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

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

Processing of hybrid laminates integrating ZrB2/SiC and SiC layers

Tape casting technique was used to develop hybrid laminates constituting by SiC and ZrB2-SiC layers; the main aim is obtaining a structure which integrate the unique properties of these materials and potentially extent their application temperature range. Multilayer with ZrB2-SiC layers stacked in between SiC ones were successfully processed. Thin cracks propagated in the composite layers without affecting SiC ones; their formation was due to residual stresses developed in the two materials because of the differences in their shrinkage and coefficients of thermal expansion. However, these cracks did not significantly affect the material properties: relative density, elastic modulus and flexural strength of hybrid laminates was indeed only slightly lower than those of laminates made up of layers with the same composition

Strategy for the development of a new stick formula without microplastics

Plastic is a synthetic, malleable, and durable material that has been used for various applications since its invention in the late 19th century. During its very long-time degradation process, mechanical and/or photochemical processes fragment plastic into increasingly smaller fragments called microplastics (MPs). In the cosmetic field, MPs are directly added to many products for various functions, including to exploit their exfoliating and structuring power. Unfortunately, it has been realized that MPs are not retained in purification plants and therefore end up in the aquatic environment causing a high problem of environmental pollution. Polyethene (PE) is the most widely used MP in cosmetics due to its use as a structuring agent, to provide consistency to formulations and as a key ingredient in lipsticks and mascaras. Given the limitations imposed by regulations and the growing demand from consumers for chemical-free and eco-friendly products, the common synthetic and petroleum-based waxes used in lipstick formulations, such as PE, must necessarily be replaced by natural waxes of plant origin. In this paper we report the development of a chemical-free and eco-friendly cosmetic stick. To achieve the goal, it was necessary to study the compatibility of the ABWAX® Revowax, natural alternative to PE, with oils and colours to predict the behaviour of these structuring waxes in more complex systems. Through a systematic comparative study, the two waxes showed similar thermal characteristics and showed similar penetration curves, presenting overall comparable performance in the MP-free finished product. We can therefore consider ABWAX® Revowax natural wax a valid alternative to PE

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

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

Wearable and interactive mixed reality solutions for fault diagnosis and assistance in manufacturing systems: Implementation and testing in an aseptic bottling line

Thanks to the spread of technologies stemming from the fourth industrial revolution, also the topic of fault diagnosis and assistance in industrial contexts has benefited. Indeed, several smart tools were developed for assisting with maintenance and troubleshooting, without interfering with operations and facilitating tasks. In line with that, the present manuscript aims at presenting a web smart solution with two possible applications installed on an Android smartphone and Microsoft HoloLens. The solution aims at alerting the operators when an alarm occurs on a machine through notifications, and then at providing the instructions needed for solving the alarm detected. The two devices were tested by the operators of an industrial aseptic bottling line consisting of five machines in real working conditions. The usability of both devices was positively rated by these users based on the System Usability Scale (SUS) and additional appropriate statements. Moreover, the in situ application brought out the main difficulties and interesting issues for the practical implementation of the solutions tested

Selective Laser Sintering versus Multi Jet Fusion: A Comprehensive Comparison Study Based on the Properties of Glass Beads-Reinforced Polyamide 12

Selective laser sintering (SLS) and multi jet fusion (MJF) are the most widespreadpowder bed fusion additive manufacturing techniques for fabricating polymericparts since they offer great designflexibility, productivity, and geometricalaccuracy. However, these technologies differ in the thermal energy source usedto melt the powders as well as the innovative use of printing agents featured inthe latter one to promote material consolidation and to avoid thermal bleeding atthe part contours. The use of a single powder made of glass beads-reinforcedpolyamide 12 (PA12/GB) for the fabrication of MJF and SLS samples makespossible a systematic comparison of the printed parts properties. A thoughtfulanalysis of the microstructure and mechanical properties of the samples revealsdifferences and peculiarities between the two technologies. SLS exhibits lowerporosity and higher mechanical performances when the parts are printed alongthe build plane thanks to the powerful heating ensured by the laser. In contrast,MJF samples show higher mechanical isotropy with greatflexural and tensilebehavior for vertically oriented parts. The role of glass beads in the materialbehavior is defined by their mechanical properties, meaning higher rigidity andlower strength compared to neat PA12, and fracture mechanism

Neurologic outcome of postanoxic refractory status epilepticus after aggressive treatment

