INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2019-231698 filed on Dec. 23, 2019 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to a method for producing a salt core.

2. Description of Related Art

In metal casting such as die casting, a core having high strength is necessary for molding a hollow component. As a material of a core, granular sodium chloride (sometimes referred to as “salt”) is suitably used. Japanese Patent Application Publication No. 07-195148 discloses a method for producing a salt core by pressure molding of granular salt as it is by using a hydraulic press (Claim 1, Paragraph 0002, Item of [Examples] and the like).

SUMMARY

In the above-described method in which granular salt as it is used as a molding material is subjected to pressure molding, coefficients of friction between salt crystals, and between a salt crystal and a die are so large that fluidity of the molding material is low in charging the molding material into the die, and hence, the molding material is difficult to charge into the die, and the degree of design freedom in shape of a core to be produced is low. When this method is employed, a core in a simple shape alone can be produced. Although there is a method in which salt is melted by heating to be charged into a die and then solidified, a large amount of energy is necessary for melting salt, and long time is necessary for solidifying the melted salt, and therefore, this method is costly, and has poor productivity.

The present disclosure provides a method for producing a salt core in which a salt core can be produced with low energy, low cost, and high productivity, and in which the salt core can be easily molded and the degree of design freedom in shape of the salt core is high.

A method for producing a salt core according to one aspect of the present disclosure includes a step (A) of adding a saturated sodium chloride aqueous solution to a sodium chloride crystal that is granular, to prepare a slurry mixed material of sodium chloride and water; a step (B) of subjecting the slurry mixed material to pressure molding to obtain a molded article; and a step (C) of drying the molded article to remove moisture.

In this aspect, a slurry mixed material of sodium chloride and water is obtained by adding a saturated sodium chloride aqueous solution (sometimes referred to as “saturated salt water”) to a granular sodium chloride crystal (sometimes referred to as the “salt crystal”). When water is added to a granular salt crystal, a part of the granular salt crystal is eluted into water, which may change an average grain size of the salt crystal, and may change suitable molding conditions, and hence it is apprehended that molding cannot be stably performed. When saturated salt water is added to a granular salt crystal, elution of a part of the granular salt crystal into water is restrained, and change of the suitable molding conditions otherwise caused by change of the average grain size of the salt crystal can be restrained, and therefore, molding can be stably performed.

Besides, as a molding material, the slurry mixed material of sodium chloride and water is used. A large part of a liquid content contained in the molding material is squeezed out through the pressure molding, but the resultant molded article contains a remaining portion of the salt water. In the step (C) of removing moisture by drying the molded article, the salt is recrystallized, and hence a core having a high density close to a single crystal can be produced.

Besides, the saturated salt water is added to the salt crystal in an amount for covering the whole surface of each granular salt crystal with a film containing salt and water (sometimes referred to as the “hydrous film”). Since the salt crystals are not in direct contact with one another but the hydrous film is disposed among these, a coefficient of friction among the salt crystals is reduced, fluidity of the molding material is increased, and hence the molding material can be easily charged into a die. Similarly, since each salt crystal and a die are not in direct contact with each other but the hydrous film is disposed therebetween, a coefficient of friction between the salt crystal and the die is reduced, and hence the molding material can be easily charged into the die.

In this aspect, in a graph corresponding to the relationship between a mass of the saturated sodium chloride aqueous solution added with respect to 100 parts by mass of the sodium chloride crystal and a flow rate of a mixed material of sodium chloride and water, when a mass added with which the flow rate starts to increase, found by increasing the mass added from 0 parts by mass, is defined as Ma parts by mass, the mass added may be over Ma parts by mass in the step A.

With the above configuration, when the mass of the saturated salt water added is over Ma parts by mass, the mixed material to be used as the molding material is in the form of a slurry having fluidity, and hence the molding material can be easily charged into a die. When the mass of the saturated salt water added is over Ma parts by mass, the resultant mixed material is a slurry in which the whole surface of each salt crystal is covered with a hydrous film, and hence, the hydrous film is disposed among salt crystals and between a salt crystal and a die, and therefore, coefficients of friction among the salt crystals and between the salt crystal and the die are reduced, and the molding material can be easily charged into the die. When the method for producing a salt core of the present disclosure is employed, since the molding material can be easily charged into the die, the degree of design freedom in shape is preferably high. Differently from a method in which salt is melted by heating to be charged into a die, and then solidified, the method for producing a salt core of the present disclosure does not require melting and solidifying steps, and therefore, a salt core can be produced with low energy, low cost and high productivity.

In this aspect, in the graph, when a mass added with which the flow rate first reaches a maximum rate, found by increasing the mass added from 0 parts by mass, is defined as Mb parts by mass, the mass of the saturated sodium chloride aqueous solution added may be equal to or larger than Mb parts by mass in the step (A).

With the above configuration, when the mass of the saturated salt water added is in a range of the Ma to Mb parts by mass, it is apprehended that the flow rate varies even if the mass of the saturated salt water added is fixed, but when the mass of the saturated salt water added is equal to or larger than Mb parts by mass, the flow rate of the resultant mixed material is stabilized, and hence molding conditions are preferably stabilized. When the mass of the saturated salt water added is equal to or larger than Mb parts by mass, the whole surface of each salt crystal is covered with the hydrous film having a suitable thickness, and the flow rate of the resultant mixed material is preferably stabilized at the maximum rate.

In this aspect, the pressure molding may be performed with a liquid content of the slurry mixed material discharged from both sides in a pressure applying direction in the step (B). With the above configuration, the pressure molding can be performed with a concentration difference of the liquid content inhibited from occurring in the material contained in the die. When this method is employed, partial increase of the coefficient of friction among the salt crystals and the coefficient of friction between the salt crystal and the die caused due to remarkable partial lowering of the concentration of the liquid content can be restrained, and therefore, the pressure can be satisfactorily applied to the whole material through the whole step of the pressure molding, and hence, the pressure molding can be satisfactorily performed.

In this aspect, a lubricant that is oily, may be applied onto an inner surface of a die and then the slurry mixed material may be charged into the die to perform the pressure molding in the step (B). With the above configuration, the friction between the salt crystal and the die can be reduced by precedently applying an oily lubricant onto the inner surface of the die, and hence pressure can be satisfactorily applied to the whole molding material. It is noted that an aqueous lubricant cannot attain the lubricating effect because the lubricant is dissolved into water contained in the molding material.

In this aspect, the lubricant may have a dynamic viscosity of 20 to 120 mPa·s.

In this aspect, a die may be provided with one or more discharge holes, and the liquid content may be discharged through the discharge holes.

According to the aspect of the present disclosure, a salt core can be produced with low energy, low cost and high productivity, and thus, a method for producing a salt core in which the salt core can be easily molded, and the degree of design freedom in shape of the salt core is high can be provided.

