Compatibility Limits of Mixed CTE-Viscosity Glass Melts: Experimental Investigation of Casting Parameters

Experimental Investigation of Casting Parameters

Abstract

In glass melting, combining glasses with differing coefficients of thermal expansion (CTE) and viscosity can generate stresses and cracking. In recycling contexts, however, full separation of glass types is sometimes challenging or economically unviable. This study examines how the processing parameters cullet size, peak temperature and dwell time influence the tolerance of mixed‑glass systems to CTE and viscosity mismatches in a casting setup, with the aim of clarifying stress‑relief mechanisms and miscibility effects. Pairs of glasses were cast in investment moulds, making tiles of 50 × 50 mm size and circa 5 mm thickness, using combinations with varying CTE mismatches at a 1:1 weight ratio. Clear glasses of known composition were used, including soda-lime, borosilicate and alumino-borosilicate types. Different cullet sizes were fired at varying peak temperatures and dwell times. Specimens were then evaluated for bonding, homogeneity, residual stress, and cracking. A heuristic mismatch index was additionally constructed to describe the combined influence of viscosity and CTE on compatibility. Results show that compatibility cannot be predicted from CTE mismatch alone. Cullet geometry, viscosity behaviour, and thermal history can strongly affect bonding and fracture likelihood. Powdered cullet and high peak temperatures (≥1120 °C) consistently increased tolerance to CTE differences by promoting enhanced flow and partial interdiffusion. The mismatch index performed well for conditions dominated by fusion but was less applicable when substantial interdiffusion occurred. Overall, the findings demonstrate that compatibility in mixed‑glass casting is process‑dependent rather than governed by fixed material thresholds, offering a basis for improved assessment and utilisation of heterogeneous waste-glass streams.