Selective Laser Sintering

Laser intensity effect on mechanical properties of wood-plastic composite parts fabricated 275. by selective laser sintering[r]

Rapid prototyping (RPT) has been termed by many manufacturing scientists and engineers to be the ultimate solution to manufacturing whereby complex, convoluted and near-impossible shapes can be modeled and developed within a very short period. Selective laser sintering (SLS) [1,2], micro-cladding [3,4] and ballistic particle manufac- turing (BPM) [5,6] are among the recently developed tech- nologies that offer the possibility of direct fabrication of metallic parts of arbitrary geometry in a single-step process. SLS is the process of melting a layer of powder by a laser beam, which scans selected regions of the layer. The powder in those regions melts and gets consolidated. After conso- lidation of one layer, another layer of powder is applied and the cycle is repeated. In such an exercise, the successive layers of solidified material formed in each cycle get joined to each other during the process and ultimately a solid form emerges, which can be used for form visualization, design feedback, functional tests and other applications.

Other studies discussed selective laser sintering of scaf- folds, however, for the experiments simple flat disks were manufactured to investigate sintering of the mate- rial used. In these cases the pore size and shape were not the specific focus, but the porosity was generated by adjusting the process parameters [16,23]. A comparison between intended design (CAD) and actual achieved structure is often missing. Williams et al. [21] designed pores of 1.75 to 2.5 mm and calculated porosities of 63.1% to 79%, while the porosity of the fabricated scaf- folds actually achieved was consistently 27 percentage points lower than the values aimed for. This was due to particle size and growth of the struts by heat conduction into adjacent powder particles.

Abstract: The availability of Additive Manufacturing (AM) technologies in particular the selective laser sintering (SLS) process has enabled the fabrication of high strength, lightweight and complex cellular lattice structures. In this study, the effective mechanical properties of SLS produced periodic lattice structures were investigated. Three different types of lattice structures were designed by repeating three types of open-form unit cells consisting of triangular prism, square prism, and hexagonal prism. A novel approach of creating the complex and conformable lattice structures using traditional modelling software such as Creo® proposed by the authors was used. Based on the predesigned lattice structures, finite element analysis (FEA) was carried out to evaluate the mechanical properties of these structures. For the experimental study, nylon samples were printed using a plastic selective laser sintering system and tested using a universal testing machine. FEA results show that lattice structures with triangular prism perform better than the other two prisms in terms of Young’s modulus to relative density ratio. Tensile tests results show good conformance with the results obtained from FEA.

Selective laser sintering (SLS) is a layer manufacturing (LM) technique and has been used to produce prototypes as well as functional components [1]. Developed by Carl Deckard for his master’s thesis at the University of Texas, SLS was patented in 1989 [2]. The technique, shown in Figure 1, uses a laser beam to selectively fuse powdered materials, such as nylon, elastomer, ceramic and metal, into a solid object through direct or indirect process. Parts are built upon a platform which sits just below the surface in a bin of the heat-fusable powder. A laser traces the pattern of the first layer, sintering it together. The platform is lowered by the height of the next layer and powder is

To increase productivity, industry has attempted to apply more computerized automation in manufacturing. Amongst the various technologies to take the industry by storm is rapid prototyping technology. RP technologies provide that bridge from product conceptualization to product realization in a reasonably fast manner, without the fuss of NC programming, jigs and fixtures. Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a part or assembly using three- dimensional computer aided design (CAD) data. RP includes various technologies out of which selective laser sintering technology is prominent because SLS allows the production of fully functional prototypes with high mechanical and thermal resistance, strength & rigidity under the extreme conditions of high temperature. Durable metal parts, mold inserts, direct Low density complex investment casting patterns can be prepared directly from CAD data.

