SELECTION AND OPTIMIZATION OF REFRACTORY COATINGS AND DRYING REGIMES FOR 3D-PRINTED PLA PATTERNS IN INVESTMENT CASTING OF ALUMINUM ALLOYS

Abstract

Investment casting using additively manufactured polymer patterns requires refractory coatings tailored to the thermal-decomposition behavior of the pattern material rather than to classical wax-pattern practice. This study evaluates four refractory coating systems for 3D-printed PLA (and ABS/PMMA) patterns used in lost-pattern casting of aluminum alloys: (1) ZrSiO4 (zircon) with a colloidal SiO2 binder, (2) SiO2 with sodium silicate (Na2SiO3, liquid glass), (3) Al2O3 with a phosphate binder, and (4) MgO- or SiO2-based compositions with a colloidal binder. Coatings were compared by thermal stability, adhesion, gas permeability, surface roughness (Ra), cost, and drying behavior. The zircon-colloidal-silica system provided the best overall balance: thermal stability up to 1600-1700 °C, adequate gas permeability during polymer burnout (250-450 °C), crack-free behavior, and a surface roughness of Ra 3-6 µm, at the cost of higher raw-material cost and longer drying time (12-24 h). Drying-parameter optimization established that a temperature of 50 ± 5 °C with forced air circulation (0.5-1.5 m/s) minimizes microporosity (from incomplete moisture removal below 40 °C) and pattern deformation (above the PLA glass-transition range of 55-65 °C), with optimal drying times of 2-4 h for coating layers of 0.3-0.6 mm and 5-6 h for 0.6-1.0 mm. These results provide a practical basis for shell-coating protocols in additive-manufacturing-based investment casting of complex-geometry aluminum parts.