1,371 research outputs found

    Users satisfaction on academic library services: measure on service quality, safety and facility

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    Library is an important and beneficial place for information and resources of knowledge. According to the previous paper, the library is an open place for all people and broadens opportunities for people of all ages. The library provided many types of equipment for people such as programming addresses the health, educational and workforce development [1]. The purpose of a library is to provide any information, research information, activities for students also to support the university curriculum and support the research at university [2]. An effective and efficient academic library system is expected to contribute significantly to users [3]

    Optimization of selective laser sintering process conditions using stable sintering region approach

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    The optimization of process parameters represents one of the major drawbacks of selective laser sintering (SLS) technology since it is largely empirical and based on performing a series of trial-and-error builds. This approach is time con-suming, costly, and it ignores the properties of starting powders. This paper provides new results into the prediction of processing conditions starting from the material properties. The stable sintering region (SSR) approach has been applied to two different polymer-based powders: a polyamide 12 filled with chopped carbon fibers and polypropylene. This study shows that the laser exposure parameters suitable for successful sintering are in a range that is significantly smaller than the SSR. For both powders, the best combination of mechanical properties, dimensional accuracy, and porosity level are in fact, achieved by using laser energy density values placed in the middle of the SSR

    Process phenomena and material properties in selective laser sintering of polymers: A review

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    Selective laser sintering (SLS) is a powder bed fusion technology that uses a laser source to melt selected regions of a polymer powder bed based on 3D model data. Components with complex geometry are then obtained using a layer-by-layer strategy. This additive manufacturing technology is a very complex process in which various multiphysical phenomena and different mechanisms occur and greatly influence both the quality and performance of printed parts. This review describes the physical phenomena involved in the SLS process such as powder spreading, the interaction between laser beam and powder bed, polymer melting, coalescence of fused powder and its densification, and polymer crystallization. Moreover, the main characterization approaches that can be useful to investigate the starting material properties are reported and discussed

    Impact of Bimodal Particle Size Distribution Ratio of Functional Calcium Carbonate Filler on Thermal and Flowability Properties of Polyamide 12

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    In previous investigations, it was shown that the melting, as well as crystallization behavior of polyamide 12, could be manipulated by adjusting the particle size distribution of calcium carbonate as a functional filler. It was demonstrated that the melt properties of this compound show a significant dependency on the filler volume-based particle size. As finer and narrower the calcium carbonate particles in the polymer matrix become, the less influence the filler has on the melting properties, influencing the melt flow less significantly than the same surface amount of broad size distribution coarse calcium carbonate filler particles. However, due to increased nucleation, the crystallization behavior on cooling showed a markedly more rapid onset in the case of fine sub-micrometer filler particle size. To control further and optimize the thermal response properties of a filling compound for improved properties in additive manufacturing processing through selective laser sintering, the possibility to combine precisely defined particle size distributions has been studied, thereby combining the benefits of each particle size range within the chosen material size distribution contributes to the matrix. The melt flow at 190 °C, the melting speed, melting and crystallization point as well as crystallization time at 170 °C were analyzed. The thermal and flow properties of a polyamide 12 matrix can potentially be optimized with a combination of a precise amount of coarse and fine calcium carbonate filler. The improvements were exemplified using a twin-screw extruder for compounding, indicating the potential for optimizing functionally filled polymer in additive manufacturing

    Calcium Carbonate as a Functional Filler in Polyamide 12 – Manipulation of the thermal and mechanical properties

