1,720,982 research outputs found
Materiali compositi: la giusta colata per uno stampo perfetto
No full textNONOSTANTE SIANO SEMPRE PIÙ UTILIZZATE, IL COSTO DELLE PARTI IN COMPOSITO CONTINUA AD ESSERE PIUTTOSTO ELEVATO, MA NON TANTO PER IL COSTO DELLA
MATERIALE, QUANTO SOPRATTUTTO PER MOTIVI LEGATI AL PROCESSO PRODUTTIVO. UNO DEI PASSAGGI
PIÙ DELICATI È QUELLO RELATIVO ALLA CREAZIONE DELLO STAMPO IN COMPOSITO OPPURE IN ALLUMINIO NECESSARIO ALLA FORMATURA DEI PEZZI IN AUTOCLAVE. DIVERSE SOLUZIONI TECNICHE SONO DISPONIBILI, MA NESSUNO SI È ANCORA AFFERMATA CON SUCCESS
Stampo dopo stampo
No full textL’UTILIZZO DI TECNICHE DI PROCESSO NUOVE, COME ANCHE LA DIFFUSIONE DI MATERIALI PIÙ FLESSIBILI RENDONO I PRODOTTI IN FIBRA DI CARBONIO SEMPRE PIÙ COMPETITIVI. IN QUESTA EVOLUZIONE CONTINUA UN RUOLO ESSENZIALE È RIVESTITO DAGLI STAMPI. PER RENDERE EVIDENTE LA LORO IMPORTANZA PER IL RAGGIUNGIMENTO DEGLI OBBIETTIVI DI COMPETITIVITÀ SUL MERCATO, IN QUESTO ARTICOLO SI DESCRIVE LA PROCEDURA DI “FORMATURA” PIÙ DIFFUSA PER ELEMENTI IN FIBRA DI CARBONIO. DETTO IN ALTRI TERMINI, COME SI TRASFORMA UNA IDEA DI FORMA IN UN PRODOTTO COMMERCIALE.
DI SOLITO ATTRAVERSO 5 PASSAGGI, CHE COINVOLGONO 2 MODELLI, 2 STAMPI E TANTA TECNOLOGI
A finned-riser design to avoid the capillarity effect in Multi Jet Fusion technology
No full textPurpose - Multi Jet Fusion process is based on a polymeric powder bed that is heated and irradiated by infra-red lamps. The layer under construction is jetted with inks to provide the desired heat management conditions for selective melting. Depending on several process variables, manufactured parts can exhibit lifting of the borders of the top surface of the shape under construction. This phenomenon is related to the capillarity effect. As a result, the top surface of MJF-manufactured parts can present a peculiar convex shape. This study proposes a solution that instead induces the capillarity effect outside of the part under construction.
Design/methodology/approach - A specific design is developed to avoid the capillarity effect in Multi Jet Fusion. It is based on an analytical model that was previously developed by the authors to estimate the shape and extent of the capillary on top surfaces of benchmark components. The proposed methodology is established by the predicted calculation of maximum values of capillarity rise and length, and safety factors. A fin-shaped geometry is designed to avoid the capillarity effect.
An experimental campaign is implemented to verify the effectiveness of the proposed solution. Prototypes are manufactured by an HP MultiJet 4200 in the original design and the so-called finned-riser design, by adding a well- dimensioned appendage on the top surface to shift the capillarity effect outside the border of the part under construction. Measurements are done by a CAM2 ScanArm contactless measuring system to achieve the real shape of top surfaces. Geomagic Control X software by 3D Systems is used to evaluate the quality ofmeasured surfaces in comparison with the expected geometry of the top plane of the benchmark.
Findings - The investigated approach involves adding an auxiliary finned-shape appendage, which acts similarly to the risers in foundry technology, to the top surface of the part that is being produced by Multi Jet Fusion technology. The procedure and rules for determining the dimensions of the fin are established based on physical
Purpose - Multi Jet Fusion process is based on a polymeric powder bed that is heated and irradiated by infra-red lamps. The layer under construction is jetted with inks to provide the desired heat management conditions for selec- tive melting. Depending on several process variables, manufactured parts can exhibit lifting of the borders of the top surface of the shape under construction. This phenomenon is related to the capillarity effect. As a result, the top surface of MJF-manufactured parts can present a peculiar convex shape. This study proposes a solution that instead induces the capillarity effect outside of the part under construction.
