Samples from both types of sand were prepared with various amounts of resin to measure the permeability, bench life and development of strength.1,7,10,11
Permeability is related to the
molding quality index (MQI) that relates to the difficulty of gas passing through a core. The results shown in Table 4 indicate that the amount of resin does not exert a high influ- ence on this parameter, but the type of sand is critical, as smaller grains are able to cluster closer together, reducing
permeability and increasing the possibility for development of defects associated with trapped gas.11-13
Bench life is ex-
pressed as the time that elapses between mixing and blow- ing the cores, and it reflects the loss in strength due to the reaction of Part I and II without catalysis (Figure 10). The polymeric reaction does not stop after the amine gas (Part III) is purged, but it continues for a certain amount of time (Figure 11). This effect is called development of resistance.
Figure 8. Gas evolved in mixtures of sand with 0.80% of resin.
Figure 9. Gas evolved in mixtures of sand with 1.15% of resin.
Figure 10. Bench life of mixtures of resins A and B.
Figure 11. Development of strength of mixtures of resins A and B.
Table 4. MQI Index of Blown Cores
International Journal of Metalcasting/Winter 11
45
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