PURPOSE: To obtain resin coated sand grains for a shell mold having a low coefft. of thermal expansion and an excellent collapsing property by limiting the weight ratio of the residue of catechol refining to be compounded with a phenolic resin.

CONSTITUTION: The residue of catechol refining is compounded at 3W20pts.wt. based on 100pts.wt. phenolic resin. The purpose cannot be achieved in this case unless the ratio does not attain 3pts.wt. and the strength of a casting mold decreases if the ratio exceeds 20pts.wt. The timing for adding the reside of catechol refining may be selected either in the stage of producing the resin, in the stage of storage or in the stage of mixing the same with sand grains. In the case of adding said residue in the stage of production, the addition is accomplished after the synthesis of the resin, that is, after the stage for condensation and dehydration under vacuum in order to maintain the characteristics of the residue of catechol refining without spoiling.