JPH079082A - Sand mold - Google Patents

Abstract

(57) [Abstract] [Purpose] To achieve high dimensional accuracy and prevent problems caused by gas generated during casting. A sand mold comprising an inner sand layer (1) forming a surface of a cavity (10) and containing a binder, and an outer sand layer (2) covering the outer side of the inner sand layer (2) and containing no binder, in a flask (2). Since the outer sand layer 2 is composed only of sand, the gas generated from the binder of the inner sand layer 1 has a small airflow resistance and can be easily discharged to the outside.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sand mold used for casting.

[0002]

2. Description of the Related Art As a sand mold used during casting, a so-called green mold using an inorganic binder such as bentonite,
A curable mold using a curable organic binder has been conventionally known. Of these, the green mold has been used for a long time because it is inexpensive, but since it contains water, it is greatly deformed and has a problem in dimensional accuracy.

On the other hand, the curable mold has high hardness and is excellent in dimensional accuracy, but has a disadvantage that it is much more expensive than the raw mold. Then, Japanese Patent Laid-Open No. 2-1555
Japanese Patent Laid-Open No. 34-34 discloses a method in which a first layer forming a cavity is formed by a curable mold, and a second layer made of green sand containing bentonite for a green mold is formed as a backing thereof.

That is, in this method, only the first layer, which requires dimensional accuracy, is made into a curable mold, and the remaining second layer is made into an inexpensive raw mold. Therefore, both high dimensional accuracy and low cost can be achieved. You can

[0005]

However, when casting is performed using the sand mold formed by the method described in the above publication, the heat of the molten metal decomposes the organic binder of the first layer to generate gas. However, the presence of bentonite in the second layer makes it difficult to secure a passage through which the gas escapes because of high ventilation resistance, and as a result, there is a problem that air bubbles are mixed in the casting. This also applies to a sand mold which is entirely made of a curable mold.

When the casting sand is reclaimed from the used sand mold, the first layer of sand containing the organic binder and the second layer of raw sand containing bentonite are mixed, so that the number of man-hours required for regeneration increases and There is also the problem that the properties of the sand produced are not stable. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a sand mold that has high dimensional accuracy and that can prevent problems caused by generated gas.

[0007]

The sand mold of the present invention for solving the above-mentioned problems comprises an inner sand layer forming a cavity surface and containing a binder, and an outer sand layer covering the outside of the inner sand layer and containing no binder. It is characterized in that it is provided inside.

[0008]

The sand mold of the present invention is composed of an inner sand layer, an outer sand layer and a flask. Of these, the inner sand layer forms the surface of the cavity, and it is desirable that the thickness of the inner sand layer is such that it is not destroyed by heat before the molten metal is poured and solidified. In the sand mold of the present invention, the inner sand layer contains the binder. Therefore, the sand grains are bonded together by the binder, and the mold becomes hard and has excellent dimensional accuracy. Examples of the binder include an inorganic binder such as bentonite and water glass, an organic binder such as a phenol resin, and the like.

Further, in the sand mold of the present invention, the outer sand layer enclosing the inner sand layer does not contain a binder. Therefore, the gas generated when the binder of the inner sand layer is decomposed by the heat of the molten metal easily escapes to the outside through the gap between the sand grains of the outer sand layer, so that it is possible to prevent bubbles from being generated in the casting. Further, since the dimensional accuracy is ensured in the inner sand layer, the sand grains in the outer sand layer have almost no effect on the dimensional accuracy of the casting or the surface roughness of the casting. Therefore, if sand grains having a large grain size are used as the sand grains of the outer sand layer, the size of the gap between the sand grains becomes large and the ventilation resistance becomes further smaller, so that defects due to gas can be further prevented.

In addition, zircon sand or artificial sand (for example, "Cera beads" manufactured by Naigai Ceramics Co., Ltd.)
The dimensional accuracy can be further improved by using sand having a small coefficient of thermal expansion such as (made by Japan). If the same sand is used for the inner sand layer and the outer sand layer, the binder can be easily regenerated by simply removing the binder by heating, and the cost required for regeneration can be reduced.

[0011]

EXAMPLES The present invention will be specifically described below with reference to examples. (Embodiment) FIG. 1 shows a sectional view of a sand mold of this embodiment. This sand mold is composed of an inner sand layer 1 forming the surface of a cavity 10, an outer sand layer 2 covering the outer side of the inner sand layer 1, and a casting frame 3 holding the outer sand layer 2 from the outside.

The inner sand layer 1 is composed of fuuka sand having a particle size of JIS67 mesh and phenol resin as a binder, and the binder concentration in the inner sand layer 1 is 1.5% by weight. Further, the outer sand layer 2 is composed only of fuuka sand having a grain size of JIS67 mesh and does not contain a binder. The sand mold of this example was manufactured as follows. First, sand particles containing a binder are attached to the model surface by a method such as natural dropping, blowing, or spraying, and the particles are hardened by natural hardening or gas hardening to form the inner sand layer 1.

It is necessary that the thickness of the inner sand layer 1 is set so that the inner sand layer 1 is not destroyed by heat before pouring the molten metal and solidifying it. The required thickness is almost proportional to the space between the cavities (thickness of the casting), and in the case of steel, it has the relationship shown in Table 1. Therefore, it is necessary to form the inner sand layer 1 so as to have a thickness that satisfies this required thickness in all parts.

[0014]

[Table 1] Next, a predetermined amount of sand constituting the outer sand layer 2 is laid in the flask 3, and the inner sand layer 1 formed as described above is placed thereon. Then, the sand constituting the outer sand layer 2 is further charged onto the inner sand layer 1, and the inner sand layer 1 is embedded in the sand while fluidizing the sand with air or the like. As a result, the outer sand layer 2 is formed. (Comparative example) The outer sand layer 2 has the same structure as that of the embodiment except that it is made of fuuka sand containing 7% by weight of bentonite. (Evaluation) Using the respective sand molds of the above-mentioned Examples and Comparative Examples, the same molten metal was poured and casting was performed. After the molten metal was solidified, the sand mold was broken to remove the casting, and the surface was visually observed. As a result, in the castings cast by the sand mold of the comparative example, defects due to air bubbles were generated on the surface, but the castings cast by the sand mold of the example were in good condition with no defects.

This is because, in the sand mold of the comparative example, bentonite is contained in the outer sand layer 2, so that the gas generated by the decomposition of the phenol resin contained in the inner sand layer 1 has a high ventilation resistance and, as a result, remains. It is considered that the generated gas was mixed in the casting. On the other hand, in the sand mold of the example, the ventilation resistance of the outer sand layer 2 was low and the gas was smoothly discharged to the outside, so it is considered that a good casting could be cast.

[0016]

[Effects of the Invention] That is, according to the sand mold of the present invention, it is possible to easily cast a casting having high dimensional accuracy and preventing defects due to decomposition gas. Since the thickness of the inner sand layer using the binder is 1/5 to 1/2 as compared with the conventional sand mold using the binder for the entire sand layer, the amount of the binder used is reduced and the cost is reduced. You can

Further, the sand can be easily regenerated, and the regenerated sand having good properties can be regenerated at low cost.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a sand mold according to an embodiment of the present invention.

[Explanation of symbols]

1: Inner sand layer 2: Outer sand layer 3: Casting frame 1
0: Cavity

Claims (1)

[Claims] 1. A sand mold characterized in that an inner sand layer forming a cavity surface and containing a binder, and an outer sand layer covering the outside of the inner sand layer and containing no binder are provided in a flask.