Effect of Wall Thickness on Properties of Multilayer Composite Plaster Mold for Investment Casting

In order to research whether the properties of a thin-walled plaster mold meets the production requirements in investment casting, this paper investigates a novel preparation technology for an adhesion-layer composite plaster mold. Based on a solid plaster mold, a single adhesion layer was applied to the outer surface for preparing the composite mold. An adhesion layer was deposited using silica sol as an adhesive, while refractory materials served as sand particles. Effects of specimen wall thickness on the strength, permeability, and thermal conductivity of composite mold were investigated. The results reveal positive correlation between wall thickness of the composite mold and its strength properties. Permeability and thermal conductivity, on the other hand, decrease with increasing wall thickness. Notably, at wall thickness of 3.5/4 compared to original specimen, significant improvements are observed. Specifically, compared to original solid mold, green specimens exhibit an increase of 4.64% and 7.80% in flexural and tensile strength, respectively. For fired specimens, the increases are even more remarkable, reaching 28.87% for flexural strength and 28.71% for tensile strength. Moreover, the increment of permeability and thermal conductivity with fired specimens was 169.33% and 9.45%, respectively. The monoclinic ZrO₂ (m-ZrO₂) characteristic peak appeared in the Raman spectra of the composite mold. It is shown that Zr4+ plays an important role in the accumulation of anion groups with the gypsum system, and then improves the macroscopic strength of mold. The obvious physical interface is observed between the plaster matrix and adhesive layer. It is noteworthy that the gypsum matrix and adhesion layer demonstrate varying abilities to resist structural damage, with a macroscopic fracture of gypsum matrix being more likely to occur prior to adhesion layer. Composite plaster mold exhibits a symmetrical and stable crystal structure and has better fracture deformation resistance compared to a solid mold. Fracture of the mold can be attributed to three types of failure: fiber failure, delamination failure, and damage between fibers and the matrix. These failure types are major factors influencing macro fracture of composite molds.