Аңдатпа
When heating pulverbakelite in sand-resin mixtures during the manufacturing of shell molds, its state of aggregation changes into solid–liquid–solid form. In the process of forming a sand-resin mold, to reduce the amount of binder (pulverbakelite) in the mixture and to increase strength, along with the thermal effect, static pressure was applied that varied in accordance with changing the aggregate state of the resin. The initial pressure was 0.25 MPa, and the mixture was fed from the hopper into a model plate heated to 230 °C. After 10 seconds, pressure increased to 0.35 MPa, and after another 10 seconds it decreased to 0.2 MPa. It was experimentally established that the internal stresses of castings obtained in sand-resin molds made with the use of variable pressure were minimal and uniform. Studies also showed that such castings had a high surface purity (Rz 70-80) and no internal defects.