Hovad, Emil

Thorborg, Jesper

ErhvervsPhD projektet omhandlede produktion af støbeforme i grønsand. Støbeformen giver den støbte metalkomponent sin endelige geometriske udformning. For at sikre en høj kvalitet af den støbte komponent, er det vigtigt at kontrollere fremstillingsprocessen af selve formen, s˚aledes at den er homogen og stabil. Derfor er det vigtigt at f˚a en grundlæggende forst˚aelse og viden om hvordan sandet ﬂyder og aﬂejres. Denne viden opn˚as via karakterisering af sandet med forskellige typer af forsøg. N˚ar sandet er karak-teriseret vha. forsøgene, kan disse forsøg anvendes til at kalibrere en computermodel. Den kalibrerede computermodel kan efterfølgende anvendes til simulering af DISAMATIC pro-cessens sandskud.Flydeevnen af grønsandet er vigtigt, n˚ar det strømmer ned gennem sandsiloen og fylder kammeret under sandskuddet. Flydeevnen af sandet er hovedsageligt styret af mængden af vand og bentonit, som begge formindsker ﬂydeevnen. Denne ﬂydeevne og de interne spændinger i materialet er afgørende for hvor godt en kompleks kammergeometri kan fyldes, hvor geometriske forhindringer kan være til stede. Hvis sandet stopper med at ﬂyde for tidligt, kan det forhindre formen i at blive helt fyldt eller resulterer i en forhøjet variation i densiteten, dette kan potentielt p˚avirke udformningen af den endelige met-alkomponents overﬂade. Sandkornene klæber sig sammen pga. bentonitten og vandet, hvor bentonitten skaber sm˚a broer, som klæber de individuelle sandkorn sammen. N˚ar blandingen trykkes sammen opn˚as der dermed mekaniske egenskaber, der stabiliserer for-men. Grønsandets ﬂydeevne er derfor vigtig i sandskuddet for at opn˚a en korrekt fyldning af kammeret og derefter er gode faststofmekaniske egenskaber opn˚aet under pressepro-cessen vigtige for den endelige styrke af formen. Dette er problematisk, eftersom disse to mekaniske egenskaber har et omvendt forhold, fx hvis sandet er for tørt vil ﬂydeevnen være høj, og styrken af formen vil være lav og det omvendte er tilfældet for v˚adt sand, i det mindste op til et vist vandindhold. Derfor er tilstanden af grønsandet vigtig, s˚a man kan opn˚a en optimal fyldning af formen under sandskuddet.Discrete Element Method (DEM) blev valgt som den numeriske metode, idet den diskrete karakter af metoden simulerer den granulære struktur af sandet med god overensstem-melse. DEM modellen bruger en rullemodstandsmodel for at efterligne de ikke-sfæriske kvartssandkorns modstand mod rulning og en kohæsionsmodel til at efterligne bindingen af kvartssandskornene fra bentonitten. Sandet blev karakteriseret med en ring shear tester, hvor den kunne detektere yield locus og en ny m˚ade at deﬁnere ﬂydeevnen er blevet foresl˚aet. Ring shear tester’en blev anvendt til at bestemme friktionskoeﬃcienterne anvendt i DEM modellen. Et sandbunkeforsøg blev anvendt til at undersøge den simple mekaniske opførsel af sandet, hvor karakteriseringen blev opn˚aet ud fra højden af bunken. Fra denne højde kunne DEM modellen ogs˚a blive kalibreret med henblik p˚a fremskaﬀelsen af værdien for rullemodstanden og man fandt ogs˚a kohæsions parameteren.Projektet behandlede strømmeningen af sandpartiklerne og aﬂejringen af dem ved frem-stillingen af sandformen under sandskuddet i DISAMATIC processen. Aﬂejringen af grønsandet i kammeret blev undersøgt med et særligt hulrumsdesign, hvor luftventiler blev placeret inde i hulrummene. Luftventilerne bruges til at transportere sandet via luftstrømme under sandskuddet. Ved at ændre ventilindstillingerne i kammeret og i hul-rummene var det muligt at forbedre fyldningen i de smalle passager i hulrumsdesignet, og derved forbedre den endelige sandform. Sandskuddet i hulrumsdesignet blev simuleret med DEM metoden kombineret med klassisk Computational Fluid Dynamics (CFD) til modellering af luftstrømmen i kammeret og luftstrømmen gennem luftventilerne. Disse eksperimenter og simuleringer gav indsigt i DISAMATIC processen og hvordan man kan forbedre den. Derudover blev ﬂuidiseringsegenskaberne af grønsandet undersøgt med en ﬂuidized bed test og den nyudviklede Anton Paar Powder Cell blev anvendt til at ﬁnde den ﬂuidiserede viskositet.Kommercielle aspekterViden blev erhvervet om fyldningen af kammeret med grønsand med den specielt designede hulrumsgeometri. Indstillingerne for luftventilerne sammen med lufttrykket i lufttanken gav værdifulde ideer til forbedring af fyldningen i hulrummene og derved forbedre den endelige støbeform. Desuden var det muligt at anvende den kommercielle software pakke STAR-CCM+ med en kombineret CFD-DEM-model til at simulere processen med en 3-D skive som repræsenterede geometrien med succes. Dette gør det muligt at udvikle en selvstændig kode i fremtiden, som kan simulere DISAMATIC processen. Sandskuddet i DISAMATIC processen kunne ogs˚a blive modelleret med en kontinuummodel, hvor ring shear tests kunne give indikationer af den faste mekaniske opførsel af sandet og Anton Paar Powder Cell testen kunne anvendes til at bestemme den ﬂuidiserede viskositet af sandet.The focus of the industrial PhD project was concentrated on the production of the sand mold (green sand) which gives the cast component its ﬁnal geometrical