Investigati on of the application of an additively manufactured and topologically optimized die for sheet metal bending followed by elastic recovery

[M. M. Delić]

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Izrada alata za oblikovanje lima primenom aditivnih tehnologija dobija sve više na značaju, ne samo u oblasti nauke i istraživanja već i u savremenoj industriji a čija uspešna primena zahteva poznavanje različitih metoda i tehnika. Primenom polimernih aditivno proizvedenih alata postiže se skraćenje vremena izrade a u nekim slučajevima i smanjenje cene izrade. U ovoj doktorskoj disertaciji razmatrana je mogućnost korišćenja aditivno proizvedenih polimernih alata za savijanje lima. S obzirom da alati tokom procesa oblikovanja trpe naprezanja bitan parametar su mehaničke karakteristike materijala. U prvom delu disertacije analizirane su mehaničke karakteristike dva polimerna materijala, ABS-a i Oniksa. Uticaj parametara FDM tehnologije na mehaničke karakteristike određen je testovima jednoosnog zatezanja i pritiskivanja. Na osnovu rezultata eksperimenata za materijal alata izabran je ABS i definisani su optimalni parametri aditivne proizvodnje radnih komponenti alata. Zbog toga što polimerni materijali imaju znatno slabije mehaničke karakteristike od metala pri projektovanju procesa oblikovanja sa polimernim alatima potrebno je smanjiti opterećenje koje alat trpi. Pored opterećenja problem koji postoji pri oblikovanju limova je elastična povratnost. Definisanje geometrije alata je urađeno korišćenjem metode konačnih elemenata da bi problemi prevelikog opterećenja alata i pojave elastične povratnosti bili otklonjeni. Na osnovu rezultata numeričkih eksperimenata redizajnirane su radne komponente alata za jednougaono savijanje uz kompenzaciju ugla elastične povratnosti i utvrđene granice deformacione sile za dobijanje dela od lima sa precizno definisanim uglom savijanja. Pored dokazivanja hipoteze da je moguće koristiti polimerne alate za oblikovanje čeličnog lima, sledeći naučni izazov je bio pokazati da je topološka optimizacija aditivno proizvedenih alata opravdana i primenljiva. Na osnovu numeričkih eksperimenata polimerni aditivno proizvedeni alati su optimizovani metodom topološke optimizacije uzimajući u obzir pravila konstruisanja za aditivnu proizvodnju (DFAM - Design for additive manufacturing). Eksperimenti jednougaonog savijanja lima sa aditivno proizvedenim i topološki optimizovanim alatima urađeni su u laboratorijskim i industrijskim uslovima. U laboratorijskim uslovima korišćena je savremena mašina za ispitivanje materijala dok je u industrijskim uslovima kroz studiju slučaja korišćena apkant presa. Posle eksperimentalnog dela alati i savijeni delovi su digitalizovani posredstvom 3D skenera za precizno merenje oblika dela i ugla savijanja. Rezultati eksperimenta su potvrdili sve polazne hipoteze, s obzirom da je dobro poklapanje ugla savijanja delova dobijenih numeričkom simulacijom, eksperimentima sa metalnim i polimernim alatima, kao i topološki optimizovanih, a na kraju i polimernih alata montiranih na industrijsku apkant presu. Pri tome su opterećenja na polimernim alatima bila u dozvoljenim granicama bez ikakvih trajnih plastičnih deformacija i površinskih oštećenja.

The production of sheet metal forming dies using additive technologies is gaining increasing importance, not only in the field of science and research, but also in modern industry, and the successful application requires knowledge of various methods and techniques. The use of polymer additively manufactured dies reduces production time and, in some cases, reduces production costs. This doctoral dissertation examines the possibility of using additively manufactured polymer dies for sheet metal bending. Since dies are subjected to stress during the forming process, an important parameter is the mechanical properties of the material. In the first part of the dissertation, the mechanical properties of two polymer materials, ABS and Onyx, were analyzed. The influence of FDM technology parameters on the mechanical properties was determined by uniaxial tensile and compression tests. Based on the results of the experiments, ABS was selected as the die material and the optimal parameters for additive manufacturing of the die components were defined. Because polymer materials have significantly weaker mechanical properties than metals, when designing the forming process with polymer dies, it is necessary to reduce the load that the die undergoes. In addition to the load, an existing problem in sheet metal forming is elastic recovery. The definition of the die geometry was done using the finite element method in order to eliminate the problems of excessive die loading and the occurrence of elastic recovery. Based on the results of numerical experiments, the die components for free air bending were redesigned with compensation for the elastic recovery angle and the forming load limits were determined to obtain a sheet metal part with a precisely defined bending angle. In addition to proving the hypothesis that it is possible to use polymer dies for forming steel sheet metal, the next scientific challenge was to show that the topological optimization of additively manufactured dies is justified and applicable. Based on numerical experiments, polymer additively manufactured dies were optimized using the topological optimization method, taking into account the design rules for additive manufacturing (DFAM - Design for additive manufacturing). Experiments on free air sheet metal bending with additively manufactured and topologically optimized dies were performed in laboratory and industrial conditions. In laboratory conditions, a modern material testing machine was used, while in industrial conditions, a press brake was used through a case study. After the experimental part, the dies and bent parts were digitized using a 3D scanner for precise measurement of the part shape and bending angle. The experimental results confirmed all initial hypotheses, given that the bending angle of the sheet metal parts obtained by numerical simulation, experiments with metal and polymer dies, as well as topologically optimized, and finally polymer dies mounted on an industrial press brake was in good agreement. In this case, the loads on the polymer dies were within permissible limits without any permanent plastic strain and surface damage.

aditivna proizvodnja, FDM, numeričke simulacije, elastična povratnost, 3D skeniranje, topološka optimizacija, ABS, Oniks

additive manufacturing, FDM, numerical simulations, elastic recovery, 3D scanning, topology optimization, ABS, Onyx

621.7+621.9(043.3)

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