Casting green sand mold, and method for producing cast article using it

1. A casting green sand mold comprising at least a pair of green sand mold parts each having a cavity portion, said green sand mold parts being stacked with their cavity portions aligned to constitute a metal-melt-receiving cavity;

each of said green sand mold parts being formed by casting sand containing a binder and water;

a hardening resin-containing coating layer being formed on at least a cavity surface of each green sand mold part;

said hardening resin-containing coating layer consisting of said casting sand, cured hardening resin and gas permeable pores between them;

said hardening resin consisting of a thermosetting resin selected from phenol resins, epoxy resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, polyurethanes, and thermosetting polyimides;

said gas-permeable pores having sufficient permeability to permit a gas generated by pouring a melt to escape; and

a surface layer including said coating layer in a range from the cavity surface to the depth of 5 mm having a water content, which is smaller than the average water content of a green sand mold portion excluding said surface layer, wherein the water content of said surface layer is 0. 5% to 2.5% by mass, and wherein a thickness of said coating layer is 0.5 mm or more and 3 mm or less.

2. The casting green sand mold according to claim 1 , wherein said gas-permeable pores are composed of communicating pores substantially uniformly distributed in said coating layer.

3. The casting green sand mold according to claim 1 , wherein said surface layer has permeability, measured by an accelerated method using a large orifice according to JIS Z2601, of 50-200.

4. The casting green sand mold according to claim 1 , wherein said coating layer has average hardness in a range of 50-95 when measured by a self-hardening mold hardness tester.

5. The casting green sand mold according to claim 1 , wherein the water content of said surface layer is 0.5% to 2.0% by mass.

6. The casting green sand mold according to claim 1 , wherein the water content of said surface layer is 1.3% to 2.1% by mass.

7. A method for producing cast articles comprising the steps of forming at least a pair of green sand mold parts each having a cavity portion by casting sand containing a binder and water;

applying a coating solution containing a hardening resin to at least a cavity surface of each green sand mold part;

stacking said green sand mold parts such that their cavity portions are aligned to form a cavity, thereby forming a green sand mold having a hardening resin-containing coating layer on at least the cavity surface;

curing said hardening resin before or after said mold-parts-stacking step, to form a coating layer consisting of said casting sand, cured hardening resin and gas permeable pores between them, said hardening resin consisting of a thermosetting resin selected from phenol resins, epoxy resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, polyurethanes, and thermosetting polyimides, said gas-permeable pores having sufficient permeability to permit a gas generated by pouring a melt to escape;

drying a surface layer including said coating layer simultaneously with or after curing said hardening resin, thereby making a water content in said surface layer in a range from the cavity surface to the depth of 5 mm smaller than that in the inner portion of said green sand mold, wherein the water content of said surface layer is 0.5% to 2.5% by mass, and wherein a thickness of said coating layer is 0.5 mm or more and 3 mm or less; and

pouring a metal melt into said cavity when the surface temperature of said coating layer is 50° C. or higher.

8. The method for producing cast articles according to claim 7 , wherein said green sand mold parts to which said coating solution is applied are heated before or after said mold-parts-stacking step, thereby curing said thermosetting resin and drying said surface layer.

9. The method for producing cast articles according to claim 7 , wherein the amount of said coating solution applied is 100-550 g/m 2 per a unit area.

10. The method for producing cast articles according to claim 7 , wherein said gas-permeable pores are communicating pores substantially uniformly distributed in said coating layer.

11. The method for producing cast articles according to claim 7 , wherein said surface layer has permeability, measured by an accelerated method using a large orifice according to JIS Z2601, of 50-200.

12. The method for producing cast articles according to claim 7 , wherein said coating layer has average hardness in a range of 50-95 when measured by a self-hardening mold hardness tester.