1,649 research outputs found
In the shadow of the church: the building of mosques in early medieval Syria
No full textIn his book In the Shadow of the Church: The Building of Mosques in Early Medieval Syria Mattia Guidetti examines the establishment of Muslim religious architecture within the Christian context in which it first appeared in the Syrian region, contributing to the debate on the transformation of late antique society to a Muslim one. He scrutinizes the slow process of conversion to Islam of the most important town centers by looking at religious places of both communities between the seventh and the eleventh century. The author assesses the relevancy of churches by analyzing the location of mosques and by researching phenomena of transfer of marble material from churches to mosques
Variation formulas for H-rectifiable sets
No full textWe compute a first- and second-variation formula for the area of H-rectifiable sets in the Heisenberg group along a contact flow. In particular, the formula holds for sets with locally finite H-perimeter, with no further regularity
Modelling of the capillarity effect for cylindrical shapes in Multi Jet Fusion technology
No full textMulti Jet Fusion is one of the most productive additive manufacturing technologies available on the market. This important characteristic, together with the almost isotropic mechanical resistance of fabricated products, attracts the interest of users and industries. Despite those significant advantages, parts can present some specific defects. In particular, one of the most characteristic markers of the Multi Jet Fusion process is the capillarity effect that affects the top planar surfaces of parts.
The present paper proposes an analytical model for the prediction of the capillarity effect in the case of curved edges. The model is based on the force equilibrium that is solved for circular cylindrical shapes and provides the equation of the capillary profile. An experimental activity is carried out to obtain useful data for model validation. For this purpose, a benchmark geometry is proposed that emphasises the mentioned phenomenon. Small circular cylinders are designed and then manufactured using an HP Multi Jet Fusion 4200. The parts obtained are digitalised by three-dimensional scanning. The scanned profiles of the top surfaces are analysed through statistical parameters. A preliminary result is that the capillarity effect on small circular cylindrical shapes can be considered axisymmetric.
To validate the model, measured profiles of benchmark top surfaces are compared with the calculated ones. The results obtained show that the proposed equation can predict the measured data with errors in the range of one hundredth of a millimetre
Modeling the strength of laminated parts made by fused filament fabrication additive manufacturing
No full textFused filament fabrication (FFF, also known as fused deposition modeling) is the most popular 3D printing additive manufacturing technology: cheap 3D printers are largely widespread and most polymeric materials can be used. It is probably the most versatile additive manufacturing technology. The applications range from prototyping to the production of custom parts with structural capabilities: fiber reinforced plastics can also be used. However, the knowledge of the FFF materials and the design criteria for fused filament fabricated parts are scarcely known and this results in a very common skepticism in adopting this technology for technical structural components. As for the other processing technologies for plastics, included injection molding, the mechanical properties of the components strongly depend on the manufacturing parameters. As shown by some authors, since production by FFF produces a layered structure, it is possible to model the mechanical behavior by means of the classical lamination theory largely verified and known for composites. In those papers, it was shown that the equivalent elastic properties could be predicted as a function of the lamination stacking and angles. In this paper, based on experimental results obtained on symmetric balanced angle-ply laminated samples made of polyethylene terephthalate with added glycol (PETG) and polyamide (PA) subject to tensile loads, the Tsai-Hill criterion was applied to predict the strength. After identification of the strength parameters, a good correlation of the experimental with the model parameters, for all lamination angles, was obtained
Fatigue strength of plastics components made in additive manufacturing: first experimental results
No full textEvolution of additive manufacturing (AM) techniques is making these innovative technologies more and more available and known to a larger audience. This allowed components built with AM techniques, especially metallic ones, to be effective in substituting similar components made with traditional technologies; with all the advantages of AM that make these components even more interesting in terms of performance. With plastics this process is relented also due to the chronic lack of established knowledge of the plastic materials, both in terms of strength, design criteria, both in long term behavior but also in static short-term properties. This work tries to give some useful information about the fatigue behavior of one class of material widely used with the mostly widespread AM technique for plastics, that is filament deposition modeling (FDM). The material considered is acrylonitrilebutadiene-styrene (ABS), used in countless components (electronic devices, household appliances, medical tools, and others) due to its excellent mechanical performances and relatively good workability. The property mainly analyzed in this work is fatigue behavior. Fatigue tests were performed in plane bending on specimen very similar to the type proposed and used by Nicoletto (2018) in different manufacturing and loading conditions. The obtained results offer an interesting insight into the properties of small components in ABS made by FDM and the effects of some influencing parameters: different stress-ratios were considered, as well as technological variations such as deposition direction. Experiments reveal that the scatter of fatigue data, even with the manufacturing uncertainties and defects typical of AM, can be controlled and within reasonable limits
