Removal of shrinkage depression in art castings with different wall thickness

Processy litʹâ, 2021, Tom 146, №4, p.14-21

Abstract

Received 28.09.2021

UDK 621.746.62

The purpose of the work is to eliminate the shrinkage depression in artistic castings with different wall thickness. The paper considers a fundamentally new method of preventing the formation of shrinkage depression. The casting defect has the form of a recess with rounded smooth edges on the surface in the thickened part of the casting, which distinguishes it from the shrinkage shell. Occurrence of a shrinkage depression is caused by formation in massive parts of a casting of thermal knots from which center the melt is filtered for food of shrinkage of the hardened layer on all nearest surfaces of a casting. Therefore, it was believed that when moving the melt from the thermal unit, in its center, a void and discharge is formed, which leads to the retraction and curvature of the hardened layer of metal on the surface of the casting with less strength. To prevent the formation of shrinkage depression, as a rule, use rods that reduce the thickness of the wall of the artistic casting, fillings and thickened spouts to feed the shrinkage of the thermal unit of the casting, internal and external refrigerators. But it is not always possible to install fillings due to non-compliance with the configuration of the surface of the artistic casting, internal rods for small castings are difficult to manufacture and fix in the mold. Therefore, on the example of making an artistic casting “Buffalo” when casting on melted models, a new method of removing the shrinkage depression is proposed. The paper took into account that the shrinkage shells and the porosity in the middle of the casting are not controlled in contrast to the high requirements for the accuracy of the surface shape of artistic cast products. The discharge that occurs in the center of the heat unit was eliminated by moving atmospheric air through a thin tubular needle made of austenitic stainless steel, which is used for medical injections. The tube with an outer diameter of 0.80 mm was installed in the mold with one end in the center of the thermal unit of the thickened part of the casting, and the other in the mold from the non-working surface, which is inaccessible to inspection and does not violate the aesthetic appearance of the artistic casting. Due to the connection of the other end of the tube with atmospheric air, the discharge in the center of the shrinkage does not occur. The new foundry system without a massive central feeder, which was previously fed into the thermal unit, eliminated shrinkage defects on the outer surface, reduced the cost of the foundry system, minting the surface at the junction of the feeder with the artistic casting, avoiding the formation of shrinkage depression. The results of the work are recommended for casting of artistic and jewelry products that have a lower melt temperature than in a stainless steel tubular needle.