Suchergebnisse

A precise production of sheet metal parts has always been a main goal in press shops. Highest quality demands are required especially in automotive production. Unfortunately, even today, the production is not optimal due to an ineffective approach to the springback compensation. Springback results in geometrical shape inaccuracies of the obtained product. Based on the current approach, excessive time and financial costs emerge due to corrections on the press tools. However, these corrections do not always lead to a better accuracy of the stampings. The main objective of the research is to design a modified solution of the current approach. The modified solution is designed as a methodology with a focus on the analysis and compensation of the springback with a help of a numerical simulation. To achieve the main goal, smaller sub-goals are employed. These sub-goals, or rather experiments, mainly focus on parameters, which, more or less, influence the springback phenomenon. The designed methodology is verified with real car body parts and is carried out with a help of the department of the press tools design in ŠKODA AUTO, a.s. There, the methodology is used for improving the accuracy of the stamping process of the car body parts and for improving the quality of the final product.

For at være konkurrencedygtig i fremstillings- og produktionsbranchen er det nødvendigt at kunne lave produkter af høj kvalitet med god over-fladefinish, men det er i sig selv ikke nok. For at følge med den hurtigt udviklende industri og økonomi er det også nødvendigt at reducere ressourceforbrug og øge produktiviteten. Det er praksis at anvende miljøskadelige, klorerede paraffinolier for at opretholde en effektiv produktionslinje. Ved at anvende disse olier reduceres smørefilmnedbrud som resulterer i rivninger på pladeoverfladerne. Ligeledes reduceres hyppigheden af nødvendig oppolering af værktøjerne. Derved forbedres økonomien, og materialeforbruget mindskes. Som et resultat af voksende bevidsthed om miljøhensyn og øget lovgivning, udskifter nogle virksomheder disse olier med mere miljøvenlige smøremidler og/eller tribo-systemer. Disse anstrengelser har dog kun været delvist succesfulde, da en ændring af tribo-systemet ofte leder til ændringer i de aktive smøremekanismer. Formålet med dette projekt er derfor at udvikle en offline metodik til udskiftning af miljøskadelige olier med miljøvenlige tribo-systemer. Endvidere anvendes numerisk simulering af smøring i pladeformgivningsprocesser gennem en nyligt opstillet fremgangsmåde baseret på en fuld kobling af væske- og metalflow. I dette projekt er et industrielt eksempel fra en produktionslinje hos Grundfos A/S udvalgt. Processen vedrører fremstilling af en kop i rustfrit stål og indeholder 13 trin, herunder flere dybtræk og til slut et kraveoptræk samt en strækningsreduktion. Smørefilmnedbrud finder sted på grund af stor reduktion af en vægtykkelsen under strækningsreduktionen. Til at starte med blev produktionsplatformen analyseret, og de tribologiske parametre blev identificeret. Metodikken i den detaljerede analyse af produktionen inkluderer eksperimentelt arbejde, numeriske beregninger af det termomekaniske problem og validering af resultaterne. En stripreduktionstest (SRT) blev valgt som en passende laboratorietest til at efterligne strækningsreduktionen i produktionen. Et nyt SRT-værktøj, som simulerer stempelslaget inklusiv returslag, er konstrueret og fremstillet. Der er foretaget test og analyser af hele SRT'en ved både stuetemperatur og forhøjet temperatur. Efter at have sikret procesforhold i laboratoriet svarende til produktionen blev udvalgte tribo-systemer undersøgt. Ydermere blev forbedringer til konstruktionen foreslået og afprøvet. Den foreslåede offline metodik for industriel strækningsreduktion blev efterfulgt af mere fundamental forskning. Den sidste del af afhandlingen fokuserer på forståelse af mekanismerne i en væske, der virker som trykbærer i metalformgivningsprocesser. Hertil blev finite element flow-formuleringen anvendt, idet denne tillader en stærk kobling mellem metaller og væsker. Den udviklede model blev anvendt til simulering af tykkelsesreduktion af en plade med mesaskopiske smørelommer til undersøgelse af mekanismerne, der får smøremidlet til at undslippe fra lommerne. ; In order to sustain in the competitive market in manufacturing, high-quality products with good surface finish and tight tolerances is a nec-essary but not sufficient condition. To keep pace with the high growth rate of the developing industry and the economy, it is also required to reduce the resources and increase the productivity. In order to ensure an efficient manufacturing line, the practice is to use environmentally haz-ardous chlorinated paraffin oils. By using these oils, not only the lubri-cant film breakdown, leading to scratching of the surfaces of the metal sheets, but otherwise required repolishing of the tools can be avoided, too. Thereby, financial and material resources can be reduced. At the same time, as a result of the growing awareness of environmental as-pects and increasing legislations, some companies are substituting these oils with more environmentally friendly lubricants and/or tribo-systems. However, these attempts have been only partially successful, because the change in the tribo-system often results in a shift in the active lubrication mechanism. It is therefore the aim of this project to propose an off-line methodology on how to replace environmentally hazardous oils with en-vironmentally friendly tribo-systems. While doing so, attention is also paid to numerical simulation of the lubrication in sheet metal forming operations by means of a newly proposed approach based on full cou-pling of fluid flow and metal flow. In this project, an industrial case from the production line of Grundfos A/S was selected. The process concerns manufacturing of a stainless steel cup and includes thirteen stages consisting of multiple drawings and a final collar drawing and ironing operation. Lubricant breakdown occurs due to large reduction of wall thickness during the ironing op-eration. To start with, the production platform was analysed and the tribological parameters were identified. The methodology during the detailed analysis of the production includes experimental work in pro-duction, thermo-mechanical numerical calculations and validation of the results. The Strip Reduction Test (SRT) was selected as a suitable labo-ratory test to emulate the ironing operation in the production. A new SRT tool emulating both forward and backward strokes of the ironing opera-tion has been designed and manufactured. The testing and the analyses of the entire SRT at room temperature as well as at elevated tempera-ture were performed. After ensuring similar process conditions in the laboratory, selected tribo-systems were investigated. Additionally, en-hancements for improving the current design were suggested and tested. The proposed off-line methodology for industrial ironing operations was followed by a more fundamental research. The final part of the thesis fo-cuses on understanding the behaviour of the fluids as pressure carriers in metal forming processes. For that, the finite element flow formula-tion, enabling strong coupling between metals and fluids, was adapted. Finally, the developed model was applied to thickness reduction testing of a sheet with mesoscopic surface pockets in order to investigate the mechanisms causing the escape of lubricants from the pockets.