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
An improved model to describe the repeated loading-unloading in compression of cellular materials
No full textCellular materials, often referred as foams or structural foams when used for energy absorption, are largely used to protect people and goods in the case of shocks and impacts. The detailed knowledge of their behavior is fundamental to design components for this aim. Peroni et al. (2008)-(2009) proposed a model able to describe the mechanical compression behavior of some polymeric material. Such model, based on the work by Rusch (1970), described the stress-strain curve as a sum of two contributions, the first for the elastic part and the second for the densification. More recently Avalle and Belingardi (2018) presented a more general model where the stress is calculated as a sum of three terms, one for the elasto-plastic phase, the second for the plateau, and a third for the densification. The model could include effects like density and strain-rate. However, those models allow to describe only the monotonic compression behavior: in several situations repeated impacts can happen with unloading followed by further reloading. Unfortunately unloading cannot be described by a linear relation between stress and strain (as is usually considered for metals). Unloading follows a non-linear law with a variable relation between stress and strain in the successive cycles: this requires a particularly complex model. In this work, a new model able to effectively reproduce the compression behavior of some polymeric cellular materials is presented. The model is validated and tuned on the basis of experimental tests with specimen subject to complex cycles of repeated loading and unloading. The model describes both the loading from different levels of residual compression and unloading from any value of compression level. The application to several materials justifies the generality of the method
A Decision Method to Improve the Sustainability of Post Processing in Multi Jet Fusion Additive Manufacturing
No full textThe improvements in terms of sustainability achievements by substituting additive manufacturing processes to traditional technologies have been widely investigated in literature. Nevertheless, a lack of information still exists regarding the impacts of post-processing phase of additively manufactured parts, even if this step usually has important economical, societal and environmental consequences.
The present paper analyses the case of the Multi Jet Fusion process, due to the unavoidability of part post-processing. The main impact factors of this phase are highlighted and linked to the manufacturing conditions in order to point out the opportunities to improve the overall sustainability of the process
Modelling of the capillarity effect in Multi Jet Fusion technology
No full textMulti Jet Fusion is a powder-based Additive Manufacturing technology patented by Hewlett-Packard Inc. It is characterised by the use of lamps instead of lasers to heat and melt polymers and by fusing and detailing agents that are jetted on the polymeric particles to modify and to control their heat absorption and thus selectively melt them. The high production rate and excellent mechanical properties of the manufactured parts, even in com- parison with Laser Sintering, together with the overall product quality make this technology effective for a production of small series of end-parts rather than functional prototypes.
In the present paper, the so-called capillarity effect is investigated. Capillarity derives from the interaction between the detailing and the fusing agents during the layer-based part building and it determines a deviation of the upper planar geometries when close to the border edges between the molten material and the powder bed.
A model for the estimation of the capillarity effect is here proposed by adopting the free liquid meniscus theory. A benchmark geometry was designed to be affected by the capillarity effect and then manufactured by the MJF process. Values of the contact angle and of the characteristic length of the capillary, which are necessary to implement the analytical model, were obtained by experimental measurements made on the benchmark geometry.
As a result the capillarity effect showed a dependence on the border edge orientation. The comparison be- tween calculated shapes of the plane affected by the capillarity effect through the analytical model was in accordance with the experimental measurements thus allowing a reliable prediction to be made
On the effect of irradiance on dimensional accuracy in multijet fusion additive manufacturing
No full textPurpose: The amount of radiated energy is known to be a crucial parameter in powder-bed additive manufacturing (AM) processes. The role of irradiance in the multijet fusion (MJF) process has not been addressed by any previous research, despite the key role of this process in the AM industry. The aim of this paper is to explore the relationship between irradiance and dimensional accuracy in MJF. Design/methodology/approach: An experimental activity was carried out to map the relationship between irradiance and dimensional accuracy in the MJF transformation of polyamide 12. Two specimens were used to measure the dimensional accuracy on medium and small sizes. The experiment was run using six different levels of irradiance. For each, the crystallinity degree and part density were measured. Findings: Irradiance was found to be directly proportional to part density and inversely proportional to crystallinity degree. Higher irradiance leads to an increase in the measured dimensions of parts. This highlights a predominant role of the crystallisation degree and uncontrolled peripherical sintering, in line with the previous literature on other powder-bed AM processes. The results demonstrate that different trends can be observed according to the range of sizes
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