Laser Sintering Process Research Articles

On the difference in material structure and fatigue properties of polyamide specimens produced by fused filament fabrication and selective laser sintering

The present paper describes the influence of both flexure quasi-static and fatigue loading on polyamide 12 (PA12) specimens fabricated by fused filament fabrication (FFF) and selective laser sintering (SLS) processes. Rectangular prisms (ISO 178:2010) of polymer were printed and tested under sinusoidal three-point bending fatigue loading at a frequency of 5 Hz. The differences in porosity, surface roughness, and degree of crystallinity are systematically measured and linked to the mechanical fatigue properties. Fatigue analysis in the visco-elastic domain of the polymer is fully described, from fatigue behavior to energy analysis. Here, we have shown that the fatigue properties of the FFF specimens are found to be higher than those of the SLS specimens, despite their lower degree of crystallinity (more than four times). The presence of pores and their growth during fatigue tests in the sintered PA12 specimen seem to be responsible. The fatigue loss factor analysis shows that at lower stress levels, PA12 material reveals its characteristic slight visco-elastic dissipation and heating as its lifetime was exhausted. Also, the obtained results of additively manufactured PA12 were compared with those of materials obtained by injection molding (IM) and extrusion techniques. The quasi-static flexural properties of PA12 obtained by FFF and SLS processes reveal better characteristics compared to IM and extruded specimens. However, the fatigue properties of the SLS-processed polymer are 24% and 40% less than those of materials obtained by IM and extrusion.

DMLS – An insight for unproblematic production

Three dimensional printing is a kind of production of things stratum by stratum deposition of material. It helps to cross many limitations to manufacture the product with desired features which are difficult to machine in subtractive manufacturing methods. At earlier stage of progress of three dimensional printing, objective of coming out with basic creation of some trial products, reduces the manufacturing time in reckoning. Current trend is with powder-based AM, where a part is manufactured with DMLS (Direct Metal Laser Sintering process) which helps to produce an end-use part. Every manufacturing process will be having some defects during the unorganized parameters. In this regard, this paper discusses about the complete procedure of DMLS and defects and the causing parameters to facilitate the trouble free and defect free manufacturing of parts. This papers also addresses the research scopes.

Drug Delivery Applications of Three-Dimensional Printed (3DP) Mesoporous Scaffolds.

Mesoporous materials are structures characterized by a well-ordered large pore system with uniform porous dimensions ranging between 2 and 50 nm. Typical samples are zeolite, carbon molecular sieves, porous metal oxides, organic and inorganic porous hybrid and pillared materials, silica clathrate and clathrate hydrates compounds. Improvement in biochemistry and materials science led to the design and implementation of different types of porous materials ranging from rigid to soft two-dimensional (2D) and three-dimensional (3D) skeletons. The present review focuses on the use of three-dimensional printed (3DP) mesoporous scaffolds suitable for a wide range of drug delivery applications, due to their intrinsic high surface area and high pore volume. In the first part, the importance of the porosity of materials employed for drug delivery application was discussed focusing on mesoporous materials. At the end of the introduction, hard and soft templating synthesis for the realization of ordered 2D/3D mesostructured porous materials were described. In the second part, 3DP fabrication techniques, including fused deposition modelling, material jetting as inkjet printing, electron beam melting, selective laser sintering, stereolithography and digital light processing, electrospinning, and two-photon polymerization were described. In the last section, through recent bibliographic research, a wide number of 3D printed mesoporous materials, for in vitro and in vivo drug delivery applications, most of which relate to bone cells and tissues, were presented and summarized in a table in which all the technical and bibliographical details were reported. This review highlights, to a very cross-sectional audience, how the interdisciplinarity of certain branches of knowledge, as those of materials science and nano-microfabrication are, represent a growing valuable aid in the advanced forum for the science and technology of pharmaceutics and biopharmaceutics.

