Bentonite bonded moulding sands

Sand moulded casting is still one of the most efficient and cheapest methods to produce metal parts of complex shapes. Due to their cyclic reusability and low emission of particles and organic compounds during casting, bentonite bonded sands have an excellent environmental performance. Moreover, sands discarded after a maximum of cycles can be reused as raw material for in various applications. Wet tensile strength testing is a common method to assess the quality of bentonite bonded moulding sands. For wet tensile strength testing, a specimen is first heated from above in order to simulate heat-driven moisture transport induced by the casting process. Then, stress is applied until the specimen ruptures. In the first part of this study, neutron radiography imaging was applied to moulding sands in-situ during heating and wet tensile strength testing in order to investigate the effects of water kinematics on the tensile strength. Neutron radiography allowed the localization of the rupture plane and the quantitative determination of the local water content with sub-mm resolution. Quantification of the temperature of the rupture plane and of the heat kinematics within the specimen was accomplished by temperature measurements both in-situ and ex-situ. In this way, experimental data correlating the wet tensile strength with the specific conditions of moulding sands at the rupture plane were obtained for the first time. Series of experiments with different initial sand moisture contents were conducted. The results show that the weakest location within a sand profile can be pinpointed at the interface between evaporation and condensation zone (i.e., at the 100°C isotherm), where water vaporisation starts and the water bridges connecting the sand grains collapse. During casting, however, the moulding sand undergoes irreversible alterations, which deteriorate its properties. Aim of this work, therefore, was to gain accurate insights into the relation between sand alteration and property change. To this end, the wet tensile strengths of heat pre-treated and cyclically reused sands were related to the kinematics of water within the sand as measured by in-situ neutron radiography and neutron diffraction. Sands subjected to 22 cycles of drying (T = 120°C) and remoistening (3 wt.%) revealed modifications in smectite lattice spacings and in water release behaviour from smectite interlayers. No significant change of tensile strength, however, was associated with these modifications. Contrarily, sands pre-treated with temperatures as low as 225°C revealed a reduction of tensile strength which was correlated with irreversible modifications of the structural and compositional integrity of the smectites as manifested by substantially altered water kinematics. The results show that cyclic use of moulding sand has little impact on the sand quality as long as the temperature of the sand remains low. For the practice in foundry, the results imply that a rigorous separation of heat-exposed sand is advantageous. Sands bonded with sodium- and calcium-bentonite were studies in order to unravel the influence of the exchangeable cation on the binding properties. Changes in tensile strength as well as in heat and mass transport within the sand were investigated. Heat pre-treated Ca- and Na-bentonite bonded sand revealed differences in tensile strength. Although no major differences in the swelling behaviour could be detected, Na-bentonite bonded sands have a better mechanical performance than Ca-bentonite bonded sands both before and after the pre-treatment. The measured changes in tensile strength of both Ca- and Na-bentonite bonded sand correspond well with the irreversible alterations of the smectites

Bentonite bonded moulding sands

Sand moulded casting is still one of the most efficient and cheapest methods to produce metal parts of complex shapes. Due to their cyclic reusability and low emission of particles and organic compounds during casting, bentonite bonded sands have an excellent environmental performance. Moreover, sands discarded after a maximum of cycles can be reused as raw material for in various applications. Wet tensile strength testing is a common method to assess the quality of bentonite bonded moulding sands. For wet tensile strength testing, a specimen is first heated from above in order to simulate heat-driven moisture transport induced by the casting process. Then, stress is applied until the specimen ruptures. In the first part of this study, neutron radiography imaging was applied to moulding sands in-situ during heating and wet tensile strength testing in order to investigate the effects of water kinematics on the tensile strength. Neutron radiography allowed the localization of the rupture plane and the quantitative determination of the local water content with sub-mm resolution. Quantification of the temperature of the rupture plane and of the heat kinematics within the specimen was accomplished by temperature measurements both in-situ and ex-situ. In this way, experimental data correlating the wet tensile strength with the specific conditions of moulding sands at the rupture plane were obtained for the first time. Series of experiments with different initial sand moisture contents were conducted. The results show that the weakest location within a sand profile can be pinpointed at the interface between evaporation and condensation zone (i.e., at the 100°C isotherm), where water vaporisation starts and the water bridges connecting the sand grains collapse. During casting, however, the moulding sand undergoes irreversible alterations, which deteriorate its properties. Aim of this work, therefore, was to gain accurate insights into the relation between sand alteration and property change. To this end, the wet tensile strengths of heat pre-treated and cyclically reused sands were related to the kinematics of water within the sand as measured by in-situ neutron radiography and neutron diffraction. Sands subjected to 22 cycles of drying (T = 120°C) and remoistening (3 wt.%) revealed modifications in smectite lattice spacings and in water release behaviour from smectite interlayers. No significant change of tensile strength, however, was associated with these modifications. Contrarily, sands pre-treated with temperatures as low as 225°C revealed a reduction of tensile strength which was correlated with irreversible modifications of the structural and compositional integrity of the smectites as manifested by substantially altered water kinematics. The results show that cyclic use of moulding sand has little impact on the sand quality as long as the temperature of the sand remains low. For the practice in foundry, the results imply that a rigorous separation of heat-exposed sand is advantageous. Sands bonded with sodium- and calcium-bentonite were studies in order to unravel the influence of the exchangeable cation on the binding properties. Changes in tensile strength as well as in heat and mass transport within the sand were investigated. Heat pre-treated Ca- and Na-bentonite bonded sand revealed differences in tensile strength. Although no major differences in the swelling behaviour could be detected, Na-bentonite bonded sands have a better mechanical performance than Ca-bentonite bonded sands both before and after the pre-treatment. The measured changes in tensile strength of both Ca- and Na-bentonite bonded sand correspond well with the irreversible alterations of the smectites

