Gas permeability of moulds and cores is an important factor to consider in the casting process. In foundries, gas permeability is measured by using instruments which give dimensionless numbers. This approach enables the comparison of values between samples and is often not quantified in units. In this study, a custom-made measurement system is introduced that applies Darcy?s law, where pressure gradients for different flow rates are studied. The Darcian permeability in standard cylindrical samples was determined using a method that is familiar with those in earth sciences. Two types, steady-state and unsteady-state approaches were used for the calculations, and the difference in permeability values generated by these two methods is discussed. The results of a silica sand sample with furan resin and a 3D-printed sample that consists of artificial granulous material with phenolic resin were compared.

Furan sand is one of the most commonly used chemically bonded molding materials in foundries across the world. It consists of a furfuryl alcohol-based resin and an acid-based liquid catalyst. When the molding material comes in contact with the molten metal, it undergoes a thermal shock accompanied by a certain release of volatile gases. In order to evacuate these gases, molds and cores should have optimal gas permeability values and proper venting by design. If the volatile compounds are not appropriately evacuated, they are prone to enter the melt before the first layer of solidified metal is formed which can lead to the formation of gas-related casting defects. Standard gas permeability measurements are commercially available tools used in the industry to compare and to quality control different sands, however, they only provide reference numbers without actual units. Permeability in a standard unit, m2, provides uniformity and helps the comparison of results from difference sources. In this paper, a new method using Darcy’s law (prevalent in earth sciences), was adapted to measure the gas-permeability of furan samples made of silica sand with various grain size distributions. The effect of grain size distribution on the gas-permeability of furan sand samples was studied. Gas-permeability values in m2 were then correlated with mercury-porosity measurement results to bring new light on the relation between pore size, pore volume and the permeability of molding materials.

The density of moulding mixtures used in the foundry industry plays a significant role since it influences the strength, porosity, and permeability of moulds and cores. The latter is routinely tested in foundries using different solutions to control the properties of the moulding materials that are used to make moulds and cores. In this paper, the gas permeability of sand samples was measured using a custom-made setup to obtain the gas permeability in standard units instead of the usual permeability numbers (PN) with calibrated units. The aim of the work was to explore the effect of density variations in moulding materials on their gas permeabilities. Permeability in this work is quantified in SI units, square metres [m<sup>2</sup>]. The setup works based on Darcy’s law and the numbers obtained from the measurements can be used to deduce the gas permeability, <em>k</em>, of a sample. Two furan resin bonded mixtures with the same grain size distribution were hand-rammed with varying compaction forces to obtain a variation in density. Cylindrical samples (50 × 50 mm) were prepared using a silica sand aggregate sourced from a Swedish lake. The results of the measurement provided the difference in gas permeability between the samples that have varying densities. The results of permeability were then extrapolated by modifying the viscosity value of the air passed through the sample. In order to find the effect of apparent density variation on the pore characteristics of the samples, mercury intrusion porosimetry (MIP) was also performed. The results were in line with the gas permeability measurements.

Predicting the permeability of different regions of foundry cores and molds with complex geometries will help control the regional outgassing, enabling better defect prediction in castings. In this work, foundry cores prepared with different bulk properties were characterized using X-ray microtomography, and the obtained images were analyzed to study all relevant grain and pore parameters, including but not limited to the specific surface area, specific internal volume, and tortuosity. The obtained microstructural parameters were incorporated into prevalent models used to predict the fluid flow through porous media, and their accuracy is compared with respect to experimentally measured permeability. The original Kozeny model was identified as the most suitable model to predict the permeability of sand molds. Although the model predicts permeability well, the input parameters are laborious to measure. Hence, a methodology for replacing the pore diameter and tortuosity with simple process parameters is proposed. This modified version of the original Kozeny model helps predict permeability of foundry molds and cores at different regions resulting in better defect prediction and eventual scrap reduction.

Gas generation from molding materials creates a complex atmosphere in the mold–metal interface and is one of the primary causes of defects in cast components. Moisture, crystalline water, and decomposing binders are significant gas sources. The presence of volatiles and decomposing binder in the mold also affects the rate of heat absorption from the solidifying metal during the casting process. This work presents a measurement methodology to evaluate the rate and volume of gases generated from sand mixtures in combination with the temperature distribution and applied thermal analysis. The presented results show high reproducibility of the method. The thermal analysis results provide the start and end temperature of the binder decomposition reactions and the corresponding heat absorbed in this interval. The results obtained from the presented methodology can be used to validate the models/simulation tools developed to predict the gas evolution and related transport phenomena in the sand casting process.

Accurate kinetic parameters are vital for quantifying the effect of binder decomposition on the complex phenomena occurring during the casting process. Commercial casting simulation tools often use simplified kinetic parameters that do not comprise the complex multiple reactions and their effect on gas generation in the sand core. The present work uses experimental thermal analysis techniques such as Thermogravimetry (TG) and Differential thermal analysis (DTA) to determine the kinetic parameters via approximating the entire reaction during the decomposition by multiple first-order apparent reactions. The TG and DTA results reveal a multi-stage and exothermic decomposition process in the binder degradation. The pressure build-up in cores/molds when using the obtained multi-reaction kinetic model is compared with the earlier approach of using an average model. The results indicate that pressure in the mold/core with the multi-reaction approach is estimated to be significantly higher. These results underscore the importance of precise kinetic parameters for simulating binder decomposition in casting processes.

Casting defects due to gas entrapment from molds and cores are responsible for a large amount of scrap in foundries. Accurate simulation of the gas generation and transport phenomena and the associated defect formation mechanism enables sustainable manufacturing of cast components and accurate material property prediction. This work presents a computational model of heat and gas transport in a dried furan core. The distributed gas source in the sand core was accurately defined with a novel approach for the description of the binder decomposition process. By employing experimentally obtained permeability and porosity, the gas velocity and the heat transfer in the porous sand core were computed. A local thermal non-equilibrium situation in sand core was modelled by coupling the heat equations for solid and fluid phases. The simulation results show good general agreement with the experimentally obtained total evacuated gas volume and temperature distribution in the sand core. The experimentally validated model enables prediction of the pressure build-up, temperature distribution, and the Darcy velocity at different regions of the sand core. The model can, therefore, be applied as a diagnostic tool to predict gas-related defect formation in the castings for different molding materials and cooling conditions. Furthermore, addressing the convective heat transfer in the sand core due to the gas evolution contributes to a more accurate prediction of solidification rate and material properties of a cast component.