Capabilities and Limitations of Using Desktop 3-D Printers in the Laser Sintering Process

Almost one-third of the revenues of the Additive Manufacturing (AM) machines market is generated by desktop systems. A new category of such devices are Laser Sintering (LS) machines, in which we can find a few representatives of this group. A growing interest in the use of desktop AM solutions in research and industry is visible, and therefore an evaluation of its capabilities and limitations is desirable. The presented paper focuses on comparing desktop (Sintratec S1; Sinterit Lisa) and industrial (Formiga P110) LS systems. The properties of raw materials were characterized by Scanning Electron Microscopy (SEM) and Differential Scanning Calorimetry (DSC), as well as by determining the Particle Size Distribution (PSD) and static and dynamic flowability of powder. Laser Sintering commercial sets were characterized by their mechanical properties (tensile strength), surface quality (roughness), as well as by their accuracy and porosity (computed tomography). The conducted test showed significant differences, especially in the case of material properties and manufacturing repeatability. The found differences between desktop and industrial LS systems suggest that the use of a low-budget LS system, especially for the research and production of end-use parts, has to take into account its limitations.

Laser additive manufactured Ti‐6Al‐4 V alloy: Anodizing and its effects on mechanical properties and corrosion resistance

AbstractTi‐6Al‐4 V components were developed in different orientations (horizontal and vertical) using an additive manufacturing technique called direct metal laser sintering process and heat treated under 3 different procedures (heat treatment‐1, heat treatment‐2 and heat treatment‐3). Anodizing was done at different voltages (8 V–12 V and 18 V–22 V) in the as‐sintered and heat treated conditions. Corrosion test was carried out in artificial saliva. Mechanical properties like tensile strength, elongation were tested; microstructure was analyzed using scanning electron microscopy and chemistry of corrosion products were analyzed using energy dispersive x‐ray spectroscopy. The results of corrosion tests have been correlated with the measured tensile strength and ductility. The same is also correlated with the dry sliding wear results published from our earlier research. Anodized (8 V–12 V) horizontal built‐heat treatment‐3 specimen and anodized (18 V–22 V) horizontal built‐heat treatment‐2 outperforms other combinations in terms of corrosion. Nevertheless, considering the outcome of mechanical testing, the specimen built in horizontal orientation and anodized at 18 V–22 V is preferred for biomedical applications that needs the combination of both mechanical property and corrosion resistance.

Comparison of thermal, structural and morphological properties of poly(L-lactide) and poly(L-lactide)/hydroxyapatite microspheres for laser sintering processes

A comparison of poly(l-lactide) (PLLA) and poly(l-lactide)/hydroxyapatite (PLLA/HAp) biocomposite microspheres fabricated by emulsion solvent evaporation technique designed for laser sintering (LS) applications is presented. Key properties such as thermal and structural as well as geometry, size distribution and powder flowability, which are crucial for this technique, are characterized to validate the applicability of microspheres for LS. The biocomposite microspheres turns out to be more suitable for the LS process than PLLA due to the higher thermal stability, broader sintering window and higher powder flowability.

A route to improving elongation of high-temperature laser sintered PEKK

Abstract Laser sintering (LS) is one of the most popular additive manufacturing (AM) techniques as it produces parts of complex geometry with high dimensional accuracy and good mechanical strength. However, the nature of the LS process often leads to brittle behavior characterized by a low elongation at break if compared to conventional polymer processing techniques, e.g., injection molding (IM). For poly(ether ketone ketone) (PEKK), such elongation is currently below 3%. This study determines and then optimizes the relationship between cooling time and crystallization of PEKK during LS and the resulting elongation at break. The elongation at break of PEKK was successfully improved by using shorter cooling times. The combination of the slow crystallization kinetics of PEKK and a short cooling time of 1 h increased elongation at break to 14%; this is a striking result never achieved for PAEKs in LS before. A calibration curve was developed that can be used to correlate PEKK structure and mechanical properties to cooling conditions according to the application. This methodology can also be applied to select and optimize the mechanical properties of other LS polymers sharing similar kinetics of crystallization and processing temperatures. This work suggests a great potential for a wide range of “post-processing” heat treatments to be used in AM to tailor the ultimate mechanical properties.

