Effect of Bentonite on the Strength of Shell and Cold Box Molds

Abstract

Green sand molding is a casting method that uses bentonite as a binder and is the most widely used technique for producing cast products. Green sand molding uses a green sand mold as the main mold while incorporating various other molds, such as shell or cold box molds for the core. When the core is heated by molten metal, it disintegrates into sand and mixes with green sand. The addition of cores increases the total sand volume, leading to overflow beyond the casting line capacity, and resulting in waste sand. This waste sand is subsequently reclaimed and processed into reusable reclaimed sand. Integrating the reclaimed sand into core production can establish a sustainable recycling system within the casting industry. Two primary methods for regenerating waste sand are thermal-mechanical and mechanical reclamations. Thermal-mechanical reclamation is more suitable for shell mold, whereas mechanical reclamation is more suitable for cold box mold. This distinction has led to a confusion in the application of reclaimed sand and has hindered the development of a circular recycling system. Furthermore, the underlying cause of this difference remains unclear.

This study considers residual bentonite in reclaimed sand as a critical factor contributing to the observed discrepancies in the mold performance. In shell mold, the residual bentonite causes a considerable degradation of the shell mold strength. This strength degradation is attributed to an intercalation reaction between the shell mold binder (guest) and bentonite (host). In cold box mold, bentonite residue does not cause a notable deterioration of the mold strength. In this case, the intercalation reaction is observed to a very limited extent and is not as pronounced as in shell mold. By applying the results of this study to the reclamation method, a reclamation line that facilitates a circular recycling structure can be established.