Neutron Radiographic Study of the Effect of Heat-Driven Water Transport on the Tensile Strength of Bentonite-Bonded Moulding Sand

Wet tensile testing is a common method to assess the stability of bentonite bonded moulding sands. For wet tensile testing, a specimen is first heated from above in order to simulate heat-driven moisture transport induced by the casting process. Then, tensile stress is applied until rupture. In this study, neutron radiography imaging was applied to moulding sands in-situ during heating and wet tensile testing in order to investigate the effects of water kinematics on the tensile strength. Neutron radiography allowed the localization of the rupture plane and the quantitative determination of the local water content with sub-mm resolution. Quantification of the temperature at the rupture plane and of the heat kinematics within the specimen was accomplished by temperature measurements both in-situ and ex-situ. In this way, experimental data correlating the wet tensile strength with the specific conditions of moulding sands at the rupture plane were obtained for the first time. Series of experiments with different initial sand moisture contents were conducted. The results show that the weakest location within a sand profile can be pinpointed at the interface between evaporation and condensation zone (i.e., at the 100 °C isotherm), where water vaporisation starts and the water bridges connecting the sand grains collapse. This weakest location has maximum strength, if the local water content at the rupture plane is between 5 and 9 wt.%. Less water leads to a strong decrease of wet tensile strength. More water requires an initial water content above 5 wt.%, which leads to a decrease of the tensile strength of the unheated sand

Nasszugfestigkeitsprüfungen in der Neutronenradiografie

Bentonitgebundene Formsande gehören aufgrund ihrer Umweltfreundlichkeit und Wiederverwendbarkeit zu den wichtigsten Formstoffen in Gießereien. Es liegt daher nahe, das Verhalten und die Eigenschaften dieser Formsande genau zu untersuchen. Im durchgeführten Forschungsprojekt wurden die Nasszugfestigkeiten von Formstoffproben in Echtzeit während einer Durchstrahlung mit Neutronen gemessen. Dabei konnten auch der Riss abgebildet und die lokalen Wassergehalte ermittelt werden

Effects of heat and cyclic reuse on the properties of bentonite-bonded sand

Bentonite-bonded quartz sand is the most common mould material used in foundry industry. During casting, however, the moulding sand undergoes alterations, which deteriorate its properties. Aim of this work, therefore, was to gain accurate insights into the relation between smectite alteration and property change. To this end, the wet tensile strengths of heat pre-treated and cyclically reused sands were related to the kinematics of water within the sand as measured by in-situ neutron radiography and neutron diffraction. Sands subjected to 22 cycles of drying (T = 120 °C) and remoistening (3 wt%) revealed modifications of the d values of the basal reflections of smectites. No significant change of tensile strength, however, was associated with these modifications. Contrarily, sands pre-treated with temperatures as low as 225 °C revealed a reduction of tensile strength, which was neither correlated to the loss of tightly bound water nor to dehydroxylation. For temperatures above 300 °C a correlation between the reduction of tensile strength and the loss of tightly bound water or early dehydroxylation was evident. With completing dehydroxylation of the smectites above 670 °C, total loss of wet tensile strength of the pre-heated sands was observed. The results showed that cyclic use of moulding sand had little impact on the sand quality as long as the temperature of the sand remained low. For the practice in foundry, the results imply that a rigorous separation of heat-exposed sand is advantageous