Analytical characterization of polyamide 11 used in the context of selective laser sintering: Physico-chemical correlations

Polyamide 11 is an attractive material for additive manufacturing, however, in the selective laser sintering process it is subject to significant thermally induced stress. Suitable analytical methods are necessary to indicate any changes in the material properties. In this work, thermally stressed powder samples and printed tensile bars were analyzed by size exclusion chromatography-multi angle laser light scattering and differential scanning calorimetry. Based on the molecular weight distribution, crystallization and melting temperatures, ageing states of the investigated powders can be defined and transferred to the particles of the printed components. As a result, thermal stress led to an increase in molar mass and polydispersity. Moreover, calorimetric measurements indicated a delayed nucleation process as well as a decrease in polymorphism. Additionally, polymeric material with lower polydispersity values across the tensile bars could lead to decreasing elongation at break (εB) values. For the crystallization onset temperature the opposite effect on εB was observed.

A Review of Methods Used to Reduce the Effects of High Temperature Associated with Polyamide 12 and Polypropylene Laser Sintering

The polymer laser sintering (PLS) process is one of the most promising additive manufacturing (AM) technologies for polymeric materials. However, the technique has challenges because the physical, mechanical, and chemical properties of the polymeric powder deteriorate due to the high temperatures prevailing in the build chamber during manufacture. These high temperatures cause agglomeration of powder, which leads to a decrease in the flowability of powder. There is also a related drop in the coalescence of the powder granules during PLS, which results in porosity that undermines the mechanical integrity of printed parts. Moreover, the viscosity of the melt increases due to cross-linking of molecular chains. This, in turn, increases the tensile strength of the printed components at the expense of the percentage elongation at break. Thus, high prolonged processing temperatures decrease the reusability of polymeric materials used in PLS. In this paper, a review of the studies conducted to investigate ways of reducing the effects of high temperature on polymeric powders is presented.

A process control and interlayer heating approach to reuse polyamide 12 powders and create parts with improved mechanical properties in selective laser sintering

Abstract Capable of building high-quality, complex parts directly from digital models, selective laser sintering (SLS) additive manufacture (AM) is a core method of agile manufacturing. Polyamide 12 is the most commonly and successfully used polymer powders to date in SLS due to the conforming thermal behaviors of this thermoplastic polymer. State of the art technology produces a substantial amount of un-sintered powders after the manufacturing process. Failure to recycle and reuse these aged powders not only leads to economic losses but also is environmentally unfriendly. This is particularly problematic for powders close to the heat-affected zones that go through severe thermal degradations during the laser sintering processes. Limited procedures exist for systematically reusing such extremely aged powders. This work proposes a new process control method to maximize reusability of aged and extremely aged polyamide 12 powders. Building on a previously untapped interlayer heating, preprocessing, and mixing of powder materials, we show how reclaimed polyamide 12 powders can be consistently reprinted into functional samples, with mechanical properties even superior to current industrial norms. In particular, the proposed method can yield printed samples with 18.04 % higher tensile strength and 55.29 % larger elongation at break using as much as 30 % of extremely aged powders compared to the benchmark sample.

Multiscale simulation study of laser sintering of inkjet-printed silver nanoparticle inks