Selective Laser Sintering is one of the additive layer manufacturing (ALM) technologies capable of building layer upon layer three dimensional objects by selectively sintering powder materials. Over the years, most polymeric SLS research focused either on the manufacturing process or on the characteriza- tion of the final sintered parts. Very little attention is being given to the morphology of powder materials and their influence on the sintering process and ulti- mately the part properties (Zarringhalam, Hopkinson et al. 2006; Goodridge, Hague et al. 2010). Whereas the behavior of polymeric powders has been fully in- vestigated for other conventional manufacturing pro- cess such as rotational molding, cold compaction (Truss, Han et al. 1980; Olinek, Anand et al. 2005), analysis into the optimum properties of SLS powders is missing. Therefore the aim of this paper is to inves- tigate the behavior of various powders with particu- lar attention to their ability to flow. Flowability plays a key role in the SLS process as the lack of homoge- neous and even layers leads to porous, weak SLS parts. Flowability depends on several factors: powder itself (particle size distribution, particle shape), the environmental conditions (temperature, moisture) and according to some authors the test conditions (Prescott J.K. 2000; Schulze 2006-2011). Schulze(2006-2011) reported basic concepts about bulk solid materials and flowability and he described the main physical factors affecting flowability such as adhesive strength and wall friction. Prescott and Bar-

Selective laser sintering of polycarbonate powder was performed with a 25 W carbon dioxide laser with power centered at 14 W at a beam diameter of 0 .060 inches 1.5 mm, 100 scan lines pe[r]

Abstract: The regeneration of functional tissue in osseous defects is a formidable challenge in orthopedic surgery. In the present study, a novel biomimetic composite scaffold, here called nano-hydroxyapatite (HA)/poly-ε-caprolactone (PCL) was fabricated using a selective laser sintering technique. The macrostructure, morphology, and mechanical strength of the scaffolds were characterized. Scanning electronic microscopy (SEM) showed that the nano-HA/PCL scaffolds exhibited predesigned, well-ordered macropores and interconnected micropores. The scaffolds have a range of porosity from 78.54% to 70.31%, and a corresponding compressive strength of 1.38 MPa to 3.17 MPa. Human bone marrow stromal cells were seeded onto the nano-HA/PCL or PCL scaffolds and cultured for 28 days in vitro. As indicated by the level of cell attachment and proliferation, the nano-HA/PCL showed excellent biocompatibility, comparable to that of PCL scaffolds. The hydrophilicity, mineralization, alkaline phosphatase activity, and Alizarin Red S staining indicated that the nano-HA/PCL scaffolds are more bioactive than the PCL scaffolds in vitro. Measurements of recombinant human bone morphogenetic protein-2 (rhBMP-2) release kinetics showed that after nano-HA was added, the material increased the rate of rhBMP-2 release. To investigate the in vivo biocompatibility and osteogenesis of the composite scaffolds, both nano-HA/PCL scaffolds and PCL scaffolds were implanted in rabbit femur defects for 3, 6, and 9 weeks. The wounds were studied radiographically and histologi- cally. The in vivo results showed that both nano-HA/PCL composite scaffolds and PCL scaffolds exhibited good biocompatibility. However, the nano-HA/PCL scaffolds enhanced the efficiency of new bone formation more than PCL scaffolds and fulfilled all the basic requirements of bone tissue engineering scaffolds. Thus, they show large potential for use in orthopedic and reconstructive surgery.

Selective laser sintering (SLS) is an additive manufacturing (3D printing) technique that can be applied to the anode of lithium batteries to simplify the manufacturing process and enhance the production efficiency. The specific surface nanostructures and intermetallic compounds (IMC) induced by the SLS process can improve the capacity and cycle life. In this study, a stable anode for a lithium ion battery was success- fully fabricated by the SLS process, the capacity of the battery exceeded 150 mAhg −1 after 10 cycles under a 0.1 C current rate at room tempera-

Selective laser sintering is categorized as a powder bed fusion AM process. SLS printing process divided into three main parts:1) preparation of powder/material, 2) SLS printing process and 3) post-processing. Preparation and post-processing identified as the main source of dust contribution in the laboratory [34]. Figure 4 enlighten the whole process of SLS printing process. Background data were monitored for 30 minutes before the powder preparation start. Figure 4 (a)-(d) illustrates the activity involves in the pre-printing process (30-140 minutes). The powder was weighted accordingly to the SLS machine recommendation. Figure 4(d) shows that the SLS machine is operated at 140-360 minutes. Figure 4 (f)-(h) present the post-printing task (360-480 minutes). The total time taken for this monitoring is 8 hours [33].