OBJECTIVE: To investigate neurologic outcome of patients with cardiac arrest with refractory status epilepticus (RSE) treated with a standardized aggressive protocol with antiepileptic drugs and anesthetics compared to patients with other EEG patterns. METHODS: In the prospective cohort study, 166 consecutive patients with cardiac arrest in coma were stratified according to 4 independent EEG patterns (benign, RSE, generalized periodic discharges [GPDs], malignant nonepileptiform) and multimodal prognostic indicators. Primary outcomes were survival and cerebral performance category (CPC) at 6 months. RESULTS: RSE occurred in 36 patients (21.7%) and was treated with an aggressive standardized protocol as long as multimodal prognostic indicators were not unfavorable. RSE started after 3 \ub1 2.3 days after cardiac arrest and lasted 4.7 \ub1 4.3 days. A benign EEG pattern was recorded in 76 patients (45.8%); a periodic pattern (GPDs) was seen in 13 patients (7.8%); and a malignant nonepileptiform EEG pattern was recorded in 41 patients (24.7%). The 4 EEG patterns were highly associated with different prognostic indicators (low-flow time, clinical motor seizures, N20 responses, neuron-specific enolase, neuroimaging). Survival and good neurologic outcome (CPC 1 or 2) at 6 months were 72.4% and 71.1% for benign EEG pattern, 54.3% and 44.4% for RSE, 15.4% and 0% for GPDs, and 2.4% and 0% for malignant nonepileptiform EEG pattern, respectively. CONCLUSIONS: Aggressive and prolonged treatment of RSE may be justified in patients with cardiac arrest with favorable multimodal prognostic indicators

Measurement of the total cross section and ρ-parameter from elastic scattering in pp collisions at sqrt(s)=13 TeV with the ATLAS detector

In a special run of the LHC with β⋆=2.5 km, proton–proton elastic-scattering events were recorded at sqrt(s)=13 TeV with an integrated luminosity of 340 μb^{−1} using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from −t=2.5⋅10^{−4} GeV^2 to −t=0.46 GeV^2 using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section σ_{tot}, parameters of the nuclear slope, and the ρ-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit t→0. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t-dependence. The results for σ_{tot} and ρ are σ_{tot}(pp→X)=104.7±1.1 mb ρ=0.098±0.011. The uncertainty in σ_{tot} is dominated by the luminosity measurement, and in ρ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude

Correlations between flow and transverse momentum in Xe+Xe and Pb+Pb collisions at the LHC with the ATLAS detector: A probe of the heavy-ion initial state and nuclear deformation

The correlations between flow harmonics v_n for n=2, 3, and 4 and mean transverse momentum [pT] in 129Xe+129Xe and 208Pb+208Pb collisions at sqrt(s)=5.44 and 5.02 TeV, respectively, are measured using charged particles with the ATLAS detector. The correlations are potentially sensitive to the shape and size of the initial geometry, nuclear deformation, and initial momentum anisotropy. The effects from nonflow and centrality fluctuations are minimized, respectively, via a subevent cumulant method and an event-activity selection based on particle production at very forward rapidity. The v_n−[pT] correlations show strong dependencies on centrality, harmonic number n, pT, and pseudorapidity range. Current models qualitatively describe the overall centrality- and system-dependent trends but fail to quantitatively reproduce all features of the data. In central collisions, where models generally show good agreement, the v_2−[pT] correlations are sensitive to the triaxiality of the quadruple deformation. Comparison of the model with the Pb+Pb and Xe+Xe data confirms that the 129Xe nucleus is a highly deformed triaxial ellipsoid that has neither a prolate nor oblate shape. This provides strong evidence for a triaxial deformation of the 129Xe nucleus from high-energy heavy-ion collisions

Measurements of the suppression and correlations of dijets in Pb+Pb collisions at sqrt(s_NN)=5.02 TeV

Studies of the correlations of the two highest transverse momentum (leading) jets in individual Pb+Pb collision events can provide information about the mechanism of jet quenching by the hot and dense matter created in such collisions. In Pb+Pb and pp collisions at sqrt(s_NN)=5.02 TeV, measurements of the leading dijet transverse momentum (pT) correlations are presented. Additionally, measurements in Pb+Pb collisions of the dijet pair nuclear modification factors projected along leading and subleading jet pT are made. The measurements are performed using the ATLAS detector at the LHC with 260 pb^{−1} of pp data collected in 2017 and 2.2 nb^{−1} of Pb+Pb data collected in 2015 and 2018. An unfolding procedure is applied to the two-dimensional leading and subleading jet pT distributions to account for experimental effects in the measurement of both jets. Results are provided for dijets with leading jet pT greater than 100 GeV. Measurements of the dijet-yield-normalized xJ distributions in Pb+Pb collisions show an increased fraction of imbalanced jets compared to pp collisions; these measurements are in agreement with previous measurements of the same quantity at 2.76 TeV in the overlapping kinematic range. Measurements of the absolutely normalized dijet rate in Pb+Pb and pp collisions are also presented, and show that balanced dijets are significantly more suppressed than imbalanced dijets in Pb+Pb collisions. It is observed in the measurements of the pair nuclear modification factors that the subleading jets are significantly suppressed relative to leading jets with pT between 100 and 316 GeV for all centralities in Pb+Pb collisions