Inkjet printing of metal nanoparticle (NP) inks on various substrates is a novel additive manufacturing technology which has found widespread applications in flexible electronics. The printed NP inks need to be heated through laser sintering to obtain high electrical and mechanical properties. However, the heat transfer and diffusion mechanisms of metal nanoparticles during laser sintering have not been fully understood. In this study, a multiscale model combined transient heat transfer model and molecular dynamics (MD) model is developed to simulate the laser sintering process of silver NP inks. Our simulation results show that the local temperature in printed silver film in the laser spot increases significantly, while the temperature increase in the polyimide (PI) substrate is much lower, demonstrating the feasibility of laser sintering in flexible heat-sensitive substrates. To reveal the micro sintering mechanism, the configurational evolution of silver nanoparticles during laser sintering is observed using MD model. We found that the silver atoms on the nanoparticle surface diffuse into the interfaces between adjacent nanoparticles, forming sintered necks. These necks gradually grow, reducing electrical resistivity of the sintered nanostructures. In addition, the effects of laser parameters on the sintering performance are discussed. Both increasing laser power and decreasing scanning speed can improve sintering performance. However, the laser power is much more efficient than scanning speed in improving sintering performance. Our simulation method could be used to optimize the laser sintering process.

Digital Modeling Accuracy of Direct Metal Laser Sintering Process

Products obtained by metal additive manufacturing have exceptional strength properties that can be compared with forged parts, and in some cases, even surpass them. Also, the cost and time of parts manufacture are reduced by two or even three times. Because of this, today’s leading corporations in the field of aerospace industry introducing this technology to its production. To avoid loss of funds and time, the processes of additive manufacturing should be predictable. Simufact Additive is specialized software for additive manufacturing process simulation is dedicated to solving critical issues with metal 3D printing, including significantly reducing distortion; minimize residual stress to avoid failures; optimize the build-up orientation and the support structures. It also enables us to compare simulated parts with the printed sample or measure it as a reference. In other words, the simulated deformations can be estimated concerning the reference geometry. The current work aims to study the deformation of the sample during the Direct Metal Laser Sintering (DMLS) process made from Maraging Steel MS1. Simufact Additive software was used to simulate the printing process. The main idea is to compare the results of the simulation and the real model. EOS M290 metal 3D printer was used to make a test specimen.

A New Proposal: a Digital Flow for the Construction of a Haas-Inspired Rapid Maxillary Expander (HIRME).

Maxillary expansion is a common orthodontic treatment used for the correction of posterior crossbite resulting from reduced maxillary width. Transverse maxillomandibular discrepancies are a major cause of several malocclusions and may be corrected in different manners; in particular, the rapid maxillary expansion (RME) performed in the early mixed dentition has now become a routine procedure in orthodontic practice. The aim of this study is to propose a procedure that reduces the patient cooperation as well as the lab work required in preparing a customized Haas-inspired rapid maxillary expander (HIRME) that can be anchored to deciduous teeth and can be utilized in mixed dentition with tubes on the molars and hooks and brackets on the canines. This article thus presents an expander that is completely digitally developed, from the first moment of taking the impression with an optical scanner to the final solidification phase by the use of a 3D printer. This digital flow takes place in a CAD environment and it starts with the creation of the appliance on the optical impression; this design is then exported as an stl extension and is sent to the print service to obtain a solid model of the device through a laser sintering process. This “rough” device goes through a post-processing procedure; finally, a commercial expansion screw is laser-welded. This expander has all the advantages of a cast-metal Haas-type RME that rests on deciduous teeth; moreover, it has the characteristic of being developed with a completely digitized and individualized process, for the mouth of the young patient, as well as being made completely of cobalt-chrome, thus ensuring greater adaptability and stability in the patient’s mouth.

Influence of selective laser sintering process parameters on microstructure and physicochemical properties of poly(vinyl alcohol) for the production of scaffolds