Abstract—Selective Laser Sintering is one of the most advanced and promising technologies of Additive Manufacturing known to mankind. The accessibility of SLS to the college students, faculty and independent researchers is limited due to prohibitively high costs. Through this paper an attempt has been made to chalk out the methodology used to design a SLS Printer for polymers thereby addressing the accessibility problem. Alternatively, this methodology is relevant to anyone, who is interested in SLS Technology and building a machine based on it. Through a careful study, it was established that laser system is one of the highest contributors to the overall asset cost. Therefore, it was treated as a primary target for cost reduction . Diode laser was used as an alternative to commonly used CO 2 laser. This selection led to a

10.6 µm is much more easier to be absorbed by most of ceramic materials, but the diameter of laser spot is much larger than that of the fiber laser [14, 15]. Therefore, fiber laser is more suited for processing with higher accuracy. However, silica is almost non-absorbent to Nd:YAG fiber [16, 17]. The study on powder absorptivity to laser is aimed to improve the process of the selective laser sintering (SLS) , and it allows one to understand the mechanism of the interaction between laser and materials which is crucial to find a more uniform and suitable processing window for SLS [18]. Therefore, the solution of improving the poor ceramic materials absorptivity to laser was proposed. To investigate the influence of carbon additive to ceramic material, 3D test specimens were successfully fabricated and characterized via the SLS process.

Selective laser sintering (SLS) enables the fast, flexible and cost-efficient production of parts directly from 3D CAD data. Unlike more established machine tools, there is a marked lack of process monitoring and feedback control of key process variables. In-situ analysis techniques permit the emergence of repair techniques, in-process optimization of production parameters, and will also serve to save time and material. In this study, optical coherence tomography (OCT) is used for the first time to evaluate components produced by SLS. Using a Polyamide-PA2200, surface defects are analyzed and the limiting factors associated with the measurement technique are quantified. OCT is shown to be a useful technique for evaluating surface irregularities alongside sub-surface defects that have resulted from poor sintering or non-homogeneous powder spreading. We demonstrate detection and quantification of surface defects such as cracks, pores and voids on a ~30 μm scale. Furthermore, we show that this technique can resolve ‘built-in’ fine features within a 200 to 400 μm depth below the surface, covering typical layer thicknesses used by this process. This capability paves the way for real-time monitoring of the SLS process for assurance, or even dynamic correction of defects during the build.

Selective laser sintering(SLS) is one of a few rapid prototyping(RP) techniques, which enables fabrication of three-dimensional(3D) parts with arbitrary shapes directly from metal powder with no or minimal post-processing[1−3]. In this method, an object is created by selectively sintering or/and melting thin layers of powder with a scanning laser beam according to CAD data[4,5]. The main advantages associated with this technique are high design flexibility, excellent process capabilities, and time- and cost-saving features[6−8]. Currently, metallic SLS process has been commercially available to produce high performance engineering parts, e.g. functional prototypes and low-volume tooling for

regarding the role of hydrogen in the sintering of aluminium and its alloys. While some investigators have claimed that hydrogen has little influence on the sintering of aluminium and alloys, others have shown that it has a deleterious effect on its sinterability with the explanation that water vapour associated with hydrogen acts as a stabiliser of hydrated alumina thus inhibiting shrinkage [106]. While no convincing inference could be drawn from the available literature on the effect of atmosphere on the sintering response of aluminium and its alloys, Schaffer and co-workers noted that anecdotal evidence from the industry indicates that nitrogen is always the preferred atmosphere for the sintering of aluminium and its alloys because of its low cost. Schaffer and co-investigators [106] examined the sintering of aluminium alloys of varying compositions in vacuum, argon, wet and dry nitrogen, nitrogen-5%hydrogen and argon- 5%hydrogen mixtures and proffered explanations as to why moisture is deleterious and the formation of aluminium nitride is essential. Fig. 27 presents the results of this investigation. The solid line represents no change in density between the green and sintered state. Hence, points above and below the line confirm the occurrence of net shrinkage and net expansion of the sintered parts respectively. It can be seen that shrinkage only occurs for all green densities under nitrogen atmosphere, whereas volumetric expansion of sintered parts occurred when 5% H 2 was

