Effect of Resin Reactivity and Storage Conditions on the Properties of Foundry Cores Based on Phenolic Resole Binders

This study investigates the influence of binder reactivity and storage conditions on the mechanical and technological properties of foundry cores made with two phenol–formaldehyde resole resins differing in reactivity. Cylindrical specimens were produced using the Alpha-Set process and tested for tensile and splitting strength, gas permeability, abrasion resistance, and thermal deformation after 1 h of curing and 24 h of storage under three climatic conditions (temperature (°C) and relative humidity (RH) are given using standard notation): A (25 °C, 90% RH), B (0 °C, 10% RH), and C (15 °C, 50% RH). The bench life of the mixtures ranged from 6.5 min for the high-reactivity resin to 8.0 min for the low-reactivity resin, indicating that higher reactivity shortens the technological window but enables faster strength development. After 48 h of curing under moderate conditions (C), cores made with the high-reactivity resin achieved a tensile strength of 0.51 MPa, compared with 0.35 MPa for the low-reactivity binder. Under high humidity (A), this trend reversed—the less reactive resin reached a higher splitting strength (4.27 MPa vs. 2.95 MPa) and exhibited lower friability (0.9% vs. 3.0%), confirming greater resistance to moisture-induced plasticization. The high-reactivity resin, however, showed smaller maximum thermal deformation (≈−2 mm vs. ≈−5.5 mm), although the onset of instability occurred earlier during heating. In cold and dry conditions (B), the low-reactivity resin provided higher tensile strength (0.23 MPa vs. 0.12 MPa) and lower friability, whereas the high-reactivity resin exhibited lower thermal distortion. Binder selection should therefore reflect the prioritized property—dimensional stability or mechanical strength and abrasion resistance. A comparative evaluation of all measured parameters indicated that the most balanced performance was obtained under moderate storage conditions (15 °C, 50% RH), representing the optimal processing route for phenolic resin-bonded cores.