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    Controlling and adjusting the thermal response properties of a polymeric compound is a key driver for improving its usability for an additive manufacturing process such as selective laser sintering, optimising the final part density as well as hardness. Adding mineral fillers to polymers was originally aimed at cost reduction. However, fillers are often used to fulfil a functional role, such as improved thermal or mechanical properties of the polymer composite. The influence of particle size distribution, filler morphology and filler amount of calcium carbonate as a mineral functional filler particle on the thermal properties in a compression-moulded polyamide 12 matrix was investigated. The possibility to combine precisely defined particle size distributions, thereby combining the benefits which each particle size range within the chosen material size distribution contributes to the matrix, was evaluated. The loss of ductility, occurring as a common downside of the introduction of a non-flexing mineral filler, has been investigated. The effect of an optimised coupling agent on the mechanical properties of a compression-moulded compound, containing polyamide 12 filled with surface modified calcium carbonate was also determined. Mineral filler modifier, namely 6- -caprolactam, L-arginine and glutamic acid, were chosen to investigate their coupling effect. The melt flowability at 190°C, the melting speed, melting and crystallisation point, degradation temperature, crystallisation time at 170 and 172°C, as well as the elongation at break were analysed. The melt properties of a polyamide 12 matrix show a significant dependency on the filler volume-based particle size. The finer and the narrower the particle size distribution of the calcium carbonate particles in the polymer matrix become, the less influence the filler has on the melting properties, influencing the melt flow less significantly than the same surface amount of broad size distribution coarse calcium carbonate filler particles. However, due to increased nucleation, the crystallisation behaviour on cooling showed a markedly more rapid onset in the case of fine sub-micrometre filler particle size. The overall thermal and flow properties can be optimised, by adjusting the morphology and particle size distribution of a coarse and fine calcium carbonate filler, blended in an optimised functional filler ratio. With an optimised surface modifying agent and/or filler morphology, 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 the correct amount of an optimised surface modifier. The improvements were demonstrated with a twin-screw extruded compound, indicating the potential for the usage of a functional filler within a polymer matrix as an optimised composite for additive manufacturing

    Development of a polyamide12/Carbon nanotube nanocomposite for laser sintering

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    This thesis presents a comprehensive study on the preparation, processing and characterisation of a polymer nanocomposite for laser sintering. Well-dispersed nanocomposite powder with near-spherical morphology and suitable particle size for processing by laser sintering was successfully produced by a novel method of coating individual polyamide 12 (PA12) powder particles with carbon nanotubes (CNTs). Mechanical specimens produced by laser sintering the PA12-CNT powder showed good definition and no distortion. Compared to the laser sintered PA12 parts, PA12-CNT nanocomposite parts showed enhanced flexural, impact and tensile properties without sacrificing elongation at break. This enhancement may be attributed to the well-dispersed CNTs in the PA12 matrix and corresponding denser laser sintered parts. By adding the CNTs into the PA12 powder, the thermal conductivity was increased. This could increase the fusion of the powder and flow of melted PA12, resulting in denser and stronger parts. Parallel oscillatory rheology tests were completed in co-operation with Toyota Technological Institute (TTI), Japan. Results demonstrated that CNTs have a significant effect on the melt rheological properties of the PA12-CNT nanocomposites, as the storage modulus G’, loss modulus G’’ and viscosity [eta] of the PA12-CNT nanocomposites increased compared to neat PA12. To evaluate the dispersion of CNTs in the laser sintered PA12-CNT nanocomposite parts, three dimensional transmission electron microscopy (3D-TEM) was carried out at National Institute for Materials Science (NIMS), Japan. Results revealed that the CNTs were dispersed well in the laser sintered PA12-CNT parts without agglomerates, which is very important for the mechanical enhancement by nanofillers. The method used in this work III appears to be a cost-efficient and effective way to produce polymer nanocomposite powders for laser sintering, while maintaining the optimum powder morphology for the laser sintering process and enhancing the mechanical properties of the laser sintered parts

    Effect of Powder Bed Fusion Laser Sintering on Dimensional Accuracy and Tensile Properties of Reused Polyamide 11

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    Polyamide 11 (PA11) is a plant-based nylon made from castor beans. Powder bed fusion laser sintering (PBF-LS) is an additive manufacturing process used for PA11 which allows for the reuse of the unsintered powder. The unsintered powder is mixed with virgin powders at different refresh rates, a process which has been studied extensively for most semi-crystalline polyamides. However, there is lack of information on the effect of using 100% reused PA11 powder and the effect of the number of times it is reused on its own, during powder bed fusion laser sintering. This paper investigates the effect of reusing PA11 powder in PBF-LS and the effect of the number of times it is reused on the dimensional accuracy, density and thermal and tensile properties. From the 100% virgin powder to the third reuse of the powder, there is a decrease in powder wastage, crystallinity and tensile strength. These are associated with the polymerisation and cross-linking process of polymer chains, upon exposure to high temperatures. This results in a higher molecular weight and, hence, a higher density. From the fourth reuse to the tenth reuse, the opposite is observed, which is associated with an increase in high-viscosity unmolten particles, resulting in defects in the PBF-LS parts
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