon nitride whisker (TPU/Al2O3/GO/Si3N4ws) composites by investigating the synergistic enhancement of fillers with different dimensions, and solving the problem of excessive porosity, material delamination, and low tensile strength during sintering of semi‐crystalline polymer TPU problems. The results show that the synergistic effect of multidimensional fillers leads to the tensile strength of TPU composites up to 12.1 MPa at 8.9 wt% Al2O3, 0.1 wt% GO and 1.0 wt% Si3N4ws hybrid fillers, which is 38.9% higher than that of pure TPU. In addition, the elongation at break of TPU/Al2O3/GO/Si3N4ws composites was maintained at 254%, which still has good toughness. The effect of fillers on the selective laser sintering (SLS) process, thermal stability, melting, and crystallization behaviors of the composites was also investigated in this work. In general, the multidimensional filler compounding synergistically constructing a three‐dimensional network structure has some improvement on the thermomechanical behavior of TPU composites with a fixed mass component addition.

Additive manufacturing of flame retardant polyamide 12 with high mechanical properties from regenerated powder

PurposeThis paper aims to develop flame-retardant (FR) polyamide 12 (PA12) nanocomposite from regenerated powder via selective laser sintering (SLS), an additive manufacturing technique.Design/methodology/approachFirst, the morphology, processibility, thermal and mechanical properties of PA12 regenerated powder, consisting of 50 wt% new and 50 wt% recycled powder, as well as corresponding printed specimens, were evaluated to characterize the effects of previous SLS processing. Second, flame-retardant PA12 was developed by incorporating both single and binary halogen-free flame retardants into the regenerated powder.FindingsIt was found that the printed specimens from regenerated powder had much higher tensile and impact properties compared to specimens made from new powder, which is attributed to better particulate fusion and coalescence realized in higher temperature SLS printing. The effect of FRs on thermal, mechanical and flame retardant properties of the PA12 composites/nanocomposites was investigated systematically. It was found that the nanoclay, as a synergist, improved both flame-retardant and mechanical properties of PA12. UL94 standard rating of V-0 was achieved for the printed nanocomposite by incorporating 1 wt% nanoclay into 15 wt% phosphinates FR. Moreover, on average, the tensile and impact strength of the nanocomposite were increased by 26.13% and 17.09%, respectively, in XY, YZ and Z printing orientations as compared to the equivalent flame retardant composite with 20 wt% of the phosphinates FR.Originality/valueThis paper fulfills the need to develop flame retardant parts via SLS technology with waste feedstock. It also addresses the challenge of developing flame retardant materials without obviously compromising the mechanical properties by making use of the synergistic effect of nanoclay and organic phosphinates.

Preparation of carbon black/graphene nanosheets/PP composites with 3D separated conductive networks based on selective laser sintering

AbstractThe main research goal of high‐performance conductive polymer composites is to improve electrical conductivity and reduce the percolation threshold. Designing an effective conductive network is one of the fundamental ways to improve the conductivity. In this study, polypropylene composite powders coated with carbon black (CB) and graphene nanosheets (GNSs) were prepared by a combination of ultrasonic dispersion and liquid‐phase deposition, and sintered using a selective laser sintering process to prepare polypropylene composites with three‐dimensional separated conductive networks. The synergistic effect of binary compound fillers of CB and GNSs was utilized to further improve the electrical conductivity of the composites. When the filler loading was 2% and the ratio of CB to graphene was 7:3, the electrical conductivity of the polypropylene composite was 1.315 × 10−3 S/m, which was eight orders of magnitude higher than that of pure polypropylene. In addition, the tensile strength and elongation at break of CB/GNSs/PP composites were improved by 36.4% and 20.77% with excellent mechanical properties. The effects of fillers on the SLS process, thermal stability, melting, and crystallization behavior of the composites were investigated. This study provides an effective and feasible strategy for the preparation of high‐performance conductive polymer composites using SLS 3D printing process.