Purpose This study aims to investigate the production of scaffolds by selective laser sintering (SLS) using poly(vinyl alcohol) (PVA) polymer, for in vitro studies, a relatively new and growing area in which scaffolds could be used in the design of three-dimensional models for in vitro disease model or tissue equivalent for safety and effectiveness tests. Design/methodology/approach The influence of the SLS process parameters laser power, 26 W and 32 W, and number of laser scans, 1, 2, 4 and 6, on the surface microstructure of the samples and on the degree of crystallinity and chemical stability of PVA material, was investigated using powder with particle size of 20-320 µm. Laser sintered PVA samples were subjected to cell culture tests using osteoblastic cells derived from human osteosarcoma (SaOs-2). Findings The laser power has no significant influence on the microstructure of the laser-sintered samples, however the number of scans has a considerable influence on the sintering degree; the SLS process causes a decrease in the degree of crystallinity and changes the chemical structure of the as-received PVA, especially when using higher laser power and more number of scans. Preliminary in vitro cell culture tests show that the laser-sintered PVA material is biocompatible with SaOs-2 cells. Originality/value SLS offers good potential for the fabrication of scaffolds and thus, may be applied as an alternative to conventional scaffold fabrication processes to overcome their limitations.

Laser sintering of graphene nanoplatelets encapsulated polyamide powders

This paper presents a comprehensive study in the fabrication and safety of new nanocomposite powders for laser sintering. The nanocomposite powder is based on a core-shell structure where nanoparticles (graphene nanoplatelets, GNP) are encapsulated on the surface of the polymeric particles (polyamides PA12) in a thin layer of poly(vinyl alcohol) (PVA). Powder rheology data as well as SEM and TEM showed that GNP were dispersed well in the PVA coating and improved flow. Half time crystallisation kinetics was used to determine differences induced in the polymer and the laser sintering process by the presence of GNP. Nanosafety aspects, critical in a manufacturing environment, are also considered here and exposure monitoring tests were carried out. Results confirmed a low nanoparticle air exposure and therefore confirmed the successful surface encapsulation of the GNP in nanocomposite powders. The laser-sintered 0.1GNP/PVA-PA12 parts showed enhanced mechanical properties in tensile, compression and 3-point bending test.

Influence of the Surface Modification of Calcium Carbonate on Polyamide 12 Composites.

In previous investigations, it was found that the thermal properties of a polyamide 12 compound can be manipulated, using a designed filler, to improve the melting as well as crystallization behavior, determined for selective laser sintering. A common downside of the introduction of a non-flexing mineral filler is the reduction of the mechanical properties, such as ductility. This paper investigates the influence of content and surface modification of limestone on the mechanical properties. The aim is to understand the effect of an optimized coupling agent on the properties of a compound, containing polyamide 12 filled with 10 wt % of surface modified calcium carbonate. A range of four mineral filler modifications was chosen to investigate their coupling effect, namely 6-amino hexanoic acid, ε-caprolactam, l-arginine or glutamic acid. The in advance surface modified fillers were then each used in combination with the polyamide 12 in a twin-screw extrusion process. With an optimized surface modifying agent, the tensile strength as well as elongation at break can be improved in comparison with uncoated filler implementation, such that up to 60% of the loss of ductility and toughness of a final part when using an untreated filler could be regained using an optimized surface modifier at a correct amount. With the tested filler grade and the specific tested filler amount, the optimized amount of 6-amino hexanoic acid was approx. 2.5 mmol of treatment agent per 100 m2 of CaCO3. These found improvements in a twin-screw extruded polyamide 12 compound show the possible usage of modified calcium carbonate as a functional filler in additive manufacturing and can potentially be transferred in a subsequent investigation in the selective laser sintering process.

Effects of various processing parameters on the mechanical properties of sisal fiber/PES composites produced via selective laser sintering

A new type of sustainable material, i.e., a sisal fiber/poly-ether sulfone composite (SFPC), which is energy-efficient, environmentally friendly, and has a low cost, was developed for laser sintering additive manufacturing. This study was performed to explore the effects of the processing parameters on the SFPC composite parts produced via selective laser sintering (SLS). The effects of the laser sintering processing parameters, i.e., the preheating temperature, laser power, and scan speed, were studied. Bending and tensile testing of the SFPC specimens was successfully performed via SLS. The effect of the processing parameters on the SLS in terms of the mechanical strength of the laser-sintered parts was investigated. The results determined that the processing parameters had a significant effect on the mechanical strength of the sintered SFPC parts. When the preheating temperature and laser power were increased in the processing SLS system, the mechanical strength of the sintered SFPC parts was significantly increased. However, the scanning speed had an inverse proportional relationship to the mechanical strength of the SFPC SLS parts.

