During the process of mixing and molding

the bonding strength of sodium silicate sand is reduced due to the loss of surface water.In severe conditions

it is failure for mold making

resulting in waste of molding sand.The water glass was modified with glycerin

and the quality guarantee period（-interval time from sodium silicate sand is mixed to be able to make qualified mould）of the molding sand prepared with glycerin modified water glass and unmodified water glass was measured

and effects of glycerin on the strength

hygroscopicity and residual strength at 800 ℃ were analyzed.The results reveal that the bench life working time of molding sand made from water glass with 3% glycerin is 2 times that of unmodified ones.In the range of glycerin content 0%～3% in water glass

the tensile strength of water glass bonded sand can be increased by 7% per 1% glycerin

and the binder content of water glass bonded sand can be reduced

however the change of residual tensile strength of the molding sand can be neglected.With glycerin modified water glass

the hygroscopicity of water glass bonded sand is not changed obviously.The results show that the most prominent effect of glycerin on the modification of water glass is that it can prolong effectively the bench life working time of water glass bonded sand.