shape. In order to ensure a high quality of the cast component, it is important to control the manufacturing process of the mold itself so that it is homogeneous and stable. Therefore gaining a basic understanding of how the ﬂow and deposition of green sand should be characterized and modelled was important, so that it could be used for simulation of the manufacturing process of the sand mold.The ﬂowability of the green sand is important when the sand ﬂows down through the hopper ﬁlling the chamber with sand during the sand shot. The ﬂowability of green sand is mostly governed by the amount of water and bentonite which both decrease it. The ﬂowability and the internal forces thus control how well you can ﬁll a complex mold geom-etry in which shadowing from ribs and other geometric obstacles may be present. If the ﬂow stops prematurely it might hinder the mould from being completely ﬁlled or result in too high variation in the material density which could inﬂuence the ﬁnal surface of the cast part. The wet bridges created by the bentonite makes the sand grains stick together where the bentonite and water make the green sand very cohesive and by squeezing the mixture it obtains mechanical properties that stabilizes the mold to acquire a strong mold for the casting process. Therefore the green sand ﬂowability is important during the sand shot for a proper ﬁlling of the chamber, and subsequently the solid mechanical proper-ties during the squeezing process are important for the ﬁnal strength of the mold. This is problematic since these mechanical behaviours have an inverse relationship, e.g if the green sand is too dry then the green sand ﬂowability will be very high and the strength of the mold will be low and vice versa at least for the wet green sand up to a certain water content level. Therefore, obtaining the correct green sand condition and improving the ﬁlling of the mold during the sand shot are of great importance.The Discrete Element Method (DEM) was chosen as the numerical model since the dis-crete nature of the method simulates the granular structure of the green sand with good agreement. The DEM model uses a rolling resistance model to emulate the non-spherical quartz sand particles’ resistance to rolling as well as a cohesive model to emulate the binding of the quartz sand particles from the bentonite.The green sand was characterized with a ring shear tester where the yield locus was found and a new way to deﬁne the ﬂowability was suggested. The ring shear tester was used to obtain the static friction coeﬃcients for the DEM model. A sand pile experiment was used to investigate the simple mechanical behaviour of green sand from the measured height. From this height the DEM model was also calibrated with respect to obtaining the values of the rolling resistance and obtaining the parameter in cohesive model.The project dealt with the ﬂow of the sand particles and the deposition of sand dur-ing the production of sand molds using the sand shot in the DISAMATIC process. The deposition of the green sand in the chamber was investigated with a special cavity design where air vents were placed inside the cavities. The air vents are used to transport the green sand with an airﬂow during the sand shot. By changing the air vents settings in the chamber and in the cavities it was possible to improve the ﬁlling in the narrow passages in the cavity design, thereby improving the ﬁnal sand mold as well. The sand shot with the cavity design was simulated by the discrete element method (DEM) modelling the ﬂow of the green sand combined with classical computational ﬂuid dynamics (CFD) for modelling the airﬂow in the chamber and the airﬂow through the air vents. These experiments and simulations gave beneﬁcial insights to the DISAMATIC process and how to improve it. Additionally ﬂuidization properties of green sand were investigated with a ﬂuidized bed and the newly developed Anton Paar Powder Cell was used to obtain the ﬂuidized viscosity.Commercial aspectsKnowledge was acquired about the ﬁlling of the mold chamber with green sand in a special designed cavity geometry. The settings of the air vents together with the air pressure initially applied in the air tank gave valuable ideas for improving the ﬁlling in the cavities thereby improving the ﬁnal mold. Furthermore, it was possible to apply the commercial software of STAR-CCM+ using the combined CFD-DEM model to simulate the process with a 3-D slice representation of the geometry successfully. This makes it more feasible to develop a stand-alone code in the future for simulating the DISAMATIC process. The sand shot in the DISAMATIC process might also be modelled with a continuum model where the ring shear tester could give indications of the solid mechanical behaviour of the green sand and the Anton Paar Powder Cell could be used for obtaining the ﬂuidized viscosities of the green sand

Online Research Database In Technology

Numerical simulation of flow and compression of green sand

http://orbit.dtu.dk/ws/files/138513591/Emil_Hovad_PhD_Thesis.pdf

Numerical simulation of flow and compression of green sand

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