SLS specimens resulting in new findings on the mechanical behaviour of nylon-12 . The main source of failure is attributed to geometric discontinuity or stress concentration. This form of discontinuity usually takes the form of a sharp change of geometry, opening, hole, notch, crack, etc [7]. Modelling and analysis of these discontinuities is meaningful, as it will build on the understanding of their behaviour within Selective Laser Sintered (SLS) parts and will contribute to their enhanced life and performance. The effect to which un-melted particles influence the onset and direction of the propagation of microcracks in SLS printed engineering parts is presented in this study. How the degree of particle melt (DPM) in SLS parts affect and control both crack initiation and propagation is one of the aims of this study. This paper is structured as follows. Firstly, a brief background on additive man- ufacturing, laser sintering process and properties of nylon-12 is provided. Secondly, tests and results conducted by using the extended finite element method (XFEM) in nylon-12 samples with different arrangements and degrees of particle melt are presented. Finally, discussion of results and concluding remarks are provided.

A distinction must be made between the production of metal and ceramic parts through rapid investment casting with (polystyrene) sacrificial patterns. When producing metal parts, the sacrificial polystyrene patterns have the shape of the part to be produced. From the sacrificial pattern, a plaster mould (see Liu et al. [12] or Niino and Yamada [13]), but generally a ceramic moulding shell is sometimes fabricated. Finally, the moulds are used to fabricate the metal parts through a casting process, e.g. vacuum pressure casting of aluminium parts, as applied by Hongjun et al. [14]. Applications of this technology can be found in the production of titanium, aluminium, steel alloys or super alloys for competitive motorsports (Cevolinni et al. [15]). When producing ceramic parts, the sacrificial polystyrene patterns have the negative geometry of the parts to be produced. Through high pressure slip casting, followed by debinding of the polystyrene and a furnace sintering treatment, Si 3 N 4

larger, however, a smaller sample size was chosen due to the unknown factor of the incorporation of the micro-foil materials (Childs et al., 2004). The variables that were manipulated for the pulse laser SLS subprocess were power and pulse duration. The values selected for the initial phase were similar to those used in other studies, though few studies have been performed using a pulse laser (Su et al., 2003). The sample size was 16 for the same reasoning as the continuous laser. The frequency and travel speed were held constant to produce a constant distance between pulses. The pulses were spaced 0.25mm apart to achieve a 75% overlap. The spaces in the samples were filled with a 316 SS powder of mesh size 125-325. Argon shielding was supplied through a shielding nozzle at a flow rate of 0.014 cumecs. The lasers were then used to join the materials with a 1mm focal spot size by scanning the surface of the micro-foils and powder at certain parameter settings seen in Table 1 and 2. The joining was done perpendicular to the direction of the micro-foil materials. Figure 15, shows a graphic representation of the SLS subprocesses.

Abstract - Compared with conventional material removal manufacturing technologies, rapid prototyping is a layer-based material addition process and can produce a 3-D freeform object with a CAD-defined geometric model directly. Due to their comparatively high rapidity and flexibility, however, they have also been used in various manufacturing and nonmanufacturing applications. This process is highly influenced by powder and laser parameters such as laser power, scan rate, spot size and layer thickness. The aim of this research is to improve the performance of the SLS process by optimizing the control of process parameters that have very strong influence on the quality of the built part. Therefore a study on fabricating a part with CL91RW powder has been performed by selective laser sintering on process parameter Orientation and layer thickness.In order to determine critical states of the sintering parameters, analysis of variances has applied while optimization of the parameters affecting the surface quality and dimension accuracy were investigated.