Experimental investigating and numerical simulations of the thermal behavior and process optimization for selective laser sintering of PA6

In this paper, the thermal behavior and process optimization for polyamide 6 during the selective laser sintering process were systematically investigated via numerical simulations and experimental testing. The effects of laser power and scanning speed were studied, and the simulation results showed that the increasing laser power and decreasing scanning speed led to the increase of the maximum temperature and the three-dimensional size of molten pool. Then, the single-layer experiments under different process parameters were carried out to verify the accuracy of the model. In addition, the energy density was applied to evaluate the combined effect of laser power and scanning speed on the thermal behavior. A process map for parameter optimization was obtained based on the predicted results, and the optimized parameters were in the region that the powder can be remelt resulting in an enough depth of molten pool, while the maximum temperature was below the decomposition temperature. Meanwhile, the specimen selected in the remelting region showed the highest tensile strength.

Experimental and Numerical Analysis of Additively Manufactured Inconel 718 Coupons With Lattice Structure

Abstract In the present paper, two lattice geometries suitable for near surface and double wall cooling were developed and tested. The first type of unit cell consisted of six ligaments of 0.5 mm diameter joined at a common vertex near the middle. The second type of unit cell was derived from the first type by adding four mutually perpendicular ligaments in the middle plane. Two lattice configurations, referred to as L1 and L2, respectively, were obtained by repeating the corresponding unit cell in streamwise and spanwise directions in an inline fashion. Test coupons consisting of these lattice geometries embedded inside rectangular cooling channel with dimensions of 2.54 mm height, 38.07 mm width, and 38.1 mm in length were fabricated using Inconel 718 powder and selective laser sintering (SLS) process. The heat transfer and pressure drop performance was then evaluated using steady-state tests with constant wall temperature boundary condition and for channel Reynolds number ranging from 2800 to 15,000. The lattices depicted a higher heat transfer compared with a smooth channel and both the heat transfer and pressure drop increased with a decrease in the porosity from L1 to L2. Steady-state conjugate numerical results revealed formation of prominent vortical structures in the inter-unit cell spaces, which diverted the flow toward the top end wall and created an asymmetric heat transfer between the two end walls. In conclusion, these lattice structures provided an augmented heat transfer while favorably redistributing the coolant within channel.

Semi-supervised deep learning based framework for assessing manufacturability of cellular structures in direct metal laser sintering process

In recent years, metal cellular structures have drawn attentions in various industrial sectors due to their design freedoms and abilities to achieve multi-functional mechanical properties. However, metal cellular structures are difficult to fabricate due to their complex geometries, even with modern additive manufacturing technologies such as the direct metal laser sintering (DMLS) process. Assessing the manufacturability of metal cellular structures via a DMLS process is a challenging task as the geometric features of the structures are complex. Besides, via a DMLS process, the manufacturability also depends on the cumulative deformation of the layers during the manufacturing process. Existing methods on Design for Additive Manufacturing (DFAM) provide design guidelines that are based on past successful printed designs. However, they are not effective in predicting the manufacturability of metal cellular structures. In this paper, we propose a semi-supervised deep learning based manufacturability assessment (SSDLMA) framework to assess whether a metal cellular structure can be successfully manufactured from a given DMLS process. To enable efficient learning, we represent the complex cellular structures as 3D binary arrays with a simple yet efficient voxelisation method. We then train a deep learning based classifier using only a small amount of experimental data by adopting a semi-supervised learning approach. By running real experiments and comparing with existing DFAM methods and machine learning models, we demonstrate the advantages of the proposed SSDLMA framework. The proposed framework can be extended to predict the manufacturability of various other complex geometries beyond cellular structure in a reliable way even with a small number of training data.