Design/methodology/approach - A specific design is developed to avoid the capillarity effect in Multi Jet Fusion. It is based on an analytical model that was previously developed by the authors to estimate the shape and extent of the capillary on top surfaces of benchmark components. The proposed methodology is established by the predicted calculation of maximum values of capillarity rise and length, and safety factors. A fin-shaped geometry is designed to avoid the capillarity effect.
An experimental campaign is implemented to verify the effectiveness of the proposed solution. Prototypes are manufactured by an HP MultiJet 4200 in the original design and the so-called finned-riser design, by adding a well- dimensioned appendage on the top surface to shift the capillarity effect outside the border of the part under construction. Measurements are done by a CAM2 ScanArm contactless measuring system to achieve the real shape of top surfaces. Geomagic Control X software by 3D Systems is used to evaluate the quality of considerations and process modelling. The method is then applied to a prototype part, which is designed to highlight the effectiveness of the finned-riser design for improving the dimensional accuracy of the top surfaces of products manufactured by the MJF process.
Experimental measurements of top surfaces of the original benchmark are com- pared to the same ones in the case of the finned-riser benchmark. Reported results are satisfactory, the capillary effect occurred in the fins outside the border edges of the part. Further developments are planned to extend the proposed design.
Originality/value - Multi Jet Fusion technology is attracting large interest from manufacturers to produce mass customised products. The quality of manufactured parts could be affected by peculiar defects related to process parameters. The present work is showing a method to avoid the capillarity effect. It is based on an original analytical model developed by the authors and imple- mented successfully in the case of a benchmark geometry
Modelling of the multi-jet fusion capillarity effect on close facing edges
Full text linkPurpose – The purpose of this paper is to develop a physical model able to predict the shape of the capillarity effect in multi-jet fusion when two facing edges mutually affect each other. The work also aims at testing the consistency of such a model with experimental observations. Design/methodology/approach – An analytical model of the phenomenon is developed considering the surface tension of the polymer melt adhering to the unfused powder. The general equilibrium equations are solved by imposing the boundary conditions corresponding to the case of two close facing edges, in which the shapes of the menisci are mutually influenced. The analytical model is validated through an experimental activity. Specifically, a set of parallelepipeds with variable width was manufactured using an HP Multi Jet Fusion 4200. The morphologies of capillarities were captured via three-dimensional scanning and compared with those predicted by the model.
Findings – The results of this study demonstrate that the average error to the experimental capillarity profile is lower than that obtained by existing methods. Particularly, considerable improvements are achieved as far as the maximum capillarity height is concerned. The manufactured specimens exhibit a change in slope near the edges, which is arguably attributable to coating powder and other effects not included in the analytical model. Originality/value – The model presented in this study differs in hypotheses from previous methods in literature by assuming a null derivative of the capillarity shape in the central point of the meniscus. This allows for a more accurate prediction of the defect morphology in the case of close facing edges
Intelligent orientation of parts based on defect prediction in Multi Jet Fusion process
No full textMulti Jet Fusion technology allows the rapid production of high-quality polymeric parts. This process has attracted the inter- est of several companies involved in rapid prototyping, mass customisation, and small-batch production. The components produced via Multi Jet Fusion can exhibit some characteristic defects depending on the part geometry and orientation, the process parameters, and the environmental conditions. First of all, the most characteristic defects related to the process and their main causes are analysed. Five numerical descriptors are proposed to qualitatively predict the impact of these defects based on the digital representation of a part. Then, an intelligent system is developed to replicate the decision-making pro- cess of a human expert orienting parts before nesting. Particularly, a genetic algorithm-based approach is used to identify a pseudo-optimal part orientation under a set of objectives defined by the user. The fitness function is obtained as a weighted sum of the above-mentioned descriptors. Finally, a case study demonstrates the effectiveness of the proposed method in overcoming the characteristic defects of the process via part orienting
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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